Transition Flow and Aero-

acoustic Analysis of NACA0018

Satish Kumar B, Fred Mendonça,

Ghuiyeon Kim, Hogeon Kim

Transition Flow and Aero-

acoustic Analysis of NACA0018

Satish Kumar B, Fred Mendonça,

Ghuiyeon Kim, Hogeon Kim

Transition Flow and Aero-

acoustic Analysis of NACA0018

Satish Kumar B, Fred Mendonça,

Ghuiyeon Kim, Hogeon Kim

Introduction

Geometry & Computational Domain

Meshing Details

Boundary Conditions

Steady State Analysis – Preliminary Study

Unsteady LES

Acoustic & Spectral Analysis

Comparison with Experiments

References

Contents

Introduction

• Whistling noise from the side mirror at high speed is an ongoing

serious issue for both design and aero-acoustic performance of a

vehicle.

• Design changes in the side mirror for reducing its contribution to

the total drag of vehicle and also to improve the fuel economy

potentially cause a discrete noise by flow transition from Laminar to

Turbulent via the growth of Tollmein - Schlichting (T-S) instability

waves.

• Simple case of flow over NACA0018 aerofoil at Re=1.6e5 is

considered to analyze the complex features of flow transition and

its associated noise at fundamental level.

Aerofoil and Computational Domain

Aerofoil: NACA0018

Aerofoil Angle of Attack (AOA): 6 Degrees

Aerofoil Chord Length(CL):0.08 m

Aerofoil Span:0.16 m (2CL)

Free stream Diameter: 2 m (25CL)

Trailing Edge Thickness:8e-5 m (0.002CL)

Mesh Modeling

Mesh Models

Surface Remesher

Trimmer

Prism Layer Mesher

Reference Values:

Prism Layer Stretching: 1.1

Base Size: 4 mm

Maximum cell size: 1600 %

Number of prism layers: 15

Prism layer thickness: 1 mm

Surface size:

Relative min. size: 0.5 mm

Relative target size: 64 mm

Template Growth Rate:

Default growth rate: Slow

Boundary growth rate: Medium

Mesh Volumetric Controls

2 mm

Mesh Volume

• Number of cells: 11 Million

• Y+ approximately 1 on complete airfoil surface • Prism Layers: 15

• Prism Layer Thickness: 1 mm

• Prism Layer Stretching: 1.1

• Predominantly Hexahedral in the free stream domain

Steady State Physics – Preliminary analysis Physics Models:

Air

Three Dimensional

Steady State

Ideal Gas

Segregated Flow Solver

Segregated Energy Solver

K-Omega SST Turbulence

All Y + wall Treatment

Reference Values:

Reference Pressure:101325 Pa

Initial Conditions:

Static Pressure:0.0 Pa (Gauge)

Static Temperature:300 k

Turbulent Intensity:0.01

Turbulent Viscosity Ratio:10

Velocity:[30,0,0] m/s

Boundary Conditions

Boundary Conditions:

Free Stream

Mach Number:0.0875

Static Temperature: 300 K

Pressure: 0 Pa

Turbulence Intensity: 0.01

Turbulent Viscosity Ratio: 10

Free Stream Non Reflecting B.C

Advantageous than Reflecting

B.C such as

Velocity Inlet

Pressure Inlet or Outlet

Steady State Mesh Frequency Cut Off

• Measure of mesh ability in terms of resolution to capture the turbulent flow

structures in the frequency of interest.

• Demonstrates ability of mesh to predict well beyond 1kHz in the boundary

layer

• Defined in terms of Isotropic Fluctuating component of Velocity and the Cell

Dimension in direction of interest.

2 / 3( )

2MC

kf Hz

D

Steady State Scalar Contours (Z=0.08 m)

Turbulent Viscosity Ratio

Velocity Magnitude

Steady State Pressure Coefficients

Unsteady LES Physics

Physics Models:

Air

Three Dimensional

Implicit Unsteady

Ideal Gas

Segregated Flow Solver

Segregated Energy Solver

LES Turbulence

WALE (Wall Adapting Local Eddy) Sub grid

Scale

All Y + wall Treatment

Aero acoustics

Ffwocs Williams-Hawkings

Reference Values:

Reference Pressure:101325 Pa

Initial Conditions:

Started from Steady RANS Calculation

1Time Step[s]=

10*Maximum Frequency Resolution [Hz]

Highest Frequency to be resolved: 10,000 Hz

Time Step: 1e-5 s

Unsteady Pressure Coefficients : Instantaneous

Indicates suction-side inception and

growth of T-S instabilities

Suction side: Breakdown to

turbulence

No instabilities indicate laminar

flow on pressure side and leading

edge suction side

Pressure side : Breakdown

to turbulence

Pressure Coefficients: Unsteady Mean Vs Steady

Scalar Contours: Wall Shear Stress (suction side)

Scalar Contours: Suction Side Q-Criterion (3D

Vorticity)

Velocity contours on Iso-surface of Q= +10

Computed Instantaneous vorticity field at TE

Acoustic Analysis

• Free- space Green’s function based FW-H solver used in STAR-

CCM+ environment for the computation of sound propagation.

