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

Euromech Colloquium 509: Vehicle Aerodynamics. External Aerodynamics of Railway Vehicles, Trucks, Buses and Cars - Proceedings

During the 509th Colloquium of the Euromech society, held from March 24th & 25th at TU Berlin, fifty leading researchers from all over europe discussed various topics affecting both road vehicle as well as railway vehicle aerodynamics, especially drag reduction (with road vehicles), cross wind stability (with trains) and wake analysis (with both). With the increasing service speed of modern high-speed railway traffic, aerodynamic aspects are gaining importance. The aerodynamic research topics comprise both pure performance improvements, such as the continuous lowering of aerodynamic drag for energy efficiency, as well as safety relevant topics, such as cross-wind stability. The latter topic was most recently brought to attention when a swiss narrow-gauge train overturned during the severe storm Kyrill in january 2007. The shape of the train head usually has largest influence on cross wind stability. Slipstream effects of passing trains cause aerodynamic loads on objects and passengers waiting at platforms. The strength of the slipstream is determined by both the boundary layer development along the length of the train and the wake developing behind the tail of the train. Since high-speed trains can be considered to be as smooth as technically possible, attention is drawn to the wake region. The wake of the train again is also one important factor for the total drag of a train. Due to the fact that trains are bidirectional, optimisation of the leading car of a train with respect to drag and cross wind performance while simultaneously minimising the wake of the train for drag and slipstream performance is a great challenge. Modern optimisation tools are used to aid this multi-parameter multi-constraint design optimisation in conjunction with both CFD and wind tunnel investigations. Since many of the aerodynamic effects in the railway sector are of similar importance to road vehicles, the aim of the colloquium is to bridge the application of shape optimisation principles between rail- and road vehicles. Particular topics to be addressed in the colloquium are: Drag, Energy consumption and emissions: Due to increase in energy cost, drag reduction has gained focus in the past years and attention will grow in the future. Pressure induced drag is of common importance for both rail- and road vehicles. The optimisation of head- and tail shape for road vehicles as well as for bi-directional vehicles (trains) is in the focus. Interference drag between adjacent components shall also be treated. Slipstream Effects: Are a safety issue for high-train operation (Prams sucked into track due to train-induced draught flows) when trains passing platforms at high speeds. For Road vehicles, the ride stability of overtaking cars is influenced by the wake of the leading trucks and busses. Common interest is the minimisation of wake effects for both rail and road vehicles. Cross-Wind Safety, Ride stability under strong winds: Both are safety issues for rail- and road vehicles. Aerodynamic forces shall be minimised (roll moment for trains and also yaw moment for road vehicles). Strategies for Vehicle shape optimisation (head, tail and roof shape) in order to minimise aerodynamic moments. Possibilities of Flow control. Optimisation strategies: Parametrisation, analyses (CFD), Optimisation tools and methods, Application to Drag, Cross-Wind, Ride stability and Snow issue

Aerodynamic performances of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models. Additional tests were conducted with geometry modifications as spoilers and underbody diffusers

Novel methods of drag reduction for squareback road vehicles

Road vehicles are still largely a consumer product and as such the styling of a vehicle becomes a significant factor in how commercially successful a vehicle will become. The influence of styling combined with the numerous other factors to consider in a vehicle development programme means that the optimum aerodynamic package is not possible in real world applications. Aerodynamicists are continually looking for more discrete and innovative ways to reduce the drag of a vehicle. The current thesis adds to this work by investigating the influence of active flow control devices on the aerodynamic drag of square back style road vehicles. A number of different types of flow control are reviewed and the performance of synthetic jets and pulsed jets are investigated on a simple 2D cylinder flow case experimentally. A simplified ¼ scale vehicle model is equipped with active flow control actuators and their effects on the body drag investigated. The influence of the global wake size and the smaller scale in-wake structures on vehicle drag is investigated and discussed. Modification of a large vortex structure in the lower half of the wake is found to be a dominant mechanism by which model base pressure can be influenced. The total gains in power available are calculated and the potential for incorporating active flow control devices in current road vehicles is reviewed. Due to practicality limitations the active flow control devices are currently ruled out for implementation on a road vehicle. The knowledge gained about the vehicle model wake flow topology is later used to create drag reductions using a simple and discrete passive device. The passive modifications act to support claims made about the influence of in wake structures on the global base pressures and vehicle drag. The devices are also tested at full scale where modifications to the vehicle body forces were also observed

The influence of turbulence on the aerodynamic optimisation of bluff body road vehicles

