the tale of two tangles: dynamics of "kolmogorov" and "vinen" turbulences in 4...
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Euromech 491, Exeter 2007. The Tale of Two Tangles: Dynamics of "Kolmogorov" and "Vinen" turbulences in 4 He near T =0. Paul Walmsley, Steve May, Alexander Levchenko, Andrei Golov (thanks: Henry Hall, JOE VINEN). Different types of tangles and their dissipation at T =0 - PowerPoint PPT PresentationTRANSCRIPT
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The Tale of Two Tangles: Dynamics of "Kolmogorov" and "Vinen"
turbulences in 4He near T=0
Paul Walmsley, Steve May, Alexander Levchenko, Andrei Golov
(thanks: Henry Hall, JOE VINEN)
Euromech 491, Exeter 2007
1. Different types of tangles and their dissipation at T=0 2. Production of random and structured tangles3. Detection of turbulence by ballistic vortex rings4. Results for both types of tangles5. Conclusions
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In classical turbulence dissipation is via vorticity and viscosity :
In superfluid, turbulence is made of quantized vortices and their tangles of density L
Superfluid 4He has zero molecular viscosity, = 0
Conversion of flow energy into heat is mediated by quantized vortices:(’ – “effective kinematic viscosity”)
dE/dt = -’(L)2
dE/dt = - 2
Introduction
Circulation quantum, = h/m = 10-3 cm2s-1
Core a0 ~ 0.1 nm
L = 10 – 105 cm-2
l = L-1/2 = 0.03 – 3 mm
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Random (“Vinen”) vs. structured (“Kolmogorov”) state
No correlations between vortices, hence only one length scale, l = L-1/2
All energy in vortex line tension
Vinen’s equation: dL/dt = - L2
Free decay: E(t) ~ t -1
L(t) = 1.2 ’-1t -1
Expectations: ’ ~
Eddies of different sizes >> l
Most of energy in the largest eddy
If the largest eddy saturates at d and decays within turnover time:
Free decay: E(t) ~ t -2
L(t) = (1.5/)d ’-1/2 t -3/2
Expectations: T > 1K, ’ ~ T = 0, ’ - ?
Dissipation: -dE/dt = ’(L)2
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Simulations by Tsubota, Araki, Nemirovskii (PRB 2000)
T = 1.6 K T = 0
dphononemission
kl = L-1/2Quasi-classical Quantum
Kolmogorov Kelvin waves(Svistunov PRB 1995)
0.03 mm - 3 mm4.5 cm ~ 40 nm
T = 1.6 K
Bottleneck? (L’vov, Nazarenko, Rudenko PRB 2007)
T = 0
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Possible scenarios in 4He at T=0
The nature of the transfer of energy from Kolmogorov to Kelvin cascade is debated:
- Accumulation of energy/vorticity at scale ~ l (L’vov, Nazarenko, Rudenko, PRB2007): ’(Vinen) / ’(Kolmogorov) ~ (ln(l/a))5 ~ 106
- Reconnections should ease the problem (Kozik-Svistunov, cond-mat 2007):’(Vinen) / ’(Kolmogorov) ~ ln(l/a) ~ 15
Kolmogorov cascade Kelvin-wave cascade
’(Vinen) ~ ’(Kolmogorov) - ?
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From Kolmogorov to Kelvin-wave cascade (Kozik & Svistunov, 2007)
SII ~ vv crossover to QT
reconnections of vortex bundles
reconnections between neighbors
in the bundle
self – reconnections
(vortex ring generation)
purely non-linear cascade of Kelvin waves
(no reconnections)
length scale
phonon radiation
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Random (“Vinen”) vs. structured (“Kolmogorov”) state
No correlations between vortices, hence only one length scale, l = L-1/2
All energy in vortex line tension
Vinen’s equation: dL/dt = - L2
Free decay: E(t) ~ t -1
L(t) = 1.2 ’-1t -1
Expectations: ’ ~
Eddies of different sizes >> l
Most of energy in the largest eddy
If the largest eddy saturates at d and decays within turnover time:
Free decay: E(t) ~ t -2
L(t) = (1.5/)d ’-1/2 t -3/2
Expectations: T > 1K, ’ ~ T = 0, ’ - ?
