GLOBAL MODE BEHAVIOR OF THE STREAMWISE VELOCITY IN WAKES

We characterize experimentally and numerically the two-dimensional str ucture of the global mode of the longitudinal component of the velocit y in flow behind obstacles of different geometrical shapes. Measuremen ts in a low velocity water tunnel and numerical simulations of the lon gitudinal component V-x of the oscillating velocity are carried out in the near wake region of the how where vortex shedding is regular. We find that in the range of Reynolds numbers studied, the position (X(ma x), Y-max) of the maximum of the global mode of V-x shifts very little away from the central line in the y direction. The amplitude A(max) o f the maximum of the global mode scales as (R - Rc)(1/2) in a range of R - R(c) larger then in previous studies. We also confirm that X(max) of the global mode scales as (R - R(c))(-1/2) in the same range of Re ynolds numbers. An analysis of the initial slope of the global mode is presented.