Is vortex method inducing *specific* frequencies?

    • johendrik.thysen
      I’m performing LES simulations of indoor ventilation flow, with the supply inlet as visualised in the image (the room is only partially shown). The inlet geometry gradually reduces in height. I used the vortex method for inlet velocity fluctuations, with the number of vortices equal to 1000 (maximum value you can set in Fluent; although I am aware now that one can set it higher) and the number of grid cells in the inlet plane around 12,000. The inlet plane is perpendicular to the flow direction.
      On the right-hand side of the image, the 1D energy spectrum of the streamwise velocity obtained at the monitoring point indicated with the cross symbol (located close to the room inlet) is depicted. As you can see, the spectrum contains high-energy frequencies at a fixed frequency interval (2.17 Hz, 4.34 Hz, 6.52 Hz, ...: separation = 2.18 Hz). My question is, why do we see these discrete frequency peaks?
      Some observations I made:
      From inspection of the instantaneous velocity magnitude contours over time (shown in the image at different times t0, t1, t2), I can see that the velocity at the inlet is fluctuating with a frequency equal to the lowest specific frequency detected in the spectrum (~ 2.17 Hz).
      So, I assumed the Vortex Method causes these fluctuations and I tried to find out how such specific frequencies may be induced by this method. The only information in the Fluent manual related to a time scale in the Vortex Method is: “The sign of the circulation of each vortex is changed randomly each characteristic time scale \tau. In the general implementation of the vortex method, this time scale represents the time necessary for a 2D vortex convected by the bulk velocity in the boundary normal direction to travel along n times its mean characteristic 2D size (\sigma_m), where n is fixed equal to 100 from numerical testing.” I tried to estimate this time scale but it did not match with the detected frequencies (I made some assumptions so my calculation could also not be accurate).
      Further away from the inlet and in the room, the energy spectra do not show such discrete frequencies.
      I also found that the spectrum remains identical in the case:
      the spectrum is obtained from the y-velocity (lateral direction)
      a change in the subgrid-scale model is made (WALE or dynamic kinetic energy)
      the grid is even more refined (i.e. even more cells over the inlet plane)

      Thank you very much for considering my question.
      Best regards

    • Kalyan Goparaju
      Ansys Employee


      It seems that the snapshots you provided are not showing presence of any resolved turbulent structures.. Maybe it would be useful to check:

      1)how do structures at the inlet look (say, look at velocity component orthogonal to the mean flow)

      2)that resolved turbulent structures indeed exist in the domain (can visualize them by vorticity magnitude, Q-criterion isosurface or just the field of not-x-velocity component …).  And in general it’s a good practice with any scale resolving simulation.

      3)If it is a velocity inlet, the velocity contour plot on the inlet plane with the "boundary values" option will show the generated synthetics. Is it, perhaps, completely off?

      4) Is the working fluid air at high pressure? For air at normal conditions the Reynolds number is too low.



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