Markus Bussmann

On a three-dimensional volume tracking model of droplet impact

A three-dimensional model has been developed of droplet impact onto asymmetric surface geometries. The model is based on RIPPLE, and combines a fixed-grid control volume discretization of the flow equations with a volume tracking algorithm to track the droplet free surface. Surface tension is modeled as a volume force acting on fluid near the free surface. Contact angles are applied as a boundary condition at the contact line. The results of two scenarios are presented, of the oblique impact of a 2 mm water droplet at 1 m/sec onto a 45° incline, and of a similar impact of a droplet onto a sharp edge. Photographs are presented of such impacts, against which the numerical results are compared. The contact angle boundary condition is applied in one of two ways. For the impact onto an incline, the temporal variation of contact angles at the leading and trailing edges of the droplet was measured from photographs. This data is applied as a boundary condition to the simulation, and an interpolation scheme propos...

Bio-Microarray Fabrication Techniques—A Review

ABSTRACT Microarrays with biomolecules (e.g., DNA and proteins), cells, and tissues immobilized on solid substrates are important tools for biological research, including genomics, proteomics, and cell analysis. In this paper, the current state of microarray fabrication is reviewed. According to spot formation techniques, methods are categorized as “contact printing” and “non-contact printing.” Contact printing is a widely used technology, comprising methods such as contact pin printing and microstamping. These methods have many advantages, including reproducibility of printed spots and facile maintenance, as well as drawbacks, including low-throughput fabrication of arrays. Non-contact printing techniques are newer and more varied, comprising photochemistry-based methods, laser writing, electrospray deposition, and inkjet technologies. These technologies emerged from other applications and have the potential to increase microarray fabrication throughput; however, there are several challenges in applying them to microarray fabrication, including interference from satellite drops and biomolecule denaturization.

Modeling the splash of a droplet impacting a solid surface

A numerical model is used to simulate the fingering and splashing of a droplet impacting a solid surface. A methodology is presented for perturbing the velocity of fluid near the solid surface at a time shortly after impact. Simulation results are presented of the impact of molten tin, water, and heptane droplets, and compared with photographs of corresponding impacts. Agreement between simulation and experiment is good for a wide range of behaviors. An expression for a splashing threshold predicts the behavior of the molten tin. The results of water and especially heptane, however, suggest that the contact angle plays an important role, and that the expression may be applicable only to impacts characterized by a relatively low value of the Ohnesorge number. Various experimental data of the number of fingers about an impacting droplet agree well with predictions of a previously published correlation derived from application of Rayleigh–Taylor instability theory.

A mesh-dependent model for applying dynamic contact angles to VOF simulations

Typical VOF algorithms rely on an implicit slip that scales with mesh refinement, to allow contact lines to move along no-slip boundaries. As a result, solutions of contact line phenomena vary continuously with mesh spacing; this paper presents examples of that variation. A mesh-dependent dynamic contact angle model is then presented, that is based on fundamental hydrodynamics and serves as a more appropriate boundary condition at a moving contact line. This new boundary condition eliminates the stress singularity at the contact line; the resulting problem is thus well-posed and yields solutions that converge with mesh refinement. Numerical results are presented of a solid plate withdrawing from a fluid pool, and of spontaneous droplet spread at small capillary and Reynolds numbers.

Multicomponent droplet evaporation at intermediate Reynolds numbers

Abstract The convective evaporation of a binary hydrocarbon droplet (decane-hexadecane) in air at 1000 K and at a pressure of 10 atmospheres has been studied using numerical methods. All transient effects including droplet size and velocity variations, heat and mass transfer within the liquid phase, and thermophysical property variations with temperature and concentration in both phases are included in the analysis. As the rate controlling process, liquid phase mass transfer is examined in detail. It is demonstrated that the existing drag coefficient, Sherwood number, and Nusselt number correlations originally developed for single-component droplets can be used for multicomponent droplets as well.

Advecting normal vectors: A new method for calculating interface normals and curvatures when modeling two-phase flows

In simulating two-phase flows, interface normal vectors and curvatures are needed for modeling surface tension. In the traditional approach, these quantities are calculated from the spatial derivatives of a scalar function (e.g. the volume-of-fluid or the level set function) at any instant in time. The orders of accuracy of normals and curvatures calculated from these functions are studied. A new method for calculating these quantities is then presented, where the interface unit normals are advected along with whatever function represents the interface, and curvatures are calculated directly from these advected normals. To illustrate this new approach, the volume-of-fluid method is used to represent the interface and the advected normals are used for interface reconstruction. The accuracy and performance of the new method are demonstrated via test cases with prescribed velocity fields. The results are compared with those of traditional approaches.

A numerical study of steady flow and temperature fields within a melt spinning puddle

A numerical solution of the momentum and energy equations yields steady flow and temperature fields within a melt spinning puddle. The model accounts for inertial, viscous, surface tension, and wetting effects, and relies on a temperature-dependent viscosity to solidify ribbon to an amorphous state. A reference simulation is presented, followed by results that examine the effect of varying individual parameters. Results demonstrate a strong influence of nozzle wetting on puddle size; significant recirculation upstream of the nozzle slot; and the presence of both recirculation and an unrelated bump downstream of the slot.

Modeling High Density Ratio Incompressible Interfacial Flows

We present an approach to modeling incompressible interfacial flows on fixed meshes that yields solutions at any density ratio. There are two aspects of the methodology that are crucial for obtaining accurate high density ratio solutions: a consistent approach to mass and momentum conservation, by using mass flux information from an interface advection algorithm as the basis for the momentum advection calculation, and a careful evaluation of pressure gradients near the interface. Our particular implementation couples a volume tracking algorithm with a predictor/projection solution of the flow equations on unstructured meshes. We present the methodology, and then the results of several calculations.Copyright

Irrigation dynamics associated with positive pressure, apical negative pressure and passive ultrasonic irrigations: A computational fluid dynamics analysis

Complexities in root canal anatomy and surface adherent biofilm structures remain as challenges in endodontic disinfection. The ability of an irrigant to penetrate into the apical region of a canal, along with its interaction with the root canal walls, will aid in endodontic disinfection. The aim of this study was to qualitatively examine the irrigation dynamics of syringe irrigation with different needle tip designs (open-ended and closed-ended), apical negative pressure irrigation with the EndoVac® system, and passive ultrasonic-assisted irrigation, using a computational fluid dynamics model. Syringe-based irrigation with a side-vented needle showed a higher wall shear stress than the open-ended but was localised to a small region of the canal wall. The apical negative pressure mode of irrigation generated the lowest wall shear stress, while the passive-ultrasonic irrigation group showed the highest wall shear stress along with the greatest magnitude of velocity.

New smoothed particle hydrodynamics (SPH) formulation for modeling heat conduction with solidification and melting

ABSTRACT When modeling the phase change, the latent heat released (absorbed) during solidification (melting) must be included in the heat transfer equation. In this paper, different smoothed particle hydrodynamics (SPH) methods for the implementation of latent heat, in the context of transient heat conduction, are derived and tested. First, SPH discretizations of two finite element methods are presented, but these prove to be computationally expensive. Then, by starting from a simple approximation and enhancing accuracy using different numerical treatments, a new SPH method is introduced, that is fast and easy to implement. An evaluation of this new method on various analytical and numerical results confirms its accuracy and robustness.