Nicolas Gourdain

A Time-Domain Harmonic Balance Method for Rotor/Stator Interactions

In the absence of instabilities, the large deterministic scales of turbomachinery flows resulting from the periodic rotation of blades can be considered periodic in time. Such flows are not simulated with enough efficiency when using classical unsteady techniques as a transient regime must be bypassed. New techniques, dedicated to time-periodic flows and based on Fourier analysis, have been developed recently. Among these, harmonic balance methods cast a time-periodic flow computation in several coupled steady flow computations. A time-domain harmonic balance method is derived and adapted to phase lag periodic conditions to allow the simulation of only one blade passage per row regardless of row blade counts. Sophisticated space and time interpolations are involved and detailed. The test case is a single stage subsonic compressor. A convergence study of the present harmonic balance is performed and compared with a reference well-resolved classical unsteady flow simulation. The results show, on one hand, the good behavior of the harmonic balance and its ability to correctly predict global quantities as well as local flow pattern; on the other hand, the simulation time is drastically reduced.

Unsteady Simulation of an Axial Compressor Stage With Casing and Blade Passive Treatments

This paper deals with the numerical simulation of technologies to increase the compressor performances. The objective is to extend the stable operating range of an axial compressor stage using passive control devices located in the tip region. First, the behavior of the tip leakage flow is investigated in the compressor without control. The simulation shows an increase in the interaction between the tip leakage flow and the main flow when the mass flow is reduced, a phenomenon responsible for the development of a large flow blockage region at the rotor leading edge. A separation of the rotor suction side boundary layer is also observed at near stall conditions. Then, two approaches are tested in order to control these flows in the tip region. The first one is a casing treatment with nonaxisymmetric slots. The method showed a good ability to control the tip leakage flow but failed to reduce the boundary layer separation on the suction side. However, an increase in the operability was observed but with a penalty for the efficiency. The second approach is a blade treatment that consists of a longitudinal groove built in the tip of each rotor blade. The simulation pointed out that the device is able to control partially all the critical flows with no penalty for the efficiency. Finally, some recommendations for the design of passive treatments are presented.

Effect of Tip Clearance Dimensions and Control of Unsteady Flows in a Multi-Stage High-Pressure Compressor

This paper describes the investigations performed to better understand unsteady flows that develop in a three-stage high-pressure compressor. More specifically, this study focuses on rotor-stator interactions and tip leakage flow effects on overall performance and aerodynamic stability. The investigation method is based on three-dimensional unsteady RANS simulations, considering the natural spatial periodicity of the compressor. Indeed, all information related to rotor-stator interactions can be computed. A comparison is first done with experimental measurements to outline the capacity of the numerical method to predict overall performance and unsteady flows. The results show that the simulation correctly estimates most flow features in the multistage compressor. Then numerical data obtained for three configurations of the same compressor are analyzed and compared. Configurations 1 and 2 consider two sets of tip clearance dimensions and a casing treatment based on a honeycomb design is applied for configuration 3. Detailed investigations of the flow at the same operating line show that the tip leakage flow is responsible for the loss of stability in the last stage. An increase by 30% of the tip clearance dimension dramatically reduces the stable operating range (by 40% with respect to the standard configuration). A modal analysis shows that the stall process in this case involves the perturbation of the flow in the last rotor by upstream stator wakes, leading to the development of a rotating instability. The control device designed and investigated in this study allows for reducing the sensitivity of the compressor to tip leakage flow by recovering the initial stable operating range.

Steady/Unsteady Reynolds-Averaged Navier–Stokes and Large Eddy Simulations of a Turbine Blade at High Subsonic Outlet Mach Number

Reynolds-averaged Navier–Stokes (RANS), unsteady RANS (URANS), and large eddy simulation (LES) numerical approaches are clear candidates for the understanding of turbine blade flows. For such blades, the flow unsteady nature appears critical in certain situations and URANS or LES should provide more physical understanding as illustrated here for a laboratory high outlet subsonic Mach blade specifically designed to ease numerical validation. Although RANS offers good estimates of the mean isentropic Mach number and boundary layer thickness, LES and URANS are the only approaches that reproduce the trailing edge flow. URANS predicts the mean trailing edge wake but only LES offers a detailed view of the flow. Indeed, LESs identify flow phenomena in agreement with the experiment, with sound waves emitted from the trailing edge separation point that propagate upstream and interact with the lower blade suction side. [DOI: 10.1115/1.4028493]

Large eddy simulation of flows in industrial compressors: a path from 2015 to 2035.

