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SUMMARY:The behaviour of particle pairs in kinematic simulations - Thomson
 \, D (Met Office)
DTSTART:20081002T161000Z
DTEND:20081002T164000Z
UID:TALK13868@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:The way pairs of particles separate is an important aspect of 
 turbulent mixing which has often been explored using the technique of kine
 matic simulation. However kinematic simulation is not like real turbulnce 
 in that the Fourier modes are independent and the smaller eddies are not a
 dvected (or 'swept') by the large eddies. Our aim here is to explore this 
 aspect of kinematic simulation both theoretically and numerically.\n\nThe 
 fact that the small eddies are not swept by the large eddies\, but the par
 ticles in the flow are so swept\, means that particle pairs are swept thro
 ugh the smaller eddies by the large eddies. This is expected to alter the 
 time scale on which the relative velocity of the particles fluctuates. A s
 imple argument then shows that the mean square separation of pairs is expe
 cted to grow\, not as t cubed as expected following Richardson\, but as t 
 to the sixth power. This is confirmed in numerical simulations where we ad
 d a mean flow to the kinematic flow field to exaggerate the problem caused
  by lack of sweeping (with the eddies not being advected by the mean flow)
 . Without the mean flow the situation is more complex with a significant c
 ontribution to the separation process from locations where the velocity is
  small and where there is no sweeping issue. This leads to a separation gr
 owing like t to the power 9/2. The time dependence of the kinematic flow f
 ield can also lead to a wider range of behaviours.\n\nThe work described h
 ere is not especially new (we published the main idea in 2005) but it rema
 ins controversial and we hope the talk will generate some discussion of th
 e ideas involved. 
LOCATION:Seminar Room 1\, Newton Institute
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