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Journal Article

Wind Tunnel and Track Tests of Class 8 Tractors Pulling Single and Tandem Trailers Fitted with Side Skirts and Boat-tails

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.
Technical Paper

Truck Aerodynamics Reborn - Lessons from the Past

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.
Technical Paper

The Use of Performance-Simulation Maps to Improve Lap Times through Wind Tunnel Testing

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.
Technical Paper

The Unsteady Wind Environment of Road Vehicles, Part Two: Effects on Vehicle Development and Simulation of Turbulence

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.
Technical Paper

The Unsteady Wind Environment of Road Vehicles, Part One: A Review of the On-road Turbulent Wind Environment

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.
Technical Paper

The Two-Variable Boundary-Interference Correction Applied to Automotive Aerodynamic Data

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.
Technical Paper

The Effect of Handlebar Fairings on Motorcycle Aerodynamics

A series of wind tunnel experiments were used to quantify the mean and unsteady aerodynamic forces on a motorcycle fitted with a handlebar fairing and a handlebar windshield. The results have shown that the positive aerodynamic yaw damping of the steerable front frame tends to stabilize wobble mode when a handlebar device is present. The mean front-frame yawing moments as a function of yaw angle were harder to interpret without use of a dynamic simulation. Static arguments were advanced to suggest that cross-wind response may be reduced with a fairing or windshield mounted.
Technical Paper

The Effect of Front-Edge Rounding and Rear-Edge Shaping on the Aerodynamic Drag of Bluff Vehicles in Ground Proximity

Wind tunnel measurements on a rectangular vehicle-like shape and on two detailed, scale-model trucks have been employed to define the front and rear edge geometries that minimize aerodynamic drag. Optimum configurations are identified with sufficient detail for commercial vehicle design purposes. Comparisons of the model-scale measurements with limited measurements on a full-scale straight truck in a large wind tunnel support the interpretation of these test results.
Technical Paper

The Aerodynamic Performance of Automotive Underbody Diffusers

This paper examines the aerodynamic behaviour of plane-walled, single-plane-expansion, underbody diffusers fitted to a wind-tunnel model of a wheel-less, simple body having automobile proportions. The measurements were performed over a moving-belt assembly in the Pilot Wind Tunnel of the National Research Council of Canada (NRC). The purposes of the investigation were: to understand the governing physics of automotive underbody diffusers operating in ground proximity, to examine the effect of moving-ground and fixed-ground simulations on the behaviour of such diffusers and on the corresponding vehicle downforce and drag, to map the performance of simple, quasi-two-dimensional diffusers when used to produce downforce or drag reduction.
Technical Paper

Pickup Truck Aerodynamics - Keep Your Tailgate Up

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.
Technical Paper

Model and Full-Scale Wind Tunnel Tests of Second-Generation Aerodynamic Fuel Saving Devices for Tractor-Trailers

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.
Technical Paper

Full-Scale Wind Tunnel Tests of Production and Prototype, Second-Generation Aerodynamic Drag-Reducing Devices for Tractor-Trailers

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).
Technical Paper

Effect of Ambient Conditions on the Measured Top Speed of a Winston Cup Car

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.
Technical Paper

Correlation Experience with the SAE Wind Tunnel Test Procedure for Trucks and Buses

Six-component aerodynamic force and moment data are compared from tests of four 1/10-scale truck models at four wind tunnels on the North American Continent. Each model was tested in both a standard and a fuel saver configuration. The differences observed between tunnels were consistent for each aerodynamic component regardless of truck or configuration. The zero-yaw drag coefficients from each of the four tunnels were within ± 4 percent of the mean coefficients. The coefficient magnitudes for the other aerodynamic components showed variations that were often many times larger than those for drag. The variations between the incremental coefficients for all six aerodynamic components were less than those found for the coefficient magnitudes. A qualified comparison was made between the average drag coefficient magnitudes from the wind tunnels and those available from some on-road coastdown tests of the full-scale vehicles.