Steady and Unsteady Numerical Characterization of the Secondary Flow Structures of a Highly Loaded Low-Pressure Compressor Stage

peer reviewedThis paper presents the numerical characterization of a highly loaded compressor by means of 3D unsteady RANS simulations. The focus is on critical flow structures and their evolution at different operating points of the machine. First, the numerical setup and mesh quality are presented to support the reliability of the provided results. The comparison against experiments is then described for this purpose. Later, a full description of the unsteady behavior of the machine is provided, giving special attention to the two regions where the most critical features are expected: the rotor hub wall and the casing. Rotor–stator interactions are then investigated and the role of the inlet guide vane (IGV) is finally discussed. Results are analyzed at design and near-stall conditions, with a focus on the behavior close to the stability limit at 100% speed

Measurements of Turbulence in Compressible Low-Density Flows at the Inlet of a Transonic Linear Cascade With and Without Unsteady Wakes

editorial reviewedIn the present work, hot-wire anemometry was employed for the characterization of the turbulent field at the inlet of a high-speed low-pressure turbine cascade, in terms of turbulence intensity and integral length scales. This work addresses two major topics relevant to the turbomachinery field: the application of hot-wire anemometry in transonic and rarefied flow regimes and the decoupling of the deterministic and the stochastic fluctuations when measuring unsteady phenomena. In compressible and rarefied flows, a hot-wire is strongly sensitive to both density and velocity fluctuations, and the commonly used Nusselt–Reynolds correlations are not valid. In this article, a nondimensional calibration methodology, based on Nusselt, Reynolds, and Knudsen numbers, was coupled with a sensitivity analysis and employed to postprocess the experimental dataset, allowing to decouple the fluctuations of density and velocity and to compute the turbulence parameters. In the presence of unsteady wakes generated upstream of the cascade, two different phase-locked averaging techniques were employed to distinguish the wake deterministic fluctuations from the background turbulence intensity

Metabarcoding data reveal vertical multitaxa variation in topsoil communities during the colonization of deglaciated forelands

Ice-free areas are expanding worldwide due to dramatic glacier shrinkage and are undergoing rapid colonization by multiple lifeforms, thus representing key environments to study ecosystem development. It has been proposed that the colonization dynamics of deglaciated terrains is different between surface and deep soils but that the heterogeneity between communities inhabiting surface and deep soils decreases through time. Nevertheless, tests of this hypothesis remain scarce, and it is unclear whether patterns are consistent among different taxonomic groups. Here, we used environmental DNA metabarcoding to test whether community diversity and composition of six groups (Eukaryota, Bacteria, Mycota, Collembola, Insecta, and Oligochaeta) differ between the surface (0–5 cm) and deeper (7.5–20 cm) soil at different stages of development and across five Alpine glaciers. Taxonomic diversity increased with time since glacier retreat and with soil evolution. The pattern was consistent across groups and soil depths. For Eukaryota and Mycota, alpha-diversity was highest at the surface. Time since glacier retreat explained more variation of community composition than depth. Beta-diversity between surface and deep layers decreased with time since glacier retreat, supporting the hypothesis that the first 20 cm of soil tends to homogenize through time. Several molecular operational taxonomic units of bacteria and fungi were significant indicators of specific depths and/or soil development stages, confirming the strong functional variation of microbial communities through time and depth. The complexity of community patterns highlights the importance of integrating information from multiple taxonomic groups to unravel community variation in response to ongoing global changes

Aero-thermal performance of a film-cooled high pressure turbine blade/vane: a test case for numerical codes validation

The present thesis concentrates on the experimental activity carried in the frame of two different measurement campaigns aiming to create an experimental test-case for turbomachinery codes validation. The first experimental activity was carried out at the University of Bergamo and dealt with the characterization of the aero-thermal performance of a film-cooled high pressure turbine rotor blade. Aerodynamic measurements, performed by means of a 5-hole miniaturized pressure probe, evidenced a marked asymmetry of the secondary flows between the upper and the lower semi-channels. Injection seemed not to affect at all the secondary flows pattern: at every injection rate, the cooling jets kept attached to the endwall surface and confined in the boundary layer. Such a result was confirmed by the adiabatic effectiveness distribution which was retrieved by thermochromic liquid crystals. The high performance of the cooling system had to be related to the extreme tangential arrangement of the holes. The second measurement campaign was held at the von Karman Institute for Fluid Dynamics (VKI) (Belgium). A film-cooled transonic turbine vane was investigated in a five blades linear cascade configuration and at engine-like conditions. The inlet free-stream turbulence was fully characterized by means of hot-wire anemometry. The aerodynamic performance of the cascade was assessed by traversing a 3-hole pressure probe in the downstream section. Injection was found to slightly enhance total pressure wakes. Thin-film thermometers have been used to retrieve the blade convective heat transfer coefficient (h) distribution. The non-cooled tests demonstrated that the tripping effect of the film-cooling holes is the responsible for a transition of the boundary layer. The thermal protection of the suction side always increases with the injection rate while the pressure side is showing values of h higher than those of the non-cooled case: at low injection rates, the breaks down the boundary layer

