Heavy Truck Drag Reduction Obtained from Devices Installed on the Trailer

The flow-field around a “common” European heavy truck, equipped with several different trailer devices, is investigated using steady and unsteady simulations. This work demonstrates how with simple devices added on the trailer it is possible to strongly decrease the aerodynamic drag over 10%, with an increase of overall dimensions below 1% without any change to the load capacity of the trailer. Several devices, installed on the trailer, are tested on a target vehicle and the shape of the “airbag”, the “fin”, the “boat tail” and the “front-rear trailer device” has been optimized to achieve the maximum in drag reduction in front wind. The performance of the optimized devices are tested also in cross wind conditions with the yaw angle varying from 0° to 30°. The truck equipped with the front-rear trailer device is also investigated using time variant simulation with yaw angle of 0°, 5°, 10°. Detached eddy simulation (DES) with one equation Spalart-Allmaras as turbulence model is used to perform the analysis of the unsteady flow; the two equation k-ω SST turbulence model is selected for the steady-state RANS simulations. The unsteady simulations are performed using computational meshes on the order of 40 million of elements while RANS simulation are done with mesh of 10 million of elements. The analysis is performed in the open-source CFD framework OpenFOAM®. This is the accepted version of Salati L. , Cheli F. , Schito P., Heavy Truck Drag Reduction Obtained from Devices Installed on the Trailer (SAE Int. J. Commer. Veh., 2015, 8:2, 747-780). The final publication is available at: http://dx.doi. org/10.4271%2F2015-01-2898 Figure 1. Comparison between European and American regulation regarding heavy truck maximum dimension. The main areas that influence the overall drag coefficient of the heavy truck are: • the front corner radius of the tractor (Palowski [4]); • the tractor-trailer gap (Hucho [5]); • the underbody of both trailer and tractor (Buchheim et al.[6], and Howell [7]); • the interaction between the tractor and the trailer (Gilhaus[8]). A wide range of devices can be developed for drag reduction on the tractor while for the trailer less solutions are practically used in the industry due to the limits of the current regulation and for technical issues (for example the access to crucial area of the trailer for maintenance, etc.). Also economic reasons influence the current “box” shape of the trailer: box shaped trailers have higher load capacity compared to stream-line shape, easy access to the merchandise, easy delivery as a container on shipping boat, street dimension (height of the tunnel), etc. Another crucial aspect to take into account is the safety requirement of these devices: they must not increase the risk of injury for the passengers in case of crash with others vehicles. The most common drag reducing devices are: • cab roof fairings and side fairings (9-17% drag reduction Drollinger [9]; Cooper [10]; Leuschen and Cooper [11]); • trailer-front fairing (7-10% drag reduction Garry [12]; Watkinset al. [13]); • tractor and trailer side-skirts (4-6% drag reduction Garry [14]; Ingram [15]); • boat-tail and base flaps (7.5 % drag reduction McCallen et al. [16]; Cooper [10]). In recent years some studies have been performed on active drag reduction devices, such as boundary layer suction, but their complexity and the necessity to provide energy for their operation did not allowed to these devices to reach the market. Another interesting way to reduce drag for the heavy truck transport is platooning as shown in Schito [17]. This solution requires a significant change in the infrastructure and in the driving behavior of the vehicle surrounding the platoon. For all these reasons commercial platooning is currently not available. One of the main issues in the truck aerodynamics, is the clearance gap that has to be provided between tractor and trailer. If the gap is very small, the turning capacity of the vehicle will be strongly affected, while, with a large gap, the vehicle tends to behave as two separate bodies that generate twice the drag of a single body. This effect is amplified in cross-wind when the vehicle is yawed relative to the wind direction (Gilhaus [8]). In this paper external devices that can be applied to the existing truck shape will be studied and developed in order to increase the aerodynamic efficiency of the truck, decreasing the fuel consumption of the vehicle with both economic and environmental benefit. The idea is to keep fixed the internal load capacity of the trailer and no radical changes on the truck body shape will be introduced: the proposed devices will be fixed to the existing trailer geometries. External devices can be easily applied to existing trucks with limited cost and high benefit in terms of fuels saving and installation costs. It must also be noticed that in this way no significant change must be done to logistic centers or shipping boats or in general to the whole commercial transport system. No modifications are proposed on the tractor since the modifications may influence also other features such as cooling, combustion efficiency and engine area arrangement. The analysis will be conducted using open-source CFD software as OpenFOAM® and Paraview® [19], analyzing the flow around the truck using state-of-the-art solvers and methods.