Basis Of Simulations Research Articles

Modeling of brake pad-disc interface with emphasis to dynamics and deformation of structures

The frictional behavior at local contacts in an automotive brake system was analysed on the basis of computer simulation by movable cellular automata method. The boundary conditions of the model were adjusted to experimental observations obtained by TEM. The model proved to be adequate for simulating mechanical mixing and velocity accommodation at the pad-disc interface. Dynamics of particle interaction were visualized by showing rotation angles and velocity vectors. The model provided information on the development of plastic deformation for metal-on-metal contacts and on crack formation at graphite lamellae of cast iron disc. Results are in agreement with conventional friction theories.

Computer models in room acoustics: The ray tracing method and the auralization algorithms

Computer algorithms are described for constructing virtual acoustic models of various rooms that should satisfy some specific sound quality criteria. The algorithms are based on the ray tracing method, which, in the general case, allows calculation of the amplitude of an acoustic ray that survived multiple reflections from arbitrary curved surfaces. As a result, calculations of room acoustics are reduced to tracing the trajectories of all the acoustic rays in the course of their propagation with multiple reflections from reflecting surfaces to the point of their complete decay. For this approach to be used, the following physical properties of a room should be known: the geometry of the reflecting surfaces, the absorption and diffusion coefficients on each of these surfaces, and the decay law for rays propagating in air. The proposed models allow for the solution of the important problem of architectural acoustics called the auralization problem, i.e., to predict how any given audio segment will sound in any given hall on the basis of computer simulation alone, without any full-scale testing in specific halls.

Detection of the chaotic process distorted by the white noise using BDS statistics

A nonparametric criterion for the detection of a chaotic process observed against the background of white noise has been proposed with due regard for the dependence of the process values. BDS statistic is used for the numerical account of the degree of dependence of values of the process observed. It was shown that this criterion refined the conventional estimate of the chaotic process stealthiness that was obtained using the energy criterion for detecting deterministic signals of the unknown “waveform” and random processes. Characteristics of a nonparametric detector of chaotic processes with a different degree of the dependence of its values were obtained on the basis of computer simulation under condition of the exposure to additive white Gaussian noises of different intensity.

Bone Resorption Induced by Fluid Flow

A model where bone resorption is driven by stimulus from fluid flow is developed and used as a basis for computer simulations, which are compared with experiments. Models for bone remodeling are usually based on the state of stress, strain, or energy density of the bone tissue as the stimulus for remodeling. We believe that there is experimental support for an additional pathway, where an increase in the amount of osteoclasts, and thus osteolysis, is caused by the time history of fluid flow velocity, fluid pressure, or other parameters related to fluid flow at the bone/soft tissue interface of the porosities in the bone.

Minimizing the Effect of Noise in High-Speed Reversal of a Magnetic Dipole

The effect of noise on the reversal of a magnetic dipole is investigated on the basis of computer simulation of the Landau-Lifshits equation. It is demonstrated that at the reversal by the pulse with sinusoidal shape, there exists the optimal duration, which minimizes the mean reversal time (MRT) and the standard deviation (SD, jitter). Both the MRT and the jitter significantly depend on the angle  between the reversal magnetic field and the anisotropy axis. At the optimal angle the MRT can be decreased by a factor of 7 for damping =1 and up to 2 orders of magnitude for =0.01, and the jitter can be decreased from 1 to 3 orders of magnitude in comparison with the uniaxial symmetry case. It has been demonstrated that fluctuations can not only decrease the reversal time, as it has been known before for the magnetic systems and is correct for small angles only, but it can also significantly, up to the factor of two, increase the reversal time.

Simulation of the deflection of 200- and 450-GeV protons by a bent germanium crystal

It is shown that the result obtained by numerically solving the kinetic Fokker-Planck equation in the transverse-energy space on the basis of computer simulations of channeled-particle trajectories describes well the results of an experiment that studied the deflection of 200- and 450-GeV protons by a bent germanium crystal.

