This paper presents the impact of an axially tilted variable stator vane platform on penny cavity flow and passage flow, with the aid of both optical and pneumatic measurements in an annular cascade wind tunnel as well as steady CFD analyses. Variable stator vanes (VSVs) in axial compressors require a clearance from the endwalls. This means that penny cavities around the vane platform are inevitable. Production and assembly deviations can result in a vane platform which is tilted about the circumferential axis. Due to this deformation, backward facing steps occur on the platform edge. Penny cavity and main flow in geometries with and without platform tilting were compared in an annular cascade wind tunnel, which comprises a single row of 30 VSVs. Detailed particle image velocimetry (PIV) measurements were conducted inside the penny cavity and in the vane passage. Steady pressure and velocity data was obtained by two-dimensional multi-hole pressure probe traverses in the inflow and the outflow. Furthermore, pneumatic measurements were carried out using pressure taps inside the penny cavity. Additionally, oil flow visualization was conducted on the airfoil, hub, and penny cavity surfaces. Steady CFD simulations with boundary conditions, according to the measurements, have been benchmarked against experimental data. The results show that tilting the VSV platform reduces the mass flow into and out of the penny cavity. By decreasing penny cavity leakage, platform tilting also affects the passage flow where it leads to a reduced turbulence level and total pressure loss in the leakage flow region. In summary, the paper demonstrates the influence of penny platform tilting on cavity flow and passage flow and provides new insights into the mechanisms of penny cavity-associated losses.

This paper presents detailed measurements and post-test simulations of the penny cavity leakage flow and its interaction with the mainstream flow in an annular cascade wind tunnel. The annular cascade wind tunnel consists of a single row of 30 variable stator vanes, derived from a high-pressure compressor stator with inner and outer vane disks, called pennies, which - when assembled in the hub and casing walls - leave cylindrical-shaped ring gaps called penny cavities. The wind tunnel runs at a Mach number of 0.34 at the stator inlet and a Reynolds number of 3.82 x 105 based on axial chord length at 50% span. Two different penny gap sizes on the hub are compared to a reference case without a penny gap. Detailed 2D-traverses were performed with multi-hole pressure and hot-wire-probes covering 2.5 passages in the inflow and outflow of the stator row. Pressure taps were embedded in the airfoil surface and inside the penny cavity. Surface oil flow measurements were conducted with different colors for the vane suction side, pressure side, hub and the penny cavity to detect the secondary flow phenomena. Reynolds- averaged Navier-Stokes (RANS) simulations, using the measured boundary conditions, were compared to experimental data. As a result, a relative increase in the total pressure loss coefficient of 1.9% for the nominal and 6.8% for the double penny gap was measured compared to no-penny cavity. The additional penny losses are limited to the lower 40% span. The post-test simulations are in good agreement with the measurements, showing that the outflow from the penny cavity on the suction side generates vortices, which cause additional losses. The penny vortices are detected in the outlet plane by an increase in turbulence intensity and streamwise vorticity. However, the additional penny losses are overestimated in the simulation by up to 7.3%. A change in the pressure fields with an increasing penny gap size, both around the airfoil and inside the penny cavity, can be seen in the numerical and experimental results. The outflow regions of the penny cavity, estimated by simulations, are confirmed by the results of the surface oil flow measurements. In summary, this paper consolidates previous numerical analyses carried out by the authors [13-16] on penny cavity leakage flow effects with experimental data for different penny gap sizes.

This paper presents a description of Detached Eddy Simulations being carried out on a variable stator vane with a penny-cavity in order to determine the secondary flow phenomena in the main flowpath. Variable stator vanes are common in multi-stage compressors to prevent flow separations on rotor and stator blades at off-design operation points. The bearing of the stators at hub and tip generate unavoidable circular-shaped ring gaps, which are called penny-cavities. The aim of this paper is to determine secondary flow phenomena in variable stator vanes on an annular cascade testbed resulting from the throughflow of the penny-cavities. Reynolds-Averaged-Navier-Stokes simulations and scale resolving Detached-Eddy-Simulations of a variable stator vane with hub penny-cavity were therefore performed using Ansys CFX. The results of these simulations will be compared to corresponding simulations without penny-cavity. The study shows secondary flow phenomena, which are comparable to the interaction of a transverse jet in a free stream. Due to the low momentum ratio of R = 0.5, the jet immediately veers in the direction of the main flow. The typical vortices which develop from a transverse jet in a free stream are identified. The steady RANS simulation shows an asymmetrical counter-rotating vortex pair. A lack of unsteady secondary flow interaction can be seen in the RANS simulations in contrast to the Detached-Eddy-Simulations, which resolve large turbulent scales. Hence an interaction between the counter-rotating vortex pair and the unsteady shear layer vortices in the stator is visible. In the Detached Eddy Simulations the counter-rotating vortex pair is superimposed by the unsteady shear-layer vortices. The vortices produce significant additional mixing losses, which will be shown in detail. By comparing simulations with and without penny-cavity, the penny-cavity losses are quantified. In conclusion, this paper will help design engineers become more aware of the significance of the penny-cavity with variable stator vanes.