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Jeffrey D. Taylor, Douglas F. Hunsaker
Journal of Aircraft pp 1-14; https://doi.org/10.2514/1.c036374

Abstract:
As contemporary aerostructural research for aircraft design trends toward high-fidelity computational methods, aerostructural solutions based on theory are often neglected or forgotten. In fact, in many modern aerostructural wing optimization studies, the elliptic lift distribution is used as a reference in place of theoretical aerostructural solutions with more appropriate constraints. In this paper, the authors review several theoretical aerostructural solutions that could be used as reference cases for wing design studies, and these are compared to high-fidelity solutions with similar constraints. Solutions are presented for studies with 1) constraints related to the wing integrated bending moment, 2) constraints related to the wing root bending moment, and 3) structural constraints combined with operational constraints related to either wing stall or wing loading. It is shown that, under appropriate design constraints, theoretical solutions for the optimum lift distribution may capture aerostructural coupling sufficiently to serve as appropriate reference cases for higher-fidelity solvers. A comparison of theoretical and high-fidelity solutions for the optimum wingspan and corresponding drag reveals important insights into the effects of certain aerodynamic and structural parameters and constraints on the aerodynamic and structural coupling involved in aerostructural wing design and optimization.
, Fábio Morgado, Marco Fossati, Walter T. Maier, Brian C. Munguía, , Adrien Loseille
AIAA Journal pp 1-12; https://doi.org/10.2514/1.j060470

Abstract:
Predicting shock/shock and shock/boundary-layer interactions in gas flows that surround high-speed vehicles is key in aerodynamic design. Under typical hypersonic conditions, these flow structures are influenced by complex nonequilibrium phenomena leading to high-temperature effects. In this work, the conceptual Bedford wing-body vehicle is studied to analyze flow patterns in shock/shock and shock/boundary-layer interactions with thermochemical nonequilibrium. A parametric computational fluid dynamics study is carried out for different hypersonic operating conditions, with respect to the freestream Mach number. Simulations are performed with the SU2-NEMO solver coupled to the Mutation++ library, which provides all the necessary thermodynamic, kinetic, and transport properties of the mixture and chemical species. The Adaptive Mesh Generation library is used for automatic anisotropic mesh adaptation. Numerical results show that increasing the freestream Mach number from 4 to 10 leads to changes in the shock layer, locations of shock impingement, and regions of boundary-layer separation. Despite these changes, the change in freestream Mach number has little impact on the overall shock interaction structures.
Huiying Zhang, Xiaohua Wu
AIAA Journal pp 1-15; https://doi.org/10.2514/1.j060505

Abstract:
Velocity gradient tensor invariants are used to extract flow physics near the turbulent and nonturbulent interfaces (TNTIs) of cylinder and airfoil wakes with vortex shedding in conjunction with spatially developing direct numerical simulation. Conditional sampling is performed on fuzzy-cluster-method-resolved TNTIs using a novel subzone approach in which each instantaneous TNTI is subdivided into four categories: trough, bulge, leading edge, and trailing edge. Results of the conditionally sampled statistics, topology, and orientation of TNTI local structures suggest that wake TNTI properties depend more heavily on the degree of vortex shedding and relatively less on the degree of wake symmetry. The present subzone-sampled joint probability density functions of the second and the third invariants of the velocity gradient tensor are compared with existing jet and mixing layer observations, and new insights are extracted. Random relative orientation between the vorticity vector and the TNTI normal is observed in the trough subzone of the present wake TNTIs, which casts doubts on the notion of full vortex structure confinement. The turbulent flow near the trailing edge subzone of wake TNTI is found to be the most effective in enstrophy production, whereas the turbulent flow in the leading-edge portion is the least effective.
Mahmudul Hasan,
AIAA Journal pp 1-12; https://doi.org/10.2514/1.j060372

Abstract:
Uncertainty in the microstructures has a significant influence on the material properties. The microstructural uncertainty arises from the fluctuations that occur during thermomechanical processing and can alter the expected material properties and performance by propagating over multiple length scales. It can even lead to the material failure if the deviations in the critical properties exceed a certain limit. We introduce a linear programming (LP) based method to quantify the effects of the microstructure uncertainty on the desired material properties of the titanium–7 wt % aluminum alloy, which is a candidate material for aerospace applications. The microstructure is represented using the orientation distribution function (ODF) approach. The LP problem solves for the mean values and covariance of the ODFs that maximize a volume-averaged linear material property. However, the analytical procedure is not applicable for maximizing nonlinear material properties where microstructural uncertainties are present. Therefore, an artificial neural network based sampling method is developed to estimate the mean values and covariance of the ODFs that satisfy design constraints and maximize the volume-averaged nonlinear material properties. A couple of other design problems are also illustrated to clarify the applications of the proposed models for both linear and nonlinear properties.
Hugo Fournier, Paolo Massioni, Minh Tu Pham, Laurent Bako, Robin Vernay, Michele Colombo
Journal of Guidance, Control, and Dynamics pp 1-15; https://doi.org/10.2514/1.g006084

Abstract:
This work concerns the problem of gust load alleviation of a flexible aircraft by focusing on Airbus’s XRF1 aircraft concept, with a fully actuated wing. The aircraft is equipped with a lidar sensor, which measures the wind velocity ahead of it, together with standard sensors. Based on the available measurements, controllers are then designed by H∞ and μ syntheses, with emphasis put on the multiple-input multiple-output robustness in order to reduce a selected set of loads due to the wind, hence potentially saving mass in the aircraft design. A state-space model of the flexible aircraft is obtained by means of an aeroelastic computation and system identification from frequency data. The controllers’ performance is evaluated through their capability to reduce shear force, bending, and torsion moments on different locations of the aircraft in response to different types of discrete gusts and continuous turbulence; the constraints of the sensors and the actuators are taken into account. The gain in performance due to the use of lidar is assessed, and a tradeoff is discussed concerning the optimal measurement distance. Finally, the closed-loop robustness is assessed by simulations where different types of uncertainties are applied to the system.
Peter R. Forsyth, David Gillespie, Matthew McGilvray
Journal of Thermophysics and Heat Transfer pp 1-11; https://doi.org/10.2514/1.t6175

