Review of Research on Vehicles Aerodynamic Drag Reduction Methods

Recent spikes in fuel prices and concern regarding greenhouse gas emissions, automotive design engineers are faced with the immediate task of introducing more efficient aerodynamic designs vehicles. The aerodynamic drags of a road vehicle is responsible for a large part of the vehicle’s fuel consumption and contribute up to 50% of the total vehicle fuel consumption at highway speeds. Review on the research performance of active and passive flow control on the vehicle aerodynamic drag reduction is reported in this paper. This review intends to provide information on the current approaches and their efficiency in reducing pressure drag of ground vehicles. The review mainly focuses on the methods employed to prevent or delay air flow separation at the rear end of vehicle. Researches carried out by a number of researchers with regard to active and passive flow controls method on vehicle and their effect on aerodynamic drag in terms of drag coefficient (CD) was highlighted. Passive methods i.e. Vortex Generator (VG), spoiler and splitter and active flow controls i.e. steady blowing, suction and air jet are among the methods had been reviewed. In addition several attempts to couple these flow control methods were also reviewed. Several aspects of aerodynamic drag that need for further investigation as to assist for vehicles aerodynamic design and for practical reasons were highlighted. Progressive research on active flow control was observed due to its flexibility for wide range of application without body shape modification

Computational investigation into the sensitivity of a simplified vehicle wake to small base geometry changes

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

Analysis of Novel Techniques of Drag Reduction and Stability Increase for Sport Utility Vehicles using Computational Fluid Dynamics

The main objective of this study is to investigate ways to reduce the aerodynamic drag coefficient and to increase the stability of road vehicles using three-dimensional Computational Fluid Dynamics (CFD) simulation. Two baseline models, the Ahmed body and the Land Rover Discovery 4, were used in these simulations. The effects of model scale and slant angle were investigated for the Ahmed body in addition to a new technique to measure the drag coefficient used in the experiments has been investigated numerically in this study. Many new aerodynamic devices and external design modifications were used for the Land Rover Discovery 4. ANSYS Meshing was used to create a variety of mesh cases for mesh optimization and ANSYS Fluent software was used to simulate all models. Different sizes of computational domain were used in order to study the effect of the blockage ratio on the aerodynamic behaviour. The range of Reynolds numbers used in this study for the Ahmed body was between 3 × 105 and 30 × 105 similar to the experimental studies. The uniform free stream velocity of air at the inlet ranging from 100km/h to 140km/h was used for the Land Rover Discovery 4. Reynolds-averaged Navier–Stokes equations (RANS) and Large Eddy Simulation (LES) turbulence models were used to establish the most appropriate turbulence model for the Ahmed body geometry. Only RANS was used for the Land Rover Discovery 4. In general, the trend of drag coefficient as a function of the Reynolds number for the Ahmed body was in good agreement with the experiments, whereas LES simulation results were closer to the experimental data. The drag and lift coefficients obtained from ANSYS Fluent for the baseline of the Land Rover Discovery 4 were validated with experimental data. It is found that the use of modern aerodynamic add-on devices and modifications has a significant effect in reducing the aerodynamic drag coefficient

Aerodynamic design of electric and hybrid vehicles: A guidebook

A typical present-day subcompact electric hybrid vehicle (EHV), operating on an SAE J227a D driving cycle, consumes up to 35% of its road energy requirement overcoming aerodynamic resistance. The application of an integrated system design approach, where drag reduction is an important design parameter, can increase the cycle range by more than 15%. This guidebook highlights a logic strategy for including aerodynamic drag reduction in the design of electric and hybrid vehicles to the degree appropriate to the mission requirements. Backup information and procedures are included in order to implement the strategy. Elements of the procedure are based on extensive wind tunnel tests involving generic subscale models and full-scale prototype EHVs. The user need not have any previous aerodynamic background. By necessity, the procedure utilizes many generic approximations and assumptions resulting in various levels of uncertainty. Dealing with these uncertainties, however, is a key feature of the strategy

