Experimental and Numerical Investigation of Coherent Structures in Turbulent Wake Flows

摘要

Large scale coherent structures have been observed in two-dimensional wakes behind bluff bodies as well as non-vortex shedding bodies. There is a strong indication that these large scale structures are related to the two-dimensional instability modes of the slowly diverging mean wake flow. The Orr-Sommerfeld equation admits two solution modes for the two-dimensional plane wake. These are the sinuous mode with antisymmetric streamwise fluctuations and the varicose mode with symmetric streamwise fluctuations. The wake response to controlled sinuous and varicose types of disturbance waves was investigated. Sinuous disturbances at several amplitudes and frequencies were introduced to the wake of a flat plate by oscillating a small trailing edge flap. The Strouhal numbers of the disturbance waves were specially chosen so that the downstream location of the neutral point was well within the range of measurements. The streamwise variation of the half width of the wake an the centerline deficit was dependent on the amplitude level and showed dramatic deviations, starting near the neutral point, at large forcing levels from the well known square root behavior of the unforced case. The measured coherent Reynolds stresses were observed to change sign in the neighborhood of the neutral point as predicted from linear stability theory. The extent of the validity of linear stability theory was investigated. The wake was also forced with a sinuous disturbance of lower Strouhal number that was amplified over the entire range of measurements. (edc)