Martin A. Passmore

An experimental study of unsteady vehicle aerodynamics

Abstract The transient response of a vehicle to a wind disturbance is of importance to car drivers since low level inputs can result in poor vehicle refinement, and extreme effects can result in path deviation. This paper investigates the use of an oscillating aerofoil gust generator to simulate the transient aerodynamic effects produced on a car-type bluff body during a simplified sinusoidal side gust interaction. A simplified bluff body was exposed to a range of sinusoidal cross-wind excitations corresponding to a reduced frequency of between 0.09 and 0.71 based on the model length. Unsteady measurements of surface pressure are processed to determine the side force and yaw moment and the aerodynamic magnification (χa) is calculated by comparing the transient response with a quasi-steady prediction. The transient yaw moment response is shown to exceed the quasi-steady by as much as 30 per cent. The transient side force is generally significantly less than the quasi-steady value except at the lowest frequency tested. The change in response is attributed to changes in the strength of the front and rear pillar vortices and to changes in phase relative to the quasi-steady response.

Design methodology and performance of an indraft wind tunnel

The design methodology and performance of Loughborough University’s new 1·9m × 1·3m, indraft wind tunnel is discussed in the following paper. To overcome severe spatial and financial constraints, a novel configuration was employed, with the inlet and exit placed adjacent to each other and opened to atmosphere. Using a fine filter mesh, honeycomb, two turbulence reduction screens and a contraction ratio of 7·3, flow uniformity in the working area of the jet at 40ms-1 is shown to be within 0·3% deviation from the mean velocity, with turbulence intensity in the region of 0·15%. Working section boundary layer characteristics are shown to be consistent with that of a turbulent boundary layer growing along a flat plate, which originates at the point of inflection of the contraction. A maximum velocity of 46ms-1 was achieved from a 140kW motor, compared to a prediction of 44ms-1, giving an energy ratio of 1·42. Comparison between theoretical and measured performance metrics indicate differences between the way modules perform when part of a wind tunnel system compared to data gathered from test rigs.

Experimental studies of the aerodynamics of spinning and stationary footballs

The accurate discrimination of the aerodynamic parameters affecting the flight of sports balls is essential in the product development process. Aerodynamic studies reported to date have been limited, primarily because of the inherent difficulty of making accurate measurements on a moving or spinning ball. Manufacturers therefore generally rely on field trials to determine ball performance, but the approach is time-consuming and subject to considerable variability. The current paper describes the development of a method for mounting stationary and spinning footballs in a wind tunnel to enable accurate force data to be obtained. The technique is applied to a number of footballs with differing constructions and the results reported. Significant differences in performance are noted for both stationary and spinning balls and the importance of the ball orientation to the flow is highlighted. To put the aerodynamic data into context the results are applied in a flight model to predict the potential differences in the behaviour of each ball in the air. The aerodynamic differences are shown to have a considerable effect on the flight path and the effect of orientation is shown to be particularly significant when a ball is rotating slowly. Though the techniques reported here are applied to a football they are equally applicable to other ball types.

Aerodynamic Drag of a Compact SUV as Measured On-Road and in the Wind Tunnel

Growing concerns about the environmental impact of road vehicles will lead to a reduction in the aerodynamic drag for all passenger cars. This includes Sport Utility Vehicles (SUVs) and light trucks which have relatively high drag coefficients and large frontal area. The wind tunnel remains the tool of choice for the vehicle aerodynamicist, but it is important that the benefits obtained in the wind tunnel reflect improvements to the vehicle on the road. Coastdown measurements obtained using a Land Rover Freelander, in various configurations, have been made to determine aerodynamic drag and these have been compared with wind tunnel data for the same vehicle. Repeatability of the coastdown data, the effects of drag variation near to zero yaw and asymmetry in the drag-yaw data on the results from coastdown testing are assessed. Alternative blockage corrections for the wind tunnel measurements are examined. A reasonable correlation between wind tunnel and on-road aerodynamic drag data is established for the configurations tested.

A COMPARISON OF ON-ROAD AERODYNAMIC DRAG MEASUREMENTS WITH WIND-TUNNEL DATA FROM PININFARINA AND MIRA

The principal development tool for the vehicle aerodynamicist continues to be the full-scale wind tunnel. It is expected that this will continue for many years in the absence of a reliable alternative. As a true simulation of conditions on the road, the conventional full-scale wind tunnel has limitations. For example, the ground is fixed relative to the vehicle, allowing an unrepresentative boundary layer to develop, and the wheels of the test vehicle do not rotate. These limitations are known to influence measured aerodynamic data. In order to improve the representation of road conditions in the wind tunnel, most of the techniques used have attempted to control the ground plane boundary layer. Only at model scale has the introduction of a moving ground plane and rotating wheels been widely adopted. The Pininfarina full-scale wind tunnel now incorporates the Ground Effect Simulation System which allows testing with a moving belt and rotating wheels. A major feature of this facility is that test vehicles can be easily installed with only minor modifications. This paper compares aerodynamic drag measurements for a large saloon, in various configurations, obtained both in the wind tunnel and on the road. The wind tunnel results are presented for various ground simulations. These are: moving belt with rotating wheels and stationary belt with fixed wheels at Pininfarina, and the conventional fixed ground in the MIRA full-scale wind tunnel. The on-road data is derived from coastdown tests.

