The mass and spacing ratios of parallel circular cylinder structures change over a wide range in engineering applications. In this study, the effects of mass ratio, spacing ratio and helical wires on ...the wake galloping of tandem circular cylinders are investigated by LES turbulence model at the subcritical Reynolds number regime. The upstream cylinder is fixed and the downstream one is free to respond in the transverse direction. The characteristics of wake galloping of tandem circular cylinders are systematically studied for the cases of mass ratios m∗ = 1.8, 200 and spacing ratios x0/D = 4, 8 and 12. A critical velocity of wake galloping is derived as a function of mass ratio and lift coefficient slope using a wake-stiffness-based model. The suppression of wake galloping by the 4-start helical wires is examined and the maximum amplitude of vibration is found to decrease by 63% because of the positive added damping generated by helical wires.
•For the wake-induced vibration of tandem circular cylinders, the resonant and galloping regimes are combined with low mass ratio m∗ = 1.8 and small spacing ratios x0/D = 4, but separated with high mass ratio m∗ = 200 for x0/D = 4 to 12.•An analytical model for predicting the critical velocity of wake galloping is derived as a function of mass ratio m∗ and lift coefficient slope α using a wake-stiffness-based model. The critical velocity of wake galloping increases with m∗ and decreases with α .•The 4-start helical wires with a pitch ratio of 8 and a diameter ratio of 0.1 are used to suppress the wake galloping of tandem circular cylinder successfully. The positive added damping is generated by helical wires, which is the main reason of wake galloping suppression.
A new analytical wake model for wind turbines, considering ambient turbulence intensity, thrust coefficient and yaw angle effects, is proposed from numerical and analytical studies. First, eight ...simulations by the Reynolds Stress Model are conducted for different thrust coefficients, yaw angles and ambient turbulence intensities. The wake deflection, mean velocity and turbulence intensity in the wakes are systematically investigated. A new wake deflection model is then proposed to analytically predict the wake center trajectory in the yawed condition. Finally, the effects of yaw angle are incorporated in the Gaussian-based wake model. The wake deflection, velocity deficit and added turbulence intensity in the wake predicted by the proposed model show good agreement with the numerical results. The model parameters are determined as the function of ambient turbulence intensity and thrust coefficient, which enables the model to have good applicability under various conditions.
In this study, advanced hydrodynamic models are proposed to predict dynamic response of a floating offshore wind turbine (FOWT) in combined wave and current conditions and validated by laboratory and ...full-scale semi-submersible platforms. Firstly, hydrodynamic coefficient models are introduced to evaluate the added mass and drag coefficients in a wide range of Reynolds numbers. An advanced hydrodynamic model is then proposed to calculate the drag force of cylinder in combined wave and current conditions. The proposed model is validated by the water tank tests in the current-only, wave-only and current-wave conditions and is used to investigate the effect of current on the dynamic response of FOWT. Finally, the full-scale semi-submersible platform used in the Fukushima demonstration project is investigated. It is found that the predicted dynamic responses of platform by the proposed hydrodynamic models are improved by the directional spreading function of the sea wave spectrum and show favorable agreement with the field measurement.
Large eddy simulation is applied to study the turbulent flow fields over a 3-D hill and a 2-D ridge with smooth ground. The mean velocities as well as the turbulent fluctuations are computed. Fairly ...good agreements with experiments validate the accuracy of the numerical model applied in this study. The spectra of the velocities in the wake of the 3-D hill and 2-D ridge, and those over the flat terrain, are calculated. Spectra of velocity fluctuations at selected locations in the wake of topographic features are well reproduced. The examination of spectra shades some lights on the dynamics of the turbulent flow. It is found that the spectra of fluids in the wake are sensitive to the oncoming turbulence condition for the 2-D ridge, which is not true for the 3-D hill. Non-isotropic characteristics of turbulence in the near-wake of topographic features are examined using ratios of Reynolds stress which are also compared with those in IEC 61400-1 (2005).
•Mean wind profiles over 3D-hill and 2D-ridge are well reproduced by LES.•Normal Reynolds stresses at selected locations are well reproduced by LES.•Power spectra of velocity fluctuations are well reproduced by LES.•Non-isotropic turbulence in the near-wake of topographic are examined.
Tornado vortices are investigated using large-eddy simulations of a single vortex evolving into a multi-vortex. Four typical tornado configurations, including a weak vortex, a vortex breakdown, a ...vortex touch-down and a multi-vortex, are investigated to provide detailed information on the turbulent flow fields. The force balances in the radial and vertical directions are also evaluated using axsymmetrically and time averaged results. The local corner swirl ratio is used as an index to describe the surface intensification and the geometry of tornado vortices; the similarity between the simulated tornadoes and a full-scale tornado in nature is examined. Good agreements between the data of scaled tornado in simulation and the observed data of the full-scale tornado in nature are achieved.
•Four typical tornado vortices are successfully simulated using LES.•Characteristics of force balances in the tornado-like vortices are summarized.•Scaled flow fields agree well with those for the Spencer tornado.
