Compatibility of Characteristic Boundary Conditions with Radial Equilibrium in Turbomachinery Simulations

Phil. Trans. R. Soc. A.

et al. 2014

Self Cite

One contribution of 13 to a Theme Issue 'Aerodynamics, computers and the environment' . 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 Reynoldsaveraged 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 wallresolved 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. IntroductionThe aeronautic industry faces a formidable challenge by targeting in 2050 a reduction of CO 2 and NO x 2014 The Author(s) Published by the Royal Society. All rights reserved. emissions by 75% and 90% per kilometre and per passenger, respectively (compared to the 2000 level). Ultra-high bypass ratio (BR > 20) turbofan and ultra-high overall pressure ratio (OPR > 70) core engines have the potential for such significant reductions in fuel burn. However, after many years of optimization, the design of ever more efficient gas turbines now requires understanding of the complex unsteady flow appearing within each individual component and in an integrated fashion. Among all components, the compressor remains a critical part of a gas turbine, especially regarding its efficiency and stability. Owing to the adverse pressure gradients, compressors are naturally subjected to unstable flows such as surge and rotating stall [1], which can potentially lead to mechanical failure. These aerodynamic instabilities impose serious constraints on the design (the so-called 'surge margin') and thus on performance. In this context, maximizing the efficiency of compressors remains particularly complex and the improvement of the compressor robustness and performance thus requires good understanding of the flow physics.Complementary to experimental investigations, the numerical simulation of flows, commonly referred to as computational fluid dynamics (CFD), is...

Section: (C) Boundary Conditionsmentioning

confidence: 99%

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

Gourdain

Sicot

Phil. Trans. R. Soc. A.

et al. 2014

Self Cite

“…As turbulent measurements will be available, a sensibility study will have to be performed. Static pressure is imposed with NSCBC conditions are imposed at outlet, where a radial equilibrium naturally occurs (Koupper et al, 2014) [16]. Static pressure at outlet is adjusted to match the experimental total pressure ratio at station 21A, downstream of the stator.…”

Section: Les: Configuration Methodology and Numericsmentioning

confidence: 99%

Comparison of LES and RANS predictions with experimental results of the fan of a turbofan

Odier

European Conference on Turbomachinery Fluid Dynamics and Hermodynamics

Gicquel

et al. 2017

This paper aims at validating LES capability if applied to an actual turbofan configuration at nominal regime, if compared to RANS and experimental measurements. For assessment, averaged radial profiles are compared in 3 axial planes-before the stage, between the rotor blade and stator vanes, and downstream of the stator. RANS and LES results are in very good agreement, but found to be shifted compared to the measurements and for some quantities. An analysis of the unsteady axial velocity is then proposed, investigating root-mean square of axial velocity. Tip-leakage, as well as two boundary layer transitions are evidenced in the rotor. An estimation of the integral turbulent timescale is finally proposed in the whole domain, using autocorrelation of the axial velocity. Suctionside horseshoe vortices are found to be very coherent, as well as the stator corner vortices. Regions of large timescale are moreover evidenced between rotor and stator wakes.

“…To ensure no reflection of acoustic waves at the inlet and at the outlet of the domain, Navier-Stokes Characteristic Boundary Conditions (NSCBC) are employed (Poinsot and Lele, 1992). It has recently been shown that a characteristic outlet boundary condition, including transverse terms, is particularly suited for turbomachinery flows since it allows the radial equilibrium to naturally take place around the target static pressure value (Koupper et al, 2014). Moreover for an axial compressor at low mass flow rate, a small variation of exit pressure leads to a relatively large variation of mass flow.…”

Section: Computational Domainmentioning

confidence: 99%

Analysis of a high-pressure multistage axial compressor at off-design conditions with coarse Large Eddy Simulations

Laborderie

European Conference on Turbomachinery Fluid Dynamics and Hermodynamics

Gicquel

2017

This paper aims at evaluating Large Eddy Simulations (LES) for the prediction of the performance line and flow at off-design conditions in a multistage high-pressure compressor. A coarse and an intermediate grid are specifically investigated, since their associated computational cost appears affordable in an industrial context. Several operating conditions of the 3.5 stages high-pressure compressor CREATE are simulated, then results are compared to experimental data and to an existing URANS simulation. Both grids yield iso-speed performance lines close to experimental measurements, but only the intermediate one is able to correctly predict the experimental point at lowest mass flow rate. The unstable regime is specifically investigated in the last stage of the intermediate grid, showing the presence of rotating instabilities. Their amount and spinning velocity are similar to experimental observations and previous URANS results. Hence coarse LES appears as an interesting tradeoff for off-design predictions of flow in a multistage compressor.