# how to compute velocity wrt to local cylindrical coord. system

 QUESTION: How to compute values of radial and axial components at two exit planeswith respect to local cylindrical coordinate systems anchored at the centers of each exit plane?See Figure-0.gif for illustration.http://www.fluentusers.com/support/solutions/958/Figure-0.gifhttp://www.fluentusers.com/support/solutions/958/Figure-0.gifANSWER: According to Fluent documentation, Chapter 29.2 Velocity Reporting Options,----------------------------------------------- * Cylindrical velocities: These velocities are the axial, radial, and tangential components based on the following coordinate systems: .... o For 3D problems involving a single cell zone, the coordinate system is defined by the rotation axis and origin specified in the Fluid panel. o For problems involving multiple zones (e.g., multiple reference frames or sliding meshes), the coordinate system is defined by the rotation axis specified in the Fluid (or Solid) panel for the ``reference zone''. The reference zone is chosen in the Reference Values panel, as described in Section 28.8. Recall that for 2D problems, you will specify only the axis origin; the direction is always the axial direction.----------------------------------------The approach, then, is to use multiple reference frames, and separate the single fluid zone (see Figure-1.gif) into three zones, as shown in Figure-2.gif.http://www.fluentusers.com/support/solutions/958/Figure-1.gifhttp://www.fluentusers.com/support/solutions/958/Figure-1.gifhttp://www.fluentusers.com/support/solutions/958/Figure-2.gifhttp://www.fluentusers.com/support/solutions/958/Figure-2.gifThis can be achieved in Fluent by using cell separation based on mark tool, in conjunction with Adaption registers:first, mark desired cells based on their location (found under Adapt>Region menu), then separate fluid zone based on created registers (found under Grid>Separate>Cells).Next, define coordinate system for each of the two separated fluid zones, fluid_zone-1 and fluid_zone-2 (shown in Figure-2.gif).Click on Define>Boundary Conditions, select a fluid zone, click on Set... and provide values for Rotation axis origin and direction.Tip 1:you can use Plane tool to help you set the "rotation" (i.e.axial) direction: o first, allign the plane tool with exit plane (see Figure-3.gif)http://www.fluentusers.com/support/solutions/958/Figure-3.gifhttp://www.fluentusers.com/support/solutions/958/Figure-3.gif o second, enable Porous Zone in the Fluid panel, and click on Update from Plane Tool o third, copy the values for Direction-1 under Rotation Axis Direction o fourth, deselect Porous Zone in the Fluid panel o repeat the steps for the other fluid zone.Tip 2: to define center of the exit plane, i.e. axis origin, you can use Fluent Surface Integrals to compute facet average x, y, z coordinates (found under Report>Surface Integrals) for each of the two exits.Finally, select fluid_zone-1 as reference zone (in Report>Reference Values...) and compute average axial and radial velocities for exit-1.Next, select fluid_zone-2 as reference zone (in Report>Reference Values...) and compute average axial and radial velocities for exit-2.