Getting Correct Drag Coefficient

I am trying to get a drag coefficient. For some reason, I am getting a number that doesn't really make sense. I will place the pictures of what I did.

I imported the geometry, created an enclosure, and suppressed the geometry. I suppressed the geometry in an effort to make "walls" out of the geometry.

I added a surface area report definition. I selected the option enclosure_enclosure. I believe it did select the right thing to calculate for drag.

Once again I added another report definition, I added a drag coefficient. I selected the enclosure_enclosure option. It, however, did not select the geometry like the surface area report. I'm not sure if this is right or wrong.

These are the calculation parameters I put in.

I generated vectors. I believe that the velocity vectors makes sense.

Here are the results. The only number that makes sense is the surface area. I found the surface area from Autodesk Inventor. The original number is in in^2, but a conversion was made and it verifies that the surface area is correct.

I used omni calculator to check the results.

This confirmed that the numbers dont make sense.


What can I do to fix this? This would be much appreciated.

Answers

  • Hello,

    1. Do you have a converged solution? Please share plots of your residuals and drag / lift coefficient as a function of iterations
    2. What reference values are you using?
    3. Do you have sufficient resolution in your boundary layer mesh?

    The lift and drag coefficients in such simulations are very dependent on all of these questions. Please double check these and see if they address your issues.

    Thanks.

    Karthik

  • I believe I do have a converged solution. Im not sure how to get the drag/lift coefficient as a function of iterations. How do you do it? Here are the residuals:

    Reference values:

    Here is the mesh quality information. I used the evaluate mesh quality button on fluent.


  • DrAmineDrAmine GermanyForum Coordinator

    Mesh resolution is not enough. Residuals are still high and reference values need to be adjusted as reference area and density and velocity are to get dimensionsles parameters

  • How do I improve the mesh quality? So I need to manually change the reference surface area? It doesnt automatically get changed with the geometry? What does it mean that the residuals are high? Im confused on why velocity and density need to get dimensionless parameters. Im really new to Ansys and would appreciate it if you could help out.

  • DrAmineDrAmine GermanyForum Coordinator

    The definition of drag coefficient requires a reference area as well as a reference stagnation pressure.


    Improve mesh in preprocessing. The wall is very coarse.


    Your residuals are high. Moreover at that speed I will include turbulence modeling.

  • I still dont know what high residuals means. Why is turbulence modeling necessary?

  • Please see the residual plot. Your continuity, momentum, and energy residuals are high. They should, for a good converged solution, go down by at least 3 orders of magnitude.

    What is your inlet Reynolds number?

    Thanks,

    Karthik

Sign In or Register to comment.