Simulation of the interaction between blood flow and atherosclerotic plaque

It has been well accepted that over 50% of cerebral ischemic events are the result of rupture of vulnerable carotid atheroma and subsequent thrombosis. Such strokes are potentially preventable by carotid interventions. Selection of patients for intervention is currently based on the severity of carotid luminal stenosis. It has been, however, widely accepted that luminal stenosis alone may not be an adequate predictor of risk. To evaluate the effects of degree of luminal stenosis and plaque morphology on plaque stability, we used a coupled nonlinear time-dependent model with flow-plaque interaction simulation to perform flow and stress/strain analysis for stenotic artery with a plaque. The Navier-Stokes equations in the Arbitrary Lagrangian-Eulerian (ALE) formulation were used as the governing equations for the fluid. The Ogden strain energy function was used for both the fibrous cap and the lipid pool. The plaque Principal stresses and flow conditions were calculated for every case when varying the fibrous cap thickness from 0.1 to 2mm and the degree of luminal stenosis from 10% to 90%. Severe stenosis led to high flow velocities and high shear stresses, but a low or even negative pressure at the throat of the stenosis. Higher degree of stenosis and thinner fibrous cap led to larger plaque stresses, and a 50% decrease of fibrous cap thickness resulted in a 200% increase of maximum stress. This model suggests that fibrous cap thickness is critically related to plaque vulnerability and that, even within presence of moderate stenosis, may play an important role in the future risk stratification of those patients when identified in vivo using high resolution MR imaging.

Relative Stability of closo-closo, closo-nido, and nido-nido Macropolyhedral Boranes: The Role of Orbital Compatibility

The concept of orbital compatibility is used to explain the relative energies of different macropolyhedral structural patterns such as closo-closo, closo-nido, and nido-nido. A large polyhedral borane condenses preferentially with a smaller polyhedron owing to orbital compatibility. Calculations carried out at the B3LYP/6-31G* level show that the macropolyhedron closo(12)-closo(6) is the most preferred structural pattern among the face-sharing closo-closo systems. The relative stabilities of four-shared-atom closo-closo, three-shared-atom closo-closo, three-shared-atom closo-nido, edge-sharing closo-nido, and edge-sharing nido-nido structures are in accordance with the difference in the number of vertices of the individual polyhedra of the macropolyhedra. When the difference in the number of vertices of the individual polyhedra is large, the stability of the macropolyhedra is also large. Calculations further show that the orbital compatibility plays an important role in deciding the stability of the macropolyhedral boranes with more than two polyhedral units. The dependence of the orbital compatibility on the relative stability of the macropolyhedron varies with other factors such as inherent stability of the individual polyhedron and steric factors.

Time-domain criteria for theL_{2}-stability of nonstationary feedback systems

Criteria for the L2-stability of linear and nonlinear time-varying feedback systems are given. These are conditions in the time domain involving the solution of certain associated matrix Riccati equations and permitting the use of a very general class of L2-operators as multipliers.

Approximation of the perturbation equations of a quasi-linear hyperbolic system in the neighborhood of a bicharacteristic

In 1956 Whitham gave a nonlinear theory for computing the intensity of an acoustic pulse of an arbitrary shape. The theory has been used very successfully in computing the intensity of the sonic bang produced by a supersonic plane. [4.] derived an approximate quasi-linear equation for the propagation of a short wave in a compressible medium. These two methods are essentially nonlinear approximations of the perturbation equations of the system of gas-dynamic equations in the neighborhood of a bicharacteristic curve (or rays) for weak unsteady disturbances superimposed on a given steady solution. In this paper we have derived an approximate quasi-linear equation which is an approximation of perturbation equations in the neighborhood of a bicharacteristic curve for a weak pulse governed by a general system of first order quasi-linear partial differential equations in m + 1 independent variables (t, x1,…, xm) and derived Gubkin's result as a particular case when the system of equations consists of the equations of an unsteady motion of a compressible gas. We have also discussed the form of the approximate equation describing the waves propagating upsteam in an arbitrary multidimensional transonic flow.

Time-Domain Criteria For L2-Stability Of Nonstationary Feedback-Systems

Criteria for the L2-stability of linear and nonlinear time-varying feedback systems are given. These are conditions in the time domain involving the solution of certain associated matrix Riccati equations and permitting the use of a very general class of L2-operators as multipliers.

Stability and transition in the flow of polymer solutions

The linear stability analysis of a plane Couette flow of viscoelastic fluid have been studied with the emphasis on two dimensional disturbances with wave number k similar to Re-1/2, where Re is Reynolds number based on maximum velocity and channel width. We employ three models to represent the dilute polymer solution: the classical Oldroyd-B model, the Oldroyd-B model with artificial diffusivity and the non-homogeneous polymer model. The result of the linear stability analysis is found to be sensitive to the polymer model used. While the plane Couette flow is found to be stable to infinitesimal disturbances for the first two models, the last one exhibits a linear instability.