Interpretation of EMG integral or RMS and estimates of “neuromuscular efficiency” can be misleading in fatiguing contraction

In occupational and sports physiology, reduction of neuromuscular efficiency (NME) and elevation of amplitude characteristics, such as root mean square (RMS) or integral of surface electromyographic (EMG) signals detected during fatiguing submaximal contraction are often related to changes in neural drive. However, there is data showing changes in the EMG integral ( I EMG) and RMS due to peripheral factors. Causes for these changes are not fully understood. On the basis of computer simulation, we demonstrate that lengthening of intracellular action potential (IAP) profile typical for fatiguing contraction could affect EMG amplitude characteristics stronger than alteration in neural drive (central factors) defined by number of active motor units (MUs) and their firing rates. Thus, relation of these EMG amplitude characteristics only to central mechanisms can be misleading. It was also found that to discriminate between changes in RMS or I EMG due to alterations in neural drive from changes due to alterations in peripheral factors it is better to normalize RMS of EMG signals to the RMS of M-wave. In massive muscles, such normalization is more appropriate than normalization to either peak-to-peak amplitude or area of M-wave proposed in literature.

Application of Proportional Activation Approach to oxidative phosphorylation

Proportional Activation Approach (PAA) [1] is a simple quantitative method allowing to determine the proportional activation of the producer (P) and consumer (C) of some intermediate metabolite M by some external factor X. M can be e.g. ATP, ΔΨ or NADH, while X can be e.g. a hormon or neural/electrical stimulation of muscle. The proportional activation of C and P ((ΔC/C)/(ΔP/P)) is quantified by the proportional activation coefficient. Application of PAA to the oxidative phosphorylation demonstrates clearly that: 1. ΔΨ production and consumption during stimulation of isolated hepatocytes by vasopressin [1]; 2. NADH production and consumption during stimulation of isolated hepatocytes by vasopressin [2]; 3. ΔΨ production and consumption during electrical stimulation of rat skeletal muscle [3]; 4. ATP production and consumption during stimulation of perfused heart by adrenaline [4] - are directly activated to a similar extent. These findings confirm the so-called parallel activation idea, saying that different elements of the oxidative phosphorylation system are activated in parallel during low-to-high work transition in different cell types, that was proposed on the basis of computer simulations using a dynamic model of oxidative phosphorylation [5,6].[1] Korzeniewski B, Harper M-E, Brand MD (1995) Biochim Biophys Acta 1229, 315-322.[2] Brown CG, Lakin-Thomas PL, Brand MD (1990) Eur J Biochem 192, 355-362.[3] Rolfe DF, Newman JBM, Clark MG, Brand MD (1999) Am J Physiol 276, C692-C699.[4] Korzeniewski B, Deschodt-Arsac V, Calmettes G, Franconi J-M, Diolez P (2008) Biochem J 413, 343-347.[5] Korzeniewski B (1998) Biochem J 330, 1189-1195.[6] Korzeniewski B (2006) Am J Physiol 291, H1466-H1474.

The moderator effect that wasn't there: Statistical problems in ambivalence research.

Ambivalence researchers often collapse separate measures of positivity and negativity into a single numerical index of ambivalence and refer to it as objective, operative, or potential ambivalence. The authors argue that this univariate approach to ambivalence models undermines the validity of subsequent statistical analyses because it confounds the effects of the index and its components. To remedy this situation, they demonstrate how the assumptions underlying the indices derived from the conflicting reactions model and similarity-intensity model can be tested using a multivariate approach to ambivalence models. On the basis of computer simulations and reanalyses of published moderator effects, the authors show that the frequently reported moderating influence of ambivalence on attitude effects may be a statistical artifact resulting from unmodeled correlations of positivity and negativity with attitude and the dependent variable. On the basis of extensive power analyses, they conclude that it may be extremely difficult to detect moderator effects of ambivalence in observational data. Therefore, they encourage ambivalence researchers to take an experimental approach to study design and a multivariate approach to data analysis.

Numerical Simulation of Mechanically Mixed Layer Formation at Local Contacts of an Automotive Brake System

Processes taking place at local contacts in an automotive brake system are analyzed on the basis of computer simulation with the method of movable cellular automata. The conditions of a mechanically mixed layer (MML) formation on the tribosurfaces and the influence of the layer on the friction coefficient are investigated. The results show that the MML formation leads to the stabilization of the coefficient of friction at a convenient range (0.3–0.4) for brake application. The presence of graphite particles in the MML decreases the critical value of local normal stress for switching from stick-slip to smooth relative motion with MML formation.