• Aerofoil surface is considered as the impermeable dipolar

source.

• Receiver location is chosen as the same point considered in

previous computations and experiments to compare and validate

the SPL at tonal frequency.

• The acoustic pressure signal build at the receiver location is

generated from the integration of signals from the all the source

elements of aerofoil surface.

Fast Fourier Transform of Radiated Pressure at

FW-H Receiver ( L: STAR-CCM+ , R: CFD Reference)

Spectral Analysis

Point Spectra:

• Point located above Suction side near the trailing edge at approx.

0.8*chord

• Shows peak at 2358 Hz

Surface Spectra:

• Pressure and Suction sides

• Shows localized excitations at various selected frequencies

Symmetry Plane Spectra

• Shows localized excitations at various selected frequencies

• Shows localized and near-field radiation (directivity) patterns

Suction Side Pressure Spectra

1000Hz 1500Hz 2000Hz

3000Hz 2500Hz 2358 Hz

2358 Hz

Pressure Side Pressure Spectra

1000Hz 1500Hz 2000Hz

3000Hz 2500Hz 2358 Hz

2358 Hz

Symmetry Plane Pressure Spectra & Near-field

radiation

2358 Hz

8mm

4mm

1mm

2mm

1000Hz 1500Hz 2000Hz

3000Hz 2500Hz 2358 Hz

Direct Propagation

2000Hz

2500Hz

Comparison with Experimental Data

Highlights of Experimental Work

Author/Journal,Year Flow Measurement Flow Visualization Aero-acoustics

/Noise

T. Nakano et al.

/JWE,2007 PIV Liquid Crystal Coating

Condenser Microphone

(20-8000 Hz)

@ Bottom wall of AWT

Y. Takagi et al.

/ JSV,2006 PIV Liquid Crystal Coating

Condenser Microphone

(20-8000 Hz)

@Bottom wall of AWT

Fujisawa et al.

/TVSJ,2002 PIV Smoke

Sound Level Meter

10 mm underneath of

Top wall of AWT

Spectrum of Aerodynamic Noise CFD Vs Expt.

Turbulent Stress (urms / Uo)

CFD Vs PIV (Nakano et al.)

PIV

STAR-CCM+

Kim & Lee

Turbulent Stress (vrms / Uo)

CFD Vs PIV (Nakano et al.)

PIV

STAR-CCM+

Kim & Lee

Turbulent Stress (u’v’ / Uo2)

CFD Vs PIV (Nakano et al.)

PIV

STAR-CCM+

Kim & Lee

References

H-J Kim , S. Lee , N. Fujisawa., 2006. Computation of unsteady flow and

aerodynamic noise of NACA0018 airfoil using large-eddy simulation.

International Journal of Heat and Fluid Flow 27, pp229-242.

T. Nakano , N. Fujisawa , Y. Oguma , Y. Takagi , S. Lee., 2007. Experimental

study on flow and noise Characteristics of NACA0018 airfoil.

Journal of Wind Engineering and Industrial Aerodynamics 95, pp511-531.

Y. Takagi , N. Fujisawa , T. Nakano , A.Nashimoto., 2006. Cylinder wake

influence on the tonal noise and Aerodynamic characteristics of a NACA0018

airfoil. Journal of Sound and Vibration 297, pp563-577.

Tomimatsu S , Fujisawa N., 2002. Measurement of Aerodynamic Noise and

Unsteady Flow Field around a Symmetric Airfoil.

Journal of Visualization Vol.5, No.4 ,pp381-388.

References

Mendonca, F., Read, A., Caro, S., Debatin, K. and Caruelle, B.2005.

Aeroacoustic Simulation of Double Diaphragm Orifices in an Aircraft Climate

Cooling System.

AIAA-2005-2976.

STAR-CCM+ Version 6.06.015 User Guide and Methodology Manuals, CD-

adapco, London, UK, 2011.