This dissertation presents the experimental investigation of coherent structures which were generated by active and passive separation control devices in a turbulent backward-facing step (BFS) flow. The Reynolds number was Re h = 2.0×10 4 , based on the free-stream velocity and the backward-facing step height. Three types of flow control devices, which are referred to as acoustic tube (AT), oscillating flap (OF) and vortex generators (VGs), were implemented independently on the backward-facing step in order to control the turbulent flow separation downstream of the step. For each implementation of the devices, 2D-2C planar particle image velocimetry was used to measure the separated shear layer, recirculation region and reattachment area downstream. Moreover, 3D-3C(t) time-resolved tomographic particle image velocimetry was employed, within a three-dimensional volume at a high sampling frequency, to measure the three-dimensional fluid motions in the separated shear layer downstream of the vortex generators. The first flow control device under investigation in the present work was the acoustic tube. As an active flow control device, the acoustic tube generated periodic pressure perturbations influencing the flow through a spanwise thin slot close to the separation edge at an actuation frequency of f a = 100 Hz, which was close to the most-amplified frequency of the shedding instability of the turbulent shear layer. The flow control results show that the acoustic tube can suppress the recirculation region and reduce the reattachment length by 43.7%. The rolling-up and pairing processes of spanwise vortices lead to an increase of the Reynolds shear stress. The coherent structures are extracted by proper orthogonal decomposition (POD) and are represented by two pairs of the spatial modes. Both the primary and secondary series of vortices iv are reconstructed as travelling waves corresponding to the actuation frequency f a and the overtone frequency 2• f a , respectively. The second flow control device under investigation was the oscillating flap. The oscillating flap was driven by periodic Ampere force and thereby generated small perturbations over the separation edge at a perturbation frequency of f p = 55 Hz, which was close to the half of the most-amplified frequency of the shedding instability of the turbulent shear layer. The time-averaged velocity fields show that the small perturbations can reduce the recirculation region behind the step and reduce the reattachment length by 31.0%. The phase-averaged results indicate a small streamwise ejection with a velocity of 0.7• U 0 when the oscillating flap moves downwards. The rolling-up and breakdown processes of the spanwise vortices result in considerable increases of coherent and incoherent parts of the Reynolds shear stress. The coherent structures in the turbulent shear layer are further analyzed by snapshot proper orthogonal decomposition (POD) and spatial dynamic mode decomposition (DMD) in energy and dynamic approaches complementarily. It is shown that the per...