Abstract:
Transient thermochromic liquid crystal (TLC) experiments can provide high-fidelity spatially resolved heat transfer data for complex geometries, particularly where infrared techniques cannot be applied. One challenge when applying transient methods to internal geometries is the local definition of the driving gas temperature. The transient nature of the streamwise driving gas temperature profile has led to comparisons with steady-state computational fluid dynamics being questioned. This paper explores simulating the temporal behavior of transient TLC experiments directly. A novel technique is developed to account for differences in the gas and solid time scales, where surface temperature is calculated at each spatial location analytically from the surface heat flux, using an impulse response method assuming one dimensional, semi-infinite conduction. Postprocessing of the simulated surface temperature history is performed using the same method as experimentally, allowing for direct comparison. This analytical thermal boundary condition (ATBC) is applied to simulate a transient TLC experiment of a stationary superscaled rib turbulated internal cooling passage in a gas turbine engine. Traditional steady-state simulations were also performed with constant temporal and spatial temperature boundary conditions. Results show that calculations using the new ATBC and traditional steady-state method give very similar Nusselt number distributions and mean values in relation to the experimental data, suggesting the larger discrepancy between simulations and the experiments is not the definition of the driving gas temperature. Analysis of transient variation of Nusselt number indicated brief highly localized maximum variations up to 40%, although this was not found to significantly affect the mean values, and passage-averaged values converged to within 0.5% of the final value within 0.2 s.
Eugene P. Bonfiglio, Mark Wallace, Eric Gustafson, Min-Kun Chung, Evgeniy Sklyanskiy, Devin Kipp
Journal of Spacecraft and Rockets pp 1-10; https://doi.org/10.2514/1.a35181

Abstract:
Early in operational testing for the InSight mission to Mars, it was discovered that the final maneuver to target the entry-interface point was unexpectedly sensitive to planned atmosphere updates that would be based on real-time measurements of the Martian atmosphere by Mars Reconnaissance Orbiter. Upon investigation, the team realized that the Phoenix mission also discovered this sensitivity during operational testing. Further investigation identified that the sensitivity was a result of the fact that both the entry flight path angle and ground target were being held fixed in the maneuver design process, constraining the maneuver in a way that forced the entry time to change to compensate for changes to the nominal trajectory from updating the atmosphere model. The final maneuver was planned for 22 h before entry, at which point it is very expensive to change entry time. The study also revealed that any unguided Mars entry, descent, and landing mission would be impacted by this sensitivity if it used real-time atmosphere observations to model the nominal expected atmosphere used for maneuver targeting of the entry-interface point. This paper discusses the results of that investigation and presents a number of mitigations as well as the consequences of ignoring the sensitivity.
, Yusuke Oki, Yuichi Tsuda
Journal of Guidance, Control, and Dynamics pp 1-23; https://doi.org/10.2514/1.g005564

Abstract:
The orbital motion of spacecraft around asteroids is strongly disturbed primarily because of solar radiation pressure (SRP) and higher-order gravitational forces. To achieve stable orbits in such an environment, the implementation of frozen orbits is a promising approach. This research derives semi-analytical solutions of frozen orbits subject to SRP and zonal gravity up to the fourth order based on singly averaged Lagrange planetary equations. Moreover, the stability of frozen orbits is characterized analytically by introducing linearized variational equations, revealing the complex eigenstructures of the proposed frozen orbits. These analytical theories identify several different types of stable frozen orbits, which are further validated via high-fidelity numerical simulations. Consequently, this paper demonstrates the feasibility and intriguing dynamic characteristics of frozen orbits around asteroids.
Ananthalakshmy K. Moorthy, John J. Blandino, Michael A. Demetriou, Nikolaos A. Gatsonis
Journal of Spacecraft and Rockets pp 1-17; https://doi.org/10.2514/1.a34975

Abstract:
A wide variety of missions could be enabled by extended orbital flight in extreme low Earth orbit, defined as an altitude range of 150–250 km. This study investigates the feasibility of a nanosatellite (mass <10 kg) using a propulsive, attitude control system in conjunction with a primary propulsion system to extend mission life. The primary propulsion system consists of a pair of electrospray thrusters providing a combined thrust of 0.12 mN at 1 W. Pulsed plasma thrusters are used for attitude control. The mission consists of two phases. In Phase I, a 4U CubeSat is deployed from a 414 km orbit and uses the primary propulsion system to deorbit to an initial altitude within the targeted range of 244±10 km. Phase I lasts 12.73 days, with the propulsion system consuming 5.6 g of propellant to deliver a ΔV of 28.12 m/s. In Phase II the mission is maintained until the remaining 25.2 g of propellant is consumed. Phase II lasts for 30.27 days, corresponding to a ΔV of 57.22 m/s with a mean altitude of 244 km. Using this approach, a primary mission life of 30.27 days could be achieved, compared with 3.1 days without primary propulsion.
Wei Dong, Chunyan Wang, Jianan Wang,
Journal of Guidance, Control, and Dynamics pp 1-15; https://doi.org/10.2514/1.g005971

Abstract:
In this paper, a distributed three-dimensional (3D) nonsingular cooperative guidance law is designed for multiple missiles with different seeker’s field-of-view (FOV) constraints to achieve salvo attack against a stationary target. The guidance law is derived from an extended biased proportional navigation guidance (PNG). The PNG is augmented by a cooperative guidance term to ensure that times-to-go of the missile group reach consensus in a fixed time before interception. Two auxiliary functions are included into the biased guidance term to guarantee nonsingularity of the guidance law and meet the FOV constraints. The proposed design does not require guidance law switching that is used in some existing biased or multistage guidance designs, which allows for a smooth guidance command. A feasible impact time set is given for the cooperative mission when applying the proposed guidance law. The nonsingularity, FOV constraint satisfaction, and closed-loop stability of the proposed guidance law are theoretically analyzed and proved. Moreover, the criteria for selecting all guidance parameters are provided to facilitate the guidance design. Finally, numerical simulations with comparative studies are conducted to verify the effectiveness of the proposed guidance law and demonstrate the advantages over existing cooperative guidance laws.
Peng Zhao, Heinz Erzberger, Yongming Liu
Journal of Guidance, Control, and Dynamics pp 1-19; https://doi.org/10.2514/1.g005825

