The Effect of Passive Base Ventilation on the Aerodynamic Drag of a Generic SUV Vehicle

Perry A., Pavia G., Passmore M.

As vehicle manufacturers work to reduce energy consumption of all types of vehicles, external vehicle aerodynamics has become increasingly important. Whilst production vehicle shape optimisation methods are well developed, the need to make further advances requires deeper understanding of the highly three-dimensional flow around bluff bodies. In this paper, the wake flow of a generic bluff body, the Windsor body, based on a square-back car geometry, was investigated by means of balance measurements, surface pressure measurements and 2D particle image velocimetry planes. Changes in the wake topology are triggered by the application of short tapers (4 % of the model length) to the top and bottom edges of the base, representing a shape optimisation that is realistic for many modern production vehicles. The base drag is calculated and correlated with the aerodynamic drag data. The results not only show the effectiveness of such small devices in modifying the time average topology of the wake but also shed some light on the effects produced by different levels of upwash and downwash on the bi-stable nature of the wake itself.

Pavia G., Passmore M., Gaylard A.

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.

Hirst T., Li C., Yang Y., Brands E., Zha G.

Wood A., Passmore M., Forbes D., Wood D., Gaylard A.

The pressure on the base of a vehicle is a major contributor to the aerodynamic drag of all practical vehicle geometries, and for some vehicles, such as an SUV, it is particularly important because it can account for up to 50% of the overall drag.Understanding the mechanisms that influence the base pressure and developing our simulation tools to ensure that base pressure is accurately predicted are essential requirements for the vehicle design and engineering process.This paper reports an experimental study to investigate the base pressure on a specifically designed generic SUV model.The results from ¼ scale wind tunnel tests include force and moment data, surface pressures over the base region and particle image velocimetry (PIV) in the wake.Results are presented for the vehicle in different ride height, underfloor roughness and wheel configurations and the paper includes some description of the experimental errors.Some initial CFD simulations are also reported.

Perry A., Passmore M., Finney A.

Bartow W.B., Moreyra A.C., Hirst T., Woyczynski G.H., Lefebvre A., Zha G.

Grandemange M., Mary A., Gohlke M., Cadot O.

The separated flow past the square-back model used in the experiments of Ahmed et al. (1984) is controlled using flaps at the end of the top and bottom faces. A parametric study of the flow regarding the slant angle of the flaps is performed from pressure and force measurements as well as particle image velocimetry. When the bottom flap orientation is fixed, variations in the top slant angle indicate a quadratic dependence of drag versus lift. This relationship presents self-similarities when modifying the bottom flap angle. It is furthermore observed that the lift is an affine function of both slant angles and the drag is a second-order polynomial containing a coupling term between the two angles. The evolution of the drag, depending on both angles, is discussed. The contribution of the wake size, lift-induced drag as well as the local drag induced by the inclination of the flaps is interpreted.

Howell J., Passmore M., Tuplin S.

Various techniques to reduce the aerodynamic drag of bluff bodies through the mechanism of base pressure recovery have been investigated.These include, for example, boat-tailing, base cavities and base bleed.In this study a simple body representing a car shape is modified to include tapering of the rear upper body on both roof and sides.The effects of taper angle and taper length on drag and lift characteristics are investigated.It is shown that a significant drag reduction can be obtained with moderate taper angles.An unexpected feature is a drag rise at a particular taper length.Pressure data obtained on the rear surfaces and some wake flow visualisation using PIV are presented.

Perry A., Passmore M.

