Bernhard Weigand

Bernhard Weigand
Universität Stuttgart · Institute of Aerospace Thermodynamics

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615
Publications
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6,122
Citations

Publications

Publications (615)
Article
Full-text available
This paper reports on particle image velocimetry (PIV) measurements in compressible accelerated wake flows generated by two different central injector types, which are mounted in a convergent-divergent nozzle. The injectors differ by the extent of their trailing edge located either in the subsonic (injector A) or supersonic flow region (injector B)...
Article
Droplet grouping is important in technical applications and in nature where more than one droplet is seen. Despite its relevance for such problems, the fundamentals of the grouping processes are not yet fully understood. Initial conditions that expedite or impede the formation of droplet groups have been studied, but a thorough investigation of the...
Article
A numerical model was developed for predicting the heat and mass transfer during condensation within a charge air cooler under different operating conditions. Numerical modeling of the charge air cooler was performed in a commercial CFD software with the condensation model implemented as a User Code. The semi-theoretical condensation model is based...
Article
The blade lean technique has been extensively employed in the design of modern gas turbine blades. Since swirl cooling is a promising alternative for internal cooling techniques at the blade leading edge due to its strongly-enhanced heat transfer, it is thereby vital to completely understand the fundamental flow and heat transfer behavior of swirli...
Article
Full-text available
The condition for the formation of droplet groups in liquid sprays is poorly understood. This study looks at a simplified model system consisting of two iso-propanol droplets of equal diameter, Dd0, in tandem, separated initially by a center-to-center distance, a20, and moving in the direction of gravity with an initial velocity, Vd0>Vt, where Vt i...
Article
Full-text available
An unsteady analytical solution is proposed to predict the spreading rate of the crown generated by an impacting droplet onto wetted walls. The modelling strategy is based on the direct integration of the boundary layer correction into the potential flow solution that leads to the well-established square-root time dependence. The original potential...
Article
The development of modern gas turbines for aircraft propulsion and power generation demands ever-increasing efficiency, which can be achieved by rising the turbine inlet temperature. Therefore, turbine components and especially the leading edge of turbine blades are exposed to particular high thermal loads with temperatures that are well above the...
Article
This work presents an improved estimation of the local measurement uncertainty of transient heat transfer experiments using Thermochromic Liquid Crystals (TLC). Additional signal analysis steps and a Monte Carlo (MC) approach are added to the evaluation routine. As a result, the local uncertainty in the indication time of the maximum green intensit...
Article
Full-text available
Two-wire thermocouples are often used for temperature measurements. Under transient conditions, measurement errors can occur due to capacitive inertia and heat conduction along the stem of the thermocouples. The present study presents a correction of these thermocouple measurement errors caused by transient inertia and conductive effects using a si...
Article
The motion of two dissimilar spherical non-evaporating liquid droplets moving vertically along their line of centers is analysed mathematically. The stimulus for the study is evidence from the literature that, under appropriate operating conditions, droplet grouping can occur. Carefully controlled experiments, aimed at providing a paradigm for unde...
Article
In this paper the solidification of a liquid around an array of cooled horizontal cylinders has been investigating experimentally and analytically. The horizontal cylinders can be arranged in various configurations. In this paper one typical example for such a configuration is studied. This is a cyclic system cylinders with cylinders of a diameter...
Article
Full-text available
We present a dual network model to simulate coupled single-phase flow and energy transport in porous media including conditions under which local thermal equilibrium cannot be assumed. The models target applications such as the simulation of catalytic reactors, micro-fluidic experiments, or micro-cooling devices. The new technique is based on a rec...
Article
The basis of many turbulence models in computational fluid dynamics is the linear Boussinesq hypothesis that assumes an alignment between the mean strain rate tensor and the Reynolds stress tensor. The validity of this main assumption is analyzed for the test case of an infinite tube bundle with periodic boundary conditions at Reb≈34,800. This work...
Article
Full-text available
In this article, a mathematical study is presented on electroosmotic flow (EOF) of power law fluids driven by an external electric field, where the heterogeneity of a microslit is created by multiple wavy triangular modulated polarized wall hurdles. The primary aim of this paper is to demonstrate and depict the mixing performance that generating mo...
Article
This study shows that the LASER Pattern Shift Method (LPSM) is a powerful measurement technique for film thickness measurements. In this paper, the approach of the LPSM is extended, which is now able to measure the lamella thickness during droplet impacts on smooth surfaces using the naturally occurring air bubble shadows. With the help of this rat...
Article
Full-text available
We study the vertical impact of a droplet onto a cubic pillar of comparable size placed on a flat surface, by means of numerical simulations and experiments. Strikingly, during the impact a large volume of air is trapped around the pillar side faces. Impingement upon different positions of the pillar top surface strongly influences the size and the...
Article
Full-text available
Medical inhalers have been used for the treatment of a wide range of respiratory diseases including the COVID-19 pandemic. In this study, we investigate the annular liquid jet breakup with a coaxial supersonic gas jet using Large Eddy Simulations. This contributes a further understanding and improvement of medical inhaler designs. The liquid is suc...