Dissipation: -dE/dt = ’(L)2
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Available information
Towed grid in 4He(Oregon):Vibrating grid in 3He-B
(Lancaster): 0.0 0.5 1.0 1.5 2.00.01
0.1
1
n/
nn/
Towed Grid(Stalp et al. 2002)
(t-3/2)Vinen & Niemela
(t-1)
Tsubota et al. (t-1)(2000-2003)
' /
T (K)
Schwarz (counterflow turb.)
3He-B vibrating grid(Bradley et al., 2006):
?
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Experimental challenges:
- How to produce turbulence at T < 1K?
- How to detect it?
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Vortex rings are nucleated by such ions at T < 1 K; electron stays trapped by vortex (binding energy ~ 50 K)
Ring dynamics: E ~ R , v ~ 1/R
Rings as injected: E0 = 30 eV, R0 = 0.8 m, v = 11 cm/s
In liquid helium, an injected electron creates a bubble of radius ~ 20 A
Charged rings have large capture diameter ~ 1m (c.f. typical inter-vortex distance of ~ 30 - 3000 m)
E
Ions in helium
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Turbulence detection: We developed techniques to measure L by scattering a beam of probe particles:
1. Free ions (T > 0.8 K), trapping diameter ~ 0.1 m
2. Charged quantized vortex rings (T < 0.8 K), trapping diameter ~1 m
Rotating cryostat was used to calibrate trapping diameter vs. electric field and temperature
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Am
plit
ude
of P
ulse
(p
A)
(rad/s)
T=140 mK, P=0Pulse length=0.1 sDiverging field:
10 V/cm 20 V/cmI=I
0exp(-2D/)
Cross-sections :10 V/cm: 2.14 ± 0.17 m20 V/cm: 3.45 ± 0.21 m
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Turbulence production:
We developed techniques to produce either structured or random tangles:
1. Impulsive spin-down to rest (works at any temperatures)Energy injected at the largest scale (structured tangle)
2. Jet of free ions in stationary helium (T > 0.8 K)Energy injected at the largest scale (structured tangle)
3. Beam of small vortex rings in stationary helium (T< 0.8 K)Energy mainly injected on scale << l (random tangle)
d phononsk
l = L-1/2Quasi-classical Quantum
Kolmogorov Kelvin waves0.03 - 3 mm4.5 cm ~ 40 nm
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4.5 cm
Experimental Cell
We can inject rings from the side
We can also inject rings from the bottom
We can create an array of vortices by rotating the
cryostat
The experiment is a cube with sides of length 4.5 cm containing 4He (P = 0.1 bar).
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1. Random Tangles Produced by Charged Vortex Rings
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.0
0.2
0.4
0.6
0.8
1.0
Cu
rre
nt (
pA
)
Time (s)
Pulse Spacing (s): 5 10 16 25 50 150 500
4.5 cm
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1 10 100 1000100
101
102
103
V
ortex
Lin
e D
ensi
ty, L (cm
-2)
Time between pulses, t (s)
Vertical Pulses:20 V/cm field
0.1 s, -160 V 0.3 s, -160 V 0.3 s, -125 V
10 V/cm field 0.3 s, -160 V
L = 2(t)-1
1 10 100 1000100
101
102
103
Horizontal Pulses:20 V/cm field
0.1 s, -400 V 0.3 s, -400 V
V
ortex
Lin
e D
ensi
ty, L (cm
-2)
Time between pulses, t (s)
Vertical Pulses:20 V/cm field
0.1 s, -160 V 0.3 s, -160 V 0.3 s, -125 V
10 V/cm field 0.3 s, -160 V
’ver = 0.17
’hor = 0.13
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0 10 20 30 40
0.0
0.2
0.4
0.6
0.8
Cur
rent
(pA
)
Time after stopping main injection, t (s)
Tangle decay
We probe the decay after a long injection by sending a short pulse a time, t, after stopping injection.
