We show how you can calculate the flow rate value in a 2D planar model boundary conditions based on velocity values.
When working on a 3D model, estimating the flow rate that passes through a BC is easy based on the velocity inlet. For example, if the user imposes a normal to the boundary and uniform velocity of 1 m/s in a pipe of 25 mm diameter. Then, the Mass flowrate will be:
Area = (pi/4)*d^2 = 0.000491 m2
Volumetric Flow = 0.000491 m3/s
If the Flow is air, the user may assume that the density is 1.225 kg/m3. So:
Mass Flowrate = 0.000601 kg/s
On the other hand, sometimes doing the calculation for a 2D planar model Boundary Condition may need some clarification. Therefore, the first thing to know is that Ansys Fluent, by default, imposes a 1 m depth to a planar model. So, if the inlet boundary condition has a length of 25 mm, the effective area of this BC is:
Area = 0.025 m x 1 m = 0.025 m2 (Which Can be verified through the Ansys Fluen reports)
Figure 1. Inlet length = 25 mm
Figure 2. Area report from Ansys Fluent
Therefore, for a normal to the boundary and uniform velocity of 1 m/s, the volumetric Flow is 0.025 m3/s, which leads to a mass flow rate of 0.0306 kg/s (Air with density = 1.225 kg/m3). The mass flowrate value can be checked under Ansys Fluent reports, as shown in the following picture:
Figure 3. Mass flow rate report from Ansys Fluent.