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SUMMARY:The structure of the velocity and passive scalar mixing in a multi
 ple opposed jets reactor - Danaila\, L (CORIA)
DTSTART:20080930T110000Z
DTEND:20080930T113000Z
UID:TALK13838@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:We document an experimental investigation of a confined chambe
 r in which fluid is injected through two sets of 16 opposed jets that issu
 e from top/bottom boundary porous planes. The investigated Reynolds number
 s\, based on injection velocity and jet diameter are up to 28\,000. The hi
 gh Reynolds numbers and impinging configuration of the flow produce very i
 ntense turbulence levels and a turbulence with zero mean velocity in the c
 entral region of the reactor. The analysis is done for basically two geome
 tries: opposed jets with strong backflow\, and opposed jets with very slow
  backflow. Particles Image Velocimetry (PIV) measurements in different pla
 nes allowed for a characterization of the mean and fluctuating velocity fi
 elds. Fluid recirculation in the reactor creates annular shear layers. The
  central region of the reactor includes stagnation regions\, where mean ve
 rtical velocity gradients are very strong with low local mean velocity val
 ues\, leading to high rms-to-mean velocity ratios. Such gradients are resp
 onsible for considerable kinetic-energy production\, that sharply peaks in
  the central region. A particular attention is paid to the determination o
 f the small-scales characteristics (energy dissipation rate) in different 
 points of the flow\, which is done using both PIV (using indirect methods\
 , i.e. inertial-range information) and LDV (Laser Doppler Anemometry). Ine
 rtial-range behaviour is discussed\, in terms of second and third-order st
 ructure functions\, and a critical comparison with classical (forced) flow
 s is done.\n\nA passive scalar (Sc=1.3) is injected in the flow\, in an al
 ternate sequence (Z=0 and Z=1) among each two opposed jets and measured us
 ing Laser-Induced Fluorescence. Scalar structure functions are investigate
 d\, as well as the extent to which isotropy and homogeneity are adequate a
 pproximations for this flow. Flow visualisations exhibit very sharp scalar
  gradients at the frontier among two opposed jets. The dynamics of these r
 egions closely follows that dictated by the back-and-forth motion develope
 d in the central region\, due to the opposed jets instabilities. Thus\, th
 e scalar is directly injected at the level of small scales\, whereas the v
 elocity field itself is injected over a whole range of scales. This direct
 ly leads to a very effective mixing in the stagnation region of the oppose
 d jets. 
LOCATION:Seminar Room 1\, Newton Institute
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