Final report 2015
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Transcript of Final report 2015
A preliminary study of natural supercavitation in SAFL water tunnel
Siyao Shao, Ashish Karn, Roger Arndt, Jiarong Hong
Catalogues• Objectives and model descriptions
Project objectives (Slide 3 ) Model types and water tunnel description (Slide 4) Nomenclatures and acronym list (Slide 5)
• Experiments with Backward Facing Model (BFM) Experimental conditions (Slide 6) Sample images (Slide 7) Cavity’s evolution under vacuuming (Slide 8) Choking phenomena (Slide 9) Supercavity’s closure type (Slide 10-11) A comparison across different cavitator diameters (Slide 12-15)
• Experiments with Forward Facing Model (FFM) Experimental conditions (Slide 16) Sample images (Slide 17) Choking phenomena (Slide 18)
• A comparison across ventilated and natural supercavitation Dimension and shape (Slide 19-21)
• Future directions (Slide 22)
Project objectives
• Measurement of the minimum cavitation number with different models;• Obtaining of natural supercavity’s geometry digital images;• Observation of supercavity’s evolution under choking;• Effects of blockage on the natural supercavities; • A comparison across ventilated and natural supercavities’ geometries;• A preliminary study of natural supercavities’ closure.
Model types and water tunnel description
SAFL high-speed water tunnel
Test section geometry:L: 1.2m, W×H: 190mm×190mm
Maximum velocity attainable:Vinf=20m/s.
Forward facing model Backward facing model
Nomenclature and acronym list• Nomenclature • Acronym list
Cavitation number:
Froude number:
Blockage ratio:
D: test section diameterdc: cavitator diameterP(Po): test section pressurePv: vaporous pressure (natural supercavitation)Pc: cavity pressure (ventilated supercavitation)V: flow speed
FFM: Forward facing modelBFM: Backward facing modelTV: Twin vortexQV: Quad vortexRJ: Re-entrant jetFC: Foamy cavity
Pressure difference:or
Experimental conditions with BFM
SAFL
Test section 190x190D (mm) 214.4dc (mm) 20.0 30.0 40.0
D/dc 10.72 7.15 5.36
Velocity range, V (m/s) 9.1-11.4 8.4-10.8 7.5-10.5
Fr range 20.6-25.6 15.5-20.0 11.9-16.8
Lowest stable operation pressure, P (kPa) 15
σmin, theory 0.1996 0.3174 0.4556σv, attainable 0.2490 0.3535 0.5050
*: maximum operational velocity was set as 12.0m/s for the stable operating and safety concern, excluded cases operated under unstable conditions (P<15kPa);**: referred from Ellison 2011’s work;***: measured and calculated from a linear regression method, in the experiment operation, this value could bounce around this measured value.
Sample images
dc=20mm
dc=30mm
dc=40mm
σv=0.25
σv=0.35
σv=0.50
K-H Instability has strong effect on 20mm’s experiment (high flow speed)
Cavity’s evolution under vacuuming
• Vacuuming leads to the development of natural supercavity;• Rapid transition from foamy cavity to supercavity;• Reference case: 40mm cavitator, BFM.
Pres
sure
redu
ctio
n
Dense bubbly flow
Bubbly flow with spiraling bubble jets
Foamy cavity
Supercavity
ChokingOnce choking cavitation number is attained, further vacuuming does not influence it. Instead, there is a velocity drop to satisfy the same choking cavitation number*.
* the Pressure difference-dynamic pressure line provides the method of the measurement of choking/minimum cavitation number (the slope of this regression line corresponding to the choking cavitation number).
Pressure
reducti
on after
choking (th
e
slope o
f the l
inear
regres
sion lin
e is th
e
minimum/ch
oking cavita
tion nu
mber,
σ choking=0.50
50)
Cavity closures’ evolution: before choking (similar V)Stage 1: Dense bubbly flow
Stage 2: Bubble jet
Stage 3 (1): Foamy cavity & bubble jet
Stage 3 (2): Re-entrant jet & bubble jet
Stage 3 (3): Vortices (Quad vortex)
Stage 3 (4): Vortices (Twin vortex)
Stage 4: Twin vortex
σv=1.2607, P=43.9kPa,V=8.1m/s
σv=0.9116, P=32.5kPa,V=8.1m/s
σv=0.5173, P=19.4kPa,V=8.0m/s
σv=0.5327, P=19.0kPa,V=7.8m/s
σv=0.5070, P=18.5kPa,V=7.9m/s
σv=0.5067, P=18.6kPaV=7.9m/s
σv=0.4979, P=17.6kPaV=7.7m/s
*: velocity descending is due to blockage effect;**: 40mm cavitator, BFM
A evolution of natural supercavity closure (Under choking)TV
TV
TV
QV
QV
RJ & Torodial bubbles
RJ & Torodial bubbles
σv=0.4979, V=7.7m/s
σv=0.5041, V=8.4m/s
σv=0.4976, V=8.7m/s
σv=0.5029, V=9.3m/s
σv=0.5004, V=9.6m/s
σv=0.5047, V=10.1m/s
σv=0.4990, V=10.4m/s*: 40mm cavitator, BFM and minimum cavitation number measured is 0.5050
Comparison of obtained choking cavitation numbers with Brennen’s simulations
Cavity geometry analysis
• Only supercavity data (under choking condition) presented;• Unlike Brennen’s choking definition (infinite cavity geometry), choking in the
experiment corresponding to the flow velocity start to drop continuously to fit the ‘choking lines’ of each cavitator size;
• rc/(L/2) shows a better agreement with Brennen’s result compared to ventilated cases;
• rc/(L/2) only reflects the front part (before maximum diameter) of the cavity’s geometry.
