Günter Brenn - Academia.edu (original) (raw)

Papers by Günter Brenn

Cornell University - arXiv, Mar 27, 2018

Recently it has been proposed to use colliding drops for producing advanced particles or well def... more Recently it has been proposed to use colliding drops for producing advanced particles or well defined capsules, or to perform chemical reactions where the merged drops constitute a micro-reactor. For all these promising applications it is essential to determine whether the merged drops remain stable after the collision, forming a single entity, or if they break up. This topic, widely investigated for binary drop collisions of miscible and immiscible liquids, is quite unexplored for ternary drop collisions. The current study aims to close this gap by experimentally investigating collisions of three equal-sized drops of the same liquid, arranged centrisymmetrically. To do this, three drop generators are simultaneously operated to obtain controlled ternary drop collisions. The collision outcomes are observed on photographs and compared to those of binary collisions. Similar to binary collisions, a regime map is built, showing coalescence and bouncing as well as reflexive and stretching separation. Differences are observed in the transitions between these regimes.

Monthly Notices of the Royal Astronomical Society

A new class of one-dimensional solar wind models is developed within the general polytropic, sing... more A new class of one-dimensional solar wind models is developed within the general polytropic, single-fluid hydrodynamic framework. The particular case of quasi-adiabatic radial expansion with a localized heating source is considered. We consider analytical solutions with continuous Mach number over the entire radial domain while allowing for jumps in the flow velocity, density, and temperature, provided that there exists an external source of energy in the vicinity of the critical point that supports such jumps in physical quantities. This is substantially distinct from both the standard Parker solar wind model and the original nozzle solutions, where such discontinuous solutions are not permissible. We obtain novel sample analytic solutions of the governing equations corresponding to both slow and fast winds.

Jahrestreffen der ProcessNet-Fachgruppen Computational Fluid Dynamics, Mischvorgänge und Rheologie, Feb 24, 2014

Meeting of experts of the GVC Group of Experts "Multiphase flows'', 1999

Symp. 20th Annivers. Phase-Doppler Anemometry, 1995

International Journal of Multiphase Flow, 2022

Journal of Fluid Mechanics, 2022

We develop a model to predict the fragmentation limit of drops colliding off-centre. The predicti... more We develop a model to predict the fragmentation limit of drops colliding off-centre. The prediction is excellent over a wide range of liquid properties and it can be used without adjusting any parameter. The so-called stretching separation is attributed to the extension of the merged drop above a critical aspect ratio of 3.25. The evolution of this aspect ratio is influenced by the liquid viscosity and can be interpreted via an energy balance. This approach is then adapted to drop–jet collisions, which we model as consecutive drop–drop collisions. The fragmentation criterion is similar to that observed for drop–drop collisions, while the evolution of the stretched jet aspect ratio is modified to account for the different flow fields and geometry.

This submission is supplementary material in the form of data and codes used in and for the manus... more This submission is supplementary material in the form of data and codes used in and for the manuscript 'A Bayesian Approach to Blood Rheological Uncertainties in Aortic Hemodynamics' submitted to the International Journal of Numerical Methods in Biomedical Engineering (currently under review).

Journal of Non-Newtonian Fluid Mechanics, 2022

Bubbles rising in viscoelastic liquids may exhibit a jump discontinuity of the rise velocity as a... more Bubbles rising in viscoelastic liquids may exhibit a jump discontinuity of the rise velocity as a critical bubble volume is exceeded. The phenomenon has been extensively investigated in the literature, both by means of experiments as well as via numerical simulations. The occurrence of the velocity jump has been associated with a change of the bubble shape under the formation of a pointed tip at the rear end and to the appearance of a so-called negative wake with the liquid velocity behind the bubble, pointing in the opposite direction to that in viscous Newtonian fluids. We revisit this topic, starting with a review of the state of knowledge on the interrelations between the mentioned characteristic features. In search for a convincing explanation of the jump phenomenon, we performed detailed numerical simulations of the transient rise of single bubbles in 3D, allowing for a local polymer molecular conformation tensor analysis. The latter shows that polymer molecules traveling along the upper bubble hemisphere are stretched in the circumferential direction, due to the flow kinematics. Then, depending on the relaxation time scale of the polymer, the stored elastic energy is either unloaded essentially above or below the bubble's equator. In the former case, this slows down the bubble, while the bubble gets accelerated otherwise. In this latter case, the relative velocity of the polymer molecules against the bubble is increased, giving rise to a self-amplification of the effect and thus causing the bubble rise velocity to jump to a higher level. Detailed experimental velocity measurements in the liquid field around the bubble confirmed the conclusion that the ratio of the time scale of the Lagrangian transport of polymer molecules along the bubble contour to the relaxation time scale of the polymer molecules determines the subor supercritical state of the bubble motion.

