Simulation and experimental observations

The flow of resin through a fibrous reinforcement is usually considered to follow Darcy s law for flow through a porous medium. Assuming low Reynolds number flow, a Newtonian fluid, and that the impregnated section of the reinforcement is fully saturated, Darcy s law can be written 

The conservation of solid and fluid mass for a thickness-varying process implies the following continuity equation  

It should be noted that the Darcy velocity is different from the flow front velocity (v), but they are related through the local preform porosity (0)  

From these equations it can be observed that three material parameters are needed to be able to simulate and predict the infiltration of a part the resin viscosity (/i), the reinforcement porosity (0), and the permeability of the reinforcement (K). The porosity of the reinforcement is related to the fibre volume fraction Vj) as  

The balance of forces in the mould during LCM processes is typically assumed to follow Terzaghi s relation [2]  

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To further improve the agreement between simulations and experimental observations, the parameters Cj ck od 3 in the Chen-Kim model were slightly modified. For modification of these parameters the ratio of the Eulerian macro length scale to the impeller blade height has been employed. The property can be compared in geometric similar vessels. For more details the reader is referred to the original paper by Jenne and Reuss [ 1 ]. 

After presenting each process in more detail in the next section, the following section will detail the physics governing the process and present work on the process simulation and experimental observations. A review of the current use of these processes in civil engineering will follow, before highlighting possible future trends and research areas. 

The simulation and experimental observations of thermal conductivity or thermal conductance aaoss polymer-CNT interfaces are few and have been attempted recently. The pico-second transient absorption spectra have been measured to deduce interface thermal conductance for carbon nanotubes suspended in surfactant micelles in water. The experimental findings have been analyzed using the MD simulations of heat transfer from a carbon nanotube to a model hydrocarbon liquid surrounding it. The heat transport in a nanotube composite material has been found to be limited mainly by exceptionally small interface thermal conductance 12 MW/m K. The net thermal conductivity of the composite thus has been found to be significantly smaller than the intrinsically high thermal conductivity... 

There has been extensive effort in recent years to use coordinated experimental and simulation studies of polymer melts to better understand the connection between polymer motion and conformational dynamics. Although no experimental method directly measures conformational dynamics, several experimental probes of molecular motion are spatially local or are sensitive to local motions in polymers. Coordinated simulation and experimental studies of local motion in polymers have been conducted for dielectric relaxation,152-158 dynamic neutron scattering,157,159-164 and NMR spin-lattice relaxation.17,152,165-168 A particularly important outcome of these studies is the improved understanding of the relationship between the probed motions of the polymer chains and the underlying conformational dynamics that leads to observed motions. In the following discussion, we will focus on the... 

Fig. 4. Simulated ESR spectrum of a flavin semiquinone considering only the hyperfine interactions from the strongly coupled (N(5) and N(10) nitrogens (left) and and experimentally observed ESR spectrum of a deuterated flavodoxin neutral semiquinone in HjO (right)...

As shown in Fig. 18, the simulation conductivity data were generally consistent with the experimental results. However, there are appreciable differences between the simulation and experimental results. At some points, the differences can be 100%. The other observation is that the simulation must underestimate the activation energies of the conduction. The primary reason for this discrepancy is that these simulation models do not take into account interaction between the membrane itself and its environment. In reality, the water uptake at elevated temperatures may be greater than that at room temperature. In the simulations, it was assumed that both... 

The intrinsic 3D interfacial curvature in compositionally asymmetric block copolymers provides extra degrees of freedom for the phase behavior in hexagonally structured microdomains. It is now well established that confinement of a cylinderforming block copolymer to a thickness other than the characteristic structure dimension in bulk, together with surface fields, can cause the microstructure to deviate from that of the corresponding bulk material. Surface structures in Fig. 1 are examples of simulated [57-59] and experimentally observed morphologies [40, 49, 60-62] that are formed in thin films of bulk cylinder-forming block copolymers. 