Abstract:
A new method for efficient trajectory planning to resolve potential conflicts among multiple aircraft is proposed. A brief review of aircraft trajectory planning and conflict resolution methods is given first. Next, a new method is proposed that is based on a probabilistic conflict risk map using the predicted probability of conflict with the intention information of the intruders. The risk-map-based method allows the path planning algorithm to simultaneously account for many uncertainties affecting safety, such as positioning error, wind variability, and human errors. Following this, A* algorithm is used to find the cost-minimized trajectory for a single aircraft by considering all other aircraft as intruders. Search heuristic method is implemented to iterate the A* algorithm for all aircraft to optimize the trajectory planning. Convergence and computation efficiency of the proposed method are investigated in detail. Numerical examples are used to illustrate the effectiveness of the proposed method under several important scenarios for air traffic control, such as wind effects, non-cooperative aircraft, minimum disturbance of pilots, and deconflict with flight intent information. Several conclusions are drawn based on the proposed method.
Daisuke Nakata, Kiyoshi Kinefuchi, Hitoshi Sakai, Suyalatu
Journal of Propulsion and Power pp 1-9; https://doi.org/10.2514/1.b38187

Abstract:
A high-efficiency concentric tubular-type resistojet with potential application to short-term orbit-raising maneuvers has been fabricated by 3-D printing and demonstrated. The propellant flows through multiple layers of cylindrical shells, with this structure also functioning as a single-piece heater. A 6-cm-high cylinder was realized with a wall thickness of 0.2 mm, using Inconel 718. A nodal thermal analysis was performed to identify the upper-limit current at a temperature limit of the wall material, and it was revealed that an outlet gas temperature of 871 K can be achieved with 77 A of current at 0.2 g/s of mass flow rate. The designed heater was combined with a boron nitride insulator and a stainless-steel housing, and thrust was measured in a vacuum chamber with nitrogen as the propellant. At a mass flow rate of 0.2 g/s and 75 A of current, an outlet temperature of 747 K, a specific impulse of 108 s, and a heater efficiency of 72% were achieved. These results with nitrogen propellant were used to predict the performance of a tungsten-made resistojet with a hydrogen propellant, and a specific impulse of over 700 s can be expected at a heater temperature of 2000 K.
Tadayoshi Shoyama, Yutaka Wada, Takafumi Matsui
Journal of Spacecraft and Rockets pp 1-9; https://doi.org/10.2514/1.a35040

Abstract:
A hybrid rockoon, which is a hybrid rocket launched from a high-altitude balloon, is proposed. The rocket system configuration was studied for single-stage or multiple-stage rockets, and the performance and range safety requirements were considered. The propellants of the hybrid-rocket motor are a low-melting-point thermoplastic fuel and nitrous oxide, which are beneficial, owing to their mechanical properties at cold temperatures experienced in high-altitude environments. Three-dimensional launch trajectory analyses were performed for science missions aimed at sampling cosmic dust levitating in the upper stratosphere. The apogee altitude can be significantly increased by elevating the altitude of the launch point because the problem of low thrust level of the hybrid rocket is solved by increasing the nozzle expansion ratio. Suborbital trajectories with multiple apogee points and orbital missions to extremely low Earth orbits are presented, which provide a long flight path in the upper atmosphere. The sequence of events and flight characteristics of the proposed hybrid rockoon system are discussed, and necessary technological improvements in the structure and propulsion systems are presented.
Anil Kumar Rout, Soumya Ranjan Nanda, Niranjan Sahoo, Pankaj Kalita, Vinayak Kulkarni
Journal of Thermophysics and Heat Transfer pp 1-10; https://doi.org/10.2514/1.t6269

Abstract:
The knowledge of surface heat flux over aerodynamic surfaces is highly desirable for high-speed applications. Impulse test facilities like shock tubes and shock tunnels are invariantly employed for this where the aerodynamic test models experience step/ramp heat loads. Contrary to conventional methods, the usage of an advanced soft computing technique through an adaptive neuro-fuzzy inference system (ANFIS), for recovery of such surface heat loads, is theme of this paper. A coaxial thermal sensor is fabricated in house from chromel and constantan alloy. This E-type thermal probe is subjected to known heat flux (2–3.5 W) of laser light in an exclusive experimental setup, and the temperature responses are recorded. The simulations are also performed to get the temperature history for these heat loads. The experimental and computational results, either separately or together, are used to train the ANFIS network. The time-averaged values of heat flux obtained from ANFIS-based recovery shows excellent agreement in trend and magnitude (uncertainty band of ±5%) with the applied heat load. The present studies demonstrate the possible use of a soft computing technique for heat flux recovery in short-duration experiments within a desired accuracy level by using training data obtained experimentally or computationally or both.
Karin W. Fulford, Dale Ferguson, Ryan Hoffmann, Vanessa Murray, Daniel Engelhart, Elena Plis
Journal of Spacecraft and Rockets pp 1-5; https://doi.org/10.2514/1.a35106

Abstract:
GPS satellites undergo surface contamination on the solar array coverglasses from repeated arcing events. Using NASA Air Force Spacecraft Charging Analyzer Program (Nascap-2 K) spacecraft charging simulation software, a GPS Block IIF satellite model was constructed and analyzed in realistic Medium Earth Orbit environments. GPS Block IIF satellites have Qioptiq CMG-type coverglasses (as do all other GPS satellites). The Nascap-2 K model with CMG coverglasses charges to high differential levels in maximum charging environments, in the range above the arcing threshold, as determined by studies at the Air Force Research Laboratory (AFRL), and so arcing is confirmed by theory. This finding agrees with onboard Los Alamos National Laboratory measurements and Arecibo observational data for GPS satellites. Other AFRL results show that CMX-type coverglasses, being more bulk-conductive, should charge less and perhaps mitigate arcing on the solar arrays. A Nascap-2 K model using CMX coverglasses is shown to charge differentially much less than CMG, and not reach levels above the arcing threshold. In the simulation, the commonly used CMG coverglass charges quickly, exceeding its arcing voltage threshold of 1500 V in about 1 h and 10 min. In comparison, CMX results indicate an ability to remain well under its arcing threshold throughout the orbit.
Simon Peterschmitt, Denis Packan
Journal of Propulsion and Power pp 1-10; https://doi.org/10.2514/1.b38156

