Soon-Duck Kwon

A T-shaped piezoelectric cantilever for fluid energy harvesting

This paper proposes a T-shaped piezoelectric cantilever for generating electric power from fluid flow. The working principle of the device is based on aeroelastic flutter and utilizes a bimorph cantilever with T-shape which hastens occurrence of flutter at a low fluid speed. A prototype device (100×60×30 mm3) was tested in a wind tunnel. The device was found to provide power from a wind speed of 4 m/s and a continuous peak electrical power output of 4.0 mW. The simplicity of the present device consisting of only a bimorph cantilever is considered to be cost effective.

Effect of vehicle weight on natural frequencies of bridges measured from traffic-induced vibration

Recently, ambient vibration test (AVT) is widely used to estimate dynamic characteristics of large civil structures. Dynamic characteristics can be affected by various environmental factors such as humidity, intensity of wind, and temperature. Besides these environmental conditions, the mass of vehicles may change the measured values when traffic-induced vibration is used as a source of AVT for bridges. The effect of vehicle mass on dynamic characteristics is investigated through traffic-induced vibration tests on three bridges; (1) three-span suspension bridge (128m +404m + 128m), (2) five-span continuous steel box girder bridge (59m + 3@95m + 59m), (3) simply supported plate girder bridge (46m). Acceleration histories of each measurement location under normal traffic are recorded for 30 minutes at field. These recorded histories are divided into individual vibrations and are combined into two groups according to the level of vibration ; one by heavy vehicles such as trucks and buses and the other by light vehicles such as passenger cars. Separate processing of the two groups of signals shows that, for the middle and long-span bridges, the difference can be hardly detected, but, for the short span bridges whose mass is relatively small, the measured natural frequencies can change up to 5.4%.

Electromagnetic energy harvester with repulsively stacked multilayer magnets for low frequency vibrations

This paper investigates the applicability of an electromagnetic generator with repulsively stacked magnets for harvesting energy from traffic-induced bridge vibrations. First, the governing equation for electro-mechanical coupling is presented. The magnetic field for repulsive pole arrangements is discussed and the model is validated from a magnet falling test. The detailed design, fabrication, and test results of a prototype device are presented in the paper. An experimental vibration shaker test is conducted to assess the performance of the energy harvester. Field test and numerical simulation at the 3rd Nongro Bridge in South Korea shows that the device can generate an average power of 0.12 mW from an input rms acceleration of 0.25 m s−2 at 4.10 Hz. With further frequency tuning and design improvement, an average power of 0.98 mW could be potentially harvested from the ambient vibration of the bridge.

Wind farm power maximization based on a cooperative static game approach

The objective of this study is to improve the cost-effectiveness and production efficiency of wind farms using cooperative control. The key factors in determining the power production and the loading for a wind turbine are the nacelle yaw and blade pitch angles. However, the nacelle and blade angles may adjust the wake direction and intensity in a way that may adversely affect the performance of other wind turbines in the wind farm. Conventional wind-turbine control methods maximize the power production of a single turbine, but can lower the overall wind-farm power efficiency due to wake interference. This paper introduces a cooperative game concept to derive the power production of individual wind turbine so that the total wind-farm power efficiency is optimized. Based on a wake interaction model relating the yaw offset angles and the induction factors of wind turbines to the wind speeds experienced by the wind turbines, an optimization problem is formulated with the objective of maximizing the sum of the power production of a wind farm. A steepest descent algorithm is applied to find the optimal combination of yaw offset angles and the induction factors that increases the total wind farm power production. Numerical simulations show that the cooperative control strategy can increase the power productions in a wind farm.

Sequential numerical procedures for predicting flutter velocity of bridge sections

A sequential numerical procedure for predicting the flutter onset velocity of bridges without wind tunnel tests is presented. The proposed procedure consists of two phases. In the first phase, the unsteady aerodynamic forces acting on moving rigid bodies are evaluated from the finite element analyses. The Reynolds numbers of the simulated flows range from 400 to 2400. In the second phase, the flutter onset velocities are estimated by solving the aeroelastic equations. The results of numerical simulation for the four generic bridge deck sections show fairly good agreement with those of experiments despite the discrepancy in Reynolds number between the computations and experiments.

