Probing of Complex Flows and Addressing Related Turbulence Modeling Issues in the Laboratory and in the Ocean

Abstract-
The seminar consists of two parts. In the first, PIV measurements are used for examining the complex flow structure caused by blade-wake and wake-wake interactions in multi-stage turbomachines. In order to provide unobstructed view on a flow with multiple blade rows, the experiments are performed in a facility containing fluid with the same refractive index as the transparent blades. The results consist of detailed distributions of phase (and passage) averaged velocity, Reynolds and deterministic stresses, turbulent kinetic energy and dissipation rate. The observed phenomena include phase-dependent flow non-uniformities and their impact on the deterministic stresses, chopping of upstream wakes and the resulting variations in stage performance, effect of wake impingement on the structure of a boundary layer, and formation turbulent hot spots at intersections of sheared wakes.

The second part focuses on the flow structure and turbulence in the bottom boundary layer of the coastal ocean. Several oceanic PIV data sets, representing periods when the mean currents are higher, of the same order, and much weaker than the wave-induced motions are examined. Second order structure functions are used for calculating the Reynolds stresses free of wave contamination. Methods for estimating the dissipation rate are compared, including estimates based on available velocity gradients and curve fitting to the energy spectra. Vertical gradients of the horizontal velocity dominate the dissipation rate and are higher than the horizontal gradients of vertical velocity. The turbulent energy spectra display substantial anisotropy at all scales, resulting in unreliable estimates of dissipation rate based on a line-fit to the energy spectrum, especially at moderate to weak flow rates. The resulting Taylor micro-scale Reynolds numbers are: 300-440 for the high, 68-83 for the moderate, and 14-37 for the weak mean currents. The moderate-weak turbulence levels are typical to the calm weather conditions in the Atlantic coast, due to the low velocities and limited range of length scales. Spatial filtering of the data enables us to calculate the SGS stresses, dissipation and model coefficients. At high turbulence levels the SGS dissipation is close to the viscous dissipation. However, in the moderate and weak conditions, the SGS dissipation is more than an order of magnitude smaller than the viscous dissipation rate. The seminar will conclude with a brief description of recent 3-D measurements of the flow around a swimming zoo-plankton using digital holography.

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