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Description: In this work, modeling of the subfilter scalar dissipation rate is addressed. First, the best set of quantities to write a model is determined using the concept of optimal estimators. This study shows that the best approach is to assume a proportionality between the turbulent time scale and turbulent scalar mixing time scale. It is shown that the turbulent time scale should be defined by the subfilter kinetic energy. To define the coefficient appearing in th...
Description: Drag reduction, or the mean velocity increase in a turbulent flow at a fixed pressure drop through the addition of tiny amounts (several parts per million) of high-molecular-weight polymers (Thoms effect), has been known already for more than 60 years. Long ago it was understood that this effect is related to supramolecular structures formed in the flow. Recent experiments by Chu, Shaqfeh, and associates, where the motion of supramolecular structures was dir...
Description: The bubble formation sites in film boiling are the nodes of the instability occurring at the liquid-vapor interface. We perform a linear stability analysis with a time dependent base state, accounting for the growth of the film due to evaporation, in order to identify the most dominant wavelength. Choosing a domain size of five times the wavelength predicted by Berenson [ASME J. Heat Transfer 83, 351 (1961) ] and an initial liquid-vapor interface profile hav...
Description: We apply an efficient method of forced imbibition to (nano-)capillaries, coated internally with a polymer brush, to derive the change in permeability and suction force, corresponding to different grafting densities and lengths of the polymer chains. While the fluid is modeled by simple point particles interacting with Lennard-Jones forces, the (end-grafted, fully flexible) polymers, which form the brush coating, are described by a standard bead-spring model....
Description: Motivated by the destabilization of a two-fluid layer flowing in a microchannel for efficient mixing or droplet formation, we study quantitatively the linear stability of the interface between two liquids subjected to an electric field parallel or normal to the flat interface. In the case of fast electric charge relaxation times, the equations for the perturbation can be significantly reduced [ A. K. Uguz, O. Ozen, and N. Aubry, Phys. Fluids 20, 031702 (2008...
Description: The rupture of thin water films (thickness of ∼ 100 μm) spreading radially outward on a solid surface was studied experimentally. Circular water films were produced by directing a 1 mm diameter water jet onto a flat horizontal plate for 10 ms. Jet impact velocity was varied from 1.6 to 6.1 m/s, giving Reynolds numbers from 1600 to 6100. Three different plates were used (glass, Plexiglas, and wax) to vary substrate wettability. Both rough and smooth wax surfa...
Description: Recent measurements and theory suggest that some solid/liquid surfaces can exhibit slip, at times to a surprising degree. Proposed mechanisms for slip include both intrinsic molecular slip as well as “effective” slip over complex and multiphase interfaces (i.e., covered with “lubricating” nanobubbles or trapped gas pockets). Electrokinetic flow velocities can be significantly enhanced over slipping surfaces as the high shear rates within the double layer dri...
Description: We perform boundary integral simulations to understand the removal of Newtonian liquids from a model gravure cell. Two different configurations are considered. In the first configuration, there is a free surface and an outlet boundary, and the liquid is driven out of a cavity by a combination of horizontal substrate motion and an imposed pressure gradient; a similar model was used by Powell et al. [Trans. IChemeE, Part C 78, 61 (2000) ]. The percentage of li...
Description: A model for the motion of two coupled spherical-cap droplets subject to periodic forcing is studied. The inviscid unforced model is a conservative second-order system, similar to Duffing’s equation. Surface tension resists the inertia of deformations from the spherical shape. Steady states of the system are parametrized by the total combined volume of the two droplet caps. The family of equilibria exhibits a classical pitchfork bifurcation, where a single le...