TURBULENCE ANISOTROPY AND NEAR-WALL MODELING IN PREDICTING 3-DIMENSIONAL SHEAR-FLOWS

Three-dimensional turbulent dow in a circular-to-rectangular transitio n duct is studied to assess the role of near-wall modeling and turbule nce anisotropy in predicting the origin and growth of longitudinal vor ticity and the secondary motion with which it is associated. Calculati ons are carried out using the standard k-epsilon model and the Reynold s stress transport closure of Gibson and Launder (GL), both of which u se wall functions. The computed solutions are compared with experiment al data and with calculations previously reported by the authors which employed the near-wall version of the GL model proposed by Launder an d Shima (LSH). These comparisons lead to the conclusion that accurate description of most three-dimensional turbulent flows, regardless of t heir origin, would require turbulence models that 1) resolve the near- wall flow and 2) account for anisotropy of the Reynolds stresses. Furt her evidence to support the latter conclusion is provided by employing the LSH solution to evaluate the various terms in the mean longitudin al-vorticity equation. It is shown that, vortex stretching, vortex ske wing, and generation and destruction of vorticity by Reynolds stresses are all dominant in one region or another.