Abstract:
The electron-cyclotron resonance thruster with magnetic nozzle relies on two successive energy transfer processes: first from electromagnetic energy to electron thermal energy, facilitated by a coupling structure; and second from electron thermal energy to ion directed kinetic energy, facilitated by a diverging magnetic field. The nature and geometry of the coupling structure are crucial to the first energy transfer process. This paper presents an experimental study of the performance of an electron-cyclotron resonance thruster with magnetic nozzle, equipped either with a waveguide-coupling structure or with a coaxial-coupling structure. The necessity of thrust balance measurements to perform such a comparison is demonstrated. The low coupling efficiency from microwave power to the plasma achieved by waveguide coupling is found to result in very large uncertainty with respect to the deposited power. A method to significantly reduce this uncertainty is proposed and implemented. Thrust balance measurements indicate 500 μN for the coaxial-coupled thruster and 240 μN for the waveguide-coupled thruster, both operated at 25 W of deposited microwave power and a mass flow rate of 98 μg/s of xenon. Electrostatic probe measurements reveal that this difference can be explained by a difference in ion energy. The results emphasize the critical role of the coupling structure, which may have been previously overlooked.
Ken Fujii, Akiko Matsuo, Junichi Oki, Hideyuki Taguchi, Takahiro Chiga, Yutaka Ikeda
Journal of Spacecraft and Rockets pp 1-11; https://doi.org/10.2514/1.a35018

Abstract:
This paper examines thermal response behind the high-Mach integrated control (HIMICO) experiment’s engine. In this experimental aircraft, instruments must be protected from heat load due to exhaust gas; therefore, a coupling calculation between the fluid and the wall is conducted to confirm the performance of HIMICO’s thermal protection system (TPS). First, the validity of the coupling calculation is confirmed through comparison with aerodynamic heating on a hollow cylinder. The present calculation result can reproduce the surface temperature distribution on the cylinder better than previous work has managed because we consider the turbulence effect. Second, one-dimensional heat-transfer analysis is conducted on the external nozzle, and the appropriateness of the calculation result is confirmed through comparison with the ramjet-engine experiment. Finally, a coupling calculation between the fluid and the wall is conducted to investigate the local temperature distribution. The calculation result indicates that the temperature increase easily meets design requirements and that TPS performance is sufficient.
, Pallavi Rastogi, Ashish Bhatt
Journal of Aircraft pp 1-11; https://doi.org/10.2514/1.c036370

Abstract:
Aircraft infrared (IR) signature studies are complex, due to their dependence on several parameters, e.g., line of sight (LOS) and viewing aspect (LOS^). There is no equivalent counterpart of the well-known radar cross section (RCS) that can be easily used for obtaining IR lock-on range, RLO,λ1−λ2. This study introduces the concept of IR cross section (IRCS)λ1−λ2 of complete aircraft as seen by seeker in λ1–λ2 band in LOS^ and of aircraft part in an orthogonal view. The IR solid angle (ωIR,λ1−λ2) subtended by (IRCS)λ1−λ2 of complete aircraft and of an aircraft part is also introduced, which is the basis for redefined general criterion for lock-on by IR-guided missile. Equality based on ωIR,λ1−λ2 at aircraft part level and at complete aircraft level is the basis for obtaining RLO,λ1−λ2 of complete aircraft in terms of its (IRCS)λ1−λ2. Dimensionless scalar factor, ΠIRCS,λ1−λ2, converting visual area to (IRCS)λ1−λ2 is studied, for temperatures ranging from close to ambient to maximum afterburning (reheat) mode of aeroengine. The ΠIRCS,λ1−λ2 enables assessment of effectiveness of IR seekers with single or dual bands, for all-aspect engagement and for rear-view only.
Adam P. Herrmann,
Journal of Aerospace Information Systems pp 1-13; https://doi.org/10.2514/1.i010992

Abstract:
This work explores on-board planning for the single spacecraft, multiple ground station Earth-observing satellite scheduling problem through artificial neural network function approximation of state–action value estimates generated by Monte Carlo tree search (MCTS). An extensive hyperparameter search is conducted for MCTS on the basis of performance, safety, and downlink opportunity utilization to determine the best hyperparameter combination for data generation. A hyperparameter search is also conducted on neural network architectures. The learned behavior of each network is explored, and each network architecture’s robustness to orbits and epochs outside of the training distributions is investigated. Furthermore, each algorithm is compared with a genetic algorithm, which serves to provide a baseline for optimality. MCTS is shown to compute near-optimal solutions in comparison to the genetic algorithm. The state–action value networks are shown to match or exceed the performance of MCTS in six orders of magnitude less execution time, showing promise for execution on board spacecraft.
Chenyu Lu, Zhitan Zhou, Xiaoyang Liang, Guigao Le
Journal of Spacecraft and Rockets pp 1-8; https://doi.org/10.2514/1.a35128

Abstract:
This paper investigates the thermal environment of launch pads during rocket takeoff. The rocket plume model is set up using the three-dimensional Navier–Stokes equations and realizable k−ε turbulence model. The aluminium oxide particles in the plume are considered by the Eulerian dispersed phase model. Comparing with the experimental data, the accuracy of the numerical model can be verified. On this basis, in total 16 numerical cases are performed to analyze the influence of the rocket flight altitude and lateral drift on the temperature of the launch pad. The results show that the launch pad has a more severe thermal environment at the flight altitudes from 3 to 20 m. Because of the rocket drift, a high-temperature gas flow layer is formed on the launch pad, which results in a dramatic increase in the temperature of the cross beam. At this location, the maximum temperature can reach up to 3900 K, 30% higher than the value without rocket drift. The method in this paper can provide an effective way to evaluate the thermal environment of the launch pad during rocket launching and have a great value on the thermal protection system design.
, Tigran Mkhoyan, Iren Mkhoyan,
Journal of Guidance, Control, and Dynamics, Volume 44, pp 1649-1662; https://doi.org/10.2514/1.g005870

Abstract:
This paper deals with the simultaneous gust and maneuver load alleviation problem of a seamless active morphing wing. The incremental nonlinear dynamic inversion with quadratic programming control allocation and virtual shape functions (denoted as INDI-QP-V) is proposed to fulfill this goal. The designed control allocator provides an optimal solution while satisfying actuator position constraints, rate constraints, and relative position constraints. Virtual shape functions ensure the smoothness of the morphing wing at every moment. In the presence of model uncertainties, external disturbances, and control allocation errors, the closed-loop stability is guaranteed in the Lyapunov sense. Wind tunnel tests demonstrate that INDI-QP-V can make the seamless wing morph actively to resist “1-cos” gusts and modify the spanwise lift distribution at the same time. The wing root shear force and bending moment have been alleviated by more than 44% despite unexpected actuator fault and nonlinear backlash. Moreover, during the experiment, all the input constraints were satisfied, the wing shape was smooth all the time, and the control law was executed in real time. Furthermore, as compared with the linear quadratic Gaussian control, the hardware implementation of INDI-QP-V is easier; the robust performance of INDI-QP-V is also superior.
Thomas A. Fitzgibbon, Mark A. Woodgate, George N. Barakos, Richard H. Markiewicz
AIAA Journal, Volume 59, pp 3431-3447; https://doi.org/10.2514/1.j060175

