Multi-Length Scale Turbulence Models.

Development of a new two-length scale turbulence model is described. This work was undertaken because the performance of current models for certain flows appears to be limited by an inadequate treatment of the turbulence length scale. One flow for which current turbulence models are not adequate is the initial developing region of a plane mixing layer. Available mixing layer data is briefly reviewed. An improved ability to analyze this flow is required for improved prediction of the near field of a jet for a wide range of applications including those for STOL aircraft applications. The work was performed in the context of developing an improved turbulence model for general application to complex three-dimensional jets. The new model is based on the physical observation that the turbulence shear stress and the turbulence energy production are associated primarily with the large scale eddies, while most of the turbulence energy is dissipated by small scale eddies near the high frequency end of the turbulence energy spectrum. Therefore, except for flows close to equilibrium, separate turbulence length scales are required to characterize the large and small scale motions.