• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.22 no.5
• /
• pp.573-583
• /
• 1998

Effects of the oscillating incoming flow on vortex shedding and lock-on behind a square cylinder are investigated using numerical simulations at a Reynolds number of 100. Vortex shedding occurred at low forcing frequencies of the incoming flow similar to the natural vortex shedding. As the forcing frequency further increases, the shedding frequency decreases to the half of the forcing freqnency. For a sufficiently large frequency, vortex shedding returns to the natural vortex shedding irrespective of the forcing amplitude. Also, the lock-on region becomes wider with higher forcing amplitudes. The phase diagram between the drag and lift shows a simple periodic behavior in the lock-on region, while a complicated periodic phase relation is observed when there is no lock-on.

• Proceedings of the KSME Conference
• /
• 2007.05a
• /
• pp.415-420
• /
• 2007

Creatures in nature flap their wings to generate fluid dynamic forces that are required for the locomotion. Small-size creatures do not use flapping wings. Thus, it is questionable at which Reynolds number the propulsion using the flapping wings are effective. In this paper, the onset conditions of the thrust generation from the combined motion of flat plates (heaving, pitching in the motion and also tandem, biplane in the array) is investigated using a Lattice Boltzmann method. To solve the pitching motion of the plate on the regularly spaced lattices, 2-D moving boundary condition was implemented. The present method is validated by comparing the wake patterns behind a oscillating circular cylinder and its hydrodynamic characteristics with the CFD results. Present method can be applied to the design of micro flapping propulsors for biomedical use.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.10 no.5
• /
• pp.706-712
• /
• 1986

The unsteay heat transfer from an isothermal circular cylinder in fluctuating cross flow is simulated numerically, for the case where the amplitude of the oscillating velocity is small compared with the mean velocity. By solving the linear perturbation equations derived from the unsteady full Navier-Stokes and the energy equations, the amplitude and the phase of heat transfer response are obtained in the range of Reynolds number R$_{3}$ < 40. The effects of the velocity, the cylinder radius and the frequency on the response are expressed graphically in terms of the normalized velocity and the cylinder radius.

• Proceedings of the KSME Conference
• /
• 2007.05a
• /
• pp.429-434
• /
• 2007

This paper presents an experimental and parametric study of a biomimetic fish robot actuated by the Lightweight Piezo-composite Actuator(LIPCA). The biomimetic aspects in this work are the oscillating tail beat motion and shape of caudal fin. Caudal fins that resemble fins of BCF(Body and Caudal fin) mode fish were made in order to perform parametric study concerning the effect of caudal fin characteristics on thrust production at an operating frequency range. The observed caudal fin characteristics are the shape, area, and aspect ratio. It was found that a high aspect ratio caudal fin contributes to high swimming speed. The fish robot was propelled by artificial caudal fins shaped after thunniform-fish and mackerel caudal fins, which have relatively high aspect ratio, produced swimming speed as high as 2.364 cm/s and 2.519 cm/s, respectively, for 300 Vpp input voltage excited at 0.9 Hz. Thrust performance of the biomimetic fish robot was examined by Strouhal number, Froude number, Reynolds number, and Net forward force.

• 한국가시화정보학회:학술대회논문집
• /
• 2001.12a
• /
• pp.86-99
• /
• 2001

Vortex lock-on or resonance in the flow behind a circular cylinder is investigated from a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K\acute{a}rm\acute{a}n$ and streamwise vortices in the wake-transition regime at the Reynolds number 360. Streamwise vortices at the lock-on and natural shedding states are observed, as well as the changes in the wake region with the change of the shedding frequency of lock-on state. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. At the lock-on state, the $K\acute{a}rm\acute{a}n$ vortices are observed to be more disordered by the increased strength and spanwise wavelength of the streamwise vortices, which leads to a strong three-dimensional motion. Recirculation and vortex formation region at the lock-on state is reduced as the oscillating frequency is increased. By comparing the Reynolds stresses at the lock-on and natural shedding states, $\bar{u'u'}\;and \;\bar{u'u'}$ at the lock-on state are concentrated on the shear layer around the cylinder. The $\bar{u'u'}\;at\;f_o/f_n=2.0$ has a large value near the centerline, compared with that of other cases. Considering the traces of maximum of u', in the wake region near the cylinder, wake width at the lock-on state is wider than that at the natural shedding state.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.9 no.4
• /
• pp.194-200
• /
• 1997

A two-dimensional shallow-water flow around a cavity driven by a sinusoidally oscillating external flow was studied numerically. A container model of "T" shape was constructed in the numerical computation for comparison with the experimental observation. The numerical computation shows that the aspect ratio of the cavity is not much affecting the overall flow pattern, and for the aspect ratio 2, the deep region of the cavity has a stagnant flow motion. At larger Reynolds number, the flow field is characterized by many small vortices which are not present in the flow visualization. The flow pattern in the external region is in good agreement with the experimentally recorded particle trajectories. It turns out that two large coherent vortices situated in the exterior region of the cavity are responsible for clockwise and counterclockwise drift motions, in large scale, of particles.particles.

