External Air Flow for a Truck: Exercise 4—Assigning Boundary Conditions
For boundary conditions, the fluid behavior and properties are specified at all bounding surfaces of the fluid domain.
Specifying Inlet Boundary Conditions
1. Under 
Boundary Conditions > 
General Boundaries, select outside_dir1_min.
Boundary Conditions > General Boundaries, select outside_dir1_min.2. In the Model panel, select Specified Velocity from Flow drop-down list.
3. Select Cartesian under Method and enter 20,0,0 for Velocity.
Specifying Outlet Boundary Conditions
1. Under Boundary Conditions > General Boundaries, select outside_dir1_max.
Boundary Conditions > General Boundaries, select outside_dir1_max.2. In the Model tab, for Flow, select the following values for the options listed:
◦ Flow — Specified Pressure Outlet
◦ Pressure — 0 Pa
Specifying Other Boundary Conditions
1. In the Flow Analysis Tree, under Boundary Conditions > General Boundaries, select outside_dir2_max, outside_dir3_max, and outside_dir3_min.
Boundary Conditions > General Boundaries, select outside_dir2_max, outside_dir3_max, and outside_dir3_min.2. In the Model tab, for Flow, select the following values for the options listed:
◦ Flow — Specified Pressure Outlet
◦ Under Velocity Profile, set Back Flow Velocity(optional) to 20,0,0