bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical study of pressure and velocity fluctuations in nearly isotropic turbulence
Authors: Schumann U. , Patterson G. S.
Published: April 2006

The spectral method of Orszag & Patterson has been extended to calculate the static pressure fluctuations in incompressible homogeneous decaying turbulence at Reynolds numbers Reλ [lsim ] 35. In real space 323 points are treated. Several cases starting from different isotropic initial conditions have been studied. Some departure from isotropy exists owing to the small number of modes at small wavenumbers. Root-mean-square pressure fluctuations, pressure gradients and integral length scales have been evaluated. The results agree rather well with predictions based on velocity statistics and on the assumption of normality. The normality assumption has been tested extensively for the simulated fields and found to be approximately valid as far as fourth-order velocity correlations are concerned. In addition, a model for the dissipation tensor has been proposed. The application of the present method to the study of the return of axisymmetric turbulence to isotropy is described in the companion paper.
Hydromagnetic Stokes flow past a rotating sphere
Authors: Sastry V. U. K. , Rao K. V. Rama
Published: April 2006

In the present investigation we consider hydromagnetic Stokes flow past a rotating sphere. The magnetic field is produced by a magnetic pole placed at the centre of the sphere. The problem is analysed by a combination of perturbation and numerical methods. It is seen that the flow reversal (due to rotation) at the rear portion of the sphere is enhanced as the strength of the magnetic field increases. In addition, we obtain the simultaneous effects of rotation and a magnetic field on the streamlines.
Shock waves at a slow-fast gas interface
Authors: Abd-El-Fattah A. M. , Henderson L. F.
Published: April 2006

This paper describes the results of our experiments with shock waves refracting at a CO2/CH4 interface. The refraction is slow-fast because the speed of sound in the incident gas (CO2) is less than that in the transmitting gas (CH4). We found three phenomena which apparently have not been reported before and which all have free precursor shocks in their wave systems; schlieren photographs of them are presented. As a result of the present and earlier work, we can assert that there exist at least four different free precursor refractions. Theoretical studies suggest that the slow-fast phenomena can be conveniently classified into three groups characterized by different ranges of values of the inverse strength ζi of the incident shock i. The classification may be an exhaustive list of the phenomena, at least when the gases are nearly perfect, but we cannot be sure. We present experimental data on all the phenomena in each group, including data on the transition conditions from one wave system to another both within and across the groups.
Instabilities of convection rolls in a fluid of moderate Prandtl number
Authors: Busse F. H. , Clever R. M.
Published: April 2006

The instabilities of two-dimensional convection rolls in a horizontal fluid layer heated from below are investigated in the case when the Prandtl number is seven or lower. Two new mechanisms of instability are described theoretically as well as experimentally. The knot instability causes the transition to spoke-pattern convection at higher Rayleigh numbers while the skewed varicose instability accomplishes a change to larger horizontal wavelengths of the convection rolls. Both instabilities disappear in the limits of small and large Prandtl number. Although the experimental methods fail in realizing closely the infinitely conducting boundaries assumed in the theory, the observations agree in all qualitative aspects with the theoretical predictions.
Steady long slender droplets in two-dimensional straining motion
Authors: Hinch E. J. , Acrivos A.
Published: April 2006

The recent analysis by Acrivos & Lo (1978) concerning the breakup of a long slender droplet in an axisymmetric straining motion is extended to the case of a two-dimensional hyperbolic flow. It is found that, although the cross-section of the droplet becomes significantly non-circular, the theoretical criterion for breakup is effectively the same as in the axisymmetric case. The theoretical predictions are in good agreement with the available experimental results.
Pressure-fluctuation measurements on an oscillating circular cylinder
Authors: Bearman P. W. , Currie I. G.
Published: April 2006

Measurements are presented of the fluctuating pressure recorded at a point 90° from the mean position of the forward stagnation point on a circular cylinder oscillating in a water flow. The aspect ratio of the cylinder was 9·5 and the turbulence level in the free-stream was 5·5%. The cylinder Reynolds number was 2·4 × 104 and the cylinder was forced to oscillate transverse to the main flow at amplitudes up to 1·33 cylinder diameters. The reduced velocity was varied over the range 3–18 and the experiments spanned the vortex-shedding lock-in range. Measurements of phase difference between pressure and displacement show that the maximum out-of-phase lift force occurs at an amplitude of about half a diameter. Good agreement is found between measurements on forced and freely oscillating cylinders. A simple potential-flow model gives reasonable predictions of the pressure fluctuations at the body frequency and at twice the body frequency at reduced velocities away from lock-in.
A vortex theory of animal flight. Part 1. The vortex wake of a hovering animal
Authors: Rayner J. M. V.
Published: April 2006

The distribution of vorticity in the wake of a hovering bird or insect is considered. The wake is modelled by a chain of coaxial small-cored circular vortex rings stacked one upon another; each member of the chain is generated by a single wing-stroke. Circulation is determined by the animal's weight and the time for which a single ring must provide lift; ring size is calculated from the circulation distribution on the animal's wing. The theory is equally applicable to birds and insects, although the mechanism of ring formation differs. This approach avoids the use of lift and drag coefficients and is not bound by the constraints of steady-state aerodynamics; it gives a wake configuration in agreement with experimental observations. The classical momentum jet approach has steady momentum flux in the wake, and is difficult to relate to the wing motions of a hovering bird or insect; the vortex wake can be related to the momentum jet, but adjacent vortex elements are disjoint and momentum flux is periodic.

