Nanosecond (ns) and alternating current (ac) dielectric barrier discharge (DBD) plasma actuators are studied in a transient separation control setup. The effects of forcing on flow over a NACA 0012 airfoil in stall (AoA = 18◦) at a Reynolds number of 740,000 are compared. The flow response to single pulse forcing with time scales much less than the convective time is investigated with the aid of 2-D PIV. The experiment is carried out by phase averaging over different time delays after the actuation pulse ranging t/Tconv = 0 to 10. Pulse energy is varied by a factor of three, but minimal impact on the two actuation mechanisms is observed. The results indicate a somewhat surprising similarity in the global flow response to ns-DBD (thermal) and ac-DBD (momentum) actuation mechanisms. The location, structure and strength of the excited vortical structures are compared and local variations originating from different actuation mechanisms are observed. Ns-DBD induced perturbations are found to be stronger at a given level of electrical energy. However the differences in global flow response are small.