Characteristics of turbulence in free convection flow past a vertical plate.

Abstract

An experimental and theoretical investigation of the turbulent free convection boundary layer on a vertical plane surface in air has been conducted. The experimental investigation comprised observations of both the streamwise development from a laminar state to a 'fully developed' turbulent flow and the lateral structure of the turbulent flow at Grashof numbers up to 7x 1010. Measurements were taken of the probability density distributions of temperature and streamwise velocity as well as power spectra of these quantities. The results show that a periodic flow structure, present in the early stages of the transition, disappears as the intensities of temperature and velocity increase to a maximum in the midstage of the transition and then decay. Observations in the 'fully turbulent' flow suggest that the flow has a lateral structure similar to that of a forced convection flow: a viscous sublayer with mean temperature profiles linearly dependent on the distance from the plate, a buffer layer which includes the maximum of mean velocity profiles, and a turbulent layer where the power spectra of temperature and velocity contain an inertial subrange. The theoretical investigation comprised a study of the governing equations and the application of several turbulence hypotheses to the prediction of the boundary layer flow. Solutions for lateral profiles and for the streamwise development of velocity and temperature fields agreed reasonably well with experimental data although there was some disagreement on the heat-transfer rates. Energy balances of the mean kinetic energy and turbulence kinetic energy of the turbulent flow were also predicted. Measurements of the flow were performed with a hot-wire anemometer and thermocouple sensor in conjunction with digital data processing. A large part of the work was devoted to the development of suitable data processing techniques.