two scale modeling of superfluid turbulence tomasz lipniacki
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Two scale modeling of superfluid turbulence Tomasz Lipniacki. He II: -- normal component, -- superfluid component, -- superfluid vortices. Relative proportion of normal fluid and superfluid as a function of t emperature. Solutions of single vortex motion in LIA - PowerPoint PPT PresentationTRANSCRIPT
Two scale modeling of superfluid turbulence Tomasz Lipniacki
He II: -- normal component,-- superfluid component,-- superfluid vortices.
Relative proportion of normal fluid and superfluid as a function of temperature.
vortex line core diameter ~ 1Å
• Solutions of single vortex motion in LIA
• Fenomenological models of anisotropic turbulence
Localized Induction Approximation
nbssss cct ),(
binormal - ,normal-
,tangent-
, and where
bssns
ts
ssss
cc
dd
dtd
c - curvature- torsion
sss ),( tIdeal vortex
)()()(),( utttWt ss
Quasi static solutions:
Totally integrable, equivalent to non-linear Schrödinger equation.Countable (infinite) family of invariants:
)(),(),(),( uttutctc
dcccdcdcddL 222
222 '
4,,,,
Figure 1
:
Hasimoto soliton on ideal vortex (1972)
sss ),( t
20
20
0
000
4 ,2
,2/)(sech
cu
utccc
Kida 1981
sss ),( tGeneral solutions of in terms of elliptic integrals including shapes such as
circular vortex ringhelicoidal and toroidal filamentplane sinusoidal filamentEuler’s elasticaHasimoto soliton
Quasi static solutions for quantum vortices ?
Quantum vortex shrinks:
0
2dc
)(),( nbssss ct
Quasi-static solutionsLipniacki JFM2003
)0,()()(),( sWs ttt
)(),( ),(),( tctc
)0,()(0
2
sωVtnb dcc
0
2222
0
232
222
0
20
dccc
cccccc
dccccccc
nbbtnnt , , ccFrenet Seret equations
In the case of pure translation we get analytic solution
220
220
0
,
)tanh( ),sech(
cBcA
ABAcc
AAtcdRdRt /))ln(cosh(,)(qsin,)cos(q),( 220
s
where 220q ),sech( AcAR
Vtnb
0
2)( dcc
0
22
22
dccc
cc
2/
For 254.2/ Solution has no self-crossings
Quasi-static solution: pure rotation
Vortex asymptotically wraps over coneswith opening angle
0
3/13/2
00
3/1 )cot()(,23)(,||)(
KzK
Kr
for in cylindrical coordinates ),,( zr
/)cot(,2
where
00
303 K
)0('),0(),0('),0( 0000 cccc
Quasi-static solution: general case
For vortex wraps over paraboloids
0
3/203/2
00
3/1 3)(,23)(,||)(
KTzK
Kr
Results in general case
I. 4-parametric class of solutions, determined by initial condition
)0( ),0( ),0( ),0( cc
II. Each solution corresponds to a specific isometric transformation (t)related analytically to initial condition.
III. The asymptotic for
is related analytically via transformation (t) to initial condition.
Shape-preserving solutionsLipniacki, Phys. Fluids. 2003
ssss ),( t
, ,2 tt
Equation
is invariant under the transformation
If the initial condition is scale-free, than the solution is self-similar
t
ttt Ss )(),(
In general scale-free curve is a sum of two logarithmic spirals on coaxial cones; there is four parametric class of such curves.
Shape-preserving solutions
t
ttt Ss )(),(
. ,1 ,1t
lt
Ttt
Kt
c
solutions withdecreasing scale
222
2
22
0
2222
0
232
TlTldKTTKK
TKTKKK
KTK
KlKldKKKKTKTKTK
l
l
ssss ),( t
Analytic result: shape-preserving solution
In the case when transformation is a pure homothety we get analytic solution in implicit form:
KK
KKpKKKKdKl
K
K
T ,
)()/ln()(
0
2200
22
scale growing 1p 0T scale, decreasing 1 p
Shape preserving solution: general case
scale growing 1p scale decreasing 1pLogarithmic spirals on cones
Results in general case
I. 4-parametric class of solutions, determined by initial condition
)0( ),0( ),0(K ),0( TTK
II. Each solution corresponds to a specific similarity transformation
III. The asymptotics for
is given the initial condition of the original problem (with time).
l
Self-similar solution for vortex in Navier-Stokes?
