Tip Noise Passive Control in Low Mach Number Axial Fan · 2014-11-13 · Axial fan family • Test model and demonstrator based on AC90/6 fan. unswept rotor fan in ducted configuration

Haifa, Israel, October 25 2012

XI Israeli Symposium of Jet Engines and Gas Turbines

Stefano BIANCHI

Tip Noise Passive Control in Low Mach

Number Axial Fan

Fläkt Woods Ltd FMGroup @ DMA-URLS

Credits

The present research was done in the context of the contract FW-DMA10-11, between

Fläkt Woods Ltd and Dipartimento di Ingegneria Meccanica e Aerospaziale “Sapienza”

University of Roma

Alessandro CORSINI

Anthony G. SHEARD

Outlines of civil aircraft issues

Introduction - Launch crews, on aircraft carriers, are exposed to excessive noise levels during

takeoffs and landings leading to costly and escalating hearing loss compensation

programs.

Pictures Marine Nationale

- The Secretary of the US Navy, has estimated a total potential liability of

$350 million should litigants prevail in lawsuits involving jet noise.

Outlines on Military aircrafts ssues

Introduction

The goal of engines noise reduction is considered of high importance.

- Typically launch/recovery support personnel can be

exposed to brutal acoustic loads of up to 150 dBA. Each

launch typically involves a 30 sec average mil power

exposure (occasionally full AB).

- The carrier deck personnel may experience up to 200

launches/recoveries per 12 hr duty shift.

Picture US Navy

- For high by pass ratio engines turbine noise account

only during the Landing operation, as the TO thrust is

exerted mainly by the fan.

- Low bpr engines are more senstive to turbine noise,

even for TO operation.

- In low bpr engines turbine noise is comparable

with the rear component of compressor noise:

directed roughly to the engine side.

NASA report: FS-1999-07-003-GRC

Rolls-Royce, 2006

Techniques and concepts for tip treatment, a first review (ii)

anti-vortex devices as end-plates at the tip

originally inspired by the technique developed for the control of tip vortex and induced drag

reduction in aircraft wings, also used as anti-vortex devices for catamaran hulls

Boeing, blended winglets F2005, rear winglets Karman end-plate

• During the last decade have appeared several passive noise control concepts based

on the blade tip modifications by means of anti-vortex appendages

Axial fan family

• Test model and demonstrator based on AC90/6 fan.

unswept rotor fan in ducted configuration

in service experiences demonstrated good acoustics performance

AC90/6 blade profiles modified ARA-D

Design concept

Arbitrary vortex with

radially increasing

work distribution

Utip about 50 m/s

FMGroup @ DMA-Sapienza

Conventional end-plate design

• Base-line solution for the tip treatments, Corsini et al. (2006, 2007), defined a blade

configuration with use of tip appendages to implement a control on leakage flow

rotor AC90/6/TF tip blade section modified by

an end-plate on pressure surface with square-

tail trailing edge

tip blade section is locally thickened of a factor

3:1 wrt maximum thickness at the tip of datum

blade

According to the theory behind the end-plate design, this

dimension was chosen as the reference radial dimension of

leakage vortex to be controlled approx. 0.2 0.1 blade span,

(Inoue et al., 1986) (Corsini et al., 2004).

TF end-plate


Analysis on inner workings of TF end-plates, tip leakage vortex bursting

• This finding was in accordance with the existence of a vortex breakdown


Ro = V /(r W)

for a confined vortex the scales are:

r , radial distance from the vortex axis at which

the swirl velocity was at its maximum

in accord with characteristic viscous

length scale Leibovich (1984)

V = waTLV, was taken to be the axial velocity

W, be the rotation rate in the wing-tip

vortices due to the solid body rotation-like

structure (estimated near the vortex centre).

