The influence of the Reynolds number on the natural transition of boundary layers over underwater axisymmetric bodies is studied using numerical approaches. This is a fundamental problem in fluid mechanics, and is of great significance in practical engineering problems. The transition locations are predicted over diameter Reynolds numbers ranging from 1.79×10⁵ to 2.32×10⁸ for eight different forebody shapes. The transition onsets are predicted using the semi-empirical eN method based on the linear stability theory (LST), and the wall pressure fluctuation spectra are estimated. The effects of the forebody shapes and the Reynolds numbers on the transition location are studied. At the same Reynolds number, the forebody shape has great influence on transition. As the Reynolds number increases, the changes in the dimensionless transition location are qualitatively similar for the different forebody shapes. The dimensionless transition location shifts closer to the leading edge as the Reynolds number increases, and is more sensitive at lower Reynolds numbers. However, the quantitative changes in transition location for different forebody shapes are distinctly different. Consequently, the sequential order of the transition locations for the eight forebody shapes is not fixed, but changes dramatically with increasing Reynolds number. This irregularity in the sequential order of the transition locations is called the "Reynolds number effect." Finally, the fundamental causes of this effect are analyzed.

The entrance loss of capillary flow at the nanoscale is crucial but often overlooked. This study investigates the entrance loss of capillary flow in narrow slit nanochannels using molecular dynamics simulations. The results show that the early stage of capillary flow is determined by entrance loss. During this period, capillary length increases linearly while the capillary velocity remains constant. The effect of length-dependent friction loss becomes more apparent in the subsequent stages, causing the capillary length to deviate from linear and the capillary velocity to decrease. Roscoe's equation, which describes the flow through an infinitely thin slit, is used to model the entrance loss. Finite element simulations of flow through slits of varying height and length demonstrate the validity of Roscoe's equation in the continuum theory framework. Based on this, a capillary flow model is proposed that can accurately depict the hydrodynamic behavior of capillary flow. Additionally, an approximate model ignoring the friction loss is proposed that predicts the linear increase of capillary length at the early stage. Theoretical analysis shows that the effect of entrance loss on capillary velocity is limited to the early stage, while the effect on capillary length can be extended to a large scale. Overall, the results of this study and the proposed models provide important theoretical support for applications related to capillary flow in nanoslits. The study emphasizes the importance of considering entrance loss in the early stages of capillary flow and demonstrates the applicability of Roscoe's equation in modeling capillary flow in nanochannels.

In this study, a two-dimensional numerical simulation is conducted to investigate the characteristics of gas flow induced by an electrohydrodynamic (EHD) pump with needle-ring-net electrodes. A needle electrode and a ring electrode are used as the high-voltage electrode, and a net electrode is used as the grounding one. The electric field distribution, space charge distribution and flow field distribution behavior were simulated and analyzed in detail. The simulation results were in good agreement with experimentally measured data. The influence of key parameters including applied voltage, electrode configurations and channel diameter on the flow characteristics and energy efficiency of EHD pump were studied systematically. The results showed that the most pronounced electric field strength locates at region around the needle tip and the edge of the ring electrode, while there is not obvious evidence showing more space charge located at vicinity of ring electrode. The airflow velocity at the net pores is higher than that at the central circular hole. Flow velocity and energy conversion efficiency of the pump monotonically increases with applied voltage. A combinational effect of tip-ring distance, ring inner diameter and pump channel size should be considered to design the EHD pump to achieve a maximum efficiency. The results also showed that an optimal energy conversion efficiency of 4.26 % can be achieved which is higher than most of the other EHD pumps (0.11~2.56 %). The proposed model can serve as an efficient tool for the design and optimization of the needle-ring-net EHD gas pumps.

