As of Q2 2023, a considerable knowledge gap exists within the literature on van-trailer aerodynamics.

With a global effort under way to transition the world’s transport sector to electric vehicles, it is ever more important to understand the aerodynamics of van-trailer combinations and explore options for novel drag reduction as fears mount within the consumer market that electric vehicles are not suitable for towing due to significant range detriment.

The Irish government under its ambitious 2021 climate action plan aim to half greenhouse gas (GHG) emissions by 2030 and achieve net zero by 2050¹.

As of 2021, the Irish transport sector accounts for approximately 20% of the country’s GHG emissions¹, highlighting the need for continued work on and research into drag reduction technology.

Aerodynamic drag is the most-dominant resistance force opposing a ground vehicle’s motion when travelling at speeds above 80 km/h². Vehicles with large drag coefficients are particularly affected when travelling at highway and motorway speeds ranging from 80-120 km/h.

EU law requires that vehicles towing a trailer are to restrict their speeds to 80 km/h or less. When a van and trailer are travelling at 80 km/h, or above, the resistance due to aerodynamic drag is well above half the vehicle’s total resistance.

Drag reduction through the application of appendable devices can play a key role in reducing fuel consumption and improving driving range for electric vehicles, particularly when towing a trailer.

As a general rule of thumb, a 20% drag reduction would materialise as a 10% fuel saving when travelling at highway speeds³. While the aerodynamics of flatbed trailers largely depend on their cargo, theoretically, they may spend half their time unloaded while returning to their starting location.

Few studies exist within the literature outlining the aerodynamics of this type of trailer when towed by vans, and none detail how their drag can be reduced through the application of appendable devices. This article aims to inform consumers and manufacturers of the drag saving potential for flatbed trailers using such devices.

Computational setup and numerical methodology

The airflow over ground vehicles can be modelled using a continuum approach as only the macroscopic interactions are of interest, with any given fluid element being the average of many fluid particles in both space and time.

Air was modelled as an isothermal Newtonian fluid with constant viscosity and density. The constant density assumption was justified as the investigated flows had Mach numbers below 0.3, minimising compressibility effects.

The governing equations that form the basis for CFD simulations are the Navier–Stokes equations (Equations (1) and (2)), which are derived from the principles of mass and momentum conservation. 𝑢_i is the velocity component in the 𝑥_i direction, 𝜌 is the fluid density, P is pressure, t is time, and 𝜇 is viscosity. 

The CFD results detailed throughout this article were realised using ANSYS Fluent, which solves the Reynolds-Averaged Navier–Stokes (RANS) equations with the help of turbulence models such as the 𝑘−𝜔 SST model⁴, where k is the turbulent kinetic energy and 𝜔 is the specific rate of dissipation in this two-equation turbulence model.

Fluent produces force measurements for the simulated geometry once the pressure is calculated at each cell in the vicinity of the geometry. The formulas for the two main force coefficients used throughout this article are detailed in Equations (3) and (4). 𝐶_D and 𝐶_L are the drag and lift coefficients, respectively, where V is the free-stream velocity, while 𝐹_D and 𝐹_L are the respective drag and lift forces. The A term represents the projected frontal area of the vehicle. Note that a 0.001 𝐶_D change is regarded as a 1 drag count change.

 (3) and (4)

Results and discussion

3.1. Flatbed trailer 1.0 

To effectively investigate the aerodynamics of flatbed trailers, a generic model for a flatbed trailer was established that closely represented most flatbeds in operation in Ireland and the UK.

Flatbed Trailer 1.0 (FBT1) was a twin-axle, torsion suspension trailer with a 3.7 m x 1.92 m bed raised 670mm off the ground. The trailer was attached to a generic electric van model.

Figure 1 outlines FBT1’s geometry. Note that the wheels of both the trailer and van use slick tyres with alloy covers to improve aerodynamics and to remove the requirement for sliding mesh or moving reference frame modelling. The aerodynamic results for FBT1 are outlined in Table 1.

For reference, the standalone van had a drag coefficient of 0.331 and a lift coefficient of 0.323. Note that the frontal area for the van-trailer combination was approximately equal to that of the standalone van, which was the case for all configurations to follow unless stated otherwise. Therefore, all drag reductions will be discussed explicitly in terms of %C_D changes and not drag area (C_DA).

When attached, the trailer increases the body’s drag by 40.8%, while also significantly reducing the amount of downforce generated by the combination. In general, when towing a trailer, the vehicle combination has increased levels of lift, which can lead to driving stability issues particularly for unloaded lightweight trailers travelling at high speeds in crosswinds or when cornering.