Effect of noise on the high-speed reversal of single-domain uniaxial magnetic nanoparticles

The effect of noise on the reversal of a single-domain uniaxial magnetic nanoparticle is studied on the basis of computer simulation of the Landau-Lifshits equation. The interplay between resonant activationlike and noise-delayed switching effects is investigated. Namely, it is demonstrated that for the dipole reversal by the pulse with smooth fronts, there exists the optimal pulse width, which minimizes the mean reversal time MRT and the standard deviation SD. Both the MRT and the SD significantly depend on the angle between the reversal magnetic field and the anisotropy axis. In comparison with the uniaxial symmetry case at the optimal angle =45°, the MRT can be decreased from a factor of seven up to two orders of magnitude, and the jitter can be decreased from one to three orders of magnitude for damping ranging from 1 to 0.01. It is demonstrated that for =5° the increase in temperature from 4 to 300 K leads to doubling the MRT due to the noise-delayed switching effect.

Study of a novel sample injection method (floating electrokinetic supercharging) for high‐performance microchip electrophoresis of DNA fragments

Aiming to achieve high-performance analysis of DNA fragments using microchip electrophoresis, we developed a novel sample injection method, which was given the name of floating electrokinetic supercharging (FEKS). In the method, electrokinetic injection (EKI) and ITP preconcentration of samples was performed in a separation channel, connecting two reservoir ports (P3 and P4) on a cross-geometry microchip. At these two stages, side channels, crossing the separation channel, and their ports (P1 and P2) were electrically floated. After the ITP-stacked zones passed the cross-part, they were eluted for detection by using leading ions from P1 and P2 that enabled electrophoresis mode changing rapidly from ITP to zone electrophoresis (ZE). Possible sample leakage at the cross-part toward P1 and P2 was studied in detail on the basis of computer simulation using a CFD-ACE+ software and real experiments, through which it was validated that the analyte recovery to the separation channel was almost complete. The FEKS method successfully contributed to higher resolution and shorter analysis time of DNA fragments on the cross-microchip owing to more rapid switching from ITP status to ZE separation in comparison with our previous EKS procedure realized on a single-channel microchip. Without any degradation of resolution, the achieved LODs were on average ten times better than using conventional pinched injection.

Gradient and vorticity banding

Banded structures of macroscopic dimensions can be induced by simple shear flow in many different types of soft matter systems. Depending on whether these bands extend along the or direction, the transition is referred to as gradient banding or vorticity banding, respectively. The main features of can be understood on the basis of a relatively simple constitutive equation. This minimal model for will be discussed in some detail, and its predictions are shown to explain many of the experimentally observed features. The minimal model assumes a decrease of the shear stress of the homogeneously sheared system with increasing shear rate within a certain shear-rate interval. The possible microscopic origin of the severe shear-thinning behaviour that is necessary for the resulting nonmonotonic flow curves is discussed for a few particular systems. Deviations between experimental observations and predictions by the minimal model are due to obvious simplifications within the scope of the minimal model. The most serious simplifications are the neglect of concentration dependence of the shear stress (or on other degrees of freedom) and of the elastic contributions to the stress, normal stresses, and the possibility of shear-induced phase transitions. The consequences of coupling of stress and concentration will be analyzed in some detail. In contrast to predictions of the minimal model, when coupling to concentration is important, a flow instability can occur that does not require strong shear thinning. Gradient is sometimes also observed in glassy- and gel-like systems, as well as in shear-thickening systems. Possible mechanisms that could be at the origin of gradient-band formation in such systems are discussed. Gradient can also occur in strongly entangled polymeric systems. Banding in these systems is discussed on the basis of computer simulations. Vorticity is less well understood and less extensively investigated experimentally as compared to banding. Possible scenarios that are at the origin of will be discussed. Among other systems, the observed vorticity-banding transition in rod-like colloids is discussed in some detail. It is argued, on the basis of experimental observations for these colloidal systems, that the vorticity-banding instability for such colloidal suspensions is probably related to an elastic instability, reminiscent of the Weissenberg effect in polymeric systems. This mechanism might explain in discontinuously shear-thickening systems and could be at work in other vorticity-banding systems as well. This overview does not include time-dependent phenomena like oscillations and chaotic behaviour.

