Pseudo-volumetric reconstruction of a variable-density turbulent jet and the fine scales

Pseudo-volumetric reconstruction of an air jet using a TR-SPIV dataset and Taylor’s hypothesis. Vortices are visualized using the Q-criterion with a threshold set at a value three times the mean. x1 – jet axial direction. (x2,x3) – in-plane directions at a jet cross-section.

Fluid turbulence is inherently a three-dimensional (3D) phenomenon that comprises spatially-coherent structures. To study turbulence is to understand the mutual interactions among these coherent structures and also on how they are affected by large-scale forcings and boundary conditions imposed on the flow.

In this project, I am interested in the dynamics and kinematics of the fine scales in a variable-density turbulent jet; the jet flow is non-Boussinesq (with an initial Atwood number of 0.6) in which the density field is not passive and modifies the flow. To obtain a 3D dataset of the jet, I use a combination of time-resolved stereoscopic particle image velocimetry (TR-SPIV) and quantitative laser-induced fluorescence (LIF) at a jet cross-section. Using Taylor’s hypothesis, a pseudo-volumetric representation of the jet volume is reconstructed from the time-resolved density and velocity data. An example of this reconstruction is shown in the figure above for the control case of a Boussinesq air jet. The fine scales are tubular vortical structures that are coherent in space.

With the 3D dataset, the following questions concerning the role of large density gradients in driving the turbulence are asked: (1) What flow topologies are dominant? Are they different from constant-density turbulent flows? (2) How is the vortex-stretching mechanism responsible for a forward energy cascade affected?  (3) What improvements are needed in existing turbulence closure models in order to account for such variable-density effects?



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