Abstract:
Optimization methods in conjunction with computational fluid dynamics are a key tool in advancing current rotor design. High-fidelity optimization of unsteady rotor flows in forward flight, however, is a challenging problem due to the high computational resources required. To minimize the computational costs, a fully turbulent, overset, adjoint harmonic-balance optimization framework has been developed, which maintains the fidelity of the Navier–Stokes equations. The framework is demonstrated in the aerodynamic redesign of the AH-64A rotor blade. An analysis of the optimized rotor blade is presented, including the key design features that contribute to the performance benefits in each of the examined design conditions. In particular, the benefits and drawbacks of rotor designs with an offloaded blade tip have been discussed. The formulation of the optimization objective function, blade surface parameterization, and treatment of trim were seen to have an impact on the final planform shape; and they have been deemed to be key in obtaining a practical rotor design suitable for use on real-life helicopters.
Teng Long, Zhao Wei, Renhe Shi, Yufei Wu
AIAA Journal, Volume 59, pp 3465-3479; https://doi.org/10.2514/1.j059915

Abstract:
Design optimization problems with black-box computation-intensive objective and constraints are extremely challenging in engineering practices. To address this issue, an efficient metamodel-based optimization strategy using parallel adaptive kriging method with constraint aggregation (PAKM-CA) is proposed. In PAKM-CA, the complex expensive constraints are aggregated using the Kreisselmeier and Steinhauser (KS) function. Besides, based on the notion of Pareto nondomination in terms of objective optimality and KS function feasibility, a novel parallel comprehensive feasible expected improvement (PCFEI) function considering the correlations of sample points is developed to effectively determine the sequential infill sample points. The infill sample points with the highest PCFEI function values are selected to dynamically refine the kriging metamodels, which simultaneously improves the optimality and feasibility of optimization. Moreover, the optimization time can be further reduced via the parallel sampling framework of PCFEI. Then the convergence and efficiency merits of PAKM-CA are demonstrated via comparing with competitive state-of-the-art metamodel-based constrained optimization methods on numerical benchmarks. Finally, PAKM-CA is applied to a practical long-range slender guided rocket multidisciplinary design optimization problem to illustrate its effectiveness and practicality for solving real-world engineering problems.
Wanqian Xu, Junlong Zhang, Chenguang Zhong, Juntao Chang, Wen Bao
AIAA Journal, Volume 59, pp 3517-3528; https://doi.org/10.2514/1.j060532

Abstract:
An algorithm of noncontact portable temperature measurement sensors that can be used for scramjet combustion chamber measurement is developed. The two-wire colorimetric temperature measurement is analyzed and found unable to be applied in the scramjet combustor due to its weak noise resistance. Then the inversion capabilities of the OH (hydroxyl) radical emission spectrum database of the multilayer perceptron and the convolutional neural network are compared. Considering the influence of noise, the influence of adding different proportions of random Gaussian noise on the network prediction results is compared. After adding 5% random Gaussian noise to the convolutional neural network, the regression error of the temperature prediction in the range of 500–4000 K is less than 50 K. As a method verification, the experiment of the ground combustion chamber of the scramjet under M=6 working condition is processed. Compared with the tunable diode laser absorption spectroscopy measurement result, the measurement temperature of the convolutional neural network is about 400 K higher, and the root mean square error is close to the measurement result of tunable diode laser absorption spectroscopy.
José Jiménez-Varona, Gabriel Liaño, José L. Castillo, Pedro L. García-Ybarra
AIAA Journal, Volume 59, pp 3375-3386; https://doi.org/10.2514/1.j059987

Abstract:
The theoretical solution of the flowfield past an axisymmetric body flying at a high angle of attack at subsonic flow conditions is a challenging problem since it entails large areas of boundary-layer separation and a complex vortex sheet structure. At high angles of attack, the flow is asymmetric and shows a dependence on the orientation angle, provided the body surface has sufficient roughness. Regarding theoretical simulations based on the unsteady Reynolds-averaged Navier–Stokes equations, eddy-viscosity turbulence models fail to simulate the unsteady flow structure in the rear zone of the body, yielding sectional side forces far different from those measured in experiments. Solutions obtained in this work by using Reynolds stress turbulence models combined with scale-adaptive simulation past an ogive-cylinder configuration show their ability to reproduce the essential features of the unsteady flow in the rear body. The model appears to be a suitable tool to investigate the complexities of this type of flow.
Samith Sirimanna, Balachandran Thanatheepan, Dongsu Lee, Shivang Agrawal, Yangxue Yu, Yuyao Wang, Aaron Anderson, Arijit Banerjee, Kiruba Haran
Journal of Propulsion and Power, Volume 37, pp 733-747; https://doi.org/10.2514/1.b38195

Abstract:
Electric aircraft propulsion is a growing research area that looks into achieving propulsion through fully electric or hybrid electric systems while achieving low CO2 emissions. The system-level benefit gained by different electric and hybrid-electric propulsion schemes depends heavily on the performance of system-level components in the electric drive-train, including the electric motor, gear box, motor drive, protection systems, as well as the thermal management system. When comparing motor topologies, it is important to understand performance measures such as efficiency and specific power on a drive system level. Many different motor types have been qualitatively compared and can be found in the literature. To guide appropriate component selection, this paper presents details of a quantitative study for a given electric propulsion drive system. A Pareto optimal front for a notional drive system of a 1.5 MW electrical propulsor with different motor types is generated and compared. An optimization algorithm coupled with an electromagnetic finite element analysis software tool was used to optimize the induction motor, switched reluctance motor, wound rotor synchronous motor, permanent magnet synchronous motor (PMSM), slotless PMSM, permanent-magnet-assisted synchronous reluctance motor, brushless DC motor, and brushless doubly fed reluctance motor types for efficiency and specific power. Overall advantages considering system-level efficiency, specific power, and a few other key metrics such as origin of losses, cooling complexity, manufacturing tolerance, and fault tolerance are discussed. This gives an indication of the relative performance of different motor types and confirms the overall advantage of PM motor topologies in aircraft propulsion.
Michael R. Natisin, Henry L. Zamora, Zachary A. Holley, N. Ivan Arnold, Will A. McGehee, Michael R. Holmes,
Journal of Propulsion and Power, Volume 37, pp 650-659; https://doi.org/10.2514/1.b38160

