Pressure drop prediction in a long straight pipe

When the pressure drop is caused by shear losses along the pipe walls instead of form drag due to obstructions, the following technique should be used to calculate an accurate pressure drop:

• Use an entry length of approximately 25 pipe diameters upstream of the test section. This is to ensure fully developed flow at the entry of the test section. (This is only necessary if a velocity or flow rate boundary condition is used. If a pressure drop is specified across the pipe, then the entrance length is not necessary.)

How to calculate entry length

• For laminar flow, calculate entry length using Le / d = .06 * Re (d).

• For turbulent flow, calculate entry length using Le / d ~ 4.4 Re (d) 1/6

• Use symmetry to reduce overall model size, if possible.

• There are two meshing strategies. One uses an extruded mesh, and the other uses an unstructured (tetrahedral) mesh:

• Extruded Mesh: Use Automatic Sizing to prescribe the best default distribution. Click the Extrude button on the Mesh dialog, and assign an extruded mesh to the pipe. Use the default advection scheme and turbulence model. Run the analysis 100-200 iterations.

• Tetrahedral Mesh: Apply a surface mesh size to the pipe wall such that there are eight nodes for every 90 degrees of arc. Apply a volume mesh size to the pipe that is two times the surface mesh size.

• On the Control tab of the Solve dialog, click Solution Control. Click the Advanced button in the Intelligent Solution Control group. Move the slider to Tight. This allows more iterations to occur before reaching convergence. Run the analysis at least 600 iterations.