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Dielectric barrier discharge (DBD) plasma actuators can generate a wall jet without moving parts through interaction between ionized and neutral molecules in an electric field. The coupling between electro-hydrodynamic, turbulence, and viscous effects in the flow boundary layer remains unclear and deserves careful investigation. We present an experimental investigation of momentum injection by DBD actuators in a U_external = 5 m/s and U_external = 11 m/s co-flow and counter-flow configuration over a range of VAC = 12 kV - 19.5 kV peak-to-peak at a frequency of 2 kHz. In the co-flow configuration, the DBD actuator adds momentum to the boundary layer, similar to an electrohydrodynamic (EHD) jet in quiescent conditions. In the counter-flow configuration, flow separation is observed at free stream velocity U_external = 5 m/s. The momentum displacement in the counter-flow configuration is ~ 6x greater than EHD jet momentum in a quiescent environment. Both co-flow and counter-flow momentum injections show diminishing effects with increased external flow speed. This work highlights that the resulting flow pattern is not a simple superposition of the EHD jet and the free stream but is determined by a balance between the inertial, viscous and Coulombic forces of the EHD and the external flow. The velocity profiles and momentum characteristics can be used to validate numerical models and inform the design of DBD actuators for active flow control.