On the evaluation of stability of rare earth oxides as face coats for investment casting of titanium

Attempts have been made to evaluate the thermal stability of rare earth oxide face coats against liquid titanium. Determination of microhardness profiles and concentration profiles of oxygen and metallic constituents of oxide in investment cast titanium rods has allowed grActation of relative stability of rare earth oxides. The relative stability of evaluated oxides in the order of increasing stability follows the sequence CeO2 — ZrO2 — Gd2O3 — didymium oxide — Sm2O3 —Nd2O3 — Y2O3. The grading does not follow the free energy data of the formation of these oxides. A better correlation with the experimental observations is obtained when the solubility of the metallic species in titanium is also taken into consideration.

Interactions of L-serine at the active site of serine hydroxymethyltransferases: induction of thermal stability

Serine hydroxymethyltransferase (SHMT), EC 2.1.2.1, exhibits broad substrate and reaction specificity. In addition to cleaving many 3-hydroxyamino acids to glycine and an aldehyde, the enzyme also catalyzed the decarboxylation, transamination and racemization of several substrate analogues of amino acids. To elucidate the mechanism of interaction of substrates, especially L-serine with the enzyme, a comparative study of interaction of L-serine with the enzyme from sheep liver and Escherichia coli, was carried out. The heat stability of both the enzymes was enhanced in the presence of serine, although to different extents. Thermal denaturation monitored by spectral changes indicated an alteration in the apparent T, of sheep liver and E. coli SHMTs from 55 +/- 1 degrees C to 72 +/- 3 degrees C at 40 mM serine and from 67 +/- 1 degrees C to 72 +/- 1 degrees C at 20 mM serine, respectively. Using stopped flow spectrophotometry k values of (49 +/- 5)(.)10(-3) s(-1) and (69 +/- 7).10(-3) s(-1) for sheep liver and E. coli enzymes were determined at 50 mM serine. The binding of serine monitored by intrinsic fluorescence and sedimentation velocity measurements indicated that there was no generalized change in the structure of both proteins. However, visible CD measurements indicated a change in the asymmetric environment of pyridoxal 5'-phosphate at the active site upon binding of serine to both the enzymes. The formation of an external aldimine was accompanied by a change in the secondary structure of the enzymes monitored by far UV-CD spectra. Titration microcalorimetric studies in the presence of serine (8 mM) also demonstrated a single class of binding and the conformational changes accompanying the binding of serine to the enzyme resulted in a more compact structure leading to increased thermal stability of the enzyme.

Phase stability and dielectric properties of ceramics in the Pb(Zn1/3Nb2/3)O3-Pb(Ni1/3Nb2/3)O3-PbTio3 relaxor ferroelectric system

The perovskite structure in Pb(Zn1/3Nb2/3)O3 can be stabilized by the addition of Pb(Ni1/3Nb2/3)O3 and PbTiO3.Pb(Ni1/3Nb2/3)O3 assists in lowering the sintering temperature and shifting the Curie temperature of ceramics while PbTiO3 helps to optimize the dielectric properties. The phase stability and dielectric properties of several compositions in the Pb(Zn1/3Nb2/3)O3-Pb(Ni1/3Nb2/3)O3-PbTiO3 ternary relaxor ferroelectric system were investigated for possible capacitor applications. The effect of calcining and sintering temperature on the stability of perovskite phase in PZN rich compositions was studied extensively as a function of composition. The boundary line separating perovskite and mixed phases was determined for compositions near PZN. Several compositions can be sintered below 1050°C. The dielectric properties of compositions near the mixed phase boundary showed strong dependence on the percentage of pyrochlore phase. Compositions with a dielectric constant of 12.500 at room temperature have been identified which meet Z5T and Y5U specifications for dielectric constant and tan δ.

The stability of transmembrane helices: A molecular dynamics study on the isolated helices of bacteriorhodopsin

Bacteriorhodopsin (bR) continues to be a proven testing ground for the study of integral membrane proteins (IMPs). It is important to study the stability of the individual helices of bR, as they are postulated to exist as independently stable transmembmne helices (TMHs) and also for their utility as templates for modeling other IMPs with the postulated seven-helix bundle topology. Toward this purpose, the seven helices of bR have been studied by molecular dynamics simulation in this study. The suitability of using the backbone-dependent rotamer library of side-chain conformations arrived at from the data base of globular protein structures in the case TMHs has been tested by another set of ? helix simulations with the side-chain orientations taken from this library. The influence of the residue's net charge oil the helix stability was examined by simulating the helices III, IV, and VI (from both of the above sets of helices) with zero net charge on the side chains. The results of these 20 simulations demonstrate in general the stability of the isolated helices of bR in conformity with the two-stage hypothesis of IMP folding. However, the helices I, II, V, and VII are more stable than the other three helices. The helical nature of certain regions of III, IV, and VI are influenced by factors such as the net charge and orientation of several residues. It is seen that the residues Arg, Lys, Asp, and Glu (charged residues), and Ser, Thr, Gly, and Pro, play a crucial role in the stability of the helices of bR. The backbone-dependent rotamer library for the side chains is found to be suitable for the study of TMHs in IMP. (C) 1996 John Wiley & Sons, Inc.

