Experimental determination of preferred instability modes in a mechanically excited thermal plume by ultrasound scattering

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 mm to l = 54 mm and the second from l = 20 mm to l = 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.