Signal applied to injector:
50 s initial injection t
Probe pulse
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1 10 100 1000101
102
103
Vor
tex
Line
Den
sity
(cm
-2)
t (s)
Decay of tangle generated by short pulses
Decay of tangle generated by long pulse
t-1
Tangle decay
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Tangle Growth & Decay in Centre of Cell
1 10102
103
Vor
tex
Line
Den
sity
, L
(cm
-2)
Time after injection from left, t (s)
L=(0.08 t)-1
We can probe the growth of the tangle by first sending a pulse from the left tip and then use a pulse from the bottom tip to probe the vortex line density in the centre of the cell.
The tangle grows and fills the whole cell. L~1/t, agrees well with our other measurements.
1 sinjection
0.3 s probe pulse a time,t, after injection from left
10 V/cmfield
Maximum line density occurs at about 4 seconds
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Tangle decay: varying temperature
1 10 100 1000101
102
103
Vortex
Lin
e D
ensi
ty (cm
-2)
Time between pulses (s)
T (K) 0.61 0.50 0.30 0.15
Bottom tip to top collector
For T = 0.08 K – 0.5 K, ’ = (0.15 ± 0.03)
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Stopping rotation2. Structured tangles
Impulsive stopping rotation:
(from a vortex array to L=0 through 3D turbulence)
~ 1 rad/s = 0
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Horizontal vs. vertical direction
100 101 102 103 104101
102
103
104 = 1.5 rad/s0.5 rad/s
0.15 rad/s
Lt,
cm-2
t, s
t -3/2
0.05 rad/s
T = 0.15 K
100 101 102 103 104101
102
103
104
1.5 rad/s
= 0.5 rad/s
0.15 rad/s
La,
cm
-2
t, s
t -3/2
0.05 rad/sT = 0.15 K
Horizontal Vertical
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Scaling with Angular Velocity
10-1 100 101 102 103 104
101
102
103
104
105
axial
horizontal
L-3
/2, c
m-2s3
/2
t
0.05 rad/s 0.15 rad/s 0.5 rad/s 1.5 rad/s1.5 rad/s1.5 rad/s 1.5 rad/s
5x106(t)-3/2
one initial revolution
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Low vs. High Temperature: horizontal
10-1 100 101 102 103 104
101
102
103
104
105
5x105(t)-3/2
L-3
/2, c
m-2s3
/2
t
T = 0.15 K: 0.05 rad/s 0.15 rad/s 0.5 rad/s 1.5 rad/s1.5 rad/s1.5 rad/s 1.5 rad/s
T = 1.6 K: 0.5 rad/s 1.5 rad/s
5x106(t)-3/2
Horizontal measurement
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High Temperatures: spin-down vs. ion-injection
1 10 100102
103
104
105
T = 1.60 KHorizontal
1.5 rad/s 0.5 rad/s Ion-induced, 20 V/cm towed grid
Vortex
Lin
e D
ensi
ty (cm
-2)
Time (s)
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Low vs. High Temperatures
1 10 100 1000 10000101
102
103
104
105
Ions and Spin-down, 1.60 K' = 0.2
Spin-down, 0.15 K' = 0.003
Vo
rte
x L
ine
De
nsi
ty (
cm-2)
Time (s)
Towed grid (Oregon), 1.6K
Ions, 0.15 K' = 0.13
Spin-down from 1.5 rad/svs. Ion-induced tangles
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Low vs. High Temperature
“Kolomogorov”(structured)tangle
“Vinen”(random)tangle
0.0 0.5 1.0 1.5 2.010-3
10-2
10-1
100 '
/
T (K)
Transverse Axial
Towed grid Theory
n
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Summary• We have used charged vortex rings to probe turbulence in superfluid 4He in the T=0 limit.
• The decay of a tangle produced by either injected current or impulsive spin-down have been studied.
• Random tangles decay as L = t-1. This is consistent with Vinen’s equation with the effective kinematic viscosity of 0.15 .
• Structured tangles decay as L ~ t-3/2 which is consistent with a developed Kolmogorov cascade saturated at cell size. The effective kinematic viscosity is 0.003 .
• ‘(random) / ’(Kolmogorov) ~ 50. Bottleneck between the two cascades? However, not as huge an effect as if reconnections were suppressed.
• Techniques of great potential. More detailed studies to follow.