# setting the hydraulic diameter when inlet is split by a symmetry plane

 A common question is whether it is necessary to change the value for the hydraulic diameter when splitting a flow boundary condition by a symmetry plane. For example, for a square with the side length of 2a, the hydraulic diameter is 2a, but if it is split in half by a symmetry plane, the hydraulic diameter of one half would be 4a/3. So if the square B.C. is divided in the middle by a symmetry plane, what is the hydraulic diameter to be entered in the boundary conditions panel when using Intensity and Turbulence Diameter to specify the turbulence at an inlet.The hydraulic diameter should be entered as if the symmetry plane is not there, which in the example from the problem description would be 2a. The reason for this is that the hydraulic diameter is used by Eq. 6.2-2 in the Fluent 6.1 User's Guide to determine a length scale for the turbulence. The length scale is based on the mixing length for fully developed turbulent pipe flow. The actual physical size of the duct remains unchanged by the presence of the symmetry boundary, therefore the length scale of the turbulence, which is estimated based on the actual physical size of the duct, should also remain unchanged by the presence of the symmetry boundary.