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Hot Cold Water Water. Hydrodynamics in ICF: The Rayleigh-Taylor Instability. Splitter plate. Exit plenum. Meshes. 1.0 m. Flow straightener. 2005 HEDP Summer School

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slide1

Hot Cold

Water Water

Hydrodynamics in ICF: The Rayleigh-Taylor Instability

Splitter plate

Exit plenum

Meshes

1.0 m

Flow

straightener

2005 HEDP Summer School

W. KraftPhd,Msc, A. BanerjeePhd, N. MueschkePhd, Msc, P. RamaprabhuPhd, P. WilsonPhd, L. DeChantPhd, K. LeichtMsc, D. SniderPhd

And Professor M.J. Andrews

Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123

( mandrews@mengr.tamu.edu)

This research is sponsored by the National Nuclear Security Administration

under the Stewardship Science Academic Alliances program through

DOE Research Grant # DE-FG03-99DP00276/A000 and DE-FG03-02NA00060.

0.2 m

UG Research Asst. (6ft tall)

PIV/PLIF Image field

8 cm Hort.

x 6 cm Vert.

0.0125 cm/pix

0.3 m

Air

Inlet ducting

He

PIV particle concentration:

3mL/2000 L

E-type thermocouples

Nickel-Chromium and Constantan

Junction diameter of 0.01-0.02 cm

Response time 0.001 s/oC

Acquisition rate 100 Hz.

Flow channel

Fans

Camera

640H x 480V Pixels

1200 Image Capacity on Board

  • Rayleigh-Taylor Instability: Background
  • Rayleigh-Taylor instability (R-T) occurs when a density gradient is accelerated by a pressure gradient such that .
  • When a heavy fluid rests above a light fluid under the influence of gravity, the density interface is unstable to infinitesimal perturbations.
  • The resulting flow evolves in three stages:
      • Exponential growth of infinitesimal perturbations
      • Nonlinear saturation of perturbations
      • Transition to turbulence and self-similar growth
  • RT flows occur in the ejecta of supernovae, in atmospheric flows, and in the ablation interface of Inertial Confinement Fusion capsules.

Lasers

Two 120 mJ

15 Hz pulse

Sample rate: 30/sec.

Air/Helium Facility as of December 8, 2003.

Flow direction

Cold water

At = 10-3

DT = 5oC

U0 = 4.4 cm/s

Warm water

Side View

10 cm

2.0 m

1.0 m

Figure shows a snapshot of the experiment, with nigrosene dye added to the cold water stream. The evolution of the mix is quadratic in x (downstream coordinate), with the mix width depending on the Atwood number (At), and the acceleration due to gravity. In this experiment,the distance downstream can be related to time through the Taylor hypothesis.

Airin

Exit

Plenum

Vents

0.6 m

Hein

Splitter Plate

Meshes

Flow Straighteners

Exit

Plenum

Vents

0.4 m

(a)

2.0 m

1.0 m

1.0 m

Top View

Fluid 1, f1 = 100 %

(b)

Fluid 2, f1 = 0 %

(c)

Background image is an experimentally captured PLIF image ~ 35 cm downstream.

Low Atwood number water channel ( At ~ 10-3 )

Statistically steady experiment with long run times

Cold and warm water enter through separate inlet plenums. As they pass through flow straighteners and wire meshes, they are kept separated by a splitter plate. As the different-density streams leave the edge of the splitter plate, they form an unstable interface resulting in a statistically-steady R-T mix.

High Atwood number He/Air gas channel, ( At ≤ 0.75 )

PIV: Velocity measurements

PIV-S : Visualization and measurement using seed particles (x = 35 cm)

PIV-Scalar (PIV-S), a variant of conventional PIV, was developed to simultaneously measure density and velocity fields in a R-T mix.

Evolution of mixing layer centerline velocity fluctuations. Vertical velocity fluctuations (in the direction of the density gradient ) are significantly larger the stream wise fluctuations.Late time velocity fluctuations grow linearly once the flow is self-similar

Image Analysis: Volume fraction, mixing layer width measurement

Consecutive grayscale images (a), separated by t = 0.033 s, of seed particles are cross-correlated to yield a velocity vector field (b). The corresponding out-of-plane component of vorticity (c) shows regions of positive and negative vorticity concentrated within the R-T rollup. Density information may be obtained from (a) through local window averages of particle concentration.

Volume fraction ( f1 ) profiles from the mixing layer shown on the right using 400 images , where f1+ f2 = 100 %

Evolution of velocity fluctuation profiles across the mix width as the mixing layer grows

Rayleigh-Taylor mixing layer with Atwood #, At = 0.035

Velocity – density correlations inside the Rayleigh-Taylor mixing layer (a) At x ~ 2.5 cm, and (b) at x ~ 35 cm. < ’v’> is strongly correlated at both locations while < ’u’> is only weakly correlated near the splitter plate

 = 0.07

Velocity power spectra at x = 35 cm (obtained from PIV) show the vertical velocity component dominating over horizontal velocity fluctuations. A developing inertial range (k-5/3) and a dissipative range (k-3) at the high-wavenumber end is visible.

Downstream evolution of the mixture fraction

(i.e. density) profile across the mixing layer

Determination of the growth constant, , for self-similar growth of the Rayleigh-Taylor instability where the half mix width, h =  At g t 2