DOI: 10.1615/TSFP9
EXPERIMENTS ON TURBULENCE MODULATION BY SURFACTANT ADDITIVES IN A PRECESSING SPHERE
SINOPSIS
To investigate the physical mechanism of turbulence modulation due to a small amount of surfactant additives, which constitute thread-like micelles in a solution, we conduct well-controlled laboratory experiments. More concretely, using the precession of a vessel to drive the flow of a fluid confined within its spherical cavity (the radius, a, of which is 90 mm), we have constructed an experimental platform with high reproducibility of flow states being ensured. Using this platform, we conduct experiments on turbulence modulation in a dilute (50 ppm) solution of a cationic surfactant (cetyltrimethylammonium chloride; CTAC). When the Reynolds number, Re = a2Ωs/ν = O(104), where Ωs and ν are the magnitude of the spin angular velocity and the kinematic viscosity of the confined fluid, the turbulence intensity is, indeed, significantly reduced, but the turbulence modulation occurs locally in space. On the other hand, in this range of Re, the surfactant additive hardly affects the transition process from steady to unsteady flows. Namely, this turbulence modulation due to the surfactant additive appears under rather limited conditions. Therefore, these experimental observations will help us to reveal the physical mechanism of the turbulence modulation.