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Initial Hydrodynamic Results from the Princeton MRI Experiment
M.J. Burin1,3, H. Ji2,3, J. Goodman1,3, E. Schartman2, W. Liu2,3
1. Princeton University Department of Astrophysical Sciences2. Princeton Plasma Physics Laboratory3. Center for Magnetic Self-Organization (CMSO)
CMSO general meeting Oct. 5-7 2005
Outline
Motivation and background
Brief description of apparatus
Result I. obtaining a Keplerian-like* flow
Result II. on subcritical hydrodynamic shear instability
Progress report on Gallium use (towards MRI)
Motivation: The Cause of Turbulence in Astrophysical Accretion Disks: What Instability(s) are Relevant?
•If sufficiently ionized and magnetized MRI probably responsible, but perhaps not solely
•If not? (e.g. some protostellar disks). The presence and relevance of various hydrodynamic instabilities is uncertain; subcritical shear instabilities are possible, but simulations and experiments so far appear to disagree.
•Hope: that a laboratory experiment which can create suitable velocity/shear profiles with sufficient Re/Rm can simulate accretion disk type instabilities in a controlled and repeatable environment.
?Taylor-Couette Flow
Experimental Apparatus: Overview
Rotating vessel components are of cast acrylic except for inner cylinder.
Phenolic gear (1/6)connects to motor via belt
Teflon seal (1/5)
(Tie-rods are usedto reinforce the outer cylinder)
Inter-nested stainlesssteel shafts aligned by cam followers.Shafts are co-supported with roller/thrust collars
Roller/Thrust Bearing
Roller Bearing
Note: this is mechanically nontrivial
Inner cylinder: R1=7cm, 1 < 4000rpmOuter cylinder: R2=21cm, 2 < 500rpmChamber height: H=28cm; Aspect Ratio ~ 2; ‘wide-gap’
Unique: Independently-Driven Intermediate Rings as End-Caps
Fluids: water, Gallium alloy (soon)
When we operate at full speeds:Max Re ~ 107
Max Rem ~ 10-100
Experimental Apparatus: Detail of Rotating Vessel
Rotating Apparatus Design: Use of Intermediary Rings to Reduce Ekman Circulation
In most experiments the end caps rotate as a solid body with the outer cylinder. A resulting pressure imbalance drives a circulation, which efficiently (and undesirably) transports angular momentum radially.
data(from prototype)
ideal (pure radial flux)
It was found numerically that just a couple intermediary rings, rotating at intermediary speeds, can significantly reduce the Ekman effect.
(Kageyama et al. 2004)
Use of Intermediary Rings to Reduce Ekman Circulation: Initial Results
Use of differentially rotating end rings successfully reduces Ekman circulation, allowing the ideal profile to be approached, which in turn allows for Keplerian-like profiles to be realized.
w/ solid end-caps
w/ differentialend-caps
Low-Speed “PIV” Data: Re ~ 2 x 105
2D simulation: Re ~3600 by W. Liu
Results submitted to Experiments in Fluids, August 2005
Being able to establish a high-Re Keplerian-like shear flow, we ask: * what instabilities/fluctuations may now be observed? * what sort/amount of transport results from them? in the absence of magnetic, kinetic, and stratification effects.
This is a very reduced but fundamental question, with a sparse and debatable history… It needs to be addressed first.
Because:
* The question is relevant for (at least) cool unmagnetized disks
* The question is relevant to a laboratory study of MRI: E.g., Is the background state is already turbulent?
Subcritical Shear Instability: Background Comments
Subcritical Shear Instability: Recent Observations (?)
dr
drS
CentrifugallyUnstable-1 < R < 0
Kep
leri
an
R > 0 Cyclonic Case
Richard & Zhan 1999, Richard 2001
Also relevant:Rotation Number R
SR
2
w/ shear
“?”grey areas of subcritical instability
Ray
leig
h st
abili
ty b
ound
ary
?
?
Example point
R =
-4/3
Subcritical Shear Instability: what does the data mean?I.e., What makes for an instability signature (“x”)?
u~
The stability boundary data given (“x”s) do not appear to represent a sudden subcritical transitionto relatively high fluctuation levels
Unconvincing. Boundary flows (e.g. Ekman circulation) are suspect
Not exactly sudden
Not exactly increasing
U
u~
Re0
8 %
Subcritical Shear Instability: Simulations
• ‘Shearing-box’ simulations by Balbus, Hawley, and Stone/Winters (1996/1999) conclude hydrodynamic stability even for slightly centrifugally stable flows. (Re ~ 104) • New numerical work by Lesur & Longaretti (2005) give a similar claim.• New work also ongoing by J. Stone and collaborators …
Rg = Critical Refor transition
Lesur & Longaretti (2005)
Subcritical Shear Instability: An Experimental Test
Ray
leig
h st
abili
ty b
ound
ary
From Keplerian-likeconditions to a centrifugally-unstable regime … and back again(check for hysteresis).
Note: actual Re #’s used are about two orders of magnitude higher
Fluctuations start and return to near-quiescent levels (~1%)
•Initial evidence for subcritical stability
•No evidence for hysteresis
Re ~ 1e6R ~ -1.05
Initial Result: No Significant Fluctuation Power for high Re Keplerian-like Flow
V
data from midplane
V
v~
Conclusions
• Ekman circulation can be significantly reduced in a wide-gap rotating flow, allowing for Keplerian-like profiles to be obtained.
• There is no significant hydrodynamic turbulence in a Keplerian-like flow up to Re ~ 1e6. This is in accord with simulations and in tentative disagreement with the experimental data of Richard & Zahn. More thorough experiments are planned.
And then onto MHD and MRI …
Addendum: Plans for MRI study
• Replacement of vessel with stainless steel version to withstand large rotation pressures (~25 atm.)
• Creation of axial B field (~ 6000 G) via 6 electromagnets
• Diagnostics: pickup coils for magnetic fluctuations and motor torque (via load cells) for gross radial angular momentum transport. Possible use of ultrasound and acoustics to assess the interior flow state. An invasive fin featuring pressure and Hall probes may be used as well.
• Some initial Gallium data may be seen later this month at APS-DPP.
** Go on Lab Tour ** See Poster by E. Schartman **