Numerical investigation of the influence of viscoelasticity on drop deformation in shear
Numerical simulations and experimental data are compared for the investigation of the influence of viscoelasticity on drop deformation in shear. A viscoelastic drop suspended in a Newtonian liquid, or a Newtonian drop suspended in a viscoelastic liquid, is sheared and investigated for transients, relaxation after cessation of shear flow, and step‐up in shear rate. The numerical simulations are conducted at parameters chosen to model the experiments. We use the volume of fluid (VOF) continuum surface force (CSF) algorithm for situations dominated by shear. For drop relaxation experiments, we use the paraboloid representation of the interface in the surface tension force (PROST) algorithm. The Oldroyd‐B and Giesekus constitutive models are implemented. An interesting result is that by stepping up in the capillary number gradually, a stationary states is achieved at higher capillary numbers than without the graduated steps. The experimental work is described in Verhulst, Moldenaers and Cardinaels [l]. We pre...
Influence of viscoelasticity on drop deformation and orientation in shear flow. Part 2: Dynamics
Abstract An experimental study of drop dynamics under shear is conducted for five fluid pairs: a reference Newtonian system, two systems with a viscoelastic drop in a Newtonian matrix, one with a Newtonian drop in a viscoelastic matrix, all at drop to matrix viscosity ratio λ = 1.5 , and a separate case at λ = 0.75 . The viscoelastic liquids are either a Boger fluid or a shear-thinning viscoelastic fluid satisfying an Ellis model. Deborah numbers in the range 1–2 and a range of capillary numbers from low to above breakup conditions are addressed. The results focus on three aspects: relaxation after cessation of shear, a new viscoelastic drop breakup scenario, and the effect of shear flow history on drop breakup. Numerical simulations with the 3D volume-of-fluid PROST method complement the experimental results.
Effect of startup conditions on drop breakup under shear with inertia
A spherical drop is suspended in another liquid and sheared between parallel walls. Numerical simulations for drop deformation under simple shear typically begin with the drop placed in an established flow. For Stokes flow, the drop evolves in a monotone manner to a stationary state or to breakup. Experimental and numerical studies of this result in a critical curve which delineates the capillary number for breakup at each viscosity ratio. With inertia, however, the history depends on the initial condition. Direct numerical simulations are performed to examine the effect of inertia through the influence of initial data on critical conditions. An abrupt startup induces drop oscillations which are sufficiently large to lead to breakup , while a gentle startup can allow the drop to reach a stationary state. An oscillator model is developed for the manner in which momentum is transferred to the drop and how it is distributed into surface deformation and dissipation as a function of the Reynolds number and capillary number.