impact: void collapse and jet formation
TRANSCRIPT
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Impact:Void collapse and jet formation
Detlef LohseDevaraj van der MeerRaymond Bergmann
Gabriel Caballero • Phys. Rev. Lett. 93, 198003 (2004)• Nature 432, 689 (2004)• Phys. Rev. Lett. 96, 154505 (2006)• Phys. Rev. Lett. 99, 018001 (2007)
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Astroid impact on earth
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Craters …on the moon
Moltke Tycho central peak
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Craters
Mars explorer, January 2004
…on Mars
Arizona
…on earth
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Speculation on craterformation
Source:Jan Smit,Amsterdam,Dept. Geology
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What’s really going on?
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Downscaled experiments: Impact of steel ball on fine sand
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Problem: reproducibility
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Controlled experiments
Ball dropped on decompactified, very fine sand
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Ball at release point Maximum jet height
Jet height > Release height !
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Jet height vs release height
Siggi Thoroddsen, and Amy Shen, Phys. Fluids 13, 4 (2001):
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Impact of ball on decompactified sand
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Impact of ball on decompactified sand
3 events: • Impact creates splash
• A jet is formed
• Granular eruption
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Planetary impact:
V = 20 km/s d = 1m-150 km Y ≈ 1 kbar
HS experiments:
V = 0.1-1 km/s d = 0.2-2 cm Y ≈ 50 bar
Our experiments:
V = 0-10 m/s d = 1-5 cm Y < 20 Pa
Impact: planetary vs. lab
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How to look into the sand?
1. Analogy to (opaque) liquid2. “2D” experiments (falling cylinder)3. Discrete particle simulations
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1. Ball or drop impact on water
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Air entrainment through impact
11 cm
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Detlef LohsePhys. Today 56, No. 2, p. 36 (2003)
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Mechanism
1. Void formation2. Void collapse due to hydrostatic pressure3. Jet formations at singularity point4. Bubble formation
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Quantitative analysis of void collapse in liquid
Phys. Rev. Lett. 96, 154505 (2006)
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Pulled disk impacting on a liquid
linear motor
1m
0.5m
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vimpact = 1.0 m/s
Rdisk = 0.03 m
Pulled disk through a liquid
Fr = v2impact /Rdisk g = 3.4
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Void profiles as function of time
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Dimensional analysis
Relevant parameters: • disk radius Rdisk
• mean velocity V • gravity g
Irrelevant parameters: • surface tension (We)
• viscosity (Re)
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hs
Rdisk⇠ Fr1/2
ds ⇠ hs
Dimensional analysis
Closure time ts ~ Rdisk1/2 / g1/2
Depth at closure time hs ~ V ts
������
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Experimental & numerical scaling law
hs/R =1.0 Fr1/2
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Boundary Integral simulation
Potentialtheory
Fr = 7.8
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Boundary Integral simulation
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Comparison BI simulation with experiment
t=-48ms t=35ms t=57ms
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Comparison BI simulation with experiment
t=88ms t=115ms t=131ms
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Simplified potential flow analysis: 2D Rayleigh-Plesset equtation
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expnum
Rs/R
disk
t – tsexp
num
theory ~ (ts– t)1/2
log(
Rs/R
disk
)
log(t – ts)
sgdhhh)hh(h =++∞
2212 log
( ) 21/s
disk
ttRh
−∝
( ) 022
22 ==+ h
dtd)hh(h
Rayleigh-type singularity t --> ts
At the end, ln to –inf neglect the rest, simplifies, great agreement
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vimpact ≈ 0.5 m/s
Rdisk = 0.03 m
Slow impact:
Toroidal bubble!
Fr = 0.8
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vimpact ≈ 3 m/s
Rdisk = 0.01 m Fr = 100
Fast impact:
Surface seal
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What exactly happens at collapse?
singularity
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Focusing of energy à jets
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Vimpact = 1.0 m/sHdisk = 0.03 m
Very close to pinch-off
12800 fps
45 mm
Zoom in, to increase 12.8 fps, capillary waves, instrability
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Vimpact = 1.0 m/sHdisk = 0.03 m
Even closer to pinch-off
48000 fps
6 mm
1. Capillary instability?2. Instability through fast air-flow?
Instability clearer, 48 fps, air rushing out, Kelvin Helmholtz, frequency bubble cloud +/- 10 kHz, 1 mm bubble radius, pure lyinertial collapse of the neck
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Outwards airflow in the end
Air flow reverses!