The evolution of the wake starting from rest is considered by releasing vortex rings at appropriate time intervals and allowing them to interact in their own velocity fields. The resulting configuration depends on the feathering parameterf(which depends on the animal's morphology);fincreases with body size. At the lower end of the wake rings coalesce to form a single large vortex, which breaks away from the rest of the wake at intervals. Wake contraction depends onf; the minimum areal contraction of one-half (as in momentum-jet theory) occurs only in the limitf→ 0, but values calculated for smaller insects of just over one-half suggest that the momentum jet may be a good approximation to the wake whenfis small.

Induced power in hovering is calculated as the limit of the mean rate of increase of wake kinetic energy as time progresses. It can be related to the classical momentum-jet induced power by a simple conversion factor. For an insect or hummingbird the usual momentum-jet estimate may be between 10 and 15% too low, but for a bird it may be as much as 50% too low. This suggests that few, if any, birds are able to sustain aerobic hovering, and that as small a value offas possible would be necessary if the bird were to hover.

Tip losses (energy cost of the vortex-ring wake compared with the equivalent momentum jet) are negligible for insects, but can be in the range 15–20% for birds.
The approach of a vortex pair to a plane surface in inviscid fluid
Authors: Saffman P. G.
Published: April 2006

It is shown that a symmetrical vortex pair consisting of equal and opposite vortices approaching a plane wall at right angles must approach the wall monotonically in the absence of viscous effects. An approximate calculation is carried out for uniform vortices in which the vortices are assumed to be deformed into ellipses whose axis ratio is determined by the local rate of strain according to the results of Moore & Saffman (1971).
Nonlinear properties of convection rolls in a horizontal layer rotating about a vertical axis
Authors: Clever R. M. , Busse F. H.
Published: April 2006

Steady finite amplitude two-dimensional solutions are obtained for the problem of convection in a horizontal fluid layer heated from below and rotating about its vertical axis. Rigid boundaries with prescribed constant temperatures are assumed and the solutions are obtained numerically by the Galerkin method. The existence of steady subcritical finite amplitude solutions is demonstrated for Prandtl numbers P < 1. A stability analysis of the finite amplitude solutions is performed by superimposing arbitrary three-dimensional disturbances. A strong reduction in the domain of stable rolls occurs as the rotation rate is increased. The reduction is most pronounced at low Prandtl numbers. The numerical analysis confirms the small amplitude results of Küppers & Lortz (1969) that all two-dimensional solutions become unstable when the dimensionless rotation rate Ω exceeds a value of about 27 at P ≃ ∞. A brief discussion is given of the three-dimensional time-dependent forms of convection which are realized at rotation rates exceeding the critical value.
Propagation of perturbations in the viscous sublayer and adjacent wall region
Authors: Kreplin Hans-Peter , Eckelmann Helmut
Published: April 2006

The fluctuating velocity components, u and w, and their gradients at the wall,
\[
(\partial u/\partial y)|_{w}\quad {\rm and}\quad (\partial w/\partial y)|_{w},
\]
were measured in a fully developed turbulent channel flow using hot-film probes. The data were taken in an oil channel at a low Reynolds number which allowed velocity measurements very close to the wall. From simultaneous measurements of these signals it could be deduced that coherent flow structures, inclined to the wall, travel downstream. Space-time correlations imply that these are rotating structures. The spanwise distance between these structures was found to be z+ ≈ 50 and they could be observed over a streamwise distance of Δx+ > 1000.
An experimental investigation of natural convection in the melted region around a heated horizontal cylinder
Authors: Bathelt A. G. , Viskanta R. , Leidenfrost W.
Published: April 2006

Melting from an electrically heated horizontal cylinder embedded in a paraffin (n-octadecane, fusion temperature 301·3 °K) has been studied experimentally. The shape of the solid-liquid interface has been determined photographically, and the local heat transfer coefficients have been measured using a shadowgraph technique. The experiments provide conclusive evidence of the important role played by natural convection in melting a solid due to an embedded cylindrical heat source. The four distinct pieces of quantitative evidence which contribute to this conclusion are the melt shape, surface temperature, local and average heat transfer coefficients and their variation with time.

The experimental findings prove the importance of natural convection in phase change problems involving melting and indicate that continued practice of neglecting the effects in the analysis of such problems does not appear reasonable. Natural convection should be considered in analysis and design of systems involving phase change.
A theory for wave-power absorption by two independently oscillating bodies
Authors: Srokosz M. A. , Evans D. V.
Published: April 2006

In a recent paper (Evans 1976) a theory was presented for the behaviour of an oscillating two-dimensional cylinder of any shape which was capable of absorbing energy from a given regular sinusoidal wave. In particular an expression was derived for the efficiency of power absorption of the cylinder when oscillating in a single mode in terms of properties of the solution of the so-called radiation problem in which the cylinder is forced to oscillate in the appropriate mode in the absence of the incident wave train.

