{"id":171023,"date":"2023-06-05T07:05:31","date_gmt":"2023-06-05T07:05:31","guid":{"rendered":"\/knowledge\/forums\/topic\/i-have-a-circular-inlet-boundary-on-which-i-prescribe-the-velocity-as-cylindrical-velocity-components-the-x-axis-is-normal-to-the-inlet-plane-and-used-as-rotation-axis-in-the-axis-definition-the-a\/"},"modified":"2023-07-31T12:29:53","modified_gmt":"2023-07-31T12:29:53","slug":"i-have-a-circular-inlet-boundary-on-which-i-prescribe-the-velocity-as-cylindrical-velocity-components-the-x-axis-is-normal-to-the-inlet-plane-and-used-as-rotation-axis-in-the-axis-definition-the-a","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/knowledge\/forums\/topic\/i-have-a-circular-inlet-boundary-on-which-i-prescribe-the-velocity-as-cylindrical-velocity-components-the-x-axis-is-normal-to-the-inlet-plane-and-used-as-rotation-axis-in-the-axis-definition-the-a\/","title":{"rendered":"I have a circular inlet boundary on which I prescribe the velocity as “Cylindrical Velocity Components”. The x-axis is normal to the inlet plane and used as rotation axis in the axis definition. The axial and theta velocity components are defined as expressions depending on the radius coordinate r. When I plot the expressions over r, everything seems fine, but when I run the simulation, the velocity components are not applied correctly and do not match the functions that I prescribed. What could be the reason for this?"},"content":{"rendered":"

CFX evaluates the radius coordinate r in the global coordinate system if the boundary has not been associated to any other (local) coordinate frame on the \u201cBasic Settings\u201d tab of the boundary condition. This is the case also if you select the option \u201cCyl. Vel. Components\u201d to prescribe the inflow velocity on the \u201cBoundary Details\u201d tab. The definition of the rotation axis on this tab only defines the orientation of the three velocity components \u201cAxial\u201d, \u201cRadial\u201d, and \u201cTheta\u201d. If you access a coordinate in the expressions used to prescribe the velocity components, this axis definition has no effect on the coordinate system in which the coordinate is evaluated \u2013 it will be the global coordinate system by default. To evaluate coordinates in the local coordinate system which is defined by the centroid and normal of the inlet boundary (i.e., the system which defines the orientation of the cylindrical velocity components), you must first define a corresponding local coordinate frame, e.g., using the \u201cPoint and Normal\u201d option (see chapter 25.2.1. Coordinate Frame: Option in the CFX-Pre User\u2019s Guide). If you then assign this to the inflow boundary on the \u201cBasic Settings\u201d tab, all coordinates are evaluated in local coordinate system. To check prior to the simulation if the velocity components are prescribed correctly, you can use the plot feature on the \u201cPlot Options\u201d tab of the boundary condition, see chapter 14.2.6. Boundary Plot Options Tab in the CFX-Pre User\u2019s Guide.<\/p>\n","protected":false},"template":"","class_list":["post-171023","topic","type-topic","status-publish","hentry","topic-tag-17-2","topic-tag-cfx","topic-tag-coordinate-system","topic-tag-fluid-dynamics","topic-tag-general-cfx","topic-tag-local-coordinate-system"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t