Figure 1:  Rendered image of FBT1’s exterior geometry and underside.

Table 1: FBT1’s drag and lift coefficients. 

To help understand the increase in drag, Figure 2 is presented, which shows the drag force accumulation along the combination’s length.

As expected, there was a large contribution from the van’s front; however, the rear surfaces of both the van and trailer were shown to have substantial contributions, with the van rear carrying 25%, and the trailer rear 9% of the total drag.

This highlights the need for appendable drag-reduction devices on rear surfaces. Other areas for drag reduction devices include the trailer’s drawbar and face, along with the wheels and axle package. The main cause of drag on the rear surfaces is the under-pressured vacuum wake that follows them.

Figure 2:  Accumulated drag distribution for FBT1.

3.2. Initial geometry modifications 

The initial aero development of FBT1 consisted of three geometry modifications detailed in Figure 3. To understand the effect the trailer’s mudguards had on its drag coefficient, they were removed to see if any C_D reduction could be realised.

Mudguards are a legal requirement for trailers with exposed wheels as they play a key role in preventing foreign and self-soiling.

Table 2 shows that the removal of the mudguards did not reduce the drag coefficient significantly, reporting only a two count drag reduction. This was because, when the mudguards were removed, the drag on the trailer’s front wheels and rear face increased by approximately 2% each, which offset the lost 4% contribution previously observed for the mudguards on FBT1.

Figure 3: Rendered images of the initial geometry modifications. No mudguards (top left), underside covered (top right), and aero drawbar (bottom left and right).

Table 2: Aerodynamic results for the initial geometry modifications to FBT1.  

The aero drawbar design offered no significant drag reduction. This was because the smooth underside of the new drawbar encouraged large amounts of flow to move quickly under the drawbar, curl up, and then, stagnate on the underside of the trailer.

This increased drag on the underside, while also decreasing downforce due the increased high pressure underneath the trailer bed. This drag increase was then offset by the drag reduction experienced on the trailer face due to the newly shaped upper drawbar surface.

It was, therefore, previously advantageous that a large amount of the flow moved up through the drawbar and out over the bed, avoiding the underside. The best-performing initial modification was underside covered, in which a 6.9% drag reduction was observed, accompanied by a 47.4% increase in downforce.

Here, the deficiencies of the aero drawbar were amended, and the fast-moving underside flow was prevented from stagnating on the underside beams.

Unlike for passenger vehicles, fitting a smooth undertray to a trailer is often very feasible, and EU regulators should consider making such a device mandatory for all new flatbed trailers.

3.3. Front face changes and added panelling

It is common for flatbed trailer manufacturers to offer configuration options for their trailers such as ladder racks, rear gates, and droppable sides. For users in the construction sector, the Ladder Rack Face (LRF) with a Dropside and Rear Gate (D&RG) trailer configuration is a common choice.

To better understand the aerodynamic attributes of these modifications, five new simulations were performed with their results and geometries detailed in Figure 4 and Table 3.

The area of the LRF, LRF with D&RG, and LRF with D&RG with Cover all had a slightly increased frontal area of 2.9% about the baseline FBT1. This was due to the protruding side posts of the ladder rack.

Figure 4: Rendered images of the front face changes and added panelling. Ladder rack face (top), LRF with dropsides and rear gate (middle left), LRF with D&RG with cover (middle right), alt front face (bottom left), and alt front face with hollow (bottom right).

A substantial 27% drag increase was observed for the LRF as the fast-moving flow leaving the van’s latter surfaces collided heavily with the outer frame of the ladder rack. Interestingly, the biggest contributor to the additional drag was the top section of the ladder rack, even though it had a relatively small area in comparison to the larger lower panel.

With the addition of the dropsides and the rear gate, the drag coefficient was seen to increase by another six counts. The LRF with D&RG with cover is a design in which a mountable cover that can roll up for storage is fit atop the dropsides and rear gate. The cover design offered a 20 count drag reduction when compared to the LRF with D&RG as the drag force on the trailer rear was significantly mitigated. 

Table 3: Aerodynamic results for the front face changes and added panelling.

The alt front face configuration only incurred a 2.6% drag increase on the baseline FBT1; this interesting result highlighted the importance of having any vertical bed features confined within the wake of the van.

