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At least 199 records · Page 11Linked to original sources
PubMed2003

Linear stability analysis in a liquid layer with a surface velocity gradient.

A case of combined planar Couette-Poiseuille flow corresponding to vanishing horizontal flux has been generalized by the introduction of a model for the surface velocity gradient. A relation corresponding to the Orr-Sommerfeld equation has been derived for this model. The critical value of the surface velocity gradient has been obtained. At the critical point, the corresponding critical Reynolds number equals infinity. Using an approximated method we estimated the behavior of the critical Reynolds number for a slightly overcritical surface velocity gradient.

Journal Article↗
PubMed2003

Stability analysis of spatiotemporal cnoidal waves in cubic nonlinear media.

We analyze numerically the modulational instability of spatiotemporal cnoidal waves of cn, dn, and sn types that are periodic along a single space coordinate and are uniform in time. The band of possible increments is calculated for all three types of cnoidal waves as a function of parameter describing the degree of localization of the wave field energy. It is shown that this band transforms into a set of discrete values for waves of cn and dn types in the limit of strong spatial localization. Simulation of perturbed cnoidal-wave propagation revealed suppression of collapse and multiple-wave filamentation on the developed stage of instability. Different instability scenarios are considered in detail.

Journal Article↗
PubMed2004

Threshold values, stability analysis, and high-q asymptotics for the coloring problem on random graphs.

We consider the problem of coloring Erdös-Rényi and regular random graphs of finite connectivity using q colors. It has been studied so far using the cavity approach within the so-called one-step replica symmetry breaking (1RSB) ansatz. We derive a general criterion for the validity of this ansatz and, applying it to the ground state, we provide evidence that the 1RSB solution gives exact threshold values c(q) for the transition from the colorable to the uncolorable phase with q colors. We also study the asymptotic thresholds for q>>1 finding c(q) =2q ln q-ln q-1+o (1) in perfect agreement with rigorous mathematical bounds, as well as the nature of excited states, and give a global phase diagram of the problem.

Journal Article↗
PubMed2002

Application of the moment condition to noise simulation and to stability analysis.

It is well-known that low frequency noises (flicker FM and random walk FM) are not stationary; it is not possible to define either the mean value or the (true) variance. Therefore, the use of a stationary approach yields convergence problems unless a low cut-off frequency is introduced, the physical meaning of which is not clear. As an example, in the case of random walk FM, the mean frequency of an oscillator does not converge if the analysis duration tends toward infinity. However, linear drifts appear if a phase sequence of random walk FM is observed over a duration smaller than the inverse of its low cut-off frequency. Moreover, the estimators, which are devoted to these non-stationary processes (i.e., the Hadamard variance), are insensitive to linear frequency drifts and converge for lower frequency noises (f(-4) FM). The moment condition explains the link between insensitivity to drifts and convergence for low frequency noises in a stationary approach. This condition may be summarized by the following consideration: the divergence effect of a low frequency noise for the lowest frequencies induces a false drift with random drift coefficients; the lower the low cut-off frequency, the higher the variance of the coefficients of this drift. These variances may be known by theoretical calculations. The order of the drift is directly linked to the power law of the noise. The moment condition will be demonstrated and applied for creating new estimators (new variances) and for simulating low frequency noises with a very low cut-off frequency.

Journal Article↗
PubMed2004

Indirect adaptive control for synchronous generator: comparison of MLP/RBF neural networks approach with Lyapunov stability analysis.

This paper compares two indirect adaptive neurocontrollers, namely a multilayer perceptron neurocontroller (MLPNC) and a radial basis function neurocontroller (RBFNC) to control a synchronous generator. The different damping and transient performances of two neurocontrollers are compared with those of conventional linear controllers, and analyzed based on the Lyapunov direct method.

Neural Networks, Computer↗
PubMed2006

Stability analysis of a wastewater treatment plant with saturated control.

This paper presents a saturated proportional controller that achieves depollution of wastewater in a continuous anaerobic digester. This goal is reached by defining a region of the state-space where the depollution is achieved and forcing attractivity and invariance of this region. The control variable is the dilution rate and the controlled variable is a linear combination (S(lambda)) of the substrates concentrations, that could be the chemical oxygen demand or the biological oxygen demand, depending on the value of A. No measurement of the substrates concentrations in the input flow is required: the only necessary measurement is S(lambda).

Bacteria, Anaerobic↗
PubMed1984

Stabilized analysis of antidepressant drugs by solvent-recycled liquid chromatography: procedure and proposed resolution mechanisms for chromatography.

