Turbulent Coherent Structures in a Thermally Stable Boundary Layer


ABSTRACT 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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Available from: Owen Williams, Sep 27, 2015
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    ABSTRACT: The critical Richardson number, Ric, is used in studies of stably stratified turbulence as a measure of flow laminarization. The accepted range of Ric is between 0.2 and 1. A growing body of experimental and observational data indicates, however, that turbulence survives for Ri ≫ 1. This result is supported by a new spectral theory of turbulence that accounts for strong anisotropy and waves. The anisotropization results in the enhanced horizontal mixing of both momentum and scalar. Internal wave contribution preserves vertical momentum mixing above its molecular level. In the absence of laminarization, Ric becomes devoid of its conventional meaning. Copyright © 2007 Royal Meteorological Society
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    ABSTRACT: Observations made in a well-developed, thermally stratified, horizontal, flat- plate boundary layer are used to study the effects of buoyancy on the mean flow and turbulence structure. These are represented in a similarity framework obtained from the concept of local equilibrium in a fully developed turbulent flow. Mean velocity and temperature profiles in both the inner and outer layers are strongly dependent on the thermal stratification, the former suggesting an increase in the thickness of the viscous sublayer with increasing stability. The coefficients of skin friction and heat transfer, on the other hand, decrease with increasing stability.
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