The flow control of a circular cylinder by using a flexible filament has been numerically investigated in this work. The cylinder is either fixed or elastically mounted, and the filament is attached to the base of the cylinder. Its leading end is fixed and its trailing end is free to flap. To execute the numerical simulation and deal with the fluid-structure interaction (FSI) of the filament as well, an improved immersed boundary-lattice Boltzmann method (IB-LBM) is presented. As compared to the conventional IB-LBM for handling the FSI of a filament, the current method can incorporate the mass effect of the filament and no user-defined spring parameter is needed to calculate the interaction force on the filament. After validating the employed method, the effects of the filament on the flow control of the cylinder are systematically examined by varying the bending coefficient (Kb) and length (L) of the filament. The laminar flow with a Reynolds number of 150 is considered in this study. Based on the numeri...

Flow control of a circular cylinder by using an attached flexible filament

Forward flights of a bilaterally symmetrically flapping butterfly modeled as a four-link rigid-body system consisting of a thorax, an abdomen, and left and right wings are numerically simulated. The joint motions of the butterflies are adopted from experimental observations. Three kinds of the simulations, distinguished by ways to determine the position and attitude of the thorax, are carried out: a tethered simulation, a prescribed simulation, and free-flight simulations. The upward and streamwise forces as well as the wake structures in the tethered simulation, where the thorax of the butterfly is fixed, reasonably agree with those in the corresponding tethered experiment. In the prescribed simulation, where the thoracic trajectories as well as the joint angles are given by those observed in a free-flight experiment, it is confirmed that the butterfly can produce enough forces to achieve the flapping flights. Moreover, coherent vortical structures in the wake and those on the wings are identified. The generation of the aerodynamic forces due to the vortical structures are also clarified. In the free-flight simulation, where only the joint angles are given as periodic functions of time, it is found that the free flight is longitudinally unstable because the butterfly cannot maintain the attitude in a proper range. Focusing on the abdominal mass, which largely varies owing to feeding and metabolizing, we have shown that the abdominal motion plays an important role in periodic flights. The necessity of control of the thoracic attitude for periodic flights and maneuverability is also discussed.

/pdf/aerodynamic-forces-and-vortical-structures-in-flapping-vbsuepl01d.pdf

Aerodynamic forces and vortical structures in flapping butterfly's forward flight

Vortex-induced vibration and structure instability for a circular cylinder with flexible splitter plates

The effect of a splitter plate on the flow around a finite prism

Many insects are well adapted to long-distance migration despite the larger energetic costs of flight for small body sizes. To optimize wing design for next-generation flying micro-robots, we analyse butterfly wing shapes and wing orientations at full scale using numerical simulations and in a low-speed wind tunnel at 2, 3.5 and 5 m s−1. The results indicate that wing orientations which maximize wing span lead to the highest glide performance, with lift to drag ratios up to 6.28, while spreading the fore-wings forward can increase the maximum lift produced and thus improve versatility. We discuss the implications for flying micro-robots and how the results assist in understanding the behaviour of the butterfly species tested.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsfs.2016.0087

Aerodynamic evaluation of wing shape and wing orientation in four butterfly species using numerical simulations and a low-speed wind tunnel, and its implications for the design of flying micro-robots.

Flow-induced vibration control of a circular cylinder by using flexible and rigid splitter plates

The passive oscillations of inverted flags are investigated both experimentally and theoretically in this paper. First, the force and energy distributions of inverted flags, which contain elastic and inertia components, are analysed based on the experimental data. Two main differences between inverted and conventional flags are found: (1) the elastic energy of a conventional flag is concentrated near the free end, while the fixed end of an inverted flag presents the largest elastic energy; and (2) the elastic component is several orders of magnitude greater than the inertia component for an inverted flag, while they are of the same magnitude for a conventional flag. Second, a linear analysis shows that the critical flow velocities obtained from the experiments at small mass ratios are scattered around the theoretical curve of wavenumber parallel inverted flags, the theoretical results show that two of all coupled flapping modes are dominant with most parameters. These findings could contribute to a better understanding of the passive oscillations of inverted flags.

Passive oscillations of inverted flags in a uniform flow

The dye visualization experiments show that a dual leading-edge vortex (LEV) structure exists on the suction side of a simplified butterfly model of Papilio ulysses at α = 8°−12°. Furthermore, the results of particle image velocimetry (PIV) measurement indicate that the axial velocity of the primary (outer) vortex core reaches the lower extreme value while a transition from a “wake-like” to a “jet-like” axial velocity profile occurs. The work reveals for the first time the existence of dual LEV structure on the butterfly-like forward-sweep wing configuration.

Dual leading-edge vortex structure for flow over a simplified butterfly model

Hydrogen-bubble visualization technique was applied in the investigation of vortex structure for flow over a heaving cylinder attached with a flexible tail along the wake central-line in a water channel. Wake structures have been mapped in the flexible tail length-frequency (L/D, St) phase space with the flexible tail length of L/D = 2–5 and the oscillation Strouhal number of St = 0–0.34. Four wake modes were identified as: (1) 2S_Karman vortex mode—a Karman-vortex-like structure with two single vortices formed per cycle in the near wake; (2) 2S_reverse Karman vortex mode—a reverse Karman-vortex-like structure with two single vortices per cycle; (3) 2P mode—a bifurcated vortex pair structure with two pairs of vortices per cycle; (4) P + S mode—an unstable vortex structure with three vortices per cycle as a transient mode. Moreover, the typical case of each wake mode was further examined by particle image velocimetry, and the evolutions of vortex structures for the four wake modes were studied in detail.

Yao-Wei Hu

Papers

Flow-induced vibration control of a circular cylinder by using flexible and rigid splitter plates

Passive oscillations of inverted flags in a uniform flow

Dual leading-edge vortex structure for flow over a simplified butterfly model

Vortex structure for flow over a heaving cylinder with a flexible tail

Flow-structure interactions of two tandem inverted flags in a water tunnel