Zhufei Li

Unsteady Behaviors of a Hypersonic Inlet Caused by Throttling in Shock Tunnel

A two-dimensional hypersonic inlet/isolator model that exhibits self-starting characteristics is tested with different exit throttling ratios at a freestream Mach number of 5.9 in a shock tunnel. Various flow characteristics are observed and measured by applying simultaneous high-speed Schlieren imaging and surface pressure measurements. The results indicate that the backpressure generated by the throttling device can be tolerated, and the inlet can maintain the starting mode at low throttling ratios, whereas unstart flows are initiated from the near-choke throttling ratios, and a shock wave oscillation appears. The Schlieren movie demonstrates that the upstream-propagating shocks in the duct play important roles during the oscillation cycles and that the formations of the upstream-propagating shocks are related to the downstream-propagating compression waves/shock waves that encounter the throttling section. The frequency of the shock wave oscillation increases with increasing exit throttling ratios, pri...

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.

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...

Effects of Attack Angle on Starting Performance of a Hypersonic Inlet

Since the boundary of an inlet starting Mach number cannot be obtained easily in high speed wind tunnels. The correlations between changing the attack angle and changing the freestream Mach number which affect the starting performance of a 2-D hypersonic inlet were investigated. The results show that the attack angle limit for unstarting and self-starting of the inlet at Mach 5.9 flow are in the range of 12o-14o and 0o-1o, respectively. The effective flow Mach numbers at the entrance of the inner contraction of the inlet are reasonably consistent at the unstarting limit and the self-starting limit, either by changing attack angles or freestream Mach numbers. One can try to change the attack angle within a certain range in wind tunnels to obtain the starting behaviors of a 2-D inlet at different freestream Mach numbers.

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.

Oscillatory Behaviors of a Hypersonic Inlet with Trips

A hypersonic inlet should be operated in a started mode for the efficient operation of an air-breathing propulsion system [1]. Various factors, either in design or in usage, may cause the hypersonic inlet to unstart, such as a large internal contraction ratio, low operating Mach number, delay of boundary layer transition, and high back pressure in the combustor, etc. When unstarted, a typical type of oscillatory flow (known as buzz) is often observed [2–5]. Violent shock oscillation, prominent pressure fluctuation, and substantial supersonic airflow spillage can occur in the oscillation flow [2, 3, 5], which are not only harmful to the engine performance but also highly detrimental to structural safety and flight control. Therefore the prevention of the oscillatory flow is important to hypersonic flights, and such a tough task is a complicated engineering problem that is intensively related to complex fluid mechanism [6–8]. As an important method to enhance boundary layer transition and help-to-start capabilities, trips are widely applied to hypersonic inlets design and practical flights [9–11]. The tripping mechanism requires the formation of streamwise vorticity on a scale within the boundary layer; hence the vortexes in trip wakes would act as that of micro vortex generators [12] to some extent. In Valdivia’s experiments [8], vortex generators were fixed on the side walls of a two-dimensional inlet/isolator model to reduce the movement scales of isolator shocks. However how such trips affect inlet oscillatory flow was overlooked. To the best of the authors’ knowledge, effects of trips have hardly been considered in the previous researches of hypersonic inlet oscillation. Consequently the present work is to experimentally investigate the effects of trips on the oscillatory behaviors of a hypersonic inlet, in which an axisymmetric inlet model was chosen to avoid the sidewall complexity.

A Combined CFD/Characteristic Method for Prediction and Design of Hypersonic Inlet with Nose Bluntness

Leading edge bluntness is widely used in hypersonic inlet design for thermal protection[1]. Detailed research of leading edge bluntness on hypersonic inlet has been concentrated on shock shape correlation[2], boundary layer flow[3], inlet performance[4], etc. It is well known that blunted noses cause detached bow shocks which generate subsonic regions around the noses and entropy layers in the flowfield.

Tests of Hypersonic Inlet Oscillatory Flows in a Shock Tunnel

For efficient operation, hypersonic air breathing engine requires the inlet to operate in a starting mode [1]. High backpressure induced by the combustion may cause the inlet to unstart in the engine actual operation [2].When unstarted, shock wave oscillations are typically observed in the inlet, a phenomenon known as buzz.