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Josef Hasslberger

Josef Hasslberger
University of the Bundeswehr Munich · Institute of Applied Mathematics and Scientific Computing

Dr.-Ing.

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78
Publications
7,222
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506
Citations

Publications

Publications (78)
Article
Full-text available
The influence of water droplet injection on the propagation rate of statistically planar stoichiometric n-heptane-air flames has been analysed based on three-dimensional carrier phase Direct Numerical Simulations for different turbulence intensities and different initially mono-sized droplets. It has been found that most water droplets do not compl...
Article
The effects of droplet inertia on the reaction zone structure, overall burning rate, and flame surface area during interaction of water droplets with a statistically planar turbulent premixed stoichiometric n-heptane-air flame based on three-dimensional carrier phase Direct Numerical Simulations have been analysed. Different initially mono-sized dr...
Article
We study turbulent emulsions and the emulsification process in homogeneous isotropic turbulence (HIT) using direct numerical simulations (DNS) in combination with the volume of fluid method (VOF). For generating a turbulent flow field, we employ a linear forcing approach augmented by a proportional‐integral‐derivative (PID) controller, which ensure...
Article
Although overall increasing computing power allows for higher resolution in Large-Eddy Simulation (LES), an appropriate choice of the subgrid-model is still decisive for the simulation quality. The relevance of the subgrid-model increases even further, if transported quantities are used in additional thermo-chemical models, which are coupled to the...
Article
The influence of non-Newtonian fluid behavior and the Eötvös number on conditional and unconditional second-order structure functions of bubbly channel flows has been investigated by conducting a series of direct numerical simulations at a friction Reynolds number of 127.3. Two Eötvös numbers have been considered (Eo = 0.3125 and Eo = 3.75) togethe...
Article
Hydrodynamic instabilities caused by shock-flame interactions are a fundamental challenge in the accurate prediction of explosion loads in the context of nuclear and process plant safety. To investigate the Richtmyer–Meshkov instability, a series of three-dimensional numerical simulations of shock-flame interactions are performed, including lean, s...
Article
Three-dimensional carrier-phase Direct Numerical Simulations (DNS), combined with a Lagrangian representation of individual droplets, have been employed in this parametric study to examine the physical effects of liquid water mist interacting with laminar and turbulent premixed stoichiometric n-heptane/air flames. Significant reductions of flame te...
Article
Full-text available
The methodology to quantify the numerical dissipation in Under-resolved DNS (UDNS) based on the balance of the kinetic energy equation by Schranner et al. (Comput Fluids 114:84–97, 2015), has been examined in this study. Furthermore, this methodology has been extended for considering active scalars, based on both balance of the kinetic energy and t...
Article
The present study deals with the application of a sub-grid activity sensor to an eddy viscosity type base model in the context of Large Eddy Simulation (LES). The coherent structure function is used to build this sensor in combination with explicit test filtering. The proposed sensor features two main advantages: First, it attenuates the sub-grid s...
Article
Full-text available
The influence of non-Newtonian fluid behavior on the flow statistics of turbulent bubble-laden downflow in a vertical channel is investigated. A Direct Numerical Simulation (DNS) study is conducted for power-law fluids with power-law indexes of 0.7 (shear-thinning), 1 (Newtonian) and 1.3 (shear-thickening) in the liquid phase at a gas volume fracti...
Article
Gene Expression Programming (GEP), a branch of machine learning, is based on the idea to iteratively improve a population of candidate solutions using an evolutionary process built on the survival-of-the-fittest concept. The GEP approach was initially applied with encouraging results to the modeling of the unclosed tensors in the context of RANS (R...
Article
To investigate the coherent structures in turbulent wall-bounded bubbly flows, a local flow topology analysis has been performed in this work. Using the invariants of the velocity-gradient tensor, all possible small-scale flow structures can be categorized into two nodal and two focal topologies for incompressible flows. The analysed direct numeric...
Article
Full-text available
The statistical behaviours of the invariants of the velocity gradient tensor and flow topologies for Rayleigh-Bénard convection of Newtonian fluids in cubic enclosures have been analysed using Direct Numerical Simulations (DNS) for a range of different values of Rayleigh (i.e. [Formula: see text]) and Prandtl (i.e. [Formula: see text] and 320) numb...
Article
Three-dimensional Direct Numerical Simulations (DNS) data has been used to analyse the gaseous phase combustion behaviour in a V-shaped flame configuration where fuel is supplied in the form of droplets such that an overall (i.e. liquid+gaseous) equivalence ratio of unity is maintained in the unburned gas. The analysis has been carried out for diff...
Article
Full-text available
Due to the continuous increase in available computing power, the Large Eddy Simulation (LES) of two-phase flows started to receive more attention in recent years. Well-established models from single-phase flows are often used to close the sub-grid scale convective momentum transport and recently some modifications have been suggested to account for...
Article
The statistics of flame–droplet interaction have been investigated for spherically expanding spray flames based on a three-dimensional DNS database. The source terms due to two-phase coupling arising from droplet evaporation in different governing equations for the gaseous carrier phase are considered to examine the effects of droplet diameter and...
Article
Uncontrolled hydrogen/air explosions pose a central problem in nuclear and process plant safety research. The Richtmyer–Meshkov Instability (RMI) can be an important contributing factor to flame acceleration and subsequently the deflagration-to-detonation transition. In this context, the RMI is caused by the interaction of a sharp pressure gradient...
Chapter
The methodology to quantify the numerical dissipation in implicit/explicit LES and under-resolved DNS (UDNS) based on the balance of the kinetic energy equation by Cadieux et al. [1] has been examined for Rayleigh-Bénard convection (RBC) in this study. To this end, this approach has been extended here for scalar transport (i.e., thermal variance) i...
Chapter
