Abstract Density disturbances in the freestream of the University of Southern Queensland’s Mach 6 wind tunnel ( $$\rho _{\infty } \approx {34\, \hbox {gm}^{-3}}$$ ρ ∞ ≈ 34 gm - 3 ) have been measured using a focused laser differential interferometer (FLDI). The direct contribution of the turbulent shear layer from the Mach 6 nozzle to the FLDI signal was largely eliminated by mechanically shielding the FLDI beams from these effects. This improvement significantly enhanced the low wavenumber FLDI spectra which allowed a von Kármán spectrum fit and demonstrated a $$-5/3$$ - 5 / 3 roll-off in the inertial subrange and enabled the identification of the integral length scale (28–29 mm). The normalised root-mean-square density fluctuations were found to change over the flow duration (typically between 0.4 and $$0.6\%$$ 0.6 % ) for the 1–250 kHz frequency range which corresponds to the wavenumber range of 6– 1600 $${\mathrm{m}}^{-1}$$ m - 1 in this Mach 6 flow. Previous disturbance measurements using intrusive methods have identified a narrowband 3–4 kHz disturbance that is first measured in the core flow about 65 ms after the flow begins and remains until the flow terminates. The onset of this narrowband disturbance was previously correlated with transition-to-turbulence in the subsonic test gas supply to the nozzle. This correlation was investigated further herein, and the 3–4 kHz feature was inferred to be entropy mode disturbances by showing the departure of the FLDI measurements from Pitot pressure measurements. Through the comparison of FLDI and Pitot pressure data, Pitot pressure probes were demonstrated to produce a poor estimate of the static pressure fluctuations when non-isentropic disturbances are non-negligible. Graphic abstract

中文翻译：

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高超声速风洞中自由流密度波动的测量

摘要 南昆士兰大学 6 马赫风洞自由流密度扰动 ( $$\rho _{\infty } \approx {34\, \hbox {gm}^{-3}}$$ ρ ∞ ≈ 34 gm - 3 ) 已使用聚焦激光差分干涉仪 (FLDI) 进行测量。来自 6 马赫喷嘴的湍流剪切层对 FLDI 信号的直接贡献通过机械屏蔽 FLDI 光束免受这些影响而在很大程度上被消除。这种改进显着增强了低波数 FLDI 光谱，这允许 von Kármán 光谱拟合，并在惯性子范围内展示了 $$-5/3$$ - 5 / 3 的滚降，并能够识别积分长度尺度（28- 29 毫米）。发现归一化的均方根密度波动会随着流动持续时间而变化（通常在 0.4 到 $$0.6\%$$ 0. 6 % ) 用于 1–250 kHz 频率范围，对应于 6– 1600 $${\mathrm{m}}^{-1}$$ m - 1 的波数范围在这个马赫 6 流中。以前使用侵入式方法进行的扰动测量已经确定了窄带 3-4 kHz 扰动，该扰动首先在核心流开始后约 65 ms 进行测量，并一直持续到流终止。这种窄带扰动的开始以前与喷嘴亚音速测试气体供应中的过渡到湍流有关。本文进一步研究了这种相关性，通过显示 FLDI 测量值与皮托管压力测量值的偏离，将 3-4 kHz 特征推断为熵模扰动。通过比较FLDI和Pitot压力数据，当非等熵扰动不可忽略时，皮托管压力探头被证明对静态压力波动的估计很差。图形摘要