• International Journal of Naval Architecture and Ocean Engineering
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• v.7 no.5
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• pp.906-919
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• 2015

The Deep Draft Semi-Submersible (DDS) concepts are known for their favourable vertical motion performance. However, the DDS may experience critical Vortex-Induced Motion (VIM) stemming from the fluctuating forces on the columns. In order to investigate the current-induced excitation forces of VIM, an experimental study of flow characteristics around four square-section cylinders in a square configuration is presented. A number of column spacing ratios and array attack angles were considered to investigate the parametric influences. The results comprise flow patterns, drag and lift forces, as well as Strouhal numbers. It is shown that both the drag and lift forces acting on the cylinders are slightly different between the various L/D values, and the fluctuating forces peak at L/D = 4.14. The lift force of downstream cylinders reaches its maximum at around ${\alpha}=15^{\circ}$. Furthermore, the flow around circular-section-cylinder arrays is also discussed in comparison with that of square cylinders.

• Wind and Structures
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• v.17 no.4
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• pp.379-397
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• 2013

Large Eddy Simulations (LES) were carried out to investigate the aerodynamic characteristics of a rectangular cylinder with side ratio B/D=5 at Reynolds number Re=22,000 (based on cylinder thickness). Particular attention was devoted to the effects of velocity shear in the oncoming flow. Time-averaged and unsteady flow patterns around the cylinder were studied to enhance understanding of the effects of velocity shear. The simulation results showed that the Strouhal number has no significant variation with oncoming velocity shear, while the peak fluctuation frequency of the drag coefficient becomes identical to that of the lift coefficient with increase in velocity shear. The intermittently-reattached flow that features the aerodynamics of the 5:1 rectangular cylinder in non-shear flow becomes more stably reattached on the high-velocity side, and more stably separated on the low-velocity side. Both the mean and fluctuating drag coefficients increase slightly with increase in velocity shear. The mean and fluctuating lift and moment coefficients increase almost linearly with velocity shear. Lift force acts from the high-velocity side to the low-velocity side, which is similar to that of a circular cylinder but opposite to that of a square cylinder under the same oncoming shear flow.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.11
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• pp.849-854
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• 2013

Experimental study was conducted to investigate aerodynamic interference effect of wing tip vortex in formation flight of high speed aircraft. In formation flight, wing tip vortex produced by leading aircraft can affect on the aerodynamic characteristics of trailing aircraft. The interference effect of flow is varied with distances between wing tips of leading and trailing aircraft. It is confirmed, in this study, that the interference of wing tip vortex generated from the leading aircraft makes the aerodynamic forces and moments of the trailing aircraft with the vertical or horizontal positions of the trailing aircraft. Especially, the lift coefficients of trailing aircraft were highly increased at y/b=-0.125, z/b=0.0 or deeply decreased at y/b=-0.5, z/b=0.38. The interfering pattern of wing tip vortices from two aircraft is precisely observed.

• Wind and Structures
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• v.30 no.5
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• pp.525-532
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• 2020

This work numerically investigates the effects of Reynolds number Re_D (= 100 - 150), cross-sectional aspect ratio AR = ( 0.25 -1.0), and attack angle α (= 0° - 90°) on the forces, Strouhal number, and wake of an elliptical cylinder, where Re_D is based on the freestream velocity and cylinder cross-section height normal to the freestream flow, AR is the ratio of the minor axis to the major axis of the elliptical cylinder, and α is the angle between the cylinder major axis and the incoming flow. At Re_D = 100, two distinct wake structures are identified, namely 'Steady wake' (pattern I) and 'Karman wake followed by a steady wake (pattern II)' when AR and α are varied in the ranges specified. When Re_D is increased to 150, an additional wake pattern, 'Karman wake followed by secondary wake (pattern III)' materializes. Pattern I is characterized by two steady bubbles forming behind the cylinder. Pattern II features Karman vortex street immediately behind the cylinder, with the vortex street transmuting to two steady shear layers downstream. Inflection angle α_i = 32°, 37.5° and 45° are identified for AR = 0.25, 0.5 and 0.75, respectively, where the wake asymmetry is the greatest. The α_i effectively distinguishes the dependence on α and AR of force and vortex shedding frequency at either Re_D. In Pattern III, the Karman street forming behind the cylinder is modified to a secondary vortex street. At a given AR and α, Re_D = 150 renders higher fluctuating lift and Strouhal number than Re_D = 100.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.6
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• pp.653-663
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• 2012

The study considers the high maneuverability flight and path optimization is conducted to investigate the appropriate generation of the lift and thrust considering the angle of the stroke plane. The path optimization problem is defined according to the various purposes of the high maneuverability flight. The flying purposes are to maximize thrust force, lift force and both lift and thrust forces. The flapping motion of the airfoil is made by a combined sinusoidal plunging and pitching motion in each problem. The optimization process is carried out by using well-defined surrogate models. The surrogate model is determined by the results of two-dimensional computational fluid dynamics analysis. The Kriging method is used to make the surrogate model and a genetic algorithm is utilized to optimize the surrogate model. The optimization results show the flapping motions for the high maneuverable flight. The effects on the generation of lift and thrust forces are confirmed by analyzing the vortex.

