Bettina Frohnapfel

Bettina Frohnapfel
Karlsruhe Institute of Technology | KIT · Institute of Fluid Mechanics

About

149
Publications
19,218
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1,413
Citations
Introduction
Additional affiliations
May 2012 - present
Karlsruhe Institute of Technology
Position
  • Professor (Full)

Publications

Publications (149)
Article
Full-text available
Many engineering applications rely on lubricated gaps where the hydrodynamic pressure distribution is influenced by cavitation phenomena and elastic deformations. To obtain details about the conditions within the lubricated gap, solvers are required that can model cavitation and elastic deformation effects efficiently when a large amount of discret...
Article
Full-text available
Heterogeneous roughness in the form of streamwise aligned strips is known to generate large scale secondary motions under turbulent flow conditions that can induce the intriguing feature of larger flow rates above rough than smooth surface parts. The hydrodynamical definition of a surface roughness includes a large scale separation between the roug...
Conference Paper
Full-text available
We investigate the active turbulent boundary layer control (BLC) scheme of uniform blowing also known as micro-blowing technique applied on airfoils. In this contribution we will show the path relating numerical investigations to experimental investigations which we are currently preparing. A significant effort was put in understanding the differen...
Preprint
Full-text available
Turbulent mixed convection in channel flows with heterogeneous surfaces is studied using direct numerical simulations. The relative importance between buoyancy and shear effects, characterized by the bulk Richardson number $Ri_b$, is varied in order to cover the flow regimes of forced, mixed and natural convection, which are associated with differe...
Article
Full-text available
The question whether spatial resolution effects should be accounted for when performing hot-wire (or particle image velocimetry) measurements in turbulent wall-bounded flows over rough surfaces in the fully rough regime is addressed and answered by exploiting an existing direct numerical simulation database of open-channel flows at Reτ=500\document...
Article
This work presents direct numerical simulations (DNS) of a circular turbulent jet impinging on rough plates. The roughness is once resolved through an immersed boundary method (IBM) and once modeled through a parametric forcing approach (PFA) which accounts for surface roughness effects by applying a forcing term into the Navier–Stokes equations wi...
Article
Full-text available
While existing engineering tools enable us to predict how homogeneous surface roughness alters drag and heat transfer of near-wall turbulent flows to a certain extent, these tools cannot be reliably applied for heterogeneous rough surfaces. Nevertheless, heterogeneous roughness is a key feature of many applications. In the present work we focus on...
Article
Full-text available
Background: The human heart is a masterpiece of the highest complexity coordinating multi-physics aspects on a multi-scale range. Thus, modeling the cardiac function in silico to reproduce physiological characteristics and diseases remains challenging. Especially the complex simulation of the blood's hemodynamics and its interaction with the myocar...
Article
We extend a diffuse interface phase-field method for two-phase flow simulations so as to include interfacial heat transfer and the thermal Marangoni effect. The set of governing equations hold non-standard terms, which originally stem from the underlying variational consideration of the total free energy of the two-phase system. It consists of the...
Article
Full-text available
Wet clutches are widely used in power transmission, but lack of the fact of an energy loss in open state condition. The flow conditions in the fluid flow of an open wet clutches are analyzed by analytical means. The requisite simplifications that result in an analytically integrateable solution are stated in detail. Special emphasis is put on the r...
Article
Full-text available
A detailed analysis of the effects of uniform blowing, uniform suction and body-force damping on the turbulent boundary layer developing around a NACA4412 airfoil at moderate Reynolds number is presented. The flow over the suction and the pressure sides of the airfoil is subjected to a non-uniform adverse pressure gradient and a moderate favourable...
Preprint
Full-text available
Many engineering applications rely on lubricated gaps where the hydrodynamic pressure distribution is influenced by cavitation phenomena and elastic deformations. To obtain details about the conditions within the lubricated gap, solvers are required that can model cavitation and elastic deformation effects efficiently when a large amount of discret...
Article
Full-text available
Inhomogeneous rough surfaces in which strips of roughness alternate with smooth-wall strips are known to generate large-scale secondary motions. Those secondary motions are strongest if the strip width is of the order of the half-channel height and they generate a spatial wall shear stress distribution whose mean value can significantly exceed the...