The mixing characteristics of hydrogen and air are vital to combustion performance. Excellent hydrogen-air mixing is required to avoid hot spots in the reactivity of hydrogen in a combustion chamber. The present study aims to explore a mixing enhancement mechanism for a hydrogen transverse jet in which a rib is added in front of the jet. A schlieren technique is used to visualize the flow field of the improved hydrogen jet, and the combustion performance of the improved flame stabilizer is studied. The results show that the penetration depth and mixing performance of the hydrogen jet are improved. At its outset, the hydrogen jet flows like a free jet downstream of the rib. The flow pattern of the hydrogen jet is then changed by the shear layer between the low-velocity region and the mainstream. Ideal mixing performance is ultimately achieved under the strong effect of the mainstream. Combustion experiments show that the mixing and combustion performance is greatly improved by the rib in front of the jet. This study provides an important theoretical basis for the design of gaseous fuel combustors.

A detailed aeroacoustic analysis of the flow induced by the clearance between the fan tip and the shroud is performed in a scale-model fan stage of an Ultra High Bypass Ratio turbofan engine. Wall-modeled Large Eddy Simulation has been performed at approach condition, which corresponds to a fully subsonic operating point. The contributions of the tip-gap noise to the total fan noise are investigated using the Ffowcs Williams and Hawkings analogy. The surface is split into two parts; the tip region and the rest of the blade in order to analyze the acoustic contributions of these two regions separately. It is shown that the tip-gap region generates a significant noise component above 2kHz, which corresponds to approximate 1.2 times the blade passing frequency. Two separate tip-leakage vortices are identified in the fan tip region. The dominant noise sources in the tip-gap region are observed at the trailing edge of the fan blade. The wall pressure spectra in the tip-gap region and the coherence of pressure fluctuations between monitor points at different positions in this region show an acoustic contribution of the tip-leakage flow at two different frequency ranges. The first range corresponds to medium frequencies between 2kHz and 9kHz, and the second range corresponds to high frequencies between 10kHz and 25kHz. The analysis of dynamic mode tracking, fluctuating pressure and velocity spectra, and instantaneous flow fields relates specific structures in the tip-gap flow to their spectral signature and paves the way for further analytical modeling of tip-gap noise sources.

The instabilities in a rotor system partially filled with a fluid can have an exponentially increasing amplitude, and this can cause catastrophic damage. Numerous theoretical models have been proposed and numerous experiments have been conducted to investigate the mechanisms of this phenomenon. However, the explanation of the existence of the first unstable region induced by a viscous incompressible fluid is unclear and only one solving method, a standard finite difference procedure, was proposed in 1991 for solving the instabilities in a system containing a symmetric rotor partially filled with a viscous incompressible fluid. To better understand the mechanisms of the instability induced by the viscous fluid, based on the linearized two-dimensional Navier--Stokes equations, this system's differential equations are transferred to solve the characteristic equations with boundary conditions. A \textsc{Matlab} boundary value problem (BVP) solver bvp5c proposed in 2008 is an efficient tool to solve this problem by uncoupling the boundary conditions with unknown initial guess. Applying this approach to a rotor system allows the instability regions to be obtained. In this study, first, the radial and tangential velocities and pressure fluctuations along the radial direction of a disk filled with fluid were examined. Then, parametric analysis of the effect of the Reynolds number $\textit{Re}_{cr}$, filling ratio $H$, damping ratio $C$, and mass ratio $m$ on the system's stability was conducted. Using this calculation method allowed the first exploration of some new laws regarding the instabilities.

Using microbubbles coated by a thin shell as ultrasound contrast agents for ultrasound diagnosis improves image resolution. Since numerous microbubbles are used in clinical practice, understanding the acoustic properties of liquids containingmultiplemicrobubbles is important. However, interactions between ultrasound and numerous coated microbubbles have not been fully investigated theoretically. Additionally, ultrasound contrast agents with shells made of various materials have been developed. Recently, an equation of motion that considers the anisotropy of the shell was proposed [Chabouh et al., J. Acoust. Soc. Am., 149 (2021), 1240], and the effect of shell anisotropy on the resonance of the oscillating bubbles was reported. In this study, we derived a nonlinear wave equation describing ultrasound propagation in liquids containing numerous coated microbubbles based on the method of multiple scales by expanding Chabouh's equation of motion for a single bubble. This was achieved by considering shell anisotropy in the volumetric average equation for the liquid and gas phases. Shell anisotropy was observed to affect the advection, nonlinearity, attenuation, and dispersion of ultrasound. In particular, the attenuation effects increased or decreased depending on the anisotropic shell elasticity.