In the present paper this theory is extended to two independent cylinders of arbitrary shape each oscillating in a single mode and capable of absorbing energy in that mode. A general expression for the efficiency is derived which depends on properties of the solution to a new radiation problem, in which one cylinder is forced to oscillate in the presence of the other cylinder, which is held fixed in its equilibrium position. In this case, the efficiency also depends on cross-coupling coefficients related to the force on the fixed cylinder due to the motion of the oscillating cylinder.

It is shown that the cylinders can be tuned to absorb all the incident wave energy at a given frequency even for symmetric cylinders, in contrast to the single symmetric cylinder, for which the maximum efficiency has been shown to be 50%.

The general solution to the new radiation problem is derived in terms of the solution to the radiation problem for a single cylinder, by assuming that the cylinders are far enough apart for local wave effects to be negligible.

The special case of two widely spaced rolling vertical plates is considered in detail and curves showing the variation of efficiency with wavelength are given for a variety of plate spacings and points of rotation.
Bénard convection in a finite box: secondary and imperfect bifurcations
Authors: Hall P. , Walton I. C.
Published: April 2006

In the solution given by Hall & Walton (1977) for Bénard convection in a two-dimensional box with slightly imperfectly insulated side walls it was shown that there were certain critical values 2Lc of the length 2L of the box at which two modes became unstable simultaneously. In this paper we show that in the neighbourhood of Lc a secondary transition takes place which may be a bifurcation or a smooth transition depending on the boundary conditions.
Laminar flow in the entrance region of a smooth pipe
Authors: Mohanty A. K. , Asthana S. B. L.
Published: April 2006

The entrance region has been divided into two parts, the inlet region and the filled region. At the end of the inlet region, the boundary layers meet at the pipe axis but the velocity profiles are not yet similar. In the filled region, adjustment of the completely viscous profile takes place until the Poiseuille similar profile is attained at the end of it. The boundary-layer equations in the inlet region and the Navier-Stokes equations with order-of-magnitude analysis in the filled region are solved using fourth-degree velocity profiles. The total length of the entrance region so obtained is ξ = x/R Re = 0·150, whereas the boundary layers are observed to meet at approximately one-quarter of the entrance length, i.e. at ξ = 0·036. Experiments reported in the paper corroborate the analytical results.
Theory of premixed-flame propagation in large-scale turbulence
Authors: Clavin P. , Williams F. A.
Published: April 2006

A statistical theory is developed for the structure and propagation velocity of premixed flames in turbulent flows with scales large compared with the laminar flame thickness. The analysis, free of usual closure assumptions, involves a regular perturbation for small values of the ratio of laminar flame thickness to turbulence scale, termed the scale ratio ε, and a singular perturbation for large values of the non-dimensional activation temperature β. Any effects of the flame on the flow are considered to be given. In this initial study, molecular coefficients for diffusion of heat and reactants are set equal. The results identify convective-diffusive and reactive-diffusive zones in the flame and predict thickening of the flame by turbulence through streamwise displacement of the reactive-diffusive zone. Profiles for intensities of temperature fluctuations and for streamwise turbulent transport are obtained. A fundamental quantity occurring in the analysis is the longitudinal displacement of the reactive-diffusive zone in an Eulerian frame by turbulent fluctuations, and to first order in the scale ratio this equals the longitudinal displacement of fluid elements in an Eulerian frame by turbulent fluctuations, herein termed simply the Eulerian displacement. To first order in the scale ratio it is found that, if the Eulerian displacement experiences the same type of statistical non-stationarity as the corresponding Lagrangian displacement, then the diffusion approximation is valid for streamwise turbulent transport but the turbulent flame thickens as time increases, while if the Eulerian displacement is statistically stationary then the diffusion approximation necessitates a negative coefficient of diffusion in part of the flame but the flame thickness remains constant. By carrying the analysis to second order in the scale ratio it is shown that the turbulent-flame speed exceeds the laminar-flame speed by an amount proportional to the mean square of the transverse gradient of the Eulerian displacement. This result can be understood from the mechanistic viewpoint of a wrinkled laminar flame in terms of the increase in flame area produced by turbulence. Thus the theory provides a precise statistical quantification of the model of the wrinkled laminar flame for describing structures of turbulent flames.
Surface-roughness effects on the mean flow past circular cylinders
Authors: Güven O. , Farell C. , Patel V. C.
Published: April 2006

Measurements of mean-pressure distributions and boundary-layer development on rough-walled circular cylinders in a uniform stream are described. Five sizes of distributed sandpaper roughness have been tested over the Reynolds-number range 7 × 104to 5·5 × 105. The results are examined together with those of previous investigators, and the observed roughness effects are discussed in the light of boundary-layer theory. It is found that there is a significant influence of surface roughness on the mean-pressure distribution even at very large Reynolds numbers. This observation is supported by an extension of the Stratford–Townsend theory of turbulent boundary-layer separation to the case of circular cylinders with distributed roughness. The pressure rise to separation is shown to be closely related, as expected, to the characteristics of the boundary layer, smaller pressure rises being associated with thicker boundary layers with greater momentum deficits. Larger roughness gives rise to a thicker and more retarded boundary layer which separates earlier and with a smaller pressure recovery.
A family of steady, translating vortex pairs with distributed vorticity
Authors: Pierrehumbert R. T.
Published: April 2006

