• Wind and Structures
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• v.38 no.5
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• pp.381-398
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• 2024

This study presents a mode analysis of 3D turbulent velocity data around a square-section building model to identify the dynamic system for Kármán-type vortex shedding. Proper orthogonal decomposition (POD) was first performed to extract the significant 3D modes. Magnitude-squared coherence was then applied to detect the phase consistency between the modes, which were roughly divided into three groups. Group 1 (modes 1-4) depicted the main vortex shedding on the wake of the building, with mode 2 being controlled by the inflow fluctuation. Group 2 exhibited complex wake vortexes and single-sided vortex phenomena, while Group 3 exhibited more complicated phenomena, including flow separation. Subsequently, a third-order polynomial regression model was used to fit the dynamics system of modes 1, 3, and 4, which revealed average trend of the state trajectory. The two limit cycles of the regression model depicted the two rotation directions of Kármán-type vortex. Furthermore, two characteristic periods were identified from the trajectory generated by the regression model, which indicates fast and slow motions of the wake vortex. This study provides valuable insights into 3D mode morphology and dynamics of Kármán-type vortex shedding that helps to improve design and efficiency of structures in turbulent flow.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.10
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• pp.3251-3261
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• 1996

Flows were measured in an unshrouded centrifugal impeller. By using a single slanted hot-wire probe and a Kiel probe mounted on the impeller hub disk, the 3-D relative velocities and the rotary stagnation pressures were measured in seven circumferential planes between the inlet and outlet of the impeller rotating at 700 rpm, which diameter is 0.39 meter, and the static pressures and the slip factor at the impeller outlet were estimated from the measured values. Measurements were made for three flow rates corresponding to zero incidence and two others with the greater and the smaller one than zero. From the measured data in these flow rates, the followings were investigated in the impeller passage, the variation of the primary and secondary flows, the leakage flows, the wake's position and its size, the static pressure rise and the loss production mechanism. Furthermore the static pressure and the slip factor were compared with the results of inviscid Quasi-3D calculation.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.759-764
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• 2003

Numerical simulations are conducted to investigate the mechanism of hovering flight by single flapping wing, and to examine the effect of the phase difference between the fore- and hindwings in hovering flight by two flapping wings. The numerical method used is based on an immersed boundary method in Cartesian coordinates. The Reynolds number considered is Re=150 based on the maximum translational velocity and chord length of the wing. For single flapping wing, the stroke plane angles are $0^{\circ}$, $30^{\circ}$, $60^{\circ}$, $75^{\circ}$ and $90^{\circ}$ and the downstroke angles of attack are varied for each stroke angle. Results show that for each stroke plane angle, there is an optimal angle of attack to maximize the vertical force. Below the stroke angle of $60^{\circ}$, wake capturing reduces the negative vertical force during the upstroke. For two flapping wings, The phase lags of the hindwing are $0^{\circ}$, $90^{\circ}$, $180^{\circ}$ and $270^{\circ}$. The amplitudes of the stroke are 2.5 and 4.0 times the chord length at each phase lag. The results show that maximum vertical force is generated when the phase lag is zero, and the amplitude of the vertical force is minimum at the phase lag of $180^{\circ}$.

• 한국신재생에너지학회:학술대회논문집
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• 2011.11a
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• pp.40.2-40.2
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• 2011

A Counter-rotating wind turbine is one of the new concepts that are proposed to increase the performance of a wind turbine. It has two rotors rotating in the same axis, and it is known that its power coefficient can reach to 0.64 in the ideal case. While the BEMT is widely used to analyze the aerodynamic performance of wind turbines, the analysis of the counter-rotating wind turbine by using it is limited due to the aerodynamic interaction between the two rotors. In this study, the vortex lattice method is used to consider the effect of the front rotor on the rear rotor of the counter-rotating wind turbine and calculate the aerodynamic performance of it. The power and thrust sharing in the two rotors of the counter-rotating wind turbine are predicted and the total power and thrust are compared with that of a single rotor. Moreover, the wake convection and expansion rate is also compared with that of a single rotor.

