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Technical Paper

Comparison of On-Road and Wind-Tunnel Tests for Tractor-Trailer Aerodynamic Devices, and Fuel Savings Predictions

Wind tunnels which are large enough for full-scale trucks are rare, and the cost of satisfactorily-detailed models for smaller tunnels is high. The work presented shows the results from the application of a method which provides an over-the-road evaluation of the incremental changes in fuel consumption and drag coefficient produced following the addition of a variety of aerodynamic drag reducing devices to a tractor-trailer truck combination. The devices tested were an aerodynamic sunvisor, a roof-mounted air deflector, cab extenders, cab skirts, a trailer nose fairing, a set of trailer quads (quarter-rounds), and trailer skirts which were mounted on a low-forward-entry tractor and high box-van trailer. The significant differences between the wind tunnel and on-road drag reductions suggest that the effects of on-road wind turbulence can substantially reduce the wind tunnel results even though a 1.5% turbulence intensity level was used in the tunnel experiments.
Technical Paper

Design of Adaptive Airfoil Control for Unmanned Aerial Vehicles using Smart Materials

Smart material is a suitable candidate for adaptive airfoil design as it can be customized to generate a specific response to a combination of inputs. Shape memory alloy (SMA) in particular is lightweight, produces high force and large deflection which makes it a suitable candidate for actuator in the adaptive airfoil design. By attaching SMA wires inside the airfoil, they can be activated to alter the shape of the airfoil. Placement of the actuator is crucial in obtaining the desired change of the airfoil camber. This paper proposed a design for the morphing wing aimed at changing the camber of the airfoil during cruise in order to increase the lift-to-drag ratio. Finite Element Method (FEM) analysis predicted the deformed airfoil geometry when the SMA wires were fully actuated. Numerical results are presented along with issues related to the fabrication of the morphing wing and implementation of the SMA actuator.
Technical Paper

The Effect of Upstream Turbulence on an Exposed Wheel Wake

As open-wheeled racing cars frequently race in close proximity, a limiting factor on the ability to overtake is the aerodynamic performance of the vehicle while operating in a leading car's wake. Whilst various studies have examined the effectiveness of wings operating in turbulent flow, there has been limited research undertaken on the aerodynamic effect of such conditions on wheels. This study describes the influence of upstream turbulence on the wake flow features of an isolated wheel, since the flow field of a wheel will generally be turbulent (due to the wakes of upstream cars and/or bodywork). Pressure distributions and velocity vector plots are examined, which were obtained using a four-hole pressure-sensitive Cobra probe on a traverse 2.5 diameters downstream of the wheel axle line, in smooth and turbulent flow.
Technical Paper

Development of a Small-Scale Aeroacoustic Open Jet, Open Return Wind Tunnel for Cavity Noise and Component Testing

A small-scale aeroacoustic wind tunnel has been designed and built to investigate tonal cavity noise in the frequency range applicable to passenger vehicles; 1 - 16 kHz. The tunnel is required for testing associated with an investigation into tonal cavity noise on passenger-vehicle wing mirrors. It was designed to operate in the low subsonic speed range (60 - 140 km/h) with a nozzle exit cross-sectional area of 0.02 m2 and a 4:1 aspect ratio. The design was intended to achieve a smooth, quiet flow facility. In this paper the design process is summarised and the factors leading to particular design decisions are detailed. An initial evaluation has shown that only minimal changes are required to achieve very smooth, even flow at the nozzle exit at all required test speeds. The acoustic design needs further work as there is a significant amount of flow noise at the nozzle exit between 1 and 13 kHz.
Technical Paper

Periodic Structures within the Formation Region of Trailing Vortices

Water tow-tank tests were performed for the Ahmed model at a range of “high-drag” backlight angles at Reynolds numbers of up to 1.3 × 105. Dye was injected just upstream of the c-pillars and visualizations were recorded with a submerged CCD camera moving with the model. Discrete sub-vortices were found to be shed periodically along the length of the c-pillar at Strouhal numbers (based on square root of frontal area) between 8 and 12. These sub-vortices were observed to undergo vortex pairing and then to roll up into the familiar c-pillar vortices. These observations are consistent with previously published observations for delta wings. Wind tunnel tests were performed in order to provide Reynolds numbers of up to 1.6 × 106. These revealed some spectral features which could be due to the shedding and pairing of discrete vortices from the c-pillar but the evidence was much less conclusive than at low Reynolds number.
Technical Paper

