Turbulent Coherent Structures in a Thermally Stable Boundary Layer



The effects of thermal stability on coherent structures in turbulent flat plate boundary layers are examined experimen-tally. Thermocouple and DPIV measurements are reported over a Richardson number range 0 < Ri δ < 0.2. The reduc-tion in wall shear and the damping of the turbulent stresses with increasing stability are qualitatively similar to that found by Ohya et al. (1996) including the major changes observed when the flow enters the strongly stable regime. In contrast, a critical bulk Richardson number of 0.05 is observed,which is much lower than the value of 0.25 found in this earlier study. In the weakly stable regime, hairpin vortices are seen to con-tinue to populate the near-wall region and are elongated in the streamwise direction creating a smaller angle of inclination to the wall. With increasing stability, the angle of these struc-tures continues to decrease and they are confined closer to the wall. In our experiments, the strongly stable flows show no evidence of large scale structures, or the presence of gravity waves.

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