Research in the field of trailer aerodynamics and drag reduction technologies is conducted at the faculty of Aerospace Engineering of Delft University of Technology, The Netherlands. Together with PART and its partners multiple full-scale tests have been conducted successfully investigating the performances of different drag reduction devices for trailers.

General flow behavior

With the aid of Computational Fluid Dynamics (CFD), numerical simulations of the flow around a tractor-trailer combination are conducted to identify the critical regions responsible for aerodynamic drag. Typical aerodynamic characteristics, indicative for the aerodynamic performance of a vehicle, are easily visualized. Figure 1 illustrates the streamlines at the underside of a vehicle. This numerical simulation indicates three regions responsible for high drag contributions: the area between the tractor and the trailer, the underside and the rear of the trailer.

Figure 1: Streamlines around a tractor-trailer combination.

Trailer Underside solutions

Base on the outcome of the numerical simulations, several aerodynamic drag reduction devices are designed and tested. A short selection is discussed below.

1. SideWings

Wind tunnel experiments

The wind tunnel is used as a design tool in order to obtain the drag reduction solution for the underside of the trailer. Many aerodynamic add-ons for the underside of a trailer are designed and tested in the wind tunnel with a scaled truck model. A series of wind tunnel experiments was conducted in the Low Turbulence Tunnel at Delft University of Technology. During the experimental measurements more than 100 different aerodynamic devices, such as belly boxes, underbody and diffusors, were designed and tested. The best performing solution is discussed below.

Figure 2: SideWings with an incorporated flow conductor (top left), standard side skirts (bottom left) and wind tunnel results (right).

Standard side skirts with open wheels deliver 8.5% drag reduction, as plot in figure 2. The SideWing configuration, straight skirts with an optimized profile generated the highest drag reduction: from 14% up to 17%. The SideWing’s excellent aerodynamic performance is created by the optimized profile of its front panel: the flow conductor, see figure 2. The wing-shaped element captures the airflow behind the tractor, creating a thrust in the driving direction, and preventing flow separation. This unique patented wing shape almost doubles the performance of existing solutions.

Fuel economy test on circuit and public road

An elaborate test phase with the Ephicas SideWing was executed in 2010 and 2011, both on a test track as on public roads during daily operational activities together with several transport companies, illustrated in figure 3. The SideWings proved an average fuel saving of 1.5 liter per 100 km or 3.9 kg per 100 km less CO2 emissions, corresponding with a difference of 5%. With higher wind velocities, i.e. a wind velocity higher than 8 m/s, a fuel saving of more than 2 liter per 100 km is measured.

Figure 3: SideWings tested on the circuit.

2. Aerodynamic mud flaps

A second technology for the underside of the trailer, aerodynamic mud aps, was executed on track. During a one-day-test, based on the SAE Type II test protocol, a fuel saving of 0.3 liter per 100 km is measured when mud flaps were only mounted behind the rear axles of the trailer.

Figure 4: Aerodynamic mud flaps.

Trailer rear-end solutions

Next to the underside, the rear of the trailer has a large contribution to the total drag built-up of a tractor-trailer combination. To reduce the drag, and thus improve the fuel economy, several aerodynamic devices and flow manipulation techniques can be applied.

1. Tails

A well-known example of such a trailer rear-end device is a boat tail, which can be described as a tapered elongation of the trailer.

Numerical simulations and wind tunnel experiments

In order to get insight in the flow characteristics and flow behavior at the rear of a tractor-trailer combination, initial numerical simulations are conducted using Computational Fluid Dynamics (CFD). With the aid of these simulations a first impression of the potential drag reduction by application of a boat tail is easily obtained: drag reduction of 12% is simulated.

Figure 5: Numerical simulations (left) and wind tunnel experiments (right) of the boat tail concept.

During wind tunnel experiments different boat tail con figurations are analyzed in order to determine potential drag reductions. Next, the influence and necessity of a bottom panel, variations in the slant angle of the tail panels, the cavity (open, closed, half-open) and offset of the bottom panel are determined. From this wind tunnel experiments it is concluded that an open-cavity boat tail including a bottom panel gives the best performances. With this configuration a drag reduction of 12% is obtained.

Stepped tail

A stepped tail can be described as an aerodynamic add-on comprising 4 rectangular plates that are mounted on the rear surface of the trailer with a certain inset with respect to the edges. With the aid of numerical simulations and wind tunnel experiments on stepped tails an analysis is made of standard and stepped on a simplified truck model. From the results it is concluded that longer tails will result in higher drag reduction, as illustrated in the figure below for stepped tails. The best performing stepped tail generates a drag reduction of 10%, while the drag reductions of the shorter versions are negligible. A standard tail generates a drag reduction of almost 40% on the same simplified truck model.

