Vortices around Dragonfly Wings

Dragonfly beats its wings independently, resulting in its superior maneuverability. Depending on the magnitude of phase difference between the fore- and hind-wings of dragonfly, the vortical structures and their interaction with wings become significantly changed, and so does the aerodynamic performance. In this study, we consider hovering flights of modelled dragonfly with three different phase differences (phi=-90, 90, 180 degrees). The three-dimensional wing shape is based on that of Aeschna juncea (Norberg, 1972), and the Reynolds number is 1,000 based on the maximum translational velocity and mean chord length. The numerical method is based on an immersed boundary method (Kim et al., 2001). In counter-stroke (phi=180 degree), the wing-tip vortices from both wings are connected in the wake, generating an entangled wing-tip vortex (e-WTV). A strong downward motion induced by this vortex decreases the lift force in the following downstroke (Kweon and Choi, 2008). When the fore-wing leads the hind-wing (phi=90 degree), the hind-wing is submerged in the vortices generated by the fore-wing and suffers from their induced downwash flow throughout the downstroke, resulting in a significant reduction of lift force. On the other hand, when the hind-wing leads the fore-wing (phi=-90 degree), the e-WTV is found only near the start of hind-wing upstroke. In the following downstroke of hind-wing, most of the e-WTV disappears and the hind-wing is little affected by this vortex, which produces relatively large lift force.Comment: Gallery of Fluid Motio

Large-eddy simulation of flow around an airfoil on a structured mesh

The diversity of flow characteristics encountered in a flow over an airfoil near maximum lift taxes the presently available statistical turbulence models. This work describes our first attempt to apply the technique of large-eddy simulation to a flow of aeronautical interest. The challenge for this simulation comes from the high Reynolds number of the flow as well as the variety of flow regimes encountered, including a thin laminar boundary layer at the nose, transition, boundary layer growth under adverse pressure gradient, incipient separation near the trailing edge, and merging of two shear layers at the trailing edge. The flow configuration chosen is a NACA 4412 airfoil near maximum lift. The corresponding angle of attack was determined independently by Wadcock (1987) and Hastings & Williams (1984, 1987) to be close to 12 deg. The simulation matches the chord Reynolds number U(sub infinity)c/v = 1.64 x 10(exp 6) of Wadcock's experiment

Hydrodynamic role of longitudinal dorsal ridges in a leatherback turtle swimming

Leatherback sea turtles (Dermochelys coriacea) are known to have a superior diving ability and be highly adapted to pelagic swimming. They have five longitudinal ridges on their carapace. Although it was conjectured that these ridges might be an adaptation for flow control, no rigorous study has been performed to understand their hydrodynamic roles. Here we show that these ridges are slightly misaligned to the streamlines around the body to generate streamwise vortices, and suppress or delay flow separation on the carapace, resulting in enhanced hydrodynamic performances during different modes of swimming. Our results suggest that shapes of some morphological features of living creatures, like the longitudinal ridges of the leatherback turtles, need not be streamlined for excellent hydro- or aerodynamic performances, contrary to our common physical intuition.ope

Very large-scale structures and their effects on the wall shear-stress fluctuations in a turbulent channel flow up to

Direct numerical simulation of a fully developed turbulent channel flow has been carried out at three Reynolds numbers, 180, 395, and 640, base

Control of Flow Separation in a Turbulent Boundary Layer Using Time-Periodic Forcing

A time-periodic blowing/suction is provided to control turbulent separation in a boundary layer using direct numerical simulation. The blowing/suction is given just before the separation point, and its nondimensional forcing frequency ranges from F* = fL(b)/U-infinity = 0.28-8.75, where f is the forcing frequency, L-b is the streamwise length of uncontrolled separation bubble, and U1 is the freestream velocity. The size of separation bubble is minimum at F* = 0.5. At low forcing frequencies of F* <= 0.5, vortices generated by the forcing travel downstream at convection velocity of 0.32-0.35 U1, bring high momentum toward the wall, and reduce the size of separation bubble. However, at high forcing frequencies of F* >= 1.56, flow separation disappears and appears in time during the forcing period. This phenomenon occurs due to high wall-pressure gradients alternating favorably and adversely in time. A potential flow theory indicates that this rapid change of the wall pressure in time occurs through an inviscid mechanism. Finally, it is shown that this high-frequency forcing requires a large control input power due to high pressure work.N

Aerodynamics of a golf ball with grooves

In this study, we investigate the aerodynamics of our newly designed golf ball that does not have dimples but grooves on its surface. The smooth part of its surface is approximately 1.7 times that of a golf ball with dimples. We directly measure the drag and lift forces on two versions of this golf ball in the ranges of real golf-ball velocity and rotational speed, and compare them with those of smooth and dimpled balls. At zero spin, the drag coefficient of our balls shows a rapid fall-off at a Reynolds number similar to that of a dimpled ball and maintains nearly a constant value (lower by 50% than that of smooth ball) at higher Reynolds numbers. At non-zero spin, the lift-to-drag ratio of one version of our ball is higher by 5%-20% in the supercritical Reynolds number regime than that of a dimpled ball, but it is lower otherwise. With the measured drag and lift forces, we predict the trajectories of our balls and compare them to those of smooth and dimpled balls for the same initial conditions. The flight distances of our balls are larger by 148%-202% than that of smooth ball and shorter by 6%-10% than that of a dimpled ball.close0