Flow visualization with smoke particles
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Back to granular matter:
Rayleigh-Plesset type model for collapse of sand void
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Cavity collapse
Sand pressure
Initial conditions
.
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Rayleigh-type dynamics of cavity collapse
Euler equation in cylindrical coordinates
2D slice at depth z
Equation for 2D collapsing void
Continuity equation and boundary conditions r v(r) = R(t) R(t)
.
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Rayleigh model at high impact velocity
bubble formation !
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Experiments vs. hydrodynamic theory
T = -21ms T = 37ms T = 78ms
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T = 100ms T = 116ms T = 191ms
Experiments vs. hydrodynamic theory
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How to look into the sand?
1. Analogy to water2. “2D” experiments (falling cylinder)3. Discrete particle simulations
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2D experimental setup
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2D experiment: high impact velocity
Just as in water:1. void formation2. void collapse3. two jets (sheets in 2D)4. bubble formation
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3. Discrete particle simulations
• soft sphere code• N = 1000000• ds = 0.5 mm• db = 15 mm• quasi 2D (8 grains thick)• pre-fluidized
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Discrete particle simulation
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3D discrete particle simulation
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Does sandbed support weight?
prepared sand
D. Lohse, R.Rauhe, D. van der Meer, R. Bergmann, Nature 432, 689 (2004)
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Sandbed does not support weight
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“Dry quick sand”
Packing density only 41% !No!
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Myth from Lawrence of Arabia…
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final depth ~ mass
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Jet height vs mass: threshold behavior
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A force model to explain the observations
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Model: Coulomb friction
Coulomb friction
Force balance
Final depth
Solution
P
?
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Depth vs time
experiments
+ model
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Large-Fr impact on sand
Surface seal, just as in
water
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Oblique impact
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Oblique impact
45°v = 3.7m/s
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Oblique impact
45°v = 3.7m/s
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Conclusions
Hydrodynamic description seems to work at least semiquantitatively (for soft sand)
Series of events in both liquid and sand:1. void formation2. void collapse3. two jets 4. bubble formation
D. Lohse et al., Phys. Rev. Lett. 93, 198003 (2004)
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Granular void collapse analyzed by…
•Experiment•Analogy to liquid•Boundary Integral simulations•Dimensional analysis•Discrete particle simulations•Simple continuum Rayleigh type model
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Breakdown of hydrodynamic description
… at large enough compactification of sand when strong enough force chains will have built up.
But how?
- sudden breakdown?
- continuous breakdown?
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Is this the full story?
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Large-Fr impact on sand
Surface seal
Fr=100
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Analyse effect of ambient air
Air
Pneumatic release mechanism
Vacuumpump
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Effect of ambient pressure on…
• … splash• … jet• …penetration depth
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Splash depends on
ambient pressure
Ejectie 9 mbar calibratie
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Jet much less pronounced
under reduced pressure!
1000 mbar25 mbarsee also Royer et al.,Nature Phys. 1, 164 (2005)
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D = 2.5cm ; Fr = 32 ; t = 159ms
25 50 100 150 200 300 400 600 800 1000
Pressure (mbar)
Effect of ambient air pressure
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Jet height vs ambient pressure: saturation effects: two regimes
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Ball trajectory in sand
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Final depth of intruder vs p
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Final depth described by force balance model
1000 mbar
400 mbar
25 mbar
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Coulomb friction coefficient depends on ambient pressure
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Final depth correlated with jet heightTwo regimes:
Regime 1 (low p):Jet height in-creases linearly with final depth
Regime 2 (high p):Jet height inde-pendent of final depth
How to explain thesetwo regimes ?
High pressure (1 bar)
Low pressure (25 mbar)
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Closure time
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Closure time: nearly constant
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Trajectories: when closure?Impact velocity
Closure time1 bar
400 mbar
100 mbar
25 mbar
High pressures:Identical trajectories until closure time
Low pressures:Trajectories deviate substantially
→ same jet height (regime 2)
→ final depth determines jet height (regime 1)
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Final question:
What causes the sphere to penetrate less at lower pressures (i.e., the friction reduction)?
