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Transition of hypersonic boundary layer flow is an important problem in aerodynamic research. To obtain the natural transition position using the flow stability analysis method, it is necessary to analysis the neutral curve and the perturbation growth rule. Accurate calculation of the basic flow is the basis and prerequisite for stability analysis. This paper proposes that surface radiation should be considered in the analysis of hypersonic boundary layer flow stability due to the high temperatures at the vehicle surface caused by aerodynamic heating. In order to obtain the influence of surface radiation on the flow stability, we assume that the surface satisfies the energy balance and the gas satisfies calorimetrically perfect gas properties. To simplify the analysis, gas radiation has been neglected. The boundary layer analysis of a hypersonic flat plate with and without radiation has been performed by a combination of flow stability theory and direct numerical simulation. It is found that high temperature radiation on the surface causes energy to transfer outwards. The results showed that surface temperature decreases along the streamwise direction and is no longer an adiabatic surface temperature. Instability interval decreases and the maximum growth rate increases. The range of the instability region expressed in dimensionless frequency values moves to lower frequency as the Mach number increases. Maximum growth rate shows a tendency to increase and then decrease as the Mach number increases. [ABSTRACT FROM AUTHOR]

Polymer film melt stretching exhibits a unique stress–strain behavior, featuring a stress plateau followed by a sudden increase with increasing the strain. This phenomenon poses a challenge to conventional melt-extension theories and methods. To address this, we derive a simplified flow theory according to the symmetric characteristics of thin melt-stretching, and propose a modified multi-mode Leonov model (m-Leonov) that accounts for molecular orientation effects, a crucial factor in accurately simulating the process. In order to improve the computational efficiency, an iterative algorithm was developed to decouple the interdependence between governing and constitutive equations. The computational results reveal that the viscoelastic nature of the film contributes to a delay in the transformation of stretching forces, leading to the observed stress plateau. Significantly, our m-Leonov model successfully predicts both the plateau and the subsequent abrupt increase in stress, across various temperatures and stretching rates. These simulations closely agree with experimental data, underlining the robustness of our approach. Our findings would enhance the understanding of rheological complexities in film stretching, offering potential applications in various materials and polymer processing. [ABSTRACT FROM AUTHOR]

During high-speed water-exit of the supercavitating projectile, the cavity interacts with the free surface and collapses, with instantaneous high collapse pressure impacting on the projectile. In order to study the cavity collapse characteristics during high-speed water-exit of the supercavitating projectile, the numerical study based on the Reynolds-averaged equation and the volume of fluid multiphase flow model is conducted in this paper. The results show that the cavity near the free surface will gradually become larger with the movement of the projectile during water-exit of the supercavitating projectile. The existence of attitude angles will cause the asymmetry of cavity to collapse. The cavity on the upstream side will first collapse and generate collapse pressure, while the cavity on the downstream side will collapse later but generate higher collapse pressure. The asymmetry of the cavity collapse becomes stronger with the increasing attitude angles. The time interval of the collapse pressure on the downstream and upstream sides of the projectile becomes shorter close to the projectile tail. [ABSTRACT FROM AUTHOR]

Instabilities of high-enthalpy flat-plate boundary layers in thermochemical equilibrium conditions are studied using a linear stability theory (LST) and parabolized stability equations. A supersonic radiation mode is found to be unstable at the downstream location of the second mode. This mode synchronizes with the second mode as its phase velocity approaches c r = 1 − 1 / M a . The amplification rate is expected to be comparable to the second mode when M a > 14 and T w < 14 T e . The results of disturbance evolution indicate that the radiation mode is generated by the second mode through a synchronization process. The coefficient for the amplitude response of the radiation mode to the forcing second mode is around 1. After the radiation mode is excited, the second mode itself still exists in the boundary layer, leading to a co-existence of dual unstable modes. Owing to the competition between the two modes, the disturbance amplitudes exhibit significant oscillations. A two-mode amplitude prediction model based on the LST is proposed by properly superposing the two modes' amplitudes. The accurate prediction of the N-factor demonstrates that the proposed model can be used to predict a high-enthalpy boundary layer transition involving these two modes. [ABSTRACT FROM AUTHOR]

For hypersonic boundary-layer flow, the effects of entropy layer on the boundary layer are investigated by theoretical analysis combining with direct numerical simulation over 5 ° blunt cones. The boundary-layer similarity is first discovered after entropy swallowing. Entropy swallowing is the phenomenon that the entropy layer gets swallowed by the developing boundary layer downstream. Compared with sharp cones, the boundary layer under entropy layer can be divided into two regions—the outer layer and inner layer. The outer layer is strongly affected by the entropy layer, whereas the inner layer near the wall is mainly dominated by viscous effects. It is found that after entropy swallowing, the entropy-layer effects do not fully disappear. The boundary-layer-edge variables of the blunt cone reach sharp-cone values at entropy swallowing point, while the flow inside the boundary layer takes a longer distance to reach sharp-cone solutions entirely. After that, the entropy-layer effects vanish. The blunt-cone boundary-layer similarity then shows up. Linear stability theory results show that, for blunt cones, the lower branches of the neutral curves always overlap with those of sharp cones, which further proves the existence of the inner layer. The neutral curves of blunt and sharp cones entirely overlap at somewhere downstream of entropy swallowing point, and at this location, the first-mode growth-rate curves of blunt and sharp cones completely overlap. The second-mode growth rates, however, are found to overlap somewhere further, at the location where the blunt-cone similarity shows up. [ABSTRACT FROM AUTHOR]

