Etc09 Id239

Experimental aero-acoustic characterization of axial fan fitted
with shaped tip end-plates

Stefano BIANCHI, Alessandro CORSINI A. G. SHEARD

FMGroup @ DMA-URLS Fläkt Woods Ltd

9th European Conference on Turbomachinery, ETC9

Istabul, Turkey, March 25 2011

Experimental aero-acoustic characterization of axial fan fitted with shaped tip end-plates

Background motivations

• Tip gap plays a detrimental role affecting axial flow rotor performance (Fukano and
Takamatsu, 1986), (Furukawa et al., 1999)

• A number of studies pointed out that the influence of tip leakage flow on rotor
aero-acoustic signature is similar in low speed ventilating equipment as well as in
high- bypass-ratio turbofan engines

• Recognized tip noise generation mechanisms in axial decelerating turbomachines
convection of the primary tip vortex producing tonal noise owing to vortex-stator
interactions
convection of large-scale fluctuations downwind the trailing edge producing scattering
and broadband noise

• Consequently there is a strong motivation to look for deliberate aerodynamic design
to minimize the negative effects of tip gap

• The target to tackle is to manage the tip-clearance flow in a manner that
reduces the self-generated noise without sacrificing aerodynamic efficiency

ETC9 – March 2011
FMGroup @ DMA-Sapienza


Aims

• Recent studies investigated the application
of end-plates of constant thickness to the blade
tips of a family of industrial fans (Corsini et al., 2006 & 2007)
Flakt Woods Ltd fan detail
Amaral et al., GT2006-90345, 2006

• Aeroacoustic & aerodynamic analyses revealed for the family of tip featured rotors:
i.  benefits in terms of aeroacoustic performance
ii.  a diminution in aerodynamic performance in association with the occurrence of
leakage vortex bursting over the tip end-plate

• The goals of the present work are:
to describe the evolution of a design concept for tip end-plate oriented to
the implementation of passive flow control strategies e.g. leakage vortex swirl level

to assess and validate through combined CFD&EFD the merit of the developed
passive noise-by-flow control technologies

ETC9 – March 2011


Axial fan family
• Family of commercially available cooling fans AC90/6

unswept rotor fan in ducted configuration
in service experiences demonstrated good acoustics performance

Design concept
Arbitrary vortex with
radially increasing
work distribution
AC90/6 blade profiles modified ARA-D

Utip about 50 m/s


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

• Techniques appeared to date are generally designed to accomplish the goal by
reducing the leakage flow, affecting the primary-secondary flow momentum
transfer, or unloading the tip blade sections

In the ambit of the so-called passive noise control approach, based on geometrical
modifications of the rotor-casing, three concepts emerge

• The use of swept blade stacking line in design methods (Wadia et al., 1998) is known
to improve stall-free operating margin (Vad, 2001), (Corsini and Rispoli, 2004)

• The use of casing treatments over the blade tip reported to improve stable flow
range by weakening the tip leakage vortex
e.g. grooves and slots (Takata and Tsukuda, 1977) (Smith and Cumpsty, 1984), or
stepped tip gaps (Thompson et al., 1998).
or
in the ambit of fan technologies recirculating vanes, and annular rings have
been proposed as anti-stall devices (Jensen, 1986)

ETC9 – March 2011


Techniques and concepts for tip treatment, a first review (ii)
• During the last decade have appeared several passive noise control concepts based
on the blade tip modifications by means of anti-vortex appendages

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

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

ETC9 – March 2011


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

TF end-plate

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).

