Reduction of drag in heavy vehicles with two different types of advanced side skirts

doi:10.1016/j.jweia.2016.04.009

Journal of Wind Engineering and Industrial Aerodynamics

Volume 155, August 2016, Pages 36-46

https://doi.org/10.1016/j.jweia.2016.04.009 Get rights and content

Highlights

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Modified side skirts are proposed to reduce the aerodynamic drag of heavy vehicles.
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Drag coefficient of the vehicle is reduced by more than 5% in wind tunnel tests.
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Flow characteristics of vehicle with and without side skirts are compared with LES.

Abstract

Investigating the aerodynamic reduction of drag in heavy vehicles, such as trucks or tractor-trailers, has considerable significance given the strong influence on related industries. The underbody flow that passes through the underside of heavy vehicles induces considerable drag while interacting with rolling wheels and other structures. Nonetheless, the reduction of drag caused by underbody flow has received less attention than that attributed to upper and forebody flows. Side skirts are common underbody drag-reduction devices that consist of straight panels curtaining the underspace between the front and rear wheels to control the underbody flow in the ground clearance. In this study, we propose two different types of side skirts with flaps or additional inclined inner panels to maximize drag reduction. Effects of these devices are quantitatively evaluated by wind tunnel tests and computational fluid dynamics analysis. In wind tunnel tests with 1/8 scaled-down vehicle models, drag coefficient is reduced by more than 5% for both side skirts. Effects of various physical dimensions or angle variations on drag reduction are determined. Large-eddy simulation (LES) estimated similar drag reduction with reduced vortical activities, loss of streamwise momentum, strength of turbulent kinetic energy and global pressure difference, compared to the case without side skirts.

Introduction

The oncoming shortage of fossil fuels have encouraged research on energy saving, especially fuel saving with effective flow control methods. Among these issues, the aerodynamic reduction of drag in heavy vehicles, such as trucks or tractor-trailers, has considerable practical significance owing to the strong influence on the industry and logistics. Many studies have been conducted to reduce the aerodynamic drag in heavy vehicles (Allan, 1981, Ahmed et al., 1985, Hucho and Sovran, 1993, Cooper, 2003). The drag caused by underbody flow passing through the underside of heavy vehicles deserves as much attention as that attributed to upper and forebody flows. The flow passing through the underbody interacts with complicated undercarriage structures, such as rolling tires, axles, frames, and various mechanical devices. Hence, considerable aerodynamic drag is induced by the underbody flow. Wood (2006) reported that the underbody flow of a tractor-trailer contributes approximately 30% to the total aerodynamic drag. Wickern et al. (1997) found that rolling tires account for 25% of the total aerodynamic drag in a passenger car. Drag is significantly increased when tires of multiple truck or tractor-trailer are exposed to large ground clearance. Choi et al. (2014) recently mentioned that large-scale flow structures existing in the relatively high ground clearance of heavy vehicles actively interact with the vehicle underbody, working as a non-negligible source of aerodynamic drag. Therefore, the underbody flow passing through the ground clearance between the vehicle underbody and the ground needs to be controlled effectively to substantially reduce the drag of heavy vehicles. Cooper and Leuschen (2005) demonstrated effective fuel savings of drag-reducing add-on devices including trailer skirts. The resultant drag coefficient was reduced about 9.5% with the attachment of long side skirts at a wind speed of 24.6 m/s and zero yaw angle. Landman et al. (2009) specified practical limitations in achieving drag reduction of a modern tractor-trailer by adopting add-on devices, including side skirts, a full gap seal, and tapered rear panels. Drag coefficient of the tested side skirts was reduced in the range from 15.7% to 19.9%. Ortega et al. (2013) conducted full-scale wind tunnel tests to evaluate the effect of flow-control devices including various trailer skirts on the drag reduction of three heavy vehicles. The top performing side skirts reduced wind-averaged drag coefficient by 0.062. Belly box is another type of underspace structure for enclosing wheels. This belly box reduced drag coefficient by 38% (Storms et al., 2004). Leuschen (2013) carried out wind tunnel experiments for tractor-trailer models with rolling wheels to examine the ground effect and influence of spinning wheels. McAuliffe (2015) performed wind-tunnel tests to evaluate the aerodynamic performances of various tractor-trailer vehicle configurations using 30% scale-down model, in which spinning wheels were reproduced to simulate the ground-effect on the underbody flow with side skirts.

