Modeling of microburst outflows using impinging jet and cooling source approaches and their comparison

Microbursts have been simulated and studied using different physical and numerical modeling methods. In the present study, the steady impinging jet model was comprehensively studied by using a 2-foot-diameter (0.61 m) microburst simulator available in the Department of Aerospace Engineering at Iowa State University. Point measurements and Particle Image Velocimetry (PIV) results revealed a detailed picture of the overall flow and distribution of velocity and turbulence in the outflow of the steady impinging jet. Comparisons suggested that the average wind velocity profile of the steady impinging jet matched well with those derived from field data and previous research. FFT of the velocity time-history and instantaneous PIV results implied that the outflow consisted of low-frequency periodic shedding of vortices and the steady impinging jet model could be seen as an ensemble average of a series of simulated microburst events. Due to lack of time-dependent evolutionary information of the steady impinging jet model, a transient impinging jet model was studied to capture the transient features which were then compared with those of the cooling-source model by performing numerical simulations. Transient features of the transient impinging jet model and cooling source model showed several differences mainly related to the different formation and transportation process of the primary vortex. Ground surface pressure distributions were found to be different due to different forcing parameter of the two models. Comparison with the field data suggested that both models resembled the dynamic features of a real microburst outflow. However, results showed that the cooling source model could produce a reasonable instantaneous radial velocity profile at maximum wind condition, while the transient impinging jet model resulted in a large deviation. Finally, merits and demerits of each modeling methods were discussed.