Coupling road vehicle aerodynamics and dynamics in simulation

A fully coupled system in which a vehicle s aerodynamic and handling responses can be simulated has been designed and evaluated using a severe crosswind test. Simulations of this type provide vehicle manufacturers with a useful alternative to on road tests, which are usually performed at a late stage in the development process with a proto- type vehicle. The proposed simulations could be performed much earlier and help to identify and resolve any aerodynamic sensitivities and safety concerns before significant resources are place in the design. It was shown that for the simulation of an artificial, on-track crosswind event, the use of the fully coupled system was unnecessary. A simplified, one-way coupled system, in which there is no feedback from the vehicle s dynamics to the aerodynamic simulation was sufficient in order to capture the vehicle s path deviation. The realistic properties of the vehicle and accurately calibrated driver model prevented any large attitude changes whilst immersed in the gust, from which variations to the aerodynamics could arise. It was suggested that this system may be more suited to other vehicle geometries more sensitive to yaw motions or applications where a high positional accuracy of the vehicle is required

Vehicle surface contamination, unsteady flow and aerodynamic drag

The rear surfaces of blunt-ended vehicles, such as SUVs, are vulnerable to the build-up of contaminants thrown up from wet road surfaces by their tyres. This can compromise drivers’ vision, vehicle visibility, sensor performance and aesthetics. Vision will be reduced if the rear screen and lenses of camera systems become obscured. Similarly, sensing methods such as Light Detection and Ranging [LIDAR], introduced to support higher-level Advanced Driver Assistance Systems [ADAS] and autonomous driving are also vulnerable to contaminant accumulation. In addition, vehicle users may find that dirt is transferred to their hands and clothes as they access the rear load space. Finally, rapid soiling of external surfaces can be perceived as degrading the aesthetics of premium vehicles. Such deposition is a manifestation of unsteady aerodynamics – particularly the interaction between tyre spray, wheel wakes and the vehicle rear wake. These wake structures also strongly influence aerodynamic drag which, in turn affects CO2 emissions for Internal Combustion Engine [ICE] powered cars and the range of Battery Electric Vehicles [BEV]. Hence, automotive manufacturers need a simulation approach that can be used to minimise these characteristics concurrently during vehicle development. This work met that need by developing and deploying an innovative simulation process which predicts both contaminant accumulation and drag at the same time, by numerically representing unsteady aerodynamics, tyre spray and surface water behaviour. It is now integrated into the vehicle development process at Jaguar Land Rover [J/LR] where it is being used to develop new cars. This has been achieved by using a series of novel simplified vehicle geometry and spray systems to incrementally develop and validate the simulation strategy. The work culminated with its application to a production vehicle and subsequent validation against full scale experiments, providing the first quantification of accuracy for simulations of rear surface contamination. This novel simulation approach is combined with original experiments to show that reduced vehicle ride heights can lead to increased rear surface contamination, by reducing underbody flow and moving the vehicle wake closer to the highly contaminated wheel wakes. This provides a challenge for vehicle developers as lower ride heights are used to reduce aerodynamic drag; an increasingly important objective for both ICE and BEV product development, to support lower CO2 emissions and enhanced range, respectively. Finally, the first evidence is presented to suggest that aerodynamically improved underfloors can increase rear surface contamination, or at least redistribute it towards the lower regions of the vehicle rear, such as the bumper. This raises a risk for future BEVs which combine aerodynamically advantageous smooth underfloors with vulnerable ADAS features, such as rear bumper mounted LIDAR

Computational fluid dynamics based optimisation of emergency response vehicles

Formal optimisation studies of the aerodynamic design of Emergency Response Vehicles, typically encountered within the United Kingdom, were undertaken. The objectives of the study were to optimise the aerodynamics of the Emergency Response Vehicles such as Ambulance and Police cars, in terms of drag force. A combination of wind tunnel tests and the Computational Fluid Dynamics (CFD) simulations were used to analyse the flow field and aerodynamic characteristics of Emergency Response Vehicles. The experimental data were used to validate the computer simulations and the good agreement observed gave confidence in the results obtained. Results from computer simulations on the scale models and full-scale models, were also characteristically similar to those of the validated scale model. Computational Fluid Dynamics (CFD) was combined with an efficient optimisation framework to minimize the drag force of three different types of Emergency Response Vehicles, Ambulance Van Conversion, Police Van Conversion and Police Sedan car Conversion. The benefits of employing an airfoil-based roof design and Bezier curve fitting approach which minimizes the deleterious aerodynamic effects of the required front and rear light-bars, were investigated. Optimal Latin Hypercube (OLH) Design of Experiments, the Multipoint Approximation Method (MAM) and surrogate modelling were used for the optimisation. Optimisation results demonstrated a clear improvement of the aerodynamic design of the Emergency Response Vehicles named above. It was also clearly demonstrated that improving the aerodynamic design of Emergency Response Vehicles roof offers a significant opportunity for reducing the fuel consumption and emissions for Emergency Response Vehicles

