After experiencing high-temperature ablation, the thermal protection material on the surface of hypersonic vehicle exhibits a pattern of distributed roughness element. However, the effect of such pattern on hypersonic boundary layer transitionand its influencing mechanism have not been recognized at present, which has attracted the attention of researchers. Wind tunnel experiments are carried out based on the ∅0.5 m Mach number 6 Ludwieg tube wind tunnel of Huazhong University of Scienceand Technology. The evolution of instability waves in the hypersonic boundary layer of the flat plate is studied in the cases of smoothand distributed roughness elements with four heights of 0.5, 1.0, 1.5, 2.0 mmand various of Reynolds number at 5.39 × 10⁶ m⁻¹ to 1.72 × 10⁷ m⁻¹. The effects of distributed roughness element heightand incoming Reynolds numbers on the boundary layer instability of a flat plate are clarified. The experimental results show that the second mode instability waves dominate the instability of the hypersonic boundary layer in the case of distributed roughness element, and the frequency range is about 60 kHz to 120 kHz. For the distributed roughness element which is lower than the local boundary layer thickness, the height factor has little influence on the transition position of the hypersonic boundary layer. The influence of height of the distributed roughness element on the flow velocity of the second mode instability waves in the hypersonic plate boundary layer is non-monotonic, and there is a critical height that has the greatest influence. Under the inflow conditions of different Reynolds numbers, the characteristic evolution of the second mode instability waves in the smoothand roughness cases is basically the same, and the boundary layer transition positions of roughness elements at different heights have little difference with the inflow of the same Reynolds number.

The HyTRV lift body is a standard model designed for studying boundary layer transition issues in three-dimensional complex geometry at hypersonic flow, with characteristics similar to those of a real aircraft. Currently, research on this model mainly relies on theoretical analysis and numerical calculations, lacking experimental data for validation. To provide wind tunnel test data for theoretical verification and control of boundary layer transition on the HyTRV lift body, we used high-frequency pressure sensors and high-speed infrared cameras to conduct experimental measurements of boundary layer stability in the Mach number 6 Ludwieg tube wind tunnel. The study analyzed the instability characteristics of the boundary layer in the flow separation region, the transverse flow region, the waist transverse flow region, and the shoulder flow separation region of the HyTRV model, with a focus on investigating instability waves in the transverse flow region of the model belly. The experimental results show that the vortex region of the HyTRV model belly exhibits weak low-frequency instability waves at 10-30 kHz, while high-frequency instability waves of 150-250 kHz were measured in the cross-flow region of the belly. At a 0° angle of attack, the propagation speed of this instability wave was determined to be 722.9 m/s, suggesting that this instability wave is a high-frequency unstable mode of the secondary crossflow instability. The process of boundary layer transition was observed in the waist crossflow region and the shoulder vortex region, with no obvious instability wave characteristics found. Infrared measurement results show good agreement between the model leading edge and trailing edge transition fronts and direct numerical simulation results. Additionally, typical sawtooth-like streaky transition arrays were observed on the model leading edge, consistent with the conclusion that cross-flow instability dominates unstable transition in the cross-flow region of the belly.

The dual-throat Ludwieg tube wind tunnel can effectively eliminate the disturbances caused by the opening process of the fast-acting valve, but will significantly reduce the effective running time of wind tunnel. Additionally, due to the limitations of the material of the fast-acting valve, it is difficult to further increase the heating temperature of the storage section of the Ludwieg tube wind tunnel. To solve this problem, this paper proposes a novel design for the dual-throat Ludwieg tube wind tunnel by placing the annular heater in the settling chamber to achieve an integrated design of homogeneous mixing and heating of flow. Firstly, unsteady numerical simulation is used to verify the feasibility of the dual-throat Ludwieg wind tunnel with the new layout. Then, the start-up characteristics of the wind tunnel is analyzed, and the variation of Mach number and pressure at different stations during the running of the wind tunnel is quantitatively studied. Finally, the effect of the heater on the running process of the tunnel is explored. The results show that the effective operating time of dual-throat Ludwieg tube wind tunnel with a heater in the settling chamber is up to 80 ms, an increase of 23% compared to that of the traditional dual-throat layout. Additionally, when the heater temperature is increased from 434 K to 1 234 K, the maximum deviation of Mach number in the core region at the exit of the second nozzle can be decreased by 0.21%, the root mean square deviation can be decreased by 0.005, and the stagnation temperature can be increased by nearly 270 K, and the testing capability of the wind tunnel is enhanced effectively.

