Laser-induced fluorescence measurements of buoyancy driven mixing in tilted tubes

The front of a light fluid rising into a miscible heavier fluid inside a long tilted tube has been analyzed by laser-induced fluorescence. Both the local concentration field and the front velocity V-f have been studied in the inertial flow regime as a function of the tilt angle theta for a constant density contrast (At=4x10(-3)). We demonstrate experimentally that the velocity V-f is directly related to the local density contrast C-f by the relation V-f proportional to(C-f)(0.5). This relation reflects a local instantaneous equilibrium between inertia and buoyancy; it is valid in the transient relaxation phase as well as in the quasistationary regime reached thereafter. (C) 2006 American Institute of Physics.