Abstract:
The overall propulsion efficiency for ion-mode electrospray thrusters has been predicted to be as high as 90%; however, experimental measurements currently fall far short of these predictions. Further complicating this is that for passively fed electrospray thrusters, the mass flow rate, which is required to obtain the propulsion efficiency and specific impulse, is not directly controlled or measured, and so this parameter is typically estimated by assuming all mass loss is due to the emitted ion current. Presented here is a detailed investigation into the efficiencies associated with a porous-media-based electrospray thruster operated in the purely ionic regime using the ionic-liquid propellant 1-ethyl-3-methylimidazolium tetrafluoroborate in both positive and negative ion emission modes. Measurements of performance metrics that affect thruster efficiency are discussed, including the transmission, angular, polydispersive, energy, and mass utilization efficiencies, in order to determine their impact on overall efficiency. The overall propulsion efficiency and specific impulse are also calculated using a variety of methods to better investigate how the assumptions made to estimate the mass flow rate affect these parameters. These results suggest that the efficiency of these devices may primarily be limited by the presence of additional mass loss mechanisms other than ion emission occurring during thruster operation.
Mar Cols Margenet, Patrick Kenneally, Hanspeter Schaub,
Journal of Aerospace Information Systems, Volume 18, pp 596-604; https://doi.org/10.2514/1.i010827

Abstract:
This paper describes the design and implementation of Black Lion, a purely software-based, distributed environment for integrated testing of independent spacecraft mission models. The Black Lion simulation environment is architected to be reconfigurable, allowing for any number of heterogenous software models, across one or multiple computing platforms, to be integrated into a single spacecraft simulation. This architecture enables seamless integration of legacy software models that were never designed to work together into mission-wide simulations. A flat-sat scenario is used to showcase the capabilities of Black Lion. In this flat-sat scenario, Black Lion synchronizes and ties together the following components: a ground system model, a spacecraft physical simulation, and a flight processor emulator in which the flight software application executes. The numerical simulation presented in this paper showcases the closed-loop behavior of the entire spacecraft system.
V. I. Yazhini, Balusamy Kathiravan, T. M. Muruganandam,
Journal of Propulsion and Power, Volume 37, pp 780-791; https://doi.org/10.2514/1.b38217

Abstract:
Experiments have been carried out to investigate the effect of cowl length variation on performance characteristics of a single expansion ramp nozzle. The performance parameters were estimated for cowl lengths of 0, 25, 50, 75, and 100% with respect to the horizontal length of the ramp. Experiments were conducted for different nozzle pressure ratios ranging between 1.5 and 9. The wall static pressure distribution data were measured from the tests to estimate the various performance parameters, such as axial thrust, normal force, gross thrust, thrust vectoring angle, and coefficient of pitching moment. High-speed schlieren imaging was used to visualize the flow separation and shock patterns and to measure the jet width. The flow was separated from the ramp wall up to a nozzle pressure ratio of 3 for all cowl cases. The shorter cowl length delays the downstream movement of shock-induced boundary separation inside the nozzle as compared to the longer cowl. The cowl trailing-edge flow was more underexpanded than the ramp tip flow. As cowl length increases, the increased restriction results in higher axial thrust and also increases the normal force. The pitching moment and thrust vectoring were dominated by normal force. Overall, as the nozzle pressure ratio increases, the axial force and jet width increase, whereas the normal force and the pitching moment increase up to a certain level and then decrease. As the cowl length increases, the axial thrust, normal thrust, pitching moment, and thrust vector angle increase, while the jet width decreases.
Alexis J. Harroun, Stephen D. Heister, Joseph H. Ruf
Journal of Propulsion and Power, Volume 37, pp 660-673; https://doi.org/10.2514/1.b38244

Abstract:
A computational and experimental study was conducted on nozzle geometries for rocket application rotating detonation engines (RDEs). Three geometries, including a nozzleless blunt body typically employed in RDE combustor hot-fire testing and two aerospike nozzles, were investigated. Simulations of the exhaust flow of a rotating high-frequency, high-pressure ratio wave based on rocket RDE test results were related to comparable constant-pressure conditions. Computational and experimental results showed the high momentum added by the highest-pressure detonation products influences the exhaust plume differently than a comparable steady flowfield fed by the same average product gas flow rate. In particular, the RDE exhaust flow tended to enhance entrainment on the nozzleless blunt body recirculation region and delay flow separation on nozzle expansion surfaces due to overexpansion compared to a constant-pressure engine. Results have important ramifications for isolating RDE combustor performance from nozzle effects and must be considered for future design of nozzle geometries to exploit the high-frequency, high-pressure ratio outflow of an RDE.
Nicholson K. Koukpaizan, Ari Glezer, Marilyn J. Smith
AIAA Journal, Volume 59, pp 3638-3656; https://doi.org/10.2514/1.j060182

Abstract:
Fluidically oscillating jet actuators that generate sweeping jets when supplied with a pressurized fluid have been used to mitigate separation and reduce drag in a range of flow control applications. Their implementation in future flight vehicles would require fundamental understanding and accurate predictive techniques of the physics of their internal flow and jet formation. The present investigations focus on high-fidelity, time-accurate simulations to characterize the flow physics of the actuator in quiescent conditions. An important element of the present simulations is to demonstrate the ability of the computational fluid dynamics (CFD) solver to predict the jet characteristics and provide a basis for the development of improved boundary conditions (BC) without entirely resolving the geometrical features of the fluidic device. The CFD-predicted oscillation frequencies of the engendered jets were found to be in excellent agreement with experiments, even on two-dimensional meshes. The study revealed that three-dimensional simulations are required to capture some of the flow features of the sweeping jet such as the double peak in time-averaged velocity distributions downstream of the actuator’s orifice that were measured in experiments. Several approaches for modeling the actuator were implemented and assessed in quiescent conditions. The evaluation of a boundary condition at the device throat, based on the phase-averaged flow variables, provides the basis for devising surface-based boundary conditions. The influence and necessity of including turbulent characteristics as part of the boundary conditions have also been identified.
Andrey V. Boiko, Kirill V. Demyanko, Stanislav V. Kirilovskiy, Yuri M. Nechepurenko, Tatiana V. Poplavskaya
AIAA Journal, Volume 59, pp 3598-3610; https://doi.org/10.2514/1.j060174

