The life of Taylor\u96Couette flow structures in 3D

Time-resolved tomographic PIV was used to investigate the time evolution of turbulent flow structures in Taylor-Couette flow. Turbulence is created by the shear due to exact counter rotation of the cylinders, where the mean velocity is zero in the bulk flow. This enables to observe the structures longer than many other turbulent flow types, sometimes during their whole life-time. Results showed that the structures are produced around streaks of positive and negative velocities. Larger structures appear in different shapes. Most dominant ones have tube-like shape and aligned in the azimuthal direction of the cylinders. Since the measurements are time-resolved in a 3D volume, it is possible to track individual structures over time and observe evolution of their shapes. The large scale structures are found to be advancing despite the zero mean velocity

The life of Taylor–Couette flow structures in 3D

Time-resolved tomographic PIV was used to investigate the time evolution of turbulent flow structures in Taylor-Couette flow. Turbulence is created by the shear due to exact counter rotation of the cylinders, where the mean velocity is zero in the bulk flow. This enables to observe the structures longer than many other turbulent flow types, sometimes during their whole life-time. Results showed that the structures are produced around streaks of positive and negative velocities. Larger structures appear in different shapes. Most dominant ones have tube-like shape and aligned in the azimuthal direction of the cylinders. Since the measurements are time-resolved in a 3D volume, it is possible to track individual structures over time and observe evolution of their shapes. The large scale structures are found to be advancing despite the zero mean velocity.Process and EnergyMechanical, Maritime and Materials Engineerin

Large-scale structure transitions in turbulent Taylor–Couette flow

We report on the experimental investigation of the large-scale instantaneous flow structures in turbulent Taylor–Couette flow using tomographic particle image velocimetry. The results indicate three distinct regimes for counter-rotating flow within a shear Reynolds number range of 11000<ReS<47000 . Close to only inner cylinder rotation, large-scale structures are aligned in the azimuthal direction, similar to Taylor vortices. Near the point of only outer cylinder rotation, we observe columnar vortical structures in the axial direction, which are associated with small Rossby numbers. This is the first time such columnar structures are reported in a fully turbulent Taylor–Couette flow. A transition between these two regimes is observed around the point of exact counter-rotation, where the instantaneous azimuthal structures are inclined with respect to the walls. Furthermore, it is shown that the reported transitions in the turbulent flow structure modify the angular momentum transport, thereby affecting the torque scaling.Fluid MechanicsSupport Process and Energ

Turbulent Taylor–Couette flow over riblets: Drag reduction and the effect of bulk fluid rotation

A Taylor–Couette facility was used to measure the drag reduction of a riblet surface on the inner cylinder. The drag on the surfaces of the inner and outer cylinders is determined from the measured torque when the cylinders are in exact counter-rotation. The three velocity components in the instantaneous flow field were obtained by tomographic PIV and indicate that the friction coefficients are strongly influenced by the flow regimes and structures. The riblet surface changes the friction at the inner-cylinder wall, which generates an average bulk fluid rotation. A simple model is proposed to distinguish drag changes due to the rotation effect and the riblet effect, as a function of the measured drag change ??w/?w,0 and shear Reynolds number Res . An uncorrected maximum drag reduction of 5.3 % was found at Res=4.7×104 that corresponds to riblet spacing Reynolds number s+=14 . For these conditions, the model predicts an azimuthal bulk velocity shift of 1.4 %, which is confirmed by PIV measurements. This shift indicates a drag change due to a rotation effect of ?1.9 %, resulting in a net maximum drag reduction of 3.4 %. The results correspond well with earlier reported results and demonstrate that the Taylor–Couette facility is a suitable and accurate measurement tool to characterize the drag performance of surfaces.Process and EnergyMechanical, Maritime and Materials Engineerin

Experimental study of surface modification in a fully turbulent Taylor-Couette flow

Friction measurements were performed in a Taylor-Couette setup. Drag reduction was obtained with a riblet surface and indicated a drag reduction for a wide range of shear Reynolds numbers, with a maximum of 5.3% at Re_s=47000 (s+=14). Tomographic PIV verified that the friction coefficients are strongly related to the flow regimes and structures. The bulk fluid rotation was changed by the application of the riblets, as the wall-bounded flow conditions at the inner cylinder wall were changed due to the surface modification and is called the rotation effect. A simple model was used to indicate the averaged bulk velocity shift (1.4%), after which the drag changes due to the rotation effect (-1.9%) and the riblet effect (-3.4%) were determined. The bulk velocity shift of 1.4% was verified by PIV measurements. Compliant surfaces will be further investigated to check their required conditions for drag reduction of wall-bounded flows.Process and EnergyMechanical, Maritime and Materials Engineerin

An experimental investigation into the drag reduction performance of dimpled plates in a fully turbulent channel flow

Dimpled surfaces have gained increasing attention in recent years for their potential to reduce turbulent skin friction, a capability previously acknowledged for its beneficial implications on heat transfer. As a passive drag reduction method, dimpled surfaces offer significant advantages for marine applications due to their effectiveness and practical applicability. However, despite numerous studies, conflicting opinions and inconsistent drag reduction rates persist in the literature. This paper addresses these ambiguities and offers valuable insights into the effectiveness of dimpled surfaces for drag reduction in fully turbulent flows. We conducted an extensive experimental investigation involving various dimple configurations, including different depth-to-diameter ratios, diameters and orientations, utilising a specialised Fully Turbulent Flow Channel facility and a Particle Image Velocimetry system. Our findings demonstrated that circular dimple geometries, particularly those with low depth ratios, can achieve significant drag reduction of up to 27% as the Reynolds number increases. These results highlight the substantial potential of dimpled surfaces for improving energy efficiency in marine applications, where skin friction accounts for a significant portion of the total drag experienced by large vessels.Support Marine and Transport Techolog