Refinement Zones for Commercial Heavy Trucks

This section will describe the commercial heavy truck best practice setup. Note that many of the logics and setup schemes are related to the passenger car setups. It is recommended to first read through the passenger vehicle section.

Commercial heavy truck best practices are divided into long-nose and cab-over configurations due to their aerodynamic differences. Below, a general overview is given, followed by specific pointers for long-nose and cab-over.

Domain and resolution levels

The Resolution Level setup is adjusted to the increased size of commercial trucks, keeping RL6 as the general offset on the whole vehicle region. Due to the larger size, an RL6 of 6mm is recommended. This would give the following resolution zones:

Refinement Level Vozel Size (mm)

RL 0: 384 mm
RL 1: 192 mm
RL 2: 96 mm
RL 3: 48 mm
RL 4: 24 mm
RL 5: 12 mm
RL 6: 6 mm
RL 7: 3 mm
RL 8: 1.5 mm

Due to the enlarged surface area, the domain dimensions need to be enlarged to ensure a blockage ratio of less than 0.5%.

As the proportions of a commercial truck including trailer differ, the following is recommended for estimating the domain extensions:

Table 1.
Recommended domain size tractor-trailer combination in lengths
+x	2
-x	4
+y	1 (or 12 widths)
-y	1 (or 12 widths)
+z	1.5 (or 20 heights)

VWT will create a default tunnel which is sized to provide a blockage ratio of 0.5% and is adequate for standard open road simulations.

Vehicle box refinement for outer regions (RL 1 though 3)

For commercial trucks, the general resolution strategy for the outer 3 refinement regions is similar, with a shift towards the rear of the tunnel to capture the wake structures. In summary as before, it is recommended that RL1 is at least:

X: 3 vehicle lengths downstream, 1 vehicle length upstream of the vehicle
Y: at least one vehicle width off to the sides
Z: at least one vehicle height above the vehicle

RL2 and RL3 are scaled down from there. It is recommended that RL3 end one length behind the vehicle.

The outer resolution levels RL1- RL3 are thus large boxes encompassing the wake of the truck, as seen in the figure below:

Major regions of resolution

For the inner resolution levels, zoom in to specific parts and go through them step by step.

RL4 (24 mm) is placed in the far wake of the trailer, the underbody flow over the whole vehicle, around the tractor, and in the engine bay:

RL5 (12 mm) is placed around more specific features, such as around mirrors (in addition to custom zones), in the interface gap, around the trailer wheels and landing gear, and in the near wake. Note the focus on the tractor to capture flow features.

Note that the cowl region is covered too:

RL6 is used around interface gaps, shear layers, cooling pack flow and bumper lower shear layer. More details below.

Note that the region around the porous media (including the post-PM flow) as well as the bumper shear layer have dedicated regions to cover:

Offset refinement overall

The trailer and the aero surfaces will in general have an RL6 offset of 4 cells (4x 6 mm). The engine bay and underbody in regions that do not directly touch the main underbody flow can be in RL5 if needed for voxel count reduction. Specific regions are selected for having RL7 and/or a larger number of RL6 offsets. These regions will be defined in the long-nose and cab-over sections of this chapter.

Refinement details for all trucks

The following refinement boxes or custom regions are recommended for all truck types. It is geometry dependent whether a box or a custom region is needed. Some regions have already been mentioned.

Tractor-trailer interface gaps: The shear layer in the tractor-trailer as well as inter-trailer gaps is important and needs to be resolved in RL6 (6 mm). If the geometry is axis aligned such that a box region suffices to capture the shear layer, this is ok. Else a custom region can be created out of a simplified shape, like a morphed shape.

Figure 10. Regions in the interface gaps that need to be covered, in green
Trailer base: The base of the trailer needs to be covered all around (top, sides, bottom including bars) using refinement boxes in RL6. This should cover the boundary layer and the separating shear layer over the edge for the first 0.4m.

Figure 11. Resolution coverage at the base of the trailer, not the finest region around separation
Shear layers off the front bumper: At the bottom of the front bumper, the flow typically separates, and the shear layer is interacting with the underbody elements of the tractor. This shear layer should be captured in RL6 using a custom refinement zone. Any element that is impinging the main flow should have 8 x RL6 offset voxels as well for the first 2m underneath the tractor.