In this paper the effects of a rough underbody on the rear wake structure of a simplified squareback model (the Windsor model) is investigated using balance measurements, base pressure measurements and two and three component planar PIV. The work forms part of a larger study to develop understanding of the mechanisms that influence overall base pressure and hence the resulting aerodynamic drag. In the work reported in this paper the impact of a rough underbody on the base pressure and wake flow structures is quantified at three different ground clearances. The underbody roughness has been created through the addition of five roughness strips to the underbody of the model and the effects on the wake at ground clearances of 10.3%, 17.3% and 24.2% of the model height are assessed. All work has been carried out in the Loughborough University Large Wind Tunnel with a scale model giving a blockage ratio of 4.4% for a smooth under-body or 4.5% with the maximum thickness roughness strips. The tests are conducted with a fixed ground plane. Results are presented for the base pressure distribution and these are compared against the stream-wise PIV results. This work demonstrates the need for rough underbody structures to be considered during base pressure investigations before any model scale work is conducted due to their influence on the wake structures. Copyright © 2013 SAE International.

Littlewood R.P., Passmore M.A.

A large contribution to the aerodynamic drag of a vehicle arises from the failure to fully recover pressure in the wake region, especially on squareback configurations. A degree of base pressure recovery can be achieved through careful shape optimisation, but the freedom of an automotive aerodynamicist to implement significant shape changes is limited by a variety of additional factors such styling, ergonomics and loading capacity. Active flow control technologies present the potential to create flow field modifications without the need for external shape changes and have received much attention in previous years within the aeronautical industry and, more recently, within the automotive industry. In this work the influence of steady blowing applied at a variety of angles on the roof trailing edge of a simplified ¼ scale squareback style vehicle has been investigated. Hot-wire anemometry, force balance measurements, surface pressure measurements and PIV have been used to investigate the effects of the steady blowing on the vehicle wake structures and the resulting body forces. The energy consumption of the steady jet is calculated and is used to deduce an aerodynamic drag power change. Results show that overall gains can be achieved; however, the large mass flow rate required restricts the applicability of the technique to road vehicles. Means by which the mass flow rate requirements of the jet may be reduced are discussed and suggestions for further work put forward.

Howell J., Sims-Williams D., Sprot A., Hamlin F., Dominy R.

Various techniques to reduce the aerodynamic drag of bluff bodies through the mechanism of base pressure recovery have been investigated. These include, for example, boat-tailing, base cavities and base bleed. In this study an Ahmed body in squareback configuration is modified to include a base cavity of variable depth, which can be ventilated by slots. The investigation is conducted in freestream and in ground proximity. It is shown that, with a plain cavity, the overall body drag is reduced for a wide range of cavity depths, but a distinct minimum drag condition is obtained. On adding ventilation slots a comparable drag reduction is achieved but at a greatly reduced cavity depth. Pressure data in the cavity is used to determine the base drag component and shows that the device drag component is significant. Modifications of the slot geometry to reduce this drag component and the effects of slot distribution are investigated. Some flow visualisation using PIV for different cavity configurations is also presented.

Irving Brown Y.A., Windsor S., Gaylard A.P.

Littlewood R., Passmore M.

Kowata S., Ha J., Yoshioka S., Kato T., Kohama Y.

Howell J., Le Good G.

Shuib M.S., Tan M.B., Mahmood J.I., Devadason J.J., Hassan M.F.

Zhao J., Su C., Liu X., Yuan X., Li W., Wang Y.

Airflow separation at the rear area of the automobile is the main source of automobile aerodynamic drag. To suppress the airflow separation, minimize the aerodynamic drag of the automobile, realize energy saving and emission reduction, a bionic drag reduction device was designed based on the dorsal fin of the orca with low drag characteristics. A numerical computation method was established to maximize the drag reduction performance of the bionic device, and the parametric modeling of the bionic drag reduction device was carried out. The design of experiments, the Kriging surrogate model, and an optimization algorithm were used to optimize the bionic drag reduction device. The validity of the optimization design was validated by the wind tunnel test. Finally, the mechanism and effectiveness of the bionic device in reducing aerodynamic drag were investigated through the comparison of flow field. The results show that the optimized bionic drag reduction device can delay the airflow separation and effectively reduce turbulence intensity of the automobile. According to the wind tunnel test, the aerodynamic drag coefficient of the optimized model is reduced by 6.16% compared with the original model.

