Numerical Investigation of Reynolds Number and Pitch Ratio Effect on

Lock-in Ability of an Aeroacoustic Field in Ducted Flows

Dept. of Mechanical and Manufacturing EngineeringTrinity College Dublin

Cristina Paduano

Aeroacoustic Resonance of Bluff Bodies in Ducted Flows Noise intensification It can occur when a Gas Flow in a duct/cavity exhibits Periodic Vortices

Vortex shedding Duct acoustic mode

HYDRODYNAMIC

Vortex shedding at acoustic frequency

=

Tonal noise is emitted

Vorte

x she

ddin

g fre

quen

cy

LOCK-IN

Flow velocity

flow

𝒇 𝒂𝒅𝒖𝒄𝒕

Off resonance

Off resonance

NOISE SELF-SUSTAINS and

ENHANCES

Aeroacoustic Resonance Behaviour of Tube Array

10 15 20 25 300

500

1000

1500

2000

V (m/s)

P a (Pa)

10 15 20 25 300

100

200

300

400

500

V (m/s)

Freq

uenc

y (H

z)

Pressure measurements (heat exchanger)

UNPREDICTABLE VELOCITYEXTENTS OF LOCK IN RANGE UNKNOWN

Velocity measurements (heat exchanger)

140 dB

(images from Finnegan -2011)

“Tube array resonance occurs when the energy available in the flow(dynamic head) overcomes the acoustic damping of the system” - (Feenstra et al.- 2006)

Conditions for Resonance

(Hall, Ziada, Weaver data -2003)

Lock-in map (EXPERIMENTAL DATA)Co

nditi

ons f

or re

sona

nce

Amplitude of the acoustic wave

Frequency ratio

This research: Reynolds number and Pitch ratio

• To understand aeroacoustic resonance in tube array it is necessary to understand the strength of the sound sources formed around the tubes.

• Numerous experimental study for reduced array configuration (single -2- 4 cylinders) used a fixed width test section ( 1 fa) and varied fv.

Research Motivations and Objectives• Mechanism of lock in is not yet clear

• Effect of turbulence increasing and variation of the vortices patterns were indicated as possible parameters contributing to resonance of tube array (Fitzpatrick -1980, Ziada-1989). However many experiments focused more on variation of frequency ratio.

Is there a flow characteristic which causes Lock in to occur ?

Does the aeroacoustic resonance of 2 and 4 cylinders configuration represent the aeroacustic resonance of tube array ?

Vorte

x she

ddin

g fre

quen

cy

Flow velocity

1Vortices incoherentstructure

Coherent acoustic sources Vortices

incoherentstructure

LOCK INFLOW

STRUCTURE

CFD Simulation of Aeroacoustic Resonance

ACOUSTICS IS

“ COMPRESSIBLE”

INCOMPRESSIBLEFLOW

(uRANS, SST) += OSCILLATING VELOCITY (BOUNDARY CONDITION)

Hydrodynamic Analogy (Tan ,Thompson, Hourigan-2003)

TRASVERSAL ACOUSTIC WAVE replaced by the Flow OSCILLATION which it causesRESONANCE: chosen to be in LOCK-IN ratio with

=Asin(2t)

ApplicationTwo cylinders in tandem

Four cylinders in square

In line multiple cylinder arrayVo

rtex s

hedd

ing

frequ

ency

Flow velocity

1

Pre-coinc. resonance

Coinc. resonance

IMPOSED LOCK IN CONDITION

FLO

W S

TRU

CTU

RE V

ARIA

TIO

NTURBULENCE EFFECT

Mean flow velocity variation applied (i.e. RE variation 10000-36000)

VORTICES CONVECTIVE VEL. VARIATION Variation of vortices convective velocity is

obtained by varying the pitch ratio L/D 2.5-3.

(Configuration analysed – Re and pitch as Finnegan-2011)

Reynolds number Normalized frequency f/fv

Normalized frequency f/fvReynolds number

Pres

sure

, Pas

cals

Pres

sure

, Pas

cals

PreCoincidence /=1.2

Coincidence /=0.85

Two Cylinder Resonance- Reynolds number dependency

Lock in only occurring above Re 27000 –Reynolds number dependency

LOCK-IN and Velocity contours

% V inlet

Normalized velocity WITHOUT EXCITATION

% V inlet

Normalized velocity case NOT LOCKED IN (Re=10000)

Normalized velocitycase LOCKED IN (Re=36000)

Normalized velocity WITHOUT EXCITATION

% V inlet% V inlet

EXPERIMENTAL ACOUSTIC POWER

Acoustic PowerNUMERICAL ACOUSTIC POWER

(Finnegan, Meskell and Ziada data-2010)

PreCoincidence <

Coincidence >

Sinks (Flow takes energy from acoustics) Sources (Flow puts energy into acoustics)

PreCoincidence <

Coincidence >

Four Cylinder Resonance - Summary of Results

Normalized frequency f/fv

Coincidence /=0.85 PICTH 2.5• Lock in only occurring at

Coincidence and for all Reynolds numbers

PICTH 3• Lock in only occurring at

Coincidence ONLY at the higher Reynolds number

Pres

sure

, Pas

cals

Reynolds number

Coincidence (Finnegan, Meskell and Ziada data-2010)

Multiple Cylinder Array Resonance - Summary of Results

Coincidence /=0.85 –Pitch L/D 2.5 PICTH 2.5• Lock in only occurring at

Coincidence and for all Reynolds numbers

PICTH 3• Lock in NEVER OCCURRING

(Finnegan, Meskell and Ziada data-2010)Coincidence

Conclusions

The cylinder configurations analysed have shown a different resonance response to the similar lock in excitation;

The onset of resonance appeared to be influenced by the Reynolds number (Two cylinders case) and influenced by the variation of the cylinders Pitch ratio

(Four cylinders case);

The frequency ratio could not be the only parameter instigating acoustic resonance, the flow condition (i.e. Turbulence and Vortices Convective Velocity) should be considered as well.

RE Pre-Coinc. Coinc.

Two Cylinders(L/D 2.5)

12000

36000

No resonance

Resonance

No resonance

Resonance

Four Cylinders(L/D 2.5)

12000

36000

No resonance

No resonance

Resonance

Resonance

Four Cylinders(L/D 3)

12000

36000

No resonance

No resonance

No resonance

Resonance

Array(L/D 2.5)

12000

36000

No resonance

No resonance

Resonance

Resonance

Array(L/D 3)

12000

36000

No resonance

No resonance

No resonance

No resonance