Tech Briefs

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The Periodic Boundary Condition in ADINA CFD
In this News we demonstrate the option of imposing periodic boundary conditions in ADINA CFD. These boundary conditions are designed for flows in which the physical geometry and the expected flow pattern are periodic in space. We illustrate this feature by presenting two examples.
The first example is the twodimensional cascade flow shown in Figure 1. This analysis is rather simple but illustrative.
Figure 1 Twodimensional cascade flow: schematic
Due to the symmetry in flow geometry, the computation can be performed over just one passage with a translation periodic boundary condition. The mesh used is shown in Figure 2.
Figure 2 Two dimensional cascade flow: mesh
Results of this analysis are shown in Figures 3 and 4.
Figure 3 Two dimensional cascade flow: velocity vector plot
Figure 4 Two dimensional cascade flow: pressure band plot
Our second example is the propeller driven flow in the pipe shown in Figure 5.
Figure 5 Rotating propeller in a pipe: schematic
There are 8 blades on this propeller set symmetrically on a central drum. Since the geometry and flow conditions exhibit rotational symmetry, we apply a rotational periodic boundary condition, choosing a small part of the domain to perform the computation, as shown in Figures 6 and 7.
Figure 6 Computational domain with periodic boundary condition, visualized with EnSight
Figure 7 Mesh used in computational domain, visualized with EnSight
The above movie shows the velocity and pressure results. Figure 8 shows the pressure calculated over the small domain using the periodic boundary condition option with the small domain computation. The pressure calculated using the entire geometry, i.e. the full domain, is shown in Figure 9. The results are the same, but using the small domain saves time and requires less memory. It should also be noted that the use of the periodic boundary condition option does not require that the meshes match exactly at the periodic boundaries.
Figure 8 Pressure around the blades, small domain computation, visualized with EnSight
Figure 9 Pressure around the blades, full geometry computation, visualized with EnSight
The periodic boundary condition option available in ADINA for CFD calculations is a convenient and efficient tool for the analysis of symmetric flows, allowing analysts to obtain accurate results with considerably less effort, computational time and memory. In fact, this option can be of great importance to solve complex 3D flows in many geometries at a reasonable cost.
Keywords:
Cyclic symmetry, substructuring, periodic, boundary condition, symmetric flow, FCBI, CFD, propeller, driven flow, pumps