• Journal of the Korean Society of Visualization
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• v.8 no.3
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• pp.12-21
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• 2010

Swirling flows are found in very wide range of applications, for examples, cyclone separators, spraying machines, heat exchangers and jet pumps, ect. Relatively, little work has been done on the swirl flow using flow visualization techniques. This study deals with many visualization techniques to study on swirling flow. These techniques are related to oil films methods, smoke, dye liquids, liquid crystal, stroboscope light, smoke wire, white light, naphthalene sublimation, LDV(lase doppler Velocimetry) and PIV(particle image velocimetry). The present work has handled single, annular, carved tube, swirl expansion and swirl wake using several visualization methods in the vertical and horizontal circular tube.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.104-109
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• 2000

The modes of oscillation and radiated acoustic fields of compressible flows over open cavities are investigated computationally. The compressible Navier-Stokes equations are solved for two-dimensional cavities with laminar boundary layers upstream. The high-order and high-resolution numerical schemes are used for the evaluation of spatial derivatives and the time integration. Physically correct numerical boundary conditions are implemented to produce time-accurate solutions in the whole computation domain. The computational domain is large enough to directly resolve a portion of the radiated acoutic field. The results show a transition from a shear layer mode, for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear layer mode is well characterized by Rossiter modes and these oscillations lead to intense upstream acoustic radiation dominated by a single frequency. The wake mode is characterized instead by a large-scale vortex shedding. Acoustic radiation is more intense, with multiple frequencies present.

• Journal of Advanced Marine Engineering and Technology
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• v.36 no.3
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• pp.358-367
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• 2012

A study on tidal turbine using CFD simulation has been an economical and reliable method. However, large flow fields with multi-turbine arrays require high computer performance. Actuator disc theory therefore is widely applied. Actuator disc is the concept that imitates actual turbine by means of an energy absorption disc which has the same dimension and characteristics. Turbines installed in array may have disturbance effects on one another. Thus, the subject of this study is to analyze the far wake of these tidal turbines and compare to single turbine case. The main objects are to analyze two turbines positioned longitudinally at different spaces.

• Journal of the Society of Naval Architects of Korea
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• v.41 no.1
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• pp.15-22
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• 2004

Twin-skeg type stern shapes are recently adopted for very large commercial ships. However it is difficult to apply a CFD system to a hull form having twin-skeg, since grid topology around a twin-skeg type stern is more complicated than that of a conventional single-screw ship, or of an open-shaft type twin-screw ship with center-skeg. In the present study a surface mesh generator and a multi-block field grid generation program have been developed for twin-skeg type stern. Furthermore, multi-block flow solvers are utilized for potential and viscous flow analysis around a twin-skeg type stern The present computational system is applied to a 15,000TEU container ship with twin-skeg to prove the applicability. Wave profiles and wake distribution are calculated using the developed flow analysis tools and the results are compared with towing tank measurements.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.638-642
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• 2008

Computational methods based on the solution of the flow model are widely used for the analysis of lowspeed, inviscid, attached-flow problems. Most of such methods are based on the implementation of the internal Dirichlet boundary condition. In this paper, the time-domain panel method uses the piecewise constant source and doublet singularities. The present method utilizes the time-stepping loop to simulate the unsteady motion of the rotary wing blade. The wake geometry is calculated as part of the solution with no special treatment. To validate the results of aerodynamic characteristics, the typical blade was chosen such as, Caradonna-Tung blade and present results were compared with the experimental data and the other numerical results in the single blade condition and two blade condition. This isolated rotor blade model consisted of a two bladed rotor with untwisted, rectangular planform blade. Computed flow-field solutions were presented for various section of the blade in the hovering mode.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.9 s.252
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• pp.921-928
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• 2006

Wake-induced boundary-layer transition on a NACA0012 airfoil with zero angle of attack is experimentally investigated in periodically passing wakes under the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensities $(Tu_{\infty})$ at the leading edge of the airfoil are 0.5 and 3.5%, respectively. The Reynolds number (Rec) based on chord length (C) of the airfoil is $2.0{\times}10^5$, and Strouhal number (Stc) of the passing wake is about 1.4. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The patch under the high free-stream turbulence $(Tu_{\infty}=3.5%)$ grows more greatly in laminar-like regions compared with that under the low turbulence $(Tu_{\infty}=0.5%)$ in laminar regions. The former, however, does not greatly change the turbulence level in very near-wall region while the latter does it. At further downstream, the former interacts vigorously with high environmental turbulence inside the pre-existing transitional boundary layer and gradually loses its identification, whereas the latter keeps growing in the laminar boundary layer. The calmed region is more clearly observed under the lower free-stream turbulence level and with the receding wakes.