The alt front face was shadowed by the van, and its solid face ensured that any stagnation or flow build-up on the face interfered with the van’s wake, producing a positive drag reduction.

It is interesting to note how the alt face hollow configuration had a 36 count drag increase over the solid alt front face. The expected result would be that the hollow section in the face would have reduced flow stagnation and, hence, reduced drag. This highlights the importance of enabling interference effects within the gap to attain favourable drag reduction.

3.4. Additional geometry modifications 

This section investigated the impact of additional geometry modifications on flatbed trailers and assessed their aerodynamic influence. The changes examined included altering the bed texture, adding a full side skirt, incorporating a generic load, and installing a mesh cage on the flatbed.

The full side skirt system (Figure 5) did not realise the amount of drag reduction that was expected given the device’s wide adoption on tractor-trailers. Table 4 outlines just a 0.4% drag reduction for the side skirt device. This was because the side skirt blocked the escape of any flow that moved up from the ground and stagnated on the front of the wheels.

A better design would be to have a partial side skirt, in which breaks occur near the wheels to ensure the flow can escape the wheel wells. The alternative bed texture configuration consisted of numerous 8mm-high mounds with 40mm diameters. This was proposed to see if the bed texture affects drag. As shown in the results, this was not the case, and it can be concluded that bed texture has an insignificant effect on C_D.

Figure 5: Images of the additional geometry modifications to FBT1.

FBT1-loaded was proposed to identify how C_D for the vehicle combination would be affected when loaded with a generic body. The load was placed in the centre of the trailer and was shadowed by the van on its sides only, being allowed to protrude considerably above the van.

As a result, the frontal area for the configuration increased by 17.5%. A manageable 11.6% C_D increase was observed; however, when combined with the frontal area increases, the drag force was seen to rise considerably.

As outlined previously, when the load was shadowed fully or partially by the van, the C_D increase could be mitigated. Overall, the most interesting result was FBT1-mesh sides, in which a huge drag coefficient increase of 40.8% was observed.

Note also that there was a frontal area increase for FBT1-mesh sides of 2.6%. This outlines a very important conclusion, that employing meshed faces, sides, or gates on a trailer can considerably increase drag. Removing the mesh sides when unneeded (storing them flat on the trailer bed) will bring about substantial fuel savings.

Table 4: Aerodynamic results for the additional geometry modifications.

3.4. Drawbar length test

One of the most critical dimensions of any flatbed trailer is its drawbar length, with it having a significant impact and the trailer’s handling and turning circle.

Shorter drawbars enable the trailer to react quickly when the towing vehicle begins to change direction, while longer drawbars have delayed reactions.

While most people are aware that a longer drawbar will increase drag, they do not have a quantification for the C_D penalty per mm change in drawbar length.

The original FBT1 had a drawbar length of 1,350mm. Five additional drawbar lengths were studied (Figure 6) to observe the drag variation on the vehicle combination. Figure 7 below outlines the results.

Figure 6: Image of the different drawbar lengths used in the drawbar length tests.

A clear drag-increasing trend was seen for the longer drawbars, except for the mid-range length drawbars, in which the drag began to plateau. A similar plateauing effect occurred for box trailers in a study previously conducted by the authors.

The explanation for this effect is that, when away from the extremities of the drawbar, such as being very close to the van, or very far away, a region exists in which the flow field behind the van is quite constant. This region exists after the intense part of the wake near the van’s rear and before the region in which the flow has sufficiently wrapped around the van from both the sides and underside.

The overall trend is considerably linear, with a 250mm change in drawbar length accounting for a 7-8 count change in C_D. This approximately equates to a 1.7% drag change per 250mm.

When working with box trailers, a different metric exists, in which the C_D change per millimetre is much higher due to the significant interference effects occurring between the van and box trailer.

The explanation for the drag reduction for shorter drawbars is due to the trailer experiencing an enhanced drafting effect from the van, while the increasing trend for longer drawbars is a result of increased stagnation on the drawbar and trailer face due to its added exposure.

In conclusion, the relationship between drag and drawbar length is linear, with drag varying noticeably, but not drastically, due to the low profile of the flatbed trailer.

Figure 7: Graph of C_D vs. drawbar length for FBT1.

3.5. Novel aero concept with trailer fitted devices (NACTFD)

The NACTFD consisted of six unique appendable devices combined with an additional aftermarket geometry modification. The six devices are outlined in the bottom image of Figure 8, while the geometry modification, which consisted of cutting out rectangular slots in the trailer’s rear face, can be seen in the top image.

