Trailer skirts aerodynamics
One of the main contributors to fuel consumption on a heavy-goods vehicle like a truck is the aerodynamic resistance (drag) due to its shape. A truck generates a large wake behind the trailer and poor quality flow on the underside. One way to improve air flow quality of underside is to include trailer side skirts. In particular, the side skirts can prevent flow circulation between the wheels and elements of the underside. The effect is a reduction in air entrainment on the underside and therefore a reduction in overall aerodynamic drag of the truck.
Semi-trailer truck side skirts aerodynamic
The area around the truck wheels and the truck underside is characterised by a very turbulent and chaotic flow. This is due the presence of different geometrical features that are not profiled for minimum aerodynamic drag like the truck axles, underside beams to sustain the rear carriage and oil tanks. At the same time, the rotating wheels are naturally increasing the level of turbulence in the area by means of introducing an additional unsteady wake.
The figure above shows the flow energy level around the wheels of a truck without side skirts. The colours indicate the level of the total pressure coefficient (Cp0), which is the sum of dynamic and static pressure. Red areas indicate where the Cp0 reaches its maximum value of 1, meaning that the flow as the same energy of the free stream. Low levels of Cp0 (in blue) indicate that the flow energy is dissipated and lost, like in wakes and recirculation zone, contributing to an increase of overall drag. The image shows the impact of the aerodynamic wake created by the wheels on the local flow energy.
Adding skirts to the truck underside has multiple beneficial effects on the flow quality, increasing the flow energy (levels of Cp0). The trailer skirts aerodynamics is helping in cutting the recirculating flow regions and reducing the wake behind the wheels. Skirts also help keeping the flow attached to the truck side surface for longer, delivering higher quality flow to the rear of the vehicle. Sometimes this high quality flow can be directly turned into the truck rear wake with curved rear channels (1) integrated inside the side skirts design, this significantly contributes to the rear wake reduction. Also the front portion of the side skirt can be profiled (2) to increase its effectiveness.
Semi-trailer truck side skirts simulation
Computational Fluid Dynamics (CFD) is a great tool to evaluate the effect of the truck side skirts. In particular a CFD simulation can be used to study areas with recirculating flow, like the wake behind the wheels, and the overall distribution of flow energy. The figure below shows the results of a CFD simulation for a truck without side skirts, where iso-surfaces of Cp0=0 are used to show low energy areas. In addition to the large wake behind the trailer, there are significant low energy regions underneath the truck.
The area of poor energy flow in the image above are highlighted by a low level of Cp0 (around 0). As described previously, by adding side skirts to the trailer, the flow quality can be significantly improved. The picture below shows the CFD results of a simulation for a truck with side skirts, where the low energy regions of the underside are reduced.
Areas of flow recirculation are clearly still there, but those are now much smaller and absorb far less energy. Moreover, higher energy flow is delivered to the rear of the vehicle. This is evident when making a comparison of Cp0 between a case without trailer skirts (shown below on the left) and one with side skirts (on the right).. The difference between the two simulations is shown in the delta plot of SimWorks Manager, where the Cp0 of the second simulation is subtracted from that of the first simulation.
In the plane towards the rear of the track, the red areas in the delta picture show regions where the Cp0 is higher and therefore the flow energy is higher. The overall result is that the truck with skirts has less energy losses and consequently a reduced overall coefficient of drag. SimWorks Manager has advanced functionalities to plot and calculate the difference between CFD simulations of different aerodynamic configurations to help identify the best design.
Rear wake control
The aerodynamics of a traditional semi-trailer truck are dictated by its bluff body shape with a flat rear face. This configuration leads to the generation of large unsteady flow structures at the back of the trailer as well as on the sides. While the flow on the front and sides of the truck can remain attached to the surface and have small recirculation areas, it becomes very lossy and unsteady when it detaches from the trailer back face, in the form of low energy and low pressure wakes. This generates a net positive difference in pressure between the front and back of the truck, ultimately causing significant levels of aerodynamic drag.
There are many ways of reducing the truck wake and therefore its overall drag. Different aerodynamic fairings can been added to the rear of trucks to help delay the flow separation and therefore reduce the size of the wake. For example, the figure below shows a truck where flaps added to the rear end which have the effect of turning the flow from the side surfaces towards the centre and delay the point where it detaches from the trailer itself, thus reducing the overall wake size.
Such devices can be very effective in reducing the drag but generally affect the usability of the truck trailer by making the operations of loading and unloading more difficult. However, as the reduction of fuel consumption and pollutants emission will become ever more important in the future, aerodynamic devices like the ones shown above will become more and more common.
Economy of semi-trailer skirts and rear wake control devices
As we have seen, lower side skirts can produce a significant drag reduction with reductions of fuel consumption up to 5%. While this can seem like a marginal gain, for a professional truck transporter, it is a significant reduction in costs over the course of a year for a very small economic investment (around $3000 for a truck side skirts kit).
Moving to more complicated solutions like the 3 dimensional shaped side skirts, with integrated rear channel integrated, the price can increase significantly for a marginal gain in terms of further drag reduction. For example, a 3 dimensional shaped kit can cost around $8000, offering a marginal further reduction in drag with respect to simple side skirts.
How will the future look like...
The aerodynamic solutions presented above will be progressively included into the design of the next generation trucks. This way it will become economically viable to afford those solutions while the main truck manufacturers will be in a better position to meet the ever more stringent pollutant emission standards.
The most effective way of reducing the truck aerodynamic wake and hence its drag is indeed to redesign its shape to be more aerodynamically profiled. New recent heavy goods vehicle designs include decisive features aimed at reducing the drag coefficient like profiled top truck surfaces, aerodynamics fairings embedded into the front, fully enclosed tyre and profiled side skirts.
Future truck concepts like the one shown above will be designed to be aerodynamically efficient, especially for those adopting an electrical drive train system. It is very likely that the trailer trucks of the future will have a very different look compared to the current designs.