Flow Induced Noise Generation By Partial Cavities · Flow Induced Noise Generation By Partial...

Flow Induced Noise Generation By Partial Cavities

Harish Ganesh, Juliana Wu and Steven L. CeccioUniversity of Michigan, Ann Arbor

USA2nd FLINOVIA Symposium

Penn-State University, State College, PA

Sponsor: Office of Naval ResearchProgram Manager: Dr. Ki-Han Kim

Motivation

Partial/Cloud cavitation: Significant source of noise, performance deterioration, and erosion

Mechanisms of transition, shedding, and their relationship to underlying flow

Void fraction flow field measurements for CFD code validations

Cavitation Dynamics on NACA0015 Hydrofoil

SuperCavitation

Sheet Cavitation

L/c = 2/3TYPE 2

SheddingL/c < 2/3

TYPE 1SheddingL/c > 2/3

Cavitation type depends upon attack angle ( ) and cavitation number ( )

( )

( )

NACA 0015 Cavitation Map Arndt et al. (2000)

Physical mechanisms and associated acoustics

Suction side surface pressure transducer data from Kjeldsen, Arndt & Effertz (2000) α = 7 degrees

Type 1

Type 2

Arndt et. al.- Cavitation Dynamics on NACA0015 Hydrofoil

What causes the abrupt change in dynamics?

Flow loop: Michigan 9” water tunnel with reduced area test sectionNACA0015 Hydrofoil: AR = 1.5 and Chord = 50 mm Flow conditions: U0= 8 m/s, po = 20 – 120 kPa, = 0.4 - 4, Dissolved Oxygen ~ 50% SatMeasurements: Inflow quantities, acoustic pressure using hydrophone (B-K)Cavitation visualization: High-speed videosVoid fraction measurements: Time resolved X-ray densitometry

Present Study

X-ray Densitometry21 cm square test section

Test section area reduced to achieve lower attenuation through water

Mäkiharju, S.A., “The Dynamics of Ventilated Partical Cavities Over a Wide Range of Reynolds Numbers and Quantitative 2D X-ray Densitometry for Multiphase Flow”, 2012, Ph.D. Thesis, University of Michigan, Ann Arbor, USA

Cavitation Observation

High speedvideos from Top and side synchronized with hydrophone

Top

Side

Filmed at 7500 fps and played back at 15 fps

Shock Collapse?

Shock Collapse

Reducing /2

Time

Incipient Cavitation

X-ray measurements of incipient cavity (0-50%)

Side

Filmed at 1000 fps and played back at 15 fps

Filmed at 7500 fps and played back at 15 fps

Type 2 Shedding: HS Video ( =10 )

High speedvideos from Top and side synchronized with hydrophone

Top

Side

Filmed at 7500 fps and played back at 15 fps

10°

5.8

Top

SideShedding is not spanwiseuniformLength is nearly constant

Re-entrant liquid flow induced shedding

Type 2 Shedding: X-ray ( =10 )X-ray measurements synchronized with hydrophone (0-100%)

Side

Filmed at 1000 fps and played back at 15 fps

10°, 5.8

Type 2 Shedding: Spectral Contentσ0 / 2 = 5.8 = 10 degrees

Morse-Wavelet-transform

Type 1 Shedding: HS Video ( =7 )

High speedvideos from Top and side synchronized with hydrophone

Top

Side

Filmed at 7500 fps and played back at 15 fps

7°, 4.2

Cycle BeginsCavity fillsMax length (L1)LE pinch offRoll-up and growth

Collapse and growth arrest (L2)

Top

Side Can lead to lift and drag changes

Length oscillates between cycles

Multi-modal

Type 1 Shedding: HS Video ( =10 )

High speedvideos from Top and side synchronized with hydrophone

Filmed at 7500 fps and played back at 15 fps

10°, 4.1Top

Side

Cycle BeginsGrowth L1

Growth L2

Rollup and growthCollapse and arrest

# of steps can change

Length oscillates thrice between cycles

Multi-modal

Type 1 Shedding: X-ray ( =7 )X-ray measurements synchronized with hydrophone (0-100%)