The devices were designed following extensive post-processing of FBT1 with in depth analysis of high-drag zones suitable for appendable devices. The underside drawbar cover was used to prevent the flow from travelling up through the drawbar and stagnating on the trailer face, while the axle wind deflectors were fit downstream to reduce stagnation effects on the axles in the presence of the additional flow being redirected underneath by the drawbar cover.

The axle wind deflectors offered an appreciable 7.5 drag count reduction. This drag saving is achievable under all trailer loading conditions, which outlines the effectiveness of such a simple device and the need to have it implemented on all trailers that have exposed axle packages.

The underside front deflectors combined with the rear wheel boat tails were fit to help guide the flow around the trailer’s wheels and ensure drag on the front and aft surfaces was minimised.

Together, they offered a 24.3 drag count reduction, which was a combination of the drag savings on the trailer’s wheel wells and mudguards. This, however, was slightly offset by a drag increase on the front and back wheels totalling 6.9 drag counts. This increase was due to the added flow leaving the underside of the front deflector and impacting directly the bottom of the wheels, with more concentration than in the original FBT1.

A device not used that would have added notable drag reduction in conjunction with the other devices is a full undertray (discussed in Section 3.2). Overall, the combined savings produced a 34 count C_D reduction, amounting to a 7.3% drag savings (Table 5).

Table 5: Aerodynamic results for the novel aero concept with trailer fitted devices.

Figure 8: Images of FBT1-NACTFD and the drag-reducing devices fit to the trailer.

3.6. Novel aero concept with van-fitted device (NACVFD)

As the van’s rear accounted for a significant proportion (25%) of the total drag on the van and trailer, it was necessary to fit a novel drag-reducing device to the rear to see how much drag it could negate.

The Multi-Stage Converging Cavity (MSCC), shown in Figure 9, is an appendable device fixed to the doors of the van that can retract inwards along a rail system, enabling the user to open the doors when retracted.

The device shown is made in halves, and when the doors open, each half moves off with the door it is attached to. The mounting mechanism along with the retracting design is not detailed here for IP reasons.

The device’s working principle is to increase the base pressure on the van’s rear. It does this in two ways: first, it uses its length to tap into a high-pressure zone that always occurs downstream of the van, and second, it uses multiple converging stages to help the flow converge to a point behind the van, which also helps spread the higher pressure evenly over the entire rear surface of the van.

The top image of Figure 10 highlights the first drag-reducing mechanism. The isosurface of Cp = 0.005 outlines the fringes of the high-pressure bubble that exists downstream of the rear surface. The existence of these higher-pressure zones is analogous to the flow around a cylinder, in which, downstream of the cylinder, behind the wake, a relatively high-pressure zone exists as the flow converts its kinetic energy back into pressure energy.

The longest, and last, converging cavity on the MSCC punches a hole into this zone and carries the higher pressure back onto the base of the van. The highest pressure can be seen on the outer edges of the last cavity in which red surfaces are located nearest the isosurface.

The converging nature of the device then allows this pressure to spread out evenly over the van’s rear and within the device. The second drag-reducing principle is shown in the bottom image, in which the converging shape helps the flow converge to a point behind the van.

Figure 9: Images of the multi-stage converging cavity (MSCC) fitted to FBT1-NACVFD.

The drag-reducing ability of the device is evident from a near 70 count reduction observed for the rear of the van. The device enabled other components like the trailer’s drawbar and face to experience drag reductions on account of the improved flow regime around the MSCC, reducing flow stagnation on these parts.

The MSCC itself reported an 11 count drag force, which could be reduced through further design optimisation, such as varying the lengths and angles of the multiple cavities.

Overall, the device reduced drag by a substantial 17.8% (Table 6), which highlights the benefit of appending such a device, even if it adds some configuration overhead to the driver when accessing the rear.

The MSCC is a universal drag-reducing appendable device, meaning it can be fit to most ground vehicles. Further research is being conducted into its application on trucks, buses, standalone vans, and other commonly used ground vehicles.

Table 6: Aerodynamic results for the novel aero concept with van fitted devices.

Figure 10: Contour plot of pressure coefficient with isosurface of Cp = 0.005 (top) and velocity magnitude plot along the symmetry plane of FBT1-NACVFD (bottom).

3.7. Final design

To conclude the results, a final design that incorporated both the van and trailer fitted devices was created. The final design reported a drag coefficient of 0.352, which amounts to a near 25% drag reduction on the baseline FBT1.

