Several experimental studies have shown that large scale orgamed vortex rings are produced in
reattaching separated flows. Understanding of the interactions between these vortex rings are much
important in the development of such flows in the downstream. An experimental work from a
backward facing step have been made in the control of mixing by the use of techniques that affect
the behavior of the vortex formed in the recirculating zone. Active controlling in the form of
periodic flow perturbations caused by changing the step angle has been found to be effective in
forming the flow over backward facing step by encouraging 1 &scouraging pairing of the large
scale structures that are responsible for much of the entrainment. Also, such a technique allows
higher pressure at the base and hence low base drag which may provide useful design guidance for
perojectiles. A finite difFerence solution using boundary layer equations with marching technique
was also performed for comparison with experimental results. A flow visualization gives some
idea for the spreading of the flow over the step. Applying the present active mixing control
technique leads to a shorter reattachment length and faster spreading which is particularly
important in dump combustors, since a shorter recirculating zone allows a shorter overall design of
the system.