Wake Turbulent Data
The flow past a circular cylinder is examined using the Particle Image Velocimetry (PIV) technique at a subcritical Reynolds number of 3,900. Both the base flow case (no control) and the controlled case where a small control cylinder is placed immediately outside of the wake shear layer have been studied. Results show that the control cylinder periodically diverts the high speed free-stream into the innermost part of the vortex formation region. Consequently, a reduction in size of the recirculation region is observed, which is accompanied by a significant reduction in the base pressure fluctuation.The measurements involve three steps. First the instantaneous velocity field in a selected plane of the flow field is obtained using a conventional PIV technique. The velocity is sampled with a period T= tk+1 -tk, where tk is the measurement time instant. Second the continuous velocity V (x,y,t) and vorticity. This reconstruction is accurate provided that the temporal acquisition rate is sufficient to resolve the temporal flow scales, i.e it satisfies the Nyquist criterion. Third, taking the vorticity field as a means to represent the coherent structures, phase aligned and averaged. It has been shown that a continuous waveform may be reconstructed from its samples by means of a suitable interpolation scheme between the samples. Therefore, this phase realignment allows us to analyze the data using the traditional phase-averaging algorithm. Following the terminology used previously by Cantwell and Coles (1983), the instantaneous turbulent flow data is decomposed into three components: a global average defined by ensemble average (represented by quantities with the subscript f), a periodic term (represented by quantities topped with the symbol ~), and the random fluctuation term (represented by quantities with the subscript ').
The near wake turbulence data is represented in the following:
1. Centerline distribution of the mean axial velocity and turbulent intensities with and without control
2. Whole-field distribution of the global turbulent intensities
3. Whole-field distribution of the periodic fluctuations
4. Whole-field distribution of the random fluctuations
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