Turbulence characteristics of the B\"{o}dewadt layer in a large enclosed rotor-stator system

A three-dimensional (3D) direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed annular rotor-stator cavity characterized by a large aspect ratio G=(b-a)/h=18.32 and a small radius ratio a/b=0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Omega under consideration is equivalent to the rotational Reynolds number Re=Omegab2/nu=9.5 x 104, where nu is the kinematic viscosity of the fluid. This corresponds to a value at which an experiment carried out at the laboratory has shown that the stator boundary layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the 3D computed solution with velocity measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence at this Reynolds number, by comparing the turbulence properties with available data in the literature. An approximately self-similar boundary layer behavior is observed along the stator side. The reduction of the structural parameter a1 under the typical value 0.15 and the variation in the wall-normal direction of the different characteristic angles show that this boundary layer is three-dimensional. A quadrant analysis of conditionally averaged velocities is performed to identify the contributions of different events (ejections and sweeps) on the Reynolds shear stress producing vortical structures. The asymmetries observed in the conditionally averaged quadrant analysis are dominated by Reynolds stress-producing events in this B\"{o}dewadt layer. Moreover, case 1 vortices (with a positive wall induced velocity) are found to be the major source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson.Comment: 16 page

Unsteady Aerodynamics and Blade-Row Interactions in the Embedded Stage of an Axial Compressor

In a mature engineering field like compressor aerodynamics, the most accessible advances in machine technology, translating to performance and efficiency, have been discovered and have found industry design applications. As the community continues to make progress, increasingly challenging aspects of the involved physics must be exploited. Modern turbomachinery operates with larger bypass ratios, smaller cores, and lighter, thinner, and more flexible materials resulting in the maintenance of higher operating pressures and temperatures. As the performance and efficiency of these machines continues to climb, the same technological advances reinforce challenges like forced-response vibration, high-cycle fatigue of engine components, and large relative tip clearances in an engine core. Accounting for these challenges increasingly depends on the investigation of the unsteady domain for solutions. Tools at the disposal of the designer include progressively improving computational simulations through both computational resources and attainable model fidelity. As essential as these tools are for modern turbomachinery design, the confidence in their results is only as good as the experimental data used to validate them. The objective of this research is the experimental investigation and characterization of the transient aerodynamics and blade-row interactions near forced-response resonant vibratory operating conditions in a multi-stage environment. Experimental methods are focused on fast-response pressure transducers with the high frequency response capable of capturing the unsteady pressure fluctuations associated with the high-speed rotation and blade-pass frequency of a modern high-pressure core axial compressor. Investigation is centered on an engine-representative embedded rear stage, with adjacent stages establishing realistic flow conditions and resulting boundary conditions for model comparison. Aerodynamic characterization of several flow conditions and the examination of the effect of a reduced vane-count stator configuration upstream of the embedded stage are performed with measurements of the embedded rotor at the casing endwall and rotor exit plane, as well as within a passage of the embedded stator. Circumferential vane traverse around stationary instrumentation provide a full vane passage of phase-locked, time-resolved pressure measurements of the rotor aerodynamics and the unsteady loading of the embedded stator is distinguished for a single vane position. Results from this investigation identify and describe the inception and trajectory of tip clearance flows, including the tip leakage vortex and double-leakage tip clearance flow. Evidence of an upstream vane wake interaction with the rotor occurs for limited regions of vane passage positions. Spectral analyses and pressure unsteadiness provide further insight into the blade-row interactions

Large eddy simulation and measurements of turbulent enclosed rotor-stator flows

International audienceTurbulent flows are studied in an actual enclosed rotor-stator configuration with a rotating hub and a stationary shroud. Besides its fundamental importance - the disk boundary layer is one of the simplest platforms for investigating the underlying structure of three-dimensional boundary layers - this cavity models more complex configurations relevant to rotating machinery. Large Eddy Simulation (LES) is performed using a Spectral Vanishing Viscosity (SVV) technique which is shown leading to stable discretizations without sacrificing the formal accuracy of the spectral approximation. Numerical results and velocity measurements have been favorably compared for a large range of rotational Reynolds numbers up to one million in an annular cavity of curvature parameter Rm=(b+a)/(b-a)=1.8 and of aspect ratio G=(b-a)/h=5, where a and b are respectively the inner and outer radii of the rotating disk and h is the interdisk spacing. In the detailed picture of the flow structure that emerges, the turbulence is mainly confined in the boundary layers including in the Stewartson layer along the external cylinder. For Reynolds numbers larger than 0.1 million, the stator boundary layer is turbulent over most of the cavity. On the other hand, the rotor layer becomes progressively turbulent from the outer radial locations although the rotating hub is shown to destabilize the inner part of the boundary layers. The isosurface maps of the Q-criterion reveal that the three-dimensional spiral arms observed in the unstable laminar regime evolve to more axisymmetric structures when turbulence occurs. At Re equal to one million, the flow is fully turbulent and the anisotropy invariant map highlights turbulence structuring, which can be either a ``cigar-shaped'' structuring aligned on the tangential direction or a ``pancake-shaped'' structuring depending on the axial location. The reduction of the structural parameter a1 (the ratio of the magnitude of the shear stress vector to twice the turbulence kinetic energy) under the typical limit 0.15, as well as the misalignment between the shear stress vector and the mean velocity gradient vector, highlight the three-dimensional nature of both rotor and stator boundary layers with a degree of three-dimensionality much higher than in the idealized system studied by Lygren and Andersson (2001-2006)

