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Performance Comparison of Finite-Volume and Spectral/ hp Methods for - - PowerPoint PPT Presentation

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Performance Comparison of Finite-Volume and Spectral/ hp Methods for LES of Representative Gas Turbine Combustor Aerodynamics Vishal Saini v.saini@lboro.ac.uk PhD Student Loughborough University Background Focus on low emission aircraft

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SLIDE 1

Performance Comparison of Finite-Volume and Spectral/hp Methods for LES of Representative Gas Turbine Combustor Aerodynamics

Vishal Saini

v.saini@lboro.ac.uk

PhD Student Loughborough University

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SLIDE 2

Background

  • Focus on low emission aircraft gas turbines
  • Gas turbine combustion systems
  • Aerodynamics underpin the combustion processes.
  • V. Saini
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SLIDE 3

Introduction

https://www.rolls-royce.com/products-and- services/civil-aerospace/airlines/trent-700.aspx

  • V. Saini

~1% of engine- cost and weight

Combustor

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SLIDE 4

Introduction

https://www.rolls-royce.com/products-and- services/civil-aerospace/airlines/trent-700.aspx

Jets in cross-flow RCZ - swirling flow M < 0.3

  • V. Saini

~1% of engine- cost and weight

Combustor

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SLIDE 5

Combustor LES

Challenges:

  • Complex geometry
  • Injector, Cooling holes
  • Multi-physics

How is it done?

  • RR’s in-house code PRECISE

(at most 2nd order accurate)

  • Need to improve computational

efficiency of the LES

Rolls-Royce. The Jet Engine. Wiley 2015.

  • V. Saini
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SLIDE 6

Present Project

Gap: Benefit of high-order schemes for LES on complex geometries. Aim: Objectively evaluate the accuracy and cost of high-order LES on gas turbine combustor relevant geometries. Challenges:

  • “Objectively”: evaluation of fair measures of cost and accuracy,
  • “high-order LES”: LES methodology for high-order methods,
  • “relevant geometries”: generating a high-order mesh.

Method: Evaluate the accuracy benefit for given cost and cost benefit for given accuracy using available packages.

  • V. Saini
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SLIDE 7

Research Path

Advanced cases Canonical case Literature Review

Taylor-Green Vortex t

  • M. Dianat et al.

(ERCOFTAC 2014)

Radial Jets in Cross Flows Injector Swirling Flow

  • V. Saini
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SLIDE 8

Performance Comparison of Finite-Volume and Spectral/hp Methods for LES of Representative Gas Turbine Combustor Aerodynamics

Vishal Saini

v.saini@lboro.ac.uk

PhD Student Loughborough University

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SLIDE 9

Taylor-Green Vortex (TGV)

  • Standard test case for evaluating numerical

schemes for DNS/LES

  • Complex 3D transient flow in a periodic box
  • Re=1600, M=0.1

Fig: Iso-surfaces of vorticity magnitude coloured by velocity magnitude. t*=10 t*=0

  • V. Saini

Solvers:

  • PimpleFoam, OpenFoam - Central 2nd order
  • IncNSSolver, Nektar++ - P4
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SLIDE 10
  • HO LES: iLES, SVV.
  • Hexahedral mesh:
  • 643
  • “low” resolution
  • 323
  • “very-low” resolution

643

TGV

A general point: Use the right preconditioners. Observed 2-5x speed-up by replacing “Diagonal” with “LowEnergyBlock”.

  • V. Saini
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SLIDE 11

TGV: Cost vs. Error

643 High-order vs. varying 2nd order

  • Similar accuracy achieved by 4x coarser

mesh using P4

  • For similar accuracy, N++ ~8-9x faster
  • For given cost, N++ ~11x low in error

323 High-order vs. varying 2nd order

  • For similar accuracy, N++ ~8-9x faster
  • For given cost, N++ ~2.5x low in error

11x 8x

  • V. Saini
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SLIDE 12

Radial Jets in Cross Flow (R-JICF)

TGV caveats:

  • Simple Geometry
  • Lacks turbulence equilibrium

Core Annulus 6 Ports

  • V. Saini

R-JICF:

  • More realistic/relevant flow features
  • boundary layers, jet shear layers,
  • vortex shedding, high levels of mixing.
  • Studied by A. Spencer (LDA) and D. Hollis

(PIV) at L’boro on a water-based rig. Jets in cross-flow

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SLIDE 13

R-JICF

Simulation parameters:

  • Velocity Ratio, !

" / ! $ = 5,

  • Bleed Ratio, ̇

&" / ̇ &' = 0.5,

  • Jet Re, ()" ~ 2.2×10/

Solvers:

  • PimpleFoam - Blended 2nd upwind & 2nd

central (40:60), WALE

  • Nektar++ (IncNSSolver) - P4, SVV

(Power Kernel) Other: Time step 5e-6s, Linear solver tolerances 1e-7 for u, 1e-6 for p.

  • V. Saini

Core Annulus 6 Ports

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SLIDE 14

R-JICF meshing

High-order mesh:

  • Prepared the coarse mesh
  • Pointwise
  • Elevate the information to high-order
  • Spherigons in NekMesh
  • V. Saini
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SLIDE 15
  • Similar simulation cost
  • Similar distribution
  • Mixed: Hexs, Tets, Pyramids

R-JICF mesh

OF: 16M cells N++: 130k Ele. (~8.5M sol. points)

X Y

P4

  • V. Saini
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SLIDE 16

R-JICF instantaneous

N++: 130k Ele. (~8.5M sol. points) OF: 16M cells

  • V. Saini

Animations

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SLIDE 17

R-JICF spectra

  • V. Saini
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SLIDE 18

R-JICF spectra

1x 0.97x

  • V. Saini
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SLIDE 19

R-JICF spectra

1x 0.97x Refine 1.5x per dimension: 48M cells

  • V. Saini
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SLIDE 20

R-JICF spectra

1x 0.97x 3.5x Refine 1.5x per dimension: 48M cells

  • V. Saini
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SLIDE 21

R-JICF mean quantities

  • V. Saini
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SLIDE 22

Conclusions

  • Present project aims to quantify the benefits (if any) of the High-Order LES on

combustor relevant geometries.

  • For TGV case, P4 LES were found to be 8x cheaper for given accuracy and

2.5-10x more accurate for a given cost.

  • For R-JICF case, P4 LES resolved broader turbulent scales at a key location

for a given cost. Equivalent spectrum was obtained by a 3.5x more expensive 2nd order simulation. Mean quantities are less distinctive but in favour of high-

  • rder. Further investigation on R-JICF case is ongoing.
  • V. Saini
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SLIDE 23

Vishal Saini v.saini@lboro.ac.uk

Thank you for the attention. Questions?

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SLIDE 24

Acknowledgements

  • Centre for Doctoral Training in Gas Turbine Aerodynamics
  • EPSRC
  • Rolls-Royce plc.
  • HPC Midlands+
  • Nektar++ team
  • V. Saini