ETC9 – March 2011


Analysis on inner workings of TF end-plates, tip flow survey
Corsini, Rispoli and Sheard, J. of Power and Energy, 221/5, (2007)

datum rotor AC90/6/TF
Helicity distributions and
vortex core paths

D operating point

Turbulence intensity evolution within
the leakage flow region

D operating point

datum rotor AC90/6/TF

ETC9 – March 2011


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

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

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)
design point
Ro
Ro = V /(r Ω)

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

Ω, be the rotation rate in the wing-tip breakdown region

vortices due to the solid body rotation-like
structure (estimated near the vortex centre).
fraction of chord

ETC9 – March 2011


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

• The present 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

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

• 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)

ETC9 – March 2011


I generation of end-plate Variable Thickness VTE. Design concept
Corsini, A., Rispoli, F., Sheard, A. G., “Shaping of tip end-plate to control leakage vortex swirl in axial flow fans”,
Transactions of ASME, Journal of Turbomachinery, vol. 132, 3, paper 031005, pp 1-9, 2010.

The goal is to enhance near-axis swirl by reconfiguring the end-plate at the tip with a view to
influencing the momentum transfer from the leakage flow

The rationale was to compute the end-plate thickness distribution (tep(sc)) by
combining a simplified law for the tip-gap pressure drop with a stability criterion of TLV
by prescribing a safe chord-wise distribution of the vortex Rossby number

The pressure losses within the tip gap at each
AC90/6 fans
chord-wise abscissa sc was modelled by studying
datum
the leakage flow as a two-dimensional flow
orthogonal to the chord line

The pressure drop could thus be expressed as a TF end-plate
function of:

-  gap geometry, (height & width)
TFvte end-plate
-  kinetic energy of the leakage flow

-  friction factor which is itself a function of
leakage flow Reynolds number, Regap

ETC9 – March 2011


Test evaluation of end-plate aerodynamics & aeroacoustics

Ro
TLV rotation number

TF

breakdown region

fraction of chord

TLV core paths

TF

TFvte
datum



Test evaluation of end-plate aerodynamics & aeroacoustics

Δp(Pa)
Ro
TLV rotation number

ηstat
TF

breakdown region

fraction of chord
volume flow (m3/s)

TLV core paths

TF

TFvte
datum



II generation of end-plate for Multiple Vortex Corsini A, Rispoli F & Sheard AG, A Meridional
Fan, Patent granted No. GB 2452104, July
Breakdown MVB, design goals (2009)

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 fraction of chord

ETC9 – March 2011


End-plate for Multiple Vortex Breakdown MVB Corsini A, Rispoli F & Sheard AG, A Meridional
Fan, Patent granted No. GB 2452104, July
concept rationale (2009)

Rossby number definition for a confined vortex

TLV rotational frequency and length measures datum
given by Rain (1954) and Lakshminarayana (1970)
AC90/6 fans
Leakage flow modelled as a two-dimensional flow
TF end-plate
orthogonal to the chord line

The TLV Rossby number could be TFvte end-plate
expressed along the blade chord as a
function of:

-  gap geometry, (height & width)

-  kinetic energy of the leakage flow MVB end-plate

-  friction factor which is itself a function of
leakage flow Reynolds number, Regap

- local blade load conditions

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

ETC9 – March 2011


MVB design concept, sensitivity analyses

sensitivity on tip gap

sensitivity on lift @ blade tip

ETC9 – March 2011


Investigation methodology, summary

The experimental and computational investigation focused on the aerodynamic
performance of the end-plate configurations, and provided an overview of the far-field
acoustics

The methodology that was adopted combined an accurate computational modelling of
the leakage-flow phenomena (under design condition), with the measurement of the fan
acoustic behaviours

ETC9 – March 2011


Numerical technique

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-ε 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

ETC9 – March 2011


datum MVB tip leakage flow survey
TLV detection helicity criterion (i)

TF

TFvte

ETC9 – March 2011


datum MVB tip leakage flow survey
TLV detection helicity criterion (i)

MVB

TF

TFvte

Identical TLV onset point wrt to datum and TF
Vortex core path deformation according to the
enhancement/detriment of swirl

ETC9 – March 2011


MVB tip leakage flow survey
TLV helicity & 3D streamlines
TLV streamlines (ii)
datum TF TFvte

swirl enhancement

No evidences of helicity inversion
swirl detriment

No evidences of separation cores, TLV
goes through the VB onset without
actually bursting

