EFFECTS OF STRAIN-RATE AND SUBGRID DISSIPATION RATE ON ALIGNMENT TRENDS BETWEEN LARGE AND SMALL SCALES IN TURBULENT DUCT FLOW

HPIV measurements of a turbulent flow in a square duct are used for studying the 3-D geometric relations between SGS stresses, filtered strain-rates and vorticity. Conditional sampling is performed based on the strain-rate magnitude and SGS dissipation, including negative values. In regions of high dissipation, the flow is more likely to be in axisymmetric expansion with vorticity strongly aligned with the intermediate strain-rate eigendirection. The alignment between the most extensive SGS stress and the most compressive strain-rate (Tao et al 2000) varies with both the dissipation and strain-rate magnitude, with increased peak values of probability density function (pdf) of angles in regions of high dissipation and strain-rate magnitude. The relative orientation pdfs of the other eigendirections exhibit a more distinguishable bimodal behavior in regions of high dissipation and strain-rate magnitude. However the alignments still differ from eddy viscosity models. The paper also presents the alignment pdf between the measured stresses and model stresses by similarity SGS models. It is shown that the non-linear model (linearized similarity model) provides much better predictions of the alignment, a trend that persists in conditionally sampled data as well.