For reference, the standalone van had a drag coefficient of 0.331, which highlights that, when both the van and trailer utilised appendable devices, the added drag due to the trailer can be significantly mitigated and brought close to full negation.

Conclusion

In conclusion, a generic dual-axle flatbed trailer was studied to find ways to reduce drag using appendable devices. The initial work revealed that most of the drag originated from the back of the van, with notable contributions from the trailer’s wheels.

Following some initial geometry modifications, a full trailer undertray was shown to offer a near 7% drag reduction. A study into the drag effects of ladder racks, dropsides, and rear gates was conducted in which large drag increases for ladder racks protruding from the van were observed, while ladder racks that shadowed the van performed better.

It was demonstrated that a solid face rack could outperform a hollow face rack due to the interference effects between the rack’s face and the van’s wake.

Additional geometry modifications included a full side skirt system, which did not reduce drag noticeably, as it prevented the flow exiting from the wheel wells. An alternate bed texture was used to see if it would affect the drag coefficient of the trailer, but no noticeable C_D reduction was observed.

The most interesting modification was the use of mesh sides and mesh gates on the trailer, in which the drag increased by a substantial 41%, highlighting that mesh sides should be stored away when not needed.

A drawbar length test found a near-linear relationship between the drawbar length and drag. A C_D change of approximately 1.7% per 250mm change in length was noted.

Six unique appendable devices were fitted to the flatbed trailer, which together offered a 7.3% drag reduction. The most effective devices were those attached to the axle and those placed in front and behind the trailer’s wheels.

To reduce the drag on the rear of the van, a new novel device known as the MSCC was introduced to reduce drag by nearly 18%.

The device significantly increased the van’s base pressure and improved the flow regime around other devices in its vicinity. To finish, a final design using a combination of both the van and trailer devices was made that offered a 25% drag reduction, which nearly fully negated the drag additions associated with adding the flatbed trailer to the van.

5. Future work

As mentioned, there is ongoing research into the new rear multi-cavity device and its application on a broad range of road vehicles. Figures 11 & 12 outline a prototype, three cavity device fitted to a commercial van.

The device is mounted to the vehicle using a towbar rack. On-road drag measurements have confirmed that this device can reduce the vehicle’s drag by nearly 20% when travelling at 100 km/h.

For vehicles without towbars, these multi-cavity devices can be mounted using technology similar to that used for bicycle racks. Research into the mounting methodology, the device’s materials, its structural design and most importantly its shape (for optimised drag reduction) are ongoing, with plans to bring a multi-cavity device to market to help Ireland and the rest of the world reduce its transport emissions and reduce fuel costs for road users.

Figure 11: Three-piece multi-cavity device mounted to a commercial van.

Figure 12: Aerial photograph of the prototype multi-cavity device on the road.

Funding

This research was funded by Research Ireland (formerly The Irish Research Council) Grant Number EPSPG/2022/213 and Research Ireland (formerly Science Foundation Ireland) Grant Number 22/NCF/EI/11277.

Authors: Michael Gerard Connolly, Alojz Ivankovic and Malachy J O’Rourke, School of Mechanical and Materials Engineering, University College Dublin, D04 PR94, Dublin.

References

1) Ireland-Climate Action Plan. 2021. Available online: https://www.gov.ie/en/publication/6223e-climate-action-plan-2021/ (accessed on 20 October 2023).

2) Wood, R.M.; Bauer, S.X.S. Simple and Low-Cost Aerodynamic Drag Reduction Devices for Tractor-Trailer Trucks. SAE Trans. 2003, 112, 143-160.

3) Ekman, P.; Gardhagen, R.; Virdung, T.; Karlsson, M. Aerodynamic Drag Reduction of a Light Truck—From Conceptual Design to Full Scale Road Tests; SAE International: Warrendale, PA, USA, 2016; p. 2016-01-1594. [CrossRef]

4) Menter, F. Zonal Two Equation k-w Turbulence Models For Aerodynamic Flows. In Proceedings of the 23rd Fluid Dynamics, Plasmadynamics, and Lasers Conference, Orlando, FL, USA, 6-9 July 1993.

Main) Connolly, Michael Gerard, Malachy J. O’Rourke, and Alojz Ivankovic. 2023. 'Reducing Aerodynamic Drag on Flatbed Trailers for Passenger Vehicles Using Novel Appendable Devices' Fluids 8, no. 11: 289. https://doi.org/10.3390/fluids8110289