Next: Results Up: COMPUTATIONS OF UNSTEADY MULTISTAGE Previous: Algorithm

# Geometry and Grid

The þstage compressor geometry used in this study models the midspan geometry of an experiment by Dring (AGARD, 1989). The experimental configuration consists of an inlet guide vane followed by two rotor/stator pairs. There are 44 airfoils in each of the rows leading to a 1:1 ratio of airfoils down the compressor. As it would be prohibitively expensive to compute the flow through the entire 220 airfoil system, the flow has been computed only through one passage and periodicity has been used to model the other 43 passages. The axial gaps between airfoil rows in the experimental configuration are approximately 50% of the average axial chord. In this study the flow through the compressor has been computed with the same midspan airfoil geometry, but with varying axial gaps.

In Gundy-Burlet, et. al. (1989, 1990), a parabolic arc inlet guide vane was used because the actual vane geometry was unavailable. The vane geometry has recently become available and is used in this calculation. The first and second stages of the compressor are similar, except that the first-stage rotor is closed 3 degrees from axial relative to the second stage rotor. This reduces the angle of attack of the first stage rotor. The airfoil sections are all defined by NACA 65-series airfoils imposed on a circular-arc mean camber line. The average axial chord is 4 inches.

A zonal grid system is used to discretize the flowfield within the þ stage compressor. Figure 1 shows the zonal grid system used for the 20% gap case. In Fig. 1, every other point in the grid has been plotted for clarity. There are two grids associated with each airfoil. An inner, body-centered "O" grid is used to resolve the flow near the airfoil. The thin-layer Navier-Stokes equations are solved on the inner grids. The grid points of the inner grids are clustered near the airfoil to resolve the viscous terms. The Euler equations are solved on the outer sheared cartesian "H" grids. The rotor and stator grids are allowed to slip past each other to simulate the relative motion between rotor and stator airfoils. In addition to the two grids used for each airfoil, there is also an inlet and an exit grid, thus yielding a total of 12 grids.

In order to generate inner grids that are wholly contained by the outer grids and yet are not distorted, it was necessary to overlap the rotor and stator outer grids in the gap regions for the 20% axial gap case. This can be seen in the 20% axial gap grid shown in Fig. 1. This required a modification to the grid generator and algorithm, and permits study of turbomachines with small axial gaps.

A coarse grid configuration has been used to validate workstation results. The inner grids are dimensioned . The outer grids have varying number of points in the axial direction, but they all have 61 points in the circumferential direction. The inlet and outlet grids have 28 and 30 points in the axial direction respectively. The outer grids associated with an airfoil average 77 points in the axial direction. This leads to a total of 50367 points for all zones in the coarse grid configuration.

Fine grids are used to obtain detailed data regarding the steady and unsteady flow structure in the compressor. The inner grids are dimensioned . The outer grids have varying number of points in the axial direction because of the change in axial gap and axial extent of each airfoil, but they all have 87 points in the circumferential direction. The inlet and outlet grids have 40 and 42 points in the axial direction respectively. The outer grids associated with an airfoil average 99 points in the axial direction for the 20% gap case, 101 points for the 35% gap case and 110 points for the 50% gap case. This leads to a total of 97279 points for all zones in the fine grid configuration for the 20% gap case, 98323 points for the 35% gap case and 102064 points for the 50% gap case.

Next: Results Up: COMPUTATIONS OF UNSTEADY MULTISTAGE Previous: Algorithm

Karen L. Gundy-Burlet
Wed Apr 9 12:58:06 PDT 1997