The types and levels of disturbances in hypersonic freestream have significant effects on the flow phenomena such as boundary layer transition and shock/boundary layer interactions. However, the understanding of the disturbances in hypersonic wind tunnels is currently limited, which restricts the relevant research. In response to this situation, two methods, the high-temperature hot-wire with a high over-heat ratio and the conventional hot-wire with variable over-heat ratios, were conducted to measure the freestream disturbances in the Mach number 6 hypersonic wind tunnel at Huazhong University of Science and Technology. The relative error of the disturbance amplitude derived by the two methods does not exceed 15%, and the normalized root mean square values of the mass flow rate and total temperature fluctuations at different Reynolds numbers are between 0.77%-1.25% and 0.1%-0.18%, respectively. The hypothesis that acoustic disturbances dominate in conventional wind tunnels was verified by using the fluctuation diagram of compressible hot-wire. Meanwhile, the direction of the acoustic disturbance was determined to be between 120° and 140°, which translates to a sound source velocity between 0.67 and 0.78 of the freestream velocities. Additionally, the spectrum of static pressure caused by the acoustic wave below 40 kHz was obtained using the mass flow spectrum of the high-temperature hot-wire. It emerged that the amplitude in low-frequency disturbance below 10 kHz had a magnitude of 10^-8 and the high-frequency spectrum roll-off had a slope of -5/3. This study verified the viability of using constant-temperature anemometers for freestream disturbance measurement in hypersonic wind tunnels, and obtained the disturbance types and amplitudes of distribution characteristics in the frequency domain.

Hypersonic boundary layer transition is one of the key problems in aerodynamics. Macroscopic distributed roughness elements pattern will be produced due to surface high temperature ablation of thermal protection system on the flight vehicle, but its influence on the evolution mechanism of instability waves in the hypersonic boundary layer is not clear now. Based on the ∅0.5 m Mach number 6 Ludwieg tube wind tunnel of Huazhong University of Science and Technology, this paper explores the influence of location and width of distributed roughness elements on the evolution characteristics and transition position of hypersonic boundary layer instability waves of a 7° half-angle sharp cone model at zero angle of attack. PCB sensors and infrared thermography are used. The experimental results show that placing the distributed roughness elements in front of the synchronization point will promote the evolution of the second mode unstable wave; the closer the roughness element is to the leading edge, the smaller the influence of its width on the evolution of downstream instability waves. However, as its distribution position moves downstream, its effect on promoting transition is weakened, and its effect on inhibition of the nonlinear interaction of instability waves is more obvious, then the transition position moves backward.

Hypersonic wind tunnels are essential in the fundamental research of hypersonic aerodynamics. However, the low operating efficiency, low flow-field quality, and high construction and operating costs of conventional hypersonic wind tunnels make delicate experimental investigations difficult. This paper aims to design a low-cost and large-diameter research-oriented hypersonic wind tunnel based on a Φ 0.5 m Mach 6 Ludwieg tube. The design of the storage tube, fast-opening valve, and Laval nozzle is analyzed using numerical simulations, particularly emphasizing the unsteady starting process of the Ludwieg tube tunnel with a bent storage tube. Preliminary freestream calibration of the Ludwieg tube is also conducted. Results show that the hypersonic Ludwieg wind tunnel controlled by the fast-opening valve can obtain high flow quality in the test section. The bent storage tube slightly affects the propagation intensity of the expansion waves locally. However, it does not affect the propagation speed of expansion waves within the storage tube and the freestream quality in the test section. The Mach number distribution in the test section is reasonably excellent. The Pitot probe results show that the intensity of freestream pressure disturbances is lower than in similar facilities in Germany and UAS. The wind tunnel, therefore, shows promising potential in experimental studies of hypersonic aerodynamics. Hopefully, this study will shed light on constructing a low-cost, research-oriented hypersonic wind tunnel.

Hypersonic Ludwieg tube tunnel is one of the most important ground facilities for hypersonic aerodynamic research. To some extent the high requirement of a large laboratory space due to the long straight storage tube of the Ludwieg tube tunnel precludes the establishment of this hypersonic aerodynamic facility for fundamental research purposes. To tackle this issue, this work reports the aerodynamic design of a hypersonic Ludwieg tube tunnel with a double-bent storage tube, and special emphasis was placed on the tunnel’s unsteady starting process and the influence of the double-bent storage tube on the flow field. Based on the aerodynamic design a Mach 6 Ludwieg tube tunnel with a double-bent storage tube has been newly built and the first shakedown results agree well with the numerical prediction. It reveals that the propagation of expansion waves in the storage tube is affected by the double-bent tube, resulting in a weak total pressure fluctuation in the storage tube. By choosing a proper diameter ratio between the bend and the storage tube, the influence of the storage tube bend can be neglected.