The importance of rear pillar geometry on fastback wake structures

The wake of a fastback type passenger vehicle is characterised by trailing vortices from the rear pillars of the vehicle. These vortices strongly influence all the aerodynamic coefficients. Working at model scale, using two configurations of the Davis model with different rear pillar radii, (sharp edged and 10 mm radius) the flow fields over the rear half of the models were investigated using balance measurements, flow visualisations, surface pressure and PIV (Particle Image Velocimetry) measurements. For a small geometry change between the two models, the changes to the aerodynamic loads and wake flow structures were unexpectedly large with significant differences to the strength and location of the trailing vortices in both the time averaged and unsteady results. The square edged model produced a flow field similar to that found on an Ahmed model with a sub-critical backlight angle. The round edged model produced a flow structure dominated by trailing vortices that mix with the wake behind the base of the model and is weaker. This flow structure was more unsteady than that of the square edged model. Consequently, although both models can be described as having a wake dominated by trailing vortices, there are significant differences to both the steady state and unsteady flow fields that have not been described previously. This also shows that the fastback wake structure described by Ahmed is not definitive

Characterisation of wake bi-stability for a square-back geometry with rotating wheels

In this paper the effects produced by the wheels on the bi-stable reflectional symmetry breaking (RSB) mode seen for the wake of a square-back geometry (Grandemange et al. [11]) are investigated considering a modified version of the Windsor body already studied in Perry et al. [18]. The contribution of the wheels and their rotation to the changes in the base pressure distribution and the wake topology is characterised by means of pressure tappings and 2D-3C particle image velocimetry. Balance measurements are used to further characterise the changes in the strength of the RSB mode. For the pure square-back configuration, the results show a general increase of the base drag as a consequence of the strengthening of the suction over the lower portion of base, due to the formation of a pair of counter rotating vortices acting close to the bottom trailing edge. At the same time, the RSB mode is weakened, leading to a reduction in the fluctuations recorded for the lateral component of the aerodynamic force. The sensitivity of the RSB mode to small changes in the shape of the model’s trailing edges is characterised by looking at the effects produced by short tapers, with a slant angle of 12° and a chord equal to 4% of the model length, applied to either the horizontal or the vertical trailing edges. The results show that the RSB mode disappears when the effect of the wheels is paired to the upwash generated by the slanted surface (when applied to the bottom trailing edge), although it is still clearly visible when the tapers are applied to the side edges of the base, in contrast with the results reported by Pavia et al. [16] for the same geometry without wheels

The measurement of transient aerodynamics using an oscillating model facility

A method for the estimation of transient aerodynamic data from dynamic wind tunnel tests has been developed and employed in the study of the unsteady response of simple automotive type bodies. The paper describes the facility and analysis techniques employed and reports the results of a parametric study of model rear slant angle and of the influence of C-pillar strakes. The model is shown to exhibit damped, self-sustained and self-excited behaviour. The transient results are compared with quasi-steady predictions based on conventional tunnel balance data through the calculation of derivative magnification factors. For all slant angles tested the results show that the quasi-steady prediction is a poor estimate of the real transient behaviour. In addition the slant angle is shown to have significant effect on the level of unsteadiness. The addition of Cpillar strakes is shown to stabilise the flow with even small height strakes yielding responses well below that of steady-state

Drag levels and energy requirements on a SCUBA diver

The popularity of sport diving has increased rapidly since its inception in the 1950’s. Over this period, the trend has been to increase the amount of equipment carried by the diver. There are many undoubted safety advantages associated with the additional kit, but under some conditions, it can impose an additional burden in the form of increased drag. The purpose of this paper is to identify the drag penalties for a number of simple SCUBA configurations. This is achieved through scale model experiments conducted in a wind tunnel. Some comments on the associated energy requirements are made, and from these, the effect on a diver’s bottom time is briefly addressed. The configurations tested include a study of the effect of the equipment configuration and the effect of small changes to the diver incidence. The tests show that the addition of a pony cylinder gives a 10% increase in drag compared to a conventional octopus set-up. When a dive knife, large torch and a Surface Marker Bouy (SMB) are also added this increases to 29%. Over the range tested, the average effect of swimming at a head up incidence to the flow is to increase the drag coefficient by 0.013/degree. This amounts to 16% at 5 degrees and 49% at 15 degrees. Estimates of the effect of the drag changes on bottom time show that particularly at the higher speeds the drag increases result in approximately similar percentage reductions in bottom time. Some simple suggestions for drag reduction are proposed

Aerodynamic drag reduction of a simplified squareback vehicle using steady blowing

