Fluids

Fluids

NEGATIVE DRAG FORCE

    • felice_lucivero
      Subscriber
      Hi everyone,ni'm doing CFD analysis of an aircraft. In particular, i'm interested in canopy's aerodynamic.nI've calculated drag force of the whole aircraft and of the canopy. In the second case, drag force is negative.nI don't understand why.nCan you help me?n
    • Surya Deb
      Ansys Employee
      Hello, nFor 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.nAlso please check the Wall Y+ values to ensure that they are not high. nAre 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.nRegards,nSuryann
    • felice_lucivero
      Subscriber
      ni've set 8 boundary layers around the aircraft.nThe 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?nYes, 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.nI'll be waiting for updates,nThank you in advance n
    • Surya Deb
      Ansys Employee
      nIn 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.nPlease also check that the cell size should not change too much from the boundary layers to the internal cells.nAnother way is to dynamically coarsen/refine the cells based on velocity gradients.nThat way, the velocity gradients would be captured without too much refinement for the entire domain.nAlso plot the velocity distribution around the body of interest in the cells neighboring it. nDo you see a good and smooth representation of the velocity gradient or is it very sharp and choppy?nRegards,nSuryan
    • Surya Deb
      Ansys Employee
      nIn 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.nRegards,nSuryann
    • felice_lucivero
      Subscriber
      Hello Array,nI've already used first layer thickness and first layer is quite small. (difference between layer and internal cells is circa 5%).nWhen 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?nI've attached the image of velocity contours near canopy, tell me if there's something wrong.nArraynn
    • Surya Deb
      Ansys Employee
      nYou 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. nYou will find more information in the link below:nnAlso , can you please embed the image instead of attaching it as we are not supposed to download attachments.nRegards,nSuryann
Viewing 6 reply threads
  • You must be logged in to reply to this topic.