Chungwook Sim

The Influence of Stable Boundary Layer Flows on Wind Turbine Fatigue Loads

Near-neutral atmospheric stability conditions form the basis for wind turbine design. This is surprising since such near-neutral conditions occur in so-called transition periods only twice each day (around sunrise and sunset). Unstable conditions occur during the day and stable conditions occur generally at night. During nighttime stable conditions, turbulence is typically generated by shear and destroyed by negative buoyancy. Wind shear (both magnitude and direction) under stable conditions is much larger in comparison to that during neutral conditions. Moreover, stable boundary layer (SBL) flows are often accompanied by low-level jets (LLJs); these LLJs can be low enough to impact today’s large utility-scale turbines and thus influence loads. This study compares turbulence, turbine loads, and accumulated fatigue damage for a utility-scale wind turbine in stable versus neutral atmospheric conditions. Our focus is on the varying simulated atmospheric flows that result from (i) different surface cooling rates (which control buoyancy destruction); and (ii) different geostrophic winds (which control shear generation). Inflow turbulence time series are generated and applied over the rotor plane of a 90-meter hub-height 5MW wind turbine based on Large-Eddy Simulation (LES) with refined dynamic sub-grid scale modeling; similarly, neutral boundary layer flows are generated using conventional Fourier techniques for comparison. These simulated wind velocity fields are then fed into an aeroelastic model of the selected wind turbine and turbine fatigue loads are analyzed. Some differences are seen between fatigue loads resulting from neutral versus stable conditions but these are not significant especially when missing high-frequency content in the inflow turbulence from LES-generated flows is augmented by fractal interpolation.

A Comparison of Wind Turbine Load Statistics for Inflow Turbulence Fields Based on Conventional Spectral Methods and Large Eddy Simulation

Efficient spatial and temporal resolution of simulated inflow velocity fields is important in order to derive wind turbine load statistics for design. There are not many published studies that have addressed the issue of such optimal space-time resolution. This study investigates turbine extreme and fatigue load statistics for a utility-scale 5MW wind turbine. Load statistics, spectra, and time-frequency analysis representations are compared for various alternative space and time resolutions employed in inflow turbulence field simulation. Conclusions are drawn regarding adequate resolution in space of the inflow turbulence simulated on the rotor plane prior to extracting turbine load statistics. Similarly, conclusions are drawn with regard to what constitutes adequate temporal filtering to preserve turbine load statistics. This first study employs conventional Fourier-based spectral methods for simulating velocity fields for neutral atmospheric stability conditions. In the second part of this study, large eddy simulation (LES) is employed with fairly coarse resolutions in space and time, justified on the basis of the earlier Fourier-based stochastic simulations, to again establish turbine load statistics. A comparison of extreme and fatigue load statistics is presented for the two approaches used in inflow field generation. The use of LES-generated flows to establish turbine load statistics in this manner is computationally more expensive but the study is partly justified in order to evaluate the ability of LES to be used as an alternative to the more conventional Fourier-based stochastic approaches. A more compelling reason for using LES is that for the stable boundary layer, it is not possible to generate realistic inflow velocity fields except by using LES. This study sets the stage for future turbine load computations in such stable conditions where low-level jets, large speed and direction shears across the rotor, etc. can possibly cause large turbine loads.

April 16, 2016 Ecuador Earthquake Damage Assessment Survey

A damage assessment survey of 169 low-rise reinforced concrete buildings was conducted following the 16 April 2016 Ecuador earthquake. Forty-four percent of the buildings surveyed sustained severe structural damage. Using the collected data, seismic vulnerability indices were calculated to examine their correlation with damage observations. It was found that 92% of the buildings with observed severe structural damage had calculated wall and column index pairs (WI, CI) that satisfied the relation WI+CI/2 < 0.2%. The frequency of damage was lower for higher-priority index values, defined as the sum of CI+WI. Furthermore, frequency of damage in buildings with captive columns was observed to decrease with window height-to-column height ratios of more than 20%.

