Study of the aerodynamics of a full-scale tractor-trailer

2.1. Material

The overall setup of the Volvo VNL860 prototype tractor and NRC trailer in the Wind Tunnel as illustrated in Fig. 1. Drive-axle wheels of tractor are placed upon specially-made chock platforms that are connected to the balance. These chock platforms allow the vehicle to be rotated in yaw at angles of up to 12°. The front steering-axle wheels of the tractor and the axle wheels of the trailer are supported off-balance on horizontally unconstrained air bearings. The air bearing pads slide over floatation support surfaces mounted on the floor of the test section. The flotation support plates provide a smooth surface that is level with respect to gravity. In this configuration, drag force, side force, and yawing moment measurements are available from the six-component balance. The dimensions of the tractor, the dimensions for the trailer, and the test section layout are included in Table 1.

Fig. 1Setup for the full-scale tractor-trailer wind tunnel experiment with the Volvo VNL860 tractor at a yaw angle of 10°

2.2. Research method

Advancements in automotive solutions are being fueled by governmental mandates and escalating consumer demands for vehicles that exhibit greater energy efficiency [4]. Reducing aerodynamic drag represents one effective strategy for enhancing energy efficiency. A diverse range of test methodologies are utilized by industry professionals, researchers, and governmental bodies to assess the aerodynamics of heavy-duty trucks [5]. Data were acquired at each specified angle for 60 seconds, following a period of dwell time to allow the wind speed, and balance to stabilize. Balance data were acquired at 20 Hz. Data were acquired at each specified angle for 60 seconds, following a period of dwell time to allow the wind speed balance to stabilize. Balance data were acquired at 20 Hz. These limits correspond to a change in drag coefficient of 0.0021, a change in side force coefficient of 0.0052, and a change in yaw moment coefficient of 0.0007. It is well-established that aerodynamic drag can be managed through two primary mechanisms: enhancing pressure recovery in the wake region and minimizing separation across the vehicle’s body surfaces [5].

3.1. Test program

This article presents an analysis of aerodynamic drag sensitivity for various add-on devices utilized in heavy commercial vehicles [6]. Modifying the shapes of heavy vehicles to lower aerodynamic drag can lead to reduced fuel consumption [7]. The test campaign involved force, moment taken over a range of wind speeds and yaw angles. The test program included 10 tests with a tractor-trailer combination. The test plan is shown in Table 1 and lists the various aerodynamic packages, or drag reduction technologies that were installed for a given configuration. The five aerodynamic packages (AP) that were evaluated throughout the test are shown in Fig. 2. These include the adjustable air deflector (aero package 1), the black side extender extensions (2), the main white side extenders (3), the tractor skirt extensions (4), and the sun shade (5). Photographs of the vehicle, configuration-specific test notes, and a tabulation of the drag coefficients at 0° yaw are shown in Table 2. As described in Table 2, the air deflector position was optimized for the space width between the tractor and trailer. The bottom mounting hole on the air deflector arm and the third (middle) hole on the air deflector mounting bracket were used throughout the test.

Fig. 2Aerodynamic packages that were removed or installed during the test

3.2. Balance coefficients

The resistance exerted by air on a moving vehicle is termed aerodynamic drag force [8]. These measured data are transformed into the body axis and are provided as coefficients. The yaw direction ($\psi$) is positive clockwise when viewed from above. The yaw angle is corrected for wall effects using standard wall corrections. Hence, to diminish the drag force, alterations can solely be made to the Coefficient of Drag (Cd) and the frontal area (A), as both air density and vehicle velocity are predetermined by atmospheric conditions and regulatory speed limits for trucks. To decrease aerodynamic drag, the key lies in reducing the product of Cd × A [9].

4.1. Baseline vehicle combinations

Force and moment coefficients for the original vehicle state are compared with (Configuration 1) and without (Configuration 2) pressure taps in Fig. 3. Fig. 3(a) plots the speed sweep results and Fig. 3(b) plots the yaw sweep conducted in both configurations. The wind speed in Fig. 3(a) has been corrected for blockage effects and the yaw angle shown in Fig. 3(b) has been corrected for wall effects.