Bubble instability is one of the complications of this process. Only recently did this matter receive theoretical attention. As pointed out by Jung and Hyun (28), there are three characteristic bubble instabilities axisymmetric draw resonance, helical instability, and metastability where the bubble alternates between steady states, and the freeze line moves from one position to another. Using linear stability analysis, Cain and Denn (62) showed that multiple steady state solutions are possible for the same set of conditions, as pointed out earlier. However, in order to study the dynamic or time-dependent changes of the process, transient solutions are needed. This was recently achieved by Hyun et al. (65), who succeeded in quite accurately simulating the experimentally observed draw resonance (28). 

Direct MD simulations of the observed Stokes shifts and corresponding solvation time scales for several proteins were reported recently [188, 199, 202, 203]. Overall, significant discrepancies exist between simulation results and experimental observations, but some general features are promising. Here, we summarize one of our recent MD studies of W7 in apomyoglobin with linear response and direct nonequilibrium calculations and highlight the critical findings, as well as point out extensive improvement required in theoretical model [199]. 

RF pulses and timings were identical to those used in the spectral simulation. Overall, Figure 11 demonstrated good agreement between simulations and experimental data. The small deviations between the simulation and the phantom data were likely caused by T2 differences between different GABA proton groups and line shape distortions from eddy currents (not accounted for in the simulations). The calculated increase in GABA intensity (18%) at 3.01 ppm in the difference spectrum for PRESS+4 compared well with observed a 17% increase in phantom spectrum. 

The basic concepts underlying the methods of data analysis discussed here are illustrated in Figure 9-1. The results of an experiment are data. A model is a description of the processes taking place in the experimental system being observed, which defines a mathematical relationship between the independent variables and the results. The model also defines physical parameters as variables to be fitted. With plausible initial values of the parameters, the mathematical relationships are used to obtain simulated data, which are compared with the experimental data. The values of the parameters are then varied until an optimal fit is obtained of the simulated and experimental results. 

Detection of multiple quantum transitions in c.w. experiments is well known. Extension to high order transitions is not promising, since the transitions observed are a sensitive function of r.f. field strength. This leads to difficult spectral simulations and experimental problems of saturation and sample heating. 

Computer simulation of molecular dynamics is concerned with solving numerically the simultaneous equations of motion for a few hundred atoms or molecules that interact via specified potentials. One thus obtains the coordinates and velocities of the ensemble as a function of time that describe the structure and correlations of the sample. If a model of the induced polarizabilities is adopted, the spectral lineshapes can be obtained, often with certain quantum corrections [425,426]. One primary concern is, of course, to account as accurately as possible for the pairwise interactions so that by carefully comparing the calculated with the measured band shapes, new information concerning the effects of irreducible contributions of inter-molecular potential and cluster polarizabilities can be identified eventually. Pioneering work has pointed out significant effects of irreducible long-range forces of the Axilrod-Teller triple-dipole type [10]. Very recently, on the basis of combined computer simulation and experimental CILS studies, claims have been made that irreducible three-body contributions are observable, for example, in dense krypton [221]. 

Some general properties for stochastic errors have been established for impedance measurements through experimental observation and simulations. The results described here correspond to additive time-domain errors. The comparison between simulations and experimental results obtained via Fourier analysis supports the suggestion that the nature of experimental time-domain errors is likely to be additive rather than proportional ... 

To close this Section we comment on two papers that do not fit under any neat heading. The first of these is by Xiao et al,261 who study the final stages of the collapse of an unstable bubble or cavity using MD simulations of an equilibrated Lennard-Jones fluid from which a sphere of molecules has been removed. They find that the temperature inside this bubble can reach up to an equivalent of 6000 K for water. It is at these temperatures that sonolumines-cence is observed experimentally. The mechanism of bubble collapse is found to be oscillatory in time, in agreement with classical hydrodynamics predictions and experimental observation. The second paper, by Lue,262 studies the collision statistics of hard hypersphere fluids by MD in 3, 4 and 5 dimensions. Equations of state, self-diffusion coefficients, shear viscosities and thermal conductivities are determined as functions of density. Exact expressions for the mean-free path in terms of the average collision time and the compressibility factor in terms of collision rate are also derived. Work such as this, abstract as it may appear, may be valuable in the development of microscopic theories of fluid transport as well as provide insight into transport processes in general. 

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