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.

In this paper, the modelling of leakages through a compressor stator penny cavity, and their effect on the aerodynamics within the compressor are studied. The penny, sometimes also referred to as ‘button’, is the cylindrical platform feature of a variable stator normally found between a vane’s airfoil and spindle. The pennies nominally lie recessed into the compressor endwalls at hub and casing, with a surrounding clearance to ensure the vane’s stagger angle can be adjusted. RANS-simulations, with these clearances included, have shown a significant impact from the penny cavity leakages on compressor efficiency and surge line. Neglecting this secondary flow path through the penny cavities results in an under prediction of the losses close to the endwalls. The prediction of the penny cavity effect on the stator row is based on a Reynolds-Averaged-Navier-Stokes (RANS) study, using a hybrid structured-unstructured mesh to provide adequate resolution of the local flow phenomena. The complex geometry and pressure field result in flows that are unevenly distributed within the penny cavity. The outflow or leakage is focused in a concentrated area leading to a high local velocity that strongly impacts the stator losses and turning. Since such geometries lie beyond the normal validated cases, the modelling uncertainties are discussed and the plausibility of the results is checked. In order to provide an experimental database and validate the turbulent mixing of leakage and main flow, which is seen as the main contributor to loss production, a validation test case — ‘Jet-In-Crossflow’ was chosen. As well as the standard RANS code, this validation case was run as a time-accurate high-order Lattice Boltzmann (LBM) simulation (PowerFLOW), using Very-Large-Eddy-Simulation (VLES) turbulence modelling. The LBM simulation showed significant unsteady flow features and was considerably closer to the test data than the RANS calculations. A future test campaign, currently being prepared at the annular cascade test facility of the Institute of Jet Propulsion and Turbomachinery (IST) at RWTH Aachen university, will be briefly presented. This focuses on investigating the penny flows in a typical engine design.

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, whichwhen assembled in the hub and casing wallsleave 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 10 5 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. Reynoldsaveraged 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.

Variable guide vanes are airfoils, normally in the front stages of multistage compressors, that can be restaggered to extend the compressor’s operational range. To allow the variable guide vanes to rotate the design will inevitably include a gap or cavity between the vane’s rotating mounting feature (Penny) and the stationary inner and outer sidewalls. These penny cavities cause additional leakages which impact losses, and airfoil turning, so reducing a compressor’s efficiency and stability. A compressor model which accurately simulates the penny cavity leakage is central to improving the design. This paper presents a study looking into how to accurately include penny cavity leakage effects during the design of multistage compressors. Multiple blade-row RANS-based flow simulations are the current state-of-the-art standard during the design of multistage compressors. However, it is unlikely that such models have the numerical accuracy to simulate the penny cavity leakage effects in detail: firstly the RANS-turbulence model cannot accurately recreate the turbulent mixing which takes place between the leakage and the main flow, and secondly because a typical multiple blade-row mesh is too coarse to resolve the details of the much smaller penny cavity. To compensate for this, the numerical modelling of penny cavities in the design of a compressor would need to be adjusted. On the other hand, a dedicated hybrid LES-model can more accurately simulate the secondary flows with its significant turbulent mixing, at the cost of computational capacity. In this paper, high resolution hybrid LES-simulations have been used as a benchmark to adjust RANS-calculations typical for the design of a multistage compressor. The paper presents the following steps: Using a standard Jet-in-Crossflow test case, a high resolution model was evaluated using both RANS and a hybrid LES model, and compared against measurements. The flow structures were analyzed and compared to measurements of this test case available from literature. These show that the hybrid LES-model performed significantly better than the RANS-model, in being able to predict the jet impact and flow structures. For a second model consisting of a generic compressor variable vane with penny cavity. RANS and hybrid LES simulations were performed with a highly refined mesh in the region of the penny cavity. The modelling is described in detail and the resulting penny cavity effects compared. Finally, the vane with standard mesh and penny cavity was run using RANS-turbulence CFD and compared to the above. From this conclusions were drawn on how to transfer experience from the higher-fidelity turbulence model to a more industry-standard RANS model, which could for instance be used during the design phase of a multistage compressor.

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.