please anyone, can you help me to model the drag coeffient of a 2D cylinder using fluent for various Reynolds numbers as I'm unable to obtain accurate results for Reynolds number greater than 100, my aim is to try to compare the experimental results of Re against Cd with those obtained using fluent. I have tried so many options, but to no avail. I would be grateful if anyone has a step by step guide on how i should model this problem, please note I'm not an expert on fluent, as I've only just started using it recently. Thank you john Lynam

Make sure you're running Fluent in transient mode. Above a very low Reynolds number, the shedding behind a cylinder is transient. Re = 100 is above that limit.

Also, try refining your mesh near the surface of the cylinder and in the wake. Errors associated with a coarse mesh can show themselves as 'numerical viscosity', making the computational Reynolds number much less than intended - and thus damping the oscillations.

Look at Schlichting, Boundary Layer Theory, (mine is 4th edition, 1960), p. 17, for some nice pictures of the flow you should compute at Re = 100.

[alessios123]

Hi guys, i'm tring to calculate the drag force coefficient in a 3-d case and the fluid is behind a cylinder.
the fluid is:
density = 800 kg/m^3
dinamic viscosity = 0.005 kg/ms
velocity = 8 m/s

the cylinder is:
diameter = 13 mm
length = 300 mm
stiffness modulus = 200000 MPa
poisson coefficient = 0.3

to calculate the drag coefficient i push: solve/monitors/force/drag coefficient.....
the direction of the vector of force is in parallel to the flow flowing

i set the reference value:
Area = 0.013*0.3 that is the product for diameter and the length of the cylinder,
Length = 0.013 that is the diameter
then i set other values: viscosity, velocity, density of fluid as i wrote upforward.

i put 90 nodes around the circonference of the cylinder, and 25 along to the length of the cylinder.

the algorytm used is LES, because this is a 3d case
I used PISO
and second order upwind for momentum...

the Cd value is incresing but after 0.02 seconds it is still to 0.7.

How can i do, please help me.
Alessio
Thank to all