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PubMed2003

Numerical viscosity and resolution of high-order weighted essentially nonoscillatory schemes for compressible flows with high Reynolds numbers.

A quantitative study is carried out in this paper to investigate the size of numerical viscosities and the resolution power of high-order weighted essentially nonoscillatory (WENO) schemes for solving one- and two-dimensional Navier-Stokes equations for compressible gas dynamics with high Reynolds numbers. A one-dimensional shock tube problem, a one-dimensional example with parameters motivated by supernova and laser experiments, and a two-dimensional Rayleigh-Taylor instability problem are used as numerical test problems. For the two-dimensional Rayleigh-Taylor instability problem, or similar problems with small-scale structures, the details of the small structures are determined by the physical viscosity (therefore, the Reynolds number) in the Navier-Stokes equations. Thus, to obtain faithful resolution to these small-scale structures, the numerical viscosity inherent in the scheme must be small enough so that the physical viscosity dominates. A careful mesh refinement study is performed to capture the threshold mesh for full resolution, for specific Reynolds numbers, when WENO schemes of different orders of accuracy are used. It is demonstrated that high-order WENO schemes are more CPU time efficient to reach the same resolution, both for the one-dimensional and two-dimensional test problems.

Journal Article↗
PubMed2006

Viscosity of a binary mixture: approach to the hydrodynamic limit.

We have used equilibrium and nonequilibrium molecular dynamics simulations to study the solute self-diffusion coefficient and the shear rate dependence of the solution viscosity in solutions of model nanocolloidal particles that range in mass ratio from mu=1 up to mu=50 and size ratio from s=1 up to s=4.03 at various concentrations. The zero shear rate viscosities and the initial rates of shear thinning were determined from data in the shear rate region in which the suspension is strongly shear thinning while the solvent remains Newtonian or is weakly shear thinning. The rate of shear thinning increased dramatically with solute volume fraction, regardless of whether the increase was due to increasing solute size or increasing the solute concentration. In a series of simulations in which the mass ratio was varied while keeping the size ratio fixed at s=1, we found that the approach of the viscosities and self-diffusion coefficients to their limiting mass ratio independent values was well described by a rather simple exponential dependence on mass ratio. The concentration dependence of the limiting infinite mass ratio values of the self-diffusion coefficients and zero shear rate viscosities were determined, and used to compute the hydrodynamic radius RH of the solute particles by various methods. The values of RH that were obtained by the different methods were reasonably consistent with each other, and indicated that the radius at which the slip boundary condition holds is slightly smaller than the cross-interaction radius between the solute and solvent particles.

Journal Article↗
PubMed2001

Shear viscosity of strongly coupled Yukawa systems on finite length scales.

The Yukawa shear viscosity has been calculated using nonequilibrium molecular dynamics. Near the viscosity minimum, we find exponential decay consistent with the Navier-Stokes equation, with significant deviations on finite length scales for larger viscosity values. The viscosity is determined to be nonlocal on a scale length consistent with the correlation length, revealing the length scales necessary for obtaining transport coefficients in the hydrodynamic limit by nonequilibrium molecular dynamics methods. Our results are quasiuniversal with respect to excess entropy for excess entropies well below unity.

Journal Article↗
PubMed2006

Anomalous viscosity of an expanding quark-gluon plasma.

We argue that an expanding quark-gluon plasma has an anomalous viscosity, which arises from interactions with dynamically generated color fields. We derive an expression for the anomalous viscosity in the turbulent plasma domain and apply it to the hydrodynamic expansion phase, when the quark-gluon plasma is near equilibrium. The anomalous viscosity dominates over the collisional viscosity for weak coupling and not too late times. This effect may provide an explanation for the apparent "nearly perfect" liquidity of the matter produced in nuclear collisions at the Relativistic Heavy Ion Collider without the assumption that it is a strongly coupled state.

Journal Article↗
PubMed2006

Measuring shear viscosity using transverse momentum correlations in relativistic nuclear collisions.

Elliptic flow measurements at the Brookhaven National Laboratory Relativistic Heavy Ion Collider suggest that quark-gluon fluid flows with very little viscosity compared to weak-coupling expectations, challenging theorists to explain why this fluid is so nearly "perfect." It is therefore vital to find quantitative experimental information on the viscosity of the fluid. We propose that measurements of transverse momentum fluctuations can be used to determine the shear viscosity. We use current data to estimate the viscosity-to-entropy ratio in the range from 0.08 to 0.3 and discuss how future measurements can reduce this uncertainty.

