Abstract
In this work, we present a new approach for coupled CFD-optics problems that consists of a combination of a finite element method (FEM) based flow solver with a ray tracing based tool for optic forces that are induced by a laser. We combined the open-source computational fluid dynamics (CFD) package FEATFLOW with the ray tracing software of the LAT-RUB to simulate optical trap configurations. We benchmark and analyze the solver first based on a configuration with a single spherical particle that is subjected to the laser forces of an optical trap. The setup is based on an experiment that is then compared to the results of our combined CFD-optics solver. As an extension of the standard procedure, we present a method with a time-stepping scheme that contains a macro step approach. The results show that this macro time-stepping scheme provides a significant acceleration while still maintaining good accuracy. A second configuration is analyzed that involves non-spherical geometries such as micro rotors. We proceed to compare simulation results of the final angular velocity of the micro rotor with experimental measurements.
Highlights
The main topic in this work is the implementation, benchmarking and numerical analysis of a combined computational fluid dynamics (CFD)-Optics solver for problems that involve a fluid component, a rigid body component and an optics component that usually is a laser used as i.e. an optical trap
Concerning the general fluid flow configurations we are dealing with low Reynolds number, laminar flow setups for which the FEATFLOW solver has been shown to produce accurate results for various fluid [6,7,5] phenomena as well as for the prediction of hydrodynamic forces flow in the context of sedimentation problems or particulate flow simulations [2]
A simple improvement technique for this simulation approach would be to estimate the hydrodynamic forces based on the Stokes model which in case of laminar flows with spherical particles expresses the hydrodynamic friction as Fh = 6πηRv, where η is the fluid viscosity, R the radius of the sphere and v the flow velocity relative to the particle
Summary
The main topic in this work is the implementation, benchmarking and numerical analysis of a combined CFD-Optics solver for problems that involve a fluid component, a rigid body component and an optics component that usually is a laser used as i.e. an optical trap. The coupling of the CFD solver with the ray tracing optics solver was done in order to enhance existing numerical simulation procedures for optical traps or tweezers Often simulations for these kind of problems are done in a vacuum whereas in real applications of the technique the objects are always embedded in a surrounding medium (i.e. a fluid such as water). A simple improvement technique for this simulation approach would be to estimate the hydrodynamic forces based on the Stokes model which in case of laminar flows with spherical particles expresses the hydrodynamic friction as Fh = 6πηRv, where η is the fluid viscosity, R the radius of the sphere and v the flow velocity relative to the particle. Apart from the two-way interaction of fluid and laser forces other advantages of using the Fictitious-Boundary Method (FBM) are the ability to include non-spherical geometry and to resolve flow features which allows to gain a better understanding of the involved microfluidics
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