The hydrodynamic mechanism of drag reduction by a rotationally oscillating cylinder with a flexible filament was explored using the penalty immersed boundary method. A simulation of a stationary cylinder without a filament was also performed for comparison. The effects of the filament length, bending rigidity, oscillatory frequency, and oscillatory amplitude on drag reduction were systematically examined. The underlying mechanism of drag reduction was characterized in terms of the shape deformation of the filament, wake pattern, pressure distribution, and flapping dynamics. Two dominant flapping modes were observed: an oscillation mode with less than half a wave on the filament, and an undulation mode with more than one wave on the filament. In the oscillation mode, drag reduction is mainly achieved by the thrust generated by the filament, accompanied by an increase of lift fluctuations. The pressure difference caused by the flapping motion between the upper and lower sides of the filament is the main cause of the thrust. In the undulation mode, drag reduction is realized by both the thrust generated by the filament and the decreased form drag of the cylinder. A filament flapping in the oscillation mode can generate greater thrust than a filament flapping in the undulation mode. A long undulatory filament with relatively low oscillatory amplitude effectively stabilizes the wake, resulting in a decrease of the lift fluctuations.

Drag reduction by a rotationally oscillating cylinder with a flexible filament

Abstract Abstract The hydrodynamic mechanism of drag reduction by a flexible hairy coating was explored using the penalty immersed boundary method. A two-dimensional flexible hairy coating is constituted by multiple flexible filaments. A simulation of a cylinder without a hairy coating at a Reynolds number of 100 was also performed for comparison. The results of the simulations show good agreement with the experimental data by Niu & Hu (Phys. Fluids, vol. 23, 2011, 101701), where maximum drag reduction of 22% was attained at a particular length, bending rigidity, coating density and coating angle of the hairy coating. The hydrodynamic mechanism of drag reduction was characterized in terms of the wake pattern, shape deformation and kinetic energy of the hairy coating. The effect of a non-uniform bending rigidity of the hairy coating on drag reduction was explored. A stable streamline shape of the hairy coating was found to delay the vortex formation and stabilize the recirculation zone, resulting in decreased form drag. Active flapping of the hairy coating with enhanced vortex shedding is adverse to drag reduction. A hairy coating with a stiff base and flexible trailing edge is beneficial to maintaining a stable shape.

Drag reduction by a flexible hairy coating

Numerical simulation of separated flows around a hybrid rigid-flexible thin plate

In this study, the k − ω S S T model is employed to evaluate the effects of flexible plates on the hydrodynamic forces acting on a circular cylinder in turbulent flows. In this regard, four flexible plates are attached to the circular cylinder. Besides, the finite volume method and finite element method have been utilized to discretize the governing equations of the fluid flow and the flexible plates, respectively. The Reynolds number based on the cylinder diameter is kept constant at R e = 10 5 . The effects of fin angle ( θ = 0 ° and   45 ° ) and the flexural rigidity ( 3 ≤ E I ∗ ≤ 20 ) on the wake structure, displacement amplitude, Strouhal number as well as lift and drag coefficients are investigated. The results showed that the flexural rigidity of plates had a significant influence on the hydrodynamic coefficients. Indeed, an increase in the non-dimensional flexural rigidity enhanced the drag coefficient, and then it approximately remained constant. For θ = 0 ° a n d E I ∗ ≤ 5 , the averaged drag coefficient is approximately constant and equal to 6.1. As the E I ∗ increases, the drag coefficient increases rapidly and reaches the value of 14.37 for E I ∗ = 20. In the second configuration ( θ = 45 o ) , values of the drag coefficient for low ( E I ∗ = 5.33 ) and high ( E I ∗ = 20 ) values of flexural rigidity are equal to 7.81 and 8.79, respectively.

Hydrodynamic characteristics of flow past a circular cylinder with four attached flexible plates in a high Reynolds number turbulent flow

A numerical study of incompressible and unsteady flow past a slotted circular cylinder was carried out in a laminar regime. Three different-shaped slits (converging, diverging, and parallel) were installed symmetrically about the cylinder's horizontal axis. A finite volume-based open-source computational toolbox, OpenFOAM, was used to compute the flow field in the Reynolds number (Re) range of 60–180. The results showed the presence of the periodic vortex shedding for all Re ranges and the slotted cylinders were effectively able to suppress it. The surface vorticity over the slotted cylinder surface increased by 16%–23% as compared to the normal cylinder. The momentum injection to the cylinder base reduced the strength of vortex shedding and shedding frequency for the slotted cylinders. Pressure recovery was improved due to an increase in the pressure coefficient which further led to a decrease in the pressure drag. The total drag coefficient for the slotted cylinders was lower than the normal cylinders for Re &gt; 70. Also, a comparatively lower Strouhal number was observed for the slotted cylinders. 

Unsteady flow past slotted circular cylinders in laminar regime: Effect of slit shapes and Reynolds number

Fins or fairings are typically streamlined structures employed to reduce the vortex-induced unsteady forces acting on a bluff body by preventing shear layer roll-up in the near-wake region. In this work, fins would refer to thin plate-like structures attached tangentially to the bluff body's top and bottom surfaces. Of particular interest here are flexible fins that can undergo static deformation or coupled fluid-elastic vibrations due to the non-linear interactions with the shear layer from a circular cylinder and the roll-up of shear layers at the trailing edge of the fin. We present a numerical analysis to realize the effect of fin flexibility on the performance with regard to vortex-induced forces by varying non-dimensional flexural rigidity, KB∈ [0.01, 10], of the fins. Two-dimensional simulations are carried out for a fixed non-dimensional fin mass ratio, m*=0.1, and Reynolds number, Re = 100. In this study, we consider two fins attached tangentially to the top and bottom surfaces of a fixed circular cylinder. We show that as the flexibility of the fins increases progressively, the stability of the fins is lost and the fins undergo a coupled flapping motion. As a function of KB, three distinct dynamic response regimes of the flexible fins are identified: (i) fixed-point stability for KB>1, (ii) periodic outward flapping 0.025≤KB≤0.1, and (iii) periodic flapping about the initial position with large amplitudes KB<0.025. Flexibility and inclination angle of fins are observed to be effective in minimizing the vortex-induced forces.

Narendran Kumar

Papers

Influence of flexible fins on vortex-induced load over a circular cylinder at low Reynolds number