Journal Article↗
PubMed1998

The responses of cytochrome redox state and energy metabolism to dehydration support a role for cytoplasmic viscosity in desiccation tolerance

To characterize the depression of metabolism in anhydrobiotes, the redox state of cytochromes and energy metabolism were studied during dehydration of soaked cowpea (Vigna unguiculata) cotyledons and pollens of Typha latifolia and Impatiens glandulifera. Between water contents (WC) of 1.0 and 0.6 g H2O/g dry weight (g/g), viscosity as measured by electron spin resonance spectroscopy increased from 0.15 to 0.27 poise. This initial water loss was accompanied by a 50% decrease in respiration rates, whereas the adenylate energy charge remained constant at 0.8, and cytochrome c oxidase (COX) remained fully oxidized. From WC of 0.6 to 0.2 g/g, viscosity increased exponentially. The adenylate energy charge declined to 0.4 in seeds and 0.2 in pollen, whereas COX became progressively reduced. At WC of less than 0.2 g/g, COX remained fully reduced, whereas respiration ceased. When dried under N2, COX remained 63% reduced in cotyledons until WC was 0.7 g/g and was fully reduced at 0.2 g/g. During drying under pure O2, the pattern of COX reduction was similar to that of air-dried tissues, although the maximum reduction was 70% in dried tissues. Thus, at WC of less than 0.6 g/g, the reduction of COX probably originates from a decreased O2 availability as a result of the increased viscosity and impeded diffusion. We suggest that viscosity is a valuable parameter to characterize the relation between desiccation and decrease in metabolism. The implications for desiccation tolerance are discussed.

Journal Article↗
PubMed2002

Viscosity minimum in bimodal concentrated suspensions under shear.

We study a model of concentrated suspensions under shear in two dimensions. Interactions between suspended particles are dominated by direct-contact viscoelastic forces and the particles are neutrally bouyant. The bimodal suspensions consist of a variable proportion between large and small droplets, with a fixed global suspended fraction. Going beyond the assumptions of the classical theory of Farris (R.J. Farris, Trans. Soc. Rheol. 12, 281 (1968)), we discuss a shear viscosity minimum, as a function of the small-to-large-particle ratio, in shear geometries imposed by external body forces and boundaries. Within a linear-response scheme, we find the dependence of the viscosity minimum on the imposed shear and the microscopic drop friction parameters. We also discuss the viscosity minimum under dynamically imposed shear applied by boundaries. We find a reduction of macroscopic viscosity with the increase of the microscopic friction parameters that is understood using a simple two-drop model. Our simulation results are qualitatively consistent with recent experiments in concentrated bimodal emulsions with a highly viscous or rigid suspended component.

Journal Article↗
PubMed1998

Partitioning the effects of temperature and kinematic viscosity on the C-start performance of adult fishes

Temperature has been shown to have a significant effect on swimming performance in teleost fish. This thermal dependence has usually been attributed to correlated changes in muscle contractile physiology. However, the physical properties of all materials, including both water and living tissues, are profoundly affected by changes in temperature. In particular, both the kinematic viscosity of water and the flexible body dynamics of the fish (independent of muscle contractile activity) are substantially higher at lower temperatures. In this study, we investigated the extent to which the observed thermal dependence of locomotor performance of fish simply reflects changes in the biophysical properties of the aqueous medium independent of the changing physiology of the animals. C-starts (escape swimming) of adult goldfish (Carassius auratus, length approximately 8 cm) were video-taped (400 frames s-1) at 5 degreesC and 20 degreesC in fresh water and at 20 degreesC in fresh water containing 1.2 % dextran (kinematic viscosity equivalent to that of water at 5 degreesC). Temperature had a significant positive effect on maximum forward velocity (m s-1), angular velocity (degrees s-1) and total distance moved, but viscosity had no detectable effect on any kinematic parameters at 20 degreesC. Since viscous forces may have more pronounced effects on smaller fish, C-starts of adult guppies (Poecilia reticulata, length approximately 2 cm) were video-taped in fresh water and in fresh water containing 1.2 % and 3. 6 % dextran at 20 degreesC. Viscosity had a significant effect on kinematic variables only at 3.6 % dextran; at 1.2 % dextran, the effects were marginal. It was concluded that most of the observed thermal dependence of C-starts at temperatures normally experienced by these fish was due to changes in physiological processes as opposed to physical changes in the environment.

Journal Article↗
PubMed1997

Effects of Whole Blood Viscosity on Atherogenesis.

