Multielement analysis of water residue: A PIXE measurement

Summary A multielement PIXE method has been developed to analyse water residue obtained by evaporation technique. In this analysis, 250 ml of water sample mixed with 200 mg of cellulose powder is evaporated on steam bath in a platinum dish. From the residue thus obtained standard pellets are made for proton irradiation to excite characteristic X-rays. Proton irradiations were performed with 2.5 MeV beams with 20 nA intensity and the characteristic X-rays were detected with a high resolution Si(Li) detector and processed by standard electronics. A synthetic standard of cellulose matrix for concentration calibration was prepared by adding different elements from atomic absorption standards and homogeneously mixing them with the matrix with methanol.Using the present methodology, for a 20 C irradiation, it is possible to analyse quantitatively 10–15 different elements (K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Br, Sr, Pb, etc.) in water samples. The procedure thus developed has been applied to study the concentration profiles of heavy elements in the municipal water supplies in the city of Dhaka and the results are compared with recommended levels of heavy elements in drinking water. Various aspects of the method and the implications of the data reported here have been discussed.

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Multielement-Analyse von Wasserrückständen: PIXE-Messung

Zusammenfassung 250 ml des Wassers wurden mit 200 mg Cellulosepulver gemischt und eingedampft. Aus dem Rückstand wurden Tabletten für die Protonenbestrahlung gepreßt. Die Bestrahlung wurde mit 2,5 MeV und 20 nA durchgeführt; die charakteristischen Röntgenstrahlen wurden mit Hilfe eines hochauflösenden Si(Li)-Detektors gemessen und elektronisch aufgearbeitet. Synthetische Standards für die Cellulosematrix wurden aus AAS-Standards hergestellt (Mischung mit Methanol). Mit dem beschriebenen Verfahren konnten bei 20-C-Bestrahlung 10–15 verschiedene Elemente (K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Br, Sr, Pb, u.a.) in Wasserproben bestimmt werden. Die Methode wurde zur Untersuchung der Konzentrationsprofile von Schwermetallen im Leitungswasser der Stadt Dhaka eingesetzt, die mit den für Trinkwasser empfohlenen Konzentrationen verglichen wurden. Verschiedene Gesichtspunkte des Verfahrens und der erhaltenen Resultate werden diskutiert.

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This research was supported in part by the International Atomic Energy Agency, Vienna, Austria     

Influence of inhomogeneous distribution of the minority carriers on the doping dependence of the threshold current in EBP lasers using n-type GaAs at 300°K

The influence of inhomogeneous distribution of the minority carriers on the doping dependence of the threshold current in electron-beam-pumped (EBP) lasers using n-type GaAs at 300°K is studied theoretically. It is found that when inhomogeneity exists, the threshold current is weakly dependent on the doping concentration in agreement with the experimental observation.     

Numerical investigation of forced convection heat transfer in unsteady flow past a row of square cylinders

This paper presents the unsteady laminar forced convection heat transfer from a row of five isothermal square cylinders placed in a side-by-side arrangement at a Reynolds number of 150. The numerical simulations are performed using a finite volume code based on the PISO algorithm in a collocated grid system. Special attention is paid to investigate the effect of the spacing between the cylinders on the overall transport processes for the separation ratios (spacing to size ratio) between 0.2 and 10. No significant interaction between the wakes is observed for spacing greater than four times the diameter at this Reynolds number. However, at smaller spacing, the wakes interact in a complicated manner resulting different thermo-hydrodynamic regimes. The vortex structures and isotherm patterns obtained are systematically presented and discussed for different separation ratios. In addition, the mean and instantaneous drag and lift coefficients, mean and local Nusselt number and Strouhal number are determined and discussed for various separation ratios. A new correlation is derived for mean Nusselt number as a function of separation ratio for such flows.     