An efficient relaxation method is developed for computing the properties of a family of vortex pairs with distributed vorticity, propagating without change of shape through a homogeneous, inviscid fluid. The numerical results indicate that a steady state exists even when the gap between vortices is arbitrarily small, and that as the gap closes the steady state approaches a limiting vortex pair with a cusp on the axis of symmetry. Comparison is made with an approximate theory due to Saffman, and agreement is found to be good until the vortices are almost touching. The energy of members of the family is computed, and possible means of experimental production are discussed.
Wave formation and heat transfer at an ice-water interface in the presence of a turbulent flow
Authors: Gilpin R. R. , Hirata T. , Cheng K. C.
Published: April 2006

Under some conditions of temperature and flow an ice-water interface in the presence of a turbulent stream has been observed to be unstable. In this paper the source and the conditions for the instability were investigated for a well-defined turbulent boundary-layer flow. It was found that the instability resulted from the interaction that occurs between a wavy surface and a turbulent flow over it. Such an interaction results in a heat transfer variation which is 90 to 180 degrees out of phase with the surface wave shape – a result which is consistent with the calculations of Thorsness & Hanratty (1979a,b).

The main factor controlling damping of the instability at an ice-water interface was found to be the rate at which heat is conducted away from the interface into the ice.

In the past it has been found that when an ice layer is melting, that is when the heat conduction in the ice is small, the ice surface is highly unstable. In the present study it was found that for a sufficiently large temperature ratio (Tf−Tw)/(T∞−Tf), a steady-state ice layer is also unstable. Furthermore it is predicted, from the present observations, that a growing ice layer with a ratio of ice-side to water-side heat fluxes of up to 2.3 could be unstable.

Under sufficiently unstable conditions waves on the ice surface grow to an amplitude at which flow separations occur near the wave crests. This results in a ‘rippled’ ice surface pattern very similar to the patterns observed on mobile bed surfaces (Kennedy 1969) or surfaces which are being dissolved into a flowing stream (Allen 1971). The development of a ‘rippled’ ice surface results in a very substantial increase in the mean heat-transfer rate which would have an important influence on predictions of ice formation in the presence of a turbulent stream.
Modulation of the amplitude of steep wind waves
Authors: Longuet-Higgins M. S.
Published: April 2006

Lake & Yuen (1978) have suggested that in very steep wind waves the modulation-frequency of the wave amplitude may correspond to the frequency of the fastest-growing subharmonic instability of a uniform train of waves whose amplitude equals the mean wave amplitude $\overline{a}$. The approximate theory of Benjamin & Feir (1967) gives this frequency as $(\overline{a}k)f_d$, where κ is the wavenumber and fd the frequency of the unperturbed waves. This expression applies strictly only to very small values of the wave steepness $\overline{a}k$.

More recently (Longuet-Higgins 1978) the present author calculated accurately all the normal-mode instabilities of steep gravity waves on deep water. In this note these calculations are used to determine the frequency of the fastest-growing sub-harmonic instabilities precisely. When compared with the experimental data of Lake & Huen, these frequencies show even closer agreement.
On wave-current interaction theories of Langmuir circulations
Authors: Leibovich S.
Published: April 2006

The Craik–Leibovich (CL) equations for Langmuir circulations are shown to be an Eulerian approximation to an exact theory of the generalized Lagrangian mean (GLM) due to Andrews and McIntyre. Derivation of the CL equations using the GLM formalism is decisively simpler than the original method. The CL theory is then compared to other wave–current interaction theories of Langmuir circulations, notably those of Garrett and of Moen.
The slumping of gravity currents
Authors: Huppert Herbert E. , Simpson John E.
Published: April 2006

Experimental results for the release of a fixed volume of one homogeneous fluid into another of slightly different density are presented. From these results and those obtained by previous experiments, it is argued that the resulting gravity current can pass through three states. There is first a slumping phase, during which the current is retarded by the counterflow in the fluid into which it is issuing. The current remains in this slumping phase until the depth ratio of current to intruded fluid is reduced to less than about 0.075. This may be followed by a (previously investigated) purely inertial phase, wherein the buoyancy force of the intruding fluid is balanced by the inertial force. Motion in the surrounding fluid plays a negligible role in this phase. There then follows a viscous phase, wherein the buoyancy force is balanced by viscous forces. It is argued and confirmed by experiment that the inertial phase is absent if viscous effects become important before the slumping phase has been completed. Relationships between spreading distance and time for each phase are obtained for all three phases for both two-dimensional and axisymmetric geometries. Some consequences of the retardation of the gravity current during the slumping phase are discussed.
On the Whitham theory of shock-wave diffraction at concave corners
Authors: Henderson L. F.
Published: April 2006

The well-known Whitham theory may be applied to shocks diffracting over concave corners, provided that the diffraction results in Mach reflexion. This paper compares the theory with data obtained during experiments with diffracting shocks. If an incident shock is classified as weak or strong in the strict sense defined by von Neumann, then it is found that the Whitham theory accurately determines the Mach number of the Mach stem at the wall for both the weak and the strong cases. The theory also has some further value for strong shocks but not for weak; it is not applicable when the diffraction is regular.
Direct measurements of skin friction in a turbulent boundary layer with a strong adverse pressure gradient
Authors: Frei D. , Thomann H.
Published: April 2006

This paper describes a new balance, suitable for direct measurement of skin friction in turbulent boundary layers with severe pressure gradients. The gaps between the floating element and the surrounding wall are filled with a liquid in order to eliminate disturbing pressure forces on the element. The resulting friction forces are measured with piezo-electric transducers with high sensitivity and extremely small element displacement.