Abstract:
The paper describes a technology designed for computing three-dimensional transonic laminar–turbulent flows at various aerodynamic configurations with the use of the general-purpose computational fluid dynamics code ANSYS Fluent and an integrated special module of computing the laminar–turbulent transition position created on the basis of the autonomous software package LOTRAN 3, developed previously by the authors. Within the framework of this technology, computations with a prolate spheroid and engine nacelle are performed for different Mach numbers (M∞=0.14–0.7 Reynolds numbers Re=2–10.38×106), and angles of attack (φ=−10 deg to +10 deg). New results are obtained on the position of the laminar–turbulent transition in boundary layers in transonic flow regimes, and the problem of the dominating transition mechanism is considered. The results obtained in the present study are demonstrated to be in good agreement with experimental data on the position of the laminar–turbulent transition available in the literature.
Linhua Lan, Weili Luo, Jing Sun, Dongying Liu, Ming-Hui Fu
AIAA Journal, Volume 59, pp 3735-3747; https://doi.org/10.2514/1.j060300

Abstract:
Analytical elastic–plastic analysis is presented to investigate the in-plane deformation of the reentrant honeycomb structures subjected to tension or compression. An analytical elastic–plastic deformation analysis is carried out and the nonlinear stress–strain relation of honeycombs is presented, reflecting the dependence of equivalent stress, equivalent Young’s modulus, and equivalent Poisson’s ratio on the deformation. The modified factors of the stress–strain relation are then presented, which depend on the shape and deformation of the honeycomb structure and are independent of the material Young’s modulus and the ratio of wall thickness to its length. It is found that, by adjusting the geometric or material parameters, we can enable this reentrant honeycomb structure to achieve a range of negative Poisson’s ratio and Young’s modulus values. A finite element model is used to indicate the effectiveness of the proposed model. A parametric study is finally carried out to investigate the effects of the geometric or material parameters on the reentrant honeycombs structure.
Haoran Li, Yufei Zhang, Haixin Chen
AIAA Journal, Volume 59, pp 3667-3681; https://doi.org/10.2514/1.j060143

Abstract:
Aerodynamic prediction of glaze-ice accretion on airfoils and wings is studied using the Reynolds-averaged Navier–Stokes method. Two separation fixed turbulence models are developed by considering the nonequilibrium characteristics of turbulence. The key ad hoc fix is a term of the local ratio of turbulent production to dissipation, which is used to amplify the destruction term of the ω equation to increase the eddy viscosity in a separating shear layer of the fully turbulent region. A shear stress limiter is adopted to appropriately simulate the beginning process of the shear layer transition when the turbulence is under development. The proposed separation fixed terms can be easily implemented into current solvers. The case of flat-plate, periodic hill, and two iced airfoils and a three-dimensional swept wing with ice accretions are numerically tested using the modified models. The results indicate that the separating shear layer fixes improve the ability of the models in predicting the stall behavior at large angles of attack. The simulated averaged flowfield and turbulence intensity distribution are consistent with experimental data.
Arnau Pont-Vílchez, Alexey Duben, Andrey Gorobets, Alistair Revell, Assensi Oliva,
AIAA Journal, Volume 59, pp 3331-3345; https://doi.org/10.2514/1.j059666

Abstract:
This paper presents a new approach for mitigating the unphysical delay in the Reynolds-averaged Navier–Stokes (RANS) to large-eddy simulation (LES) transition, often referred to as the gray area, which is a common issue for hybrid RANS–LES turbulence models such as delayed-detached eddy simulation. An existing methodology designed for improving the LES performance in complex flows is adapted and tested. This is based on reducing the numerical diffusion in critical areas for permitting a more accurate development of turbulence. The new formulation comprises both a two-dimensional sensitive velocity gradient model and an alternative definition of the subgrid length scale, which are tested both individually and in tandem, and compared with the other formulations commonly used for addressing the gray area. Four test cases are examined, a flat plate, two variants of the incompressible backward-facing step, and an open jet compressible case, all of which are selected to expose the adverse impact of numerical diffusion that this study seeks to address. Furthermore, the proposed changes are implemented in two different codes for the purpose of cross-validation. Encouraging results are observed, supporting the suitability of the new approach as a candidate for addressing the gray area issue in flows of this kind.
Donald P. Rizzetta, Miguel R. Visbal
AIAA Journal, Volume 59, pp 3346-3358; https://doi.org/10.2514/1.j060365

Abstract:
Numerical calculations were carried out in order to investigate the delay of transition to turbulence on a wing section by means of local dynamic surface deformation. Physically, the deformation may be produced by piezoelectrically driven actuators located below a compliant aerodynamic surface, which have been explored experimentally. One actuator was located in the upstream region of the wing, and it was oscillated at the most unstable frequency in order to develop small disturbances corresponding to Tollmien–Schlichting instabilities. A second controlling actuator was placed further downstream, and then it was oscillated at the same frequency but with an appropriate phase shift and modified amplitude in order to decrease the disturbance growth and delay the transition process. The configuration consists of a NLF(1) 0414F natural laminar flow wing section in subsonic flow at a chord-based Reynolds number of 1×106. Angles of attack of both 3.0 and 4.0 deg were considered. Large-eddy simulations were carried out via solution of the unsteady three-dimensional compressible Navier–Stokes equations using a high-fidelity computational scheme and an implicit time-marching approach. Two-dimensional simulations were used to develop an empirical process that was applied to determine the optimal phase shift and amplitude of the controlling actuator. Results of the simulations are described, features of the flowfields are elucidated, and comparisons are made between solutions for the uncontrolled and controlled cases in order quantify effectiveness of the control. It is shown that dynamic surface control can return approximately 20% of the upper wing surface to laminar flow that is lost to premature transition when disturbances are present.
Hiroka Rinoshika, Akira Rinoshika, Yan Zheng, Masato Akamatsu
AIAA Journal, Volume 59, pp 3359-3374; https://doi.org/10.2514/1.j060113