The presence of the pressure taps in Configuration 1 did not have a significant impact on the drag, side, and yawing moment coefficients. It should be noted that two yaw sweeps (Runs 28 and 29) were conducted for Configuration 1. Only the balance data from Run 29 are shown in Fig. 3(b). However, the pressure data between these two repeat runs are compared below. The wind-averaged drag coefficient for Configuration 1 (Run 29) was 0.527 and the wind-averaged drag coefficient for Configuration 2 (Run 32) was 0.527, based on a vehicle ground speed of 95 km/h. Additionally, the difference in the drag coefficient at 0° between these two configurations corresponded to a change in drag coefficient of 0.0024 (% difference of 0.5 %).

Fig. 3Force and moment coefficients for the baseline vehicle (Configuration 1: with pressure measurements. Configuration 2: without pressure measurements)

a) Speed sweep

b) Yaw sweep

4.2. Effect of drag reduction technologies

Throughout the test campaign, five aerodynamic packages were removed from the vehicle. These devices were shown earlier in Fig. 2. The drag coefficient at 0° and at 95 km/h for each configuration is shown in Fig. 4(a). For the yaw sweeps conducted for Configurations 1 and 2, the average of the drag coefficients for the 0° data points in the yaw sweep was used in the comparison. The change in drag (∆CD) associated wit a particular configuration was calculated by comparing the drag coefficient to a baseline reference case. Configurations 1 to 5a use Configuration 2 as a baseline, whereas Configurations 5b to 9 use Configuration 10 as a baseline. This was due to the variability associated with the re-installation of aero package 2 in Configuration 5a. The challenges associated with the positioning and repeatability of the installation of aerodynamic package 2 are documented in Table 2. Aerodynamic package 2 was adjusted in Configuration 5b and was maintained for the duration of the test.

An interesting observation was an interaction between the air deflector (aero package 1) and the side extender extensions (aero package 2). The removal of only aero package 2 in Configuration 3 resulted in a small increase in drag. When aero packages 1 and 2 were removed in Configuration 4, a larger increase in drag was observed. When only aero package 1 was removed in Configuration 5a, the magnitude of the drop was much larger than the magnitude of the increase seen for Configuration 3. This seems to indicate that the presence of one component alone (either the side extender extensions or the air deflector) has a dominant effect at controlling the flow in the gap.

The removal of aero packages 1, 2, and 3 (Configurations 6 and 7) resulted in the largest increase in drag coefficient (increase of 6 % compared to Configuration 10). The removal of the tractor skirt extensions in Configurations 7 and 8 did not have a significant impact on the drag coefficient obtained at 0° yaw, although it is expected that they would have a more significant influence when the vehicle is yawed. The removal of the sun shade in Configuration 9 resulted in a drop in drag coefficient of 0.0024, indicating that the sun shade has been optimized to have only a minor influence on drag.

Fig. 4Effect of drag reduction technologies on the blockage-corrected drag coefficient at 0° yaw and at 95 km/h

a) CD, c

b) ∆CD, c, relative to baseline. Configurations 1 to 5auseConfiguration 2 as a baseline and Configurations 5b to 9 use Configuration 10 as a baseline

Depending on the vehicle configuration and vehicle weight, the fuel consumption used for aerodynamic drag and the fuel consumption used for rolling resistance are approximately equal at 100 km/h for a vehicle weighing 97,000 lb. A rough approximation of the effect of an aerodynamic drag reduction on overall fuel consumption is that the overall fuel consumption would be reduced by half of the magnitude of the reduction in aerodynamic drag. A 6 % change in aerodynamic drag coefficient roughly corresponds to a 3 % change in overall fuel consumption, depending on vehicle speed, weight, and rolling resistance. Another method of determining the potential fuel savings from drag measurements is used by the US Environmental Protection Agency (EPA) Smart Way program. The method estimates fuel savings based on the percent change in wind-averaged drag and a conversion factor. For a vehicle travelling at 65 mph, the percent change in wind-averaged drag is divided by a factor of 1.85 to estimate the percentage of fuel savings associated with a particular technology.