AIAA Journal

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ISSN / EISSN : 0001-1452 / 1533-385X
Total articles ≅ 26,577
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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.
, 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.
Amandine Guissart, Jonas Romblad, Timotheus Nemitz, Cameron Tropea
AIAA Journal, Volume 59, pp 3611-3621; https://doi.org/10.2514/1.j060068

Abstract:
Using flight measurements conducted between altitudes of 700 and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar–turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien–Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel are postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.
Matthew G. Leibowitz, Joanna M. Austin
AIAA Journal, Volume 59, pp 3317-3330; https://doi.org/10.2514/1.j060162

Abstract:
Bow-shock standoff distances over sphere and spherically-blunted cone geometries were examined through experiments in two facilities capable of high-stagnation enthalpy hypersonic flows simulating Mars planetary entry conditions. High-speed and high-resolution schlieren images were obtained in the California Institute of Technology T5 reflected shock tunnel and the Hypervelocity Expansion Tube to examine facility independence of the measurements. Accompanying reacting Navier–Stokes simulations were carried out. A recently developed unified model for sphere and sphere–cone behavior was first verified for high-stagnation enthalpy CO2 flows through simulations with thermal and chemical nonequilibrium. Shock standoff distance measurements in both facilities were found to be in good agreement with model predictions. The need to account for the divergence of the streamlines in conical nozzles was highlighted and an existing model extended to account for changes in shock curvature between parallel and conical flows. The contributions of vibrational and chemical nonequilibrium to the stagnation-line density profile were quantified using the simulation results comparing three chemical kinetic models.
, Kivanc Ekici
AIAA Journal, Volume 59, pp 3448-3464; https://doi.org/10.2514/1.j060142

Abstract:
A computationally efficient one-shot approach with a low memory footprint is presented for unsteady optimization. The proposed technique is based on a novel and unique approach that combines local-in-time and fixed-point iteration methods to advance the unconverged primal and adjoint solutions forward and backward in time to evaluate the sensitivity of the globally time-integrated objective function. This is in some ways similar to the piggyback iterations in which primal and adjoint solutions are evaluated at a frozen design. During each cycle, the primal, adjoint, and design update problems are solved to advance the optimization problem. This new coupled approach is shown to provide significant savings in the memory footprint while reducing the computational cost of primal and adjoint evaluations per design cycle. The method is first applied to an inverse design problem for the unsteady lid-driven cavity. Following this, vortex suppression and mean drag reduction for a circular cylinder in crossflow is considered. Both of these objectives are achieved by optimizing the rotational speeds for steady or periodically oscillating excitations. For all cases presented in this work, the proposed technique is shown to provide significant reductions in memory as well as computational time. It is also shown that the unsteady optimization problem converges to the same optimal solution obtained using a conventional approach.
Julien Lefieux, Eric Garnier, Jean-Philippe Brazier, Neil D. Sandham, Antoine Durant
AIAA Journal, Volume 59, pp 3529-3545; https://doi.org/10.2514/1.j059972

Abstract:
In hypersonic flows, it is often necessary to be able to trip the transition to turbulence, upstream of air intakes, for example. Direct numerical simulations have been performed to identify the roughness-induced transition mechanisms on a wedgelike forebody at Mach 6 and unit Reynolds number Re=11 million (/m). Good agreement with the experiments performed in the Boeing/Air Force Office of Scientific Research Mach-6 Quiet Tunnel at Purdue University was obtained in terms of wall heat-flux and wall-pressure fluctuations. First, an isolated roughness was considered. The presence of the roughness in the span-inhomogeneous base flow leads to the formation of a crossflowlike vortex. High-frequency secondary instabilities of the stationary crossflow vortex are observed in the wake and are found to be responsible for the breakdown to turbulence. Spatial linear modal instability analysis of this flow has been performed at selected streamwise locations. The linear stability approach is found to give accurate predictions in terms of mode shapes, most-amplified disturbance frequencies, and growth rates, as it only underpredicts the N-factor of the most unstable mode by 10% compared to the direct numerical simulations. Unsteady simulations were then carried out for a trip array configuration and showed that it does not change the transition mechanisms, but the frequencies of the most unstable secondary instabilities were found to be higher.
, Yaping Hu, Honghu Ji, Min Zhang
AIAA Journal, Volume 59, pp 3657-3666; https://doi.org/10.2514/1.j059776

Abstract:
An inlet strut with a hot-air anti-icing system is studied experimentally and numerically to understand the heat transfer between impinged supercooled water droplets and solid surfaces. Experiments were performed in an icing research wind tunnel for dry (without water droplets) and sprayed cases. The results show that the impinged droplets can decrease the surface temperature, even on areas without droplet impingement or a water film. Theoretically, the impinged droplets exchange heat with solid surfaces through two heat flow rate terms: Q˙imp, which contains the kinematic and internal energies of the impinged droplets; and the evaporation energy Q˙evap. Adding these two terms to the solid surface when simulating the dry case provides a method to predict the surface temperature after spraying. Calculations for Q˙evap depend on the film coverage that varies due to different operating conditions. However, calculating Q˙evap for the impinged area or over the entire surface can give the upper and lower bounds of the prediction for surface temperature. The results show that both methods can give predictions with errors that are below 5% for all cases studied. The numerical results also show that both Q˙imp and Q˙evap decrease the surface temperature, whereas the effect of Q˙imp is neglectable when compared with Q˙evap.
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