Engineered fluorescent protein (FP) chimeras that modulate their fluorescence in response to changes in calcium ion (Ca²⁺) concentration are powerful tools for visualizing intracellular signaling ...activity. However, despite a decade of availability, the palette of single FP-based Ca²⁺ indicators has remained limited to a single green hue. We have expanded this palette by developing blue, improved green, and red intensiometric indicators, as well as an emission ratiometric indicator with an 11,000% ratio change. This series enables improved single-color Ca²⁺ imaging in neurons and transgenic Caenorhabditis elegans. In HeLa cells, Ca²⁺ was imaged in three subcellular compartments, and, in conjunction with a cyan FP—yellow FP—based indicator, Ca²⁺ and adenosine 5′-triphosphate were simultaneously imaged. This palette of indicators paints the way to a colorful new era of Ca²⁺ imaging.
In this study, the availability and the levelized cost of energy (LCOE) are investigated considering failure rate and downtime for onshore wind turbines in Japan. The failure mode effect analysis is ...conducted using the wind turbine failure database collected by the New Energy and Industrial Technology Department Organization (NEDO). The normalized failure rate and downtime between Europe and Japan are comparable. The occurrence rate is similar between Europe and Japan, but the downtime in Japan is much longer than that of Europe. Three cost-reduction scenarios are then proposed to improve availability and to reduce LCOE using assumed failure rate and downtime in each mode based on the industry interview and best practices in Japan. The availability is improved from 87.4% for the baseline scenario to 92.7%, 95.5% and 96.4% for the three scenarios, and LCOE is also reduced from 13.7 Yen/kWh to 11.9, 11.0 and 10.7 Yen/kWh. Finally, the probability distributions of downtime and repair cost are obtained for each failure mode. It is found that the probability distributions of the failure modes with the shortest downtime show similar probability distributions regardless of the size of the assembly. The effects of downtime and repair-cost uncertainties on LCOE are also evaluated.
This study investigates the sectional loads on an elastic semi-submersible platform for a 2 MW FOWT (floating offshore wind turbine) used in the Fukushima demonstration project. A water tank test is ...firstly carried out with an elastic model to study the dynamic responses and sectional loads of the platform in regular and irregular waves. Numerical simulations are then performed using multiple hydrodynamic bodies connected by elastic beams. The dynamic responses of the elastic model are compared to those of a rigid model to clarify the influence of the structural stiffness on the platform motion and mooring tension. The predicted sectional loads on the deck, brace and pontoon by the proposed nonlinear hydrodynamic models show good agreement with the experimental data obtained from the water tank test and a simplified formula is proposed to evaluate the distribution of the moments on the platform. Finally, the structural optimization of the elastic semi-submersible platform is conducted. The sectional moments and fatigue loadings on the pontoons are significantly reduced using the strut between the pontoons since the horizontal wave loads on the side column are dominant and the vertical wave loads acting on the platform are relatively small due to the deep draft.
The numerical simulation by LES turbulent model for translating tornadoes and those over roughness was carried out in a Ward type simulator. The tornado translation was modeled by providing a ...relative motion on the ground and the roughness was simulated through adding a momentum source in the Navier–Stokes equation. The effects of translation and roughness on the flow fields of three typical tornado-like vortices, i.e., vortex breakdown, vortex touching down and multi-vortex, were investigated and the detailed velocity distributions, Reynolds stresses and the pressure on the ground were examined. The similarity of the flow fields after the introduction of translation and ground roughness was also studied. It was found that, at the high elevation, Vc and rc shows the same trend versus the external swirl ratio for stationary and translating tornadoes. However, if the ground is rough, the core radius at high elevation changes greatly. The ground roughness will expand the size of the core. But for the very small swirl ratio cases, the ground roughness shows the effect reducing the core size. The explanation for the evolution of the flow fields due to translation and ground roughness is provided.
•Similarity law still works even tornado is translating or ground is rough.•To reproduce tornado in nature effects from translation have to be considered.•Effects from ground roughness are not same for different types of tornadoes.
The slope of a hill could affect the flow patterns significantly. In the present study, three smooth three dimensional (3D) hills (H2, H4, and H8) and three smooth two dimensional (2D) ridges (R2, ...R4, and R8), with the hill heights increased by factors of 2.0 while the hill radii are kept constant to achieve different slopes, are examined systematically using large eddy simulations (LES). The reattachment of the flow is found to become difficult as the slope increases or as the shape changes from 3D hill to 2D ridge. In addition, the differences of the flow fields between the 3D hills and the 2D ridges will become more evident as the slope increases. Moreover, the mean velocities and the fluctuations for H2 and R2 are almost the same, whereas as the hill slope increases, the differences between the 3D hills and 2D ridges become stronger, and the similarities between the 3D hills and 2D ridges almost disappear for H8 and R8. Based on information available from LES, an analytical model to predict the fractional speed-up ratio is proposed. As for the turbulence structures over the 3D hills, a spiral structure is identified, and the pitch of the spiral becomes narrow as the hill slope increases. When the hill slope further increases to H8, the spiral structures are broken into separated circular tubes. For R4, a hairpin structure is identified, whereas for R8, the hairpin structure disappears, and a vortex with a long spanwise size occurs periodically.
•Differences of flow fields between 3D hill and 2D ridge become more evident as increasing slope.•An analytical model to predict fractional speed-up ratio is proposed by fitting LES data.•For 3D hills, a spiral structure is identified.•For 2D ridges, a hairpin structure is identified when the slope is 0.64.
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