drag reduction by anionic surfactant solutions in gravity driven flow system

this paper presents efflux time experiments performed in the absence and presence of aqueous solutions of drag reducing agents (dras) when a liquid is emptied from a large open cylindrical storage tank through an exit piping system. the drag reducing agents studied are dodecyl benzene sulfonate anionic surfactant and a mixed solution of surfactant and sodium chloride counter ion. the variables studied are initial height of liquid in the tank, diameter of storage tank, and length of exit pipe. drag reduction (i.e reduction in efflux time) is found be more significant in presence of mixed solutions of surfactant and its counter ion. it has also been observed that froude umber remains constant during draining and increases upon addition of surfactant solution and also in case of mixed solutions of surfactant and its counter ion.