• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.17 no.6 s.123
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• pp.498-506
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• 2007

Using wind tunnel, experimental approaches were employed to investigate fluidelastic instability of tube bundles, subjected to uniform cross flow. There are several flow-induced vibration excitation mechanisms, such as fluidelastic instability, periodic wake shedding resonance, turbulence-induced excitation and acoustic resonance, which could cause excessive vibration in shell-and tube heat exchanges. Fluidelastic is the most important vibration excitation mechanism for heat exchanger tube bundles subjected to cross flow. The system comprised of cantilevered flexible cylinder(s) and rigid cylinders of normal square array, In order to see the characteristics of flow in tube bundles, particle image velocimetry was used. From a practical design point of view, Fluidelastic instability may be expressed simply in terms of dimensionless flow velocity and dimensionless mass-damping. The threshold flow velocity for dynamic instability of cylinder rows is evaluated and the data for design guideline is proposed for the tube bundles of normal square array.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2006.05a
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• pp.941-945
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• 2006

In the development of electricity generating wind turbines for wind farm application, only two types have survived as the methods of power regulation; stall regulation and fun span pitch control. The sound measurement procedures of IEC 61400-11 are applied to field test and evaluation of noise emission from each of 1.5 MW and 660 kW wind turbine generators (WTG) utilizing the stall regulation and the pitch control for the power regulation, respectively. Apparent sound power level, wind speed dependence and third-octave band levels are evaluated for both of WTGs. It is found that while 1.5 MW WTG using the stall control is found to emit lower sound power than 660 kW one using the pitch control at low wind speed (below 8 m/s), sound power from the former becomes greater than that of the latter in the higher wind speed. Equivalent continuous sound pressure levels (ECSPL) of the stall control type of WTG vary more widely with wind speed than those of the pitch control type of WTG These characteristics are believed to be strongly dependent on the basic difference of the airflow around the blade between the stall regulation and the pitch control types of WTG. These characteristics according to the methods of power regulation lead to the very different noise emission characteristics of WTG depending on the seasons because the average wind speed in summer is lower than the critical velocity over which the airflow on the suction side of blade in the stall types of WT are separated. These results propose that, in view of environmental noise regulation, the developer of wind farm should give enough considerations to the choice of power regulation of their WTG based on the weather conditions of potential wind farm locations.

• Journal of Bio-Environment Control
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• v.4 no.2
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• pp.167-174
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• 1995

This study was carried out to find a way of improving the windproof capability of greenhouse foundations. Generally, greenhouses are often collapsed due to the strong winds, because they are very light weight structures. In such a critical situations, the foundations are very often subjected to uplift and vibration at the same time. This paper describes both the wind disaster of greenhouses by the typhoon FAEY and the uplift resistance of greenhouse foundations. Followings are the results obtained from this study ; Judging from the view point of year round cultural aspects, it is recommended that some measures be taken for the preventions of greenhouse film ruptures because greenhouse structural damages are found to be directly associated with the local rupture of cover film. In the case of surveyed area, movable pipe-houses or pipe-houses of 1-2W type were found to be completely destroyed when the maximum instantaneous wind velocity was over 30m/sec or so. In the case of movable pipe-houses, the uplift resistance of greenhouse was expected to increase with the increase of pipe diameter and/or the embedment pipe length. But at present situations there is a limitation in raising the uplift resistance of movable pipe-house, because pipe diameters as well as pipe lengths customarily selected by farmers are quite a much limited.

• Structural Engineering and Mechanics
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• v.78 no.5
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• pp.519-528
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• 2021

One of the most efficient designs of solar trackers for photovoltaic panels is the single-axis tracker, which holds the panels along a torque tube that is driven by a motor at the central section. These trackers have evolved to become extremely slender structures due to mechanical optimization against static load and the need of cost reduction in a very competitive market. Owing to the corresponding decrease in mechanical resistance, some of these trackers have suffered aeroelastic instability even at moderate wind speeds, leading to catastrophic failures. In the present work, an analytical and experimental approach has been developed to study that phenomenon. The analytical study has led to identify the dimensionless parameters that govern the motion of the panel-tracker structure. Also, systematic wind tunnel experiments have been carried out on a 3D aeroelastic scale model. The tests have been successful in reproducing the aeroelastic phenomena arising in real-scale cases and have allowed the identification and a close characterization of the phenomenon. The main results have been the determination of the critical velocity for torsional galloping as a function of tilt angle and a calculation methodology for the optimal sizing of solar tracker shafts.

• Wind and Structures
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• v.4 no.4
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• pp.333-352
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• 2001

A two dimensional discrete vortex method (DIVEX) has been developed to predict unsteady and incompressible flow fields around closed bodies. The basis of the method is the discretisation of the vorticity field, rather than the velocity field, into a series of vortex particles that are free to move in the flow field that the particles collectively induce. This paper gives a brief description of the numerical implementation of DIVEX and presents the results of calculations on a recent suspension bridge deck section. The predictions for the static section demonstrate that the method captures the character of the flow field at different angles of incidence. In addition, flutter derivatives are obtained from simulations of the flow field around the section undergoing vertical and torsional oscillatory motion. The subsequent predictions of the critical flutter velocity compare well with those from both experiment and other computations. A brief study of the effect of flow control vanes on the aeroelastic stability of the bridge is also presented and the results from DIVEX are shown to be in accordance with previous analytical and experimental studies. In conclusion, the results indicate that DIVEX is a very useful design tool in the field of wind engineering.