Experimental study of flow past a square cylinder at high Reynolds numbers

 Measurements of two-components of velocity in the wake of a square cylinder using a hot-wire anemometer are reported. Two Reynolds numbers, namely 8700 and 17,625, have been considered. The measurements were carried out in a low-speed, low-turbulence wind tunnel. Benchmark experiments at much lower Reynolds numbers show good agreement between the present experiments and published results. At higher Reynolds numbers, the experimental data reveal anticipated trends in terms of wake recovery and turbulence decay. Both velocity and velocity fluctuations show symmetry about the wake axis. The experimental data have been compared with the large eddy simulation (LES) calculation reported by Wang et al. [University of Illinois at Urbana – Champaign (1996) Report CFD 96-03] and LDV measurements of Lyn et al. [J Fluid Mech (1995) 304: 285–319]. The agreement among the three sets is generally acceptable in terms of the time-averaged velocity components, but not the velocity fluctuations. The turbulence fluctuations in the present experiments are seen to be lower than in the referred work. The differences have been traced to factors such as the aspect ratio, blockage ratio and upstream turbulence. Experiments with increased upstream turbulence did show a reduction in the discrepancy between the present experiments and the published data. An assessment of the experimental data in terms of physical mechanisms revealed that (a) streamwise normal stresses were correlated with the vortex centers, and (b) the turbulence kinetic energy profiles are similar to the turbulence shear stress. Spectral analysis of the velocity signals was carried out in the present work. Energy transfer from the mean flow to the streamwise velocity fluctuation was confirmed in the near wake. A redistribution of the kinetic energy between the streamwise and transverse components of velocity over a longer distance downstream was subsequently observed. Received: 17 May 1999/Accepted: 29 December 1999     

On holomorphic principal bundles over a compact Riemann surface admitting a flat connection

Let G be a connected reductive linear algebraic group over , and X a compact connected Riemann surface. Let be a Levi factor of some parabolic subgroup of G, with its maximal abelian quotient. We prove that a holomorphic G-bundle over X admits a flat connection if and only if for every such L and every reduction of the structure group of to L, the -bundle obtained by extending the structure group of is topologically trivial. For , this is a well-known result of A. Weil. Received: 1 December 2000 / Revised version: 2 April 2001 / Published online: 24 September 2001     

Viscosity Solutions for a System of Integro-PDEs and Connections to Optimal Switching and Control of Jump-Diffusion Processes

We develop a viscosity solution theory for a system of nonlinear degenerate parabolic integro-partial differential equations (IPDEs) related to stochastic optimal switching and control problems or stochastic games. In the case of stochastic optimal switching and control, we prove via dynamic programming methods that the value function is a viscosity solution of the IPDEs. In our setting the value functions or the solutions of the IPDEs are not smooth, so classical verification theorems do not apply.     

On semistable principal bundles over a complex projective manifold,II

Let (X, ω) be a compact connected Kähler manifold of complex dimension d and \({E_G\,\longrightarrow\,X}\) a holomorphic principal G–bundle, where G is a connected reductive linear algebraic group defined over \({\mathbb{C}}\). Let Z(G) denote the center of G. We prove that the following three statements are equivalent:

1. (1)
   There is a parabolic subgroup \({P\,\subset\,G}\) and a holomorphic reduction of structure group \({E_P\,\subset\,E_G}\) to P, such that the corresponding L(P)/Z(G)–bundle

   $E_{L(P)/Z(G)}\,:=\,E_P(L(P)/Z(G))\,\longrightarrow\,X$

   admits a unitary flat connection, where L(P) is the Levi quotient of P.
    
2. (2)
   The adjoint vector bundle ad(E _G ) is numerically flat.
    
3. (3)
   The principal G–bundle E _G is pseudostable, and

   $\int\limits_X c_2({\rm ad}(E_G))\omega^{d-2}\,=\,0.$
    

If X is a complex projective manifold, and ω represents a rational cohomology class, then the third statement is equivalent to the statement that E _G is semistable with c ₂(ad(E _G )) = 0.

     

Solitary wave solution of the Zakharov–Kuznetsov equation in plasmas with power law nonlinearity

In this paper, we obtain an exact 1-soliton solution of the Zakharov–Kuznetsov equation, with power law nonlinearity, by the solitary wave ansatz method. A couple of conserved quantities of this equation are also calculated by using this 1-soliton solution.