Kevin P. Garry

Comparison of quasi-static and dynamic wind tunnel measurements on simplified tractor-trailer models

Abstract The results from a series of wind tunnel experiments are presented, intended to measure dynamic forces and moments on simplified commercial vehicle type models rotating about their vertical centre line axis in smooth, uniform flow. These results are compared with quasi-static measurements on the same models over a range of fixed yaw angles (the conventional technique for wind tunnel test on this type of vehicle) in order to assess the significance of attempting to simulate the essentially dynamic nature of the full scale flowfield in a wind tunnel. It is shown that the magnitude of drag and yawing moment coefficients do not change significantly even at rotational rates as high as 64° s −1 . However the dynamic coefficient versus yaw angle curves appear displaced, relative to the yaw angle axis, compared to the static data. The magnitude and sign of this “phase shift” are seen to be dependent on both yaw rate and model geometry and are evident at rotational rates as low as 0.25° s −1 .

Measurement of shock wave unsteadiness using a high-speed schlieren system and digital image processing

A new method to measure shock wave unsteadiness is presented. Time-resolved visualizations of the flow field under investigation are obtained using a high-speed schlieren optical system and the motion of the shock wave is determined by means of digital image processing. Information on the shocks unsteadiness is subsequently derived with Fourier analysis. A sample study on shock unsteadiness in a shock-wave/turbulent boundary-layer interaction with separation is included. The method presented enables a measure of shock unsteadiness at locations in the imaged flow field not accessible by intrusive methods.

The effect of skewing on the vorticity produced by an airjet vortex generator

The vorticity production by a single vortex generator jet in a turbulent boundary layer at high speed and zero pressure gradient was studied by analysing surface-flow patterns. The investigation involved a range of jet mass flow rates and skew angles while the freestream Mach number and jet pitch angle were held constant. A qualitative analysis of the skin friction patterns demonstrated the effective role of the skew angle as a bifurcation parameter. The vortex production is classified into three main regimes as the skew angle increases from zero (fully downstream) to 180° (fully upstream). Better understanding of the physical mechanism behind the transition from the first regime to the second is crucial in the design of the airjet vortex generator

DEVELOPMENT OF CONTAINER-MOUNTED DEVICES FOR REDUCING THE AERODYNAMIC DRAG OF COMMERCIAL VEHICLES*

Abstract Aerodynamic drag coefficients are presented from a series of wind-tunnel tests carried out to determine the effectiveness of various container-mounted mouldings as drag-reducing devices. A 1 6th scale Ford D1614 cab/chassis model was fitted with a movable container in order that the cab-container gap could be varied to simulate either a rigid or an articulated vehicle. Mouldings were mounted on the container forebody and took the form of: (1) convex fairings of various external radii; (2) deflector vanes mounted both forward of and around the containers leading edges, with varying “air-gap”; and (3) semicircular and quadrant sections around the containers edges. All tests were carried out in the Cranfield Department of Aerodynamics 8 ft. × 6 ft. low-speed wind tunnel using a “ground plate” simulation technique, at tunnel speeds giving 1/3 full-scale Reynolds numbers at typical motorway cruise speeds of 50 m.p.h. Zero-yaw reductions in drag coefficient by 33% for rigid and 23% for articulated vehicles were achieved, and the corresponding figures at 20° yaw were 21% and 11% for rigid and articulated vehicles, respectively. Surface oil-flow and smoke-filament flow visualisation studies were included among the tests.

An investigation of the flow characteristics in the bootdeck region of a scale model notchback saloon vehicle

Abstract The results of an experimental investigation of the bootdeck flow structure of a 36 per cent scale model notchback car are presented together with a general review of advanced laser diagnostic techniques suitable for large-scale wind tunnel flow measurement. The tests were used to characterize the flow behaviour over the Reynolds number range 0.74 × 106-4.93 × 106 in the Cranfield University 2.4 m × 1.8 m wind tunnel. The experiments involved flow visualization, rear bootdeck and backlight three-dimensional stereoscopic particle image velocimetry (PIV) measurements, and mean static and unsteady static pressure measurements. Initial results from the flow visualization suggested flow asymmetries originating in the backlight region that were sensitive to the Reynolds number. The PIV data and static pressure data, however, showed little or no sensitivity of flow to Reynolds number with consistent flow structure and levels of unsteadiness from the backlight to the rear bootdeck region. At this stage no definitive reasons can be given for the discrepancies between the flow visualization and the other data, although the flow visualization data were particularly difficult to interpret near the backlight, and the near-wake structure may be bi-stable in this Reynolds number range. More detailed three-dimensional stereoscopic, time-resolved PIV flow data are now planned to quantify conclusively the correct flow structure and its sensitivities.

