Internal mixing mechanism of a turbulent boundary layr formed by spanwise gaps between roughness element

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Measurement was made on the 3-D development of a turbulent boundary layr flow through a gap in an isolated wall-mounted roughness element to examine the internal mixing mechanism inwall shear flow. The effect of gap size was examined for a ratio of roughness height to boundary layr thickness 0.135 at a Reynolds number of 3300. The magnitude of distortion produced by the gap varies with gap width.Centerline downstrem tendency of the mean velocity profile is almost similar, independent of gap width. Mean flow and turbulence quantities shows maximum distortion at about four gap with downstream of the gap. The gap size effect on major flow features was also analyzed with both the appropriate velocity and length scales. From the off-centerline features in the horizontal plane at y^+=25, present flow field can be divided into two regions, outer layr region and internal mixing region. An internal mixing region, furthermore, can be divided into four small groups, center region, interacted region, weak interaction region and mixing region. The interaction is more intense for the large gap than for small one.Conditional averaged patterns detemined with the VITA analysis and higher-order moments are significantly affected by the gap width. In particular, the gap width effects on the peak ratio, mean event frequency and mean consecutive time for both accelerating and decelerating event at x/G=0 are completely contrary to those at x/G=4. The higher-order moments show the reverse tendency at y/delta*0.14 as the border.