Optimisation of Trailing Edge Flaps on the Base Cavity of a Vehicle for Improved Performance at Yaw

Regulations to reduce greenhouse gas emissions of passenger vehicles are becoming increasingly stringent. The aerodynamic drag is a major contributor to the vehicle\u27s total energy consumption where a large portion is attributed to the base wake. This paper optimises the angles of small trailing edge flaps on a base cavity of a full-scale sports utility vehicle placed in a wind tunnel. The trailing edge flaps are controlled using servos mounted inside the cavity. The flap angles are optimised using a surrogate model based optimisation algorithm with the objective of reducing the aerodynamic drag at different yaw angles and to create a yaw-insensitive geometry by considering several weighted yaw angles to form the driving cycle averaged drag. Low drag designs are further investigated using base pressures and wake measurements. The results show that the base pressures are symmetrised by reducing the crossflow in the wake. As the model is yawed the wake becomes increasingly downwash dominated by a large rotating windward structure which is reduced by the optimised flaps. The cycle averaged drag optimised design has a smaller increase in drag when yawed compared to a design optimised without considering yaw

Tyre Pattern Features and their Effects on Passenger Vehicle Drag

In light of the drive for energy efficiency and low CO 2 emissions, extensive research is performed to reduce vehicle aerodynamic drag. The wheels are relatively shielded from the main flow compared to the exterior of the passenger car, however, they are typically responsible for around 25% of the overall vehicle drag. This contribution is large as the wheels and tyres protrude into the flow and change the flow structure around the vehicle underbody. Given that the tyre is the first part of the wheel to get in contact with the oncoming flow, its shape and features have a significant impact on the flow pattern that develops. This study aims at identifying the general effects of two main tyre features, the longitudinal rain grooves and lateral pattern grooves, using both CFD and wind tunnel tests. This is performed by cutting generic representations of these details into identical slick tyres. Combinations of the two resulted in four physical tyre patterns that are tested on both a production and a closed rim. The test setup is reproduced in CFD taking the tyre deformation under loading into account. Due to the tyre\u27s deformation, Moving Reference Frame - grooves (MRFg) was used to model rotation, while the rim spokes were modelled with the sliding mesh approach. The results indicate that the rain grooves play a significant role in reducing drag when introduced on a slick tyre both in test and simulations, while the results from adding lateral grooves were less consistent dependent on the rim-tyre combination. The interaction between the longitudinal and lateral grooves could be observed on the overall vehicle drag. In general, CFD is able to predict the drag changes for different tyre patterns with good accuracy for the open rim, however the closed rim case proved to be more challenging

Validation of Different Fan Modelling Techniques in Computational Fluid Dynamics

The accuracy of predicting the engine bay flow field with computational fluid dynamics (CFD) is crucial for designing efficient cooling systems for heat sensitive components. The engine cooling fan is the main driving component in cases of high thermal load, such as uphill driving with a trailer, or high speed driving on a highway, when the ram air itself is no longer sufficient for cooling purposes.The most widely used fan modelling method is the Moving Reference Frame (MRF). This method can be used in steady and unsteady simulations, but has the drawback of using a fan geometry that is fixed in the global reference frame and, therefore, causing non-physical low velocity regions in the wake ofthe blades. The Rigid Body Motion (RBM or ”sliding mesh”) approach is a more accurate, but also more expensive approach, since it uses an unsteady solver. This study looks closely at the prediction of the flow field in the wake of an axial fan for different freestream velocities and fan speeds using the traditional MRF and RBM approach. In addition, a method that uses the average of flow field data for multiple MRF simulations with different fan positions is presented. Thereby the shadow of the fan blades is removed from the wake and the flow field becomes more uniform without the need of performing unsteady simulations. As a reference, measurements are performed on a vehicle fan with a 2D Laser Doppler Anemometry set-up in a small scale wind tunnel.The results show good agreement between the measurements and the RBM simulations. As expected, the MRF simulations show a distinct blade pattern in the wake flow field. This was successfully removed by the proposed averaged MRF method. Even though there are still some differences between this method and the experimental results, the average MRF method has shown to be applicable as it improves the flow field results at a relatively low computational cost

Evaluation of wind tunnel interference on numerical prediction of wheel aerodynamics

