Abstract
The effect of spacing ratio on the near-wake regions immediately behind porous disks that consisted of multiple annular elements is investigated experimentally. Annular jets through individual slots merge into groups of high-momentum regions that displace a global recirculating region downstream. Multiple wake patterns result from the same overall conditions. Once the pattern forms, it remains nearly unchanged unless the flow is disturbed. Compared to the flow behind a solid disk, three-dimensional large-scale motions are suppressed downstream and the wake motions are more axisymmetric. The patterns of jet mergings are also axisymmetric and persistent. The mean and time-dependent large- and small-scale velocity fluctuations are characteristic of individual spacing ratios and flow patterns, and they are analyzed and compared with flow visualizations. The process of how the wake settles into a certain flow pattern also is addressed
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