Skin friction measurements were taken in the turbulent boundary layer of a wind tunnel with circular cross-section at M [les ] 0·25. Severe adverse pressure gradients were generated by means of a step on the wall or, alternatively, by a conical centre body.

The new apparatus was mainly used to investigate the error of Preston tubes in adverse pressure gradients. It was necessary to develop a new measuring technique to improve the repeatability of the Preston tube readings.

The Preston tube error was found to depend on both the local pressure gradient P = (dp/dx) ν/ρ3τ and on the Preston tube diameter uτd/ν and to be independent of the upstream pressure distribution for the range of parameters covered by the experiments.
Nonlinear convection in a layer with nearly insulating boundaries
Authors: Busse F. H. , Riahi N.
Published: April 2006

A general class of solutions is studied describing three-dimensional steady convection flows in a fluid layer heated from below with boundaries of low thermal conductivity. Non-linear properties of the solutions are analysed and the physically realizable convection flow is determined by a stability analysis with respect to arbitrary three-dimensional disturbances. The most surprising result is that square-pattern convection is preferred in contrast to two-dimensional rolls that represent the only form of stable convection in a symmetric layer with highly conducting boundaries. The analysis is carried out in the limit of infinite Prandtl number and for a particular boundary configuration. But it is shown that the results hold for arbitrary Prandtl number to the order to which they have been derived and that other assumptions about the boundaries require only minor modifications as long as their thermal conductance remains low.
Discharge‐dependent pollutant dispersion in rivers: Estimation of aggregated dead zone parameters with surrogate data
Authors: Smith Paul , Beven Keith , Tawn Jonathan , Blazkova Sarka , Merta Ladislav
Published: April 2006

Much has been done to mitigate the effects of intermittent discharges of pollutants; however, pollution incidents still occur, and the downstream transport and dispersion of pollutants must be predicted. The application of transient storage models based on the advection dispersion equation is often limited by the strong dependence of the parameters on changes in discharge. In this paper a methodology is outlined for estimating the parameters of the simple aggregated dead zone model using surrogate data derived from continuous water quality measurements such as conductivity, including a full treatment of the errors and prediction uncertainties within a Bayesian framework. This methodology is demonstrated in the prediction of a tracer experiment on a reach of the river Elbe in the Czech Republic.
Immobilization of Pycnoporus sanguineus laccase on copper tetra‐aminophthalocyanine–Fe3O4 nanoparticle composite
Authors: Huang Jun , Xiao Haiyan , Li Bin , Wang Juntao , Jiang Desheng
Published: April 2006

A magnetic nanoparticle composite of CuTAPc (copper tetra‐aminophthalocyanine)–Fe3O4 was prepared by in situ complexation and was characterized by FTIR (Fourier‐transform IR) spectroscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, FESEM (field emission scanning electron microscopy) micrograph and hysteresis loop. The results showed that CuTAPc formed the covering layer on the surface of the composite. Using glutaraldehyde, laccase was covalently attached to the surface of the composite. When white‐rot‐fungus (Pycnoporus sanguineus) laccase was immobilized on the CuTAPc–Fe3O4 nanoparticle composite in PBS, the optimal reaction pH was 5.0 and the optimal temperature was 0 °C. When 2.0 mg/ml laccase solution was used, the immobilization yield was 20%. The activity, Km, kcat/Km and Vmax values of the immobilized laccase were 1.43 units/mg, 2.38×10−5 mol/l, 2.1×103 mol−1·s−1·l and 1.19×10−6 mol·l−1·min−1 respectively. The system exhibited maximum enzyme activity at pH 3.0 and at 45 °C. After storage at 4 °C for 1 month, the residual activity of the immobilized laccase was 85% of its initial activity, while that of free laccase was only 30%. During 240 min incubation at 55 °C, the activity of immobilized laccase quickly increased to a maximum after 150 min and then decreased to 95% in the next 90 min, while the activity of the free enzyme decreased monotonically to 28%. After five consecutive operations, the immobilized laccase still retained 80% of its initial activity.
Synthesis of a precursor dipeptide of RGDS (Arg‐Gly‐Asp‐Ser) catalysed by the industrial protease alcalase
Authors: Hou Rui‐Zhen , Yang Yan , Li Gang , Huang Yi‐Bing , Wang Hua , Liu Yun‐Jia , Xu Li , Zhang Xue‐Zhong
Published: April 2006