For a passenger vehicle, approximately 25% of the total\ua0aerodynamic drag\ua0originates from the wheels, making the aerodynamics of wheels a significant factor for the overall performance of a vehicle. To understand the complex flow field created by the rotational condition and geometry of these bluff-bodies, numerical simulations are often used. However, computations are frequently performed in domains that replicate open road conditions, differing from the conditions of\ua0wind tunnels. Therefore, to properly validate a\ua0CFD\ua0procedure and to correlate physical tests to numerical results, interference effects of the wind tunnel need to be investigated and their impact on the aerodynamics of wheels analysed and compared to that of open road calculations.In this study, numerical simulations on the DrivAer model were performed using different tyres and rims in both open road conditions and with the inclusion of a detailed model of a slotted walls wind tunnel. The results of the simulations are compared to experimental data, consisting of forces and flow field measurements. It was found that the inclusion of the wind tunnel in the computations improves the prediction of the flow fields, resulting in better prediction of both the absolute drag values and the drag deltas between configurations

Surrogate-based optimisation using adaptively scaled radial basis functions

Aerodynamic shape optimisation is widely used in several applications, such as road vehicles, aircraft and trains. This paper investigates the performance of two surrogate-based optimisation methods; a Proper Orthogonal Decomposition-based method and a force-based surrogate model. The generic passenger vehicle DrivAer is used as a test case where the predictive capability of the surrogate in terms of aerodynamic drag is presented. The Proper Orthogonal Decomposition-based method uses simulation results from topologically different meshes by interpolating all solutions to a common mesh for which the decomposition is calculated. Both the Proper Orthogonal Decomposition- and force-based approaches make use of Radial Basis Function interpolation. The Radial Basis Function hyperparameters are optimised using differential evolution. Additionally, the axis scaling is treated as a hyperparameter, which reduces the interpolation error by more than 50% for the investigated test case. It is shown that the force-based approach performs better than the Proper Orthogonal Decomposition method, especially at low sample counts, both with and without adaptive scaling. The sample points, from which the surrogate model is built, are determined using an optimised Latin Hypercube sampling plan. The Latin Hypercube sampling plan is extended to include both continuous and categorical values, which further improve the surrogate\u27s predictive capability when categorical design parameters, such as on/off parameters, are included in the design space. The performance of the force-based surrogate model is compared with four other gradient-free optimisation techniques: Random Sample, Differential Evolution, Nelder–Mead and Bayesian Optimisation. The surrogate model performed as good as, or better than these algorithms, for 17 out of the 18 investigated benchmark problems

Flow angularity investigations in an automotive slotted wall wind tunnel

The Volvo Cars aerodynamic wind tunnel has had a vortical flow angularity pattern in the test section since its original commissioning in 1986. The vortical flow nature persisted after an upgrade in 2006, when the fan was replaced and a moving ground system was introduced. It has been hypothesized that the cause for this flow angularity pattern was leakages around the heat exchanger installed in the settling chamber. The present paper tests this hypothesis by measuring the flow angularity in the test section before and after sealing the leakages. The findings show that the leakage path around the heat exchanger does not influence the flow angularity, and that the current pattern is different compared to the commissioning after the upgrade. This prompted an investigation of the influence from the turbulence screens, which were changed after the upgrade commissioning. These investigations indicate that the probable cause of the vortical flow angularity pattern is residual swirl from the fan. Force measurements on a reference car with and without extra induced flow angularity show that the flow angles measured in the tunnel for regular operation are most likely small enough to not have a significant effect on the measured aerodynamic forces

Experimental investigation of a two-truck platoon considering inter-vehicle distance, lateral offset and yaw

In recent years a renewed interest in platooning has emerged due to increasing pressure on vehicle manufacturers to reduce greenhouse gas emissions of their fleets. Vehicles traveling in\ua0close proximity\ua0have been studied in some depth, particularly simplified bodies and North American trucks. Still, there is a lack of understanding of the benefits of platooning for European style trucks. In this study, experiments were undertaken using two 1:6 scale detailed cab over engine tractor-trailer models in a\ua0wind tunnel\ua0with a moving ground. Surface pressures were measured on both trucks, while force measurements were taken on the model placed on the belt. Inter-vehicle distance, lateral offset, and yaw conditions were varied. Results show that a reduction of drag for the platoon is seen as the inter-vehicle distance decreases. For the leading truck, the reduction is due to an increased base pressure caused by the truck behind. The trailing truck has a more complex behavior and is sensitive to yaw changes. At short inter-vehicle distances, the leading truck loses in performance with a lateral offset, while the trailing truck gains in performance if under yaw conditions. To aid the flow analysis, numerical simulations were undertaken for some conditions studied experimentally