Figure 6: Numerical simulations (left) and wind tunnel results(right) of the stepped tail.

Rigid tail

In 2008 a first road test with a rigid tail was executed. For one year, a trailer with rigid tail was allowed to drive on public roads. Data on fuel consumption was already recorded before the tail was mounted onto the trailer, allowing a comparison of fuel consumption with and without rigid tail. During the test period, the length of the tail was varied. Initially, the rigid tail had a length of 2 m and was shortened gradually to 1.5 m and 1 m. A fuel economy of 2 liter per 100km is obtained with a rigid tail of 2 m. When the tail length is gradually diminished towards 1.5 m and 1 m, obtained fuel savings are 1.7 and 0.8 liter per 100km respectively.

Figure 7: Road test results (left) of the rigid tail (left).

Foldable and inflatable tail

More practical concepts of the tail are designed and tested on the circuit and the public roads. The first concept is tail that can be folded manually to the rear doors of the trailers, assuring the accessibility of the trailer. Circuit test indicated a fuel saving of more than one liter per 100 km. This tail is also tested during operational activities for a period of five months, resulting in a fuel saving of 1.6 liter per 100 km. This foldable tail proved to be a very effective solution that is not hindering the operational activities of the truck driver.

Figure 8: Foldable (left) and inflatable (right) tail.

A second concept is the inflatable tail that is fabricated from a flexible material. The tail is inflated with the aid of pumps to the desired shape. Circuit test with a prototype proved a fuel saving of 1 liter per 100 km.

2. Vanes

An alternative way to reduce the total drag of a tractor-trailer combination is by applying airfoil-shaped guiding vanes at the rear of the trailer. This new drag reduction device, called the guiding vanes, is developed with the aid of numerical simulations and wind tunnel experiments by Delft University of Technology. In August 2011, a first test with a full-scale prototype has been executed together with PART.

Numerical analysis and wind tunnel experiments

With the aid of Computational Fluid Dynamics an analysis of the flow around the vanes on the rear of a trailer provides more insight into the aerodynamic characteristics. Using these numerical simulations, several parameters of the vanes are simulated to get a first indication of the potential drag reduction. The shape of the airfoil is varied, as well as the position of the vanes with respect to the trailer roof, sides and bottom. Also, angles of attack are varied as an optimal angle of attack for the airfoil-shaped vanes needs to be found.

Wind tunnel experiments indicated the effect on different vane configurations on the total drag of the vehicle. Using the same variations as during numerical simulations, i.e. airfoil shape, position and angle of attack, potential drag reductions are measured. In addition, drag reductions obtained by different configurations of the guiding vanes are measured. During wind tunnel experiments potential drag reductions up to 20% were demonstrated with a complete enclosed ring, i.e. vanes at each side of the trailer-rear.

Fuel economy test on circuit

During a test weekend the guiding vanes were mounted onto a standard box trailer in order to determine the optimal configuration and corresponding fuel savings. Its impact on the fuel consumption is measured and compared with a clean trailer which was not equipped with any drag reduction device. The configuration with the top vane only indicated a fuel saving of 0.5 liter per 100 km. Also the configuration with all four vanes at the rear edges was tested. This configuration resulted in a slight improvement compared to the top vane con figuration. The reason for this can be found in the boundary layer development at the sides of the vehicle. More research is required to improve the efficiency of the vane.

3. System Drag Reduction

The System Drag Reduction (SDR), is an aerodynamic drag reduction device mounted onto the roof of a trailer at the rear end. The aerodynamic performances and the corresponding fuel savings received by application of the SDR were measured during a one-day full-scale circuit test based on the SAE Type II test protocol. An absolute fuel saving of 0.24 liter per 100 km is measured, corresponding to a saving of 1% for an average fuel consumption of 23.96 liter per 100 km for the occurring wind conditions.

4. Active Flow Control

Active flow control technologies actively manipulate the flow behavior locally to get a lower total drag of the corresponding object. One of the most known typical examples of actively manipulating the flow to prevent flow separation is boundary layer suction and blowing. Numerical simulations are conducted to analyze the effect of the different parameters involved. Optimization of the involved parameters of a continuous blowing system resulted in a maximum net drag reduction of 20%.

One suggestion is to apply pulsed blowing which is a very effective way to reduce drag. Tel Aviv University (Israel) developed a combination of continuous boundary layer suction together with pulsed blowing that has the same effect as continuous blowing but much more effective in terms of energy consumption. Active flow control technologies are still in an early stage of development in order to be successfully applied on a full-scale vehicle during circuit tests where high Reynolds numbers, cross winds, gusts and rain are occurring. Although, more fundamental and experimental research is required to fully understand the different flow mechanisms, active flow control has the potential to improve the drag levels of bluff road vehicles in the future.