Synthesis of Bz‐Arg‐Gly‐NH2 (N‐benzoylargininylglycinamide) [a precursor dipeptide of RGDS (Arg‐Gly‐Asp‐Ser)] catalysed by protease in water/organic co‐solvent systems was studied. Starting substrates were N‐benzoyl‐l‐arginine ethyl ester hydrochloride (acyl donor) and glycinamide (nucleophile). Acetonitrile was selected as the organic solvent. Alcalase, an industrial alkaline protease, was applied to the synthesis of the target dipeptide. The conditions of the synthesis reaction were optimized by examining the effects of several factors, including water content, temperature, pH, molar ratio of the substrates and reaction time, on the yield of Bz‐Arg‐Gly‐NH2. The optimum conditions were established to be pH 10.0, 45 °C, in acetonitrile/0.1 M Na2CO3/NaHCO3 buffer system (90:10, v/v) for 1 h with a dipeptide yield of 82.9%.
Regulation of nitric oxide production in snail (Lymnaea stagnalis) defence cells: a role for PKC and ERK signalling pathways
Authors: Wright Bernice , Lacchini Audrey H. , Davies Angela J. , Walker Anthony J.
Published: April 2006

Background information. Nitric oxide (NO) is an important molecule in innate immune responses. In molluscs NO is produced by mobile defence cells called haemocytes; however, the molecular mechanisms that regulate NO production in these cells is poorly understood. The present study focused on the role of cell signalling pathways in NO production by primary haemocytes from the snail Lymnaea stagnalis.

Results. When haemocytes were challenged with PMA (10 μM) or the β‐1,3‐glucan laminarin (10 mg/ml), an 8‐fold and 4‐fold increase in NO production were observed after 60 min respectively. Moreover, the NOS (NO synthase) inhibitors L‐NAME (NG‐nitro‐L‐arginine methyl ester) and L‐NMMA (NG‐monomethyl‐L‐arginine) were found to block laminarin‐ and PMA‐induced NO synthesis. Treatment of haemocytes with PMA or laminarin also increased the phosphorylation (activation) status of PKC (protein kinase C). When haemocytes were preincubated with PKC inhibitors (calphostin C or GF109203X) or inhibitors of the ERK (extracellular‐signal‐regulated kinase) pathway (PD98059 or U0126) prior to challenge, significant reductions in PKC and ERK phosphorylation and NO production were observed following exposure to laminarin or PMA. The greatest effect on NO production was seen with GF109203X and U0126, with PMA‐induced NO production inhibited by 94% and 87% and laminarin‐induced NO production by 50% and 91% respectively.

Conclusions. These data suggest that ERK and PKC comprise part of the signalling machinery that regulates NOS activation and subsequent production of NO in molluscan haemocytes. To our knowledge, this is the first report that shows a role for these signalling proteins in the generation of NO in invertebrate defence cells.
Electromechanical coupling coefficient of an ultrasonic array element
Authors: Kim Moojoon , Kim Jungsoon , Cao Wenwu
Published: April 2006

One of the most important parameters for characterizing piezoelectric materials is the so-called electromechanical coupling coefficient, k, which describes the electromechanical coupling strength. Although this parameter should be an intrinsic material parameter, it appears to depend on the aspect ratio of the resonator. There are three different values defined for three extreme geometries, k33, k33′, and kt, and they differ by more than 50%. Unfortunately, these three values cannot describe resonators of general geometries and also create conceptual confusion. Here, we provide a unified formula that will accurately describe the coupling coefficient of rectangular slender bar transducer array element with any aspect ratio.
Mechanical unfolding revisited through a simple but realistic model
Authors: West Daniel K. , Olmsted Peter D. , Paci Emanuele
Published: April 2006

Single-molecule experiments and their application to probe the mechanical resistance and related properties of proteins provide a new dimension in our knowledge of these important and complex biological molecules. Single-molecule techniques may not have yet overridden solution experiments as a method of choice to characterize biophysical and biological properties of proteins, but have stimulated a debate and contributed considerably to bridge theory and experiment. Here we demonstrate this latter contribution by illustrating the reach of some theoretical findings using a solvable but nontrivial molecular model whose properties are analogous to those of the corresponding experimental systems. In particular, we show the relationship between the thermodynamic and the mechanical properties of a protein. The simulations presented here also illustrate how forced and spontaneous unfolding occur through different pathways and that folding and unfolding rates at equilibrium cannot in general be obtained from forced unfolding experiments or simulations. We also study the relationship between the energy surface and the mechanical resistance of a protein and show how a simple analysis of the native state can predict much of the mechanical properties of a protein.
Ferroelectric-dielectric tunable composites
Authors: Sherman Vladimir O. , Tagantsev Alexander K. , Setter Nava , Iddles David , Price Tim
Published: April 2006

