## Simulation CFD

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# Reynolds number and drag force slightly high

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I am just beginning to use the software and conducting some sample similations. I have a 3D model of a 3.94 in diamater sphere in a square column of water with a horizontal velocity of 39.37 in/sec. I would like to chek the software output for the Reynolds number and drag force. Using water density of 9.33204e-5 lb-s^2/in^4 and Dynamic Viscosity of 1.45342e-7 lb-s/in^2, I calculate the Re to be 99,597.2 and drag force to be 0.3742 lbs. The wall results and summary file show drag force to be 0.637 lbs and Re to be 159876.

Why are the simulated results slighty higher?

Thanks

# Re: Reynolds number and drag force slightly high

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Hi jcwldy,

These analyses are typically highly mesh dependant, have you tried refining it and investigating the impact on the results?

Also what Advection scheme are you using? Try 5.

**Jon Wilde**

# Re: Reynolds number and drag force slightly high

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Yes, I have ran many simulations with different mesh sizes.

I am using the wall calculator to view the drag forces. Is this the correct mathod to view the drag forces?

The drag forces should be dependent on the density, fluid velocity, projected sphere area, and the drag coefficient. Is there a way to output or view a drag coefficient?

I tried simulations using a ADV scheme of 5 and the forces are still about double of what I expect to see.

Thanks

# Re: Reynolds number and drag force slightly high

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Hi jcwldy,

Could you share the support share file?

You may also want to consider using a development flag, resid_bdry_force_calc, which is being used to test a different way to calculate the wall forces. I have seen improvements with this flag. You will need to use ADV5 in conjunction with that flag to get the new outputs. The flag needs to be set at the scenario level and the default level.

Have you tried adaptation?

What Re number are you using from CFD?

It could be that the k-e/RNG turbulence models are difficient in this simulation.

-Royce

**Royce Abel**

# Re: Reynolds number and drag force slightly high

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Royce,

Thanks for your reply.

I have never implemented a development flag, but I will begin to research how. What will the development flag do for the analysis. Can you provide more information on this.

I will conduct a few runs using the adaption feature you suggested.

I am not using a Reynolds number from the simulation, I am calculating the Re using the fluid density, fluid velocity, sphere diameter, and dynamic viscosity, and using that to find the drag coefficient and calculate the drag force. The Reynolds number calculated from the analysis is higher. It is using the area of the continuum and the flow rate from the fluid velocity to calculate the Re.

How can I solve this?

I would be happy to share the support share file, but when I try to upload the file, it errors out because the file is too large. Contact me and I will link it to you.

Thank you again for your reply.