Fluid flow in a mixer

For this work we followed the tutorials 7.1 and 7.2 of StarCD. We give here the main stages:

GEOMETRY

The mixer consists of two concentric cynlider, both mounted with equally-spaced baffles. The fluid (water) is stirred by the inner cylinder and its baffles as they rotate at a speed of 300 rpm. A K-E turbulence model is used (high Reynolds number option).

MESH CREATION

The grid is created with the module 'Create a 3D grid' of PROSTAR. An outer and inner cylinder are first defined. The sliding interface will be located along a cylindrical surface where the two parts of the mesh meet. Then six and four equally-spaced baffles are created around the inner and the outer cylinder (in blue).

BOUNDARY CONDITIONS

Boundary conditions at the sliding interface must be of type "Attach" (in red). Boundaries for the inner cylinder wall are defined explicitly in order to assign a rotational speed to them. The same is not necessary for the inner bafflesas their motion is normal to the mesh and prescribed by the mesh movement operations.

Note that duplicate vertices still exist at teta=0 and teta=360. They have to be merged together. This is normally done at the end of the mesh creation stage.
However, in this particular case, the task has been deferred to this point in order to maintain regular vertex numbering when specifying the sliding interface
and moving boundaries.

Since the problem is two dimensional, all cell faces normal to the Z-directions must become part of a symmetry plane.

EVENTS SETTING

Events settings is required when a sliding mesh is used (see the manual for commands). Moving mesh operations are defined in terms of event steps and these must be stored in a special file (cgrid.cgrd file). There is two ways to handle the problem of a sliding interface: the slide-shear approach and the ASI method (arbitrary sliding interface). Both have been tried and have showed that the best one is the ASI method. Indeed, this method is shorter and more flexible. As cell connectivities at the sliding interface are automatically calculated, the set up of events and grid changing operations are greatly simplified.

The following picture illustrates how a sliding movement works. Two basic moving mesh operations are used: a change of mesh geometry  and a change of cell connectivity.

The ASI method only requires specification of changes in the mesh geometry. As changes in cell connectivity are not required, this method is easier.

cgrid.cgrd  for the slide-shear approach

*IF TIME GT 0.
*SET DPS 300. * 360. / 60.
CSYS 4
*SET TANG TIME * DPS
*GET TPOS Y 1615
*SET TOFF TANG - TPOS
CSET NEWS GRAN 4.9,9.1,,,,,2
VSET NEWS CSET
VGEN 2 0 VSET,,,0 TOFF
*SET TNEW EVEX * 10.
*GET TNOW Y 1801
*SET TOFF TNEW - TNOW
CSET NEWS GRAN 0.049 0.251,,,,,4
VSET NEWS CSET
VSET SUBS GRAN 0.249 0.251,,,,,4
VGEN 2 0 VSET,,,0. TOFF
CSET ALL
*ENDIF

cgrid.cgrd  for ASI method

*IF TIME GT 0.
*SET DPS 300. * 360. / 60.                               Convert rotating speed to degrees/sec.
CSYS 2                                                                   Activate cylindrical coordinate system
*SET TANG TIME * DPS                              Total degrees rotated by mesh
*GET TPOS Y 37                                                Get  teta coordinate of vertex n. 37
*SET TOFF TANG - TPOS                           Calculate offset angle in degrees
CSET NEWS GRAN 4.9 10.1,,,,,2                  Collect all the cells in the inner mesh
VSET NEWS CSET
VGEN 2 0 VSET,,,0 TOFF                                Change VSET vertices by angle TOFF degrees
CSET ALL
*ENDIF

RESULTS