The dielectric response of ferroelectric-dielectric composites is theoretically addressed. Dielectric permittivity, tunability (relative change of the permittivity driven by dc electric field), and loss tangent are evaluated for various composite models. The analytical results for small dielectric concentration and relative tunability are obtained in terms of the traditional electrostatic consideration. The results for large tunability are obtained numerically. A method is proposed for the evaluation of the tunability and loss at large concentrations of the dielectric. The basic idea of the method is to reformulate the effective medium approach in terms of electrical energies stored and dissipated in the composite. The important practical conclusion of the paper is that, for random ferroelectric-dielectric composite, the addition of small amounts of a linear dielectric into the tunable ferroelectric results in an increase of the tunability of the mixture. The loss tangent of such composites is shown to be virtually unaffected by the addition of moderate amounts of the low-loss dielectric. The experimental data for (Ba,Sr)TiO3 based composites are analyzed in terms of the theory developed and shown to be in a reasonable agreement with the theoretical results.
Effect of a gate insulator with a long alkyl side chain on the performance of pentacene organic thin-film transistors
Authors: Pyo Seungmoon , Lee Yoonjeong , Jeon Jihyun , Yi Mi Hye , Kwon Soon-Ki
Published: April 2006

This paper discusses some recent findings regarding the effect of a long alkyl (C18) group on the performance of organic thin-film transistors (OTFTs). Two different types of polymer gate insulators, BTDA-DACM (PI-1) and BTDA-DACM-DA18IM (PI-2), were prepared by a condensation polymerization. DA18IM with C18 alkyl group was used as a comonomer for the preparation of PI-2. We found that the C18 alkyl group of PI-2 protruded to the surface and made the surface more hydrophobic. The modified surface strongly affects the initial growth mechanism of pentacene on PI-2 and, subsequently, the device performance. The performance of pentacene OTFTs with the PI-2 was superior to that of the OTFTs with the PI-1. The OTFTs with PI-2 exhibited a carrier mobility of 0.4cm2∕Vs, current on/off ratio of 1.5×105, subthreshold slope of 2.1V∕dec. This suggested that the C18 alkyl group over the surface of PI-2 contributed to the better interface formation between PI-2 and pentacene and resulted in more favorable initial growth of pentacene and much improved performance.
Induced changes in surface band bending of n-type and p-type AlGaN by oxidation and wet chemical treatments
Authors: Lin Yow-Jon , Chu Yow-Lin , Lin Wen-Xiang , Chien Feng-Tso , Lee Chi-Sen
Published: April 2006

The surface chemistry and electrical properties of p-type and n-type AlGaN surfaces were studied via x-ray photoelectron spectroscopy before and after oxidation and wet chemical treatments. Shifts in the surface Fermi level were measured with the change in onset of the valence-band spectra. Oxidation and HF and (NH4)2Sx treatments on p-type AlGaN (n-type AlGaN) led to an increase (the reduction) in the surface band bending due to more N vacancies and N vacancies being occupied by S (i.e., donorlike states) than Al vacancies and Ga vacancies (i.e., acceptorlike states) near the p-type AlGaN (n-type AlGaN) surface region. The changes in surface chemistry indicate that oxidation and wet chemical treatments alter the surface state density through the formation of more donorlike states.
Transient injection and fast switch on in p-i-n diodes
Authors: Mnatsakanov T. T. , Tandoev A. G. , Yurkov S. N. , Levinshtein M. E. , Ivanov P. A. , Palmour J. W.
Published: April 2006

Fast and ultrafast switch-on processes in p-n diodes at very high current densities j and very short current rise time τ0 have been investigated analytically. It is demonstrated that even at relatively modest values of the rate of current density rise dj∕dt, the switch-on transient fundamentally differs from that in the “classical” quasineutral mode. The switch-on processes occur in modes with broken neutrality. The base resistivity is modulated by fast hole and electron waves that propagate toward each other at the maximum possible (saturation) velocities. The conditions for the appearance of fast waves have been established. Analytical expressions for the initial stage of propagation of these waves have been derived. Adequate computer simulation confirms the analytical results.
Spectroscopic and electrical properties of atomic layer deposition Al2O3 gate dielectric on surface pretreated Si substrate
Authors: Xu Min , Xu Cong-Hui , Ding Shi-Jin , Lu Hong-Liang , Zhang David Wei , Wang Li-Kang
Published: April 2006

Interfacial layer (IL) suppression is a big concern in atomic layer deposition (ALD) of high-k gate dielectrics. We have studied two kinds of surface pretreatments for ALD Al2O3 deposition: surface Al(CH3)3 (TMA) pretreatment (ST) and surface nitridation (SN). High resolution transmission electron microscopy images show that the IL has been effectively suppressed after performing surface pretreatments. X-ray photoelectron spectroscopy results indicate that both surface pretreatments are beneficial for compact Al2O3 deposition, however, the sample with SN shows much better interface thermal stability. And electrical characterizations demonstrate that the capacitor with SN exhibits better interface property, lower leakage current, and smaller capacitance-voltage (C-V) hysteresis. Based on transient capacitance measurement (C-t) and C-V measurement with different gate bias sweep ranges, a “shallow level trap” model is proposed to explain well the difference of C-V hysteresis between ST and SN.
Temperature dependence of the LO phonon sidebands in free exciton emission of GaN
Authors: Xu S. J. , Li G. Q. , Xiong S.-J. , Che C. M.
Published: April 2006