Connolly M.G., O'Rourke M.J., Ivankovic A.

This paper addresses the critical role of drag reduction technology in the evolution of road vehicle design amidst the ongoing climate crisis. With transportation accounting for a substantial portion of the EU's greenhouse gas emissions, the shift towards alternatively powered vehicles highlights the need for innovative solutions to extend range, reduce fuel consumption, and lower emissions. This review thoroughly outlines the literature on appendable drag reduction devices, encompassing both passive and active techniques, and their applicability across a variety of road vehicles, including light and heavy-duty transport. Methods applied to simplified bodies such as the Ahmed or other commonly studied generic bluff bodies are clearly distinguished from those applied to more detailed road vehicles, where results hold greater practical significance due to authentic geometry. A combination of both wind tunnel and CFD works are outlined with insights given into how advancements in both computing power and CFD will greatly enhance the future outputs of drag reduction research for road vehicles. Finally, an outlook is provided on the future of the technology and how increased consumer demand for configurable vehicles will encourage increased engagement between drag reduction device manufacturers and automakers to improve the device mounting process.

Aultman M., Auza-Gutierrez R., Disotell K., Duan L.

Lattice Boltzmann method (LBM) simulations were performed to capture the long-period dynamics within the wake of a realistic DrivAer fastback model with stationary and rotating wheels. The simulations showed that the wake developed as a low-pressure torus regardless of whether the wheels were rotating. This torus shrank in size on the base in the case of rotating wheels, leading to a reduction in the low-pressure footprint on the base, and consequently a 7% decrease in the total vehicle drag in comparison to the stationary wheels case. Furthermore, the lateral vortex shedding experienced a long-period switching associated with the bi-stability in both the stationary and rotating wheels cases. This bi-stability contributed to low-frequency side force oscillations (<1 Hz) in alignment with the peak motion-sickness-inducing frequency (0.2 Hz).

Varney M., Passmore M., Swakeen R., Gaylard A.

Many modern vehicles have blunt rear end geometries for design aesthetics and practicality; however, such vehicles are potentially high drag. The application of tapering; typically applied to an entire edge of the base of the geometry is widely reported as a means of reducing drag, but in many cases, this is not practical on real vehicles. In this study side tapers are applied to only part of the side edge of a simplified automotive geometry, to show the effects of practical implementations of tapers.The paper reports on a parametric study undertaken in Loughborough University’s Large Wind Tunnel with the ¼ scale Windsor model equipped with wheels. The aerodynamic effect of implementing partial side edge tapers is assessed from a full height taper to a 25% taper in both an upper and lower body configuration. These were investigated using force and moment coefficients, pressure measurements and planar particle image velocimetry (PIV). These geometries showed that the drag reductions are maximised with a 50% span, generating a vertically symmetric wake and less taper drag contribution when compared to a full span taper.

Luckhurst S., Varney M., Xia H., Passmore M.A., Gaylard A.

For vehicles with a squareback geometry, for example Sports Utility Vehicles (SUVs), base pressure drag is a large contributor to overall drag. Simple passive techniques, such as tapering, can reduce drag significantly but at a large aesthetic and functional cost. Therefore, very small base geometry changes have been investigated. An experimentally validated methodology has used Detached Eddy Simulations (DES) to obtain time-averaged and instantaneous data; allowing the effect of horizontal base slats on global forces and wake structures to be presented. The small geometry modifications have caused substantial changes to the base pressure distribution with the main mechanisms of change being identified and observed close to the model surfaces. A region of separation is seen below each slat corresponding to reduced pressure whilst high pressure regions attributed to stagnation are increased. The combined effect is a statistically significant drag reduction of 4 counts (1 count = 0.001 CD) when a slat is added at 3/4 of the base height. The results show the scope for very small changes to a simplified road vehicle, in areas that have not previously been explored, to reduce overall drag with minimal aesthetic penalties. This understanding provides the impetus for new approaches in real vehicle development.

