Control of the unsteady flow in a stator blade row interacting with upstream moving wakes

Abstract

A computational study of the unsteady flow in a 2-D stator blade row interacting with upstream rotor wakes has been carried out. A direct spectral-element Navier-Stokes solver has been used for the laminar flow regime (Re<10,000). Turbulent calculations (Re>106 are based on the Baldwin-Lomax turbulence model. The rotor wakes are represented by velocity distortions moving along the inlet boundary of the computational domain. After interception, the rotor wake migrates towards the pressure surface of the stator blades where it forms a pair of counter-rotating vortices. A moving series of such vortex pairs is the dominant form of unsteady flow over the pressure surface. The unsteady flow over the suction surface is characterized by a street of co-rotating vortices, produced in the leading edge region. These vortices consist of boundary layer fluid distorted and detached by the passing wakes.

Downstream of the leading edge, each of these vortices induces an associated, opposite-sign vortex. The blade loading fluctuations arising from wake interaction, are of two kinds. First, a strong pressure pulse occurs on the leading edge upon wake interception. This pulse is a potential flow effect associated with the excess tangential velocity in the wake. Second, a moving pattern of pressure fluctuations, associated with the vortices, is present over the blade surface. The pressure fluctuations are negative on the suction surface, and positive on the pressure surface. The unsteady flow features over the suction surface can be adequately represented by linearized perturbation calculations, where the disturbance flow associated with the wakes is linearized about a steady viscous flow. Three parameters influence the unsteady flow over the suction surface-stator blade loading, excess wake momentum in the stator frame, and wake reduced frequency.

The strength of the disturbance flow vortices is directly proportional to the wake momentum and decreases at higher reduced frequencies. An adverse pressure gradient results in stronger vortices and pressure fluctuations. On the pressure surface, the amount of unsteady flow depends on the excess wake momentum only. Strategies for controlling the unsteady flow are simulated using appropriate blade surface boundary conditions. Fluid removal from the suction surface prevents formation of vortices and reduces the associated loading disturbances. Fluid injection from the pressure surface reduces the pressure fluctuations there.