MVB

ETC9 – March 2011


MVB tip leakage flow survey, TKE & νt (iii) MVB
Turbulent kinetic energy iso-surfaces
0.8
0.8
0.2
0.2
0.8

0.2

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

MVB
Normalized turbulent viscosity νt iso-surfaces
ν _1 t

νt_2: 102 νmol;
νt_1: 5× 10 νmol νt_1 νt_1

νt_2
νt_1
νt_2
νt_2

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

ETC9 – March 2011



Experimental techniques, rig (i)
Fan tested @ 8 – 6 m3/s

Anechoic chamber BS848 p2

Noise measurements @ farfield:

•  distance = 2 rotor D from the fan section

•  six azimuthal positions

Pressure measurements @ nearfield:

•  distance d=15 mm from the trailing edge
2% of the chord

•  spanwise traversing microphone in 14
positions (10% span each)

ETC9 – March 2011



Techniques for near-field acoustic measurements, a review (i)

• Coherence-based noise source identification techniques can be used in identifying the
contribution to the global far-field emission of the source distributed in the near-field

• A number of techniques have appeared to date based on cross-correlation and
coherence analysis mainly inspired by the early publication of the Mugridge and
Morfey in 1972.

• Previously published works in this area span from 1970 and 1980, (ie. Leggat and
Siddon 1978) have been revised in more recent years for application in
turbomachinery aeroacoustics (Holste and Neise, 1997), (Fehse and Neise, 1999),
(Miles, 2006), subsonic jets (Laurendeau et al., 2007) and combustors (Bennett and
Fitzpatrick, 2007)

ETC9 – March 2011



Techniques for fan noise near-field measurements, a review (ii)

• Leggat and Siddon (Journal of Acoustical Society of America (64), 1978): pressure-noise
cross correlation to study the rotor-vortex in an axial fan.

• Mongeau et al. (Journal of Sound and Vibration (181), 1995): near- and far-field cross
correlation to isolate the aeroacoustic emissions a ducted centrifugal pump.

• Kameier and Neise (Journal of Sound and Vibration (203), 1997 ): azimuthal cross
correlation to investigate the rotating blade flow instability as a source of noise in axial
high speed turbomachine.

• J. H. Miles (NASA/TM-2006-214352, 2006): multiple microphone near- and far- field cross
correlation to separate the correlated core noise from the uncorrelated noise in a PW
4098 turbofan engine.

ETC9 – March 2011



Experimental techniques, data post-processing (ii)

Hypotesis:
-  H(t,r) Linear throug the anechoic space
- H(t,r) depends only from geometries
tip
Cross-correlation, between near- and far-field
Auto&cross spectra GSS GNN GSN

Coherence:

hub

H(t,r)
N(t,r)
Hsound(t,r) To far-field
S(t,r)
HpsdSound(t,r) D(t,r)

ETC9 – March 2011



Experimental techniques, instrumentation (iii)

ETC9 – March 2011



MVB rotor performance assessment
aerodynamics (i)

ETC9 – March 2011


aeroacoustics (ii)
operating point 8 m3/s

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

BPF

motor signature

ETC9 – March 2011


aeroacoustics (iii)

ETC9 – March 2011


Norm Freq (ʄ = f/BPF) ʄ = 1 - 7 MVB rotor performance assessment
Near-field aeroacoustic source
dissection (i)

Source characterization – Span wise
Coherence map @ 30°
AC90/6datum

AC90/6/TF

AC90/6/TFvte

ETC9 – March 2011


dissection (i)

Source characterization – Span
wise Coherence map @ 30°
AC90/6datum

AC90/6/TF

AC90/6/TFvte

ETC9 – March 2011


dissection (ii)

Source characterization – Span
wise Coherence map @ 30°
Norm Freq (ʄ = f/BPF) ʄ = 1 - 7

AC90/6/TFMVB

ETC9 – March 2011


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

ETC9 – March 2011

Acknowledgement

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 Rome

(Corsini A, Rispoli F & Sheard AG, A Meridional Fan, Patent granted No. GB 2452104, July
2009)

ETC9 – March 2011

Etc09 Id239

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