Estimation of bluff body transient aerodynamics using an oscillating model rig

A method for the estimation of transient aerodynamic derivatives from dynamic wind tunnel tests using time response data is presented in this paper. For the purposes of the study, the aerodynamic derivatives are considered to act as a stiffness and damping to the model motion. The experimental set-up consists of a simple bluff body (Davis model) constrained to oscillate with a single degree of freedom of pure yawing motion. A range of springs were used to control the oscillation frequency and hence the reduced frequency. The transient responses from dynamic wind tunnel tests are compared with quasi-steady analysis in order to investigate the effect of unsteady aerodynamics. The aerodynamic derivatives are initially estimated using the classical logarithmic decay method. The dynamic stiffness derivative exceeds that determined statically across the reduced frequency range. The damping derivative was found to be a function of free-stream speed; at low velocities it is negative but progressively increases to a positive value. With further increases in speed, a self-sustained oscillation is observed with almost constant frequency and amplitude. This result is attributed to coupling between the model wake and the model stability; however, the exact mechanism of the interaction is not fully understood. This phenomenon is under further investigation.

The aerodynamic performance of a range of FIFA-approved footballs

Much discussion surrounds the flight of a football, especially that which is perceived as irregular, and is typically done so with little understanding of the aerodynamic effects or substantive evidence of the path taken. This work establishes that for a range of FIFA-approved balls there is a significant variation in aerodynamic performance. This paper describes the methods used for mounting stationary and spinning footballs in a wind tunnel enabling accurate force data to be obtained, and the analysis techniques used. The approach has been to investigate a number of scenarios: the non-spinning Reynolds sweep, unsteady loads, orientation sensitivity (yaw sweep), and the spinning Reynolds sweep. The techniques are applied to a number of footballs with differing constructions and the results reported. To put the aerodynamic data into context, the results are applied in a flight model to predict the potential differences in the behaviour of each ball in the air. This paper concludes that, although the drag characteristics are different for each different ball tested, the simulation suggests that this has only a limited effect on the flight of the ball. It is also shown that the unsteadiness of the aerodynamic loads is unlikely to be responsible for unpredictable behaviour. However, it is also shown that there are significant differences in the lateral aerodynamic forces for a range of FIFA-approved match balls, and that these aerodynamic differences have a significant effect on the flight path for both spinning and slowly rotating balls.

On the Optimisation of Road Vehicle Leading Edge Radius in Varying Levels of Freestream Turbulence

It has been recognised that the ideal flow conditions that exist in the modern automotive wind tunnel do not accurately simulate the environment experienced by vehicles on the road. This paper investigates the effect of varying one flow parameter, freestream turbulence, and a single shape parameter, leading edge radius, on aerodynamic drag. The tests were carried out at model scale in the Loughborough University Wind Tunnel, using a very simple 2-box shape, and in the MIRA Full Scale Wind Tunnel using the MIRA squareback Reference Car. Turbulence intensities up to 5% were generated by grids and had a strong effect on transcritical Reynolds number and Reynolds sensitivity at both model scale and full scale. There was a good correlation between the results in both tunnels.

An Investigation into the Wake Structure of Square Back Vehicles and the Effect of Structure Modification on Resultant Vehicle Forces

A large contribution to the aerodynamic drag of a vehicle (30% (1) or more depending on vehicle shape) arises from the low base pressure in the wake region, especially on square- back configurations. A degree of base pressure recovery can be achieved through careful shape optimization, but the flow structures and mechanisms within the wake that cause these base pressure changes are not well understood. A more complete understanding of these mechanisms may provide opportunities for further drag reductions from both passive shape changes and in the future through the use of active flow control technologies. In this work surprisingly large changes in drag and lift coefficients of a square-back style vehicle have been measured as a result of physically small passive modifications. Tests were performed at quarter scale using a simplified vehicle model (Windsor Model) and at full scale using an MPV. The full scale vehicle was tested with and without a flat floor. During both tests the vehicle base region was fitted with a series of low profile, horizontal slats to disrupt any steady wake structures acting close to the vehicle base surface. Force balance, static pressure and PIV measurements have been used to investigate the flow structures in the vehicle wake. This paper summarizes the results and relates the global drag and lift changes that arise from the horizontal slats to the base pressure from both model and full scale and to the PIV measurements from the model tests.

Influence of Short Rear End Tapers on the Unsteady Base Pressure of a Simplified Ground Vehicle

Short tapered sections on the trailing edge of the roof, underside and sides of a vehicle are a common feature of the aerodynamic optimization process and are known to have a significant effect on the base pressure and thereby the vehicle drag. In this paper the effects of such high aspect ratio chamfers on the time-dependent base pressure are investigated. Short tapered surfaces, with a chord approximately equal to 4% of the overall model length, were applied to the trailing edges of a simplified passenger car model (the Windsor Body) and base pressure studied via an array of surface pressure tappings. Two sets of configurations were tested. In the first case, a chamfer was applied only to the top or bottom trailing edge. A combination of taper angles was also considered. In the second case, the chamfer was applied to the side edges of the model base, leaving the horizontal trailing edges squared. In all configurations both the base and the slanted surfaces were covered with pressure taps for the entire width to ensure that any asymmetry was captured and two different sampling time were considered (respectively equal to 31.5 s and 630.0 s). The results show the effects produced on the base pressure by the presence of a long period bi-stable behavior, whose characteristics were further investigated by conditional averaging the recorded data and considering the distribution of the rms pressure values recorded over the entire model base.