EXPERIMENTAL STUDY ON SEDIMENT RESUSPENSION IN TAIHU LAKE UNDER DIFFERENT HYDRODYNAMIC DISTURBANCES

Contaminants resuspension in sediments induced by wind-wave could influence the water quality in shallow lakes. Resuspension of surface sediments from the Zhushan Bay, Taihu Lake was simulated under different wind forcing by using a pneumatic annular flume in this study. Acoustic Doppler Velocimeter (ADV) was used to measure flow velocity at each wind speed, and the characteristics of sediment resuspension were studied with the layered sampling technology. The experimental results show that the flow velocity increases with wind speed obviously and 6m/s is a critical wind speed which affected hydrodynamic conditions significantly. The distribution of flow velocity and water depth is different from that in ordinary open channel. With the enhanced hydrodynamic factors, the Suspended Solids Concentration (SSC) in water increases accordingly, and the incipient velocity of sediment resuspension is about 0.21 m/s. Based on the analysis of wind speed and average SSC in water column, the quantitative relationship is obtained. The SSC of the bottom layer is higher than the content of surface layer under different hydrodynamic conditions, and there are similar distributions between SSC and flow velocity in different water layers.     

Negative Poisson's ratio polymeric and metallic foams

Foam materials based on metal and several polymers were transformed so that their cellular architecture became re-entrant, i.e. with inwardly protruding cell ribs. Foams with re-entrant structures exhibited negative Poisson's ratios as well as greater resilience than conventional foams. Foams with negative Poisson's ratios were prepared using different techniques and materials and their mechanical behaviour and structure evaluated.     

Size effects and micromechanics of a porous solid

The rigidity of rods of a polymeric foam in bending and torsion is measured as a function of diameter. The dependence of rigidity upon specimen size is found to be inconsistent with a classically viscoelastic continuum model. The Cosserat continuum, which admits additional degrees of freedom associated with rotation of the microstructure, describes the foam more accurately than the classical continuum. Evidence is presented that additional degrees of freedom associated with the deformation of the microstructure, must be incorporated in a complete continuum model of foamed materials.     

Elastic and viscoelastic behavior of chiral materials

Chiral materials are not invariant to inversions: there is a distinction between right- and left-handed material. Material properties such as piezoelectricity and pyroelectricity, represented by tensors of odd rank, can only occur in chiral materials. Chiral effects in elasticity cannot be expressed within classical elasticity since the modulus tensor, which is fourth rank, is unchanged under an inversion. We consider effects of chirality in elastic materials described by a generalized continuum representation, specifically Cosserat elasticity. Analysis of several configurations discloses a chiral material to generate reaction moments when compressed as a slab. A chiral plate bent to hyperbolic shape is predicted to exhibit size effects from the Cosserat characteristic length, and a shear force from the chirality. This analysis can be used for the interpretation of experiments on compliant chiral materials, in particular the evaluation of the elastic constants. Viscoelastic chiral solids are examined in the context of the correspondence principle.     

HYDRAULIC RESISTANCE OF SUBMERGED VEGETATION RELATED TO EFFECTIVE HEIGHT

Submerged vegetation has a significant impact on water flow velocity. Current investigations include the impact through adding drag resistance and increasing bottom roughness coefficient, which cannot elucidate the characters of real submerged vegetation. To evaluate the effects of submerged vegetation on water currents at different velocities, a laboratory experiment was conducted using three kinds of vegetations. The effective heights of these vegetations on varying flow velocities were evaluated. An equation describing the relationship between the normalized resistance of the submerged plants and the Reynolds number based on the plant effective height was then established and used to calculate the hydraulic resistance parameters of submerged plants in different stages of growth.     

Analysis of high-loss viscoelastic composites

A theoretical study of the viscoelastic properties of composites is presented with the aim of identifying structures which give rise to a combination of high stiffness and high loss tangent. Laminates with Voigt and Reuss structures, as well as composite materials attaining the Hashin-Shtrickman bounds on stiffness were evaluated by the correspondence principle. Similarly, viscoelastic properties of composites containing spherical or platelet inclusions were explored. Reuss laminates and platelet-filled materials composed of a stiff, low-loss phase and a compliant high-loss phase were found to exhibit high stiffness combined with a high loss tangent.     

Strong,Ductile Magnesium-Zinc Nanocomposites

Incorporated SiC nanoparticles are demonstrated to influence the solidification of magnesium-zinc alloys resulting in strong, ductile, and castable materials. By ultrasonically dispersing a small amount (less than 2 vol pct) of SiC nanoparticles, both the strength and ductility exhibit marked enhancement in the final casting. This unusual ductility enhancement is the result of the nanoparticles altering the selection of intermetallic phases. Using transmission electron microscopy (TEM), the MgZn₂ phase was discovered among SiC nanoparticle clusters in hypoeutectic compositions. Differential thermal analysis showed that the MgZn₂ formation resulted in elimination of other intermetallics in the Mg-4Zn nanocomposite and reduced their formation in Mg-6Zn and Mg-8Zn nanocomposites.     

Creep and creep recovery of cast aluminum alloys

Constant load uniaxial creep tests were performed on four aluminum alloys (designated M4032-2, 332, 332RR, and 333) at stresses of 31.5 MPa, 56.5 MPa, and 73 MPa and temperatures of 220°C and 260°C. Of the four materials, M4032-2 had the greatest resistance to creep, while 332RR alloy had the least. In addition to creep, the creep recovery phase was observed as well. It was found that, even for short loading periods, much of the time-dependent strain was not recoverable for all of the materials studied. Hardening was observed to occur in each of the alloys, resulting in a reduced creep rate on subsequent loadings. A constitutive equation for creep and recovery incorporating both stress and temperature dependence was developed for each of the alloys tested based on a viscous-viscoelastic model.     

Non-linear properties of polymer cellular materials with a negative Poisson's ratio

Negative Poisson's ratio polymeric cellular solids (re-entrant foams) were studied to ascertain the optimal processing procedures which give rise to the smallest value of Poisson's ratio. The non-linear stress-strain relationship was determined for both conventional and re-entrant foams; it depended upon the permanent volumetric compression achieved during the processing procedure. Poisson's ratio of re-entrant foam measured as a function of strain was found to have a relative minimum at small strains. The toughness of re-entrant foam increased with permanent volumetric compression, and hence with density.