General motors sulfate dispersion experiment: An analysis of the wind field near a road

The mean wind field and the wind fluctuations observed near a controlled, heavily traveled level roadway were investigated. The mean wind field was studied in terms of velocity defects relative to the ambient mean wind. The wind fluctuations were studied in terms of energy spectra, velocity cross spectra, excesses of velocity variances, and cross covariances over their ambient values. Two-point cross spectra and correlations were also used to study the propagation of wind fluctuations. The influence of traffic on the ambient wind field extended vertically to at least 4.5 m above ground, and horizontally to at least 30 m downwind of the road. The vertical extension was more gradual at high cross-road winds than at low cross-road winds. The wind tended to move up as it approached the road and down as it left the road, except when the cross-road wind was low, where the downward motion was reduced or reversed. This is probably due to the buoyancy effect of the heated exhaust. The traffic effects propagated outward at about 1 m sec super(-) super(1) . Thus, at cross-road winds of less than 1 m sec super(-) super(1) , the influence of traffic could be observed upwind of the road. When intense shear existed between the local wind field and an opposing traffic direction, large turbulent energy production resulted, generating large eddies in addition to the high frequency (>0.1 Hz) eddies generated by vehicular motions in the absence of intense shear. The large eddies also increased the mixing volume, so that when the ambient wind opposed the traffic direction on the upwind lanes, the pollutant concentration upwind of the road could be higher than that downwind. While the cross-covariance excesses generally agreed with the gradient transport assumption, disagreements also existed, which may not be surprising in view of the participation of large eddies in the momentum transport.