The present work describes an experimental investigation that applies stereo particle image velocimetry in a cross-plane of a turbulent channel flow that is additionally perturbed by spanwise oscillatory body forces, induced by a plasma actuator and designed to mimic the effect of spanwise wall oscillations. The experiment is aimed at retrieving the forcing-correlated scales and the turbulent flow stochastic fluctuations for the measured cross-plane. The first are macroscopic scales and require a larger investigation domain while the latter benefit of a higher resolution. Furthermore, the extended flow-field dynamic range posed a challenge on the experiment design, finally leading to an optimal tradeoff. The results of the unactuated flow compare well to the direct numerical simulations of Hoyas and Jimenez ́ (2008), while the actuated case demonstrates strong near-wall momentum addition and spanwise modulation of the streamwise flow component.

The present experimental study revolves around the applicability of a Bragg-shifted laser Doppler velocimetry profile sensor (LDV-PS) in use for open wet clutch flow scenarios, where sub-millimeter gap height and textured surfaces are present. It is shown that the LDV-PS is capable to determine angular-resolved 1D3C velocity information, with all complex flow structures, depicted properly that are present in a radial groove. For the flow measurements the sensor is tilted to $$\pm 30^{\circ }$$ ± 30 ∘ compared to the axial orientation to enable the opportunity to reconstruct angular-resolved 1D3C velocity fields from two consecutively conducted runs. This facilitates measurement results with high axial and angular resolution for the complete open clutch flow and proves for the first time, that a profile sensor is capable to extract 3C information with the mentioned method. The results show that all characteristic flow structures occurring in the investigated sub-millimeter rotor-stator gap flow can be recorded properly. This insight renders the LDV-PS a promising and straight-forward applicable means to support industry-relevant research so as to uncover formerly hidden flow features and thus contribute to advanced development approaches for the respectively considered applications. Graphical abstract

For the development of flow control techniques in wall-bounded turbulent flows, precise measurements of the flow in the proximity of the wall are essential. A laser Doppler velocity profile-sensor (LDV-PS) allows the combined measurement of tracer particle positions and velocities, which makes it a promising tool for flow measurements with high spatial resolution in flows with high velocity gradients. Therefore, the LDV-PS is employed in measurements of a turbulent channel flow at Reτ = 350. Additionally, LDV-PS measurement and signal-processing accuracies of velocity and location estimations are evaluated for various tracer-object sizes and velocities. On this basis, the turbulent channel flow measurements are evaluated and compared to reference data from direct numerical simulations. Thus, potentials of the LDV-PS are investigated for different regions of the flow, and various data processing routines as well as the experimental practice are discussed from an application perspective.

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