Download transitions in a soft-walled channel.

10th International Symposium on Turbulence and Shear Flow Phenomena (TSFP10), Chicago, USA, July, 2017
TRANSITIONS IN A SOFT-WALLED CHANNEL.
V. Kumaran
S. S. Srinivas,
Department of Chemical Engineering,
Indian Institute of Science,
Bangalore 560 012, India.
[email protected]
Department of Chemical Engineering,
Indian Institute of Science,
Bangalore 560 012, India.
[email protected]
ABSTRACT
The transition in a soft-walled channel, due to a dynamical instability caused by the coupling between the fluid flow and the wall dynamics,
is studied at Reynolds numbers less than 1000 for the hard-walled laminar-turbulent transition. Here, the Reynolds number is defined as
Re=(ρV h/η ), where ρ and η are the fluid density and viscosity, h is the channel height and V is the average flow velocity. Due to the fluid wall
coupling, the wall elasticity also affects the transition through the dimensionless parameter Σ = (ρ Gh2 /η 2 ), where G is the shear modulus of
the wall material. The channels are made of polyacrylamide gel with about 94% water and a shear modulus of about 0.75 kPa. Particle Image
Velocimetry (PIV) measurements are made across the height of the channel, which is the smallest dimension and is about 0.6 mm. The width of
the channel is much larger at about 1.3 cm, and the length is about 14 cm to ensure that the flow is fully developed before the end of the channel
where the measurements are made. There are two distinct transitions observed with very different characteristics. At a Reynolds number of
about 300, there is a soft-wall transition, after which the mean velocity profile is more plug-like with a higher gradient at the wall and lower
curvature at the center in comparison the the parabolic profile for a laminar flow. There is also a significant increase in the root mean square
of the velocities in the stream-wise and cross-stream direction, and the Reynolds stress. There is no observable normal motion at the wall, but
there is a clearly discernible tangential motion. A clear logarithmic layer is observed in the von Karman plots for the velocity profiles near the
walls, though the viscous sub-layer is note observed for distances as small as 2 in wall units. As the Reynolds number is further increased, there
is a wall-flutter transition, where visible normal motion is observed at the top (unrestrained) wall alone, but there is no motion at the bottom
wall that is fixed. In this case, there is a distinct asymmetry in the profiles of the mean and fluctuating velocities, and there is no logarithmic
layer.
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