Criterion for VB detection based on vortex swirl analysis

an interpretive criterion to detect the appearance of vortex breakdown makes use of the Rossby

number (or inverse swirl parameter) (Ito et al., 1985)

0.4 0.5 0.6 0.7 0.80.4

0.6

0.8

1

1.2

1.4

1.6

fraction of chord

Ro

design point

breakdown region

End-plates for tip leakage vortex swirl control, design goals

• The work is dedicated to sketch new concepts for the shaping (design)

of the anti-vortex tip end-plate

• The rationale is to designate a variation of end-plate thickness, in order to

implement a manipulation of the chord-wise evolution of the leakage vortex swirl

• The aim is to promote an enhancement or a detriment of near-axis swirl by influencing

the momentum transfer from the leakage flow and by inducing some waviness

into the leakage vortex trajectory, as attempted in delta-wing platform design

Srigrarom and Kurosaka (2000)


Visualization and Analytics Center for Enabling Technologies

(VACET) DOE, USA

Aircraft Vortex Manipulation,

Manipulation of the blade vortex interaction

of helicopters with nonplanar blade tips on the main rotor

Institute of Aeronautics and Astronautics, Aachen, Germany

Rossby number definition for a confined vortex

TLV rotational frequency and length measures

given by Rain (1954) and Lakshminarayana (1970)

Leakage flow modelled as a two-dimensional flow

orthogonal to the chord line

The TLV Rossby number could be

expressed along the blade chord as a

function of:

- gap geometry, (height & width)

- kinetic energy of the leakage flow

- friction factor which is itself a function of

leakage flow Reynolds number, Regap

- local blade load conditions

End-plate for Multiple Vortex Breakdown MVB

concept rationale


Corsini A, Rispoli F & Sheard AG, A Meridional

Fan, Patent granted No. GB 2452104, July

(2009)

AC90/6 fans

TFvte end-plate

datum

TF end-plate

MVB end-plate

Ro(xc)=f [tip gap, blade load, leakage flow, end-plate thickness]

The idea is to obtain an end-plate geometric able to combine

the acoustic pay-off of TF related to the mixing enhancement effect induced by the

vortex bursting

the aerodynamic performance benefit of TFvte

consequence of the healthier status of the flow

in the annulus and in the wake

The rationale was to profile the end-plate at the

blade tip according to a prescribed chordwise

distribution of the TLV Rossby number

The design Rossby number distribution would drive

the TLV to a sequence of critical condition able to

induce the onset of vortex bursting

Corsini A, Rispoli F & Sheard AG, A Meridional

Fan, Patent granted No. GB 2452104, July

(2009)

II generation of end-plate for Multiple Vortex

Breakdown MVB, design goals


fraction of chord


datum

TF

TFvte

The authors solved the Reynolds-Averaged Navier-Stokes

equations by an original parallel Multi-Grid Finite Element

flow solver, using C++ technology and libMesh libraries

The physics involved in the fluid dynamics of

incompressible 3D turbulent flows in a rotating frame of

reference is modelled with a non-linear k-e model, here in

its topology-free low-Reynolds variant.

The Finite Element flow solver uses second-order accurate

approximations in space for primary-turbulent variables.

Concerning the solution strategy, the authors solved the

Navier-Stokes and turbulence equations fully coupled. The

linear solver uses a SOR preconditioned GMRES(5)

technique.

The mesh was built using a non-orthogonal body fitted

coordinate system with block-structured topology. The

mesh consists of about 0.6 million linear hexahedral

elements.