Active Drag Reduction of Ground Vehicles Using Air-Jet Wheel Deflectors

Seven turbulence models were used to simulate the flow within the wheelhouse of a simplified vehicle body. The performance of each model was evaluated by comparing the aerodynamic coefficients obtained using computational fluid dynamics (CFD) to data collected from wind tunnel experiments. The performance of large eddy simulation (LES) and detached eddy simulation (DES) was largely dependent on the time step and grid size to accurately resolve turbulent eddies. The standard k-e, realizable k-e, k-w, DES, and LES all trended towards a drag coefficient which was 20% lower than the experimental value. In all numerical cases, the lift coefficient was found to be at least 60% greater than the experimental value, but was consistent with numerical studies by other authors. The standard k-w and SST k-w models provided results which were the most consistent with experimental data for the three different mesh sizes. Two types of flow modification devices were then added to the simplified vehicle model to assess drag reduction potential. Conventional wheel defectors are compared to air-jet wheel defectors on wheel drag and overall drag reduction capabilities. Two parametric studies are conducted on the Fabijanic body at a Reynolds number of 1.6x105: a study on the variation of the size and location of a conventional wheel defector, and a study on the jet speed and location of an air-jet wheel defector. Results show that wheel drag is decreased as the height of the conventional wheel defector is increased, and that the further the conventional wheel defector is from the wheelhouse, the more sensitive the wheel is to changes in drag coefficient. The air-jet wheel defector successfully decreases the wheel drag. The closer the air-jet is to the wheelhouse the less of an impact it has on wheel drag, but the greater the impact on the overall drag of the simplified body. A maximum overall drag reduction of 2.76% is achieved with a configuration which also results in a wheel drag reduction of 16%. Air-jet wheel defectors were then simulated on the DrivAer reference model -- an open source model which blends features of the Audi A4 and the BMW 3 Series. The air jets were found to be less impactful at low speeds, but at higher speeds, they were observed to reduce wheel drag and cause an overall drag reduction of up to 5.1%. Even though jet speeds as high as twice the driving speed were investigated, and caused relatively large reductions in wheel drag, a jet speed approximately 2/3 of the driving speed was observed to cause the greatest overall reduction

Experimental investigations of automotive surface contamination

Sports Utility Vehicles are popular in the global automotive market due to their practicality. However, these geometries have a propensity to contamination of water and dirt that generates problems with visibility, driver assistance systems, lighting and customer dissatisfaction. Contamination issues tend to be discovered during prototype testing and are therefore expensive to resolve because the geometry is largely fixed. Identifying problems at an early stage in the design through numerical simulations would resolve many of the issues. To have confidence in the simulations they must be correctly initialised and validated objectively against experimental test cases.An approach to quantifying surface contamination is identified for the first time and an extensive set of experimental controls determined. The approach uses an Ultra Violet dye to dope water and an Ultra Violet light to illuminate the fluid. The intensity of the emitted light from the dye is a function of the fluid thickness, illumination intensity and time. The approach is thoroughly explored and validated before being implemented in a pilot study that employs a quarter scale model of a simplified SUV and tested in a wind tunnel using a fully characterised spray to contaminate the base. The model base pressures and streamwise Particle Image Velocimetry planes of the wake are obtained and used alongside the results to identify mechanisms. The objective measure of mass deposition rate per area is calculated and compared to previously used measures to demonstrate its effectiveness.Once deposited on the surface, wet contaminant forms drops, rivulets and thin films. These move across the surface of the car requiring wiper systems to maintainvisibility and management techniques to remove the contaminant. Uncontrolled flows can cause distractions to the driver and reduce the effectiveness of other systems such as cabin ventilation. An essential requirement in currently available simulation methods for surface water flows is a representative contact angle model. The tilted plate method is used to obtain these at a range of Capillary numbers for different fluids and surfaces relevant to the topic. Model parameters for the simulations are identified and a novel validation method proposed. This study demonstrates that fully characterised experiments and physical objective measures are absolutely essential to enable numeric simulations to be employed with the high levels of certainty demanded by the industry.</div