Newsletter 2018.1 Index
Theme : "Mechanical Engineering Congress, 2017 Japan (MECJ-17)"
Investigation of the sustaining mechanism of turbulence of Newtonian fluids by using viscoelastic fluids
On the basis of systematic investigations of the turbulence suppression due to a small amount of surfactant additives, we can obtain important information on the sustaining mechanism of the turbulence. To show that this strategy is useful, we have experimentally investigated turbulence suppression in a precessing sphere. Combining our experimental results with previous theories on the onset of turbulence suppression by viscoelasticity of fluids, we can conclude that the sustaining mechanism of turbulence in the sphere is an energy cascading process starting from the largest-scale vortices in turbulence.
Turbulence, Laboratory experiments, Viscoelastic fluids, Rotating fluids
Movie. 1 Precession of a sphere. The word “precession” denotes the rotation of the spin axis of a rotating object. Precession of a container can sustain developed turbulence of the confined fluid. Flow in the sphere is controlled only by two dimensionless parameters: the Reynolds number, Re=a2Ωs/ν, and the Poincaré number, Po=Ωp/Ωs. Here, a is the radius of the sphere, ν is the kinematic viscosity of fluid, and Ωp and Ωs are the magnitudes of the angular velocity of the spin and precession, respectively.
Fig.1 Experimental apparatus. An acrylic container with a spherical cavity is rotated by a motor on a turntable which is rotated by another motor. A laser sheet visualizes flow on the equatorial plane of the cavity so that we can conduct flake visualization and particle image velocimetry (PIV) of the flow. A camera fixed on the turntable records the visualized flow.
Movie. 2 Turbulence of (left) water and (right) the dilute surfactant (CTAC) solution. Re=4.0×104, Po=0.1. If the working fluid is Newtonian, e.g. water, and Re is sufficiently high, fully developed turbulence is sustained for Po=O(0.1) [1,2], which is the target of this study. When we add a small amount of surfactant to this turbulence, drastic turbulence suppression occurs (see reviews [3,4] of turbulence reduction by additives). We investigate this suppressed turbulence to understand the developed turbulence of water, since the suppression is due to the absence of the sustaining mechanism of turbulence .
Fig.2 Temporally averaged enstrophy (squared vorticity) field of turbulence of (a) water and (b) the dilute surfactant (CTAC) solution. Re=4.0×104，Po=0.1. The enstrophy indicates the existence of small-scale turbulent eddies. This figure quantitatively verifies the observation of Movie 2, i.e. the strong suppression of turbulent eddies by the surfactant additive.
Fig.3 Temporally averaged velocity field of turbulence of (a) water and (b) the dilute surfactant (CTAC) solution. Re=4.0×104，Po=0.1. Large vortical structures in the right-upper and bottom-left regions, which are clearly observed in the turbulence of water, still exist in the turbulence of the CTAC solution where small-scale eddies almost completely disappear.