Article
Full-text available
The heat transfer of a single water droplet impacting on a heated hydrophobic surface is investigated numerically using a phase field method. The numerical results of the axisymmetric computations show good agreement with the dynamic spreading and subsequent bouncing of the drop observed in an experiment from literature. The influence of Weber numb...
Article
Full-text available
To mimic the unsteady vortex–wall interaction of animal propulsion in a canonical test case, a vorticity-annihilating boundary layer was examined through the spin-down of a vortex from solid-body rotation. A cylindrical, water-filled tank was rapidly stopped, and the decay of the vortex from solid-body rotation was observed by means of planar and s...
Conference Paper
Full-text available
We present a comprehensive parametric study on the flow-control scheme denoted as wall-normal uniform blowing and suction in turbulent boundary layers around airfoils. The focus is primarily put on the influence of the drag caused by the power consumption of the support system. We also assess the theoretical background of the momentum budget includ...
Article
Anisotropic fiber-reinforced composites are used in lightweight construction, which is of great industrial relevance. During mold filling of fiber suspensions, the microstructural evolution of the local fiber arrangement and orientation distribution is determined by the local velocity gradient. Based on the Folgar–Tucker equation, which describes t...
Conference Paper
Full-text available
Direct numerical simulation (DNS) is carried out to study turbulent flow over irregular rough surfaces in periodic minimal channels. A passive scalar transport equation is solved to study heat transfer over rough surfaces with the Prandtl number P r = 0.7. The generation of irregular roughness is based on a mathematical random-ization algorithm, in...
Article
The present work describes an experimental investigation that applies stereo particle image velocimetry in a cross-plane of a turbulent channel flow that is additionally perturbed by spanwise oscillatory body forces, induced by a plasma actuator and designed to mimic the effect of spanwise wall oscillations. The experiment is aimed at retrieving th...
Preprint
Full-text available
Fundamental fluid--mechanics studies and many engineering developments are based on tripped cases. Therefore, it is essential for CFD simulations to replicate the same forced transition in spite of the availability of advanced transition modelling. In the last decade, both direct and large--eddy simulations (DNS and LES) include tripping methods in...
Article
Full-text available
The present work discusses the impact of the back coupling of the fiber orientation distribution on the base flow and on the fiber orientation itself during mold filling simulations. Flows through a channel and over a backward-facing step are investigated. Different closure approximations are considered for modeling the flow-induced evolution of an...
Article
Full-text available
An extensive parametric study of turbulent boundary-layer control on airfoils via uniform blowing or suction is presented. The control is applied on either the suction or pressure side of several four-digit NACA-series airfoils. The considered parameter variations include angle of attack, Reynolds number, control intensity, airfoil camber, and airf...
Article
The interaction of droplets consisting of urea-water solution (UWS) with a wall is of interest for automotive exhaust gas after-treatment of Diesel engines by selective catalytic reduction (SCR). Since the impingement of tiny UWS droplets on the solid substrate is difficult to examine experimentally, little is known about the detailed dynamics of t...
Article
Full-text available
A DNS parametric study of streamwise‐aligned rectangular ridges is carried out in a fully developed turbulent channel flow with constant flow rate at Reb = 18000. The simulations were carried out systematically varying the ridge height h, width w and structural wavelength S. The ridges generate a strong large‐scale secondary motion, which is measur...
Chapter
Inhalt Keynote System-of-Systems-Engineering als Basis zukunftsfähiger Antriebs- und Kupplungsentwicklung . . . . .1 Systemwechselwirkungen und Modellbildung Einfluss von Betriebsgrößen auf die Schwingungsreduzierungswirkung im nasslaufenden Kupplungssystem: Empirische Modellbildung – Kennfelder und Skalierbarkeit . . . 11 Entwicklung einer Methode...
Chapter
Inhalt Keynote System-of-Systems-Engineering als Basis zukunftsfähiger Antriebs- und Kupplungsentwicklung . . . . .1 Systemwechselwirkungen und Modellbildung Einfluss von Betriebsgrößen auf die Schwingungsreduzierungswirkung im nasslaufenden Kupplungssystem: Empirische Modellbildung – Kennfelder und Skalierbarkeit . . . 11 Entwicklung einer Methode...
