Experimental and numerical investigation of coherent structures

Several significant tasks were accomplished during the first year of the grant period. The first involved completing the Reynolds stress measurements behind various wake generators and demonstrating that the previously observed variation in the spreading rate of the mean flow field could be confirmed by the Reynolds stress results. This provides convincing evidence for the lack of universality of the turbulent structure of two dimensional small deficit wakes. The model decomposition work, initiated to investigate possible cause for the lack of universality, was completed. Varicose disturbances were successfully generated using a two flap mechanism to investigate the significance of the varicose mode. The amplitude ratio of the two modes, sinuous and varicose, appear to approach an asymptotic value in the far wake. Finally, varicose effects were accounted for in the model decomposition work by solving the Orr Sommerfeld equation. The varicose mode appears to be more affected by viscosity than the sinuous mode. Numerical simulations based on the complete Navier Stokes equation are under development for a somewhat simpler flow geometry than the wake, namely the free shear layer. Considerable progress has been made toward the development of numerical methods that are applicable to such simulations. For simplicity, the temporal model is being used first. A computer program based on the spatial model is nearly finished.