In this study, the effects of entropy layer on the boundary layer over hypersonic blunt cones for a thermochemical equilibrium gas are investigated. The flow and stability characteristics of the entropy and boundary layers are presented considering the chemical reactions. It is found that the entropy layer has little influence on the inner layer inside the boundary layer. The inner layer thickness increases when chemical equilibrium is considered, which stems from the enhanced viscosity protection near the wall surface. At the leading edge of the blunt cone, due to the effects of the equilibrium gas, the temperature in the boundary and entropy layers decreases and the boundary-layer-edge parameters significantly change. The entropy layer gradually vanishes along the downstream direction, and the effects of chemical reactions are concentrated in the boundary layer. The entropy swallowing point of the equilibrium gas does not significantly differ from that of the perfect gas. For all the analyzed cases, chemical reactions stabilize the entropy layer instability modes. The instability region and the frequency range of the equilibrium gas decrease, and the growth rate of the most unstable perturbation of the equilibrium gas is considerably smaller than that of the perfect gas. Moreover, the results show that the equilibrium gas has a minor influence on the process of the blunt cone boundary layer solution tending to the sharp cone solution. [ABSTRACT FROM AUTHOR]

To investigate the unsteady shedding characteristics of a reentrant jet supercavity with a low Froude number, a high-fidelity numerical model based on the inhomogeneous multiphase model is developed to predict the complex supercavitation flow that occurs during supercavity development. The developed solver is validated quantitatively against experimental results in terms of supercavity geometry and closure mode. This study focuses on the initial generation and development process of a reentrant jet supercavity, revealing three distinct stages: foam cavity, transparent supercavity with rapid growth in dimensions, and fully developed supercavity exhibiting significant deformation. Owing to reverse flow of the gas–water mixture, interfacial instabilities arise from the unsteady cavity shedding, leading to fluctuations in supercavity shape. The types of large-scale cavity shedding observed in this work—wing-like and cloud-like—are caused by the concave deformation resulting from the reentrant jet. As the gas entrainment coefficient increases, the unsteady characteristics of pressure oscillation weaken, and the instance of wing-like cavities decreases. When the gas entrainment coefficient reaches a critical value, the twin-vortex closure mode occurs, resulting in a more stable flow behavior. In sum, we propose a theoretical model that elucidates the strength of the reentrant jet and reveals its unsteady shedding behavior during supercavity development. [ABSTRACT FROM AUTHOR]

Entropy swallowing is the phenomenon where the entropy layer is swallowed by the boundary layer developing downstream. In this work, the entropy-swallowing point over a 5 ° blunt hypersonic cone is investigated theoretically and numerically. An equation for obtaining the location of the entropy-swallowing point is derived herein that requires only the free-stream parameters. This equation also reveals the relationship between the entropy-swallowing point and the nose radius under the constant free-stream Mach number. The analytical results are entirely consistent with direct numerical simulation results. Furthermore, the influence of the Mach number and Reynolds number on the entropy-swallowing point is investigated. The location of the entropy-swallowing point depends more strongly on the Reynolds number than on the Mach number, with the entropy-swallowing point shifting further downstream as the Reynolds number increases. However, as the Mach number increases, its impact on the entropy-swallowing point decreases. Furthermore, the influence of the boundary-layer displacement also decreases with an increase of the Mach number. [ABSTRACT FROM AUTHOR]

The maneuverability of supercavitating vehicles is significantly affected by disturbance from their control surfaces during the water-entry process. In this paper, experiments and simulations on cavity evolution models and vehicle dynamic characteristics are carried out to investigate the influence of the water-entry disturbance on the water-entry maneuvering motion process of supercavitating vehicles. In the water-entry experiment of medium speed projectiles based on the high speed photography, the shapes of cavity contours are extracted and the applicability of the cavity evolution models to the problem of water-entry maneuvering motion is verified. A kind of more correct prediction model of cavity shape is chosen by the comparison. A mathematical model on the water-entry maneuvering motions of vehicles is developed based upon certain simplifications, which makes an improvement on the three-dimensional cavity topology algorithm. Afterward, the simulations on the water-entry maneuvering motion of the vehicle are carried out, from which the real-time interaction between the vehicle and the cavity is extracted and the influence law of the fin deflection angle on the vehicle dynamic characteristics is analyzed. The influence mechanism of the desynchronization of fins' contacting water on the water-entry maneuvering motions of vehicles is revealed. It is found that the fin deflection angle of 10° is the most conducive to enhancing the vehicles' ability to stabilize their attitudes during water-entry maneuver by analyzing the fin efficiency and planning parameters. [ABSTRACT FROM AUTHOR]

A semi-sealed cylindrical shell is a hollow cylinder in which one end is open and the other end is sealed. In order to systematically study the cavity evolution, the hydrodynamic characteristics, and corresponding structural response of the semi-sealed cylindrical shell during high-speed vertical water entry, a numerical simulation is carried out based on a Star-CCM+ and ABAQUS collaborative simulation method. The results show that a nested cavity is formed that presents three different morphologies as the semi-sealed cylindrical shell penetrates the water. Moreover, a jet is formed under the shell, which profoundly influences the hydrodynamic and structural characteristics. Compared with the completely sealed case, the velocity and displacement of the semi-sealed cylindrical shell are significantly changed upon water entry, and the deformation at the top wall is more prominent. [ABSTRACT FROM AUTHOR]