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This paper presents an experimental investigation into the effect of a varying downstream boundary condition on dynamic separation control in a twodimensional low-speed asymmetric diffuser. The potential for coupling between the downstream boundary condition and the separation dynamics is relevant, for example, in using separation control to enable more aggressive serpentine aircraft inlets, where the compressor may be close to the separation point. Separation control in the experiment is obtained using spanwise unsteady forcing from a single tangential actuator located directly upstream of the separation point. The downstream boundary condition simulates the dominant quasi-steady and reflection characteristics of a compressor. Although the boundary condition affects the uncontrolled pressure recovery, the optimal forcing frequency is shown to depend only on the mass flow rate and not on either the presence, impedance, or location of the downstream boundary condition. At the conditions tested herein, we therefore conclude that the mechanism underlying dynamic separation control is local in nature, and is not influenced by global system dynamics.