Vorticity Dynamics and Scalar Transport in Separated and Reattached Flow on a Blunt Plate By

Two-dimensional calculations are performed for a Reynolds number of 1000 and an effective Prandtl number of unity to study the dynamics of large scale spanwise vortices with a passive scalar field in a separation and reattaching flow over a blunt plate. A triple decomposition technique is used to isolate the quasi-periodic fluctuations related to the large scale vortices shed from the separated shear layer. Three vortex shedding modes are identified in the reattachment zone, two of which involve partial pairings. The coherent vortices shed from the reattachment zone have a dominating effect on scalar transport and act as "large scale mixers" by entraining wall scalar around their upstream periphery and free-stream scalar on their lower downstream edge. There is a definite relationship between the phase averaged perturbations (due to the presence of coherent vortices) in wall friction factor and wall scalar transport with their respective time mean maximums. On the other hand, although there does exist an explicit spatial relationship between the instantaneous stagnation point and instantaneous location of maximum wall scalar transport, there is no such relationship in the mean between the two locations, except that both are dependent on the vorticity dynamics in the reattachment region. 2 1. Introduction The understanding of the fundamental mechanisms affecting heat and mass transfer in separated and reattached flows is of vital importance in a number of engineering applications. Previous studies (Ota and Kon 1 ; Vogel and Eaton 2) have shown that the heat transfer coefficient decreases from a sharp peak at the the point of separation and then increases gradually to reach a maximum near the time mean reattachment point. The exact location of the point of maximum heat transfer with respect to the time mean reattachment point (change in sign of surface shear stress) has been the subject of much debate. Early investigators assumed that the point of maximum heat transfer coincided with the reattachment point. However, later studies (Vogel and Eaton 2 ; Sparrow et al. 3), have shown that the point of maximum heat transfer lies well upstream of the time mean reattachment point. There are a number of studies which have investigated heat transfer characteristics in different separating and reattaching geometries. Due to their relative simplicity the most common of these are the backward facing step and the blunt plate. Vogel and Eaton 2 made fluid dynamic as well as heat transfer measurements downstream of a …