Preprint
In this work we assess the applicability of the adjoint optimization technique for determining optimal surface topographies of two surfaces in relative motion in presence of a thin lubricant films that can cavitate. Among the existing numerical tools for topology optimization in engineering problems, the adjoint method represents a promising and ve...
Article
Full-text available
We carried out high-fidelity large-eddy simulations (LES) to investigate the effects of uniform blowing and uniform suction on the aerodynamic efficiency of a NACA4412 airfoil at the moderate Reynolds number based on chord length and incoming velocity of Re c = 200, 000. We found that uniform blowing applied at the suction side reduces the aerodyna...
Article
The remaining torque of disengaged wet clutches is a major source of energy loss and, therefore, an objective of current research. The present contribution describes the necessary simplifications to obtain an analytical solution of the governing equations by means of an order-of-magnitude analysis. The obtained results are brought to a dimensionles...
Article
Tools for the numerical prediction of haemodynamics in multi-disciplinary integrated heart simulations have to be based on computational models that can be solved with low computational effort and still provide physiological flow characteristics. In this context the mitral valve model is important since it strongly influences the flow kinematics, e...
Article
Full-text available
The injection of urea-water solution (UWS) sprays into engine-close selective catalytic reduction (SCR) systems faces developers with complex challenges regarding two-phase flow and deposit formation. Potential spray impact on the static mixing elements inside the exhaust duct can lead to accumulation of liquid film and solid deposits, which may re...
Article
Full-text available
A considerable number of surface texture investigations is based on pin-on-disc tribometers. This work shows that a crucial role in the reproducibility of the results, e.g. Stribeck curves, is played by the geometry of the pin surface. The investigation is based on an elastohydrodynamic model of a pin-on-disc tribometer which is validated with expe...
Chapter
Developing efficient flow control techniques remain a challenging task due to the complexity of turbulent flows in industrial applications, a relevant example of which are turbulent boundary layers (TBL) subjected to pressure gradients. In the present study, we employ high-fidelity numerical simulations to assess the impact of different control str...
Chapter
An accurate representation of complex geometries in numerical simulations is of great interest. For the numerical implementation, there are two possibilities to represent the structure of the immersed geometry. Firstly, the simulation domain can be represented by body-conforming grids following the shape of the body.
Article
Full-text available
In this work we assess the applicability of the adjoint optimization technique for determining optimal surface topographies of two surfaces in relative motion in presence of a thin lubricant films that can cavitate. Among the existing numerical tools for topology optimization in engineering problems, the adjoint method represents a promising and ve...
Article
Full-text available
Trapezoidal riblets are experimentally investigated in both their drag-reducing and drag-increasing regimes in an air turbulent channel flow facility. The nondimensional riblet spacing $$s^+$$ s + and height $$h^+$$ h + in viscous units are varied by changing the bulk Reynolds number in the range $$5 \times 10^3<{\mathrm{Re}}_b<8.5 \times 10^4$$ 5...
Article
Full-text available
On the stages of vortex decay in an impulsively stopped, rotating cylinder - Volume 885 - Frieder Kaiser, Bettina Frohnapfel, Rodolfo Ostilla-Mónico, Jochen Kriegseis, David E. Rival, Davide Gatti
Preprint
Full-text available
We derive a theory for material surfaces that maximally inhibit the diffusive transport of a dynamically active vector field, such as the linear momentum, angular momentum or vorticity, in three-dimensional unsteady flows. Along these evolving material surfaces (Lagrangian active barriers), the time-averaged change of diffusive flux generated by th...
Article
Full-text available
Turbulent flow over a surface with streamwise-elongated rough and smooth stripes is studied by means of direct numerical simulation (DNS) in a periodic plane open channel with fully resolved roughness. The goal is to understand how the mean height of roughness affects the characteristics of the secondary flow formed above a spanwise heterogeneous r...
