Large Structure in the Far Wakes of Two-Dimensional Bluff Bodies,
CALIFORNIA INST OF TECH PASADENA GRADUATE AERONAUTICAL LABS
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Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 Re 2000, a very rapid exponential decay of velocity fluctuations at the Karman vortex street frequency is observed. Beyond this region of decay, large-scale lower wave-number structure can be seen. In the far wake beyond one hundred diameters a broad band of frequencies is selectively amplified and then damped, the center of the band shifting to lower frequencies as downstream distance is increased. The far-wake structure does not depend directly on the scale or frequency of the original Karman vortices the growth of this structure is due to hydrodynamic instability of the developing mean wake profile it is not caused by amalgamation of the Karman vortices. Hot-wire measurements show that two-dimensional locally-parallel inviscid linear stability theory is adequate to explain the growth of downstream structure. Namely, measured prominent frequencies in the cylinder wake are in close agreement with those predicted by the theory, when streamwise growth of wake width is taken into account.
- Fluid Mechanics