[User Tip]Using Arbitrary Interfaces for Sliding Meshes in CFD-ACE+

[cfdace]

When using the Grid Deformation module in CFD-ACE+ with systems containing rotating parts, the system may require the grid to rotate 360 degrees. However, this is not possible since the grid will become highly skewed/stretched. To alleviate this problem, Arbitrary Interface boundary conditions may be used in CFD-ACE+ to allow the meshes to slide past one another.

The Arbitrary Interface technique enables the solver to transfer data between disconnected grids. Although similar to zonal interfaces, one of the most useful features of this condition is that arbitrary interfaces do not require point-to-point matching grids at the interface. When you pick two groups of boundaries as an arbitrary interface set, the CFD-ACE-SOLVER will compute the intersections of every face in each boundary group. The intersection will then be an interface through which information will be allowed to transfer from one zone to another. The region where the sliding meshes line up will be an interface, and the remaining area of the patch will be the boundary condition specified in the GUI, i.e. an inlet, outlet, wall, etc. The arbitrary interface option is only available for 3D grid systems. However, it can still be used for "2D" cases by extruding the geometry one cell in the third direction. By extruding the geometry, there will be extra boundary conditions as compared to the 2D case. For the top and bottom boundaries, symmetry conditions will be used and all other boundary conditions will be applied just like they would be in the 2D case. Below is a figure which shows the symmetry boundary conditions.

For this user tip, we will consider a geometry which consists of two rotating parts, one is a cylinder (the bottom roll) and the other is non-cylindrical (the top roll). These two parts rotate in opposite directions and this rotation induces flow in the surrounding fluid. If both were cylindrical, we could use Rotating Wall boundary conditions, and therefore no arbitrary interface would be required. But, since this BC only adds velocity components at the wall surface, this is not possible if the boundary is not perfectly cylindrical. Thus, we actually need to 'move' the walls and not just specify a wall velocity. The boundaries are shown below in the Figure 2. The cylinder in red is rotating counter-clockwise and the cylinder in blue is rotating clockwise. The outer boundaries, which are in black, are outlets.

Figure 2. Boundary Conditions

The arbitrary interface in this example is shown in Figure 3 as the gap between the green and purple curves. This gap will allow the inner block (grid) to rotate and slide past the outer, stationary block (grid). The size of the gap should be about one tenth of the cell size.