In this efficient extraction and isocratic liquid-chromatographic procedure for measuring eight antidepressant drugs in serum (amoxapine, 8-hydroxyamoxapine, doxepin, desmethyldoxepin, imipramine, desipramine, amitriptyline, and nortriptyline), they are extracted from serum as a group by use of disposable solid-phase cyanopropyl columns; the eluate is injected directly onto a Zorbax cyanopropyl analytical column. This sample preparation circumvents potential problems of drug instability associated with the usual evaporating/concentrating techniques. Recycling the acetic acid/acetonitrile/n-butylamine mobile phase not only maintains a stable, cost-effective system, it also prolongs column life. Standard curves for all eight drugs are linear to 1000 micrograms/L; the detection limit is 8-10 micrograms/L. For all drugs and metabolites, within-run CVs were 0.9 to 2.2%, between-run CVs 2.1 to 3.8%. Analytical recovery of the solid-phase extraction step was 85 to 100% (mean = 94.5%) for all analytes. Standards and controls, stored frozen in drug-free serum, are stable for at least six months. We also report a study of separation mechanisms.

Antidepressive Agents, Tricyclic↗
PubMed1981

[The synthesis, analysis, stability and biotransformation of N-(3-methylphenyl)-N'-cyanobenzamidine and analogues (author's transl)].

The antivirally active N-(3-methylphenyl)-N'-cyanobenzamidine (1) and some of its analogues were synthetized by the reaction of the ethyl ester of N-cyanobenzimidic acid with the corresponding arylamines. The analytical profile of 1 and of some of its analogues, which is needed for stability and biotransformation studies, was established by means of the following techniques: solubility determination, TLC, UV, IR and MS. When 1 is heated with methanolic HCl and NaOH (0.5 mol/l) for 1 h, several hydrolysis products are formed: m-toluidine, benzoic acid, benzoic acid 3-toluidide, N-(3-methylphenyl)benzamidine, N-(3-methylphenyl)-N'-carboxamidobenzamidine, benzoylurea, However, 1 is fairly stable under physiological conditions so biotransformation studies are not affected. After oral application of 1 to rats, the 3-hydroxymethyl and the 3-methyl-5-hydroxy compound as well as its glucuronide and sulphate conjugate were detected in the urine. The faeces contained considerable amounts of unchanged 1 (incomplete absorption).

Amidines↗
PubMed1997

Stability analysis of a controller for a high power ultrasonic actuator.

High power ultrasonic transducers for medical surgery require closed loop regulation of operating frequency and amplitude. This is a tutorial design paper which provides a description of the fundamental component properties and the criteria which are required to drive a power ultrasonic transducer. In addition, a classical root locus system analysis of the regulator servo in conjunction with requirements for idealizing individual blocks is also provided.

Surgical Equipment↗
PubMed2003

Why is trehalose an exceptional protein stabilizer? An analysis of the thermal stability of proteins in the presence of the compatible osmolyte trehalose.

Trehalose, a naturally occurring osmolyte, is known to be an exceptional stabilizer of proteins and helps retain the activity of enzymes in solution as well as in the freeze-dried state. To understand the mechanism of action of trehalose in detail, we have conducted a thorough investigation of its effect on the thermal stability in aqueous solutions of five well characterized proteins differing in their various physico-chemical properties. Among them, RNase A has been used as a model enzyme to investigate the effect of trehalose on the retention of enzymatic activity upon incubation at high temperatures. 2 m trehalose was observed to raise the transition temperature, Tm of RNase A by as much as 18 degrees C and Gibbs free energy by 4.8 kcal mol-1 at pH 2.5. There is a decrease in the heat capacity of protein denaturation (DeltaCp) in trehalose solutions for all the studied proteins. An increase in the DeltaG and a decrease in the DeltaCp values for all the proteins points toward a general mechanism of stabilization due to the elevation and broadening of the stability curve (DeltaG versus T). A direct correlation of the surface tension of trehalose solutions and the thermal stability of various proteins has been observed. Wyman linkage analysis indicates that at 1.5 m concentration 4-7 molecules of trehalose are excluded from the vicinity of protein molecules upon denaturation. We further show that an increase in the stability of proteins in the presence of trehalose depends upon the length of the polypeptide chain. The pH dependence data suggest that even though the charge status of a protein contributes significantly, trehalose can be expected to work as a universal stabilizer of protein conformation due to its exceptional effect on the structure and properties of solvent water compared with other sugars and polyols.

Animals↗