Standard boundary condition settings were adopted, as

previously used in studies on high-performance fans

CFD survey details


MVB tip leakage flow survey

TLV detection helicity criterion (i)

datum

TF

TFvte

MVB

Identical TLV onset point wrt to datum and TF

Vortex core path deformation according to the

enhancement/detriment of swirl


TLV helicity & 3D streamlines

datum TF TFvte

MVB tip leakage flow survey

TLV streamlines (ii)

MVB

swirl enhancement

swirl detriment

No evidences of helicity inversion

No evidences of separation cores, TLV

goes through the VB onset without

actually bursting


0.8

0.2

0.8

0.2

0.8

0.2

nt_1

nt_2

nt_1

nt_2

nt_1

nt_1

nt_2

Normalized turbulent viscosity nt iso-surfaces

Turbulent kinetic energy iso-surfaces

nt_2: 102 nmol;

nt_1: 5× 10 nmol

MVB tip leakage flow survey, TKE & nt (iii)

datum rotor AC90/6/TF AC90/6/TFvte

datum rotor AC90/6/TF AC90/6/TFvte

MVB

MVB

FLAKT WOODS Ltd


Lw dB(A)

frequency (Hz)

The noise has a broadband characteristic

Tones are due to the tip leakage flow in the

pressure side

Boundary layer separation appears in the suction

side at the trailing edge

Normalized turbulent viscosity nt iso-surfaces

Turbulent kinetic energy iso-surfaces

MVB

MVB


MVB rotor performance assessment

aeroacoustics (ii)

sound power spectra in one-third octave band narrow band (3 Hz) sound pressure level spectra

motor signature

BPF

operating point 8 m3/s



aeroacoustics (iii)

Experimental techniques, rig (i)

Fan tested @ 8 – 6 m3/s

Anechoic chamber BS848 p2

Pressure measurements @ nearfield:

• distance d=15 mm from the trailing edge

2% of the chord

• spanwise traversing microphone in 14

positions (10% span each)

Noise measurements @ farfield:

• distance = 2 rotor D from the fan section

• six azimuthal positions

= 0°

= 30°

= 45°

= 60° = 75°

= 90°

Far-field

microphone

Span traversed microphone (when operating in the rig)


Experimental techniques, data post-processing (ii)

Hsound(t,r) S(t,r)

HpsdSound(t,r)

To far-field

H(t,r) N(t,r)

D(t,r)

Hypotesis:

- H(t,r) Linear throug the anechoic space

- H(t,r) depends only from geometries

Cross-correlation, between near- and far-field

Auto&cross spectra GSS GNN GSN

NNSS

SN

GG

GCo

2

2

Coherence:

tip

hub

H(t,r)

H(t,1)

H(t,0) ∑

S(t,1)

S(t,r)

S(t,0) N(t)

Near-field Far-field




Near-field aeroacoustic source

dissection (ii)

AC90/6/TFMVB

Source characterization – Span

wise Coherence map @ 30°

Norm Freq (ʄ = f/BPF) ʄ = 1 - 7

Norm Freq (ʄ = f/BPF) ʄ = 1 - 7

datum

TF

TFvte

MVB

Conclusions

• The passive control technique explored in the study was based on blade tips that had

been modified by the addition of anti-vortex appendages as end-plates.

• The end-plate configuration aimed to control the chord-wise evolution of the leakage

vortex swirl level by preventing the occurrence of tip-leakage vortex bursting by the

enhancement of near-axis swirl.

• The leakage-flow survey has shown that the variable-thickness configuration MVB was

able to exert control over near-axis leakage vortex swirl.

As a result, although the critical Rossby number region is reached,

the end-plate configuration provides effective control over leakage vortex bursting.

• The comparative aerodynamic and aeroacoustic assessment, demonstrated

the overall gain for the investigated end-plates was found to be a consequence

of the implemented passive flow control concept acting on the tip leakage flow


Thank you!

(Corsini A, Rispoli F & Sheard AG, A Meridional Fan, Patent granted No. GB 2452104, July

2009)



aerodynamics (i)

backup slide

sensitivity on tip gap


MVB design concept, sensitivity analyses

backup slide

sensitivity on lift @ blade tip

Documents

Tip Noise Passive Control in Low Mach Number Axial Fan · 2014-11-13 · Axial fan family • Test model and demonstrator based on AC90/6 fan. unswept rotor fan in ducted configuration