# Troubleshooting: different results for different mesh on exact same geometry (Rectangular channel)

Hello All! I have been running some simulations on a simple rectangular (square) channel on ANSYS Fluent, and analytically verified the results.

I am facing two problems: (1) When I lower the mesh size (add more elements), keeping dimensions and other parameters fixed, the pressure drop somehow keeps increasing, even when the centerline velocity seems to converge.

(2) To analytically verify the ANSYS results, I used Navier Stokes for a thin rectangular channel. I am aware that for a square channel, the error should be ~10%. Using the following equations I obtained the analytical centerline velocity and pressure drop values which differ greatly from the ANSYS calculations. While I understand that the centerline velocity of fully developed flow will not be the same as the inlet velocity, I am confused as to how the analytical centerline velocity can be an order of magnitude over the inlet velocity.

Does anyone have some insight on how to resolve these two issues?

## Answers

6,847Forum CoordinatorIs your channel long enough to allow flow to develop?

21MemberMy channel width and thickness are both 1 mm, while the length is 12 mm

8,855Forum CoordinatorWhat material properties are you using in the model?

6,847Forum CoordinatorThere is a phenomenon called centerline velocity overshoot.

Also increase length of channel to ensure fully developed laminar flow. Laminar flows require longer entrance length than turbulent flows.

21MemberI am using Water as a fluid under ambient conditions, at laminar flow

8,855Forum CoordinatorCan you double check? If you didn't set water in the cell zone it'll still be using air in the model.

21MemberThis is what shows up when I first pull down cell zone

21MemberThe ANSYS calculated centerline velocity was an order of magnitude smaller than what I had calculated analytically though.

21MemberFor the entrance length, point I basically got what I input as inlet velocity (0.02 m/s):

The centerline velocities I showed in the table are mid-channel (6mm in a 12 mm channel).

6,847Forum CoordinatorAgain have you estimated the entrance length for your flow?

8,855Forum CoordinatorPut a line down the centre of the channel and plot velocity on that: that'll give the centreline velocity with position and answer whether the profile has established.

21MemberSorry, misunderstood before. Thanks for the clarification Rob.

I would estimated the length for the flow to fully develop to be approximately 2.7 mm out of the total length of 12 mm

6,847Forum CoordinatorRe check the way you are calculating streamwise velocity: hint check h.

21MemberI had the velocity in the lengthwise direction calculate at the exact center line of the thickness. The yellow line shows where the calculations were taken

6,847Forum CoordinatorI meant your "analytic" derived velocity.

8,855Forum CoordinatorThe CFD result looks to be about right, peak velocity is around twice the inlet speed and in around half the area is covered by the "high" speed region.

21MemberYes you are right. The calculation for general rectilinear channels turned out to be 0.6% from the ANSYS calculation

6,847Forum CoordinatorGreat!