Yujian Zhu

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.

Experimental and NumericalStudy of Hypersonic Type IV Shock Interaction on Blunt body with Forward FacingCavity

The effects of a forward facing cavity on the unsteady behaviors of the hypersonic type IV shock interaction on a circular cylinder are investigated both experimentally and numerically. The results show that the flow behaves to be either steady or unsteady depending on the supersonic jet impingement location. Under the conditions within the current work, two different oscillation modes, namely high frequency forward-backward oscillation and low frequency up-down oscillation modes are observed. The numerical results agree fairly well with that of the experiments, which helps to make further analysis for the mechanisms of the oscillation phenomena. The interference between the supersonic jet and the forward facing cavity plays a key role in the shock wave oscillations. Experimental results of cylinders with different length-to-diameter ratios of 4.3, 2.7 and 1.7 show that the three-dimensional effects are significant on the oscillation frequency.

Liquid jets produced by an immersed electrical explosion in round tubes

Liquid jets produced by an electrical explosion in water in round, centimeter-wide tubes are investigated experimentally and theoretically. Several jet flow patterns including sharp conical forms and annular forms are observed through high-speed photography. The general features of these patterns are presented and examined herein. It is found that the jet pattern is influenced mainly by the tube diameter, with narrow tubes producing conical jets and wide tubes producing annular jets. The jet maintains a nearly constant velocity, which increases with the explosion energy and decreases with the tube diameter and the standoff distance. A quasi-one-dimensional theoretical model that includes the bubble dynamics is proposed. With the model proposed herein, the effective explosion energy is calibrated first by matching the maximum bubble size. The results show that the confinement imposed by the tube tends to reduce the effective energy and that tubes with smaller diameters collect less effective energy. A theo...

Self-ignition induced by cylindrically imploding shock adapting to a convergent channel

A convergent channel method is applied to create an imploding arc shock as a partial representative of an overall cylindrical shock. The self-ignition phenomenon induced by such an imploding shock is investigated experimentally and numerically. Agreements between experimental and numerical results are widely reached not only for the shock imploding process but also for shock reflection and features of self-ignition, which approves the validity of the method. The induced self-ignition is found to lag behind the incident shock front and experience a shockless spontaneous process before the onset of detonation. The mechanisms are briefly addressed.

Note: A top-view optical approach for observing the coalescence of liquid drops

We developed a new device that is capable of top-view optical examination of the coalescence of liquid drops. The device exhibits great potential for visualization, particularly for the early stage of liquid bridge expansion, owing to the use of a high-speed shadowgraph technique. The fluid densities of the two approaching drops and that of the ambient fluid are carefully selected to be negligibly different, which allows the size of the generated drops to be unlimitedly large in principle. The unique system design allows the point of coalescence between two drops to serve as an undisturbed optical pathway through which to image the coalescence process. The proposed technique extended the dimensionless initial finite radius of the liquid bridge to 0.001, in contrast to 0.01 obtained for conventional optical measurements. An examination of the growth of the bridge radius for a water and oil-tetrachloroethylene system provided results similar to Paulsens power laws of the inertially limited viscous and inertial regimes. Furthermore, a miniscule shift in the center of the liquid bridge was detected at the point of crossover between the two regimes, which can be scarcely distinguished with conventional side-view techniques.

Note: A contraction channel design for planar shock wave enhancement

A two-dimensional contraction channel with a theoretically designed concave-oblique-convex wall profile is proposed to obtain a smooth planar-to-planar shock transition with shock intensity amplification that can easily overcome the limitations of a conventional shock tube. The concave segment of the wall profile, which is carefully determined based on shock dynamics theory, transforms the shock shape from an initial plane into a cylindrical arc. Then the level of shock enhancement is mainly contributed by the cylindrical shock convergence within the following oblique segment, after which the cylindrical shock is again bent back into a planar shape through the third section of the shock dynamically designed convex segment. A typical example is presented with a combination of experimental and numerical methods, where the shape of transmitted shock is almost planar and the post-shock flow has no obvious reflected waves. A quantitative investigation shows that the difference between the designed and experimental transmitted shock intensities is merely 1.4%. Thanks to its advantage that the wall profile design is insensitive to initial shock strength variations and high-temperature gas effects, this method exhibits attractive potential as an efficient approach to a certain, controllable, extreme condition of a strong shock wave with relatively uniform flow behind.

Hypersonic Shock Wave Interactions on a V-Shaped Blunt Leading Edge

An investigation of hypersonic shock wave interactions on a V-shaped blunt leading edge (which is commonly designed in hypersonic inlets) is conducted, focusing on the effects of the geometry of th...

On the Early-Stage Deformation of Liquid Drop in Shock-Induced Flow

The early-stage deformation of waterdrop exposed to shock-induced flow is experimentally investigated. High-speed photography is applied to record the development of the drop, and numerical simulations are performed as well to assist the understanding of deformation dynamics. Two deformation dynamics, the pressure mechanism and the shear mechanism, are discussed. A parameter is proposed to characterize the accumulation speed induced by shearing of the gas flow. The peaks of accumulation speed is strongly correlated with the wrinkles and liquid rings on the drop rear surface, which infers that shear might be responsible for these fine structures of drop deformation.

An Investigation of Type IV Shock Interaction Over a Blunt Body with Forward-Facing Cavity

Shock wave interaction occurs in many external and internal flow fields around hypersonic vehicles. The type IV shock interaction is one of the six types of shock interactions categorized by Edney [1] and is characterized by a supersonic jet embedded with the surrounding subsonic flow. It receives the most attention because it creates the most complex flow pattern and severe heating problem. Substantial experimental [2–4] and computational [5–8] research efforts have been made to study such type of shock interaction. Unsteady oscillations with typical frequencies of 3–10 kHz were first observed by Holden et al. [3]. With the same conditions as that of the experiments [3] however, Gaitonde and Shang [5] performed a numerical study with a modified Steger–Warming scheme and revealed that the dominant frequency was about 32 kHz. Zhong [7], Chu, and Lu [8] solved the Navier–Stokes equations using high-order schemes to analyze the characteristics of the unsteady type IV shock interaction. Unfortunately, the frequencies of their calculations were still far from that of the experiment. In addition, there has been an interesting speculation that introducing an opposing jet [9] or a forward-facing cavity [10] at the nose region of a blunt body may reduce the drag and aerothermal loads. However, it needs to be pointed out that the introduction of a cavity or an opposing jet in the stagnation-point region will inevitably bring some new flow features especially when shock interaction occurs.

A New Method of Convergent Contour Design for Planar Shock Wave Enhancement in a Shock Tube

It will be interesting and important as well if there is a method that is capable of generating a shape/strength controllable shock wave. In this paper, we propose a new method to increase the intensity of a planar shock wave smoothly by means of specially designed wall contour along an area contraction channel. Based on a shock dynamics design method suggested by the author’s group1, a further development of a planar-to-planar shock wave enhancement was challenged recently, in which a smoothly concave-to-convex shaped convergent channel was designed for the control of the shock enhancement.