On the Effects of Wind Tunnel Floor Tangential Blowing on the Aerodynamic Forces of Passenger Vehicles

Many aerodynamic wind tunnels used for testing of ground vehicles have advanced ground simulation systems to account for the relative motion between the ground and the vehicle. One commonly used approach for ground simulation is a five belt system, where moving belts are used, often in conjunction with distributed suction and tangential blowing that reduces the displacement thickness of the boundary layer along the wind tunnel floor. This paper investigates the effects from aft-belt tangential blowing in the Volvo Cars Aerodynamic wind tunnel. First the uniformity of the boundary layer thickness downstream of the blowing slots is examined in the empty tunnel. This is followed by investigations of how the measured performance of different vehicle types in several configurations, typically tested in routine aerodynamic development work, depends on whether the tangential blowing system is active or not. Numerical simulations are also used to explain the flow field origin of the force differences measured in the wind tunnel. Results show that even though the displacement thickness behind the blowers varies along the width of the blowing slots, it is significantly reduced compared to the case of no blowing; furthermore, it is also shown that deactivating the blowing altogether has an effect not only on the absolute forces but also on the deltas measured between different configurations, and that this phenomenon is more prominent if the vehicle has a larger base area

A parametric study on the influence of boundary conditions on the longitudinal pressure gradient in CFD simulations of an automotive wind tunnel

Computational fluid dynamics (CFD) is an important and extensively used tool for aerodynamic development in the vehicle industry today. Validation of virtual methods by comparison to wind tunnel experiments is a must because manufacturers aim to substitute physical tests on prototype vehicles with virtual simulations. An appropriate validation can be performed only if the wind tunnel geometry with representative boundary conditions is included in the numerical simulation, and if the flow of the empty wind tunnel is accurately predicted. One of the important flow parameters to predict is the longitudinal pressure distribution in the test section, which is dependent on both the wind tunnel geometry and the settings of the boundary layer control systems. This study investigates the effects of flow angularity at the inlet and different boundary layer control systems, namely, basic scoop suction, distributed suction, and moving belts, on the longitudinal pressure distribution in the full-scale aerodynamic wind tunnel of Volvo Cars using CFD and a systematic design of experiments approach. The study shows that the different suction systems used to reduce boundary layer thickness upstream of the vehicle have statistically significant effects on the longitudinal pressure distribution in the test section. However, the estimated drag difference induced on a typical vehicle by the difference in horizontal buoyancy between the tested settings is within the test-to-test uncertainty of the physical wind tunnel, thereby leading to the conclusion that force calculations in simulations are fairly insensitive to the tested parameters on the investigated intervals

Investigation of Wheel Ventilation-Drag using a Modular Wheel Design Concept

Passenger car fuel consumption is a constant concern for automotive companies and the contribution to fuel consumption from aerodynamics is well known. Several studies have been published on the aerodynamics of wheels. One area of wheel aerodynamics discussed in some of these earlier works is the so-called ventilation resistance. This study investigates ventilation resistance on a number of 17 inch rims, in the Volvo Cars Aerodynamic Wind Tunnel. The ventilation resistance was measured using a custom–built suspension with a tractive force measurement system installed in the Wheel Drive Units (WDUs). The study aims at identifying wheel design factors that have significant effect on the ventilation resistance for the investigated wheel size. The results show that it was possible to measure similar power requirements to rotate the wheels as was found in previous works. The magnitude of the measured ventilation resistance confirms the conclusion that this effect should be taken into account when designing a wheel. It was found that some of the rim design factors have greater influences on the ventilation resistance than others. It was also shown that one of the investigated rims had lower ventilation resistance than measured for the fully-covered wheel configuration

Passenger Car Wheel Aerodynamics

Current and future requirements for efficiency force the automotive industry to consider all aspects of reducing the energy consumption of road going vehicles. An important aspect of reducing fuel consumption is reduced aerodynamic drag, which at highway cruising speeds is dominating the total drag force for both cars and heavy goods vehicles. Furthermore, on a modern car, up to 25 percent of the aerodynamic drag originates from the wheels and wheel housings, making them important aerodynamic components.This thesis investigates the aerodynamic effects of wheels and tyres further, with focus on reducing aerodynamic drag. Both experimental and numerical tools were used in order to increase the understanding of how the wheels interact with the surrounding flow field regions, and how such interactions are affected by wheel design. Experiments were performed in the Volvo Aerodynamic Wind Tunnel and RANS and U-RANS were used in the numerical investigations.With regards to CFD, the performance of MRF for modelling wheel rotation was investigated in detail, and compared with both experimental data and sliding mesh simulations for some configurations. The steady state RANS approach using MRF was found to correlate well with experiments for several configurations, but in the case of highly closed front wheels, large discrepancies were identified. Using sliding mesh showed a potential for improved predictions of wake structures compared with experiments, but further investigations of numerical methods, suitable for modelling wheel aerodynamics in a reliable way, was recommended.Several experimental investigations on wheel design resulted in an increased understanding of important wheel design parameters, and their effect on both drag and local flow field. Radial wheel covers was found to be one of the most efficient means of reducing the drag contributions from the wheels. The effect of tyre model was found to be of equal importance as the wheel design. Despite identical size definitions, the tyre geometries were found noticeably different, giving several possible reasons for the differences in drag. Furthermore, effects on drag due to the tyre geometry changing with velocity were also investigated

