Sensible Heat Flux Measurements Near a Cold Surface.

Abstract

Abstract : A unidirectional sonic anemometer with a fine wire thermocouple in conjunction with a hot film anemometer were employed to measure the turbulent fluctuating velocities of w', u' and the fluctuating temperature T'. Covariances were evaluated to compute the turbulent heat flux and the friction velocity. Based on preliminary data, it can be noted that the values of fluctuating vertical velocity and temperature, the friction velocity, and the standard deviations of vertical and horizontal turbulent fluctuating velocities can all be correlated rather well with a single variable, i.e., the mean wind speed measured at a height of 2 m. In all the plots of friction velocity, vertical and horizontal turbulent fluctuating velocities, and the fluctuating vertical velocity and temperature vs. the mean wind speed at 2 m, the slopes are slightly lowered as the test season progressed from early summer to the winten The most striking reduction can be observed in the case of the fluctuating vertical velocity and temperature vs. mean wind speed at 2 m. During the winter period, the slope is only about one third of that during the spring summer period. In other words, under unstable conditions, for the same mean wind speed, the heat flux during the winter is only about one third of the flux that would have occurred during the spring-summer. Under stable conditions, the magnitude of the fluctuating vertical velocity and temperature is much smaller, and its value shows much greater dispersion. The values of fluctuating vertical velocity and temperature cannot be correlateed in any meaningful manner, as is the case under unstable conditions, by the mean wind speed alone. Comparisons were made with the few existing measured data or with predictions from theoretical expressions, and they were found to be in fairly good agreement in some cases and to have large divergence in others.