ON THE IMPACT OF THE TURBULENCE MODEL ON THE SECONDARY FLOW STRUCTURE OF A HIGHLY-LOADED COMPRESSOR STAGE

peer reviewedRANS turbulence models still represent a weak point of industrial simulations of turbomachinery flows. In presence of a large blade loading the turbulence model can impact severely on the prediction of the end-wall flow and secondary structures in the blade passage, with an important effect on the computed global performance and flow stability. This paper presents a turbulence model comparison for the characterization of a highly-loaded low-pressure compressor at multiple operating points using 3D RANS simulations. The turbulence models mostly employed for the design and analysis of axial compressors are considered, with the aim of providing a clear physical description of their impact on the critical flow features of the machine. It will be shown that no single turbulence model is generally better than others and the prediction depends on the operating point, with increasing discrepancies as the mass-flow is reduced

Bayesian Inference of Experimental Data for Axial Compressor Performance Assessment

International audienceAs the next generation of turbomachinery components becomes more sensitive to instrumentation intrusiveness, a reduction of the number of measurement devices required for the evaluation of performance is a possible and cost-effective way to mitigate the arising of non-mastered experimental errors. A first approach to a data assimilation methodology based on Bayesian inference is developed with the aim of reducing the instrumentation effort. A numerical model is employed to provide an initial belief of the flow, that is then updated based on experimental observations, using an ensemble Kalman filter algorithm for inverse problems. Validation of the algorithm is achieved with the usage of experimental measurements not used in the data assimilation process. The methodology is tested for a low aspect ratio axial compressor stage, showing a good prediction of the corrected compressor map, as well as a promising prediction of the inter-row radial pressure distribution and 2D flow field

Forced Response Analysis of a Highly-Loaded Low-Pressure Compressor Stage With Inlet Distortions

peer reviewedAbstract Due to modern evolutions in propulsion, in view of obtaining higher propulsive efficiency, turbofan engines are subjected to ever higher inlet flow distortions. In these systems, structural damage can be induced by distortion patterns with excitation close to the blade natural frequency. To fill the gap currently present in literature about the impact of inlet distortions on the aeroelastic behaviour of engine components, this paper presents an aerodynamic damping and forced-response analysis on a highly-loaded axial compressor subjected to intake total pressure distortions. First, a stability analysis for the vibrational mode with natural frequency close to the resonance condition at the nominal speed of the machine is performed. Blade deformations are then computed through a distortion induced forced-response analysis at different distortion amplitudes. This work aims at studying the aeromechanical behaviour of a modern compressor, highlighting that the amplitude of the blade vibration does not increase linearly with the distortion amplitude

Numerical Characterisation of a HP Compressor Stage Equipped with a Closed Shrouded Stator Cavity

International audienceAbstract In axial compressors, shrouded stator cavity flows are responsible for performance degradation due to their interaction with the power stream. The present paper aims at exploring the possibility of employing a single stage high pressure axial compressor as a test vehicle for cavity flows investigations. In a first step, the robustness of the adopted RANS approach is tested against experimental data on the closed-cavity baseline configuration (i.e. no downstream-to-upstream recirculation). In a second phase, the effect of different hub cavities layouts of different levels of realism is numerically investigated. The focus is set on the representativeness of a closed cavity configuration with injection. The cavity flow topology and impact on the overall performance are considered in the analysis. At its final extent, this paper provides numerical and experimental guidelines for the robust assessment of cavity flows topology and performance effects

Redesign of a closed-loop high-speed facility to test distortion generators

peer reviewedAircraft engine architectures are currently in the phase of a change to meet future market demands. Certain such novel architectures force engines to operate under distorted inflow conditions, which are undesirable during flight. It is important to characterize the effect of such inflow distortions in order to understand the impact on the performance of engine components. This paper discusses the design and development of a novel test-facility that has the capability to test combined total pressure and swirl distortion generators under desired flow conditions. The return duct of the high-speed closed-loop compressor rig R4 at the von Karman Institute for Fluid Dynamics (VKI) has been redesigned to incorporate a test-section where distortion generators can be tested and characterized. Thus, a compressor test-rig is modified to act also as a wind tunnel capable of testing distortion generators at engine-like conditions

Aerodynamic Performance of an Ultra-Low Aspect Ratio Centripetal Turbine Stator

An extensive measurement campaign was carried out at the von Karman Institute for Fluid Dynamics to assess the aerodynamic performance of an ultra-low aspect ratio centripetal turbine stator. The test section consisted of 18 periodic sectors of two blade passages each, tested at transonic conditions in a blow-down facility. Particle image velocimetry (2D-PIV) measurements were performed in the trailing edge area at blade mid-height. Further downstream, 16 micro virtual 3-hole pressure probes were used to measure the aerodynamic performance at about 44% of the radial chord, downstream of the trailing edge. Results describe a highly swirling flow-field characterized by a very large tangential velocity component increasing at smaller radii. The radial component decreased instead to compensate for an opening of the test section closer to its center, therefore enhancing the tangential nature of the generated flow field. Adjacent blades showed asymmetric wakes as an effect of the particular design of the nozzle. The performance analysis exhibited very high loss coefficients (always higher than 12%) which have to be related to the extremely small aspect ratio of the cascade