Experimental Investigation of the Near-Surface Flow Dynamics in Downburst-like Impinging Jets Immersed in ABL-like Winds

Abstract

Downburst winds are strong downdrafts of cold air that embed into the atmospheric boundary layer (ABL) and produce intense horizontal outflow upon impingement on the ground. They are highly transient and three-dimensional extreme wind phenomena with a limited spatiotemporal structure that often makes the anemometric measurements in nature inadequate for reconstructing their complex flow fields. In the framework of the project THUNDERR, an experimental campaign on downburst outflows has been carried out at the WindEEE Dome at Western University, Canada. The present study analyzes the three-dimensional interaction between downburst (DB) outflows produced as large-scale impinging jets and ABL winds. Most experimental, numerical and analytical models in the literature neglect this flow interplay or treat it in an oversimplistic manner through a vector superposition. We found that the generated near-surface outflow is asymmetric, and a high-intensity wind zone develops at the interface between DB and ABL winds. The time variability of the leading edge of the outflow was investigated by synchronizing all wind measurements across the testing chamber. The three-dimensional flow structure was studied using a refined grid of Cobra probes that sampled the flow at high frequencies. The passage of the primary vortex produced a significant decrease in the height of maximum radial wind speed, predominantly in the ABL-streamwise direction. The turbulence intensity was the highest in the region where DB propagates into oppositely directed ABL winds.

Experimental Simulation of Thunderstorm Profiles in an Atmospheric Boundary Layer Wind Tunnel

“…Facilities from the first group can be called 3D wind simulators due to their unique capabilities of reconstructing the entire downburst event, creating a wall radial outflow and a vortex ring with the impinging jet technique. In these facilities, different experimental simulations have been carried out: isolated downdraft, i.e., stationary downburst without background flow [8,16], translational downdraft [17,18] and both cases embedded into ABL-like winds [19]. This type of experimental test requires the same length scale for the simulated downburst and the structure; thus, large facilities are needed, which makes them expensive and consequently not very common.…”

Observation of downburst wind characteristics using the Doppler profiler and near-ground measurements

“…As awareness has increased of the potential disasters of downbursts, researchers have carried out extensive investigations on thunderstorm wind characteristics through field measurements (Chaudhuri et al, 2020;Jiang et al, 2022), wind tunnel tests (Asano et al, 2019;Fang et al, 2023) and numerical simulations (Selvam and Holmes, 1992;Iida et al, 2015). Specifically, the collaborative team of Hangan and Romanic have conducted extensive research on the climatic environment of downbursts and their interaction with boundary layer winds by comparing downburst models with full-size downbursts (Romanic et al, 2020(Romanic et al, , 2022Canepa et al, 2022). For example, Romanic et al (2020) proposed a new downburst scaling method to discuss the similarity of the transient characteristics between the downburst model simulated in the WindEEE Dome simulator at Western University and a full-scale downburst event in the Mediterranean Sea, based on the criteria of the scaling method.…”

“…However, the method ignores the effect of atmospheric boundary layer winds and can only be scaled based on a subset of the measurement points rather than the entire wind field. Canepa et al (2022) have studied the three-dimensional interaction between downburst generated by large-scale impinging jets and atmospheric boundary layer winds. The ABL winds were simulated by a series of boundary layer plates to create a uniform flow in the experimental section, and the near-surface flow dynamics were studied using particle image velocimetry (PIV) measurements.…”