Conference Paper
Full-text available
High-pressure multi-hole injectors for gasoline direct injection (GDI) engines allow a flexible optimisation of the individual spray hole properties to specific engines and combustion strategies. One degree of freedom within this spray targeting process is the mass flow rate of each spray plume, which is mainly controlled by the corresponding spray...
Conference Paper
Full-text available
When a drop impacts onto a liquid film the outcome can differ very much and is in general difficult to parameterize due to the many influencing parameters. A parameter often idealized when studying drop film interactions is the shape of the impacting drop. Whereas for one-component interactions its influence on the impact outcome is known to some e...
Conference Paper
Full-text available
The surface wettability has a significant influence on the morphology and spreading behavior during droplet impacts on dry smooth walls. On the way for predicting spreading diameters in dependency of the wetting behavior, the experimental database was extended by an experimental study in which distilled water and isopropanol droplets have impacted...
Conference Paper
Full-text available
The flourishing of applications in need of self-cleaning mechanisms increased the search for water repellent hydrophobic surfaces with induced roughness. Disclosing the small-scale interface phenomena on the wetting behavior is essential to design efficient hydrophobic materials with defined topography. On the other hand, the spreading behavior con...
Conference Paper
Full-text available
The tendency of convening and coalescing of initially distant droplets, called droplet grouping, can influence the final droplet size distribution, evaporation rates or final settling points. To better understand the factors which govern droplet grouping mechanisms, reliable direct numerical simulations are beneficial. Previous numerical studies ha...
Article
Full-text available
The interaction between the flow above and below a permeable wall is a central topic in the study of porous media. While previous investigations have provided compelling evidence of the strong coupling between the two regions, few studies have quantitatively measured the directionality, i.e. cause-and-effect relations, of this interaction. To shed...
Article
The use of swirling flows in cyclone cooling systems is a promising method for internal cooling applications for instance in the leading edge of gas turbine blades. However, vortex breakdown can occur in such flows, which is associated with an axial flow reversal. Within the scope of this investigation a numerical parameter study was conducted in o...
Patent
Die vorliegende Erfindung betrifft eine Vorrichtung und ein Verfahren zum Bestimmen der Schärfentiefe eines optischen Aufbaus, insbesondere von optischen Mikroskopen
Article
The influence of wettability on the morphology of droplet impacts onto dry surfaces is often neglected in literature, despite its significant effect on the resulting morphology. In this work, the role of wettability is investigated systematically by considering droplet impact processes on smooth dry surfaces of two different materials. The wetting...
Article
Flash atomization can occur in rocket thrusters operated at near vaccum conditions just prior to ignition when cryogenic propellants are injected into the reaction chamber. Although the morphology of flashing sprays has been widely studied, empirical observations of the primary breakup mechanisms at the micro-scale are currently not feasible. Still...
Article
A numerical investigation of the effect of the hydrogen-air reaction mechanism on the prediction of supersonic combustion was conducted. Different reaction mechanisms with different number of species and reactions were investigated at a Scramjet hypersonic flight trajectory. A generic three-dimensional supersonic combustor was investigated at fligh...
Article
Full-text available
Transient heat generation during guest adsorption and host–guest interactions is a natural phenomenon in metal–organic framework (MOF) chemistry. However, in situ tracking of such MOF released heat is an insufficiently researched field due to the fast heat dissipation to the surroundings. Herein, a facile capillary‐driven liquid‐imbibition approach...
Article
Full-text available
A facile capillary‐driven liquid‐imbibition approach is developed in article number 2006980 by Guang Yang, Roland A. Fischer, and co‐workers for in situ tracking of transient heat release at the wetting front of surface‐mounted metal–organic frameworks (SURMOFs) on cellulosic fiber papers. As an advancement to the state‐of‐the‐art in trace‐solvent...
Article
Full-text available
Two-phase flow in gas turbine compressors occurs, for example, at heavy rain flight condition or at high-fogging in stationary gas turbines. The liquid dynamic processes are independent of the application. An overview on the processes and their approach in literature is given. The focus of this study lies on the experimental investigation of the tr...
Article
Full-text available
The measurement of the apparent contact angle on structured surfaces is much more difficult to obtain than on smooth surfaces because the pinning of liquid to the roughness has a tremendous influence on the three phase contact line. The results presented here clearly show an apparent contact angle variation along the three phase contact line. Accor...
Article
This study focuses on an experimental and numerical heat transfer investigation of an impingement jet array with a concave target plate that resembles a turbine blade’s leading edge. The array consists of nine circular jets with a diameter of Djet and the target plate has a curvature radius of R = Djet . The target plate also features film cooling...
Article
Periodic vortex shedding which forms when fluid moves over a circular cylinder creates a wake which causes fluctuating unsteady forces on structures. The oscillating wake may also serve to enhance heat transfer rates from a surface on which it impinges. In this paper we investigate the passive excitation of the impinging jet's velocity field using...
Article
A simultaneous impact of three water drops (aligned with equidistant spacing) onto a solid wall covered by a thin liquid film is predicted by direct numerical simulation using the multiphase code Free Surface 3D (FS3D) which is based on the volume-of-fluid (VOF) method and uses the piecewise linear interface calculation (PLIC) method to reconstruct...
