Abstracts - KTH Mechanics

Flow visualization of relaminarization in a two-dimensional
channel flow
M. Matsubara ∗ andT.Matsuzawa ∗
In viscous flows, there exists the minimal Reynolds number at which flow sustains
turbulent state against viscous dissipation. Around this Reynolds number the Kolmogorov
scale is in same order of the large structure of turbulence correspond to flow
geometry so that, in a sense, it is more facile to observe turbulent structure and sustain
mechanism in experimental methods though there is no inertial subrange at all.
On the other hand, a great deal of research effort has been put on seeking periodical
solutions at relatively low Reynolds number in canonical flows such as channel or pipe
flows.
In this research we focus on experimental observation of relaminarization from
fully developed turbulence to laminar by the viscous effect in a two dimensional
channel flow. In previous experiments the minimal Reynolds number based on the
maximum velocity as in Poiseuille profile and half channel width has already been
estimated at about 2100 by extrapolation of decay of Reynolds stress 1 . There is
another estimation of 1000 to be determined observing turbulent spot generation with
large point-like initial disturbance 2 . Our experiment attempt to decide the minimal
Reynolds number in the channel flow and investigate disturbance structures at such
low Reynolds number.
A 2 m length test cannel with 5 mm width and 260 mm span is following a 1
m expansion channel and a 1m pre-developping channel with tripping wires at its
inlet. The expansion channel has 2 degree diffuse angle at the end walls so that
the Reynolds number is reduced 2/3 of the inlet value. Flow visualization (figure 1)
shows that at Reynolds number of 1340 the flow state is turbulence with relatively
large spanwise scale of streaky structures. Around Reynolds number is 1100, the flow
undergoes relaminarization with characteristic structures of disturbance that elongate
in the streamwise direction with forming a cluster. Downstream of the cluster a new
elongated structure is generated one after another and its upstream the structures are
disappearing. For Reynolds number less than 960 flow becomes complete laminar, so
that the minimal Reynolds number is between 960 and 1340
a Re=1340 bRe=1100
Figure 1: Flow visualization of relaminarization in two dimentional channel flow.
∗ Shinshu Univ. Mechanical systems engeneering, Wakasato 4-17-1, 380-8553 Nagano, Japan.
1 M. A. Badri Narayanan, J. Fluid Mech. 31, 609 (1968).
2 D. R. Carlson et al., J. Fluid Mech. 121, 133 (1982).
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