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 scale squareback style vehicle has been investigated. Hotwire 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

Experimental study of multiple-channel automotive underbody diffusers

Underbody diffusers are used widely in race car applications because they can significantly improve the cornering capacity of the vehicle through the generation of a downforce. They are also likely to have a wider role in reducing the drag in road vehicles as it becomes increasingly important to reduce emissions of carbon dioxide. This paper reports on a wind tunnel investigation, using a simplified bluff body model, into the effect of splitting a simple plane diffuser into multiple channels. Tests are reported for a range of diffuser geometries suitable for road and race car applications. The results for the lift, the drag, and the incremental changes to the lift-to-drag ratio are reported and discussed in terms of the underbody pressures. While broadly similar trends to the single-channel plane diffuser are seen in the multiplechannel diffuser configurations, it was found that the effect of increasing the number of channels depended on the flow regimes present in the plane diffuser. At angles just above the plane diffuser optimum, where the flow is partially separated, the multiple-channel configurations give large improvements in the downforce with minimal increase in the drag, significantly extending the performance envelope. The pressure maps indicate that the gains occur through improved diffuser pumping and pressure recovery in both the inner and the outer channels

Estimation of bluff body transient aerodynamics using an oscillating model rig

A method for the estimation of transient aerodynamic derivatives from dynamic wind tunnel tests using time response data is presented in this paper. For the purposes of the study, the aerodynamic derivatives are considered to act as a stiffness and damping to the model motion. The experimental set-up consists of a simple bluff body (Davis model) constrained to oscillate with a single degree of freedom of pure yawing motion. A range of springs were used to control the oscillation frequency and hence the reduced frequency. The transient responses from dynamic wind tunnel tests are compared with quasi-steady analysis in order to investigate the effect of unsteady aerodynamics. The aerodynamic derivatives are initially estimated using the classical logarithmic decay method. The dynamic stiffness derivative exceeds that determined statically across the reduced frequency range. The damping derivative was found to be a function of free-stream speed; at low velocities it is negative but progressively increases to a positive value. With further increases in speed, a self-sustained oscillation is observed with almost constant frequency and amplitude. This result is attributed to coupling between the model wake and the model stability; however, the exact mechanism of the interaction is not fully understood. This phenomenon is under further investigation

Characterisation of the low-frequency wake dynamics for a square-back vehicle equipped with side trailing edge tapers [Powerpoint]

Characterisation of the low-frequency wake dynamics for a square-back vehicle equipped with side trailing edge tapers [Powerpoint

On the optimisation of road vehicle leading edge radius in varying levels of freestream turbulence

It has been recognised that the ideal flow conditions that exist in the modern automotive wind tunnel do not accurately simulate the environment experienced by vehicles on the road. This paper investigates the effect of varying one flow parameter, freestream turbulence, and a single shape parameter, leading edge radius, on aerodynamic drag. The tests were carried out at model scale in the Loughborough University Wind Tunnel, using a very simple 2-box shape, and in the MIRA Full Scale Wind Tunnel using the MIRA squareback Reference Car. Turbulence intensities up to 5% were generated by grids and had a strong effect on transcritical Reynolds number and Reynolds sensitivity at both model scale and full scale. There was a good correlation between the results in both tunnels

Usage of body-fitted windows in PIV image processing

Flow close to the boundaries of bodies is often difficult to measure using particle image velocimetry (PIV); factors such as glare, velocity gradients and the body itself all present challenges in obtaining good quality results. One common problem in conventional PIV algorithms is that they are based on square grids, which for most applications is not aligned with the shape of the body. This means the body will clip part of the window resulting in fewer particles and erroneously placed vectors. Being able to align the interrogation window shape to the body boundary would remove these sources of error. This study investigates the effect that applying non-square windows has, compared to conventional square windows for three test cases: a free-field, a circular body and an airfoil shape. For each of these a number of interrogation methods are tested. For the circular body, conventional and rotated square windows were tested along with a body fitted mesh. The image was also deformed into R-θ coordinates to enable conventional square window processing, before de-warping the vector field. Square, rotated square and body fitted techniques were also tested on an airfoil shape. It was found that the body-fitted and warping methods both showed significant improvements in the boundary layer over the square meshes; although the warping process added computational expense. The meshing technique was found to have little impact on the free field as there was no body or velocity gradient. Utilising this interrogation method in conjunction with developed methods of automated edge detection and more mature processing algorithms will result in better measurements close to bodies than is possible with conventional square windows. An example application of this technique on flow around a sphere is also demonstrated