Side

7°, 4.2

Filmed at 1000 fps and played back at 15 fps

Growth arrest due to cloud collapse observed

Type 1 Shedding: X-ray ( =10 )X-ray measurements synchronized with hydrophone (0-100%)

Side

10°, 4.1

Filmed at 1000 fps and played back at 15 fps

Cavitation at trailing edge can have an effect on spectral content

σ0 / 2 = 4.1 = 10 degrees

Type 1 Shedding: Spectral Content

Morse-Wavelet-transform

Type 1 with Shocks: X-ray ( =10 )X-ray measurements synchronized with hydrophone (0-50%)

Side

Filmed at 1000 fps and played back at 15 fps

10°, 3.0

Bubbly Shock

Type 1 Shedding: Void FractionTop

Side

10°, 3.0

10°, 3.0 ′

′ ~0.3

~0.6

σ0 / 2 = 3.0 = 10 degrees

Type 1 with Shocks: Spectral Content

Morse-Wavelet-transform

Shedding Dynamics: Flow ProcessesX-ray measurements synchronized with hydrophone (0-50%)

Side

Filmed at 1000 fps and played back at 15 fps

10°, 3.0

Shedding Dynamics: Flow ProcessesWavelet transform of Acoustic Signal

Side

10°, 3.0

Shedding Dynamics: Flow ProcessesX-ray measurements (0-50%)

Side

Filmed at 1000 fps and played back at 15 fps

10°, 3.0

3 step with shocks

1 step with shocks

Does cloud collapse cause cavitation near trailing edge?

Top

Side

TS Organ Pipe Mode

StC= 6.7

StC = 0.42

StC = 0.14

StC = 0.28 = 7 degrees

Shedding Dynamics: Hydrophone ( =7 )

Top

Side

TS Organ Pipe Mode

StC= 6.7

StC = 0.48

StC = 0.12

StC = 0.24

= 10 degrees

Shedding Dynamics: Hydrophone ( =10 )

StC = 0.36

L1

Side

L2Averaged void fraction

L3L4

Shedding Dynamics: Void Fraction

StC = 0.42

StC = 0.14

StC = 0.28

= 7 degrees

L1

Side

L2Averaged void fraction

L3L4

Shedding Dynamics: Void Fraction

StC = 0.48

StC = 0.12

StC = 0.24

= 10 degrees

Conclusions

Side

Goal: To address the source of transition in cavity dynamics

1. Shed cloud collapse influences cavity growth and hence the cycle duration2. Cavity can attain different lengths in a given cycle depending upon the

nature of the shed cloud3. At lower cavitation numbers, propagating bubbly shocks are observed4. Secondary cavitation at the trailing edge is also observed

The line of demarcation between the processes can be thin, thus making the flow multi-modal.

Thanks for your attention

Cavitation

Occurs in liquids, when local pressure in close to vapor pressure

12

Partial cavitation: Occurs in separated flows attached to the object with stable cavity lengths

Cloud Cavitation: Characterized by cavity volumetric oscillations accompanied by shedding

Arndt et al.- Cavitation Dynamics on NACA0015 Hydrofoil

NACA 0015 Cavitation Map

Arndt et al. (2000)TYPE 2

SheddingL/c < 2/3

TYPE 1SheddingL/c > 2/3

SuperCavitation

Sheet Cavitation

L/c = 2/3

Type 1:St ~ 0.15 independent of σ

Type 2:Re-entrant jet induced sheddingFrequency is ~ linear with σCavity length based St ~ 0.3

f – frequencyC- Chord (also LC)V- Velocity

Cavitation type depends uponattack angle ( ) and cavitation number ( )

Ganesh et al. “Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities”, JFM, Vol. 802, 2016 Shedding cavities can exhibit both re-entrant and bubbly shock induced

sheddingWhat are the mechanisms involved in NACA0015 hydrofoil cavitation?

