Numerical Investigation of Flow Motion and Performance of A Horizontal Axis Tidal Turbine Subjected to A Steady Current

Horizontal axis tidal turbines have attracted more and more attentions nowadays, because of their convenience and low expense in construction and high efficiency in extracting tidal energy. The present study numerically investigates the flow motion and performance of a horizontal axis tidal turbine with a supporting vertical cylinder under steady current. In the numerical model, the continuous equation and incompressible Reynolds-averaged Navier-Stokes equations are solved, and the volume of fluid method is employed to track free surface motion. The RNG k-ε model is adopted to calculate turbulence transport while the fractional area/volume obstacle representation method is used to describe turbine characteristics and movement. The effects of installation elevation of tidal turbine and inlet velocity on the water elevation, and current velocity, rotating speed and resultant force on turbine are discussed. Based on the comparison of the numerical results, a better understanding of flow structure around horizontal axis tidal turbine and turbine performance is achieved.

Predicting scour beneath subsea pipelines from existing small free span depths under steady currents

An equation was developed to predict current-induced scour beneath subsea pipelines in areas with small span depths,S.Current equations for scour prediction are only applicable to partially buried pipelines.The existence of small span depths(i.e.S/D<0.3)are of concern because the capacity for scour is higher at smaller span depths.Furthermore,it is impractical to perform rectification works,such as installing grout bags,under a pipeline with a small S/D.Full-scale two-dimensional computational fluid dynamics(CFD)simulations were performed using the Reynolds-averaged Navier-Stokes approach and the Shear stress transport k-ωturbulence model.To predict the occurrence of scour,the computed maximum bed shear stress beneath the pipe was converted to the dimensionless Shields parameter,and compared with the critical Shields parameter based on the mean sediment grain size.The numerical setup was verified,and a good agreement was found between model-scale CFD data and experimental data.Field data were obtained to determine the mean grain size,far field current velocity and to measure the span depths along the surveyed pipe length.A trend line equation was fitted to the full-scale CFD data,whereby the maximum Shields parameter beneath the pipe can be calculated based on the undisturbed Shields parameter and S/D.

A NEW APPROACH TO THE EQUATIONS FOR THE STEADY STATE CURRENTS AT MICROELECTRODES:EC'PROCESSES(SECOND ORDER REACTION)

With a new approach,the general current expressions of two typical second order catalytic reactions at microelectrodes are obtained.This allows the study of fast chemical reactions and systems where the reactants are present in similar concentrations.

Reversal current observed in micro-and submicro-channel flow under non-continuous DC electric field

In practical applications of biochips and bio-sensors, electrokinetic mechanisms are commonly employed to manipulate and analyze the characteristics of single bio-molecules. To accurately and flexibly control the movement of single molecule within micro-/submicro-fluidic channels, the characteristics of current signals at the initial stage of the flow are systematically studied based on a three-electrode system. The current response of micro-/submicro-fluidic channels filled with different electrolyte solutions in non-continuous external electric field are investigated. It is found, there always exists a current reversal phenomenon, which is an inherent property of the current signals in micro/submicro-fluidics Each solution has an individual critical voltage under which the steady current value is equal to zero The interaction between the steady current and external applied voltage follows an exponential function. All these results can be attributed to the overpotentials of the electric double layer on the electrodes. These results are helpful for the design and fabrication of functional micro/nano-scale fluidic sensors and biochips.

Development of current-induced scour beneath elevated subsea pipelines

When scour occurs beneath a subsea pipeline and develops to a certain extent,the pipeline may experience vortex-induced vibrations,through which there can be a potential accumulation of fatigue damage.However,when a pipeline is laid on an uneven seabed,certain sections may have an elevation with respect to the far-field seabed,e o,at which the development of scour would vary.This work focused on predicting the development of the scour depth beneath subsea pipelines with an elevation under steady flow conditions.A range of pipe elevation-to-diameter ratios(i.e.0≤e o/D≤0.5)have been considered for laboratory experiments conducted in a sediment flume.The corresponding equilibrium scour depths and scour time scales were obtained;experimental data from published literature have been collected and added to the present study to produce a more complete analysis database.The correlation between existing empirical equations for predicting the time scale and the experimental data was assessed,resulting in a new set of constants.A new manner of converting the scour time scale into a non-dimensional form was found to aid the empirical equations in attaining a better correlation to the experimental data.Subsequently,a new empirical equation has also been proposed in this work,which accounts for the influence of e o/D on the non-dimensional scour time scale.It was found to have the best overall correlation with the experimental data.Finally,full-scale predictions of the seabed gaps and time scales were made for the Tasmanian Gas Pipeline(TGP).