Pressure Fluctuations on Automotive Rear View Mirrors

The function of a rear view mirror is a determining factor in its shape - resulting in a flat rear mirrored face. The resulting bluff body generates unsteady base pressures which generate unsteady forces, leading to movement of the mirror surface and potential image blurring. The objective of this paper was to experimentally determine the fluctuating base pressure on a standard and modified mirror. Half a full-size vehicle was utilised, fixed to the side wall of a wind tunnel. A dynamically responsive multi channel pressure system was used to record the pressures. The modification to the mirror consisted of a series of extensions to the mirror rim, to see if this method would attenuate the fluctuating base pressures. It was found that increasing the length of the extension changed the pressure pattern across the face, and the over all magnitude of the fluctuations reduced with increasing length of extension. It was recommended to further the work via phase measurements.
Technical Paper

Aerodynamic Testing of a Vented Disc Brake

Many attempts have been made to improve automotive brake cooling by increasing the pumping action of vented brake rotors, both experimentally and using computational fluid dynamics. Testing of these improvements has occurred by measuring the airflow at the outlet of a rotating brake rotor in still air, however this is a vastly different environment to the actual working condition of the rotor. Airflow around the rotor, as a result of the forward movement of the vehicle, will have a considerable effect on its pumping ability. In this paper a comparison is made between the measured airflow through a straight-vane vented disc: (1) isolated disc still air; (2) disc in still air with the wheel on; (3) disc in moving air with the wheel on; and (4) on road simulation using a ¼ car. Both time-averaged and real-time measurements are presented. In the still air tests results showed a linear relationship between rotational velocity and airflow through the disc.
Technical Paper

On The Causes of Image Blurring in External Rear View Mirrors

Effective rear view vision from external mirrors is compromised at high speed due to rotational vibration of the mirror glass. Possible causes of the mirror vibration are reviewed, including road inputs from the vehicle body and a variety of aerodynamic inputs. The latter included vibrations of the entire vehicle body, vibrations of the mirror “shell”, the turbulent flow field due to the A-pillar vortex (and to a lesser extent the approach flow) and base pressure fluctuations. Experiments are described that attempt to understand the relative influence of the causes of vibration, including road and tunnel tests with mirrors instrumented with micro accelerometers. At low frequencies, road inputs predominate, but some occur at such low frequencies that the human eye can track the moving image. At frequencies above about 20Hz the results indicate that at high speeds aerodynamics play a dominant role.
Technical Paper

Effects of Vehicle A-pillar Shape on Local Mean and Time-Varying Flow Properties

Separated flow is the main generator of aerodynamic noise in passenger vehicles. The flow around the A-pillar is central to the wind noise as many modern vehicles still have high fluctuating pressures due to flow separations in this region. Current production vehicle geometry is restricted due to the amount of three dimensionality possible in laminated windscreen glass (and door opening etc). New materials (e.g., polycarbonate) offer the possibility of more streamlined shapes which allow less or no flow separation. Therefore, a series of experimental investigations have been conducted to study the effects of the A-pillar and windshield geometry and yaw angles on the local flow and noise using a group of idealised road vehicle models. Surface mean and fluctuating pressures were measured on the side window in the A-pillar regions of all models at different Reynolds numbers and yaw angles.
Technical Paper

Transient Wind Noise

Wind noise sources are described including those from the A-pillar region, cavities and bluff bodies. Hydrodynamic pressure fluctuations results from flow separations (in such areas as the A-pillars and mirrors) that generate relatively broad band in-cabin noise. The influence on local radii of the A-pillar is outlined and shown to be a dominant factor in determining hydrodynamic pressure fluctuations in the side-glass regions. Small cavities (eg. styling or water management channels on the mirror casing) generate high-frequency acoustic tones that can also be heard in the cabin and an example of tones from a whistling mirror cavity is shown. A spectrogram of in-cabin noise obtained whilst driving in strong winds is used to illustrate the variability of noise that can be heard on-road and to consider the influence of the relative wind speed.