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Property of Valeo – Duplication prohibited February 2008 1 WP6: CD Airfoil Test-case Experimental and numerical data base October 2008 S. Moreau VEC Manager of the Fan System Core Competencies Manager of the Group Simulation Competency Center

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Property of Valeo – Duplication prohibited February 2008 2 Airfoil chord length ~10 cm Valeo CD and NACA12 airfoils, Flat Plate, V2 and V3 airfoils Nozzle exit section 50 cm x 25 cm RMP Camber 12°Thickness 4% RMP 11 26 Open-Jet Aeroacoustic Experiment in ECL Large Wind Tunnel ECL Experimental Set-up, LMFA

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Property of Valeo – Duplication prohibited February 2008 3 CD Airfoil Experimental Data Base Open-Jet Aeroacoustic Experiment in ECL Large Wind Tunnel Moreau et al, AIAA J. 2004, 2005, JFE 2005 Valeo CD Airfoil: Re 1.5 10 5 ; M 0.05 ; several angles of attack (8° focus) Far field noise measurements: Noise spectra and directivities Remote Microphone Probe (RMP) measurements: Wall pressure statistics (C p, frequency spectra, coherence, phase) Hot wire measurements: Velocity statistics (mean and RMS velocity components, Reynolds stress and frequency spectra)

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Property of Valeo – Duplication prohibited February 2008 4 CD airfoil in high loading conditions Flat plate at zero angle of attack Evidence of 2 mechanisms: vortex-shedding noise and trailing-edge noise Experimental Far Field Noise Spectrum Directivity

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Property of Valeo – Duplication prohibited February 2008 5 Wall pressure fluctuations must be statistically homogeneous is deduced from coherence measurements is deduced from the phase diagrams of streamwise cross spectra Gaussian model proposed (Roger & Moreau, AIAA 2002- 2460). Equivalent Corcos’ model Experimental Wall Pressure Statistics f -5

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Property of Valeo – Duplication prohibited February 2008 6 Hot Wire Measurements Wake Zoom Overview Inlet survey LES bc survey Shear layer survey

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Property of Valeo – Duplication prohibited February 2008 7 leading edge separation line Reattachment of the flow Separation bubble Suction side streaklines Direction of the flow turbulent laminar Flow Visualization on CD Airfoil « Oil » FlowTuft Film MVI_9612.avi Evidence of laminar flow separation at the leading edge Possible flow separation at the trailing edge

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Property of Valeo – Duplication prohibited February 2008 8 Two families of results k- , SST and V2F k- TL and WL Numerical Wall Pressure Coefficient Moreau et al, AIAA J. 2003

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Property of Valeo – Duplication prohibited February 2008 9 Two families of results k- , SST and V2F k- TL and WL Numerical Wall Friction Coefficient Moreau et al, AIAA J. 2003

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Property of Valeo – Duplication prohibited February 2008 10 Comparison of Wall Pressure Coefficients Good prediction of laminar flow separation at the leading edge No prediction of onset of trailing edge flow separation SST model

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Property of Valeo – Duplication prohibited February 2008 11 Broadband Models: Generalized Amiet - 1 3 main mechanisms considered: Tip and leakage flow are not considered yet 1 - Turbulence- interaction noise 2 - Trailing edge noise 3 - Vortex-shedding noise

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Property of Valeo – Duplication prohibited February 2008 12 Broadband Models: Generalized Amiet - 2 Turbulence Interaction Noise: Trailing Edge Noise: wall-pressure spectrum Inflow velocity statistics Spanwise correlation length Radiation integrals (including back-scattering correction)

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Property of Valeo – Duplication prohibited February 2008 13 Stanford LES Set-up & Averaged Results Choose the largest jet width (w = 50 cm) LES domain in the jet core, with velocity B.C.'s coming from RANS (only mean values) Better prediction of leading edge flow (C p ) with LES

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Property of Valeo – Duplication prohibited February 2008 14 Grid for Stanford LES Single block-structured topology Grid Size: 960 x 84 x 64 5.2 million nodes Domain Size: (4 x 2.5 x 0.1) x chord (first LES able to resolve the spanwise coherence length) Very regular, fine and orthogonal grid at LE Stanford Reference LES grid in 2D slice (2003)

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Property of Valeo – Duplication prohibited February 2008 15 Grid Quality/Parameters of Stanford LES Almost a DNS resolution in the normal direction Very regular and orthogonal grid near airfoil Grid independence of the solution verified on pressure spectra Energy-conserving hybrid finite-difference/spectral code Dynamic sub-grid-scale model