• 한국전산유체공학회:학술대회논문집
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• 1998.11a
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• pp.185-190
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• 1998

A Numerical simulation on the flow around a Rudder with blowing is performed by Finite Volume Method. The governing equations are three dimensional incompressible Navier-Stokes equation and Continuity equation, Flow field around a finite Rudder including tip vortex is simulated and the change of the lift force by blowing is analyzed.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.1
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• pp.9-18
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• 2016

The slender structures such as high rise building or marine riser are highly susceptible to dynamic force exerted by fluid-structure interactions among which vortex-induced vibration(VIV) is the main cause of dynamic unstability of the structural system. If VIV occurs in natural frequency regime of the structure, fatigue failure likely happens by so-called lock-in phenomenon. This study presents the numerical analysis of dynamic behavior of both structure and fluid in the lock-in regimes and investigates the subjacent phenomena to hold the resonance frequency in spite of the change of flow condition. Unsteady and laminar flow was considered for a two-dimensional circular cylinder which was assumed to move freely in 1 degree of freedom in the direction orthogonal to the uniform inflow. Fluid-structure interaction was implemented by solving both unsteady flow and dynamic motion of the structure sequentially in each time step where the fluid domain was remeshed considering the movement of the body. The results show reasonable agreements with previous studies and reveal characteristic features of the lock-in phenomena. Not only the lift force but also drag force are drastically increasing during the lock-in regime, the vertical displacement of the cylinder reaches up to 20% of the diameter of the cylinder. The correlation analysis between lift and vertical displacement clearly show the dramatic change of the phase difference from in-phase to out-of-phase when the cylinder experiences lock-in. From the results, it can be postulated that the change of phase difference and flow condition is responsible for the resonating behavior of the structure during lock-in.

• Journal of Korean Society of Water and Wastewater
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• v.19 no.3
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• pp.288-294
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• 2005

This study was conducted to evaluate the grit removal efficiency of tornado block-type vortex grit separator. Vortex grit separator was manufactured for this study, and it was characterized by the impeller and tornado block installed in separator. Impeller was installed to increase water velocity in the separator and tornado block was installed to increase the grit lifting efficiency. Pilot study was also conducted in A sewage treatment plant (STP) in Gyeonggi province from November 2003 to May 2004 (64 days). Major findings are as follows. 1. Impeller was proven to increase water velocity in the grit separator, especially in low flow rate. This influence will increase separation ratio of organics from grits, preventing those organics from sedimentation. 2. Sand (with 0.2~0.3mm size) removal efficiency was over 98 % and 96 %, at the flow rate of $500m^3/day$ and $750m^3/day$ under the condition that impeller rotation velocity kept at 15 rpm. Originally that grit separator was designed to have the capacity of $500m^3/day$. $750m^3/day$ was tried to investigate the performance of this type of grit separator under overload condition. Stable grit removal was still available to the extent of 150% of designed capacity. 3. It took less than 3 minutes for the grit separator to completely lift out 3 kgs of 0.2-0.3 mm sized, settled sand at the bottom to 2,060 mm high above water surface. But it showed the tendency to spend a little more time on lifting the grit as the grit size and the vertical height of the lift increased. 4. During experimental duration in A STP, it was found that the average amount of inlet grit was about 981 g/day (160~1,685 g/day) under $500m^3/day$ of operation condition, but it varied so severely during the experimental duration. After classification of discharged grit according to its size, grit with 0.3-0.42 mm size was found as largest part of output.

• Journal of the Korean Institute of Gas
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• v.19 no.4
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• pp.35-40
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• 2015

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 and the nondimensional PSD(Power Spectral Density) function with the Strouhal number, fD/U, had been derived by experimental method. The present study examined the results of the previous experimental researches for the nondimensional PSD characteristics by CFD analysis and the basis for the application of flow induced vibration to the heat recovery steam generator tube array would be prepared from the present CFD analysis. For the previous mentioned purpose, the present CFD analysis introduced a single circular cylinder and calculated with the unsteady laminar flow over the cylinder. The characteristics of vortex shedding and lift and drag fluctuation over the cylinder was investigated. The derived nondimensional PSD was compared with the results of the previous experimental researches and the characteristics of lift and drag PSD over a single circular cylinder was established from the present CFD study.

• 한국전산유체공학회:학술대회논문집
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• 2009.11a
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• pp.151-158
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• 2009

To implement the insects' flapping flight for developing flapping MAVs(micro air vehicles), the unsteady flow characteristics of the insects' forward flight is investigated. In this paper, two-dimensional FSI(Fluid-Structure Interaction) simulations are conducted to examine realistic flow features of insects' flapping flight and to examine the flexibility effects of the insect's wing. The unsteady incompressible Navier-Stokes equations with an artificial compressibility method are implemented as the fluid module while the dynamic finite element equations using a direct integration method are employed as the solid module. In order to exchange physical information to each module, the common refinement method is employed as the data transfer method. Also, a simple and efficient dynamic grid deformation technique based on Delaunay graph mapping is used to deform computational grids. Compared to the earlier researches of two-dimensional rigid wing simulations, key physical phenomena and flow patterns such as vortex pairing and vortex staying can still be observed. For example, lift is mainly generated during downstroke motion by high effective angle of attack caused by translation and lagging motion. A large amount of thrust is generated abruptly at the end of upstroke motion. However, the quantitative aspect of flow field is somewhat different. A flexible wing generates more thrust but less lift than a rigid wing. This is because the net force acting on wing surface is split into two directions due to structural flexibility. As a consequence, thrust and propulsive efficiency was enhanced considerably compared to a rigid wing. From these numerical simulations, it is seen that the wing flexibility yields a significant impact on aerodynamic characteristics.