• Journal of Aerospace System Engineering
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• v.17 no.6
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• pp.54-62
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• 2023

Flapping-wing air vehicles, well known for their free vertical take-off and excellent flight capability, are currently under intensive development and research. While most of the studies have explored the effect of various parameters of synchronized motions on the unsteady aerodynamics of flapping wings, limited attention has been given to the effect of nonsynchronous motions on the unsteady aerodynamic characteristics of flapping wings. In the present study, we conducted a numerical analysis to investigate the unsteady aerodynamic characteristics of an airfoil flapping with different frequency ratios between pitch and heave oscillations. We identified the motions and angle of attacks due to nonsynchronous motions. It was found that the synchronous motion produced thrust with zero lift, but the nonsynchronous motion generated a large lift with little drag. The aerodynamic characteristics of the airfoil undergoing the non-synchronous motion were also analyzed using the vorticity distributions and the pressure coefficient around and on the airfoil. When r was equal to 0.5, larger leading and trailing edge vortices were observed compared to the case when r was equal to 1.0, and these vortices significantly affected the aerodynamic characteristics of the airfoil undergoing the nonsynchronous motion. In future, the effect of pitch amplitude on the unsteady aerodynamic characteristics of the airfoil will be studied.

• Wind and Structures
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• v.26 no.4
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• pp.215-229
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• 2018

The appearance of a rivulet at the upper surface of a stay cable is responsible for rain-wind-induced vibration (RWIV) of cables of cable-stayed bridges. However, the formation mechanism of the upper rivulet and its aerodynamic effects on the stay cable has not been fully understood. Large eddy simulation (LES) method is used to investigate flow around and aerodynamics of a circular cylinder with an upper rivulet at a Reynolds number of 140,000. Results show that the mean lift coefficients of the circular cylinder experience three distinct stages, zero-lift stage, positive-lift stage and negative-lift stage as the rivulet located at various positions. Both pressure-induced and friction-induced aerodynamic forces on the upper rivulet are helpful for its appearance on the upside of the stay cable. The friction-induced aerodynamic forces, which have not been considered in the previous theoretical models, may not be neglected in modeling the RWIV. In positive-lift stage, the shear layer separated from the upper rivulet can reattach on the surface of the cylinder and form separation bubbles, which result in a high non-zero mean lift of the cylinder and potentially induces the occurrence of RWIV. The separation bubbles are intrinsically unsteady flow phenomena. A serial of small eddies first appears in the laminar shear layer separated from the upper rivulet, which then coalesces and reattaches on the side surface of the cylinder and eventually sheds into the wake.

• Journal of the Korean Institute of Gas
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• v.20 no.2
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• pp.23-29
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• 2016

Heat exchanger tube array in a heat recovery steam generator is exposed to the hot exhaust gas flow and it could cause the flow induced vibration, which could damage the heat exchanger tube array. It is needed for the structural safe operation of the heat exchanger to establish the characteristics of flow induced vibration in the tube array. The researches for the flow induced vibration of typical heat exchangers have been conducted by using single cicular tube or circular tube array and the nondimensional PSD(Power Spectral Density) function with the Strouhal number, fD/U, had been derived by experimental method. From the present study, the basis for the application of flow induced vibration to the heat recovery steam generator tube array would be prepared. For the previous mentioned purpose, the present CFD analysis introduced a single fin tube and calculated with the unsteady laminar flow over the single fin tube. The characteristics of vortex shedding and lift and drag fluctuation over the fin tube was investigated. The derived nondimensional lift PSD was compared with the results of the previous experimental studies and the characteristics of lift and drag PSD over a single fin tube was established from the present CFD study.

• Journal of computational fluids engineering
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• v.13 no.4
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• pp.33-38
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• 2008

For study on the unsteady blockage effect, flows around a rotationally oscillating circular cylinder with relatively low forcing frequency in closed test-section wind tunnels have been numerically investigated by solving compressible Navier-Stokes equations. The numerical scheme is based on a node-based finite-volume method with the Roe's flux-difference splitting and an implicit time-integration method coupled with dual time-step sub-iteration. The computed results of the oscillating cylinder in the test section showed that the fluctuations of lift and drag are augmented by the blockage effects. The drag further increases because of low base pressure. The pressure on the test section wall shows the harmonics having the oscillating and the shedding frequencies contained in the blockage effect.

