Numerical Simulation Of Two Dimensional Free Jet Based On Lattice Boltzmann Method

Jet flow is one of the most important contents of hydrodynamics researches. It isusually turbulent and has great research value in the denoise optimization design ofsubmarine’s waterspout and pump jet propeller. Lattice Boltzmann method is a newapproach in numerical simulation of fluid motions with the advantages of highcalculation speed, convenient boundary imposing, distinct physical schema and so on.This thesis mainly focus on the applicability of lattice Boltzmann method in thenumerical simulation of jet flow through programming with lattice Boltzmann method toanalyze the characteristics of two dimensional free jet flow.Lattice Boltzmann method is a mesoscopic method. It evolves from the microscopicview and uses the collision and transfer of particle distribution function to describe themacroscopic flow phenomenon. This thesis adopts the lattice Boltzmann LBGK (LatticeBhatnagar Gross Krook) model to simulate stable jet flow at low Reynolds numbers anduse the MRT (Multiple Relaxation Time) large eddy simulation approach, combined withlattice Boltzmann algoritthm, to simulate high Reynolds number jet flow. The Reynoldsnumber ranges from10to100000. The feasibility of lattice Boltzmann method isvalidated by comparing the simulation results of lattice Boltzmann method with resultsof the commonly used CFD software Fluent. Meanwhile this thesis discusses the impactof the change of Reynolds numbers and the pipe length on two dimensional free jet flowfield, compares the stability of LBGK model with MRT model and analyzes the turningpoint of the steady to unsteady state of the free jet flow. The main conclusions of thispaper are as follows.The lattice Boltzmann method is accurate in simulating low Reynolds number jetflows, while some discrepancies do exist in the turbulent simulation. However, it can stilldepict the characteristics of turbulent jet flow field and the vortex shedding, merging and evolution process clearly. The higher the Reynolds number is, the farther the jet canproceed downstreams and the slower the centerline velocity decays. At low Reynoldsnumbers, the pipe length has nearly no influence on the jet flow characteristics and the jetspread width becomes narrower as the Reynolds number increases. While at highReynolds numbers, the centerline velocity increases greatly as the pipe length increasesand the jet width becomes wider as the Reynolds number increases. We figure out thatRe=700is approximately the turning point of two dimensional free jet flow’s steady tounsteady state.