There is a high correlation between high whole blood viscosity and the well known risk factors for arterial occlusive disease: hypertension, hyperlipidemia, diabetes, male sex, age, smoking, and obesity. These risk factors increase whole blood viscosity, whereas the preventive factors of arterial occlusive disease such as fish oil, aspirin, alcohol, and exercise probably tend to reduce whole blood viscosity. The protective adaptation theory recently presented by Kensey and Cho1 proposed high whole blood viscosity as one of the major factors that make up the mechanical injury possibly inducing arterial occlusive disease. New diagnostic and prophylactic treatments for arterial occlusive disease are suggested. An accurate, convenient, and cost-effective blood viscometer that can be used in a clinical environment might become a useful diagnostic screening device for patients at risk for arterial occlusive disease, and it would help discover new prophylactic treatments.

Journal Article↗
PubMed1992

Viscosity and stability stijdies of hydroxypropyl methylcellulose polymer solutions.

Viscosity studies of three grades of hydroxypropyl methylcellulose polymer solutions have been done in detail. The solutions showed pseudoplastic behaviour and their viscosity increased with increase in concentration of the polymer, but rise in temperature significantly lowered the viscosity. The effect of aging on solutions' viscosity at 25 degrees C was measured.

Journal Article↗
PubMed1986

Effect of sex, age, body weight, and smoking on plasma viscosity.

The influence of age, sex, smoking, and body weight on blood plasma viscosity of a group of 639 healthy subjects was investigated in a randomized study. In order to determine the individual effects on a homogeneous collective of healthy subjects, all participants of this field study had to undergo physical examination, laboratory analysis, doppler sonography, and they were asked for a health history. The presence of disease or a disorder eliminated the participant from the study. Plasma viscosity was measured with the capillary tube plasma viscometer. Quality control according to clinical chemistry guidelines was conducted throughout the study to ensure the accuracy of the values measured being. It could be demonstrated that plasma viscosity is significantly elevated in healthy overweight subjects. However, a relationship between plasma viscosity and age, sex, and cigarette smoke inhalation could not be found.

Adult↗
PubMedDate not supplied

[The effect of energy density and viscosity of porridges on the energy intake of infants].

It has been known for at least 25 years that high energy density and low viscosity are desirable attributes of complementary foods. However, the effects of increasing energy density and reducing viscosity of gruels on infant energy intake have only been studied in the last 10 years. Works published between 1986 and 1992 were carried out with infants randomly selected in urban slums or rural areas in India and Tanzania. When gruels are given in a single meal, all of them showed higher energy intake with both high energy-dense and low viscosity gruels. But these studies do not give information about the effects of these gruels on daily energy intake from gruels and on total daily energy intake when they are given with feeding frequencies corresponding to traditional weaning practices. Works published since 1992 were conducted among infants suffering from or recovering from acute diarrhoea or severe malnutrition. A positive effect of increasing energy density of gruels was observed not only on energy intake from gruels but also on total energy intake. On the other hand, effectiveness of reducing viscosity was not clearly demonstrated. Further studies are required, particularly to identify the conditions for which high energy dense gruels prepared from bulk-reduced starchy staples are able to significantly improve infant energy intake.

Diarrhea, Infantile↗
PubMed1962

The viscosity of erythrocyte suspensions. A review of theory.

Blood and erythrocyte suspensions have non-linear pressure-flow curves and so do not possess a unique Newtonian coefficient of viscosity (or its reciprocal, the fluidity) except in the physically unrealizable limits of infinite flow rate and tube radius. However, three coefficients can be defined which are related mathematically to one another and which converge in these infinite limits. They are first, the apparent fluidity, which is proportional to the slope of the line joining any given point on the pressure-flow curve with the origin; second, the differential fluidity, which is proportional to the slope of the pressureflow curve itself at any given point; and third, the generalized fluidity which is proportional to the ratio of the shear rate to the applied stress across any given cylindrical lamina (taken here at the tube wall) within the tube. These three coefficients, which are related mathematically to one another, have been calculated from measured pressure-flow curves for erythrocyte suspensions in glass tubes, and the differential viscosity has been used to develop a simple flow model in which the shear-dependent viscosity is assumed to arise from "structural changes" in the fluid as the flow rate increases. Although the physical basis of such structural changes is uncertain, it is likely that some sort of axial redistribution of the red cells is of greatest importance at normal, physiological hematocrit values.

Erythrocytes↗
PubMed1950

The intrinsic viscosity of mixed protein systems, including studies of plasma and serum.

Experimental evidence is presented that the intrinsic viscosity of solutions of mixed proteins obeys the additive equation See PDF for Equation. The datum serves to characterize the system, and combined with other analytical techniques and fractionation procedures, enables one to analyze and characterize subfractions. The plasmas and sera of clinically "normal" individuals give intrinsic viscosity values agreeing with calculated values. The intrinsic viscosity values for pathological plasmas and sera in all cases are greater than normal and reflect the augmented levels of those proteins fibrinogen, alpha(2)-globulins, and gamma-globulins occurring in the pathological state. The method is readily adaptable to routine clinical use and furnishes a measure of the departure from normal of protein levels in serum and plasma.

