Aerodynamic analysis of a platoon of bluff bodies subjected to cross wind, a numerical investigation on the effect of drag reduction devices

Due to the awareness of climate change, more sustainable and efficient ways of transport are needed. The most polluting road transport mode in the European Union are the long-haul tractor-trailer combinations. This study will investigate the aerodynamic characteristics of three vehicles in a platoon subjected to cross wind conditions. The effect of front- and rear drag reduction devices is investigated numerically. The frontal edge radius is considered the front drag reduction device while the rear drag reduction device is represented by a boat tail. ANSYS Fluent was used to solve the RANS equations that were closed with the SST k-omega turbulence model. The GETS model was used to analyse the aerodynamic characteristics of a simplified heavy-duty vehicle. The geometry was meshed and a mesh sensitivity study showed the asymptotic behaviour of the drag as a function of mesh size. It was shown that the frontal edge radius influences the flow behaviour largely. For the radii of 0.54 m and 0.27 m, a large thrust force on the frontal edges decreased the drag of the front part of the vehicle significantly such that the drag contribution of the front part was lower than 15% of the total drag while the pressure drag at the rear delivered around$70% of the drag. Halving the frontal edge radius to 0.135 m, caused the drag contributions of the front and rear part of the vehicle to be about 45% of the total drag. Adding an inward deflected tail increased drag drastically by increasing the base pressure. Adding a cross wind component to the incoming flow increased the drag and gave a side force. The inter-vehicle distance had a large influence on the drag of the individual vehicles. At a very short distance, the lead vehicle experienced a very large drag reduction because of the presence of the high pressure region in front of the middle vehicle. This lead to a large reduction in pressure drag. When the distance is increased, the drag goes asymptotically to the value in isolation. The trailing vehicle showed opposite behaviour. The drag of the trailing vehicle was increased when the inter-vehicle distance decreased due to the fact that the streamlines were deflected inward after the middle vehicle and they experienced a deceleration due to the concave trajectory of the flow arriving at the frontal surfaces of the trailing vehicle. This lead to a decreased thrust force on the frontal edges of the trailing vehicle. For the lowest inter-vehicle distance, the trailing vehicle was positioned inside the near-wake of the middle vehicle and the drag decreased again. The middle vehicle experienced a combination of the effects on the lead and trailing vehicle, its drag remains fairly constant except for the closest distance where the negative effect on the frontal part started to decrease. These trends are similar for all platoons, the amount of drag increase or decrease however was determined by the geometric variables and the cross wind condition. The drag reductions of the different vehicles in the platoon were mainly determined by the frontal edge radius. The relative drag reductions on the lead vehicle were higher when the frontal edge radius was larger, since then the pressure drag at the rear represents a larger portion of the total drag. The drag decrease in drag counts however only slightly changed. For the large radii, the trailing vehicle experienced a drag that increased up to a value above that in isolation when the inter-vehicle distance was decreased. A trailing vehicle with the smallest frontal edge radius, 0.135 m, experienced a drag reduction. The combined effects of the lead and trailing vehicle caused the drag reductions of the middle vehicle to increase for a lower frontal edge radius. The drag reductions decreased when tails were added or deflected inwards. A tail on a specific vehicle mainly influences the vehicle driving behind the tail. Deflecting a tail decreased the drag reductions caused by the slower incoming flow field, i.e. the lower dynamic pressure experience by the following vehicles. Deflecting a tail on the lead vehicle inward has the same effect on the middle vehicle as increasing the inter-vehicle distance, it decreases the influence of the lead vehicle's wake on the middle vehicle. For a frontal edge radius of 0.27 m, the total drag reduction of a following vehicle was decreased when the tail was deflected from 0° to 6°, but it was increased when deflecting the tail from 6° to 12°. Decreasing the frontal edge radius to 0.135 m gave different results. The total drag of the following vehicle was increased when the tails of the vehicle in front were deflected more inward. Overall, the drag reductions obtained in a platoon were not much affected by the applied cross wind conditions. It is shown that the lead vehicle of a platoon redirects the flow such that the middle and trailing vehicle experience a significantly lower side force. The side force of the lead vehicle itself is not changed significantly. When inward deflected tails are applied to the lead and middle vehicle, the side force reduction on the following vehicles is diminished but still significant. The final results showed that the platoon with the worst performing vehicles in isolation, the `nnn' configuration with R = 0.135 m, experienced the largest drag reduction, namely 29%, corresponding to 1223 drag counts. If this is extrapolated to a vehicle with an initial drag coefficient as high as a real heavy-duty vehicle, this could lead to a fuel saving of 2.48 L/100km. Considering the absolute drag coefficients, the platoon with the best performing vehicles was still the optimal platoon with the lowest drag coefficient, namely the `t12t12t12' platoon with the largest frontal edge radius. This aerodynamic analysis investigated the effect of the drag reduction devices in a platoon of bluff bodies subjected to cross wind conditions. It was shown that the frontal edge radius can determine whether the following vehicles experience a drag reduction or a drag increase. Tails reduced the drag of the individual vehicles but did also decrease the drag reductions experienced in the platoon. The drag reductions were still present under cross wind conditions. Considering the set-up of platoon formations, vehicles with the most streamlined front should be placed as lead vehicle. Vehicles with large tail angles should be used as trailing vehicle.