I am trying to simulate flow around a circular cylinder using ANSYS FLUENT. I did formulated my problem and solved till convergence. However, I was unable to validate the existing coefficient of drag (Cd) already published in the literature.
The flow was carried out with Re of 100 and the obtained Cd in my case was 0.0241 while in the literature the value in the range of 1.3-1.5 has been published.
Remember that Cd= Force in flow direction/(1/2*denisty*velocity^2*ProjectedArea)...
where Projected Area= Depth * Diameter
so in Fluent, before starting the solution, Go to Reference values and check the value of density, velocity and Area and length...
In case of 2D,
Depth is consider unity= 1m, so ProjArea= Diameter*1m=Diameter
Length= Diameter
after writing the correct values, now solve the problem
In case of 3D
Only thing which will be change is
Projected Area=Depth *Diameter
Length=Diameter
and then solve
Calculate the force and the velocity from the simulations and then calculate the drag coefficient manually. You might make a mistake to provide correct value in the reference values. Hopefully it will work.
suvash.com.au
have you checked the reference values?
you need to use the cylinder projected area and the diameter as the length.
- activate the option "Data sampling for time statistics
- Fluent will keep a running average of all variables from that time-step onwards
- after enough time-steps calculate the average Cd.
- Check whether the upstream and down stream are far enough
- the area should to surface area that used in the calculation
first go under the solution in the main menu of fluent
then choose monitors
after that choose force and make a force history with time
yoy can plot, print and save a file including the drag and lift coeff.
after that get the avarge value for the Cd from the file
Thank you very much everyone! @ Faik Hamad and @ Rafik Ouchene Thanks so much, do you mean in the Reynolds number (Re) calculation, the D must be the nominal diameter of the cylinder.
i.e. If I am simulating flow of Re=100, the Re=rho*u*DN/mu
where DN is the nominal diameter based on the cylinder and the velocity calculated will now be the input velocity.
yes, I agree with Rafik that the diameter used in Re is the nominal cylinder diameter
In case of 2d like guess the cylinder diameter bcomes the nominal diameter. Right?
You go through the following link. It is a tutorial for unsteady flow past a cylinder. Almost details were mentioned in this tutorial including time step calculation, selection of reference values etc
https://confluence.cornell.edu/display/SIMULATION/FLUENT+-+Unsteady+Flow+Past+a+Cylinder+-+Problem+Specification
When fluent works out coefficients it used the reference values provided by you. If you did not provide them, the reference length/area will most likely be 1m and 1 square metre.
If in doubt, I'd get the force (which will be calculated using the correct wall stresses and geometry - at least in most cases) and calculate the coefficents myself.
As your case is related to other on line publication, please find all constraint are same on both the cases and second type of flow you are using. As mentioned by other people about the cause, you should follow up there advice.
Remember that Cd= Force in flow direction/(1/2*denisty*velocity^2*ProjectedArea)...
where Projected Area= Depth * Diameter
so in Fluent, before starting the solution, Go to Reference values and check the value of density, velocity and Area and length...
In case of 2D,
Depth is consider unity= 1m, so ProjArea= Diameter*1m=Diameter
Length= Diameter
after writing the correct values, now solve the problem
In case of 3D
Only thing which will be change is
Projected Area=Depth *Diameter
Length=Diameter
and then solve
For 3D, I thought the Proj. Area =(( pi* DN^2)/4), where DN is nominal diameter.
see the reply of @Rafik above
Hi.
I have a problem with the reverse flow at the outlet of my geometry. I am doing a simulation to determine the water drag force on a pier inside a channel using FLUENT. I want to get the results for a Laminar flow (v=0.0001 m/sec). the simulation is going well but the drag force I got is negative, that means the reverse flow affecting on the results. I increased the elements density and I changed the geometry to be as shown in the attached picture and made the outlet far from the pier and I decreased the time step size but I still get the reverse flow warning messages. I read all the comments in this site about this problem and i did all what were suggested but unfortunately, the problem stills unsolved. Any tips please? [email protected]
Kindly check the reference values in Fluent and correct it. Most of the students who learnt CFD from me usually do this correction after arriving at wrong Cd values.
As your case is related to bridge piers and bridge piers scours on line publication, the drag coefficient for cylinders in rigid bed and sand bed is quite difference.
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