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Euromech 10 th European Turbulence Conference 29 June – 2 July 2004, Trondheim, Norway. An experimental study of bypass transition in plane Couette flow. S. AMALFI, F. LAADHARI & J. F. SCOTT Laboratoire de Mécanique des Fluides et d’Acoustique Unité Mixte de Recherche 5509

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an experimental study of bypass transition in plane couette flow

Euromech10 th European Turbulence Conference29 June – 2 July 2004, Trondheim, Norway

An experimental study of bypass transition in plane Couette flow

S. AMALFI, F. LAADHARI & J. F. SCOTTLaboratoire de Mécanique des Fluides et d’Acoustique

Unité Mixte de Recherche 5509

69 134 ECULLY, France

slide2

Introduction

  • 2 different kinds of transition to turbulence in 2D bounded flows:
  • Plane Couette flow (PCF) is of particular interest :
  • simplicity of the basic flow and the absence of linear instability.
  • Recent scenario* :
  • streamwise vortices streaks secondary instabilities turbulence
  • The aim of the present study : investigate such a scenario in PCF introducing Counter rotating pairs of streamwise vortices based on optimal criteria.

*Reddy et al. (1998), J. Fluid Mech., 365, 269-299

slide3

Outline

  • Experimental setup and perturbation generating system
  • Tomography animation and flow characteristics based on LDA measurements
  • Visualization of vortical structures and statistical analysis based on PIV measurements
  • Conclusion and perspectives
slide4

PCF : flow between one or two moving belts

  • Very few experimental PCF studies, considering several setup difficulties (moving belts stability, free stream perturbations, etc.)
  • Reynolds number of interest is 470 with .

Laminar

Motor

Encoder

Top view

Re ≤ 470

ZOOM

Turbulent

Water

Ly=2h=2.6 cm

Re ≥ 500

Lx=200 cm

Lz=30 cm

Experimental facilities

Tomography and LDA results

PIV analysis

Experimental setup

slide5

Experimental facilities

Tomography and LDA results

PIV analysis

slide6

Experimental facilities

Tomography and LDA results

PIV analysis

30°

5 mm

50 mm

40 mm

Perturbation system based on optimal perturbations*

* Butler & Farrell (1992), Phys. Fluids, 4, 1637-1650

slide7

Experimental facilities

Tomography and LDA results

PIV analysis

moving belt

plexiglass

wall

slide8

Experimental facilities

Tomography and LDA results

PIV analysis

slide9

cm/s

Y

(mm)

Time (s)

Before injection

After injection

-1

-0.5

Y

(cm)

0

0.5

1

-4

-3

-2

-1

0

1

2

3

4

Averaged streamwise velocity (cm/s)

Experimental facilities

Tomography and LDA results

PIV analysis

Instantaneous streamwise velocity

slide10

Experimental facilities

Tomography and LDA results

PIV analysis

Visualization of vortical structures

  • PIV measurements (2 000 snapshots)performed in the transverse plane. Snapshot size : 26 mm (y) x 96 mm (z) giving two velocity components : v and w (along y et z).
  • Dynamical analysis of structures possible, considering very low velocities of moving walls (3.6 cm/s) with a 4 Hz frequency acquisition
  • Normalized Angular Momentum*is a good tool to visualize vortical structures

* Michard et al. (1997), 11th Int. Symp. Turbulent Shear Flows, F. Durst, Springer, 278-290

slide11

Z

Z

Z

Z

Z

Z

Z

Z

Z

Z

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Times (s)

Time (s)

Time (s)

Time (s)

Z

(mm)

Z

(mm)

Z

(mm)

Z

(mm)

Z

(mm)

Z

(mm)

Z

(mm)

Z

(mm)

Z

(mm)

Z

(mm)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Y

(mm)

Y

(mm)

Y

(mm)

Y

(mm)

Y

(mm)

Y

(mm)

Y

(mm)

Y

(mm)

Y

(mm)

Y

(mm)

300 ≤ Time (s) ≤ 350

400 ≤ Time (s) ≤ 450

350 ≤ Time (s) ≤ 400

200 ≤ Time (s) ≤ 250

050 ≤ Time (s) ≤ 100

250 ≤ Time (s) ≤ 300

150 ≤ Time (s) ≤ 200

450 ≤ Time (s) ≤ 500

100 ≤ Time (s) ≤ 150

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Energy

(m²/s²)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Time (s)

Experimental facilities

Tomography and LDA results

PIV analysis

Time

001 ≤ Time (s) ≤ 050

slide12

Energy

(m²/s²)

Time (s)

d/h

Time (s)

Time (s)

Experimental facilities

Tomography and LDA results

PIV analysis

slide13

Conclusion and Perspectives

  • First PIV database concerning PCF
  • Transient growth of secondary instabilities has been observed using visualizations and PIV measurements
  • Mean size of structures doesn’t play a key role in the transition process
  • but vortex intensity does !
  • LDA measurements confirming PIV study
  • Comparison with other complex cases : Boundary Layer over a flat plate
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