§ 1037.532 Using computational fluid dynamics to calculate drag area (CdA).
This section describes how to use commercially available computational fluid dynamics (CFD) software to determine CdA values, subject to the provisions of §§ 1037.525 through 1037.527. This is considered to be an alternate method for both tractors and trailers.
(a) For Phase 2 vehicles, use SAE J2966 (incorporated by reference in § 1037.810), with the following clarifications and exceptions:
(1) Vehicles are subject to the requirement to meet standards based on the average of testing at yaw angles of +4.5° and −4.5°; however, you may submit your application for certification with CFD results based on only one of those yaw angles.
(2) For CFD code with a Navier-Stokes based solver, follow the additional steps in paragraph (d) of this section. For Lattice-Boltzmann based CFD code, follow the additional steps in paragraph (e) of this section.
(3) Simulate a Reynolds number of 5.1 million (based on a 102-inch trailer width) and an air speed of 65 mi/hr.
(4) Perform an open-road simulation (not the Wind Tunnel Simulation).
(5) Use a free stream turbulence intensity of 0.0%.
(6) Choose time steps that can accurately resolve intrinsic flow instabilities, consistent with good engineering judgment.
(7) The result must be drag area (CdA), not drag coefficient (Cd), based on an air speed of 65 mi/hr.