5 Replies Latest reply on Jan 27, 2016 8:28 AM by Amit Katz

    Adding a rotation region for not-a-fan objects.

    Билл Гейтс

      Hello guys! I have a simple question - most of tutorials are about fans or something like this, but what do i need to to in such situations:

      1) pipe with extension at the end is rotated about a vertical axis (1 pic), do i need to make rotation region covering only the pipe walls (2nd pic) or the walls and empty center (3rd pic)?

      2) basket with blades is rotated about a vertical axis (4 pic), do the rotation region need to cover only bottom and blades zone (5 pic) or to cover all "cylinder" around a model?

      Thanks!

        • Re: Adding a rotation region for not-a-fan objects.
          Amit Katz

          The question of whether or not to include the fluid in your rotating region will depend on what your goals are. Remember that any fluid cells inside of the rotating zone will move with the zone according to its angular velocity. In theory this is OK for some fluids because for most Newtonian fluids, that is the "steady state" nature of rotation. However this could be a problem if you're looking at developing flow, or if you have some strange geometry. It also depends on whether you are doing rotational averaging or sliding mesh. Could you tell us a little more about your application?

            • Re: Adding a rotation region for not-a-fan objects.
              Билл Гейтс

              ok there are some details:

              1) i didnt set up any goals in the study (pic 0.1 - are my calculation control options correct for this? what do i need to change to have more correct and stable calculation results?) - i need just to see how rotating tube (usual air inside) will affect in the particles wich are falling with g=9.8 from top to down (pic 0.3). i want to see will they just fall or will be pressed against the wall as in a centrifuge - dont know what calculation goals can be apllied in this situation..

              2) metal tube with roughness 0,5 mm, height ~1500mm and lesser D=400mm, rotates with angular speed 15 rad/s. (pic 0.2).

              3) first i made rotation regoin that just covered the walls of tube +5 mm (i.e rotation region was like tube model but had the thicker walls. what value is better for calculation? how much bigger must be rotation region than an object wich it covers?). so "empty" part of the tube was not affected by rotation region. results: 1.1. - speed in tube at the end of calculation, 1.2. 1.3. - airflow trajectories, 1.4, 1.5 - falling particles.

              4) now i made rotation region wich totally covers the tube (no space in the center) +5 mm to outside. other conditions are unchanged. and thats what ive got: 2.1. - speed in tube at the end of calculation, 2.2. 2.3. - airflow trajectories, 2.4, 2.5 - falling particles.

              5) so the main question - wich rotation regoin is correct in this situation? results are very different..

              6) second situation with basket is the same - it rotates and particles are falling in its center so the centrifugal force throws them through the blades outside - wich rotation region i need to use in that case?

              thanks!

              ps sorry for my crazy english - its not my native :]

                • Re: Adding a rotation region for not-a-fan objects.
                  Amit Katz

                  I think in your case it would be OK to have the rotating region cover all of the fluid inside of the tube, since this will be the steady state solution after a long time. The effect you see in things like 1.3, the inviscid core that doesn't rotate with the walls is probably because your simulation didn't run through enough iterations. Air is relatively less viscous than liquids, and so it may take a long time of the tube spinning for the angular motion to diffuse through the core of air in the center, but it will happen eventually. Just as a cup of water that's left on a spinning plate will eventually have all the liquid inside rotating as a uniform volume around the central axis.

                    • Re: Adding a rotation region for not-a-fan objects.
                      Билл Гейтс

                      Thanks, I'll try to use the "whole coveriing" rotation region. This is suitable and for "basket" as I understand?

                      And one more little question - rotation region must fit 1:1 to the model or be  a few mm bigger? For example fan D=200 and region D=200 too, or D must be for example 202-210mm? In different examples there are different ways to build it.. In the example that i attached (found on youtube) the rotation region is much bigger than helicopter blades - why?

                • Re: Adding a rotation region for not-a-fan objects.
                  Amit Katz

                  It comes down to experience really. You have to remember that every fluid cell inside the region will be forced to rotate at the specified angular velocity, so the designer must be confident that will be the case in actuality. I can't tell you exactly why some people make the boundary tight and others make it loose, you'll have to just try it and see what happens.