Experimental Investigation of Coherent Structures Generated by Active and Passive Separation Control in Turbulent Backward-Facing Step Flow

Kurzfassung

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 Reh = 2.0×104, 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) timeresolved tomographic particle image velocimetry was employed, within a threedimensional 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 fa = 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 are reconstructed as travelling waves corresponding to the actuation frequency fa and the overtone frequency 2•fa, 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 fp = 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•U0 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 periodic small perturbations enable to manipulate the large-scale separated shear layer in a flapping motion and therefore effectively reduce the reattachment length downstream of the step. The third flow control device was an array of passive vortex generators. Low-profile wedge-type vortex generators (VGs) were fixed on the surface upstream of the step in the spanwise direction, which were totally submerged within the turbulent boundary layer. The time-averaged reattachment length is reduced by 29.1% due to the VGs. Two-point crosscorrelation functions were used in order to analyze the spatial and temporal evolution of the VG-induced vortices. The Reynolds shear stress is considerably increased, which is, moreover, highly phase-correlated with the VG-induced coherent structures. POD and DMD are applied to analyze the complex vortex structures in the spatial and temporal approaches, respectively. First, two series of quasi-periodic oblique vortices in the streamwise-spanwise orientation are extracted by the POD method, which contain a highly coherent bandwidth of Strouhal number 0.3 < Sth < 0.6. Second, by applying the DMD to the POD reconstructed flow field, a single-frequency dynamic mode at Sth = 0.37 is extracted and Λ-shaped vortex structures are reconstructed in the recovery of the predominant DMD mode. Experimental results show that the three types of flow control devices are able to reduce the reattachment length by generating quasi-periodic coherent motions in the separated shear layer. These coherent structures lead to an increase of Reynolds shear stress and play an important role in the momentum transfer in the turbulent shear flow.