Blood↗
PubMed1961

Shear rate dependence of the viscosity of whole bllod and plasma.

The analysis of the shear stress/shear rate relationship, and thus the viscosity/shear rate relation, of blood and plasma shows that (i) freshly drawn whole blood has a large shear rate dependence on viscosity (viscosity falls as shear rate increases), and (ii) the shear rate dependence of viscosity of whole blood, or plasma, that has not been treated to prevent clotting is substantially greater than that of whole blood or plasma treated with anticoagulants. The influence of this phenomenon upon the fluid mechanics of the microcirculation is commented upon.

Anticoagulants↗
PubMed2003

Quantification of the effect of altering hematocrit and temperature on blood viscosity.

Rheological changes occurring with the conduct of cardiopulmonary bypass affect the distribution of blood throughout the cardiovascular system. The purpose of this study was to evaluate the effects of changing physical characteristics of fluid on the dynamics of blood flow in an in vitro model. An extracorporeal model simulating coronary vessel constriction was designed that consisted of tubing with varying internal diameters. Tubing sizes were selected as percentage reductions (11, 33, 56, and 78%) of a normal sized (3.6 mm) coronary artery. Flow rates were randomly varied between 150 and 300 mL min(-1) temperatures of 6 and 37 degrees C, and hematocrits of 0, 20, and 38%. Endpoints included viscosity, pressure drop, and volume distribution. As temperature fell from 37 to 6 degrees C, viscosity increased with hematocrit as follows: 192% at 0%, 225% at 20%, and 249% at 38%, p < .001. Pressure drop increased significantly across each tubing size ranging from 173-351%, p < .01, as fluid was cooled from 37 to 6 degrees C. However, intraconduit statistical differences in volumetric distribution of flow were not achieved. Although the induced hypothermia resulted in increases in resistance, statistical significance was only seen in the smallest lumen conduit. In conclusion, the effects of changing temperature has profound influence on fluid distribution secondary to changing blood viscosity in an in vitro model for fluid distribution. Knowledge of such flow alterations may aid in determining optimal perfusion strategies where vessel constrictions are encountered.

Blood Viscosity↗
PubMed1987

Effect of silicone oil viscosity on emulsification.

Emulsification of intravitreal silicone oil has been observed as a late complication after its use as a retinal tamponade. We compared the emulsification potential of silicone oil of different viscosities (100 centistokes [cs], 1000 cs, and 12,500 cs) and molecular composition in an in vitro model using a commercial detergent (benzalkonium chloride) and physiologic surface-active agents (human serum and lysed red blood cells). We found that silicone oils that have low-molecular-weight contents emulsify more readily than those that do not. Moreover, the extent and susceptibility to emulsification increases with decreasing viscosity or with an increase in the proportion of low-molecular-weight constituents. Hence, higher-molecular-weight (viscosity) oils or removal of low-molecular-weight components from medical grade fluid may lessen the emulsification observed clinically.

Emulsions↗
PubMed1998

Light scattering and viscosity study of heat aggregation of insulin.

Aggregation behavior and hydrodynamic parameters of insulin have been determined from static and dynamic light scattering experiments and intrinsic viscosity measurements carried out at pH 4.0, 7.5, and 9.0 in the temperature range 20-40 degrees C in aqueous solutions. The protein aggregated extensively at elevated temperatures in the acidic solutions. Intermolecular interactions were found to be attractive and to increase with temperature. The measured intrinsic viscosity [eta], diffusion coefficient D0, molecular weight M, and radius of gyration Rg exhibited the universal behavior: M[eta] = (2.4 +/- 02) x 10(-27) (Re, eta/Re, D)3(D0 eta 0/T)-3 and (D0 square root of n)-1 approximately equal to (square root of pi eta 0 xi beta/kBT) [1 + 0.201)(v/beta 3) square root of n], where n is the number of segments in the polypeptide. The effective hydrodynamic radii deduced from [eta], (Re, eta) and the same deduced from D0, (Re, D) showed a constant ratio, (Re, eta/Re, D = 1.1 +/- 0.1). Re, D/Rg = xi was found to be (0.76 +/- 0.07). From the known solvent viscosity eta 0, the segment length beta was deduced to be (10 +/- 1) A. The excluded volume was deduced to be (5 A)3 regardless of pH. The Flory-Huggins interaction parameter was found to be chi = 0.45 +/- 0.04, independent of pH and temperature.

Chemical Phenomena↗