A better understanding of turbulent unsteady flows is a necessary step towards a breakthrough in the design of modern compressors. Owing to high Reynolds numbers and very complex geometry, the flow that develops in such industrial machines is extremely hard to predict. At this time, the most popular method to simulate these flows is still based on a Reynolds-averaged Navier–Stokes approach. However, there is some evidence that this formalism is not accurate for these components, especially when a description of time-dependent turbulent flows is desired. With the increase in computing power, large eddy simulation (LES) emerges as a promising technique to improve both knowledge of complex physics and reliability of flow solver predictions. The objective of the paper is thus to give an overview of the current status of LES for industrial compressor flows as well as to propose future research axes regarding the use of LES for compressor design. While the use of wall-resolved LES for industrial multistage compressors at realistic Reynolds number should not be ready before 2035, some possibilities exist to reduce the cost of LES, such as wall modelling and the adaptation of the phase-lag condition. This paper also points out the necessity to combine LES to techniques able to tackle complex geometries. Indeed LES alone, i.e. without prior knowledge of such flows for grid construction or the prohibitive yet ideal use of fully homogeneous meshes to predict compressor flows, is quite limited today.

Towards Massively Parallel Large Eddy Simulation of Turbine Stages

The context of integrated numerical simulations of gas turbine engines by use of high-fidelity Computational Fluid Dynamic (CFD) tools recently emerged as a promising path to improve engines design and understanding. Relying on massively parallel super-computing such propositions still have to prove feasibility to efficiently take advantage of the ever increasing computing power made available worldwide. Although Large Eddy Simulation (LES) has recently proven its superiority in the context of the combustion chamber of gas turbine, methodologies need to be developed and start addressing the problem of the turbomachinery stages, if integrated simulations based on LES are to be foreseen. In the proposed work an in-house code and strategy, called TurboAVBP, is developed for turbomachinery LES thanks to the coupling of multi-copies of the unstructured compressible reacting LES solver AVBP, designed to run efficiently on high performance massively parallel architectures. Aside from the specificity of such wall bounded flows, rotor/stator LES type simulations require specific attention and the interface should not interfere with the numeric scheme to preserve proper representation of the unsteady physics crossing this interface. A tentative LES compliant solution based on moving overset grids method is proposed and evaluated in this work for high-fidelity simulation of the rotor/stator interactions. Simple test cases of increasing difficulty with reference numerical are detailed and prove the solution in handling acoustics, vortices and turbulence. The approach is then applied to the QinetiQ MT1 high-pressure transonic turbine for comparison with experimental data. Two configurations are computed: the first one is composed of 1 scaled stator section and 2 rotors while the second computation considers the geometrically accurate periodic quarter of the machine, i.e. 8 stators and 15 rotors to test scalability issues of such applications. Although under-resolved, the LES pressure profiles on the stator and rotor blades appear to be in good agreement with experimental data and are quite competitive compared to the traditional (Unsteady) Reynolds-Averaged Navier-Stokes (RANS or URANS) modeling approach. Unsteady features inherently present in these LES underline the complexity of the flow in a turbine stage and clearly demand additional diagnostics to be properly validated.Copyright

High-performance Computing to Simulate Large-scale Industrial Flows in Multistage Compressors

