DYNAMICS OF EXPERIMENTALLY SIMULATED MICROBURSTS

Laboratory-simulated microbursts are used to study the behavior of buoyancy-driven downdrafts and their associated low-altitude wind shears. The microburst flowfield is simulated by releasing an axisymmetric volume of heavy liquid into a less dense ambient surrounding and allowing it to impinge on a horizontal surface. Using particle image velocimetry, normalized horizontal and vertical velocity fields are extracted at different stages during the evolution of the dow The leading edge of the falling fluid rolls up into a vortex ring, which then impacts on the ground and expands radially outward. After the ring impinges on the surface, unsteady adverse pressure gradients cause the low momentum boundary layer to separate and roll up into secondary vortices just ahead of the main vortex. Interaction of the primary and secondary vortices, as well as azimuthal instabilities, can produce sharp spatial variations in velocity, especially near the surface. Particle image velocimetry results show that the largest radial velocities occur close to the surface. The radial velocities decay gradually with altitude and are very small at the height of the vortex ring core. The results are scaled to and compared with previously studied atmospheric microbursts that occurred on June 30, 1982, and July 11, 1988. The experimental data generally agree well with observed and numerically simulated microbursts. Thus, it appears that the results can be used in the development and modification of ground based and airborne microburst detection systems.