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The solid-wall wind tunnel boundary correction method outlined in this paper is an efficient pressure-signature method that requires few wall-mounted pressures. These pressures are used to determine the strengths of model- and wake-representing singularities that are used with the method of images to calculate the longitudinal and lateral velocity increments induced by the wind tunnel walls. Two force correction models are presented that convert these velocity increments to force and moment corrections. The performances of the correction procedures are demonstrated by their application to data from two sets of four, geometrically identical, differently sized, simplified automotive models.

A 1:10-scale wind tunnel development program was undertaken by the National Research Council of Canada and Airshield Inc. in 1994 to develop trailer side skirts that would reduce the aerodynamic drag of single and tandem trailers. Additionally, a second wind tunnel program was performed by the NRC to evaluate the fuel-saving performance of boat-tail panels when used in conjunction with the skirt-equipped single and tandem trailers. Side skirts on tandem, 8.2-m-long trailers (all model dimensions converted to full scale) were found to reduce the wind-averaged drag coefficient at 105 km/h (65 mi/h) by 0.0758. The front pair of skirts alone produced 75% of the total drag reduction from both sets of skirts and the rear pair alone produced 40% of that from both pairs. The sum of the drag reductions from front and rear skirts separately was 115% of that when both sets were fitted, suggesting an interaction between both.

This paper presents applications of the Two-Variable method for the correction of solid-wall boundary interference of both wind tunnel and CFD data for a simplified automobile model at zero yaw angle and to a flat-plate wing over a 90° angle range. The latter model has flowfields that vary from those of a streamlined body at 0° yaw to those of a bluff body at 90° yaw. The Two-Variable method utilizes measurements on the wind tunnel walls to estimate the interference velocity components induced by the solid boundaries. The correction of the forces and moments from these interference velocities are obtained by Hackett's force model. The paper compares this method to a simpler analytical method that is more practical to apply in closed-wall wind tunnels. It is shown that the effect of the wind tunnel walls or CFD domain boundaries can accurately removed by these techniques for model/domain area ratios of up to 0.15.

This paper is the first of two papers that address the simulation and effects of turbulence on surface vehicle aerodynamics. This, the first paper, focuses on the characteristics of the turbulent flow field encountered by a road vehicle. The natural wind environment is usually unsteady but is almost universally replaced by a smooth flow in both wind tunnel and computational domains. In this paper, the characteristics of turbulence in the relative-velocity co-ordinate system of a moving ground vehicle are reviewed, drawing on work from Wind Engineering experience. Data are provided on typical turbulence levels, probability density functions and velocity spectra to which vehicles are exposed. The focus is on atmospheric turbulence, however the transient flow field from the wakes of other road vehicles and roadside objects are also considered.

This paper summarises the effects of turbulence on the aerodynamics of road vehicles, including effects on forces and aero-acoustics. Data are presented showing that a different design of some vehicles may result when turbulent flow is employed. Methods for generating turbulence, focusing on physical testing in full-size wind tunnels, are discussed. The paper is Part Two of a review of turbulence and road vehicles. Part One (Cooper and Watkins, 2007) summarised the sources and nature of the turbulence experienced by surface vehicles.

The National Research Council of Canada (NRC) has completed the second round of full-scale wind tunnel tests on Class-8 tractor-trailer combinations. The primary intent of the program is to effect a reduction in greenhouse-gas emissions by reducing the fuel consumption of trucks through aerodynamic drag reduction. Add-on aerodynamic components developed at the NRC several decades ago have become important contenders for drag reduction. This program has encouraged the commercialization of these technologies and this round of tests evaluated the first commercial products. Three primary devices have been evaluated, with the combination able to reduce fuel consumption by approximately 6,667 liters (1,761 US gal) annually, based on 130,000 km (81,000 miles) traveled per tractor at a speed of 100 km/hr (62 mi/hr).

This paper provides a method that corrects errors induced by the empty-tunnel pressure distribution in the aerodynamic forces and moments measured on an automobile in a wind tunnel. The errors are a result of wake distortion caused by the gradient in pressure over the wake. The method is applicable to open-jet and closed-wall wind tunnels. However, the primary focus is on the open tunnel because its short test-section length commonly results in this wake interference. The work is a continuation of a previous paper [4] that treated drag only at zero yaw angle. The current paper extends the correction to the remaining forces, moments and model surface pressures at all yaw angles. It is shown that the use of a second measurement in the wind tunnel, made with a perturbed pressure distribution, provides sufficient information for an accurate correction. The perturbation in pressure distribution can be achieved by extending flaps into the collector flow.

