Study of free surface swirling flow of a drag-reducing surfactant solution

Swirling flows of dilute cetyltrimethyl ammonium chloride (CTAC) surfactant solutions in an open cylindrical container with a rotating disc at the bottom were experimentally investigated by a double-pulsed particle image velocimetry (PIV) system. The mass fraction of CTAC solutions ranges within 5 × 10^-5-1 × 10^-3, and the Reynolds number reaches to 4.3 × 10⁴. The relationship of the dip of the free surface center of the swirling flow with the surfactant mass fraction and drag-reduction is revealed, which provides a method for screening a dilute surfactant solution with small elasticity and drag-reducing ability. The effects of the small elasticity of the surfactant solution on flow vortex structures are also discussed. It is found that the flow pattern in the meridional plane for water with the presented high Reynolds number differs remarkably from that with low Reynolds numbers, and an inertia-driven vortex is pushed to the corner between the free surface and the cylindrical wall by a counter-rotating breakdown vortex. In the surfactant solution flow with mass fraction of 1 × 10^-3, the inertia-driven vortex locates at the corner between the bottom and the cylindrical wall while elasticity-driven reverse vortex governs the flow field.