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首页> 外文期刊>Experimental Thermal and Fluid Science: International Journal of Experimental Heat Transfer, Thermodynamics, and Fluid Mechanics >Experimental determination of preferred instability modes in a mechanically excited thermal plume by ultrasound scattering
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Experimental determination of preferred instability modes in a mechanically excited thermal plume by ultrasound scattering

机译：通过超声散射实验确定机械激发的热羽中优选的不稳定性模式

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Ultrasound scattering is used to characterize instability modes in a laminar axisymmetric thermal plume subjected to controlled axisymmetric ("varicose") disturbances. A scattered signal is detected as soon as vortical structures appear in the flow, whereas temperature inhomogeneities have almost no effects on scattering. In the absence of any disturbances, the flow remains laminar and quite stable, which was corroborated with Schlieren visualizations. Scattering peaks exhibited maxima around forcing frequencies of f = 2 Hz. By increasing the mechanical forcing frequency, the amplitude of the scattering peak decreases and disappears for higher forcing frequencies, revealing a relaminarization process. For frequencies around f=1 Hz and lower, no scattering is observed, like without forcing. The study of the normalized amplitude of the scattering peak at different structure sizes enables the identification of two ranges of preferred wavelengths of instability modes: the first ranges from l = 80 min to l = 54 nun and the second from I = 20 turn to I = 16 mm, These preferential space modes can be attributed to natural frequencies of the flow. The levels of vorticity of the preferential modes depend on the frequency of the mechanical disturbance, being maximum around f = 2 Hz. The position of the maximum of vorticity moves to higher values of frequency as the temperature is increased. When the mechanical forcing frequency increases away from the resonance frequency, vorticity decays gradually reaching very low values. High frequency disturbances do not destabilize the thermal plume, which acts as a filter. (C) 2005 Elsevier Inc. All rights reserved.
机译：超声散射用于表征层状轴对称热羽中的不稳定性模式，这些羽流受到受控的轴对称（“静脉曲张”）干扰。流动中出现旋涡结构后，便立即检测到散射信号，而温度不均匀性几乎对散射没有影响。在没有任何干扰的情况下，流量保持层流且相当稳定，这在Schlieren可视化结果中得到了证实。散射峰在强迫频率f = 2 Hz附近表现出最大值。通过增加机械施力频率，散射峰的幅度会减小，而对于较高的施力频率则消失，从而揭示了再分层过程。对于f = 1 Hz或更低的频率，没有观察到散射，就像没有强迫一样。对不同结构尺寸的散射峰的归一化幅度的研究使得能够确定失稳模式的两个优选波长范围：第一个范围从l = 80分钟到l = 54 nun，第二个范围从I = 20匝到I = 16 mm，这些优先空间模式可归因于流动的固有频率。优先模式的涡度水平取决于机械干扰的频率，在f = 2 Hz附近最大。随着温度升高，最大涡度的位置移至较高的频率值。当机械强迫频率远离共振频率增加时，涡度逐渐衰减，达到非常低的值。高频干扰不会破坏用作过滤器的热羽流的稳定性。 （C）2005 Elsevier Inc.保留所有权利。

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