Fluent 6.1.22: Deforming (MDM) and sliding (SMM) mesh exhibiting rotation
A piston pump shown in attachment <a target=_blank href="http://www.fluentusers.com/support/solutions/943/piston_pump_fig1.gif">http://www.fluentusers.com/support/solutions/943/piston_pump_fig1.gif</a>http://www.fluentusers.com/support/solutions/943/piston_pump_fig1.gif is to be modeled. The problem here is that the displacing pistons which can be modeled by layering (MDM) also rotate. The question is "Does modeling the rotation by a Moving Mesh take the rotation correctly into account, while concurrently MDM can be used to model the oscillation of the pistons?" We consider a simplified test case <a target=_blank href="http://www.fluentusers.com/support/solutions/943/cradle_geometry.jpg">http://www.fluentusers.com/support/solutions/943/cradle_geometry.jpg</a>http://www.fluentusers.com/support/solutions/943/cradle_geometry.jpg consisting of a cylinder rotating around an axis parallel to its own but offset in the radial direction. One end is a solid wall which oscillates in axial direction, while the other represents an interface which opens into another, much larger cylinder whose axis coincides with the offset position of rotation. This cradle configuration is modeled by (1) SMM (Sliding Mesh Model) in Define/Boundary Conditions/rot_fluid with 10 rpm rotation. (2) MDM (Moving deforming Mesh  layering approach) in Define/Dynamic Mesh for the bottom wall with a purely oscillatory motion prescribed by an UDF. First we verify a radial pressure gradient when the cylinder is purely revolving SMM only. Then the pressure has to be approximately defined by d(p)/d(r) = rho (vtheta)^2/r, i.e. the pressure difference is approximately 6 Pa. This value is confirmed by <a target=_blank href="http://www.fluentusers.com/support/solutions/943/cradle_SMMclip_z%3D6.jpg">http://www.fluentusers.com/support/solutions/943/cradle_SMMclip_z%3D6.jpg</a>http://www.fluentusers.com/support/solutions/943/cradle_SMMclip_z%3D6.jpg at the axial position z=6 for a radial cut through the cylinder. Second, we model the oscillatory piston without the rotation. Here no radial pressure gradient is expected, cf. <a target=_blank href="http://www.fluentusers.com/support/solutions/943/cradle_MDM_A%3D0.01clip_z%3D6.jpg">http://www.fluentusers.com/support/solutions/943/cradle_MDM_A%3D0.01clip_z%3D6.jpg</a>http://www.fluentusers.com/support/solutions/943/cradle_MDM_A%3D0.01clip_z%3D6.jpg . The amplitude of the oscillation is chosen to be small, so that the pressure variation due to compression and expansion is in the same order of magnitude as the pressure gradient due to rotation. Finally we superpose these approaches by enabling (1) and (2). We confirm the amplification of the radial pressure gradient due to rotation by the pumping cf. <a target=_blank href="http://www.fluentusers.com/support/solutions/943/cradle_SMM_MDM_A%3D0.01clip_z%3D6.jpg">http://www.fluentusers.com/support/solutions/943/cradle_SMM_MDM_A%3D0.01clip_z%3D6.jpg</a>http://www.fluentusers.com/support/solutions/943/cradle_SMM_MDM_A%3D0.01clip_z%3D6.jpg Thus the combined approach using deforming (MDM) and sliding (SMM) mesh exhibiting rotation is valid. 