ωωvω 2)( v
t
N-S is invariant to the same transformation
,/, ,2 rVVrrtt
The friction coefficient corresponds to
b
n
b
n
VVv
VV
Wing tip vortices
-- Euler equation for superfluid component
-- Navier-Stokes equation for normal component
Macroscopic description
Coupled by mutual friction force LVF nsns ~
where snns VVV and L is superfluid vortex line density.
Microscopic description of tangle superfluid vortices
Assuming that quantum tangle is “close” to statistical equilibriumderive equations for quantum line length density L and anisotropy parameters of the tangle.
Aim
Two cases
I – rapidly changing counterflow, but uniform in space
II – slowly changing counterflow, not uniform in space
Statistical equilibrium?
Model of quantum tangle“Particles” = segments of vortex line of length equal to characteristicradius of curvature. Particles are characterized by their tangent t, normal n, and binormal b vectors. Velocity of each particle is proportional to its binormal (+collective motion).Interactions of particles = reconnections.
STAdtdQVn /
Reconnections
1. Lines lost their identity: two line segments are replaced by two new line segments
2. Introduce new curvature to the system
3. Remove line-length
Motion of vortex line in the presence of counterflow
Evolution of its line-length
snns VVV
dl is
Evolution of line length density
where
I
average curvature
average curvature squared
Average binormal vector (normalized)
dL 1
Rapidly changing counterflow
Generation term: polarization of a tangle by counterflow
Degradation term: relaxation due to reconnections
Generation term: polarization of a tangle by counterflow
evolution of vortex ringof radius R and orientation in uniform counterflow
unit binormal
Statistical equilibrium assumption: distribution of f(b) is the most probable distribution giving right I i.e.
I
bIexp)( Cbf
Degradation term: relaxation due to reconnections
Reconnections produce curvature but not net binormal
Total curvature of the tangle
Reconnection frequency 2/2/51 Lcfr
Relaxation time: LfCT
rr
2
Finally
Lipniacki PRB 2001
Model prediction: there exists a critical frequency of counterflow above which tangle may not be sustained
max78.0 L max94.0 L
Quantum turbulence with high net macroscopic vorticity
nsns I VVI 0We assume now
Let denote anisotropy of the tangent
Vinen type equation for turbulence with net macroscopic vorticity
Vinen-Schwarz equation
Drift velocity LV
where
Mutual friction force
drag force
nsF
where
finally
Helium II dynamical equations
Lipniacki EJM 2006
Specific cases: stationary rotating turbulence
Pure heat driven turbulence
Pure rotation
„Sum”
Fast rotation Slow rotation
Plane Couette flow
V- normal velocityU-superfluid velocity
q- anisotropyl-line length density
“Hypothetical” vortex configuration
Summary and Conclusions
• Self-similar solutions of quantum vortex motion in LIA
• Dynamic of a quantum tangle in rapidly varying counterflow
• Dynamic of He II in quasi laminar case.
In small scale quantum turbulence, the anisotropy of the tangle in addition to its density L is key to analyzedynamic of both components.
bEVmt
i *0
22
2
Non-linear Schrodinger equation (Gross, Pitaevskii)
*2
)/(
mmA
mV
s
s
Madelung Transformation
superfluid velocity
superfluid density
)exp( iAFor
k
jk
jjss
ss xx
PVVt
V
2
20
2mVP s
kj
ssjk xxm
)ln(
4
2
2
2
Modified Euler equation
With
pressure
Quantum stress
Mixed turbulence
Kolgomorov spectrum for each fluid
3/5
3/2
)(k
CkE
- energy flux per unit mass
Two scale modeling of superfluid turbulence Tomasz Lipniacki
He II: -- normal component,-- superfluid component,-- superfluid vortices.
Relative proportion of normal fluid and superfluid as a function of temperature.
vortex line