Article
Recognizing the attention currently devoted to the environmental impact of aviation, this three-part publication series introduces two new aircraft propulsion concepts for the timeframe beyond 2030. The first part focuses on the novel steam injecting and recovering aero engine concept. In the second part, the free-piston composite cycle engine conc...
Article
Full-text available
Turbulence transportation across permeable interfaces is investigated using direct numerical simulation, and the connection between the turbulent surface flow and the pore flow is explored. The porous media domain is constructed with an in-line arranged circular cylinder array. The effects of Reynolds number and porosity are also investigated by co...
Article
Full-text available
Mixing and evaporation processes play an important role in fluid injection and disintegration. Laser-induced thermal acoustics (LITA) also known as laser-induced grating spectroscopy (LIGS) is a promising four-wave mixing technique capable to acquire speed of sound and transport properties of fluids. Since the signal intensity scales with pressure,...
Chapter
The interaction between an airflow and a water surface influences many environmental processes. For example in a rough ocean, entrained droplets from the water surface enhance transport processes above the ocean surface which can lead to the formation of hurricanes. In order to get a better understanding of the fundamental processes we perform dire...
Article
Full-text available
The influence of miscibility and liquid wettability during droplet impact onto thin wall films is investigated experimentally. Despite similar liquid properties and impact conditions, differences in the splashing limit, in the crown extension and in the duration of the ascending phase are observed. These differences are related to the interfacial t...
Poster
Summary of Activation and Deactivation (hydrophobic recovery) experiments for smooth polymer surfaces
Preprint
Full-text available
The interaction between the flow above and below a permeable wall is a central topic in the study of porous media. While previous investigations have provided compelling evidence of the strong coupling between the two regions, few studies have quantitatively measured the directionality, i.e., causal-and-effect relations, of this interaction. To she...
Article
The performance of the fluid flow, heat and contaminant transfer is numerically experimented in a slot-ventilated container in which the flow -in and -out are occurred through inlet and exit ports in combination with thermosolutal sources. The inlet source of cold air is mechanically flushed in through left vertical wall to generate mixed convectio...
Article
Turbulence in a wall-bounded flow of supercritical fluid can be significantly modulated by nonuniform body forces. This study presents direct numerical simulations performed with a nonuniform streamwise body force varying in the wall-normal direction in a fully developed channel flow. A quasilaminar state and reorganized turbulence were obtained by...
Poster
The scenario of an impacting drop onto a film is highly relevant in many natural and technical systems. A fundamental and often required parameter of these interactions is the so called splashing threshold above which secondary droplets are generated. For situations in which the liquids of drop and film differ, a general threshold correlation is un...
Article
In the current work, the turbulent flow and heat transfer in divergent swirl cooling tubes is studied numerically by using the open source CFD code OpenFOAM. The k-ω SST turbulence model with curvature correction, i.e., the k-ω SSTCC model, is used to compute the strong turbulent swirling flow. The unsteady solver “rhoPimpleFoam” is adopted in the...
Chapter
Full-text available
This chapter book summarizes the major achievements of the five topical focus areas, Structural Cooling, Aft-Body Flows, Combustion Chamber, Thrust Nozzle, and Thrust-Chamber Assembly of the Collaborative Research Center (Sonderforschungsbereich) Transregio 40. Obviously, only sample highlights of each of the more than twenty individual projects ca...
Chapter
Full-text available
Mixing characteristics of supercritical injection studies were analyzed with regard to the necessity to include diffusive fluxes. Therefore, speed of sound data from mixing jets were investigated using an adiabatic mixing model and compared to an analytic solution. In this work, we show that the generalized application of the adiabatic mixing model...
Article
The present work provides an overview on the possible phase transitions associated with supercritical fluid injection and a detailed evaluation of the mixing process between injectant fluid and quiescent ambience. The experiments cover superheated liquid disintegration, pseudo-boiling transition and single-phase jets under different nozzle pressure...
Conference Paper
Two-wire thermocouples are often used for temperature measurements. Under transient conditions, measurement errors can occur due to capacitive inertia and heat conduction along the stem of the thermocouples. The present study presents a correction of these thermocouple measurement errors caused by transient inertia and conductive effects using a si...
Conference Paper
Recognizing the attention currently devoted to the environmental impact of aviation, this three-part publication series introduces two new aircraft propulsion concepts for the timeframe beyond 2030. The first part focuses on the novel steam injecting and recovering aero engine concept. In the second part, the free-piston composite cycle engine conc...
Conference Paper
Raising the combustion temperature continues to be a major driver for increasing the overall efficiency of industrial gas turbines. Therefore, active cooling has become a necessity as gas temperatures have surpassed the allowable temperature of the base material. Thus, cooling schemes need to be improved for higher heat loads. However, the thermal...
Article
Full-text available
Details on the fall speeds of raindrops are essential in both applications and natural events, such as rain-rate retrieval and soil erosion. Here, we examine the influence of turbulence on the terminal velocity of two water drops of different sizes. For the first time, computations of droplets in turbulent surroundings are conducted with a direct n...