抽吸和压力梯度在层流边界层转捩过程中的作用

用空间模式的二次稳定性理论研究了抽吸和压力梯度对边界层三维亚谐扰动流动稳定性的影响．数值结果表明，固体边界上的抽吸有明显的层流控制作用，逆压梯度则有较强的不稳定作用．     

A DIRECT METHOD FOR THE UNIT CIRCLE PROBLEM

The unit circle problem is the problem of finding the number of eigenvalues of a non-Hermitian matrix inside and outside the unit circle . To reduce the cost of computing eigenvalues for the problem, a direct method, which is analogous to that given in [5], is proposed in this paper.     

非饱和固化淤泥的力学及体变特性试验研究

由于降雨、水位升降等自然因素,固化淤泥作为填土材料使用时,其饱和度(基质吸力)常处于变动状态,使固化淤泥力学及体变特性发生改变,对土工构筑物的安全带来潜在隐患.鉴于此,本文通过对不同基质吸力、净围压下的三轴固结排水剪切试验,探讨了基质吸力和净围压对非饱和固化淤泥力学及体变特性的影响.结果表明:随着净围压增大,基质吸力的...     

Field Study on Mobility and Persistence of Linuron and Monolinuron in Agricultural Soil

Abstract

A field site equipped with suction cup lysimeters was installed at Treviglio (BG) to assess the migration capacity of the herbicides linuron and monolinuron from topsoil to groundwater and to verify the appearance of their relevant transformation products in soil and water samples. A constant hydraulic head was applied in order to develop water saturation conditions in the upper layers. KCl was used as a tracer to evaluate water infiltration velocity through the vertical soil profile. The constant hydraulic head accelerated infiltration rates, while herbicide concentrations reached maximum contamination because soil adsorption capacity was underdeveloped. The results indicated two main processes of pesticide transport: firstly transport due mainly to water infiltration through macropores; secondly the transport driven by matrix flow. Linuron was found to be the most mobile herbicide, while chloroanilines were found to be the major transformation products of the herbicides considered.     

Dispersion of Colloidal Alumina using a Rhamnolipid Biosurfactant

Dispersability of colloidal alumina in water has been studied using a rhamnolipid containing biosurfactant. Zeta potential measurements revealed that the surface charge of alumina was altered due to adsorption of the biosurfactant and the iso‐electric‐point of alumina shifted from pH 9 to 6.3. Sedimentation tests indicated that the alumina suspension was completely dispersed for 3–5 hours in the presence of biosurfactant after which some settling was observed. Stability of the suspension in the time period studied was found to be independent of pH. Capillary suction time measurements showed that the alumina suspension was dispersed in the presence of the biosurfactant and varied with pH. Maximum dispersion is obtained in the pH range of 3.5–5 and 7–11 while a minimum is obtained around pH 6. This behavior is consistent with the changes in zeta potential in the presence of the biosurfactant and thus capillary suction time measurements appeared to be more reliable than the sedimentation tests. Optimization studies showed that about 60 mg/g of biosurfactant was necessary for best dispersion and dispersion could be done up to 40% solids. The application of a natural biosurfactant for dispersing colloidal alumina has been demonstrated.     

Prediction of entrance length and mass suction rate for a cylindrical sucking funnel

Conservation equations for mass, momentum and energy have been solved numerically for a cylindrical funnel with louvers (lateral openings on the side wall of the cylindrical funnel through which air can come into it) to compute the suction rate of air into the funnel. The nozzle placed centrally at the bottom of the cylinder ejects high‐velocity hot gaseous products so that atmospheric air gets sucked into the funnel. The objective of the work is to compute the ratio of the rate of mass suction to that of the mass ejected by the nozzle for different operating conditions and geometrical size of the funnel. From the computation it has been found that there exists optimum funnel diameter and optimum funnel height for which the mass suction is the highest. The protruding length of the nozzle into the funnel has almost no effect on the mass suction rate after a certain funnel height. The louvers opening area has a very high impact on the mass suction rate. The entrance length for such a sucking funnel is strikingly much lower compared with a simple cylindrical pipe having uniform flow at the inlet at same Reynolds number. A new correlation has been developed to propose the entrance length for a sucking pipe, the rate of mass suction into it and the exhaust plume temperature over a wide range of operating parameters that are normally encountered in a general funnel operations of naval or merchant ship. Copyright © 2009 John Wiley & Sons, Ltd.     

MHD free convective flow past semi-infinite vertical permeable wall

In this paper,the basic equations governing the flow and heat transfer of an incompressible viscous and electrically conducting fluid past a semi-infinite vertical permeable plate in the form of partia...     

Theory of Microbial Carbonate Precipitation and Its Application in Restoration of Cement-based Materials Defects

Bacterial induced carbonate mineralization has been demonstrated as a new potential method for restoration of limestones in historic buildings and monuments. We claim here the formation of calcium carbonate was controlled by extracellular polymeric substances (EPS) isolated from Bacillus pasteurii. The process of crystallization nucleation was accelerated in the presence of cells and inhibited in the presence of EPS. The CaCO₃ film deposited on cement paste surface was about 100 µm after 7 d treatment. The results of various restoring methods showed that higher decrease of water absorption of cement paste was gained in brushing application in the presence of agar, which could maintain urease with high activity in long term compared to spraying method. The coefficient of capillary suction of cement paste treated with brushing method was reduced by 90%. Mixed media consisted of sands, urea, Ca²⁺ and concentrated biomass, was injected into artificial cracks of cement paste followed by continual nutrient supplement, and CaCO₃ particles were precipitated gradually between sands particles which were combined with cement matrix. The results showed that the compressive strength of recovered specimens was restored to 84%, which demonstrated that this kind of bio‐restoration method is effective in repairing surface defects of cement‐based materials.     

Numerical computation of an impinging jet with uniform wall suction

The paper presents numerical predictions of a turbulent axisymmetric jet impinging onto a porous plate, based on a finite volume method of solving the Navier-Stokes equations for an incompressible air jet with the K–ε turbulence model. The velocity and pressure terms of the momentum equations are solved by the SIMPLE (semi-implicit method for pressure-linked equation) method. In this study, non-uniform staggered grids are used. The parameters of interest include the nozzle-to-wall distance and the suction velocity. The results of the present calculations are compared with available data reported in the literature. It is found that suction effects reduce the boundary layer thickness and increase the velocity gradient near the wall.