The operating environment in a multistage turbomachine is inherently unsteady, with interactions occurring between rotor and stator airfoils. These interactions are both viscous and inviscid in nature. The potential field of each airfoil is inviscid in nature, and mainly affects adjacent airfoil rows. The viscous field is much more complex, with wakes from upstream airfoils convecting many chords downstream to interact with airfoils and other wakes.
The effects of airfoil clocking (or indexing) on turbine performance has been investigated by Huber et al. , Griffin et al.  and Dorney and Sharma . The studies showed changes in efficiency on order of 0.5% as the stators were clocked. In that study, the highest efficiencies occurred when the first-stage stator wake impinged on the second-stage stator and the lowest occurred when the first-stage stator wake convected through the middle of the second-stage stator passage. Very little work has been published on the subject of airfoil clocking in compressors, although Gundy-Burlet and Dorney  have recently studied the effects of stator clocking on the efficiency of a multistage compressor.
Wake/wake and wake/airfoil interactions were investigated for a two-dimensional þstage compressor configuration in Gundy-Burlet . The effect of varying axial gap on the unsteady flow within the compressor was studied. Initially, it was expected that reducing the axial gap between successive airfoils would result in increased unsteady forces on the airfoils because of the stronger potential interaction. However, the results indicated that the amplitude and frequency of the unsteady forces are also dependent on the interactions between the convected wakes and the airfoils. The wake structures could either increase or decrease the amplitude and frequency of the unsteadiness on the downstream airfoils.
The purpose of this study is to determine if the combined effects of airfoil clocking (or indexing) and varying the axial gaps between adjacent blade rows can be used to alter the steady and unsteady characteristics of the flow in a multistage compressor. An unsteady, two-dimensional thin-layer Navier-Stokes zonal approach is used to perform a parametric investigation of the effects of varying axial gap and stator clocking on the unsteady flow in a compressor. Results include time-averaged pressures and pressure amplitudes, force polars and compressor efficiency.