The National Research Council of Canada (NRC) is commencing a new round of aerodynamic development of heavy trucks in partnership with Natural Resources Canada (NRCan), the Canadian Trucking Alliance (CTA) and the US Department of Energy (DOE). The program is meant to take second-generation, add-on technology from the wind tunnel to the fleet. The purpose is to reduce fuel consumption and greenhouse gas emissions. The benefit is that the fuel reductions pay the operators to improve their vehicle emissions. 1:10-scale model tests in the NRC 2m × 3m wind tunnel, followed by full-scale tests on a Navistar 9200 Day Cab with 40-foot trailer in the NRC 9m × 9m wind tunnel, were employed to develop the add-on devices of interest. The results demonstrated significant fuel savings from a combination of longer cab extenders, trailer skirts and trailer boat-tails that reduced fuel consumption as much as the contemporary aerodynamic cab packages.

The influence on aerodynamic drag of a non-uniform, streamwise pressure distribution over the wake of an automobile model in both open-jet and closed-jet wind tunnels is considered in this paper. It has long been an unsolved issue in the theory of open-jet interference and is usually not important in closed-wall wind tunnels unless the model is very long. A new, semi-empirical approach is presented that is based on the observation that the drag changes due to a pressure gradient over a wake correlate with the empty-test-section pressure-coefficient difference between the base of the vehicle and the position of wake closure. A method is demonstrated that is able to remove the effect of the pressure gradient and that is not buoyancy related. This method is applied to a range of simplified and detailed automobile shapes at model scale and at full scale in various wind tunnels, as well as to normal flat plates.

The measurement of the top speed of a racecar on a test track is commonly used in the aerodynamic development of the car and as a verification of wind tunnel results. The speed differences resulting from typical drag and lift changes will be small, requiring precise (i.e. repeatable) speed measurement. Unfortunately, changing environmental conditions over one or several days of trials will make these results unreliable at the required level of accuracy. This paper discusses the errors encountered in top speed testing and suggests methods to improve accuracy.

During the wind-tunnel development of racing vehicles, modifications are made to the vehicles in an effort to minimize lap times. The optimal configuration is usually a function of the chassis, powertrain and track; so several different aerodynamic configurations are required to maximize performance throughout a season. The results of a wind tunnel test are typically drag and lift measurements but the desired information is the change in lap time. This paper proposes a method to relate wind tunnel measurements to on-track results using a simple performance simulation. Though relatively straightforward, this technique has been observed to improve the efficiency and outcome of several wind tunnel tests.

The aerodynamic effects of the pickup truck tailgate are examined in this paper. It is shown that the removal or the lowering of the tailgate increases the aerodynamic drag of a pickup truck, increases lift by up to sixty percent and increases the yawing moment. All these changes are negative and reduce vehicle performance, albeit, only by small amounts. This finding demonstrates that the commonly seen removal of tailgates to reduce aerodynamic drag is a public misconception that should be discouraged by manufacturers and by regulators.

During the late 1970's and early 1980's considerable effort was expended in the improvement of truck aerodynamics to reduce fuel consumption. This first-generation effort focused on aerodynamic drag reduction obtained from add-on aerodynamic aids to the cab or the trailer, from improved cab shaping and from body/trailer front-end edge rounding. Rising fuel prices have renewed interest in further aerodynamic improvements. This paper will review past developments and show that several unused concepts offer potential as second-generation, add-on, fuel-saving technology. It will raise the issue of finding successful means for bringing them profitably into service, which will require concerted action by the trucking industry, manufacturers and government.

Underbody diffusers are used on racing cars to generate large downforce that will permit them to achieve reduced lap times through aerodynamically-enhanced traction. Both the configuration of these cars and their underbody flows are complex, so the design of optimum underbody geometries is a formidable task. The objective of the present study is to generate data and understanding that will facilitate design through knowledge of the relevant physics and the application of a numerical analysis that provides generalised design guidance. The approach taken is one that is traditional in the study of complex problems: to identify a less-complex but still relevant sub-problem that has the key elements and flow physics of the main one, and study it to generate a first phase of cause-and-effect relationships. In addition to having immediate utility, it can serve as the foundation upon which future research activity can be built.