Vasileios Symeonidis

Schmidt number effects in dissipative particle dynamics simulation of polymers

Simulation studies for dilute polymeric systems are presented using the dissipative particle dynamics method. By employing two different thermostats, the velocity-Verlet and Lowes scheme, we show that the Schmidt number (S(c)) of the solvent strongly affects nonequilibrium polymeric quantities. The fractional extension of wormlike chains subjected to steady shear is obtained as a function of S(c). Poiseuille flow in microchannels for fixed polymer concentration and varying number of repeated units within a chain is simulated. The nonuniform concentration profiles and their dependence on S(c) are computed. We show the effect of the bounce-forward wall boundary condition on the depletion layer thickness. A power law fit of the velocity profile in stratified Poiseuille flow in a microchannel yields wall viscosities different from bulk values derived from uniform, steady plane Couette flow. The form of the velocity profiles indicates that the slip flow model is not useful for the conditions of these calculations.

A family of time-staggered schemes for integrating hybrid DPD models for polymers: algorithms and applications

We propose new schemes for integrating the stochastic differential equations of dissipative particle dynamics (DPD) in simulations of dilute polymer solutions. The hybrid DPD models consist of hard potentials that describe the microscopic dynamics of polymers and soft potentials that describe the mesoscopic dynamics of the solvent. In particular, we develop extensions to the velocity-Verlet and Lowes approaches - two representative DPD time-integrators - following a subcycling procedure whereby the solvent is advanced with a timestep much larger than the one employed in the polymer time-integration. The introduction of relaxation parameters allows optimization studies for accuracy while maintaining the low computational complexity of standard DPD algorithms. We demonstrate through equilibrium simulations that a 10-fold gain in efficiency can be obtained with the time-staggered algorithms without loss of accuracy compared to the non-staggered schemes. We then apply the new approach to investigate the scaling response of polymers in equilibrium as well as the dynamics of λ-phage DNA molecules subjected to shear.

A spectral vanishing viscosity method for stabilizing viscoelastic flows

A new method for stabilizing viscoelastic flows is proposed suitable for high-order discretizations. It employs a mode-dependent diffusion operator that guarantees monotonicity while maintaining the formal accuracy of the discretization. Other features of the method are: a high-order time-splitting scheme, modal spectral element expansions on a single grid, and the use of a finitely extensible non-linear elastic-Peterlin (FENE-P) model. The convergence of the method is established through analytic examples and benchmark problems in two and three dimensions, and unsteady flow past a three-dimensional (3D) ellipsoid is studied at high Reynolds number.

A seamless approach to multiscale complex fluid simulation

Dissipative particle dynamics is an efficient and accurate mesoscale simulation method that bridges the gap between molecular dynamics and continuum hydrodynamics. Using time-staggered algorithms, it can simulate complex liquids and dense suspensions more than 100,000 faster than molecular dynamics.