Numerical Simulation Of Structures And Evaluation Of The Front In Unsteady Wall Jet

The front is one of the prominent features in the unsteady wall jet,as well as in different unsteady jet configurations,which is the interface between ambient and injected flow.The front of unsteady wall jet play a fundamental role in nature and engineering applications,such as the moving wall of sand in haboob dust storms,pyroclastic flow and the dust emission caused by a take off and landing aircraft.The study of the evolution of front structures in unsteady wall jet is both practically important and fundamentally interesting due to the obvious exchange of mass and energy between inside and outside of the front.In this paper direct numerical simulations of unsteady wall jet in the Boussinesq limit have been conducted to study the evolution of the front.There are two original contributions to the understanding of the dynamics of front.This is the first direct numerical simulation of a turbulent unsteady wall jet we are aware of,and this is the first time to numerically investigate the distribution and motion of inertial particles under the influence of front structures.The main results are as follows.Firstly,At relatively low Ra the flow dynamics resemble that of an isothermal unsteady wall jet in which the turbulent front is rarely investigated,and the high speed infected flow separate from the wall forming a circular frontal vortex,while as the Ra increases the flow becomes a constant flux release gravity current in which the semi-circular frontal vortex always adhere to the bottom wall.The results reveal that there are two mechanisms for frontal instability: centrifugal and local unstable stratification corresponding to the formation of lobe-cleft structures in the two extreme cases,respectivly.The front region is dominated by hairpin-like vortexes but with opposite orientation of vortex head and spanwise vorticity in the two extreme cases.Secondly,owing to the vertical ascent flow induced by the frontal vortex and the rapidly increasing bottom shear stress at the front,the erosion of near wall inertail particles is strongest at the front of the unsteady wall jet,and the lighter particles can be more easily carried away from the near wall region.Induced by the turbulent structures at front region,preferential accumulation of the inertial particles takes place just behind the cleft structures.Thirdly,when the time duration of jet is finite,the effect of viscous dissipation become relatively remarkable,the flow decays faster for the shorter time duration of jet.At this decay phase of unsteady wall jet,the front velocity decreases rapidly,and the time evolution of the front velocity agree very well with the results in gravity current,especially for higher Rayleigh number.The peak value of bottom shear stress at front decrease with time by a power function,which means the intensity of erosion by unsteady wall jet also decreases rapidly with time.