Simultaneous and instantaneous measurement of velocity and density in rayleigh-taylor mixing layers

摘要

There are two coupled primary objectives for this study of buoyancy-driven turbulence.The first objective is to create a new diagnostic for collection of measurements to capture thephysics of Rayleigh-Taylor (RT) mixing. The second objective is to use the new diagnostic tospecifically elucidate the physics of large Atwood number, ( )( )2 1 2 1 / ? ? ? ? + ? = t A , RTmixing. Both of these objectives have been satisfied through the development of a new hot-wirediagnostic to study buoyancy-driven turbulence in a statistically steady gas channel of heliumand air ( 6 . 0 03 . 0 ? ? t A ). The capability of the diagnostic to simultaneously and instantaneouslymeasure turbulent velocity and density fluctuations allows for a unique investigation into thedynamics of Rayleigh-Taylor mixing layers at large At, through measurements of turbulence andmixing statistics. The new hot-wire diagnostic uses temperature as a fluid marker for helium andair, which is possible due to the Lewis number ~ 1 (Le = ratio of thermal diffusivity to massdiffusivity) for helium and air, and the new diagnostic has been validated in an At = 0.03 mixinglayer. The energy density spectrum of v? ? ? , measured experimentally for the first time in RTmixing, is found to closely follow the energy distribution of v? , up to the Reynolds numbers investigated ( ( ) mix t h gA h ? 6 2 Re 2 / 3 = ~ 1450). Large At experiments, with At = 0.6, havealso been achieved for the first time in a miscible RT mixing layer. An asymmetric penetrationof the bubbles (rising fluid) and spikes (falling fluid) has been observed, resulting in measuredself similar growth parameters ?b = 0.060 and ?s = 0.088 for the bubbles and spikes, respectively.The first experimental measurements of turbulent velocity and density fluctuations for the largeAt case, show a strong similarity to lower At behaviors when normalized. However conditionalstatistics, which separate the bubble (light fluid) and spike (heavy fluid) dynamics, hashighlighted differences in v? ? ? and rms v? in the bubbles and spikes. Larger values of v? ? ? andrms v? were found in the downward falling spikes, which is consistent with the larger growth ratesand momentum of the spikes compared to the bubbles. These conditional statistics are a first inRT driven turbulence.