The sand bed is fluidized by the air flow around the impacting ball (Resand grains ≈ 5)!
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Impact of ball on decompactified sand
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Height of sand bed vs time at impact
Ambient air leads to
expansion of granular bed at impact: extra fluidization
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Conclusions II
- Ambient air pressure strongly influences the penetration depth of the ball and thus the jet height
- Ambient air pressure hardly affects the collapse of the cavity
- Autofluidization effect
Gabriel Caballero et al., Phys. Rev. Lett. 99, 018001 (2007)
- Two regimes: high p: trajectories unchanged up to closurelow p: trajectories deviate: jet height <-> depth
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Collaborators: • Raymond Bergmann• Gabriel Caballero• Martin van der Hoef• Hans Kuipers• Devaraj van der Meer• Rene Mikkelsen• Andrea Prosperetti• Remco Rauhe• Marijn Sandtke• Mark Stijnman• Michel Versluis• Ko van der Weele• Christiaan ZeilstraFinancial support from FOM
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Scaling for position of singularity
hs/R =0.69 Fr1/3
ttouch(z)=tget(z) + tcollapse(z)
Minimize:
hs(Fr) ~ Fr 1/3
Different from scaling law in water!
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Different scaling laws! water
hs/R =1.0 Fr1/2
sand
hs/R =0.69 Fr1/3
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High velocity impacts
v = 2.0 m/s
v = 3.6 m/s
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Oblique impact on water
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Rayleigh model: low impact velocity
Collapse without air entrainment
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Disk depth vs. Fr1/2
I’M NOT AT ALL SURE ABOUT THE EQUALITY. CHECK MCMAHON & GLASHEEN FOR THEIR DEFINITION OF <v>!!!!!!
Again refer to the big feet of the lizard.
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Rdisk
Dimensional analysis
Closure time ts ~ Rdisk1/2 / g1/2
Depth at closure time hs ~ V ts
ds ~ hs
I’M NOT AT ALL SURE ABOUT THE EQUALITY. CHECK MCMAHON & GLASHEEN FOR THEIR DEFINITION OF <v>!!!!!!
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Experimental & numerical scaling law
hs/R =1.0 Fr1/2
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Air entrainment by shaking fluid: The Faraday experiment
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Parametric instability
115 121
118 125
128
131
134
137
140
143
h1 h2 h3 h4 h5
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Void profile just
before singularity
Entrained air
How much air is entrained?
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Air entrainment
Slope 0.80
Strong tools to look at such questions as air entrainment
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Profile of void just before singularity
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Differences liquid vs soft sand
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The sound of impact
Minnaert formula:
fr ≈ 175 Hz
R0 ≈ 2.0 cm
t = 0.13 s
t = 0.14 s
t = 0.15 s
hydrophone
nb gamma = adiabatic exponent
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Preparation of sand in our experiments
• Grain size = 40µm• Let air bubble through it• Slowly turn off air stream• Resulting packing density: only 41%!
à Model system for sedimented fine sand in the desert after a sand storm
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Radius of curvature
Include correction, but nevertheless at low froude there’s a significant deviation. The observed anomalous powerlaw of the neck radius must reflect itself in the in time evolution of the void. Define R, R exp increasing with froude
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Dimensional numbers at singularity
I’M NOT AT ALL SURE ABOUT THE EQUALITY. CHECK MCMAHON & GLASHEEN FOR THEIR DEFINITION OF <v>!!!!!!
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Intrinsic scales at singularity (for water)
Below this,
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Intrinsic scales at singularity (for glycerol)
Below this,
I’M NOT AT ALL SURE ABOUT THE EQUALITY. CHECK MCMAHON & GLASHEEN FOR THEIR DEFINITION OF <v>!!!!!!
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Complete 2D-Rayleigh equation
Viscous ~ capillary: Viscous ~ inertia: Inertia ~ capillary: Purely inertially:
I’M NOT AT ALL SURE ABOUT THE EQUALITY. CHECK MCMAHON & GLASHEEN FOR THEIR DEFINITION OF <v>!!!!!!