Three representative types of underbody drag-reduction devices have been introduced to control the underbody flow of heavy vehicles: undercarriage straight skirts, belly boxes, and undercarriage wedge skirts (Choi et al., 2014). Undercarriage straight skirts are straight panels affixed to the undersides of trucks or tractor-trailers. They are usually known as side skirts. These skirts usually curtain the underspace between the forward and rear wheels of a vehicle. Side skirts not only prevent the intrusion of unexpected objects for safety reasons, but also control the underbody flow in ground clearance to reduce the aerodynamic drag caused by turbulent gap flow.

Although a few products for heavy vehicles are commercially available, an advanced type of side skirt that can maximize the drag-reduction effect needs to be developed. In the present study, two different types of advanced side skirts are proposed. The first side skirt has front or rear flaps bent inward, which are expected to guide the flow around the rolling tires and the edge of skirt panel. For effective flow guidance, the appropriate position and angle of the bent flaps for maximum drag reduction are experimentally investigated. The other side skirt has additional inclined flap panels, which are adopted to smoothen the underbody flow and isolate the complicated flows around the rolling tires or other vehicle underbody structures. The drag-reduction effects of these two side skirts are evaluated quantitatively based on both wind tunnel experiments using a 1/8 scale model and computational fluid dynamics (CFD) analysis.

Section snippets

Wind tunnel and drag-coefficient evaluation

Wind tunnel tests are conducted in a closed-return type subsonic wind tunnel with a test section of 1.8 m high, 1.5 m wide, and 4.3 m long. The maximum speed is 75 m/s and the contraction rate is 9:1. Flow uniformity and turbulence intensity are less than 0.25% and 0.2%, respectively. Freestream wind speed is monitored with a micro-manometer (FCO510, Furness Controls) connected to a pitot tube attached at the ceiling of the wind tunnel test section. The pitot probe was located at 1.1 m above the

Computational methodology and simulation results

Turbulent flow around a heavy vehicle is simulated with realistic geometric details using an in-house large-eddy simulation (LES) code, which can highlight unsteady flow physics in sufficient detail. The filtered incompressible Navier–Stokes equations are solved with a dynamic global-coefficient subgrid-scale model (You and Moin, 2009) on an unstructured grid. A fully-implicit fractional-step method is employed as a time-integration technique while all terms in the Navier–Stokes equations are

Conclusions

The effects of the two different types of side skirts proposed in this study on the reduction of drag in heavy vehicles are quantitatively evaluated through a wind tunnel experiment conducted with a 1/8 scale-down model. The maximum reduction rates in drag coefficient relative to the vehicles without the side skirts are 5.3% and 4.7% for the 15-ton truck and 40-foot trailer vehicle models installed with the tested flap-type side skirts, respectively. The drag reduction effects are maximized

Acknowledgments

This study was conducted for the 2nd year research requirements of Development of aerodynamic technologies for efficient road freight transport, which is supported by the KAIA in the, South Korea. (NTIS 1615007940).

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View full text

Reduction of drag in heavy vehicles with two different types of advanced side skirts

Highlights

Abstract

Introduction

Section snippets

Wind tunnel and drag-coefficient evaluation

Computational methodology and simulation results

Conclusions

Acknowledgments

J. Wind. Eng. Ind. Aerodyn.

Aerodynamics of road and rail vehicles

Veh. Syst. Dyn.

Aerodynamics of heavy vehicles

Ann. Rev. Fluid Mech.

Aerodynamics of road vehicles

Annu. Rev. Fluid Mech.

Considerations for the wind tunnel simulation of tractor-trailer combinations: correlation of full- and half-scale measurements

SAE Int. J. Commer. Veh.