Propagating Bubbly Shocks

Cavity Length vs / 2a

7 degrees (■)10 degrees (●)Open symbols = X-ray Filled symbols =HSV

FSL theory for thin cavity

“negative compliance”

not observed

Cavity Length

Four Modes of Cavity Shedding Identified

1. Incipient shedding2. Re-entrant flow, rear pinch off

(Type 2)3. Multi-step shedding with

shocks (Type 1)4. Shock induced shedding

(Intermittent Type 1)

It is important to note that the flow is multi-modal

Cavity Behavior

Incipient Cavitation: Void FractionTop

Side

10°, 6.3

10°, 6.3 ′

~0.15

′ ~0.04

Type 2 Shedding: HS Video ( =7 )

High speedvideos from Top and side synchronized with hydrophone

Top

Side

Filmed at 7500 fps and played back at 15 fps

7°

5.8

Shedding is not spanwiseuniformLength is nearly constant

Re-entrant liquid flow induced shedding

Type 2 Shedding: X-ray ( =7 )X-ray measurements synchronized with hydrophone (0-100%)

Side

Filmed at 1000 fps and played back at 15 fps

7°, 5.8

Type 2 Shedding: Mean Void FractionTop

Side

7°, 5.8

10°, 5.8

~0.3

~0.3

Type 2 Shedding: RMS Void FractionTop

Side

7°, 5.8 ′

10°, 5.8 ′

′ ~0.10

′ ~0.08

Type 2 Shedding: Spectral Content

σ0 / 2 = 5.8

0.40

0.28

0.14

= 10 degrees

Type 1 Shedding: Mean Void FractionTop

Side

7°, 4.1

10°, 4.1

~0.4

~0.5

Type 1 Shedding: RMS Void FractionTop

Side

7°, 4.2 ′

10°, 4.1 ′

′ ~0.20

′ ~0.20

σ0 / 2 = 4.1

0.45

0.30

0.15

= 10 degrees

Type 1 Shedding: Spectral Content

σ0 / 2 = 3.0

0.48

0.24

0.12

= 10 degrees

Type 2 with Shocks: Spectral Content

0.36

L1

Shedding Dynamics: Flow Processes

σ0 / 2 = 3.54

0.42

0.28

0.14

= 10 degrees

Next Steps

Side

Goal: To address the source of transition in cavity dynamics

1. Measure unsteady pressures beneath cavity (Shock speed and Mach number)

2. High resolution void fraction measurements 3. Examine NACA and plano-convex hydrofoil?4. Measure unsteady boundary conditions

Larger Hydrofoil with 8.25 inch span.

Bubbly Shock Speeds: t-s Diagrams

Bubbly Shock Speeds

7 degrees (■)10 degrees (●)U

S/U

O

/ 2

Void Fractions Pre- and Post-shock

7 degrees (■)10 degrees (●)

/ 2 / 2

~3 kPa

US2 (p2 p1)

L

(12 )(11)(1 2 )

Top

Side

StC From the Void Fraction at L1

StC = 0.42

StC = 0.14

StC = 0.28

σ0 / 2 = 5.85σ0 / 2 = 4.83

σ0 / 2= 3.19

= 7 degrees

Top

Side

StC From the Void Fraction at L2

StC = 0.42

StC = 0.14

StC = 0.28

σ0 / 2 = 5.85σ0 / 2 = 4.83

σ0 / 2= 3.19

= 7 degrees

Top

Side

StC From the Void Fraction at L3

StC = 0.42

StC = 0.28

StC = 0.14

σ0 / 2 = 5.85 @ L2

σ0 / 2 = 4.83

σ0 / 2= 3.19

= 7 degrees

Top

Side

StC From the Void Fraction at L4

StC = 0.42

StC = 0.28

StC = 0.14

σ0 / 2 = 5.85 @ L2

σ0 / 2 = 4.83

σ0 / 2= 3.19

= 7 degrees

Top

Side

σ0 = 0.78

Acoustic Pressure From Hydrophone

Time Expanded

U0 = 8 m/s = 7 degrees