The aim of this study is to propose a computing method to obtain a detailed simulation of the unsteady flow that develops in multistage turbomachines. The three-dimensional unsteady Reynolds-averaged Navier—Stokes equations are solved using a structured multiblock decomposition method. Although this kind of flow solver is very popular in the turbomachine community nowadays, the complex block connectivities used in meshes of industrial configurations can be penalizing for parallel computing. The computing strategy implemented in a modern flow solver is investigated in this paper, with a particular interest in mesh partitioning, communications and load balancing. Advantages and drawbacks of different computing platforms are then discussed, ranging from vector supercomputers to massively scalar platforms. Comparisons are performed regarding criteria such as the elapsed time and the electrical power consumption. The results show that the use of a large number of computing cores (>128) is heavily penalized by communications and load balancing errors, whereas computing performance with a moderate number of computing cores (<128) is mainly driven by the peak power of the architecture. To help users estimate a priori the parallel performance of a task, a tool based on an extension of Amdahl’s law is proposed, showing satisfying results when compared with observations. Finally, an unsteady flow simulation is performed in a complete three-stage compressor at the design operating point. While still far beyond industrial resources, this numerical flow simulation shows that potential breakthroughs in the design of compression systems can be expected.

Experimental and Computational Methods for Flow Investigation in High-Speed Multistage Compressor

This study takes place in the frame of a research project to better understand the flow that develops in multistage high-speed compressors. First, the paper presents the high-speed 3.5-stage compressor CREATE and the methods that are considered to increase the data reliability and the investigation capability with such realistic compressors. Second, flow simulations, achieved with three-dimensional unsteady Reynolds-averaged Navier-Stokes computations over the whole compressor spatial and temporal periodicities, are analyzed through a study of their sensitivity to some parameters such as the inlet conditions, the space and time discretization, and technological effects such as tip and hub clearances. Finally the paper focuses on the methodology used to compare the experimental and numerical results over the whole compressor periodicities. The local spatial and temporal flow structures are well estimated. The key is the advection of these structures, which interact with each other and produce a significant part of the flow fluctuations in the downstream stages. The paper presents the zone close to the casing, where losses and blockage are induced by the interaction between the tip leakage flow and the incoming wakes. This is where the numerical simulation has to be improved, in order to accurately predict the performance.

A Novel Approach to Evaluate the Benefits of Casing Treatment in Axial Compressors

Passive control devices based on casing treatments have already shown their capability to improve the flow stability in axial compressors. However, their optimization remains complex due to a partial understanding of the related physical mechanisms. In order to quantitatively assess the interaction between slots and the blade tip flow, the present paper develops a novel analysis methodology based on a control-volume approach located in the rotor tip region. This methodology may be used for analyzing the casing treatment based on both axi- and non-axisymmetric slots design. The second issue of the paper focuses on the application of the current approach to better understand the effects of axi- and non-axisymmetric grooves in three different axial compressors which differ by the flow regime (subsonic/transonic) and the smooth casing shape (cylindrical/concave). Numerical simulations are performed, and results of the current approach with and without casing treatments are compared.

Analysis of Unsteadiness on Casing Treatment Mechanisms in an Axial Compressor

Control devices based on casing treatments have already shown their capability to improve the flow stability in compressors. However their optimization remains complex due to a partial understanding of the related physical mechanisms. The present paper proposes to use a budget analysis of the Navier Stokes equations to support the understanding of such flow phenomena. Based on the original work of Shabbir and Adamczyk (2005), the strength of the present contribution is to generalize the flow analysis method to all Navier-Stokes equations, including unsteady terms. A high-pressure multistage compressor equipped with circumferential casing grooves is chosen to demonstrate the potential of this approach. Steady and unsteady Reynolds-Averaged Navier-Stokes (URANS) equations are solved with a structured multi-blocks solver. Results are then briefly compared to experimental data to validate the numerical method. The analysis of the unsteady axial momentum equation for configurations with and without casing treatment points out some of the mechanisms responsible for the stability improvement. The analysis also indicates that the flow unsteadiness generated by upstream stator wakes (stator/rotor interaction) reduces viscous efforts and increases convective forces, significantly modifying the compressor stability. Finally, the proposed post processing method shows very interesting results for the understanding of circumferential grooves and it should be also used for non-axisymmetric casing treatment configurations.Copyright