Design And Optimization Of Car Empennage With Winglet Based On Aeroelastic Analysis

Car empennage is an excellent additional device of automobile aerodynamics,which can reduce lift of automobile or improve flow-field characteristics of automobile's tail.Therefore,car empennages have been used on automobiles widely.Wingtip vortex will be produced for the three dimensional flows around the wingtip when a car empennage act in flow field.Wingtip vortex will produce additional induced drag and destroy original aerodynamic characteristics of car empennage.In order to reduce the influence of wingtip vortex and further improve the performance of a car empennage,This paper expects to achieve this intention by designing a new additional device which locates at the wingtip of a car empennage.Winglet which was first used on aircraft can produce additional lift and forward thrust for wing by using three dimensional flows around the wingtip.After investigation,its unique mechanism of action is also suitable for the application to a car empennage,which can help a car empennage to improve aerodynamic performance further.Unfortunately,there are large differences between the flow field of an automobile's tail and an aircraft,traditional aircraft's airfoils are difficult to meet design requirements of car-use winglet.Meanwhile,adding a winglet will create new aerodynamic load distribution due to the additional elastic deformation.Therefore,the design and optimization of a car-use winglet are needed.Firstly,quasi-uniform B-spline curve was applied to fit the airfoil,then the fitting airfoil was employed in building the three-dimensional model of the winglet.After that,the Bi-directional fluid-structure interaction was used to add the actual influence of static aeroelasticity on the car empennage with the winglet.Coupled simulation between steady flow filed and static structure was by System Coupling connecting the Fluent with the Static Structural.Then,optimized the airfoil and shape parameters of the new car empennage's winglet with muti-objective method under the three dimensional flows before and after considering the effect of static aeroelasticity.In the stage of optimization,in order to obtain the accurate optimal results,took samples with Latin hypercube sampling firstly.Then,fitted the approximation model by kriging method.In addition,NSGA-II was applied to perform muti-objective optimization based on approximation model.Finally,obtained the optimal solutions from two pareto-optimal frontiers according to specific design targets.3D models were reconstructed based on the optimal solutions.Automobile model,normal car empennage with end-plates and two types of new optimal car empennages were manufactured by Stereo Lithography Appearance(SLA)3D printing.Then,aerodynamic force of these models were measured at the wind tunnel laboratory in Tohoku University in Japan.The results of wind tunnel tests compared with the results of numerical optimization,showing that the numerical simulations have high fidelity.The results of wind tunnel tests and numerical simulations indicate that,through design and optimization,a winglet can reduce drag and produce additional downforce for a vehicle with an ordinary car empennage.Compared with a rigid winglet,the optimal solution set of the new car empennage,under the influence of static aeroelasticity,have the trend to increase the lift coefficient and reduce the drag coefficient of the car model.In addition,automobile's driving is a transient process,violent unsteady flow field is located at automobile's wake.Adding the winglet on the car empennage will produce additional elastic deformation which will further lead to appearance of elastic restoring force,car empennage will vibrate in the flow field due to interaction between the elastic restoring force and unsteady aerodynamic force.In order to avoid the influence about vibration of car empennage on its real aerodynamic performance and security risk even,the response of dynamic aeroelasticity about the new optimal car empennage after considering the effect of static aeroelasticity needs to be analyzed under the design speed.Firstly,fluctuation of static pressure about test point which located at the leading-edge of winglet's wingtip was measured by static pressure sensor,then took the fluctuation frequencies of related vortexes with vibration of car empennage by hot-wire anemometer at the wind tunnel laboratory in Tohoku University.After that,determined the scheme of mesh generation based on the result of force measurement in the wind tunnel experiment.Besides,System Coupling coupled Fluent and Transient Structural to analyze dynamic aeroelasticity with the method of Bi-directional fluid-structure interaction.Finally,compared the results of wind tunnel test and simulation,indicating that numerical simulation has a high accuracy.The vortex-induced vibration of the car empennage with winglet occurs in the flow field at the vehicle's tail,which is motivated by the drag vortex at the vehicle's tail and the induced vortex of the wingtip.The vibration frequency of the new car empennage is equal to its first-order modal vibration frequency.Due to that this frequency is different form the frequency of the motivated vortexes obviously,the new car empennage has no risk about resonance under the design speed.This paper designed and optimized a car empennage with winglet.After that,the dynamic aeroelastic response of the new car empennage was investigated.Finilly,a realistic and completed method of parametric modeling,optimization and analysis were obtained.That can offer a new ideal for the study about aerodynamic attachments of automobiles.