Article
Surface structuring in form of streamwise-aligned triangular ridges is investigated in the framework of a fully developed turbulent channel flow with constant wall temperatures of different values prescribed on the upper and lower walls at Re b = 18000 . Two arrangements of the ridges on both channel walls are considered – a symmetrical arrangemen...
Article
Different types of thermal boundary conditions are conceivable in numerical simulations of convective heat transfer problems. Isoflux, isothermal and a mixed-type boundary condition are compared by means of direct numerical simulations (for the lowest Reynolds number) and well-resolved large-eddy simulations of a turbulent forced convection pipe fl...
Article
Full-text available
In this work a two‐step model to investigate a thin liquid film sheared by a turbulent gas flow is presented. In a first step, a direct numerical simulation of the gas phase is conducted. The turbulent shear stress at the lower wall is stored and used as a boundary condition for the long‐wave evolution of the liquid film, which is calculated in a s...
Article
In this study, we model the heat transfer mechanism for a single nucleate vapor bubble which then expands to an elongated bubble during evaporation in a microchannel. The model is a logical mix of empirical correlations and analytical models and is defined for the two steps of bubble growth including: (a) partially confined growth, in which the bub...
Article
Bypass transition in a boundary layer subjected to freestream turbulence and distributed surface roughness is studied numerically. The distributed surface roughness is reproduced with an immersed boundary technique, and the freestream turbulence is artificially generated by a superposition of eigenmodes of the Orr–Sommerfeld and Squire equations. B...
Preprint
Full-text available
Turbulent flow over a surface with streamwise-elongated rough and smooth stripes is studied by means of direct numerical simulation (DNS) in a periodic plane channel with fully resolved roughness. The goal is to understand how the mean height of roughness affects the characteristics of the secondary flow formed above a spanwise-heterogeneous rough...
Article
Full-text available
Different types of thermal boundary conditions are conceivable in numerical simulations of convective heat transfer problems. Isoflux, isothermal and a mixed-type boundary condition are compared by means of direct numerical simulations (for the lowest Reynolds number) and well-resolved large-eddy simulations of a turbulent forced convection pipe fl...
Poster
http://www.itcp.kit.edu/modegat/ http://www.itcp.kit.edu/modegat/downloads/Program%20MODEGAT%20VI%202019-08-28.pdf
Chapter
We present a theoretical framework for describing production, transport, redistribution and dissipation of every Reynolds stress component occurring among different scales and along directions of statistical inhomogeneity. It is based on the exact budget equations for the second-order structure function tensor \(\left\langle {\delta {u}_i \delta {u...
Conference Paper
Full-text available
Secondary flows of Prandtl’s second kind are spontaneously-occurring large-scale swirling motions normal to the direction of the mean flow and are caused by Reynolds stress gradients (Bradshaw, 1987). In the present study we investigate the transport mechanisms of turbulent secondary flows by means of the transport equation of turbulent kinetic ene...
Article
The effect of azimuthally inhomogeneous surface heat flux on thermal statistics of turbulent forced convection pipe flow is studied via direct numerical and large eddy simulations. Two Prandtl numbers of Pr=0.71 and Pr=0.025 are considered together with bulk Reynolds numbers of Reb=5300,11700,19000,37700. A zero temperature gradient condition is im...
Article
In this work, the dynamics of a thin liquid film sheared by a turbulent gas flow are investigated numerically. It is known that even a constant interfacial shear stress affects film stability and dynamics. We are interested in the effect of turbulent fluctuations on the film development. A combination of a direct numerical simulation (DNS) of the t...
Article
Full-text available
Enhanced heat transfer surfaces based on cylindrically shaped pin fins with wire diameters in the range of 100 µm were analyzed. The design is based on a high pin length to diameter ratio in the range of 20–100. Correlations for thermal and fluid dynamic characteristics of these fine wire structures are not available in literature. An in-line and s...
Preprint
The scope of the present project is to quantify the effects of uniform blowing and body-force damping on turbulent boundary layers subjected to a non-uniform adverse-pressure-gradient distribution. To this end, well-resolved large-eddy simulations are employed to describe the flow around the NACA4412 airfoil at moderate Reynolds number 200, 000 bas...