Effects Of Viscous Stresses Research Articles

Numerical investigation of electrowetting-based droplet splitting in closed digital microfluidic system: Dynamics, mode, and satellite droplet

In this paper, the splitting of microdroplet in a closed electrowetting-based digital microfluidic system has been studied via a numerical model. The governing equations for the fluid flow are solved by a finite volume formulation with a two-step projection method on a fixed computational domain. The free surface of the liquid is tracked by a coupled level-set and volume-of-fluid method, with the surface tension at the free surface computed by continuum surface force scheme. Contact angle hysteresis is implemented as an essential component of electrowetting modeling, and a simplified viscous force model is adopted to evaluate the viscous stress based on the Hele-Shaw model. Excellent agreement has been achieved between the numerical and published experimental results. A parametric study has been performed in which the effects of viscous stress, channel height, static contact angles, contact angle hysteresis, and electrode size on the splitting process have been analyzed. Three distinct splitting modes, which are “splitting with satellite droplet,” “normal splitting,” and “splitting cessation,” have been discussed. Based on the competition between the curvature in the z-direction (κz) and that on the x-y plane (κxy), the physical mechanism that separates the splitting into these three modes has been revealed. More importantly, a dimensionless parameter κ̃ has been proposed, which can be used for (a) determining the splitting mode and (b) estimating satellite droplet volume for electrowetting-induced droplet splitting process.

Simulation of Lubricant Behavior by Using the Particle Method

Lubricant behavior within a head-disk clearance in recent hard disk drives was simulated by using a particle method, which is based on continuum dynamics. The lubricant was simulated as a group of particles. The surface tension and the wall-adhesion models used in the particle method were verified before applying the particle method to the lubricant behavior simulation first. That is, simple benchmark problems like liquid-droplet deformations under unbalanced surface tension and wall adhesion force were used to confirm the accuracy of the models. In the lubricant simulation, the effects of viscous stress and surface tension on the lubricant behavior were studied. The study found that viscous stress is the dominant factor in comparison with surface tension on bulk condition of viscosity; that is, the lubricant bridge is deformed by the viscous stress, not by the surface tension.

Studies of Thermally Unstable Accretion Disks around Black Holes with Adaptive Pseudospectral Domain Decomposition. I. Limit‐Cycle Behavior in the Case of Moderate Viscosity

We present a numerical method for spatially 1.5-dimensional, time-dependent studies of accretion disks around black holes. The method originates from a combination of the standard pseudospectral and adaptive domain decomposition methods found in the literature, but with a number of improvements in both the numerical and physical senses. In particular, we introduce a new treatment for the connection at the interfaces of decomposed subdomains, construct an adaptive function for the mapping between the Chebyshev-Gauss-Lobatto collocation points and the physical collocation points in each subdomain, and modify the oversimplified one-dimensional basic equations of accretion flows to account for the effects of viscous stresses in both the azimuthal and radial directions. Our method is verified by reproducing the best results obtained previously by Szuszkiewicz & Miller on the limit-cycle behavior of thermally unstable accretion disks with moderate viscosity. A new finding is that, according to our computations, the Bernoulli function of the matter in such disks is always and everywhere negative, so that outflows are unlikely to originate from these disks. We are encouraged to study the more difficult case of thermally unstable accretion disks with strong viscosity in ongoing work.

Be Star Discs: Inflow or Outflow?

AbstractIt is assumed that the dynamics of Be star discs is dominated by the effects of viscous stresses. By examining angular momentum transport in discs, we show that many, if not all observed Be star discs should be accretion discs unless (i) the disc is acted upon by another agent (e.g. magnetic fields or the stellar radiation field), or (ii) the disc cools significantly as it flows outwards.

Viscous excretion discs around Be stars

The properties of geometrically thin viscous Keplerian excretion discs around Be stars are analyzed. The inner boundary of the disc is assumed to be located at the equatorial surface of the central star. It is assumed that angular momentum is transferred by some mechanism from the interior to the equatorial surface of the central star and hence matter is supplied at the inner boundary of the disc at a certain rate (excretion rate). The matter drifts outward in the disc because the angular momentum of the matter is exchanged by the effect of viscous stress. For mass excretion rates similar to the mass-loss rate estimated for Be stars, the irradiative heating by the central star is as important for the disc structure as the viscous heating in the disc

Thermal evaporation of spherical clouds - Effects of viscous stresses

view Abstract Citations (40) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Thermal evaporation of spherical clouds - Effects of viscous stresses Draine, B. T. ; Giuliani, J. L., Jr. Abstract The thermal evaporation of a cold spherical cloud embedded in hot gas is analyzed, including the effects of viscous stresses as well as 'saturation' of the conductive heat flux, for the case of equal electron and ion temperatures. The relevant transport properties for an H-He plasma are discussed, including the modification of these transport coefficients when temperature or velocity gradients become large. The fluid-dynamical equations for steady spherical evaporation are given and cast into dimensinless form. Numerical integration of these equations and the results are discussed, emphasizing the novel features of the viscous solutions, astrophysical implications, and uncertainties in the physics. Publication: The Astrophysical Journal Pub Date: June 1984 DOI: 10.1086/162146 Bibcode: 1984ApJ...281..690D Keywords: Evaporation; Interstellar Gas; Inviscid Flow; Shock Waves; Supernova Remnants; Viscous Flow; Boltzmann Transport Equation; Mach Number; Navier-Stokes Equation; Plasma Clouds; Solar Wind; Astrophysics full text sources ADS |