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Property of Valeo – Duplication prohibited February 2008 16 Stanford LES Instantaneous Results Leading-edge separation leading to transition on suction side Laminar boundary-layer on pressure side Qualitative agreement with experimental observation

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Property of Valeo – Duplication prohibited February 2008 17 2D vs 3D LES Instantaneous Results NACA0012 Re ~ 5-9 10 5 and M~0.2 Drastic change of flow topology after transition 3D 2D

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Property of Valeo – Duplication prohibited February 2008 18 Experiment LES Stanford Broadband Noise (BBN) Sources Excellent qualitative and quantitative agreement

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Property of Valeo – Duplication prohibited February 2008 19 Stanford BBN Prediction Good agreement of both analogies with experimental data Effect of finite-chord up to 2 kHz Discrepancy between the two analogies at high frequencies Acoustic Analogies based on wall pressure statistics (Amiet) and on velocity statistics near the trailing edge (Ffowcs-Williams and Hall)

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Property of Valeo – Duplication prohibited February 2008 20 Evaluation of Other Unsteady Methods Starting from the same “LES domain” with the same RANS boundary conditions and, if possible, the same LES grid: Unsteady RANS: no unsteadiness was observed Detached Eddy Simulations (DES-SA within Fluent 6.1) Lattice Boltzmann (RANS/DNS) > Powerflow (EXA) LES with Immersed Boundary Technique Moreau et al, CTR Summer Program 2004

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Property of Valeo – Duplication prohibited February 2008 21 Grid and Simulation Parameters of SP-2004 Moreau et al, CTR Summer Program 2004 LBM:Powerflow 6.1.22 (3ddp) Grid: 73551 voxels and 1491 surfels in a 2D slice (~1.2 M in 3D) Smallest cell at LE is similar to body-fitted LES Model: RANS k- in 2D - No model in 3D Simulation parameters: (Re=1.5 10 5 ) Time step: t =2.0e-7 CPU time for 100 time steps: 5 minutes on SGI Octane (1 CPU) LES-IB: Structured Cartesian (PhD: S. Kang) Grid: 4.8M in 3D (0.15M in a 2D slice) Smallest cell at LE is ~2.5 larger than body-fitted LES Model: LES + Dynamic Procedure Simulation parameters: (Re=1.5 10 5 ) Time step: t =1.0e-4 CPU time for 100 time steps: 20 minutes on Linux cluster (8 CPU)

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Property of Valeo – Duplication prohibited February 2008 22 SP-2004 (Stanford) Grids Reference LESDES-SA LBM-DNSLES-IB 1-2 coarsening

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Property of Valeo – Duplication prohibited February 2008 23 SP-2004 (Stanford) Flow field Results Reference LES DES-SA LBM-DNS LES-IB Instantaneous Velocity Field

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Property of Valeo – Duplication prohibited February 2008 24 SP-2004 (Stanford) Wall Pressure Coefficient Moreau et al, CTR Summer Program 2004 Only IB-LES provided a complete flow field close to reference LES

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Property of Valeo – Duplication prohibited February 2008 25 SP-2004 (Stanford) Wake Velocity Profiles Moreau et al, CTR Summer Program 2004 Two coarse grids in any of the new simulations to yield good wake X-wire

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Property of Valeo – Duplication prohibited February 2008 26 SP-2004 (Stanford) Flow field Results DES gives unrealistic flow field (over production of k at LE) Grids for IB-LES need to re-visited for better TE prediction All over estimate the pressure fluctuations at low frequencies

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Property of Valeo – Duplication prohibited February 2008 27 STAR-CD LES Simulations (2005-2006) Moreau et al, AIAA 2005-2916 PISO algorithm for time discretization Central differencing and upwind MARS scheme (no stable solution could be obtained even in 2D with CDS for all grid topologies tested). To keep a CFL number below 1 throughout the computational domain, a maximum allowable time step t = 1.5 10 -5 s is used. 5 to 10 time units run to eliminate the transient and collect reliable statistics (based on the free stream velocity of 16 m/s). Smagorinsky sub-grid scale (SGS) model together with a van Driest near-wall damping and WALE SGS are used. k- based DES is selected. Evaluation of different numerical Schemes Evaluation of different SGS models Re-assessment of the DES model with a different code

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Property of Valeo – Duplication prohibited February 2008 28 Final MARS LES Grid Topology (2005) Zoom LE Zoom TE 1,115,000 cells Similar grid 1-2 coarsening as DES with Fluent 6.1 Only 5% chord span Good near-surface resolution: x + ≤ 20 ; y + ≤ 1 ; z + ≤ 10

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Property of Valeo – Duplication prohibited February 2008 29 MARS LES Instantaneous Velocity Field Stanford Reference LES STAR-CD MARS LES Similar small structures created after separation convected downstream towards the trailing edge Larger flow separation at the leading edge in MARS LES. More coherent structures at the trailing edge in MARS LES.

