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This paper outlines the creation of a facility for simulating on-road transients in a model scale, ¾ open jet, wind tunnel. Aerodynamic transients experienced on-road can be important in relation to a number of attributes including vehicle handling and aeroacoustics. The objective is to develop vehicles which are robust to the range of conditions that they will experience. In general it is cross wind transients that are of greatest significance for road vehicles. On-road transients include a range of length scales but the most important scales are in the in the 2-20 vehicle length range where there are significant levels of unsteadiness experienced, the admittance is likely to be high, and the reduced frequencies are in a band where a dynamic test is required to correctly determine vehicle response.

It is well known that in motorsport the wake from an upstream vehicle can be detrimental to the handling characteristics of a following vehicle, in particular in formulae with high levels of downforce. Previous investigations have been performed to characterize the wake from an open wheel race car and its effect on a following car, either through the use of multiple vehicles or purpose-built wake generators. This study investigates how the wake of an upstream race car impacts the aerodynamic performance of a following car in a close-following scenario. Wakes are imposed on the inlet of a CFD simulation and wake parameters (eg: velocity deficit, trailing vorticity) are directly manipulated to investigate their individual impacts on the following vehicle. The approach provides a useful alternative to the simulation of multi-vehicle cases but a better simulation could be achieved by including wake unsteadiness from the upstream vehicle.

The effect of the upstream wake of a Formula 1 car on a following vehicle has been investigated using experimental and computational methods. Multiple vehicle studies in conventional length wind tunnels pose challenges in achieving a realistic vehicle separation and the use of a short axial length wake generator provides an advantage here. Aerodynamic downforce and drag were seen to reduce, with greater force reductions experienced at shorter axial spacings. With lateral offsets, downforce recovers at a greater rate than drag, returning to the level for a vehicle in isolation for offsets greater than half a car width. The effect of the wake was investigated in CFD using multiple vehicle simulations and non-uniform inlet boundary conditions to recreate the wake. Results closely matched those for a full two-vehicle simulation provided the inlet condition included unsteady components of the onset wake.

A vehicle on the road encounters an unsteady flow due to turbulence in the natural wind, the unsteady wakes from other vehicles and as a result of traversing through the stationary wakes of road side obstacles. There is increasing concern about potential differences in aerodynamic behaviour measured in steady flow wind tunnel conditions and that which occurs for vehicles on the road. It is possible to introduce turbulence into the wind tunnel environment (e.g. by developing active turbulence generators) but on-road turbulence is wide ranging in terms of both its intensity and frequency and it would be beneficial to better understand what aspects of the turbulence are of greatest importance to the aerodynamic performance of vehicles. There has been significant recent work on the characterisation of turbulent airflow relevant to road vehicles. The simulation of this time-varying airflow is now becoming possible in wind tunnels and in CFD.

The aerodynamic development and evaluation of passenger vehicles is almost universally performed in the controlled, low turbulence environment of a wind tunnel or under similarly idealized conditions using CFD. This environment is substantially different from that which is experienced on-road due to the effects of atmospheric winds and the wake flows from other road vehicles. The scope of this work is to establish, with regard to surface pressures, if a low turbulence wind tunnel evaluation of passenger cars yields results which accurately reproduce on-road data or whether a more complete simulation of the real world is required. The test vehicle was a Rover 214, a typical European mid-sized hatchback. Data were obtained from both the MIRA full-scale wind tunnel and on the road using the same vehicle and instrumentation. The on-road data were gathered under various atmospheric wind conditions.

There are many aspects of the unsteady flow around fastback passenger cars that remain to be understood. These include the source and nature of unsteady flow structures, the relevant time-scales, the effect of geometric parameters and the impact of the unsteadiness in terms of steady and unsteady forces on the vehicle. This paper investigates large scale unsteady structures in the wake of the Ahmed form and of a scale model of a real car shape using two wind tunnels and model scales between 12.5% and 40%. The unsteadiness demonstrated only low coherence and weak periodicity and the Strouhal number of a given structure varied from tunnel to tunnel indicating a high sensitivity to external influences. Nevertheless, a novel visualization technique, used to display the results of time-accurate pressure probe measurements, was able to reveal structures involving both symmetric and anti-symmetric oscillations in the strength of the rear-pillar vortices.

Performance simulations have been performed on two alternative race car configurations to establish whether the racing regulations for the Le Mans 24-hour race and other related series provide effective equivalence between the different, eligible vehicle configurations. In particular, two different types have been investigated; the prestigious LMP900 prototype class and the less powerful but lighter LMP675 class. It is shown that around the Le Mans circuit the equivalence is remarkably effective with the two types each having the potential to achieve almost identical lap times. Because of the unusual nature of the Le Mans circuit which is dominated by long, high speed straights simulations have also been performed for a more typical road circuit. The results show that the relative performance of the lighter car is enhanced resulting in a better laptime than that of the more powerful LMP900 class.

Detailed flow-field data have been acquired using experimental and computational techniques in the wake of a 40% full-scale exposed wheel. The experimental investigation focused on taking discrete single-point measurements in the wake using a pneumatic 5-hole pressure probe. A wake integral method was used to compute the total drag force acting on the wheel. The computational aspects of the investigation used the commercially available Fluent 6.0 CFD package. A tetrahedral volume mesh was used to discretise the flow domain and the k-ε turbulence model was used for all calculations. The boundary conditions were set according to the experiment. As the tire rotates the work done on its surface shear layer leads to increased velocities and compression immediately ahead of the contact patch which results in pressure coefficients in excess of unity. This leads to an outflow from this high pressure zone; an effect that is known as jetting. The reverse effect occurs behind the contact patch.

It is well documented that the aerodynamic performance of an open wheel race car is degraded when closely following another car. The greatest performance loss is usually experienced by the front-mounted wing leading to reduced aerodynamic downforce, handling imbalance and a reduced overtaking capability. Although previous wind tunnel studies have been performed to investigate this race situation the model scales have generally been compromised in order to achieve representative separation between the two test vehicles within the confines of the wind tunnel working section and particularly within the limited length of the moving ground plane. This study addresses that issue by using a very short, bluff body to create an accurate representation of the wake flow from the leading car in order to provide additional, effective test length ahead of the instrumented model.

It is well documented that the aerodynamic downforce that is developed by a NASCAR race car is degraded when closely following another car. This may lead to handling imbalance and a reduced overtaking capability. Although wind tunnel studies have been performed to investigate this race situation those investigations have frequently been compromised in order to accommodate two or more test vehicles or models within the confines of the wind tunnel's working section. This study addresses that issue by using a very short, bluff body to create an accurate representation of the wake flow from the leading car in order to maximize the test length available to the test model.