Abstract:
Three-dimensional (3-D) flow structures around a wall-mounted short circular cylinder, into which was drilled a rear inclined hole (RIH) going from the rear surface to the top surface of the cylinder, were instantaneously measured at Reynolds number 10,720 in a water tunnel by high-resolution tomographic particle image velocimetry (Tomo-PIV). Based on the measured instantaneous 3-D velocity distribution, the 3-D vorticity field, the Q criterion, and characteristics of arch-type and tip vortices were compared between the RIH cylinders and a standard cylinder. A 3-D W-type arch vortex appeared behind the standard and RIH cylinders, and the height of the RIH cylinders was higher than that of the standard cylinder. Compared to the standard cylinder, the 3-D W-type arch vortex and large-scale vortices broke down slowly in the wake of the RIH cylinders. A 3-D orthogonal wavelet multiresolution technique was used to decompose the 3-D velocity fields from the Tomo-PIV data. The time-averaged large-scale structures of the RIH cylinders exhibited a stronger M-shape arch vortex, and a strong W-type-shape arch structure was extracted from the time-averaged intermediate-scale structures. Meanwhile, the tip vortices simultaneously existed in the large- and intermediate-scale structures. The heights of instantaneous large-scale streamwise structures were proportional to the hole height of the RIH cylinders. Large-scale spanwise vorticity components increase and the strength of vortex shedding decreases when using an RIH cylinder.
, Jonathan Fleming, Matthew Langford, Will Walton, Wing Ng, Kyle Schwartz, David Wisda, Ricardo Burdisso
AIAA Journal, Volume 59, pp 3304-3316; https://doi.org/10.2514/1.j060109

Abstract:
Wake-ingesting propellers are attractive for low-speed propulsion due to their potential fuel savings and aircraft configuration benefits. Aeroacoustic concerns must be addressed for such installed pusher-propeller configurations, though limited validation of reduced-order models has constrained designers who require rapid and accurate predictions of unsteady blade loading and noise due to nonuniform inflow. The current work introduces a benchmark experiment aimed at low-speed pusher-propeller configurations including ingestion of thick wakes such as from an unstreamlined pylon or muffler. Data derived from wind-tunnel measurements on two-bladed propellers downstream of a blunt-ended NACA0015 airfoil include inflow characterizations, on-blade unsteady pressure measurements, and far-field aeroacoustic measurements. The experimental data are compared with blade-loading predictions stemming from existing indicial gust-response functions combined with Ffowcs Williams-Hawkings calculations of acoustic sources. A good correlation is found between the performance of the gust-response functions for blade loading and for far-field noise levels. The reduced-order approach presented here demonstrates promising accuracy, especially considering its low computational expense compared to computational fluid dynamics.
Kaveh Gharibi, Ali Y. Tamijani
AIAA Journal, Volume 59, pp 3725-3734; https://doi.org/10.2514/1.j059642

Abstract:
The connection between topology optimization and load transfer is established in this work. The load transfer functions are used as an intermediate variable for topology optimization. This approach uses the total variation to minimize different objective functions such as the norm of the stress tensor and deviatoric principal stress subjected to equilibrium. To attain the topology of the microstructure, the principal load paths that follow the optimized principal stress directions are calculated. The principal vector field has singularities that are removed by an interpolation scheme. The optimal periodic microstructure is constructed using the load functions and the microstructures’ dimensions. The first advantage of this scheme is that using the load functions reduces the number of equilibrium constraints from two to one and reduces the number of variables from three stress tensor components to two load functions, leading to computational cost savings. The second advantage is that the nonlinear elliptic partial differential equations derived from the total variation equations are solved using the Gauss–Newton method, which has a quadratic convergence, speeding up the convergence toward the optimal structure. The third feature of the load-path-based optimization method is that the equilibrium and optimization problems are solved simultaneously.
Ewan Fonda-Marsland, Graham T. Roberts, Charles N. Ryan, David Gibbon
Journal of Propulsion and Power, Volume 37, pp 713-724; https://doi.org/10.2514/1.b38083

Abstract:
Experimental testing of a number of novel additively manufactured monopropellant microthrusters was conducted under atmospheric conditions using 87.5% concentration hydrogen peroxide. The aim of this work was to select a specific catalyst bed geometry for the thruster system and to investigate more general methodologies for monopropellant packed catalyst bed optimization. Characteristic velocity efficiencies approaching 0.98 were demonstrated, and performance improved for smaller beds with low aspect ratios; although, these beds flooded at lower propellant flow rates. The onset of bed flooding was used to identify physical limits of propellant flow rate supported by the catalyst. The particular propellant–catalyst pairing limit was defined by a Damköhler number of 56, independent of the bed geometry, with thermal performance peaking for the high flow rates just before flooding occurred. It is suggested that this method is extensible to other monopropellant systems, although with further work required to confirm it is a more general effect beyond thrusters using hydrogen peroxide.
Darren C. Tinker, Marsalis P. Pullen, Robin J. Osborne, Robert W. Pitz
Journal of Propulsion and Power, Volume 37, pp 748-758; https://doi.org/10.2514/1.b38303

Abstract:
Spark discharges were parametrically examined for a cylindrical air-gap electrode configuration. The aim of this study was to elucidate spark discharge characteristics, arc penetration, and exhaust plume development to guide designers of relevant ignition devices. Spark gaps ranged from 0.5 to 2.3 mm, nominal pressures ranged from 150 to 2200 kPa, and two exciter types (bipolar and unipolar) were tested. Positive correlations were observed between the pressure–distance product and multiple dependent variables: breakdown voltage, energy discharged, and percentage of sparks quenched. Positive correlations were observed between the pressure–distance quotient and various other dependent variables: spark duration, channel resistance, and plume velocity. This study also discusses the effects of quenching on electrical measurements, how these effects are nontrivial, and the subtle irregularities in electrical results that are indicative of quenching.
Yujun Leng, Nicole L. Key
Journal of Propulsion and Power, Volume 37, pp 682-692; https://doi.org/10.2514/1.b37765

Abstract:
A generalized flat plate cascade model is used in this study to predict the unsteady aerodynamics of a vibrating blade row with nonuniform blade spacing in subsonic compressible flow. The blade row is assumed to vibrate in an isolated family of blade-dominated modes. The effect of nonuniform blade spacing on compressor rotor flutter stability is demonstrated by case studies based on the geometric and flow conditions of a high-speed three-stage axial research compressor. The results show that nonuniform blade spacing can greatly alter the blades’ aerodynamic damping. At certain vibrational nodal diameters, some blades are destabilized so much that their aerodamping becomes negative. However, negative aerodamping of some blades do not necessarily lead to the instability of the whole blade row. A general multibladed system aeroelastic model is derived to study the effects of the nonuniform blade spacing on rotor stability through an eigenvalue approach. The aerodynamic influence coefficients matrix can be calculated using the generalized flat plate cascade model for a blade row with any user-specified blade spacing patterns. The case studies investigated in this paper show that alternating blade spacing and shifting only one blade position can slightly increase the stability of the least-stable eigenmode, whereas sinusoidal blade spacing has a slightly destabilizing effect. On the other hand, the eigenvectors of the least-stable mode for the nonuniformly spaced blade rows can be significantly different from the uniform blade spacing case.
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