• Journal of computational fluids engineering
• /
• v.6 no.2
• /
• pp.22-31
• /
• 2001

The two-dimensional unsteady incompressible viscous flow behind rectangular bluff bodies between two parallel plates was analyzed numerically. The steady state flow and the vortex flow behind rectangular bluff bodies in the channel were investigated for two regimes i.e., the laminar(Re = 100, 300, 500) and the turbulent flows(Re = 10⁴∼10/sup 6/). The vortex shedding was generated by a physical disturbance(6%) numerically imposed at the rear of the bluff bodies for a short time. It was observed that the perturbed flow became periodic after a transient period. And in the case of unsteady inflow, the sinusoidal pulsatile flow was applied as the inlet condition in the turbulent flow of Reynolds number of 1.0×10/sup 5/. FLUENT code was employed to solve the problems. The power-law scheme was used to get stable linearized equations and the PISO algorithm was applied to finding the solution of them.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.25 no.11
• /
• pp.1527-1534
• /
• 2001

Phase averaged velocity fields in the near wake region behind a square cylinder have been (successfully) obtained using randomly sampled PIV data sets. The Reynolds number based on the flow velocity and the vertex height was 3,900. To identify the phase information, we examined the magnitude of circulation and the center of peak vorticity. The center of vorticity was estimated from lowpass filtered vorticity contours (LES decomposition) adopting a sub-pixel searching algirithm. Due to the sinusoidal nature of firculation which is closely related to the instantaneous vorticity, the location of peak voticity fits well with a sine curve of the circulation magnitude. Conditionally-averaged velocity fields represent the barman vortex shedding phenomenon very well within 5 degrees phase uncertainty. The oscillating nature of the separated shear layer and the separation bubble at the top surface are clearly observed. With the hot-wire measurements of Strouhal frequency, we found thats the convection velocity changes its magnitude very rapidly from 25 to 75 percent of the free stream velocity along the streamwise direction when the flow passes by the recirculation region.

• International Journal of Aeronautical and Space Sciences
• /
• v.14 no.3
• /
• pp.201-209
• /
• 2013

Much numerical and experimental research has been done for the flow around an oscillating airfoil. The main research topics are vortex shedding, dynamic stall phenomenon, MAV's lift and thrust generation. Until now, researches mainly have been concentrated on analyzing the wake flow for the variation of frequency and amplitude at a low angle of attack. In this study, wake structures and acoustic wave propagation characteristics were studied for a plunging airfoil at high angle of attack. The governing equations are the Navier-Stokes equation with LES turbulence model. OHOC (Optimized High-Order Compact) scheme and 4th order Runge-Kutta method were used. The Mach number is 0.3, the Reynolds number is, and the angle of attack is from $20^{\circ}$ to $50^{\circ}$. The plunging frequency and the amplitude are from 0.05 to 0.15, and from 0.1 to 0.2, respectively. Due to the high resolution numerical method, wake vortex shedding and pressure wave propagation process, as well as the propagation characteristics of acoustic waves can be simulated. The results of frequency analysis show that the flow has the mixed characteristics of the forced plunging frequency and the vortex shedding frequency at high angle of attack.

• 한국가시화정보학회:학술대회논문집
• /
• 2007.11a
• /
• pp.134-137
• /
• 2007

The temporal evolution of wake behind a circular cylinder oscillating rotationally with a relatively high forcing frequency has been investigated experimentally using a dynamic PIV technique. Experiments were carried out with varying the frequency ratio $F_R\;(=f_f/f_n)$ in the range from 0.0 (stationary) to 1.6 at oscillation amplitude of ${\theta}_A=30^{\circ}$ and Reynolds number of $Re=4.14{\times}10^3$. Depending on the forcing condition ($F_R$), the flow was divided into three regimes; non-lock-on ($F_R=0.4$), transition ($F_R=0.8$, 1.6) and lock-on regimes ($F_R=1.0$) with markedly different flow structure in the near-wake region behind the cylinder. When the frequency ratio was less than 1.0 ($F_R{\le}1.0$), the rotational oscillatory motion of the cylinder decreased the length of the vortex formation region and enhanced the mutual interaction between large-scale vortices across the wake centerline. The entrainment of ambient fluid seemed to play an important role in controlling the near-wake flow and shear-layer instability. However, the flow characteristics changed markedly beyond the lock-on flow regime ($F_R=1.0$) due to high-frequency forcing. At $F_R=1.6$, the mutual interactions between the vortices shed from both sides of the cylinder were not so strong. Thereby, the flow entrainment and momentum transfer into the wake center region were reduced. In addition, the size of the large-scale vortices decreased since the lateral extent of the wake was suppressed.