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Technical Paper

The Aerodynamic Forces Induced on a Passenger Vehicle in Response to a Transient Cross-Wind Gust at a Relative Incidence of 30°

The aerodynamic forces induced on a generic ‘hatchback’ model have been measured as it passes through a perpendicular cross-wind jet generating a relative yaw angle of 30°. This has been done in the unique University of Durham automotive wind tunnel, which utilises the stationary model approach, with the cross-wind being introduced by means of a second jet which is separated from the main jet by a moving belt and aperture assembly. Data acquisition was by means of an array of surface pressure tappings. Transient pressure force and moment coefficients have been measured and it is shown that the side and lift forces experienced in the transient situation exceed the steady state values at corresponding yaw angles by between 10% and 20%.
Technical Paper

The Reconstruction of Periodic Pressure Fields from Point Measurements

A new method for processing data from time-accurate point measurements has been developed in order to investigate periodic elements of unsteady flow fields. The technique synchronizes the phase of measurements taken at different locations using a reference signal and collapses the spectral peak of interest onto a single frequency. The technique has been applied to data gathered using a time-accurate 5-hole probe behind a two dimensional body exhibiting vortex shedding. It has been possible to generate a sequence of instantaneous pressure and velocity fields which show the shedding of vorticity and total pressure loss to form a vortex street.
Technical Paper

An Improved Wind Tunnel Configuration for the Investigation of Aerodynamic Cross Wind Gust Response

An improved technique is described for the experimental modeling of transient cross wind gust influences on passenger vehicles. The new configuration uses a set of vertical axis shutters which open and close in a ‘Mexican wave’ fashion to scan the cross wind jet along the working section of the wind tunnel. The new arrangement dramatically increases the rate at which experiments can be performed and offers the opportunity to apply phase-averaging techniques to multiple data sets in order to reduce noise. This is a significant development as most previous test methods have suffered from poor signal to noise ratios. Experimental results are presented for transient surface pressure measurements on a simplified vehicle model which clearly demonstrate the benefits of the new technique.
Technical Paper

Wake Surveys Behind a Passenger Car Subjected to a Transient Cross-Wind Gust

Transient wake surveys have been conducted on a generic three dimensional vehicle shape. The flow conditions were those generated by the unique crosswind facility at Durham University, which imitates a vehicle passing through a sharp-edged, finite length cross-wind gust. Each survey consisted of some 7000 cross-wind gusts, with each point in the wake being phase-averaged over 20 gusts. The surveys clearly show the development of the wake structure from the familiar axial flow conditions, through the transient to a nominally steady yawed flow. Although both the structure and total pressure loss that develop in the wake flow are comparable to those found for quasi-steady flow conditions, the developing flow reveals characteristics that are not found in the quasi-steady measurements. New data are also presented with regard to the character of the gust that develops in this wind tunnel and their impact upon the reported wake measurements is discussed.
Technical Paper

An Investigation into the Flow-Field About an Exposed Racing Wheel

Detailed flow-field measurements in the wake of a 40 percent full-scale exposed wheel have been obtained using particle image velocimetry (PIV). Additional data have been acquired in the form of surface static pressure measurements acquired using the Durham University radio telemetry system. The results presented in this paper compare and contrast, both quantitatively and qualitatively, the physical differences that exist with respect to the flow structures of rotating and non-rotating wheels. Some of the ‘special’ features of the flow-field postulated by Fackrell, such as the ‘jetting’ phenomenon, have been revisited, examined and revised based on the surface static pressure and PIV data presented in this paper. The experimental observation of a flow mechanism is presented in terms of the rear jetting after the line of contact, and the effects of this have been considered and analyzed.
Technical Paper

An Investigation into Large Scale Unsteady Structures in the Wake of Real and Idealized Hatchback Car Models

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.
Technical Paper

Racing Car Wheel Aerodynamics – Comparisons between Experimental and CFD Derived Flow-Field Data

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.
Technical Paper

The Air Flow About an Exposed Racing Wheel

A radio telemetry system has been designed and developed at Durham University that enables surface pressure data to be transmitted from a rotating racing wheel to a host PC, where data post-processing is carried out. A multi-element wheel rim has been designed to allow the telemetry system to be located inside a pneumatic tire. Surface pressure distributions around the centerline of the wheel show good agreement with previous research. A flow field investigation has also been conducted, downstream of the wheel, for both stationary and rotating wheel cases. The results presented highlight some of the key features of the flow field and give confidence in the telemetry system.