[kweston]

Hi there,

We are a pair of Uni students battling through some extremely basic CFD simulations as part of our undergrad thesis. We have tried so many variations of our models and nothing seems to be working - any help on this matter is greatly appreciated!

Our first task is to undertake a mesh dependency test by looking at the RMS lift and mean drag of 2D uniform flow over a cylinder at Re=100 (extremely basic - we know!).

Our geometry domain is such that the cylinder D = 10mm, and the boundaries from the cylinder are 16D to the inflow, 20D to the outlow, 10D each way to the top and bottom wall (similar to what's used in a lot of research).

We have tried a variety of meshing techniques, but our most recent attempt looks like this:

[IMG][/IMG]

We have looked at three different meshes with the following properties

Coarse:
- Cylinder divisions = 100
- Total nodes = 8364
- Diagonal divisions = 50
- Wake element size (mm) = 4
- Wake bias factor = 5

Medium:
- Cylinder divisions = 152
- Total nodes = 17581
- Diagonal divisions = 76
- Wake element size (mm) = 3
- Wake bias factor = 5

Fine:
- Cylinder divisions = 200
- Total nodes = 31709
- Diagonal divisions = 100
- Wake element size (mm) = 2
- Wake bias factor = 5

Our inlet velocity is 0.01005 m/s and we treat the outlet as an outflow.

To record the lift and drag, we monitor (read, write and plot) Cl and Cd with the following reference values:

L = 0.01m
D = 1.00m
A = 0.01m

To obtain the RMS Lift we take the standard deviation of the lift coefficient while the cylinder is oscillating. Similarly, to obtain the mean drag coefficient we simply take the mean drag over the same period.

The tests have the following parameters

Coarse:
- time step = 0.05s
- number of time steps = 5000
- Max Local Courant number = 2.19

Medium
- time step = 0.0375s
- number of time steps = 6667
- Max Local Courant number = 2.48

Fine
- time step = 0.025s
- number of time steps = 10000
- Max Local Courant Number

Applying all these parameters, we get the following results

RMS Lift Coefficient
- Coarse = 0.237
- Medium = 0.218
- Fine = 0.189

Mean drag
- Coarse = 1.366
- Medium = 1.362
- Fine = 1.347

The change in these is values (especially the lift) is obviously too great to settle with any of these meshes. Can anyone suggest any incredibly obvious things that my partner and I are missing of straight out doing wrong? Any help is greatly appreciated!