Effects of unsteady strain rate on scalar dissipation structures in turbulent planar jets

Abstract

An analysis of previously acquired experimental data is performed with the objective of investigating the relationship between strain rate and scalar dissipation rate structures in gas-phase turbulent planar jet flows at local Reynolds numbers ranging from 1000 to 6100. The data analyzed were simultaneously acquired two-dimensional (2D) velocity and 2D conserved scalar fields. The measurements show that, in agreement with previous work, the scalar dissipation structures tend to align orthogonally to the axis of the principal compressive strain rate, and the magnitude of the strain rates acting on the dissipation layers is consistent with theoretical turbulent inner-scale values. Furthermore, the spatially resolved data show that profiles of scalar dissipation, taken across the sheet-like scalar dissipation structures, are approximately Gaussian, whereas the strain rate profiles across the same structures exhibit no characteristic shape, and rarely reach a maximum at the same location as the scalar dissipation. When the scalar dissipation layer thicknesses are cast in nondimensional form and plotted as a function of the instantaneous relative strain rate, the data at all Reynolds numbers exhibit similar characteristics. The measurements from the turbulent flow were compared to a simple one-dimensional (1D) unsteady strained laminar diffusion-layer model, where the imposed strain rate varied harmonically. The simplified model shows remarkable agreement with the experimental data as it predicts the correct trend of layer thickness with strain rate and captures the range of scalar dissipation layer thicknesses that are present in the turbulent flows. The model and experiments show that the layers with greater-than-average strain rate tend to be thicker than expected by steady-state theory, whereas at low strain rates the layers are significantly thinner than at steady state. The 1D model shows best agreement with the measurements when the imposed strain rates are allowed to be positive in the direction of the scalar gradient over some part of the oscillation cycle. This suggests that dissipation layers in turbulent flows experience significant positive strain over their lifetimes.