Exact free-surface flows for shallow-water equations .2. the compressible case

Exact free surface flows with shear in a compressible barotropic medium are found, extending the authors' earlier work for the incompressible medium. The barotropic medium is of finite extent in the vertical direction, while it is infinite in the horizontal direction. The ''shallow water'' equations for a compressible barotropic medium, subject to boundary conditions at the free surface and at the bottom, are solved in terms of double psi-series, Simple wave and time-dependent solutions are found; for the former the free surface is of arbitrary shape while for the latter it is a damping traveling wave in the horizontal direction, For other types of solutions, the height of the free surface is constant either on lines of constant acceleration or on lines of constant speed. In the case of an isothermal medium, when gamma = 1, we again find simple wave and time-dependent solutions.

Direct Cartesian-Space Solutions of Generalized Bloch Equations in the Rotating Frame

Analytical solutions of the generalized Bloch equations for an arbitrary set of initial values of the x, y, and z magnetization components are given in the rotating frame. The solutions involve the decoupling of the three coupled differential equations such that a third-order differential equation in each magnetization variable is obtained. In contrast to the previously reported solutions given by Torrey, the present attempt paves the way for more direct physical insight into the behavior of each magnetization component. Special cases have been discussed that highlight the utility of the general solutions. Representative trajectories of magnetization components are given, illustrating their behavior with respect to the values of off-resonance and initial conditions. (C) 1995 Academic Press, Inc.

Linear stability, transient growth and the role of viscosity stratification in compressible Couette flow

Linear stability and the nonmodal transient energy growth in compressible plane Couette flow are investigated for two prototype mean flows: (a) the uniform shear flow with constant viscosity, and (b) the nonuniform shear flow with stratified viscosity. Both mean flows are linearly unstable for a range of supersonic Mach numbers (M). For a given M, the critical Reynolds number (Re) is significantly smaller for the uniform shear flow than its nonuniform shear counterpart; for a given Re, the dominant instability (over all streamwise wave numbers, α) of each mean flow belongs to different modes for a range of supersonic M. An analysis of perturbation energy reveals that the instability is primarily caused by an excess transfer of energy from mean flow to perturbations. It is shown that the energy transfer from mean flow occurs close to the moving top wall for “mode I” instability, whereas it occurs in the bulk of the flow domain for “mode II.” For the nonmodal transient growth analysis, it is shown that the maximum temporal amplification of perturbation energy, Gmax, and the corresponding time scale are significantly larger for the uniform shear case compared to those for its nonuniform counterpart. For α=0, the linear stability operator can be partitioned into L∼L̅ +Re2 Lp, and the Re-dependent operator Lp is shown to have a negligibly small contribution to perturbation energy which is responsible for the validity of the well-known quadratic-scaling law in uniform shear flow: G(t∕Re)∼Re2. In contrast, the dominance of Lp is responsible for the invalidity of this scaling law in nonuniform shear flow. An inviscid reduced model, based on Ellingsen-Palm-type solution, has been shown to capture all salient features of transient energy growth of full viscous problem. For both modal and nonmodal instability, it is shown that the viscosity stratification of the underlying mean flow would lead to a delayed transition in compressible Couette flow.

The standard molar Gibbs energy of formation of YbPt3 and LuPt3 intermetallics

The standard molar Gibbs energies of formation of YbPt3 and LuPt3 intermetallic compounds have been measured in the temperature range 880 K to 1100 K using the solid-state cells:View the MathML source and View the MathML source The trifluoride of Yb is not stable in equilibrium with Yb or YbPt3. The results can be expressed by the equations: View the MathML source View the MathML source The standard molar Gibbs energy of formation of LuPt3 is −41.1 kJ · mol−1 more negative than that for YbPt3 at 1000 K. Ytterbium is divalent in the pure metal and trivalent in the intermetallic YbPt3. The energy required for the promotion of divalent Yb to the trivalent state is responsible for the less negative ΔfGmo of YbPt3. The enthalpies of formation of the two intermetallics are in reasonable agreement with Miedema's model. Because of the extraordinary stability of these compounds it is possible to reduce oxides of Yb and Lu with hydrogen in the presence of platinum at View the MathML source. The equilibrium chemical potential of oxygen corresponding to the reduction of Yb2O3 and Lu2O3 by hydrogen in the presence of platinum is presented in the form of an Ellingham diagram.

Potentiometric determination of the stability of BaCu2O2

The thermodynamic stability of the compound BaCu2O2 was determined using a solid-state galvanic cell: View the MathML source as a function of temperature in the range 970–1170 K. Single crystal BaF2 was used as the solid electrolyte. The partial pressure of oxygen in the argon gas flowing over the electrodes was 1.27 Pa. The reversible e.m.f. of the cell can be expressed by View the MathML source. The Gibbs free energy of formation of barium cuprite from component oxides according to the reaction View the MathML source is View the MathML source.