Jiming Yang

Generation of cylindrical converging shock waves based on shock dynamics theory

A simple but effective technique is proposed to generate cylindrical converging shock waves. The shock dynamics is employed to design a curved wall profile of the test section in a shock tube. When a planar shock wave propagates forward along the curved wall, the disturbances produced by the curved wall would continuously propagate along the shock surface and bend the shock wave. As an example, the wall profile for an incident shock Mach number of M0=1.2 and a converging angle of 15° is tested numerically and experimentally. Both numerical and experimental results show a perfect circular shock front, which validates our method.

On phase transition in compressible flows: modelling and validation

A physical model for compressible flows with phase transition is described, in which all the processes of phase transition, i.e. nucleation, droplet growth, droplet evaporation and de-nucleation, are incorporated. The model is focused on dilute mixtures of vapour and droplets in a carrier gas with typical maximum liquid mass fraction smaller than 0.02. The new model is based on a reinterpretation of Hills method of moments of the droplet size distribution function. Starting from the general dynamic equation, it is emphasized that nucleation or de-nucleation correspond to the rates at which droplets enter or leave droplet size space, respectively. Nucleation and de-nucleation have to be treated differently in agreement with their differences in physical nature. Attention is given to the droplet growth model that takes into account Knudsen effects and temperature differences between droplets and gas. The new phase transition model is then combined with the Euler equations and results in a new numerical method: ASCE2D. The numerical method is first applied to the problem of shock/expansion wave formation in a closed shock tube with humid nitrogen as a driver gas. Nucleation and droplet growth are induced by the expansion wave, and in turn affect the structure of the expansion wave. When the main shock, reflected from the end wall of the low-pressure section, passes the condensation zone, evaporation and de-nucleation occur. As a second example, the problem of the flow of humid nitrogen in a pulse-expansion wave tube, designed to study nucleation and droplet growth in monodisperse clouds, is investigated experimentally and numerically.

Parametric study of cylindrical converging shock waves generated based on shock dynamics theory

In our previous work, the technique of generating cylindrical converging shock waves based on shock dynamics theory was proposed. In the present work, a further study is carried out to assess the influence of several parameters including the converging angle θ0, the incident planar shock Mach number M0, and the shock tube height h on the wall profile and the converging shock wave. Combining the high-speed schlieren photography and the numerical simulation with the shock dynamics theory, the characteristics of wall profiles, cylindrical converging shock waves, and thermodynamic properties for different controllable parameters are analyzed. It is found that these parameters have great effects on shapes of the wall profile and experimental investigation favors large values of M0 and h and moderate θ0. The experimental sequences of schlieren images indicate that the shocks moving in the converging part are of circular shapes, which further verifies the method in our previous work. In addition, the changes of ...

Experimental investigation of reshocked spherical gas interfaces

The evolution of a spherical gas interface under reshock conditions is experimentally studied using the high-speed schlieren photography with high time resolutions. A number of experimental sets of helium or SF6 bubble surrounded by air for seven different end wall distances have been performed. Distinct flow structures are observed due to the additional vorticity and wave configuration caused by the reshock. In the air/helium case, the deformation of the reshocked bubble is dependent on the development of the penetrating air jet along the symmetry axis of the bubble. In general, two separate vortex rings can be observed, i.e., one develops slowly, and the other approaches and eventually impinges on the shock tube end wall. In the air/SF6 case, two SF6 jets moving in opposite directions are generated and the oscillation of the interface is observed for small end wall distances, while small scale vortex morphologies on the gas interface are found for large end wall distances. The physical mechanisms of the...

A cylindrical converging shock tube for shock-interface studies

A shock tube facility for generating a cylindrical converging shock wave is developed in this work. Based on the shock dynamics theory, a specific wall profile is designed for the test section of the shock tube to transfer a planar shock into a cylindrical one. The shock front in the converging part obtained from experiment presents a perfect circular shape, which proves the feasibility and reliability of the method. The time variations of the shock strength obtained from numerical simulation, experiment, and theoretical estimation show the desired converging effect in the shock tube test section. Particular emphasis is then placed on the problem of shock-interface interaction induced by cylindrical converging shock waves. For this purpose, membrane-less gas cylinder is adopted to form the interface between two different fluids while the laser sheet technique to visualize the flow field. The result shows that it is convenient to perform such experiments in this facility.

Measurement of a Richtmyer-Meshkov Instability at an Air- SF6 Interface in a Semiannular Shock Tube

We report the first measurements of the perturbation amplitude in the converging Richtmyer-Meshkov instability in a semiannular shock tube. At early stages, the amplitude growth agrees well with the impulsive model considering the geometrical convergence effect. A quick decrease of the growth rate at late time, even to be negative, before the reshock is observed for the first time. The reduction of the growth rate is ascribed to the Rayleigh-Taylor stabilization caused by the interface deceleration motion only presented in the converging circumstance. By reasonably evaluating the Rayleigh-Taylor stabilization, a modified model based on the Bell equation is proposed, which well predicts the perturbation growth in a converging geometry from early to late stages before the reshock. It is also found that the flow compressibility is significant in the converging Richtmyer-Meshkov instability.

