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Grid and Geometry

The grid system for the þstage compressor geometry is shown in Fig. 1. The experimental compressor geometry consists of an inlet guide vane (IGV) followed by two nearly identical stages. The only difference between the stages is that the first-stage rotor is closed tex2html_wrap_inline83 relative to the second-stage rotor. The only deviation from the original design was that the axial gaps between the rotors and stators were increased slightly to permit the incorporation of a radial-circumferential traverse system. All airfoils are defined by NACA 65-series sections with circular-arc mean camber lines. There are 44 airfoils in each row. The hub and casing are at a constant radius along the axis of the compressor. These features make the compressor ideal for computational modeling.

The computation was performed using the midspan experimental geometry. It was assumed that the flow is periodic from airfoil to airfoil in the circumferential direction, so the flow through only one airfoil-to-airfoil passage was computed. In addition, the experimental IGV geometry was unavailable at the time this calculation was initiated, so a 0.8%-thick parabolic arc airfoil was used. These assumptions are not expected to significantly affect the comparison between the computation and the experiment.

Two types of grids are used to discretize the flow about each of the airfoils, as seen in Fig. 1. An inner "O" grid, generated using an elliptical method, is used to resolve the viscous effects near the airfoil. The "O" grid is then overlaid on an algebraically generated, sheared Cartesian outer grid. The outer grids are allowed to slip past each other to model the relative motion between rotors and stators. Using this system, 12 grids are required to fully discretize the flow field through the þstage compressor.

Two different grids were used to study the effect of grid refinement on the solution. Every other point in the coarse grid is displayed in Fig. 1. The inner grids have tex2html_wrap_inline85 grid points each, and the outer grids have tex2html_wrap_inline87 grid points each. The inlet and exit grids have tex2html_wrap_inline89 and tex2html_wrap_inline91 grid points each, respectively. The total number of points in all 12 zones of the coarse grid is 53,173.

Experimental data exists only for the second stage of the compressor. In order to reduce the computational time required for a grid refinement study, grid refinement was done only in the second stage of the compressor. The second-stage inner grids were refined to have tex2html_wrap_inline93 points each while the second-stage outer grids were refined to have tex2html_wrap_inline95 grid points each. The exit-grid size was increased to tex2html_wrap_inline97 . Thus, the fine-grid cells are one quarter the area of the coarse-grid cells. The total number of points for the fine-grid system was 116,732.


next up previous
Next: Results Up: TEMPORALLY AND SPATIALLY RESOLVED Previous: Algorithm

Karen L. Gundy-Burlet
Wed Apr 9 10:52:39 PDT 1997