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Compressible flow in a suddenly expanded square duct was investigated experimentally to assess the effectiveness of the passive control in the form of the cavities. The flow parameters studied were the Mach number, nozzle pressure ratio, L/W ratio, and area ratio. The test were conducted for multiple cavities and without multiple cavities. From experimental results it is seen that the multiple cavity has a very good effect in reduction of base drag by decreasing the base suction and hence increasing the base pressure. From experimental investigation it is found that for all the L/W ratios the effect of multiple cavities are able to control the base pressure, further, it was seen that with the increase in the duct length control is becoming very effective. The wall pressure in the duct indicates that the passive control in the form of cavity do not disturb the flow field in the duct. INTRODUCTIONThe backward facing step (BFS) is one of the most fundamental configurations to study flow separation and following attachment which occurs due to the sudden expansion in the flow passage. The existence of flow separation and reattachment plays an important role in many engineering applications such as combustors, diffuser, electronic and turbine blade cooling as well as in external flows such as aircraft. For this reason, several studies on the flow separation and reattachment of the BFS geometry have been presented numerically and experimentally by many researchers in the past decades. Among these studies, the effect of Reynolds number, step height, aspect ratio has been reported for the 2D and 3D flows, which brought to insight for understanding the flow characteristics of the BFS configuration. Their attachment point spreads within a certain span along the streamwise distance, which is referred to as the reattachment zone. These three regions in whole, comprise the important features of a BFSX flow that can be altered or controlled to achieve desired outcome, such as, enhanced mixing characteristics and reduced drag, noise and vibrations. Hence it is essential to understand these flow features to control the parameter of interest to study the wake characteristics of a BFS flow. With the advancement in technology and the recent discovery of

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