A Study on Three-Dimensionality of Turbulent Heat Transfer in a Channel. Eduction of Coherent Structures by a Conditional Sampling Technique.

Abstract

Coherent structures near the wall (y+<60)in a turbulent channel flow were educed by a conditional sampling technique from the DNS data for two kinds of temperature fields : (a) the uniform heat flux assigned on the two walls (UHF) and (b) the constant spanwise mean temperature gradient imposed on the working fluid (STG). Attention was paid for roles of the quasi-streamwise vortex in heat transfer mechanisms. Flow and temperature fields around the quasi-streamwise vortex have basically two-dimensional structure. In the case of UHF, temperature fluctuation is high at right-and left-sides of the vortex core where the rotating flow motion due to the vortex has a wallnormal component. In STG, large value of the temperature fluctuation was observed at the top-and bottom-sides of the vortical structure where flow in the spanwise direction is conspicuous. In both cases, gradients of the time-mean temperature and the temperature fluctuation in the same direction have opposite signs since excessive temperature is transported by the vortical flow motion in the azimuthal direction with respect to the vortex axis. Therefore, temperature distribution for UHF and that for STG have azimuthally phase difference of about 90 degree, and they are almost similar in shape. Destruction (correlation between pressure and temperature gradient) of wall-normal turbulent heat flux and that of spanwise component are based on similar mechanisms. They are caused by the rotating motion of the vortex by which, near the center axis of the vortex, pressure fluctuation takes negative value and the gradient of temperature fluctuation becomes inverted for that of the mean temperature.