Passenger Car Wheel Aerodynamics

Current and future requirements for efficiency force the automotive industry to consider all aspects of reducing the energy consumption of road going vehicles. An important aspect of reducing fuel consumption is reduced aerodynamic drag, which at highway cruising speeds is dominating the total drag force for both cars and heavy goods vehicles. Furthermore, on a modern car, up to 25 percent of the aerodynamic drag originates from the wheels and wheel housings, making them important aerodynamic components.This thesis investigates the aerodynamic effects of wheels and tyres further, with focus on reducing aerodynamic drag. Both experimental and numerical tools were used in order to increase the understanding of how the wheels interact with the surrounding flow field regions, and how such interactions are affected by wheel design. Experiments were performed in the Volvo Aerodynamic Wind Tunnel and RANS and U-RANS were used in the numerical investigations.With regards to CFD, the performance of MRF for modelling wheel rotation was investigated in detail, and compared with both experimental data and sliding mesh simulations for some configurations. The steady state RANS approach using MRF was found to correlate well with experiments for several configurations, but in the case of highly closed front wheels, large discrepancies were identified. Using sliding mesh showed a potential for improved predictions of wake structures compared with experiments, but further investigations of numerical methods, suitable for modelling wheel aerodynamics in a reliable way, was recommended.Several experimental investigations on wheel design resulted in an increased understanding of important wheel design parameters, and their effect on both drag and local flow field. Radial wheel covers was found to be one of the most efficient means of reducing the drag contributions from the wheels. The effect of tyre model was found to be of equal importance as the wheel design. Despite identical size definitions, the tyre geometries were found noticeably different, giving several possible reasons for the differences in drag. Furthermore, effects on drag due to the tyre geometry changing with velocity were also investigated

Flow field investigation of rotating wheels on passenger cars

The importance of aerodynamics of road-going vehicles on the total driving resistance and hence fuel consumption is well known to vehicle manufactures. At present, upper body aerodynamics of passenger cars is relatively well understood. However, upper body aerodynamics only represents half of the total aerodynamic resistance on a typical passenger car. It has been shown that as much as 25 percent of the total aerodynamic drag of a passenger car originates from the wheels. Consequently it is possible to achieve important reductions in driving resistance by optimising the flow around the wheels and underbody.At present, advanced ground simulations techniques exist. Several car manufacturers perform wind tunnel experiments with moving ground and other systems for boundary layer treatment. It has been shown that such system have a large impact on the results and are essential to create accurate boundary conditions when investigating road-going vehicles in wind tunnels. A full width moving ground system has been developed for the L2 Aerodynamic Wind Tunnel at Chalmers University of Technology. It will be used for future correlation investigations between full size and scale model experiments.Detailed flow field investigations around the front and rear wheels on a full size Volvo C30 DRIVe™ passenger car have been performed in the Volvo Aerodynamic Wind Tunnel. Omni-directional pressure probes were used to map the local flow field using an automated traversing and data acquisition system. General flow field structures were identified both at the front and the rear wheels. Two large scale wakes were found at the front wheel and one at the rear wheel. A significant dependence on ground simulation was found for both the front and rear wheel flow structures as well as for the global drag. Some dependency on wheel geometry was found at the front wheel whereas the differences at the rear wheel were insignificant for the investigated configurations. A qualitative agreement between front wheel lower wake size and global drag was found. However, integrated microdrag showed that even though there was a quantitative difference in the local flow field, it did not fully explain the differences between configurations. Consequently, it is necessary to expand the experimental investigation in order to fully explain the reasons for changes in global drag.Computational Fluid Dynamics (CFD) analysis of an equivalent vehicle showed good qualitative agreement in the local flow field with some minor exceptions that need to be further investigated. The difference in global drag between the investigated wheel configurations differed by only 1 drag count between experiments and CFD, thus showing good correlation as well

Investigation of Aerodynamic Wheel Designs on a Passenger Car at Different Cooling Air Flow Conditions

Passenger cars represent the largest part of all means of personal transportation today. Thus, it is important to work towards reduced energy consumption of cars if a sustainable mobility is to be achieved. This involves many aspects of vehicle engineering; one of them being aerodynamics. This study focuses on aerodynamic drag and the contributions from the wheels at different cooling air flow configurations. Wheels and wheel housings are important for the overall aerodynamic drag on passenger cars. It has been shown that as much as 25 % of the aerodynamic drag originates from these components. Therefore, it is desirable to understand the flow structures related to the wheels and wheel housings, and how they interact with other important flow regions. This paper presents an investigation of the effects of wheel designs on aerodynamic drag at different cooling air flow configurations on a sedan type passenger car. Comparisons between numerical simulations and wind tunnel measurements are made for some of the configurations as well. Several additional wheel configurations were investigated numerically to further investigate the flow structures at the front and rear wheels. The numerical results show that the effects of radial wheel covering varied noticeably with cooling air flow configuration. In two of the configurations this resulted in a net drag increase with closed cooling air inlets. The best configuration with closed cooling air inlets generated an overall drag reduction of 29 drag counts compared with the numerical baseline with open cooling air inlets. In addition to the obvious drag reduction of closing the cooling air inlets, the main reasons for the additional decrease was limiting the drag increase at the front stagnation region and positive interference effects along the underbody and vehicle base