• Journal of Bio-Environment Control
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• v.17 no.3
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• pp.197-203
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• 2008

This study was performed to check the structural safety of modified 1-2W Greenhouses to be utilized fur growing Paprika. This type of greenhouse was derived from being remodeled by enhancing the column height of conventional 1-2W type greenhouses. According to the results of structural analysis performed by SAP-2000, there was not significant change in critical snow depth in spite of increasing the column height of 1.2 m by welding. But the critical wind velocities were shown to be $26.0\sim4l.0m/s$, which were $3\sim18%$ lower wind velocities compared with those critical velocities estimated for typical type of 1-2W greenhouse. Under the wind loads, those maximum section forces such as shear force, axial force, and bending moment, together with the deformed frame shape of strained greenhouse, were almost similar in both typical type and modified type. Maximum bending moment of column was found at eave's height of column on windward side. Under the snow loads, those maximum section forces such as shear farce, axial force, and bending moment, together with the deformed frame shape of strained greenhouse, were almost similar in both typical type and modified type. Maximum section forces except axial force was found at eave's height of column. Maximum axial force was found at inner column. Soil bearing capacity together with the total foundation resistance against wind upheaval was found to be consistently safe enough to resist to both wind load and snow load.

• Structural Engineering and Mechanics
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• v.14 no.4
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• pp.491-504
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• 2002

A state-space method is proposed to analyze the aerodynamically coupled flutter problems of long-span bridges based on the modal coordinates of structure. The theory about complex modes is applied in this paper. The general governing equation of the system is converted into a complex standard characteristic equation in a state space format, which contains only two variables. The proposed method is a single-parameter searching method about reduced velocity, and it need not choose the participating modes beforehand and has no requirement for the form of structure damping matrix. The information about variations of system characteristics with reduced velocity and wind velocity can be provided. The method is able to find automatically the lowest critical flutter velocity and give relative amplitudes, phases and energy ratios of the participating modes in the flutter motion. Moreover, the flutter analysis of Jiangyin Yangtse suspension bridge with 1385 m main span is performed. The proposed method has proved reliable in its methodology and efficient in its use.

• Wind and Structures
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• v.17 no.5
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• pp.515-535
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• 2013

The present study aims to investigate characteristics of the flow structures around the Ahmed body by using both experimental and numerical methods. Therefore, 1/4 scale Ahmed body having $25^{\circ}$ slant angle was employed. The Reynolds number based on the body height, H and the free stream velocity, U was $Re_H=1.48{\times}10^4$. Investigations were conducted in two parts. In the first part of the study, Large Eddy Simulation (LES) method was used to resolve the flow structures around the Ahmed body, numerically. In the second part of the study the particle image velocimetry (PIV) technique was used to measure instantaneous velocity fields around the Ahmed body. Time-averaged and instantaneous velocity vectors maps, streamline topology and vorticity contours of the flow fields were presented and discussed in details. Comparison of the mean and turbulent quantities of the LES results and the PIV results with the results of Lienhart et al. (2000) at different locations over the slanted surface and in the wake region of the Ahmed body were also given. Flow features such as critical points and recirculation zones in the wake region downstream of the Ahmed body were well captured. The spectra of numerically and experimentally obtained stream-wise and vertical velocity fluctuations were presented and they show good consistency with the numerical result of Minguez et al. (2008).

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.300-303
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• 2008

In the present study, the simple form of the heat transfer equation were suggested to estimate the temperature variation inside the oil pipe in order to determine the thickness of the insulating materials to retain the working oils below the critical temperature. The conservation of the thermal energy at arbitrary time were modeled to one dimensional unsteady equation with the empirical formula or data. The calculating results for non-insulation case showed that the temperature were very sensitive to the thermal convection by the velocity of the external wind. For insulation case, the insulation material which has higher density and specific heat, lower thermal conductivity should be chosen with more brighter coloring outside the pipe in order to retain the working oils below the critical temperature.

• Wind and Structures
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• v.29 no.1
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• pp.33-41
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• 2019

The interaction of head waves with an infinite row of identical, equally spaced, rectangular breakwaters is investigated in the presence of uneven bottom topography. Using linear water wave theory and matched eigenfunction expansion method, the boundary value problem is transformed into a system of linear algebraic equations which are numerically solved to know the velocity potentials completely. Utilizing this method, reflected and transmitted wave energy are computed for different physical parameters along with the wave field in the vicinity of breakwaters. It is observed that the wave field becomes more complicated when the incoming wavelength becomes smaller than the channel width. A critical ratio of the gap width to the channel width, corresponding to the inflection point of the transmitted energy variation, is identified for which 1/3 of the total energy is transmitted. Similarly, depending on the incident wavelength, there is a critical breakwater width for which a minimum energy is transmitted. Further, the accuracy of the computed results is verified by using the derived energy relation.