Kulkarni S., Edwards D.J., Parn E.A., Chapman C., Aigbavboa C.O., Cornish R.

Purpose Vehicle weight reduction represents a viable means of meeting tougher regulatory requirements designed to reduce fuel consumption and control greenhouse gas emissions. This paper aims to present an empirical and comparative analysis of lightweight magnesium materials used to replace conventional steel in passenger vehicles with internal combustion engines. The very low density of magnesium makes it a viable material for lightweighting given that it is lighter than aluminium by one-third and steel by three-fourth. Design/methodology/approach A structural evaluation case study of the “open access” Wikispeed car was undertaken. This included an assessment of material design characteristics such as bending stiffness, torsional stiffness and crashworthiness to evaluate whether magnesium provides a better alternative to the current usage of aluminium in the automotive industry. Findings The Wikispeed car had an issue with the rocker beam width/thickness (b/t) ratio, indicating failure in yield instead of buckling. By changing the specified material, Aluminium Alloy 6061-T651 to Magnesium EN-MB10020, it was revealed that vehicle mass could be reduced by an estimated 110 kg, in turn improving the fuel economy by 10 per cent. This, however, would require mechanical performance compromise unless the current design is modified. Originality/value This is the first time that a comparative analysis of material substitution has been made on the Wikispeed car. The results of such work will assist in the lowering of harmful greenhouse gas emissions and simultaneously augment fuel economy.

Varney M., Passmore M., Gaylard A.

Kabanovs A., Garmory A., Passmore M., Gaylard A.

Accurately predicting vehicle soiling is important for maintaining a clear view for the driver and on board camera and sensor systems. In this work we study the soiling process on a scale model of generic SUV body, which is a vehicle type particularly susceptible to base contamination. The Spalart-Allmaras formulation of the IDDES model is used to compute the continuous phase and the dispersed phase is computed using Lagrangian particle tracking, both concurrently with the flow-field, and also as a post-processing approach using time averaged statistical information of turbulence in a stochastic dispersion model. The results are compared against experimental data and the discrepancies discussed with regard to the predicted and measured flow field and base pressure distribution. Good agreement with experiment is shown for the contamination pattern using the fully unsteady method, but the more economic stochastic model does not recover some important details. This is attributed to the role of spatially correlated flow structures around the wheel in entraining particles into the wake that the stochastic model cannot accurately represent. This leads to the conclusion that base soiling is a function of unsteady modes, elimination of which may potentially reduce spray deposition.

Kabanovs A., Hodgson G., Garmory A., Passmore M., Gaylard A.

The motivation for this paper is to consider the effect of rear end geometry on rear soiling using a representative generic SUV body.In particular the effect of varying the top slant angle is considered using both experiment and Computational Fluid Dynamics (CFD).Previous work has shown that slant angle has a significant effect on wake shape and drag and the work here extends this to investigate the effect on rear soiling.It is hoped that this work can provide an insight into the likely effect of such geometry changes on the soiling of similarly shaped road vehicles.To increase the generality of results, and to allow comparison with previously obtained aerodynamic data, a 25% scale generic SUV model is used in the Loughborough University Large Wind Tunnel.UV doped water is sprayed from a position located at the bottom of the left rear tyre to simulate the creation of spray from this tyre.Having a single source of contamination simplifies the configuration of both experimental tests and simulations.It also improves analysis by allowing the soiling pattern from only one wheel to be seen in isolation.In order to provide further insight into the flowfield and its interaction with the spray CFD simulations are also performed at the same scale.A Detached Eddy Simulation approach is used, specifically the Spalart Allmaras formulation of the IDDES CFD model.Lagrangian particle tracking is used to model the dispersed phase.This CFD methodology has been found to give good agreement for soiling pattern with experiment for baseline cases.