Some observations on the mixing of an underexpanded rectangular jet ejector

An experimental investigation was carried out on a rectangular ejector (constant area mixing duct) with an underexpanded rectangular jet as the primary flow. From the wall static pressure measurements, the ejector performance was found to show irregular variations with the primary jet pressure. Hot-wire measurements together with schlieren photographs showed that a better performance was obtained when the flow was well mixed. The well mixed flow was found when the screech tone Strouhal number was around 0.13, which agreed with the most unstable Strouhal number of the free underexpanded rectangular jet. By incorporating a phase-locked technique with schlieren flow visualization, it is possible to photograph standing waves excited by the screech tones inside the ejector mixing duct. The interactions between the jet flow and the screech tones were studied using this flow visualization technique.