NEGATIVE DRAG FORCE

Hi everyone,

i'm doing CFD analysis of an aircraft. In particular, i'm interested in canopy's aerodynamic.

I've calculated drag force of the whole aircraft and of the canopy. In the second case, drag force is negative.

I don't understand why.

Can you help me?

Comments

  • sdebsdeb Posts: 192Forum Coordinator

    Hello,

    For force and Torque calculations, please make sure that your mesh is well resolved around the body of interest. You should ideally have 5 or more boundary layers.

    Also please check the Wall Y+ values to ensure that they are not high.

    Are you using SST k-omega for Turbulence? Then having a wall y+ around 1 should enable you to predict the forces and torques on the body in an accurate way.

    Regards,

    Surya

  • felice_luciverofelice_lucivero Posts: 3Member

    Hi @sdeb,

    i've set 8 boundary layers around the aircraft.

    The values of Wall Y+ are very high (>> 1), but i have already 8 millions cells and i don't want to increase them. Can I improve mesh quality whitout increasing cells' number?

    Yes, I'm using k-omega SST and I tried to set k-epsilon equations, which are more accurate near wall. Unfortunately i had no improvements.

    I'll be waiting for updates,

    Thank you in advance

  • sdebsdeb Posts: 192Forum Coordinator

    Hello @felice_lucivero ,

    In that case, you could try setting the first layer thickness for the boundary layers to a smaller value. I believe you have used Smooth Transition now.

    Please also check that the cell size should not change too much from the boundary layers to the internal cells.

    Another way is to dynamically coarsen/refine the cells based on velocity gradients.

    That way, the velocity gradients would be captured without too much refinement for the entire domain.

    Also plot the velocity distribution around the body of interest in the cells neighboring it.

    Do you see a good and smooth representation of the velocity gradient or is it very sharp and choppy?

    Regards,

    Surya

  • sdebsdeb Posts: 192Forum Coordinator

    Hello @felice_lucivero ,

    In addition to the above point, plot the pressure distribution as well. As a significant portion of the force might come through pressure. The rest will be viscous.

    Regards,

    Surya

  • felice_luciverofelice_lucivero Posts: 3Member

    Hello @sdeb,

    I've already used "first layer thickness" and first layer is quite small. (difference between layer and internal cells is circa 5%).

    When I tried to set dynamically coarsen/refine the cells based on velocity gradient, it asks me to define MIN and MAX values. Wich values should I insert?

    I've attached the image of velocity contours near canopy, tell me if there's something wrong.


  • sdebsdeb Posts: 192Forum Coordinator

    Hello @felice_lucivero ,

    You can try to use a Normalized value for the velocity gradients. it will then be bounded within 0 and 1. Accordingly you can specify the min/mac for coarsening/refining.

    You will find more information in the link below:


    Also , can you please embed the image instead of attaching it as we are not supposed to download attachments.

    Regards,

    Surya

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