Abstract
The objective of this study is to investigate the temporal behavior of the pressure field of a stationary dry microburst-like wind phenomenon utilizing Unsteady Reynolds-averaged Navier-Stokes (URANS) numerical simulations. Using an axisymmetric steady impinging jet model, the dry microburst-like wind is simulated from the initial release of a steady downdraft flow, till the time after the primary vortices have fully convected out of the stagnation region. The validated URANS results presented herein shed light on the temporal variation of the pressure field which is in agreement with the qualitative description obtained from field measurements. The results have an impact on understanding the wind load on structures from the initial touch-down phase of the downdraft from a microburst. The investigation is based on CFD techniques, together with a simple impinging jet model that does not include any microphysical processes. Unlike previous investigations, this study focuses on the transient pressure field from a downdraft without obstacles.
Highlights
A thunderstorm downburst is an intense transient downdraft of air that induces an outburst of damaging wind on or near the surface of the Earth
The radial distribution of Cp obtained from Unsteady Reynolds-averaged Navier-Stokes (URANS) and steady RANS simulation are extracted at ypeak, which is the height from the ground where the peak velocity is found at each radial location
The evolution of the pressure coefficient with time derived from the URANS simulation of a steady impinging jet flow has been shown to approach the RANS results after a sufficiently long time period (T = 348)
Summary
A thunderstorm downburst is an intense transient downdraft of air that induces an outburst of damaging wind on or near the surface of the Earth. Fujita [1] classified a downburst as “microburst”. Microbursts are further classified as of dry or wet type. Dry microbursts are formed in deep, dry, and well-mixed atmospheric boundary layers, while wet microbursts are formed together with thunderstorm clouds with shallow, well-mixed boundary layers and large vertical gradients of potential energy [2]. The onset of downdrafts occurs when there is cooling of air by the evaporation of rain and melting of ice, causing the air density in the evaporation and melting regions to increase. The colder and denser air accelerates vertically downward from the cloud base. Difference in density between the downdraft and the surrounding atmosphere subsequently leads to the entrainment of the surrounding atmospheric air into the downdraft core, resulting in ring vortices due to Kelvin-Helmholtz instability (KHI)
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