Aerodynamic optimisation of wiper system for high speed vehicles

Windshield wipers are developed to improve visibility during inclement weather and belong to standard safety equipments. A conventional wiping system of a vehicle consists of three components, namely motor and mechanism, wiper arm and wiper blade. The wiper arm transfers the movement to the wiper blade and cleaning is achieved by wiping water and dirt from the windscreen by a blade rubber. In order to wipe water and dirt from the windshield, the wiper blade should be forced onto the windshield with a specified force. The necessary force is obtained by a spring mechanism within the arm. As the velocity of the vehicle increases, the pressing force decreases due to the controverse aerodynamic forces occurring on the wiper arm and blade. As the lift force increases, the necessary force cannot be created to wipe the windshield. In order to minimize the aerodynamic lift force, an alternative force in the opposite direction of the lift force is needed. To beat the aerodynamic lift force acting on the wiper system, new wiper geometries are designed. Current standard products are capable of keeping the wiper performance around 160 km/h but for high-class vehicles, the manufacturers require 240 km/h or more on rainy days. In this project a new wiper prototype which can operate at around 240 km/h vehicle speed is developed with help of Computational Fluid Dynamics (CFD) analysis. 3D original wiper geometry is acquired from TEKLAS A.Ş. The flow over the windshield wiper and spoiler is simulated by a commercial flow solver (ANSYS-Fluent). Flow is modeled by the finite-volume based, incompressible, turbulent, steady flow solver. The flow is assumed to be still incompressible as the local Mach number is around 0.2, thus compressibility effects are ignored. Realizable k-ε turbulence model is used with enhanced wall functions as already used by many researchers in their studies. To reduce the computational cost a simplified geometry is used. The hood and windshield of the car is assumed as two slightly curved surfaces and only wiper blades are taken into account. To obtain reliable numerical results, several meshes with different number of elements are tested to show mesh-independence. After analysis on simplified model, original wiper geometry is taken to analysis on a half car model. The 3D computational domain consists of a half car model, wiper blades and wiper arms for both driver and passenger sides. The driver side wiper blade has a length of 600 mm and angle of 30° with the horizontal. Passenger side wiper blade has a length of 450 mm and angle of 45° with the horizontal. In this analysis it is observed that the lift force acting on the wiper system is caused by the wiper arms. For this reason increasing the down force on the wiper blades is decided to eliminate total lift force acting on the wiper system. To create a new wiper model, wiper geometry is investigated in three sections, which are spoiler profile, height of the wiper and connection between metal part and spoiler of the wiper. While modifying the spoiler curvature increasing the pressure above spoiler is aimed. To do that vortex structure above the spoiler should be reduced. Also another matter is having the maximum pressure point in a more desired position to press wiper blade down. The best profile tried here reduced lift force by almost one third compared with the original profile. Then a further design is proposed where the spoiler is slightly swept back which increases the frontal surface of the spoiler leading to a considerable reduced lift force. The second property investigated is height of the wiper blade. In this analysis wiper heights varying from 16 mm to 19 mm are investigated. It is seen that increasing the wiper height helps to decrease the lift force. Even in the analysis of the wipers with 18 and 19 mm of height, down force is acquired. A new approximation in preventing the vortex structure occurring between the metal part of the wiper and spoiler is changing the connection type between them. Three connection types are suggested. All of the modifications are found to be useful in preventing the vortex structure between the metal part and the spoiler. However the third connection type is the most effective case since it not only prevents the vortex occurring on the connection location but it also eliminates the main vortex structure on the top of the spoiler. As a result this modification provides pressing force. Original geometry has 16.6 mm height and the connection type between metal part and the spoiler is designed in a way that the metal part has sharp edges. Wiper 9_1_1 has a larger spoiler curvature than original geometry but their heights and connection types are the same. Wiper 3_5_1 has a height of 19 mm. Its spoiler curvature and connection type are identical to Wiper 9_1_1. Wiper 3_5_3 has same height and spoiler curvature as Wiper 3_5_1. The only difference between Wiper 3_5_1 and 3_5_3 is the connection type. In Wiper 3_5_3, the metal part and the spoiler are connected in a way that the metal part becomes a blunt body. The new acquired wiper geometries are investigated in further analysis. First of them is effect of the velocity on the aerodynamic forces. Original geometry and one new geometry (Wiper 3_5_1) are taken to this analysis. Five different inlet velocities are tried varying from 160 km/h to 240 km/h with an interval of 20 km/h. It is seen that reducing the inlet velocity reduces both drag and lift forces. Second analysis done in this section is effect of the angle on the aerodynamic forces. Five different blade angles are investigated varying for 10° to 50° with an interval of 10°. Original geometry and three new modified geometries are used in this analysis. For all four wiper profiles, it is observed that the variation of the drag coefficient with respect to blade angle displays the same characteristics. Drag coefficients decrease gradually by the increase of the angle. Lift coefficients reveal same variation characteristics up to the angle of 50° .At 20° angle minimum lift coefficients are obtained for all four wiper models. At 40° angle maximum lift coefficient is obtained for the wiper models with a height of 16.6 mm and finally at 50° angle maximum lift coeffcient is achieved for 19 mm high wipers. For all blade angles the modified wiper geometries have lower lift coefficients than the lift coeffcient of the original wiper geometry. Lastly the modified wiper geometries are investigated on the half car model. Three different inlet velocities are used which are 160, 200 and 240 km/h. At 240 km/h inlet velocity there is 3.7 N of lift at the driver side for the original geometry. Wiper 9_1_1 decreases lift force to 2.3 N at the driver side. Wipers with 19 mm height cause down force on the wiper system. A down force of 1.1 N for the Wiper 3_5_3 and a down force of 0.7 N for Wiper 3_5_1 are calculated. After numerical investigations a new wiper model is created which is suitable for molding. After the production pressing force distribution of the new geometry is obtained with X-sensor equipment for different blade angles. Wiper prototype is tested in Stutgart, FKFS instute's thermal wind tunnel to reveal it's performance. Soiling tests are performed. Water droplets with fluorescent agent are added to the air flow and made visible using UV- light. Experiments are done at wind-tunnel velocities varying between 160 and 240 km/h. Satisfactory results are achieved with the wind tunnel tests and the numerical solutions are validated qualitatively.