Near-Wall Flow in Turbomachinery Cascades—Results of a German Collaborative Project

This article provides a summarizing account of the results obtained in the current collabora-tive work of four research institutes concerning near-wall flow in turbomachinery. Specific questions regarding the influences of boundary layer development on blades and endwalls as well as loss mech-anisms due to secondary flow are investigated. These address skewness, periodical distortion, wake interaction and heat transfer, among others. Several test rigs with modifiable configurations are used for the experimental investigations including an axial low speed compressor, an axial high-speed wind tunnel, and an axial low-speed turbine. Approved stationary and time resolving measurements techniques are applied in combination with custom hot-film sensor-arrays. The experiments are complemented by URANS simulations, and one group focusses on turbulence-resolving simulations to elucidate the specific impact of rotation. Juxtaposing and interlacing their results the four groups provide a broad picture of the underlying phenomena, ranging from compressors to turbines, from isothermal to non-adiabatic, and from incompressible to compressible flows.The investigations reported in this article were conducted within the framework of the joint research project “Near-Wall Flow in Turbomachinery Cascades” which was funded and supported by the Deutsche Forschungsgemeinschaft (DFG) under grant number PAK 948. The responsibility for the contents of this publication lies entirely by the authors.Peer ReviewedPostprint (published version

Experimental and computational investigation of flow structure of buoyancy induced flow in heated rotating cavities

This paper presents Laser-Doppler Anemometry (LDA) measurements obtained from the Sussex Multiple Cavity test facility. This facility comprises a number of heated disc cavities with a cool bore flow and is intended to emulate the secondary air system flow in an H.P compressor. Measurements were made of the axial and tangential components of velocity over the respective range of Rossby, Rotational and Axial Reynolds numbers, (Ro, Reθ and Rez), 0.32 < Ro < 1.28, Reθ = 7.1 × 105, 1.2 × 104 < Rez < 4.8 × 104 and for the values of the buoyancy parameter (βΔT) : 0.50 < βΔT < 0.58. The frequency spectra analysis of the tangential velocity indicates the existence of pairs of vortices inside the cavities. The swirl number, Xk, calculated from these measurements show that the cavity fluid approaches solid body rotation near the shroud region. The paper also presents results from Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations for the test case where Ro = 0.64. The time-averaged LDA data and numerical results show encouraging agreement

Three-dimensional turbopump flowfield analysis

A program was conducted to develop a flow prediction method applicable to rocket turbopumps. The complex nature of a flowfield in turbopumps is described and examples of flowfields are discussed to illustrate that physics based models and analytical calculation procedures based on computational fluid dynamics (CFD) are needed to develop reliable design procedures for turbopumps. A CFD code developed at NASA ARC was used as the base code. The turbulence model and boundary conditions in the base code were modified, respectively, to: (1) compute transitional flows and account for extra rates of strain, e.g., rotation; and (2) compute surface heat transfer coefficients and allow computation through multistage turbomachines. Benchmark quality data from two and three-dimensional cascades were used to verify the code. The predictive capabilities of the present CFD code were demonstrated by computing the flow through a radial impeller and a multistage axial flow turbine. Results of the program indicate that the present code operated in a two-dimensional mode is a cost effective alternative to full three-dimensional calculations, and that it permits realistic predictions of unsteady loadings and losses for multistage machines

Design, Analysis, Optimization and Control of Rotor Tip Flows

Developments in turbomachinery focus on efficiency and reliability enhancements, while reducing the production costs. In spite of the many noteworthy experimental and numerical investigations over the past decades, the turbine tip design presents numerous challenges to the engine manufacturers, and remains the primary factor defining the machine durability for the periodic removal of the turbine components during overhaul. Due to the hot gases coming from the upstream combustion chamber, the turbine blades are subjected to temperatures far above the metal creep temperature, combined with severe thermal stresses induced within the blade material. Inadequate designs cause early tip burnouts leading to considerable performance degradations, or even a catastrophic turbine failure. Moreover, the leakage spillage, nowadays often exceeding the transonic regime, generates large aerodynamic penalties which are responsible for about one third of the turbine losses. In this view, the current doctoral research exploits the potential through the modification and optimization of the blade tip shape as a means to control the tip leakage flow aerodynamics and manage the heat load distribution over the blade profile to improve the turbine efficiency and durability

Fluid dynamics of thrust vectorable submerged nozzle

A numerical simulation of the gas-dynamic processes in the thrust vectorable nozzle of the solid rocket motor is considered. Construction of a geometric model and a generation of computational mesh, and reconstruction of model and mesh at each time step are discussed. Calculations of the flowfield of combustion products in the pre-nozzle chamber and nozzle block are carried out for various angles of nozzle rotation. The distributions of the gas dynamic quantities in the pre-nozzle volume corresponding to the outflow of the combustion products from the cylindrical channel and star-shaped channel are compared, as well as the solutions of the problem obtained with quasi-stationary and unsteady formulations. The effects of the channel shape on the distribution of flow quantities and formation of a vortical flow structure in the nozzle block are discussed