Refraction of cylindrical converging shock wave at an air/helium gaseous interface

Refraction of a cylindrical converging shock wave at an inclined air/helium interface is investigated. Experimentally, based on the shock dynamics theory, a special wall profile is designed to generate a perfectly cylindrical converging shock wave. A soap film technique is developed to form an inclined discontinuous air/helium interface, and high-speed schlieren photography is adopted to capture the flow. Numerical simulations are also performed to compare with the experimental counterparts and to show details of refraction. In this work, two initial incident angles (45° and 60°) are considered. As the incident shock converges inward, the shock intensity increases while the incident angle decreases, causing possible transitions among the wave patterns. For the case of 45°, an irregular refraction of free precursor refraction (FPR) first occurs and gradually transits into regular refraction, while for the case of 60°, various irregular refractions of twin von Neumann refraction (TNR), twin regular refracti...

Hypersonic Type-IV Shock/Shock Interactions on a Blunt Body with Forward-Facing Cavity

S HOCK interactions can cause extremely high pressure and heating in the local interaction region on a vehicle’s surface, which may severely shorten the useful life of structural components [1]. It is vital for the designers of hypersonic vehicles to understand the mechanisms associated with shock interaction phenomena. A typical milestone of early research is the contribution of Edney [2], who defined six types of shock interaction patterns known as types I–VI. Of these interaction patterns, the type-IV shock interaction results in the most severe increases in pressure and heating, and it generates a very complex flowfield. Substantial research efforts have concentrated on this type of shock interaction.Wieting and Holden [3] reported the experimental results of shock interactions acting on a cylindrical leading edge that represented the cowl of a hypersonic inlet. Holden et al. [4] measured the distributions of pressure and heat transfer on the surface of the cylinder and observed unsteady oscillations with typical frequencies of 3–10 kHz for the type-IV shock interaction. Further experimental investigations [5,6] of unsteady flow characteristics were very limited, possibly due to the challenges in measurement [7]. Meanwhile, there have been numerous numerical simulations of shock interactions in which unsteady oscillation phenomena were frequently seen [8–11]. Gaitonde and Shang [8] revealed that the dominant frequency of the oscillation was approximately 32 kHz. Zhong [10] and Chu and Lu [11] solved the Navier–Stokes equations using high-order schemes. Unfortunately, the frequencies calculated in both studies were still far from the experimental values. As one of the locations that suffers the most from aerothermal loading, the stagnation point of a blunt leading edge is also a focus of research for an effective way to reduce the load. An opposing jet [12–14] and a forward-facing cavity [15,16] in the nose region are well-known examples of techniques [17–19] to decrease the load on the nose. However, it should be noted that introducing a forwardfacing cavity or an opposing jet in the stagnation-point region will inevitably change the shape of the blunt body and produce a complex flowfield with new flow features, especially when shock interactions occur. Natural questions to ask are whether the coupling between these factors can enhance or suppress the moving flow and how the frequency might change. In the present work, the hypersonic type-IV shock interaction flow over a cylinder with a forward-facing cavity is experimentally and numerically investigated. The primary objectives are to clarify the unsteadiness of the coupled interaction flow and to clarify how the flow parameters are affected by the presence of the cavity. Using high-speed schlieren photography and surface pressure measurements, the unsteady shock oscillations can be characterized; then, the analyses for the mechanisms of the oscillation phenomena can be carried out based on the combination of experimental and numerical results.

Generation of Air/SF6 Interface with Minimum Surface Feature by Soap Film Technique

The Richtmyer-Meshkov (RM) instability occurs on an initially perturbed interface subjecting to a sudden acceleration by a shock [2]. Due to the deposition of baroclinic vorticity, the initial perturbation will grow with time, which generally intensifies the mixing between fluids and eventually induces turbulence in flow. Because of its academic significance in vortex dynamics and turbulent mixing as well as wide applications ranging from inertial confinement fusion, supernova explosions to supersonic combustion, the hydrodynamic instability becomes increasingly attractive. Specifically, several comprehensive reviews on this topic have been made [1, 2, 3].

Reflection of cylindrical converging shock wave over a plane wedge

The cylindrical converging shock reflection over a plane wedge is investigated experimentally and numerically in a specially designed shock tube which converts a planar shock into a cylindrical one. When the converging shock is moving along the wedge, both the shock strength and the incident angle are changing, which provides the possibility for the wave transition. The results show that both regular reflection (RR) and Mach reflection (MR) are found on the wedge with different initial incident angles. The wave transitions from direct Mach reflection (DiMR) to inverse Mach reflection (InMR) and further to transitioned regular reflection (TRR) are observed with appropriate initial incident angles. The instability development in the shear layer and strong vortices formation near the wall are evident, which are ascribed not only to the interaction of two shear layers but also to the shock impact and the shock converging effect. Because of the flow unsteadiness after the converging shock, the detachment crite...