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Property of Valeo – Duplication prohibited February 2008 30 MARS LES Wall Pressure Coefficient Qualitative agreement on the laminar separation bubble (good level of pressure plateau but too large extent of the bubble, 11.2% instead of 3.7%) First simulation to predict the positive pressure gradient up to mid-chord.

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Property of Valeo – Duplication prohibited February 2008 31 MARS LES Wake Velocity Profiles Excellent agreement in the near wake for the MARS LES Too large diffusion and deflection of the wake in the DES.

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Property of Valeo – Duplication prohibited February 2008 32 MARS LES Wall Pressure Spectra Too high levels everywhere No homogeneous statistics close to TE P2 P8 P14 P60 P60: mid-chord (-60 mm) P14: -14 mm P8: -8 mm P2: -2 mm Origin: TE Too large structures and coherence at the trailing edge Fluctuations are getting damped towards TE (MARS upwinding)

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Property of Valeo – Duplication prohibited February 2008 33 MARS Fine Scale Structures Turbulent re-attach. Turbulent T.E. Separation Laminar L.E. separation Iso-values of normalized Q colored by the streamwise vorticity Larger structures than reference LES

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Property of Valeo – Duplication prohibited February 2008 34 DES Simulation Issue Velocity Field Sub-grid Turbulent Viscosity Well attached flow field all the way to the trailing edge as in RANS simulations Short laminar separation bubble is not captured Transition occurs at the stagnation point (local turbulent kinetic energy overproduction in k- model).

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Property of Valeo – Duplication prohibited February 2008 35 Conclusions Summer 2005 The STAR-CD MARS LES reproduces all qualitative features of the flow encountered in the ECL experiment and simulated in the reference LES (Wang et al, Stanford 2004). A short laminar separation bubble is formed, reattaches and sheds small vortices that are convected towards the trailing edge. Evolution of the boundary layer seems to be well captured, especially the experimental positive pressure gradient up to mid-chord. Yet the laminar separation bubble is too wide and the wall pressure fluctuations are damped at the trailing edge (most likely due to MARS upwind scheme). No use for self-noise prediction. All DES found inadequate for this attached flow (transitional airfoil with a short laminar bubble). Further grid optimization required to remove instabilities with Central Differencing Scheme (CDS).

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Property of Valeo – Duplication prohibited February 2008 36 Final CDS LES Grid Topology (2006) 690,000 cells 3-4 grid coarsening in all directions 10% chord span Good near-surface resolution: x + ~ 2.5 ; y + ~ 2 ; z + ~ 3.6 3-4 coarsening Stability of CD scheme without significant oscillations Only a small jump in the turbulent viscosity D. Laurence, VKI Lecture Series 2005 Y. Addad, PhD UMIST

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Property of Valeo – Duplication prohibited February 2008 37 CDS LES Wall Pressure Coefficient Same qualitative agreement on the laminar separation bubble No significant differences between the two SGS models

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Property of Valeo – Duplication prohibited February 2008 38 CDS LES Wake Velocity Profiles Same excellent agreement in the near wake for the CDS-LES No significant differences between the two SGS models

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Property of Valeo – Duplication prohibited February 2008 39 CDS LES Wall Pressure Spectra Good overall predicition with the CD SGS model has only a moderate effect on spectra

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Property of Valeo – Duplication prohibited February 2008 40 Iso-values of normalized Q colored by the streamwise vorticity t = 0.07 s t = 0.25 s CDS Fine Scale Structures Smaller structures than with MARS scheme

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Property of Valeo – Duplication prohibited February 2008 41 SP-2006 (Stanford) Unstructured LES Excellent agreement with both structured LES and experimental data <4% difference

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Property of Valeo – Duplication prohibited February 2008 42 Conclusions Summer 2006 The STAR-CD CDS LES improves the MARS LES significantly (over dissipative in the TE region) and compares favorably with the ECL experiment and the reference LES (Wang et al, Stanford 2004). A regular grid with smooth jumps is required if coarsening is to be used to yield numerical stability and limited oscillations. (3-4) coarsening as suggested by Laurence seems to provide the best compromise and still yield reasonable grid sizes (< 1 Million nodes). Different SGS models do not yield significant differences Unstructured LES solver (CDP, Stanford) with the same numerical schemes and SGS as the reference structured LES yields similar results. But all LES still show weaknesses in the laminar flow recirculation and especially in the transition process (we have as many bubble sizes as LES) Yet in the trailing edge region, similar statistics are achieved and consequently same broadband noise prediction

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