侧壁约束下压缩激波/湍流边界层相互作用低频不稳定性实验研究

Experimental Study on Low-Frequency Instability in Sidewalls-Confined Compression Shock-Wave/ Turbulent Boundary-Layer Interactions

关键词:  进气道  侧壁约束  激波/湍流边界层相互作用  超声速压缩拐角  低频大尺度振荡

Key words:  Inlet  Sidewalls-confined  Shock-wave/turbulent boundary-layer interactions  Supersonic compression ramp  Low-frequency large-scale oscillation

摘要:

侧壁面约束条件下压缩激波/湍流边界层相互作用（Shock-Wave/Turbulent Boundary-Layer Interactions，SWTBLIs）呈现出区别于经典二维SWTBLIs的强三维特性，尤其是低频不稳定性问题自发现以来就受到了广泛关注，而其与下游压缩环境之间的关联性尚不明确。本文基于带侧壁约束的超声速压缩拐角构建三维受限的SWTBLIs流动，并通过自由射流风洞实验进行了系统研究，其中来流Ma_∞=2.5，湍流边界层发展厚度对矩形流道宽度的占比约0.08，压缩拐角在12°~24°的宽范围内变化。通过不同压缩拐角下流场结构演化及壁面压力脉动信号谱特性的对比，揭示了侧壁面约束条件下压缩拐角对SWTBLIs低频大尺度振荡时间-空间演化行为的影响规律和作用机制。研究发现：压缩拐角角度较小时，侧壁面约束导致的“拖尾效应”有助于延缓压缩拐角附近流动分离模式的过渡，并抑制压缩激波的不稳定性；压缩拐角达到20°及以上时，压缩拐角前沿逐渐发展为大尺度流动分离模式，侧壁面约束的角区将首先发展出能量集中的低频脉动，并逐步演化为两种峰值频率分别约为50Hz和200Hz量级的不同大尺度间歇性低频振荡叠加行为，且伴随着压缩激波形态的显著改变；两种低频振荡行为都属于SWTBLIs流场的全局性不稳定振荡，但沿着展向振荡强度差异显著，角区内的振荡强度远远高于中心主流区域。

Abstract:

Sidewalls-confined compression shock-wave/turbulent boundary-layer interactions（SWTBLIs） shows strong three-dimensionality（especially on the three-dimensional， low-frequency unsteadiness） which is widely focused since reported， while its relationship to the downstream compression is not clear yet. The current experimental research was engaged on a sidewall-confined， supersonic compression ramp model in a free jet wind tunnel for understanding the three-dimensional SWTBLIs. The freestream Ma_∞=2.5 and the turbulent boundary layer thickness-flow channel width δ₀/w≈0.08. The compression ramp angle varies within the range of 12°~24°. The influence that the sidewall-confined compression ramp applies to the low-frequency， large-scale oscillation and its time-spatial evolution is revealed via observation on flow structure in various ramp angle cases and spectral analysis on the time-history of wall pressure. The results proved that when the compression corner angle is small， the “smearing effect” caused by the sidewalls helps to delay the transition of the flow separation mode near the compression corner and suppress the instability of the compression shock wave. The front of the compression corner gradually develops into a large-scale flow separation mode while the compression corner reaches 20° and above. The corner area will firstly develop energy-intensive low-frequency pulsation， and gradually evolve into two peak frequencies of about 50Hz/200Hz superposition behavior of different large-scale intermittent low-frequency oscillations， accompanied by a significant change in the shape of the compressed shock wave. The two low-frequency oscillation behaviors both belong to the global unstable oscillation of the SWTBLIs flow field， but the oscillation intensity along the span direction is significantly different， and the oscillation intensity in the corner area is much higher than that in the central mainstream area.