In order to minimize sonic boom contribution, a conventional turbofan propulsion system was shape tailored to circularize its non-axisymmetric external profile, which, after the addition of a new supersonic inlet and nozzle, has led to a new supersonic propulsion system. A new, secondary, bypass with a highly complex internal geometry was created during this process. The high-flow nacelle bypass geometry includes a forward and aft fairing to direct the flow around the gearbox, a set of thin forward guide vanes, and a set of thick aft guide vanes. The aft guide vanes, which also serve structural purposes, are used to direct the flow such that the exhaust is a uniform, nearly-full annular cross-section, and to choke and then accelerate the flow to supersonic freestream conditions upon exit. A supersonic wind tunnel facility at the University of Illinois was modified and used to simulate the flow through the aft bypass at approximately 6% scale. Two models, one with and one without guide vanes, are studied. Due to facility limitations, the design operating condition could not be achieved; a series of off-design operating conditions are tested instead. Radial pressure surveys are conducted at five azimuthal stations at the inlet to the aft bypass in order to establish in-flow conditions. Static pressure taps on the model surface provide insight into the nature of the flow through the bypass on an individual channel basis. An isentropic-case comparison and mass flow rate calculations were performed. Pressure data were supplemented with surface oil flow visualization and Schlieren imagery. In the off-design conditions tested, results indicate that the channels choke successively, beginning with those that experience the greatest amount of curvature, and hence pressure losses, and ending with those that experience the least curvature, until the entire facility chokes. Also the flow through the aft bypass was found to be highly three-dimensional containing a large amount of flow separation for the conditions tested.