Figure 12. Custom zone for capturing the shear layer off the front bumper (in green), and offsets needed for impinging parts (example at green arrow)
Cooling pack flow: The flow through the grill elements and past the cooling pack is important. Use either a box or custom refinement region to capture this flow at least in RL5, preferably RL6.
Treatment of the wheels: Commercial trucks typically have more closed rims compared to passenger vehicles. For that reason, an MRF setup is not needed, and a moving wall boundary condition suffices for the rims, axles, brakes and tires.; The flow around the wheels is important, especially the front axle; the first tractor rear axle and the first trailer rear axle can receive a lot of flow. It is recommended to use on the front tires: 8 layers of RL7 (3 mm) plus 12 layers of RL6 on top of that. This can be repeated for the other axles. If voxel count needs to be reduced, remove the RL7 of any tires that are not leading into the wind, and focus on the tractor wheels only.

Considerations for long-nose trucks

Custom refinement regions

It is recommended to create the following custom refinement regions:

around the nose including the lower bumper
specific wake of lower bumper deflector
zone around the mirrors
cowl vortex region

The cowl vortex on the hood of a long-nose truck can be quite large due to the large angle between the hood and windscreen. It can extend forward of the windscreen. This vortex needs to be captured with a RL5 region box or shaped custom refinement zone. This zone needs to extend to the mirror section of the truck on the sides, as the cowl vortex commonly interacts with the flow around the mirror.

The green dotted line indicates needs for added refinement due to the separation and forming of a cowl vortex.

Note that the cowl region is not included here as it was set as a box region separately afterwards

For the offset zones, as with the general setup, focus on the nose of the tractors, the region around the wind screen and mirrors, and rounded edges that could separate, underbody parts in the main flow, and aero features such as deflectors. Optional elements such as detailed grids and surfaces that possibly have separation bubbles can also be considered.

General zones in green including nose and a-pillar, lower bumper separation zone offset in blue, critical appendages in yellow. Note the grill in red, which might need even additional resolution depending on the grill elements sizing.

Considerations for cab-over commercial trucks

Air curtain: Cab-over trucks typically can have a flow deflector around the front corners to prevent soiling onto the side-glass and door handles, called an air curtain.

Figure 18. Air curtains on cab-over trucks; This air curtain contains slots to pass flow through. A minimum of 6 cells in these slots is recommended, resulting in the use often of a RL7 (3 mm) or even RL8 offset region.
Leading edge corner flows: For cab-over trucks, the flow around the leading-edge corners is important. It is suggested to isolate the parts over the full height and set a part offset of 8 layers RL7 (3 mm) plus an offset of 8 layers of RL6 (6 mm) on top of that.

Figure 19. Offset regions for leading edge corner flow on cab-over trucks (model source FAT); Also add similar offset regions to details in the flow, such as mirrors, roof deflectors, flaps, and underbody parts in front in the main flow.

Yaw-angle notes

The current best practices for the previous truck sections are set for 0-degree yaw angle. However, for commercial truck applications, a yaw angle (for example, 5 or 10 degrees) can be an interesting case to investigate. The leeward side of the trucks will have a large wake, which would be filled in from the top/bottom of the trailer, from leeward edges of the tractor, and from cross-flow in the tractor-trailer gap.

Considerations for yaw setups:

The leeward edges should have a shaped refinement zone to capture the separation (RL6).
The wake on the leeward side should be adequately captured in RL5 and RL4. This can be achieved using combined loose wraps of 0 yaw until maximum yaw surfaces.
The tractor trailer gaps should have at least RL5 in the whole interface to cover cross-flow.

Run-time of the simulation

A full tractor-trailer combination is typically a lot longer than a passenger vehicle. 8 flow passes is recommended to settle the flow, and another 8 for measurement of the required variables.

For example, for a 16m long combination, inflow 25 m/s, there are 25/16=1.6 flow passes per second. The recommended setting would be:

Total simulation time 10s
Start of averaging time 6s (measuring between 5s and 10s)

In conclusion: mesh setup for commercial trucks

For 0 yaw, the current best practice should generate a setup of around 220-320 million voxels, which would run for 10 seconds physical time in less than 1 day on Nvidia 8 x V100 GPUs.