Simulation of Density Driven Down-Gusts and their Interaction with Street Canyons

Severe convective down-gusts or downbursts lead to flow conditions at ground level that differ from those in a typical atmospheric boundary layer flow (ABL). Wind gusts induced by downdrafts lead to high velocities near the ground and generate wall-jet-like wind profiles. When compared to the flow of an atmospheric boundary layer (ABL), these wall-jet-like winds show fundamental differences concerning their non-steady flow behavior, their wind profile with height, their complex 3d-structure and their lower level of turbulence.

Wind loadings of structures, however, are usually calculated by national standards, which are based on the assumption of a horizontal steady turbulent ABL flow. A vertical wind component is not considered and, thus, the local interaction of downdrafts with urban structures is not taken into account in these standards. Recent works showed that single buildings within a convective outflow experience a pressure field that differs from the one in an ABL. If the down-gust impinges on a built inner-city area, additional effects arise within the street canyons.

To show these effects an experimental laboratory study was performed. The gust was simulated by a parcel of gas mixture consisting of air, CO₂ and seeding particles with a higher density than the surrounding air. The particles are necessary for the velocity measurements with Laser Doppler velocimetry (LDV). The gust with a diameter of D = 240 mm was suddenly released from a collecting container by opening the bottom of the container. A ring vortex is generated in the shear layer between the falling gas parcel and the surrounding air. The gust impinged on a street canyon model with BxH = 51mm x 76 mm. The horizontal velocities within the street were measured at a distance of 0.9 D from the impingement center in the central axis of the street. The measurements at each point were rerun three times. At all points the reproducibility was very good. To be able to compare the experimental results with full-scale events, it was insured that the Reynolds number defined in Lundgren et al., 1992 was above the critical Reynolds number of 3000, which implies a density difference of Δρ≥12% for the given parameters. In the conducted experiments Δρ was approximately 26%.

One challenge in the analysis of down-gust events is their highly unsteady behavior. In many papers the time series is divided in a time varying moving average and residual fluctuations to separate the deterministic characteristics from the random turbulence as it is shown in Holmes et al., 2008. To do so, some considerations must be taken into account. For the present work an averaging time of 0.2s was determined. Each simulation lasted for about 5s. Analogously, a running turbulence intensity was defined.

As reported in many previous studies, the ring vortex spreads on the ground after touchdown (see Figure 1a+b). The high pressure region in the impingement zone results in a radial acceleration of the flow near the ground. When the gust impinges on a city model, the ring vortex propagates above the roof level and forces a gust front within the street canyon, which can be seen in Figure 1c+d.

On open terrain the peak velocities decrease rapidly with height, whereas within the street canyon the velocities remain nearly constant over the street canyon’s height. Furthermore, high velocities are retained over a longer period of time within the street. Exemplarily, the velocity time series of two points are shown in Figure 2, V_ref is the velocity of the falling gust. Maximum horizontal velocities were higher within the street canyon than on the flat plate, except for the measuring point in close proximity to the ground. In the upper part on the flat plate, only one distinct peak can be observed, which is a result of the passing ring vortex. The following outflow passes underneath this measuring point, whereas within the street high velocities are also measured after the vortex has passed. The running turbulence intensities are lower within the canyon for all measured points. This is partially due to a real reduction of the fluctuations and partially due to the reduced mean velocity on the flat plate.

The simulation of a density driven gust represents the physical characteristics of a real down-gust better than the often used mono-fluid wall jet. The density driven experiments, however, are very time-consuming and only few points could be measured in the current study. This is why the presented results will serve to validate a more extensive study with an impinging mono-fluid jet. It is planned to measure at different distances from the impingement center to make sure that maximum velocities are recorded.

The conducted experiments revealed, however, that the vertical momentum flux into the street canyon generates a gust front that is conserved over longer period of time compared to the propagation on a flat, open terrain and these additional effects may lead to wind loadings that differ from those in an ABL.

Cited Paper:

Holmes, J.D., Hangan, H.M., Schroeder, J.L., Letchford, C.W., Orwig, K.D., 2008. A forensic study of the Lubbock-Reese downdraft of 2002. Wind Struct. 11, 137–152.

Lundgren, T.S., Yao, J., Mansour, N.N., 1992. Mircoburst modelling and scaling. J. Fluid Mech. 239, 461–488.