Optimisation of air-jet vortex generators with respect to system design parameters

A simple technique is proposed that allows the definition of the geometry and characteristics of an air-jet vortex generator to be defined, using the system requirements in practical design. Typically, the aircraft designer is concerned with the mass flow-rate and air pressure requirements of any pneumatic system for inclusion to an airframe. These parameters are not congruent with those for air-jet vortex generator aerodynamic design, and therefore, some tool is required to bridge the gap. Such a tool is proposed, based on empirical methods for the prediction of air-jet vortex generator behaviour. The technique allows the comparison of the vortex strength, and the system inputs (the jet mass flow-rate and the air-jet plenum pressure) for the air-jet, for a range of jet nozzle diameters and jet velocity ratios. Through this comparison, the optimum air-jet design can be reached for a given system input.

Some effects of ground clearance and ground plane boundary layer thickness on the mean base pressure of a bluff vehicle type body

Abstract The use of a moving ground plane is an established wind tunnel technique for ground simulation in surface vehicle tests. However, the data available relating to the influence of variations in ground plane boundary layer thickness on bluff body flowfields is limited. Results from a short series of wind tunnel tests, using a moving ground facility, are presented, in which mean base pressure measurements are made on three simple bluff body models while; (i) the moving ground speed, relative to the freestream speed, and (ii) the model ground clearance, are varied. The results show that reducing the moving ground speed has a similar effect on the base pressure to a reduction in the model ground clearance. This effect on base pressure is limited to moving ground speeds less than those which result in a significant increase in the ground boundary layer displacement thickness, except in the case of a more complex tandem module body. The data for this model, chosen to simulate a typical cab-container commercial vehicle, suggest that the presence of a gap between the front and rear modules increases significantly the sensitivity of the base pressure to the moving ground speed. Data from this limited study may be useful when comparing measurements made on vehicles with particularly small ground clearances above fixed and/or moving ground planes. They may also serve as a guide to the assessment of the tolerance of a moving ground system to belt speed variations in terms of the ground boundary layer displacement thickness.

Using singular systems analysis to characterise the flow in the wake of a model passenger vehicle

Abstract As the time-dependent fluid dynamics of wakes becomes important in industrial applications such as vehicle design, so techniques need to be found that enable these dynamics to be characterised. Whilst laser Doppler anemometry and particle image velocimetry are becoming widespread in their application, they are not necessarily suitable for this application due to their low rate of data capture when air is the working fluid. In this paper, a methodology that has already been applied successfully to low Reynolds number flows is applied to a turbulent wake. This involves the use of hot-wire anemometry to capture a large number of time series of velocity throughout the wake of a model road passenger vehicle. These time series are then analysed by a mathematical analysis tool known as singular systems analysis, which enables the low-frequency components of a noisy signal to be determined. This is done in the framework of non-linear dynamical systems theory so that the underlying dynamics of the wake can be determined. From this it is possible to characterise those areas of the wake where coherent dynamical structures are present and to explore the mechanism responsible for the oscillation of the wake. The paper reviews the background to singular systems analysis systems analysis and describes the application of the technique to the characterisation of the dynamics of the wake of a model vehicle placed in an open jet wind tunnel. Results are presented for three cross-flow planes in the wake where the structure of the wake is revealed in a new light. In particular, it is clear that the traditional picture of the vortex core appear to be present around the periphery of the vortex and in other areas where shear is apparent in the mean flow. The analysis technique allows the motion of these to be tracked downstream through the wake, whereas simpler analysis techniques do not allow such tracking to be carried out.

Aerodynamic and performance characteristics of a passive leading edge Kruger flap at low Reynolds numbers

An experimental and numerical study was performed on a Clark Y aerofoil with a 10% chord leading edge Kruger flap to examine its aerodynamic performance at Reynolds numbers of 0·6 × 10 6 , 1 × 10 6 , and 1·6 × 10 6 , to help to identify the forces and moments acting on a basic configuration. A detailed comparison of the numerical and experimental data is presented in this paper. The leading edge flap was effective at high angles of attack with an increase in C L of up to 18% over a conventional no flap configuration and delayed separation by up to 3°. The moments around the Kruger flap rotation point were calculated from the numerical analysis as an initial stage in the design of a UAV passive flap system and they are also presented in the paper.

Pressure measurements on aircraft wing using phase-shifted fibre Bragg grating sensors

Experimental results of static pressure measurements from specially packaged π-phase shifted optical fibre Bragg gratings (FBGs) that were installed into a 2D, 3 element high lift wing are presented. The static pressure measurements from the fibre sensors are found to be in good agreement with theory. Further pressure calibrations are currently under investigation.