Kaiming She

Fall velocities of natural sediment particles: a simple mathematical presentation of the fall velocity law

The fall velocities of large natural sediment particles (shingle) and of commonly used model sediments (anthracites) have been measured using a video imaging technique. The study covers a Reynolds number ranging from 35 to 23,000, a significant extension to the existing database for natural sand particles (up to 1500). Making use of the extended database, a simple empirical equation has been derived giving the relationship between the particle fall velocity and sieve diameter. Additional analysis of the drag coefficient has led to a revised formula based on that of Fredsoe and Deigaard (Mechanics of Coastal Sediment Transport. Advanced Series on Ocean Engineering, World Scientific, Singapore), providing a better fit for the data obtained at high Reynolds numbers.

Tidal effects on cross-shore sediment transport on a shingle beach

ABSTRACT This paper presents a model study of the sediment transport processes of a shingle beach under the combined action of waves and tides. Parallel experiments were carried out with a constant water depth corresponding to the high water level of the tidal experiments, thus allowing a direct comparison between the two cases. It was found that the presence of the tide affected the beach behaviour differently depending on the wave climate. Under storm conditions, a rolling bar was clearly seen moving below the Mean Water Level (MWL) with both the flood and ebb tides. Under swell conditions on the flood, sediment was eroded once a critical depth had been reached but was moved onshore in the uprush to form the berm. The presence of the tide was shown to increase the size of the berm in a summer profile, and push the offshore bar further seaward in the winter profiles. The overall beach slope did not change when subjected to the wave attack without the tide but altered significantly under the combined action of the wave and the tide.

Effects of Permeability on the Performance of Mixed Sand-Gravel Beaches

This paper examines the performance of a mixed sand-gravel beach in comparison with that of a pure gravel beach through a series of controlled laboratory experiments. The gravel beach was modelled using anthracite of a uniform (single) grain size while the mixed sand-gravel beach modelled by means of graded anthracite of the same median sediment size (D 50 ). The two model beaches were tested under identical wave conditions and beach profile changes were recorded. Significant differences were observed in the performance of the two beaches. The reduced permeability of the mixed sand-gravel beach seems to have the effect of reducing the onshore transport but increasing the offshore transport.

Threshold of Motion of Natural Sediment Particles in Oscillatory Flows

Abstract The mechanism of the threshold of sediment motion under wave action has been investigated by examining the dynamic forces acting on the sediment particle. The force analysis has led to a range of nondimensional parameters of clear physical significance, which are then applied to the analysis of a range of existing experimental data. The introduction of some new nondimensional parameters has resulted in significant improvement in the definition of the threshold of motion condition. Several empirical equations involving different nondimensional parameters have been derived, providing a mathematical description of the threshold condition of the sediment motion. The accuracy of these empirical equations is examined in the context of scientific and engineering applications and a simple equation is recommended for its accuracy and ease of use.

Aerodynamics in the open channel of the Sistan-type wind-mill with vertical axis wind turbine

Purpose The purpose of this paper is to examine the details of the air mass flow and aerodynamical phenomena across a channel containing a large vertical axis wind turbine. The considered model reproduces as closely as possible the real assembly of the Sistan-type wind-mill whose top is open. The technical results of this work could be used for the restoration and operation of this assembly whose historical and architectural values are recognized. Design/methodology/approach Several inlet velocities into the channel are considered, taking into account the possible local wind resources. Calculations corresponding to Reynolds number varying between 8×105 and 4×106 are made by means of the finite volume method and turbulence is treated with the realizable k-e model. The mesh consists of a fixed part associated to the contour of the channel, interfaced with a moving one linked to the turbine itself, equipped with nine partly filled wings. Findings The relative pressure and velocity fields are presented for various dynamic and static conditions. Calculation results clearly show that the vortex phenomena present in some cases are not a source of degradation of the wind turbine’s aerodynamical performances, given its location, intensity and rotation direction. Particular attention is devoted to the air mass flow and its distribution between the inlet and the outlet sections of the channel. Originality/value The present work provides technical information useful to consider the restoration and modernization of this installation whose architecture and technical performance are very interesting. This survey complements a previous one examining the aerodynamical phenomena occurring in a modified version of this assembly with a closed top channel.