A complete turbulent solution of a semi tractor-trailer truck shows the effectiveness of Detached-Eddy Simulation (DES) to capture the unsteady separation behind a blunt body. DES combines Reynolds-averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) methods. Traditional RANS methods routinely miss the amount of separation behind a bluff body geometry. The growth and separation of the boundary layer can be adequately predicted at a low computational cost by using RANS. LES is used to resolve the large energy-containing eddies that depend heavily on the geometry.
The solution presented here is a truck traveling at a speed of 65 MPH with the airflow coming in at 10o sideslip from the left (drivers viewpoint). The Reynolds number (per length) is 53,088. The computational domain consists of a viscous surface for the ground plane and farfield conditions elsewhere. There is no symmetry in this case.
As seen in the Figure 1, the amount of turbulent structure in the vicinity of the truck is immense due to the sideslip angle of the airflow, the sharp corners of the trailer, and the large flat surfaces that encourage boundary layer growth.
The unstructured grid, created using Gridgen, took about 1 day to complete. The grid contains 5,549,999 cells - prisms in the boundary layer and tetrahedra in flowfield. Most of the cells lie within 1 axle-length of the truck. The ability to easily cluster tetrahedral cells in areas of high turbulence keeps the total number of cells manageable and lends itself to accurate turbulent calculations. The high degree of detail that is modeled is made easy by using hybrid grids.
The solution was run on a 64-node cluster in a time-accurate mode.
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Figure 1: Isosurfaces of vorticity. Truck surface colored by pressure. Air flow at 10o sideslip.
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Figure 2: Geometry around wheel.