Turbulent diffusion behind vehicles: Experimentally determined turbulence mixing parameters

Abstract

The wake of a moving vehicle was stimulated using a specially constructed wind tunnel with a moving floor. A ‘block-shaped’ model vehicle was fixed in position over the test-section floor while the floor moved at the freestream air speed to produce a uniform, shear-free, approach flow. This simulates an automobile traveling along a straight highway under calm atmospheric conditions. Vertical and lateral profiles of tracer gas concentration were obtained in the wake. Profiles were taken at distances of 30, 45 and 60 model heights downwind. The equations describing the wake theory were solved numerically to determine the ‘best’ turbulence scale lengths by using wind tunnel data taken at 30 mode heights downwind as the inflow boundary condition and comparing the numerical computation made at 60 heights downwind to the wind tunnel data taken at this location. It was found that the ‘best’ scale lengths were the vehicle width along and across the wake, and height above the surface in the vertical directions, respectively. The ROADWAY model, in which the computer code incorporates the wake theory to predict air pollution concentrations along highways, was modified with these new results and found to better predict the General Motors data than the initial version of the model.