Bounced-back effect

Lattice gas models taking into account interactions between different particles have also been developed. First neighbour interactions are generally considered but also those with Coulomb interactions have been used The influence of Coulomb forces between mobile ions in combination with the static lattice potential thus plays an important role in non-protonic conductors (e.g. NASICON ). The bounce-back effect-an ion which performs a jump is expected to return to its initial location more often when repulsive interactions are present - plays an important role in modelling of frequency dependent conductivity (see Chapter 25) this tends to reduce the low frequency conductivity relative to the high... 

Only one example will be given. It concerns a specific form of stray light that is observed when the entrance face of the CCD is protected by a transparent window. This situation arises mostly for cooled CCDs. A small fraction of the measured light is reflected off the surface of the CCD chip, hits the window, and bounces back toward another pixel of the CCD chip (Fig. 5). So the reading on the first pixel is lowered and that on the second is increased, the overall effect distorting the tme distribution in intensity. 

The vacuum interface is the source of all quantum effects. Interaction with the interface causes particles to make excursions into time and bounce back with time-reversal and randomly perturbed space coordinates. Different from classical particles, quantum objects can suffer displacement in space without time advance. They can appear to be in more than one place at the same time, as in a two-slit experiment. 

The bounce-back collision typically employed at fluid-solid boundaries, where fluid particles are turned back in the direction they came from following collision with a solid wall, causes the effective wall position to extend one half lattice unit into the fluid from the solid surface (Stockman et al., 1997). This is not a serious problem for velocity computations in slow flows, but has the potential to be a significant problem for tracer/dispersion simulations. Increasing the number of lattice points inside a flow channel can reduce this error, but is computationally very expensive. 

To minimize the particle bounce off effect, collection surfaces should also be selected carefully. Common types of impaction surfaces include membrane, fiberglass, silver membrane, Teflon and Nuclepore filter, and brass and stainless steel shim stock. Table 2.2 shows an example of the effect of selection of collection surface on the wall losses (Newton et al., 1990). In Table 2.2, the test aerosols are droplets of 1% CsCl plus 1% uranine. Three types of cascade impactors were used, including Mercer, Sierra Radial Slit Jet (SRSJ), and Lovelace Multi-Jet (LMJ). The occurrence of particle bouncing may be indicated by the presence of excess mass on the back-up filter. 

Similar assemblies have been extensively characterized for the ion pair cetyltrimethylammonium-salicylate. In both cases the micellar fibers produce slightly viscous solutions and the effect of viscoelasticity is observed if one rotates such a solution and suddenly stops the rotation, smalt particles in the solution (e.g., air bubbles) bounce back. While the bulk water is still rotating in the nonviscous solutions, the inertia of the high molecular weight threads builds up an elastic wall for the suspended particles and pushes them back. Lithium and sodium ricinolates produce helical micellar fibers of opposing chirality in toluene (Tachibaona, 1970,1978). 

Hydrophones are simply receivers of sound energy and pick up noise and multiple reflections of bottom features that bounce back and forth between the mirror effect of the air-sea interface and a bottom reflector. By proper filtering, systems have been developed to minimize the effect of multiple reflections. 

In the frozen-layer scheme, a continuum limit is applied to the group of frozen-layer particles to analytically obtain the effective form of the dissipative and random forces of interactions between the wall particles and the DPD fluid particle. An explicit rule is employed to maintain the impermeability of solid walls. Three possible rules for achieving this are illustrated in Fig. 3. For free-slip, specular reflection instead of bounce-back reflection for particles that cross the free-slip boundaries is employed. 

Comubert et al. [54] were one of the first to use the LBM to model slip velocity for bounce-back and specular reflection boundary conditions. This method has proved to be effective in microflow for moderate Kn. High order of LBM needs to be used for gas microflows at higher Kn. Recent work for modeling the gaseous flows with the LBM shows modified relaxation parameters and... 

To have an intuitive understanding of the concentration effect, we consider a suspension of hard spheres. Suppose that a portion of the suspension acquires temporarily a higher concentration than the surrounding, as shown in Pigure 3.25a. The particles in the locally concentrated region tend to move away from each other, resulting in the collision of black particles with white particles (Pig. 3.25b). Upon collision, the particles bounce back, although the motion is overdamped in a viscous environment (Pig. 3.25c). When we trace the motion of each black particle, the collision makes the square displacement smaller compared with the one in the absence of collisions. The effect of the local concentration fluctuation is, however. 

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