The change in supercavity shape after choking
For all the blockage ratios considered, supercavity size (before maximum diameter) does not change with Fr, once choking cavitation number has been attained.
• Experiments were operated under choking condition• With flow velocities’ (test section pressure) increasing, a evolution of cavity
closure occurred from TV to (QV) to RJ*.
* A similar trend was observed in the ventilated supercavitation experiment (Ashish 2015) (with the increasing of ΔP, a evolution of cavity closure occurred from TV to RJ).
σv,choking=0.5050, 40mm BFM
σv,choking=0.3535, 30mm BFM
σv,choking=0.2490, 20mm BFM
Experimental conditions with FFM
SAFL
Test section 190x190D (mm) 214.4dc (mm) 30.0 40.0
D/dc 7.15 5.36
Velocity range, V (m/s)* 8.7-10.7 7.7-10.7
Fr range 15.9-19.7 12.3-17.1
Lowest stable operation pressure, P (kPa) 15
σmin, theory** 0.3174 0.4556
σv, attainable*** 0.3205 0.4594
*: maximum operational velocity was set as 12.0m/s for the stable operating and safety concern, excluded cases operated under unstable conditions (P<15kPa);**: referred from Ellison 2011’s work;***: measured and calculated from a linear regression method, in the experiment operation, this value could bounce around this measured value.
Sample images*,**
dc=40mm, σv=0.47
dc=30mm, σv=0.31
*: smoother cavity surface observed compared to BFM experiments. (no wake signature etc.);**: no clear supercavity observed with 20mm cavitator.
100mm
100mm
ChokingOnce choking cavitation number is attained, further vacuuming does not influence it. Instead, there is a velocity drop to satisfy the same choking cavitation number* , **,***.
*: the theoretical analysis of choking cavitation number was provide by Ellison 2010 (MS thesis);**: the Pressure difference-dynamic pressure line provides the method of the measurement of choking/minimum cavitation number;***: lower choking cavitation number measured compared to the BFM experiments.
Pressure
reducti
on afte
r choking (th
e
slope o
f the l
inear re
gressi
on line i
s the
minimum/ch
oking cavita
tion n
umber)
A comparison of cavity geometry of natural supercavity to ventilated supercavity
• Only supercavity (under choking) data presented;• Both FFM and BFM shows a good agreement with Brennen’s result;• A better agreement of the cavity geometry to the Brennen’s result was
observed compared to ventilated supercavity (negligible Fr effect).
Comparison of natural and ventilated supercavitiesBFM 30mm cavitator
The cavity size and shape can be different between natural and ventilated cases even at same cavitation numbers (actual cavitation number for natural case could be lower than 0.43 according to geometry data, and a temperature drop may exists across the cavity
surface, this may leads to the cavitation number’s discrepancy).
Natural SupercavityV = 9.0m/s,
rc/(L/2)=0.15
rc/(L/2)=0.09
Ventilated SupercavityV = 4.0m/s
σc=0.40
σv=0.40
Comparison of natural and ventilated supercavitiesBFM 30mm cavitator
The cavity size and shape can be similar between natural and ventilated cases even at different cavitation numbers (A water vapor pressure drop may occurs across the surface of
natural supercavity).
Natural SupercavityV = 8.4m/s,
rc/(L/2)=0.06, Dmax=71.06mm
rc/(L/2)=0.06Dmax=77.94mm
Ventilated SupercavityV = 7.0m/s
σc=0.35
σv=0.32
Future directions of natural supercavitation study in SAFL
• Test different models to study the structure influence on the natural supercavity (choking cavitation number, cavity geometry, evolution of natural supercavity);
• Develop a stable FFM for the natural supercavitation experiments;• Develop a new shape factor which could reflect the cavity’s dimension
properly;• A detailed study into the temperature drop across the surface of natural
supercavity to make a correction of cavitation number.