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The objective of this study is to determine processes of pollution ventilation in the X-shaped street intersection in an idealized symmetric urban area for the changing approach flow direction. A unique experimental setup for simultaneous wind tunnel measurement of the flow velocity and the tracer gas concentration in a high temporal resolution is assembled. Advective horizontal and vertical scalar fluxes are computed from averaged measured velocity and concentration data within the street intersection. Vertical advective and turbulent scalar fluxes are computed from synchronized velocity and concentration signals measured in the plane above the intersection. All the results are obtained for five approach flow directions. The influence of the approach flow on the advective and turbulent fluxes is determined. The contribution of the advective and turbulent flux to the ventilation is discussed. Wind direction with the best dispersive conditions in the area is found. The quadrant analysis is applied to the synchronized signals of velocity and concentration fluctuation to determine events with the dominant contribution to the momentum flux and turbulent scalar flux.

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Abstract.It is well known that sweep and ejection are major contributors to the momentum flux in the turbulent boundary layer. In order to evaluate the contribution of these events to the total flux, the wind-tunnel data measured by PIV technique were analyzed from spatial point of view. It was revealed that both the sweep and the ejection reach up to 90% of the flux momentum inside the canyon. Moreover, the sweep and ejection pass the street in an alternative fashion. When sweep reaches maximum, the ejection falls down to minimum and vice versa. The TKE energy is also very high in these particular events, reaching 80-90% of total TKE inside the street what makes the sweep and ejection to be the most important features in the flow.

Abstract. The objective of this experimental study is to determine processes of a vertical turbulent pollution transport above the X-shaped street intersection in an idealised symmetric urban area for several approach flow directions. An experimental set-up for simultaneous measurement of the flow velocity and the tracer gas concentration in a high temporal resolution is assembled. Vertical turbulent scalar fluxes are computed from the measured data in a horizontal plane above the street intersection. The quadrant analysis was applied to the vertical turbulent pollution fluxes data. Events with dominant contribution to vertical turbulent pollution flux were detected. The mean duration, repetition frequency and the duration percentage were computed for these events. A strong influence of the approach flow direction on the the type of dominant events and their characteristics was resolved.

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