measured seafloor penetrations of a large, free-falling cylinder and comparison to model predictions...
TRANSCRIPT
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Measured Seafloor Penetrations of a Large, Free-Falling Cylinder and Comparison to
Model Predictions
by
Philip Valent, Grant Bower, Stephen Theophanis, Charles King, Michael Richardson, all Naval Research Laboratory,
Andrei Abelev, University of Southern Mississippi, and
John Bradley, OMNI Technologies, Inc.
30 October 2002
OCEANS 2002 ConferenceBiloxi, Mississippi
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Project Objective: To predict height, projected area, and volume of bottom mines protruding above mudline
Measured Seafloor Penetrations of a Large, Free-Falling Cylinder and Comparison to
Model Predictions
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Existing Predictive Model…
• Solution is deterministic
• Neglects boundary layer effects
• 2-D, limits motion to vertical plane– disproven in at-sea experiments, September and
November 2000
• Overpredicts penetration
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3-D Motion in Water Column Illustrated in 1/3rd Scale Model Tests at NSWC Carderock Division
Double click on the above images to start the video clip
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To gather data describing impact burial event in muds we needed:
• Sensors and data storage internal to model mine shell– To contain fiber optic gyro needed minimum
shell outside diameter of 0.27 m
• Test site in lake or ocean
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Designed and FabricatedFull-Size Instrumented Cylinder
• Diameter = 0.53 m
• Length = 2.40 m (L/D = 4.5)
• Weight in air = 10 kN (2,400 lbs)
• ‘Weight’ in seawater = 4.9 kN
• CM – CV = 0.104 m
• Three interchangeable noses: hemispherical, blunt, chamfered
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Cylinder Instrumentation
• Fiber Optic Gyro
• Accelerometers– 2.5 g– 4 g– 10 g
• Magnetometer
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Experiment Off Cocodrie, Louisiana,
January 2002• Water depth: 15 m• Distance above water surface: +1.0 or -0.5 m• Pitch: horizontal and 45o nose down
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Processed Data from Cocodrie Depict
3-D, Chaotic TrajectoriesDrop #OrientationHeight (m)
1level-0.5
2level-0.5
345o
-0.5
445o
+1.0
545o
+0.5
6level+1.0
7level+0.2
8level-0.5
945o
-0.5
1045o
-0.5
1145o
+0.2
January 8, 2002
January 9, 2002Drop #OrientationHeight (m)
Water Depth = 15.5 meters
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Processed Data from Two Cylinder Drops, Cocodrie, LA
Drop #1Released 0.5 mbelow water surface
Drop #5Released 0.5 mabove water surface
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Distribution of vertical speed of cylinder approaching mudline, Cocodrie, LA
0
1
2
3
4
5
6
7
8
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
Vertical velocity at mudline, m/s
Fre
qu
en
cy
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0
1
2
3
4
5
6
10 20 30 40 50 60 70 80 90
Pitch at mudline, deg
Fre
qu
en
cy
Distribution of pitch, nose down, of cylinder approaching in mud, Cocodrie, LA
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0
1
2
3
4
5
6
10 20 30 40 50 60 70 80 90
Pitch, deg
Fre
qu
en
cy
Distribution of pitch, nose down, of cylinder embedded in mud, Cocodrie, LA
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0
0.2
0.4
0.6
0.8
1
1.2
1 2 3 4 5 6 7 8 9 10 11
Test Number
He
igh
t a
bo
ve
mu
dlin
e (
m)
Measured
Predicted
Diver Measured and IMPACT28 Predicted Height Above Mudline,
Cocodrie Experiment
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Accelerometer Measured and IMPACT28 Predicted Pitch Nose Down in Mud, Cocodrie Experiment
0
5
10
15
20
25
30
35
40
1 2 3 4 5 6 7 8 9 10 11
Test Number
Pit
ch
No
se
Do
wn
(d
eg
)
Measured
Predicted
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Prediction of Impact Burial
• A predictive model improvement in progress
• Cylinders in free-fall in water column display complex 3-D behavior
• For one site, sediment impact model:– underpredicts height proud– overpredicts change in pitch angle
Summary
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Vertical Speed at Mudline Used to Initiate IMPACT28
0
1
2
3
4
5
6
1 2 3 4 5 6 7 8 9 10 11
Test Number
Ve
rtic
al S
pe
ed
(m
/se
c)