The boundary element method (BEM) is a well-established method and particularly well suited to tr... more The boundary element method (BEM) is a well-established method and particularly well suited to treat wave propagation phenomena in unbounded domains. However, the occurrence of dense system matrices is prohibitive, limiting the classical BEM to small and mid-sized problems. In the present work we propose a Chebyshev interpolation based multi-level fast multipole method (FMM) to reduce memory and computational cost of the 3D elastodynamic boundary integral operators. We present two versions for the proposed algorithm: Firstly, the direct approximation of the tensorial elastodynamic displacement and traction kernels and secondly, a version using a representation of the fundamental solutions based on scalar Helmholtz kernels. The former offers easy extensibility to more complicated kernel functions, which arise for instance in poroelastic problems. The latter minimizes the number of moment-to-local (M2L) operations and, additionally, offers the possibility to exploit the rotational invariance of the scalar kernel to further reduce memory requirements. For both approaches a directional clustering scheme in combination with a plane wave modification of the kernel function is implemented to treat the high frequency case. In order to validate the proposed numerical schemes, the FMM approximation error is investigated for both the low and high frequency regime. Furthermore, convergence results are given for a Dirichlet as well as a mixed boundary value problem in Laplace domain. Finally, the applicability of the proposed FMM to transient problems treated with the Convolution Quadrature Method is investigated.

aerodynamics of the Ahmed body

Atomization and Sprays, 2021

International audienceWe study the behaviour of a viscoelastic liquid jet. To that end, we carry ... more International audienceWe study the behaviour of a viscoelastic liquid jet. To that end, we carry out a temporal stability analysis. The eigenvalue problem has been already solved and is reported in Cottier et al. ILASS-Europe 2019, but does not enable a complete description on the jet's behaviour. In this paper, we propose two different sets of initial conditions to close the corresponding initial-value problem. These conditions are determined by formulating the different quantities that can be imposed over the base flow, and making a correspondence with the expression of these quantities found by solving the eigenvalue problem, evaluated at the initial time. Here the imposed quantities are considered to hold on the jet's surface position, on the axial flow velocity and on the axial normal extra stress within the flow. After giving a general formulation of the three initial conditions, two particular different initial configurations are distinguished: the pure deformation and the pure impulse configuration. A linearization of the conditions leads us to the determination of the analytical expressions of the first-order flow quantities in function of control parameters determining the choice of the initial configuration. The reduction of the initial conditions to the Newtonian case is valid and does not induce any physical change at the initial time. An application of the initial conditions is made upon a given liquid jet and the visualization of the flow quantities allows a comparison between the pure deformation and the pure impulse configuration

Fluid Mechanics and Its Applications, 2020

The present invited contribution to this book reviews the state of understanding and application ... more The present invited contribution to this book reviews the state of understanding and application of shape oscillations of liquid drops in a gaseous environment. The oscillations influence transport processes across the surface of spray drops, the drag as well as heat and mass transfer. The physical basics of linear and nonlinear oscillations are presented and discussed. For the linear case, the characteristic equation of the drop is derived, and the equations of motion are solved, accounting for the fact that the characteristic equation has pairs of complex conjugate solutions. The effects characterising the nonlinear case are reviewed and discussed. Shape oscillations of non-Newtonian, viscoelastic liquid drops exhibit interesting influences from time scales of the viscoelastic liquid relative to the oscillation period. The liquid elasticity may take over from surface tension as the restoring effect. Drop shape oscillations are used for measuring material properties of the drop liquid, such as dynamic viscosity and surface tension, as well as rheological and interfacial parameters. The most important measurement techniques and measured liquid properties are presented and discussed.

We investigate nonlinear oscillations of viscous Newtonian liquid drops for their relevance for t... more We investigate nonlinear oscillations of viscous Newtonian liquid drops for their relevance for transport processes across the liquid-gas interface. Theoretically we adopt the weakly nonlinear approach to account for the influence of the nonlinear motion on the drop oscillation. The theory is in progress. Experiments show a different behaviour for large and for small oscillation amplitudes, seen weakly in the oscillation frequency, but strongly in the damping rate. Liquid material parameters deduced from the oscillations agree well with the expected values when amplitudes are small.