Temperature-dependent radiative recombination of free excitons involving one or two LO phonons in GaN is investigated in detail. It is found that both phonon sidebands possess asymmetric lineshapes and their energy spacings from the zero-phonon line strongly deviate from the characteristic energy of LO phonons as the temperature increases. Furthermore, the deviation rates of one- and two-phonon sidebands are significantly different. Segall-Mahan [Phys. Rev. 171, 935 (1968)] theory, taking the exciton-photon and exciton-phonon interactions into account, is employed to calculate the sidebands of one or two LO phonons for free excitons in a wide temperature range. Excellent agreement between the theory and experiment is achieved by using only one adjustable parameter, which leads to determination of the effective mass of heavy holes (∼0.5m0).
Origin of noise in AlGaN∕GaN heterostructures in the range of 10–100MHz
Authors: Vitusevich S. A. , Danylyuk S. V. , Kurakin A. M. , Klein N. , Offenhäusser A. , Petrychuk M. V. , Belyaev A. E.
Published: April 2006

We report on the noise origin in AlGaN∕GaN heterostructures for the frequency range of 10–100MHz. High electron mobility transistor heterostructures were designed for high-power and high-frequency application and grown on SiC substrates. The structures were patterned with Ohmic transmission line model (5, 10, 15, 20, and 25micrometer working distances) contacts and were analyzed using I-V characteristics and noise figure measurements. Different possible mechanisms of noise origin were considered and investigated in detail. The results of our analysis show that the thermal noise and hot carrier noise play a minor role in the structure in the investigated frequency range. At the same time, a dominant generation-recombination (G-R) noise is revealed. Moreover, two different components of G-R noise are found demonstrating different temperature dependences and as a result different physical origins of the noise are established. A detailed analysis of potential profiles of the structure calculated self-consistently for several voltages allows us to propose a physical model for the observed noise behavior. The fluctuations of electron concentration on the first quantum level of the quantum well and the scattering of the electrons in the barrier layer play a definite role in the noise phenomena.
Degradation of the piezoelectric response of sputtered c-axis AlN thin films with traces of non-(0002) x-ray diffraction peaks
Authors: Sanz-Hervás A. , Clement M. , Iborra E. , Vergara L. , Olivares J. , Sangrador J.
Published: April 2006

We have observed the apparently anomalous behavior of sputtered AlN thin films that exhibit poor piezoelectric response in spite of having (0002) preferred orientation and narrow rocking curve with full width at half maximum below 2°; however, other AlN films with weak 0002 x-ray diffraction peak and rocking curve as wide as 8° provide a good value of the piezoelectric coefficient d31 (2.1pm∕V). The critical difference between both types of films is the presence of traces of non-(0002) reflections in the x-ray diffraction patterns of the former. Non-(0002) reflections may be related to defects (probably, inversion domains) that significantly reduce the net piezoelectric field.
Signal electronics for an atomic force microscope equipped with a double quartz tuning fork sensor
Authors: Rust H.-P. , Heyde M. , Freund H.-J.
Published: April 2006

Signal electronics equipped with a bandpass filter phase detector for noncontact atomic force microscopy (ncAFM) has been developed. A double quartz tuning fork assembly is used as a force sensor, where one fork serves as a dither tuning fork, while the other is used as a measuring tuning fork. An electrically conductive Pt90Ir10 tip enables the sensor to work in both scanning tunneling microscopy (STM) and AFM modes. Electronic circuits for self-oscillation control and for frequency detection are given in detail. Atomically resolved STM and ncAFM images of a thin alumina film on NiAl(110) are shown with the microscope cooled down to 4.5K by liquid helium.
SUBVERSIVE REVISIONS IN THE WORK OF CHARLES DE BEYS
Authors: Scott Paul
Published: April 2006

Charles de Beys is a comic author whose
L'Hospital des fous
(1636), about an eloping couple who seek refuge in a lunatic asylum, was reworked and published in 1653 as
Les Illustres fous.
It has long been accepted that the modifications to this play were essentially minor; however, the character of the hospital's director, or
concierge
, becomes the play's principal figure in the revised version. As well as having a greatly expanded role, the
concierge
is depicted in a more sinister, and even tyrannical, light in the 1653 comedy. The difference in emphasis between the two versions may be explained by Beys's own experience during the intervening period: in the mid-1630s he was incarcerated in the Bastille for several months by order of Cardinal Richelieu on suspicion of having authored the
Miliade
, a satirical poem attacking the prelate. The second drama becomes not so much a comedy depicting the folly of lovers through the escapades of outsiders who are temporarily institutionalized as a subtle commentary on the potential misuse of such places to lock away those deemed unsuitable for political reasons. This reading is supported by examining the ironic subtext that underpins Beys's sonnets dedicated to Richelieu, as well as the second play's
dedicace.
Inflammatory Myopathies
Authors: Pourmand Rahman
Published: April 2006

The inflammatory myopathies are a heterogeneous group of immune-mediated myopathies that present as pure form or in association with other autoimmune diseases or malignancy. Although they share few similarities in their clinical presentations and pathophysiologic aspects, for the most part they are different and distinctive. The 3 major subsets are: dermatomyositis (DM), polymyositis (PM), and sporadic inclusion body myositis (S-IBM).

This review highlights our understanding of these disorders, their clinical aspects, diagnosis, pathogenesis, and treatment.