Vortex-induced vibration of twin tandem square cylinders at an inclined angle of 45{degree sign} to the fluid, i.e., twin diamond cylinders of mass ratio m*=3, is numerically investigated at Reynolds number Re=100 and reduced velocity U_r=3~18. This paper focuses on the effects of cylinders' spacing ratio L* (= L/ B, where L is cylinders' center-to-center spacing, and B is the characteristic length) ranging from 2 to 6 on the oscillation responses of two-degree-of-freedom cylinders. The results indicate that the wake structure experiences two gap flow patterns, the reattachment and co-shedding regimes, and eight different wake modes. At a small spacing ( L*=2~3), the reattachment regime occurs for the lower or higher U_r with the approximate range of 3 and 16~18. Meanwhile, the reattachment regime mainly occurs for other ranges of U_r at L*=2~6. The more significant oscillation of each spacing appears in the cross-flow direction, and the maximum cross-flow amplitude of the upstream cylinder is smaller than that of the downstream cylinder. Additionally, although significant cross-flow oscillations occur at small spacings ( L*=2~3) with the U_r≈5~9 and 12~14, the intrinsic mechanisms are entirely different. As for the cross-flow oscillation characteristics of larger spacings ( L*=4~6), they are virtually similar.

Fuel/air mixture clouds have important research value in the process industry and military applications. Different from condensed explosions, blast height has a direct impact on the fuel cloud field and the detonation power field. In this paper, we establish numerical models of the detonation process of propylene-oxide (PO) clouds generated by the dispersion of 2kg fuel/air explosives at different blast heights. The process of fuel dispersion, detonation propagation and the distribution of the near-surface detonation power field are explored. Through theoretical analysis, we establish optimization models of the fuel/air explosive dispersion under different blast heights. The relationship between the proportional blast height, proportional distance and power field peaks are quantitatively revealed. The results show that the effect of cloud detonation on the ground power field is obvious. The optimal proportional blast height exists. When the cloud mass is 2kg, the optimum proportional blast height is 0.8m/kg^1/3. At the optimum blast height, the overpressure effect of cloud detonation is the strongest (the peak overpressure is 2.19 MPa, and the action time is 1.77 ms), and the temperature range of cloud detonation is the largest (the peak temperature is 1462.16K, and the action time is 2.84 ms). Under the condition that the proportional blast height is less than or equal to the optimal proportional blast height, the power field peaks show N-shaped trends with the increase of the proportional distance. When the proportional blast height > proportional ignition radius > 0.8m/kg^1/3,the peaks decrease with the increase of the proportional distance.

The compressible budget terms in the transport equations of Reynolds stresses are examined from the (Large Eddy Simulation) LES result of the film cooling. The capability of LES and the statistical post-processing procedure were first validated.The compressible Reynolds stress budget terms are then analyzed for both fan-shaped and cylindrical cooling films. The balance of all budget terms is shown. The effect of the blowing ratio on each budget term is examined. The mechanisms by which energy is extracted from the mean flow and distributed among the normal Reynolds stresses are highlighted.The sources of anisotropy in the Reynolds stress distributions are examined in detail and their relation to the flow patterns of the mean and instantaneous flow are explored.The downstream development of the Reynolds stress budgets is studied and it is shown that the jets of both fan-shaped and cylindrical films can be split into a near field and a far field with different properties.Far downstream of the cooling films, the Reynolds stress budgets near the wall present similarities with the Reynolds stress budgets in a boundary layer, while the Reynolds stress budgets further away from the wall resemble budgets in a free-shear flow. It is shown that the budgets of the Reynolds stress in the three-dimensional wall jets object of this study obey approximate similarity laws. These laws are based on easily obtained integral scales but need to be modified by suitable powers of the distance from the orifice producing the jet.