Bijels: a new class of soft materials

Bicontinuous interfacially jammed emulsion gels (‘bijels’) were proposed in 2005 as a hypothetical new class of soft materials in which interpenetrating, continuous domains of two immiscible fluids are maintained in a rigid state, by a jammed layer of colloidal particles at their interface. Such gels should have unusual material properties relevant to catalysis and other applications. Although initially proposed on the basis of computer simulation, the existence of bijels has since been confirmed in the laboratory. However, a wide gap remains between the simulated and the experimental systems. This leaves room for continuing debate on the cause of stability in these materials; in particular, it is not known whether an attraction between the colloidal particles is essential for stability.

Effects of threshold on multiple-target detection by using a modified amplitude-modulated joint transform correlator

Effects of the threshold value on the multiple-target detection performance of a modified amplitude-modulated joint transform correlator are studied on the basis of computer simulations, using human fingerprint and face images. Correlation outputs are quantitatively measured by means of a primary-to-secondary peak ratio and a peak-to-correlation-deviation ratio. The simulation results reveal that the detection of high-contrast targets can be optimized by using a low-threshold. In the case of low-contrast targets, the optimization depends on spatial-frequency contents and noise.

Computer Insight into the Molecular Level of Heart Failure. What is the Role of NCX?

Though congestive heart failure is a leading cause of death in our population, the pathophysiological mechanisms at molecular level remain to be elucidated. This paper discusses the contribution of NCX to the pathological pattern of intracellular calcium regulation and contraction on the basis of computer simulations of a virtual cell. The model comprises calcium handling mechanisms, troponin control and acto-myosin interactions. The contribution of NCX was studied by changing its activity and turning it off for some simulations.It was found that NCX helps to support diastolic function by reducing the Ca2 level during the diastole. At the same time there is a reduction in peak Cai and hence contraction. However, increased NCX activity does not seem to improve calcium handling and contraction crucially, as has been suggested by some authors.

Autoassociative and heteroassociative hippocampal CA3 model based on location dependencies derived from anatomical and physiological findings

The hippocampus is an important region for our memories. Many researchers have focused especially on the CA3 region of the hippocampus and have proposed many computational models of the CA3. Conventionally, they have mono-functionally modeled this region as autoassociative memory or heteroassociative memory. However, we derive auto- and heteroassociative functions of the CA3 from anatomical findings of recurrent collaterals in CA3 subregions and physiological findings of spike-timing dependent plasticity in dendrites. On the basis of computer simulations, we suggest that the CA3 should be modeled as auto- and heteroassociative memory.

Use of Optimal Controllers for Multidimensional Technological Objects

We study multidimensional optimal controllers for technological objects on an example of a subsystem of controlling temperature of juice-shaving mixture in an oblique diffusive plant of sugar production. On the basis of computer simulation we determine conditions, advantages and disadvantages of each of the systems with different controllers for the given object. Variants of implementation of some algorithms on modern technical automation means are given.

Resonant charge exchange involving highly excited atoms

Transition of a classical electron between two Coulomb centers is analyzed on the basis of computer simulations. The contribution to the electron transfer cross section from a tunnel electron transition is evaluated taking into account the strong mixing of highly excited electron states due to motion of Coulomb centers. The rate of transition of a highly excited electron between two Coulomb cores with a fixed separation is evaluated together with the cross section of resonant charge exchange in slow collisions. Typical times of change of the electron momentum as a result of electron motion in the field of two Coulomb centers are determined.

Simulation studies of a vacuum and temperature swing adsorption process for the removal of VOC from waste air streams

A theoretical study of the cyclic combined vacuum and temperature swing adsorption (VTSA) process for removing of volatile organic compounds (VOC) from waste air streams is done on the basis of computer simulation. The VTSA system consists of two adsorption columns with a fixed bed of activated carbon. The adsorption cycle for each column is operated in four steps: adsorption, indirect adsorbent bed heating, vacuum desorption and cooling. A nonequilibrium, nonisothermal mathematical model of the VTSA process is developed and solved using the numerical method of lines. Exemplary simulation results are presented for the system: 2-propanol–activated carbon Sorbonorit 4. The effect of desorption temperature, pressure and purge gas use on 2-propanol desorption efficiency is investigated.