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Boundary Conditions

Both natural and zonal boundary conditions are required within the zonal grid system. Zonal boundary conditions occur at the interfaces between the inner and outer grids and at the slip interfaces between outer grids. Bilinear interpolation is used to transfer information between grids along these interfaces. The natural boundary conditions occur on the vane, blade, hub and tip surfaces, as well as at the inlet and outlet boundaries. No-slip and adiabatic boundary conditions are applied on the physical surfaces within the turbine. On rotating surfaces, the no-slip boundary condition implies that the fluid velocity at the surface is set to the rotational velocity of the surface. The hub velocity is blended in the overlap regions between the rotating and stationary grids to improve algorithm stability. At the inlet, the flow angles, total pressure and the downstream-propagating Riemann invariant are specified while the upstream-propagating Riemann invariant is extrapolated from the interior. The inlet plane of the computational grid is approximately two first-stage stator chords upstream of the first-stage stator. As a significant boundary layer is present at that axial location, a boundary layer profile has been imposed at the inlet. At the exit, cross-flow velocities, the downstream-propagating Riemann invariant and the entropy are extrapolated from the grid interior. The midspan exit pressure is specified and radial equilibrium is used to determine the radial pressure distribution.

Karen L. Gundy-Burlet
Wed Apr 9 13:58:36 PDT 1997