This paper demonstrates reduction of stator unsteady loading due to forced response in a large-scale, low-speed, rotor/stator/rotor axial compressor rig by clocking the downstream rotor. Data from the rotor/stator configuration showed that the stator response due to the upstream vortical disturbance reaches a maximum when the wake impinges against the suction surface immediately downstream of the leading edge. Results from the stator/rotor configuration revealed that the stator response due to the downstream potential disturbance reaches a minimum with a slight time delay after the rotor sweeps pass the stator trailing edge. For the rotor/stator/rotor configuration, with Gap1= 10% chord and Gap2= 30% chord, results showed a 60% reduction in the stator force amplitude by clocking the downstream rotor so that the time occurrence of the maximum force due to the upstream vortical disturbance coincides with that of the minimum force due to the downstream potential disturbance. This is the first time, the authors believe, that beneficial use of flow unsteadiness is definitively demonstrated to reduce the blade unsteady loading.

This paper demonstrates reduction of stator unsteady loading due to forced response in a large-scale, low-speed, rotor/stator/rotor axial compressor rig by clocking the downstream rotor. Data from the rotor/stator configuration showed that the stator response due to the upstream vortical disturbance reaches a maximum when the wake impinges against the suction surface immediately downstream of the leading edge. Results from the stator/rotor configuration revealed that the stator response due to the downstream potential disturbance reaches a minimum with a slight time delay after the rotor sweeps pass the stator trailing edge. For the rotor/stator/rotor configuration, with Gap1 = 10 percent chord and Gap2 = 30 percent chord, results showed a 60 percent reduction in the stator force amplitude by clocking the downstream rotor so that the time occurrence of the maximum force due to the upstream vortical disturbance coincides with that of the minimum force due to the downstream potential disturbance. This is the first time, the authors believe, that beneficial use of flow unsteadiness is definitively demonstrated to reduce the blade unsteady loading.

The objective of this paper is to decompose the unsteady force on the rotor of a stator/rotor axial compressor into vortical and potential contributions, for axial gaps of 10, 20, and 30% chord between blade rows. Three methods of decomposition are proposed. The nal method adopted requires two steps. First, the potential contributed gust within the gap region is found using a panel code for stator/rotor con guration and the vortical contributed gust from the difference between that calculated by a Navier -Stokes code and the potential gust. Second, the rotor gust response is decomposed using the NavierStokes code in the rotor cascade con guration, with the vortical and potential disturbances as separate inlet boundary conditions. Results show that the rotor gust response caused by vortical contribution is dominant when the upstream wake impinges upon the rotor leading edge. The rotor gust response caused by potential contribution reaches an extremum when the stator trailing edge is closest to the rotor leading edge. Both vortical and potential contributions are important to determine the total blade response when the axial gap is less than about 30% of the chord. Nomenclatureclosure coef cients of turbulence model D = additional dissipation term of turbulence kinetic energy E, F = production and dissipation term of turbulence dissipation rate, respectively f m , , f f m m 1 2 = closure coef cients of turbulence model G k = production term of turbulence kinetic energy h = circumferential spacing of line vortices in panel method i, i = unit vector in axial direction, 21 Ï j = unit vector in tangential direction k = turbulence kinetic energy p = static pressure Re = Reynolds number based on inlet ow velocity and blade chord R t = turbulence Reynolds number S = total length of a blade surface s = coordinate along blade surface T = blade-to-blade period or spanwise moment t = time U t = wall shear velocity U`= inlet uniform velocity u = streamwise component of instantaneous velocity u = absolute ow velocity vector, ui 1 vj ub = rotor blade wheel velocity vector V = magnitude of instantaneous absolute velocity v = transverse component of instantaneous velocity W = periodic unsteady relative velocity from NS code Research Scientist, CFD and Environmental Engineering Section; currently Associate Research Scientist, National Center for High-Performance Computing, Hsinchu 300, Taiwan, ROC. ‡Doctoral Student. w = complex velocity potential x, x = position vector, axial direction y = tangential direction y 1 = wall variable used in f m Z g = axial gap z = complex number or radial direction G = circulation l = normalized turbulence length scale « = dissipation rate of turbulence kinetic energy m t = eddy viscosity coef cient, rC m f u k 2 /« s«, sk = Prandtl number of turbulence dissipation rate and kinetic energy, respectively tw = wall shear stress Subscripts B = bound circulation b = blade NS = Navier -Stokes calculation p = potential disturbance or panel code calculation v = vortical disturbance W = shedding or wake vorti...

This paper addresses the gust response on the stator of a rotor/stator axial compressor, by decomposing the response into vortical and potential contributions. Experiments were conducted in a large-scale, low-speed compressor rig, with two axial gap cases — 10% and 30% chord — and at two time-mean loadings. To determine the gust response due to potential contribution, a two-step approach was taken. First, a panel code was used to determine the gust in the mid-gap plane for the rotor/stator configuration. Then, this calculated gust served as an inlet boundary of a Reynolds-averaged Navier-Stokes code for the stator cascade configuration. The vortical contributed gust response was found by subtracting the potential contributed response from the measured response. Results show that the vortical contributed response is largest near the instant when the rotor wake impinges at the stator leading edge. The potential contributed response reaches a maximum when the rotor trailing edge is axially upstream of the stator leading edge. The vortical contributed response dominates for all cases studied.

Three-dimensional near-wake structure behind a rotor was measured using slanted hot-wire technique in a large-scale, low-speed, rotor/stator axial compressor. Unsteady flow interaction between blade rows was varied by setting the axial gap between rows at 10% and 30% of rotor chord. Results show that stronger flow interactions between blade rows, or closer axial gap, produce more pronounced time variation within the rotor wake. All parameters measured three component velocities, yaw and pitch anglesvaried strongly within the wake, and are quantified. List of symbolsC airfoil chord (same for all blades) u a , u F , u r axial, tangential and radius components of velocity in compressor Coordinates V b blade velocity at mid-span W rotor relative velocity