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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