Numerical Simulation Of Separated Flows Around Isolated Airfoil At Different Reynolds Numbers

When unmanned aerial vehicles(UAV)and other aircrafts cruise at high altitude,the smaller size,lower speed and low atmospheric density and other factors will lead to the fact that the airfoil usually operates at low Reynolds number conditions,which has a significant impact on its aerodynamic performance.The most important manifestation is that the flow of airfoil surface is usually in a laminar state,which is weak in resistance to the adverse pressure gradient.When suffering from strong adverse pressure gradient,it is prone to arise the laminar separation phenomenon.The separation of the airfoil surface boundary layer will deteriorate aerodynamic performance,decrease the lift while increasing the drag,and even cause the problem that lift-to-drag ratio drops sharply,which will affect the overall performance of the aircraft.And the large scale separation of the airfoil surface will cause stall,which will bring a great threat to the safety of flight.To improve the aerodynamic performance of the airfoil at low Reynolds number conditions,it is necessary to control the laminar flow separation,reduce or even eliminate the separation.But the spatio-temporal evolution of the flow is very complex,which is often accompanied by the transition and reattachment process.How to understand the relevant physical mechanisms in depth and to improve the accuracy of numerical simulation has become the main challenges we are facing.In the thesis,we focus on the separated transitional flow problems of low Reynolds number airfoil,and carry out a series of work by using two numerical techniques of large-eddy simulation(LES)and Reynolds-averaged Navier-Stokes(RANS)simulation.Using high precision large eddy simulation method to simulate flows around isolated NACA 0025 airfoil at three different Reynolds numbers 55 000,100 000 and 150 000,where the Mach number and angle of attack are 0.4 and 5 degree respectively.Numerical results at three working conditions are compared and analyzed to investigate the aerodynamic characteristics of separated flows.It is found that there exist two different flow regimes around airfoil,when the Reynolds number is 55 000 or 100 000,a large open separation bubble appears on the upper surface of the airfoil,and when the Reynolds number equals to 150 000,the time-averaged open separated region becomes a smaller closed separation bubble and the wake width decreases significantly.For the open separating flow regime,the curves of lift and drag coefficient fluctuates obviously,and the airfoil lift is negative.The total pressure loss in the wake region is high,as the Reynolds number increases from 55 000 to 100 000,the separation zone size and wake width are slightly reduced.When the Reynolds number increases to 150 000,the acceleration of the transition process makes the separation region from open to closed one.The rise and drag curve is more stable,the lift-to-drag ratio is obviously improved,and the wake width and total pressure loss are significantly reduced.Meanwhile the aerodynamic performance is obviously improved.No matter what kind of flow pattern,the Kelvin-Helmholtz(K-H)instability plays an important role in the process of laminar separation induced transition.However,there are differences in the process of evolution of different Reynolds numbers,the lower the Reynolds number,the larger scale of the vortex structures in the flow field,the wider wake width,and the more violent of the velocity fluctuation.So a wider distribution of Reynolds stress and a greater loss of wake flow appear.The Reynolds-averaged Navier-Stokes method is also employed to simulate airfoil flows in the same conditions,where two dimensional steady model was chosen.The SST ?-Re? transition model is relatively good for the prediction of separation flow,but insufficient for the details of the transition process.As for three working conditions of NACA 0025 airfoil,comparing to the open separated flow state,the RANS is better for the prediction of closed separation bubble flow regime.The calculation of the location of separation point,the reattachment point as well as the wake profile are in good agreement with the LES results.On the whole,the two-dimensional steady RANS can obtain the average flow field which is close to the LES one with little computational cost.