Power evaluation of flutter-based electromagnetic energy harvesters using computational fluid dynamics simulations

H- and T-shaped cross sections are known to be susceptible to rotational single-degree-of-freedom aerodynamic instabilities. Here, such self-excited aerodynamic response of a T-shaped cantilever structure is used to extract energy, which is then converted into electric power through an electromagnetic transducer. The complex fluid–structure interaction between the cantilever harvester and wind flow is analyzed numerically and experimentally. To study the dynamic response of the cantilever and estimate the power output from the harvester, numerical simulations based on the vortex particle method are performed to determine the aerodynamic damping of the harvester section and to analyze the stability behavior of the section. The estimated aerodynamic damping parameter together with the mechanical and electrical damping parameters in the harvester are used to find the critical wind speed of flutter onset as well as the optimum load resistance. Wind tunnel experiments are conducted to validate the simulation results.

Mitigating the Effects of Wind on Suspension Bridge Catwalks

In the current study, wind tunnel tests and buffeting analyses were conducted for a catwalk structure of a suspension bridge under turbulent winds. From the wind tunnel tests, it was found that the Reynolds number effects on the aerostatic coefficients were negligible for the catwalk floor systems. Simple design formulas were proposed to estimate the aerostatic coefficients of catwalks with various solidity ratios and angles of attack. The buffeting analysis results revealed that tying the catwalk to an erecting main cable may be an effective structural countermeasure to reduce the lateral displacements of the catwalk. Stay ropes fastening the catwalk and bridge tower were also found to be effective at alleviating the lateral displacements.

A data-driven approach for cooperative wind farm control

This paper discusses a data-driven, cooperative control strategy to maximize wind farm power production. Conventionally, every wind turbine in a wind farm is operated to maximize its own power production without taking into account the interactions among the wind turbines in a wind farm. Such greedy control strategy, when an upstream wind turbine attempts to maximize its power production, can significantly lower the power productions of the downstream wind turbines and, thus, reduces the overall wind farm power production. As an alternative, we propose a cooperative wind farm control strategy that determines and executes the optimum coordinated control actions that maximize the total wind farm power production. To determine the optimum coordinated control actions of the wind turbines, we employ Bayesian Ascent (BA), a probabilistic optimization method constructed based on Gaussian Process regression and the trust region concept. Wind tunnel experiments using 6 scaled wind turbine models are conducted to assess (1) the effectiveness of the cooperative control strategy in improving the power production, and (2) the efficiency of the BA algorithm in determining the optimum control actions of the wind turbines using only the input control actions and the output power measurement data.

Improvement of Aerodynamic Performance and Energy Supply of Bridges Using Small Wind Turbines

Abstract This study proposes a methodology of using small wind turbines for dual purposes, improving the aerodynamic performance of flexible bridges and wind-energy harvesting. A method for the proper placement of small wind turbines on flexible bridges was proposed according to the analogy of conventional aerodynamic appendages. The effectiveness of the proposed method was investigated using the wind-tunnel tests for a bridge girder. It was found from the tests that the wind turbine attached analogously to a fairing was effective for decreasing the vortex-induced vibration of a bridge girder. The optimal spanwise interval of the wind turbines was three times the turbine diameter depending on the distance from the girder edge. The annual energy productions of the wind turbines at the bridge were evaluated from the wind velocity and its probability model at the site. The results of the current study show the general applicability of wind turbines for the improvement of aerodynamic performance and energy su...

Wind Tunnel Test of Aerodynamic Forces and Wind Pressures Acting on Muilti-layer Radom in Active Phased Array Radar

In this paper, we investigated the sensitivity of aerostatic force coefficients of multi-layer radom in the various wind speeds. The test was conducted in KOCED Wind Tunnel Center in Chonbuk National University, and wind speeds were in the range from 5 m/s to 26 m/s in order to determine the Reynolds number independence. The test results of present multi-layer radom were not affected by the Reynolds number, The maximum positive pressure coefficient was found to be 1.08 at the center of the front of the plane in angle of attack of 0 degree, the maximum negative pressure coefficient was -2.03 at the upper right corner in angle of attack of 120 degree, while maximum drag coefficient was 1.11 in angle of attack of 180 degree.