Thermal encapsulation of large battery packs for electric vehicles operating in cold climate

Thermal management for electric vehicles is becoming ever more significant for ensuring prolonged driving range. Climatization of batteries to the optimal operating temperatures is crucial for their performance and lifetime, and therefore they need to be heated when operating in cold climate. This often results in reduced driving range. This work numerically investigates the potential of thermal encapsulation of large battery packs for electric truck applications. Vehicle-level simulations were performed under drive cycle conditions at different operating temperatures to study its influence on the battery performance. Parking–driving cycles at various ambient temperatures and a parametric study on the encapsulation characteristics were carried out to assess the energy consumption under each condition. The study shows that high thermal resistance of the insulation material significantly reduced the heat loss to the environment acclimatizing the battery pack close to near-optimal operating temperatures, which can result in potential energy savings of about 15% at −25 \ub0C when operating after a 12-h parking period

Transferencia convectiva de calor e massa em um duto preenchido com esferas compactadas

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro TecnologicoO presente trabalho é uma investigação numérica do problema da transferência simultânea de calor e massa em um duto preenchido com um meio poroso saturado. O meio poroso é constituído de esferas compactas e é submetido a um escoamento com convecção forçada. As paredes do duto são impermeáveis ao fluxo de massa e são aquecidas sob duas condições: temperatura constante e fluxo de calor uniforme. O problema hidrodinâmico é modelado através da lei de Darcy modificada, a qual computa efeitos de inércia, atrito com as paredes sólidas, efeitos viscosos e porosoidade variável. Inicialmente, o problema térmico na ausência de transferência de massa é analisado sob dois aspectos distintos. Numa primeira aproximação considera-se o fluído e o meio poroso em equilíbrio térmico local. Isto é, ambos com a mesma temperatura. Após, analisa-se a situação na qual o fluído troca calor tanto com as partículas como com as paredes do duto. O problema isotérmico da transferência de massa é avaliado e discutido a seguir. Quando o fluído tem seu conteúdo de umidade aumentado pela evaporação da água líquida presente na superfície das partículas e a sua temperatura é modificada devido as exigências do calor latente, as equações do transporte de calor e massa são acopladas e precisam ser resolvidas simultaneamente. Para este caso mostrou-se que, para a evaporação d'água no ar, a posição onde o ar satura é virtualmente indepedente do seu conteúdo de umidade inicial

High speed driving stability of road vehicles under crosswinds: an aerodynamic and vehicle dynamic parametric sensitivity analysis

Crosswinds affect vehicle driving stability and their influence increase with driving speed. To improve high speed driving stability, interdisciplinary research using unsteady aerodynamics and vehicle dynamics is necessary. The current demands of faster development times require robust virtual methods for assessing stability performance in early design phases. This paper employs a numerical one-way coupling between the two disciplines and uses a variety of realistic crosswind gust profiles for the aerodynamic simulations to output representative forces and moments on three vehicle dynamic models of different fidelity levels, ranging from a one-track model to a full multi-body dynamic model of a sports utility vehicle. An investigation on required model fidelity was conducted along with a sensitivity study to find key aerodynamic and vehicle dynamic characteristics to minimise the yaw velocity and lateral acceleration response during crosswinds. Transient aerodynamic simulations were used to model crosswind gusts at high speeds. Analysis of the forces and moments showed that rapid changing gusts generate overshoots in the yaw moment, due to the phase delay of the flow between the front and rear of the vehicle. A methodology for modelling this phase delay is proposed. The response of the vehicle was captured equally well by the enhanced model (mid-level fidelity) and the full multi-body dynamic model, while the simplest one-track model failed to emulate the correct vehicle response. The sensitivity study showed the importance of the positioning of the centre of gravity, the aerodynamic coefficient of yaw moment, wheel base, vehicle mass and yaw inertia. In addition, the axles\u27 side force steer gradients and other suspension parameters revealed potential in improving crosswind stability