A Cyberplatform for Sharing Scientific Research Data at DataCenterHub

DataCenterHub is a new solution for preserving, sharing, and discovering data produced by scientific research. Datasets are organized by experiments, with a simple common structure for metadata, file collections, and key parameters. Researchers associate annotations, reports, media, and measurements to each experiment, and interactive viewers interpret data by type and use so that they can be investigated before downloading. Parameters are extracted for discovery of key data otherwise hidden in files. DataCenterHub provides an alternative discipline-neutral solution, with the goal of helping researchers classify and share data for easy discovery and exploration.

Increasing Bridge Deck Service Life: Volume I—Technical Evaluation

Deterioration of bridge decks is a primary factor limiting the lifespan of bridges especially in cold climates where deicing salts are commonly used. While controlling deck cracking or decreasing the permeability and porosity of concrete can improve performance and service life, chloride and moisture ingress as well as cracking cannot be eliminated. Full-depth cracks which are caused by restrained shrinkage allow for corrosive conditions at early ages for both the top and bottom reinforcement mats. Therefore, the use of corrosion-resistant reinforcement is essential to mitigate deterioration of bridge decks. The objective of this research program to examine the efficacy of using alternative materials in a bridge deck from both technical and economic perspectives. For the technical evaluation (Volume I), a three phase experimental investigation was conducted considering a wide range of corrosion-resistant reinforcing materials. These materials included stainless steels, microcomposite steel, and coated steels considering a variety of metallic and nonmetallic coatings. The first phase evaluated the bond between corrosion-resistant reinforcement and concrete using lap splice tests. The second phase evaluated the cracking behavior of slabs reinforced with corrosion-resistant reinforcement. Finally, the third phase evaluated corrosion resistance under uncracked and cracked conditions using macrocell test specimens. Transverse steel was also tied to the longitudinal steel to simulate actual bridge deck conditions. Recommendations are provided on development and splice lengths for both conventional black and corrosion-resistant reinforcing steel, control of cracks widths, as well as the selection, design, and construction of corrosion-resistant reinforcement. For the economic evaluation (Volume II), a decision support methodology and associated spreadsheet tool for robust analysis of the cost-effectiveness of alternative material types for bridge deck reinforcement was developed. The two evaluation criteria are agency and user costs, and the input data that influence this criteria include the deck service life, material process, discount rate, detour length, and bridge size. The methodology incorporates analytical techniques that include life cycle analyses to evaluate the long-term cost and benefits of each material over the bridge life; Monte Carlo simulation to account for the probabilistic nature of the input variables; stochastic dominance to ascertain the probability distribution of the outcome that a specific reinforcement material is superior to others; and analytical hierarchical process to establish appropriate weights for the agency and user costs. The study methodology is demonstrated using a case study involving three reinforcement material alternatives: traditional (epoxy-coated) steel, zinc-clad steel, and stainless steel. Through this study, it is demonstrated that the use of corrosion-resistant reinforcing materials can significantly increase bridge deck life, reduce agency and user costs associated with bridge deck rehabilitation and maintenance, and thus lower the financial needs for long-term preservation of bridges.

Flexural Experiment of PSC-Steel Mixed Girders and Evaluation for Analyses on Tangentional Stiffness of Connection

This study was performed to evaluate joint behavior of prestressed concrete(PSC)-steel mixed girders through the flexural test of 14 beams according to embedded length, amount of reinforcing steel, stud arrangement, and prestressing force. All test beams were failed by turns of desertion of reinforcing steel, stud, and steel plate. From test results, prestressing force was more effective on performance of connection than stud arrangement and reinforcing steel. And the spacing of stud is also more effective than embedding length. This paper also presented 3D nonlinear analysis considering the slip of composite section as well as the static load tests of PSC-steel mixed girders. According to the slip modulus, the nonlinear analysis showed that the behavior of hybrid girders could be divided into three parts as full-composite, partial-composite and non-composite. However, the experimental results showed that the PSC-steel hybrid girders with shear connectors took the part of partial composite action in ultimate load stage. In addition, it was founded that stud shear connectors and welded reinforcements were contributed to improve the ultimate strength of hybrid girders for about 20%.