• Journal of computational fluids engineering
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• v.17 no.2
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• pp.71-77
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• 2012

Numerical simulations are carried out for flow over a circular cylinder controlled by the momentum forcing which is generated by a pair of plasma actuators symmetrically mounted on the cylinder surface. A popular and empirical plasma model is used for the spatial distribution of momentum forcing. In this study, we consider two different types of actuation, i.e., steady and unsteady (or pulsed) actuation. In the unsteady actuation, the actuation is turned on and off periodically, its frequency being a control parameter. The objective of this study is to investigate the effects of actuator location and actuation frequency on the flow structures and the forces on the cylinder. Results show that the cylinder wake can be effectively controlled by proper actuator location. For example, when the actuators are located at $120^{\circ}$ from the stagnation point, vortex shedding is completely suppressed with the boundary layer almost fully attached to the surface, resulting in drag reduction and lift elimination.

• 한국전산유체공학회:학술대회논문집
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• 2006.10a
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• pp.126-129
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• 2006

Many researchers have made an effort to explain flight mechanism of flapping insects. As a result, several unsteady mechanisms about lift generation in insect flight have been proposed. But it has a limits to elucidate insect's forward flight and abrupt thrust, because most of these are about insect's hovering flight. For this reason, the objective of this paper is to simulate "Figure-of-eight motion" of insect's wing during tethered flight for comprehending aerodynamic property in insect's forward flight.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.27-30
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• 2002

As computer capacity has been progressed continuously, the studies of the flow characteristics have been performing by the numerical methods actively. In this study, 3-dimensional unsteady incompressible Wavier-Stokes equation was solved by numerical method using the fractional step method with the fourth order compact pade' scheme to achieve high accuracy To validate the present code and algorithm, 3D flow-field around a cylinder was simulated. The drag coefficient and lift coefficient were computed and, then, compared with experiment. The present code will be tailored to LES simulation for more accurate turbulent flow analysis.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.1386-1392
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• 2000

This paper presents a new prediction method of radiated noise from grille of the airconditioning appliance. Laminar vortex sheddings behind a circular cylinder are simulated by solving two dimensional unsteady incompressible Navier-Stokes equation. The Finite Elements Method(FEM) and unstructured grid generation technique are applied to solve, the unsteady lift/drag coefficients are obtained to compute far-field noise using Lighthill's acoustic analogy. Grille is divided into some cylinder segments, and radiated noise from grille is obtained by summing noise generated from each segment. The effects of changing cross section of cylinder and grille geometry are studied. And sound pressure levels radiated from typical H-type grille are measured in KAIST anechoic wind tunnel at various inflow conditions and compared with numerical predictions.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.206-209
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• 2011

During landing approach, an airplane could experience dynamic unstable motion by the combination of a gust and elevator control to cancel the disturbances. This situation is dangerous and could lead to a loss of an airplane. In this paper, numerical analysis was used to study the effect of pitch oscillating 2-D high lift devices in a landing condition. Experimental data on a pitching naca0012 airfoil was used for code validation. Dynamic characteristics of an airfoil, single slotted flap for mid-class passenger aircraft were analyzed. Unsteady Navier-Stokes analysis was performed with Spalart-Allmaras turbulence model for separation dominant low speed flow. As a result, flow hysteresis of a flapped airfoil was more complex than that of an oscillating airfoil. So, dynamic analysis of a flap in a landing condition is very important for operational safety.

• Smart Structures and Systems
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• v.6 no.7
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• pp.867-887
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• 2010

An energy-based variational approach is used for structural dynamic modeling of the IPMC (Ionic Polymer Metal Composites) flapping wing. Dynamic characteristics of the wing are analyzed using numerical simulations. Starting with the initial design, critical parameters which have influence on the performance of the wing are identified through parametric studies. An optimization study is performed to obtain improved flapping actuation of the IPMC wing. It is shown that the optimization algorithm leads to a flapping wing with dimensions similar to the dragonfly Aeshna Multicolor wing. An unsteady aerodynamic model based on modified strip theory is used to obtain the aerodynamic forces. It is found that the IPMC wing generates sufficient lift to support its own weight and carry a small payload. It is therefore a potential candidate for flapping wing of micro air vehicles.