drag reduction by surfactant solutions in gravity driven flow systems

efflux time measurements are carried out for gravity draining of a liquid from a large cylindrical tank (where the flow is essentially laminar) through single exit pipe in the absence and presence of cetyl pyridinium chloride (cpc) surfactant solutions. the variables considered are initial height of liquid in the tank, dia. of tank, length of the exit pipe and concentration of surfactant. the dia. of exit pipe in all the cases however remained constant. drag reduction is expressed as the difference in efflux time in the absence and presence of surfactant solutions. maximum drag reduction at optimum surfactnat concentration is reported. it is observed that during draining, froude number remains constant.