In order to promote further understanding of the effects of the atmospheric environment encountered by road vehicles in the real world, a wind tunnel based investigation was conducted into the effect of small scale turbulence on the road vehicle optimisation process. An initial investigation was carried out using a I-box model with variable leading edge radii from 10mm to 100mm. Measurements of time averaged forces were made over a range of Reynolds numbers from 200,000 to 1,300,000 (based on the square root of frontal area) and free stream turbulence levels from 0.2% to 5.1%. The transcritical Reynolds number based on edge radius was established as a basis for comparison between turbulence levels. Centreline pressures and PlV vector fields are presented to provide information on separation and reattachment. The investigation was extended to a more representative 2-box model using the same radii as before and a reference model at full scale, where the edge radii varied from 25mm to 150mm and turbulence intensity from 1.8% to 4.3%. It was shown that there is a strong reduction of separation under increased turbulence, and a small increase in skin friction. A further experiment was carried out to investigate the influence of freestream turbulence on the characteristic effect of changing backIight angle on lift and drag. It is shown that there was a reduction in drag due to the action of turbulence on the separation over the backIight, which may be driven by an effect on vortex strength. Tests were also carried out on two full scale vehicles to investigate the effect of increasing turbulence intensity on front and rear spoilers, cooling drag, and A-pillar vortex flows. The observed changes were small but would often be cumulative in their effect, so that optimising a vehicle in a significantly different turbulence level could produce a difference in the total forces acting on the vehicle. These experiments have shown that the primary effect of the additional freestream turbulence introduced by grids is on the boundary layer, as was expected from the literature. The results showed that increasing the turbulence intensity made separated regions smaller, and suggested that vortices become weaker and less well defined. The work provides a basis for continuing to investigate the effect of freestream turbulence on the process of optimising the aerodynamics of road vehicles.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

On the aerodynamics of an enclosed-wheel racing car: an assessment and proposal of add-on devices for a fourth, high-performance configuration of the DrivAer model

A modern benchmark for passenger cars – DrivAer model – has provided significant contributions to aerodynamics-related topics in automotive engineering, where three categories of passenger cars have been successfully represented. However, a reference model for highperformance car configurations has not been considered appropriately yet. Technical knowledge in motorsport is also restricted due to competitiveness in performance, reputation and commercial gains. The consequence is a shortage of open-access material to be used as technical references for either motorsport community or academic research purposes. In this paper, a parametric assessment of race car aerodynamic devices are presented into four groups of studies. These are: (i) forebody strakes (dive planes), (ii) front bumper splitter, (iii) rear-end spoiler, and (iv) underbody diffuser. The simplified design of these add-ons focuses on the main parameters (such as length, position, or incidence), leading to easier manufacturing for experiments and implementation in computational studies. Consequently, a proposed model aims to address enclosed-wheel racing car categories, adapting a simplified, 35% scaled-model DrivAer Fastback shape (i.e. smooth underbody, no wheels, and with side mirrors). Experimental data were obtained at the 8ft x 6ft Cranfield Wind Tunnel using an internal balance for force and moment measurements. The aerodynamic performance of each group of add-on was assessed individually in a range of ride heights over a moving belt. All cases represent the vehicle at a zero-yaw condition, Reynolds number (car length-based) of 4.2 × 106 and Mach number equal to 0.12. The proposed high-performance configuration (DrivAer hp-F) was tested and a respective Reynolds number dependency study is also provided. In line with the open-access concept of the DrivAer model, the CAD geometry and experimental data will be made available online to the international community to support independent studies

Vehicle wakes in side wind

There is a global push to reduce the energy consumption of passenger vehicles with increasingly stringent targets and regulations. More than one-tenth of Europe\u27s greenhouse gasses are due to passenger vehicles. The aerodynamic drag is a major contributor to energy consumption responsible for more than a quarter of the vehicle\u27s energy usage. Thus, improving the aerodynamic drag will help us achieve our greenhouse gas emissions targets. Vehicle aerodynamics is typically assessed and developed in idealised conditions using low turbulence wind tunnels and numerical methods. Several aspects influencing vehicle aerodynamics are often neglected such as traffic and wind conditions. This thesis explores the effects of steady wind, or yawed flow, on the wake of vehicles. The goal is to increase the knowledge of the full wake behaviour at yaw and how it is related to the aerodynamic drag. For this, an optimisation method is used throughout this work to generate robust, low-drag, reference geometries. The optimisation is done at different yaw angles, allowing asymmetric geometries at yaw. The cycle averaged drag, which takes into account the driving cycle as well as the wind distribution, is also considered to create symmetric geometries which are insensitive to yaw. The optimisation is focused on base cavities and trailing edge modifications to these cavities. Generally, the low-drag configurations have a more balanced wake, with and without side wind, where the recirculating flow in the wake is aligned with the vehicle. The improved balance allows the wake to move more freely which often increases the large scale coherent motions of the wake. These unsteady motions are linked to increases in drag in the literature, however, in this work, improving the wake balance was found to be the more important indicator of the overall drag. At yaw, the coherent unsteady motions are reduced as a result of the wake being locked in a more stable, but higher drag, upwash or downwash dominated state. The wake becomes increasingly downwash or upwash dominated at yaw by a large rotating structure in the wake. The yaw insensitive designs have a wake that is slightly biased towards the top or bottom of the base at zero yaw to counteract the movement of the wake at yaw. Optimising the geometry without considering yaw can reduce the performance over the entire operating range. This highlights the importance of considering several operating conditions during vehicle development

Numerical Simulation of the Flow Field around Generic Formula One

The steady Reynolds-Averaged Navier-Stokes (RANS) method with the Realizable k turbulence model was used to analyze the flow field around a race car (generic Formula One). This study was conducted using the ANSYS software package. The numerical simulations were conducted at a Reynolds number based on the race car model (14.9×106). The time-averaged velocity field, flow topology, velocity magnitude, static pressure magnitude and vortex regions of the flow fields are presented in this paper. The measurements were performed on the vertical and cross-sectional planes. The results are presented graphically, showing the main characteristics of the flow field around the whole race car, whereas most previous studies only mention the flow field around individual components of race cars. The Realizable k turbulence model results showed consistency with the valuable validation data, which helps to elucidate the flow field around a model generic Formula one race car

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