Turbulent bubbly flows play an essential role in a large number of technical applications, e.g. for chemical reactors in the process industry.
Article
Full-text available
Precise evaluation of flame surface area plays a pivotal role in the fundamental understanding and accurate modelling of turbulent premixed flames. This necessity is reflected in the requirement for the instantaneous flame area evaluation of the turbulent burning velocity (by making use of Damköhler’s first hypothesis). Moreover, the information re...
Article
Direct Numerical Simulations (DNS) have been conducted to analyse velocity and temperature spectra in unsteady three-dimensional Rayleigh-Bénard convection (differentially heated horizontal walls, i.e. heated from below and cooled from above whereas the other walls are adiabatic) at high nominal Rayleigh number of viscoplastic fluids obeying a Bing...
Article
Full-text available
The effects of droplet diameter and the overall (liquid+gas) equivalence ratio on flame topology and propagation statistics in spherically expanding turbulent n-heptane spray flames have been analysed based on three-dimensional Direct Numerical Simulations (DNS) data. It has been found that the range of both mean and Gauss curvatures of the flame s...
Article
The statistical behaviors of the magnitude of the reaction progress variable gradient (alternatively known as the Surface Density Function (SDF)) and the strain rates, which affect the SDF evolution, have been analyzed using three-dimensional Direct Numerical Simulations (DNS) of spherically expanding flames in globally stoichiometric initially mon...
Conference Paper
Full-text available
This paper presents an adaptive flame-tracking shock-capturing scheme for industry-scale explosion simulations. The multi-physics multi-scale problem poses several challenges which are met by special numerical techniques. As a key element, the hybrid flame-tracking shock-capturing scheme reduces grid dependency by treating the flame as a reactive d...
Article
Full-text available
A local flow topology analysis was conducted for laminar particle-affected flows. Based on the invariants of the velocity gradient tensor, all possible flow structures can be categorized into two focal and two nodal topologies for incompressible flows. The underlying field descriptions for bubble- and droplet-affected flows in the creeping flow reg...
Article
Compared to Large Eddy Simulation (LES) of single-phase flows, which has become a mature and viable turbulence modelling technique, the LES of two-phase flows with moving immiscible interfaces is at a rather early development stage. There is no standard set of governing equations for two-phase flow LES, but rather a variety of different formulation...
Article
Laminar and turbulent spherically expanding n-heptane flames in mono-sized fuel droplet-mists have been simulated for a range of different overall equivalence ratios and droplet diameters using three-dimensional Direct Numerical Simulations (DNS). Flame wrinkling and the evolutions of flame surface area and burned gas volume have been investigated...
Article
The local flow topology analysis of the primary atomization of liquid jets has been conducted using the invariants of the velocity-gradient tensor. All possible small-scale flow structures are categorized into two focal and two nodal topologies for incompressible flows in both liquid and gaseous phases. The underlying direct numerical simulation da...
Article
This paper presents a detailed investigation of flow topologies in bubble-induced two-phase turbulence. Two freely moving and deforming air bubbles that have been suspended in liquid water under counterflow conditions have been considered for this analysis. The direct numerical simulation data considered here are based on the one-fluid formulation...
Conference Paper
This paper presents a detailed investigation of flow topologies in bubble-induced two-phase turbulence. The direct numerical simulation data considered here is based on the one-fluid formulation of the two-phase flow governing equations. To study the development of coherent structures, a local flow topology analysis is performed. Using the invarian...
Conference Paper
Precise evaluation of flame area is important for understanding and accurate modelling of turbulent premixed flames. Exemplarily, the instantaneous flame area is needed for evaluating the turbulent flame speed (by making use of Damköhler's hypothesis). Further, it is required in the context of flame surface density based modelling, or for determini...
Article
Three-dimensional direct numerical simulations with a modified single-step Arrhenius chemistry have been used to analyze spherically expanding n-heptane flames propagating into mono-sized fuel droplet mists for different droplet diameters and an overall equivalence ratio of unity. The evolutions of flame surface area and burned gas volume for both...
Conference Paper
Three-dimensional Direct Numerical Simulations (DNS) with a modified single-step Arrhenius chemistry have been used to analyse spherically expanding n-heptane flames propagating into fuel-droplet mists for different droplet diameters for an overall equivalence ratio of unity. The evolutions of flame surface area and burned gas volume for both lamin...
Conference Paper
Experimental and numerical analyses were performed to resolve the complex 3D deformation and periodic shape oscillations of deformable air bubbles in water (~ 4−5 mm diameter). Characterizing the temporal variation of the surface-to-volume ratio was the primary interest of this investigation. It is shown that the surface-to-volume ratio oscillates...
Conference Paper
In this study, Direct Numerical Simulations (DNS) have been conducted to analyse unsteady three-dimensional Rayleigh-Bénard convection of yield stress fluids obeying a Bingham model in cubic domains (i.e. differentially heated horizontal walls heated from below where other walls are adiabatic) at high values of nominal Rayleigh number. The simulati...
Thesis
In the framework of a research project funded by the German Federal Ministry of Economic Affairs and Energy (BMWi), a CFD combustion solver has been extended for the purpose of nuclear safety analysis. The methodology aims at the prediction of industry-scale hydrogen explosions like in the Fukushima-Daiichi core meltdown accident, with a particular...
Conference Paper
The promoting effect of intrinsic flame instabilities on explosive combustion is generally known. Most detailed computational studies are limited to generic configurations however. In the present numerical investigation, direct qualitative and quantitative comparison with experimental data from a laboratory-scale explosion channel is provided. Char...
Conference Paper
This work aims to quantify the effect of pressure on flame front wrinkling caused by intrinsic flame instabilities in lean hydrogen-air explosions. Work concerning pressure dependency was conducted, however only for fuels other than pure hydrogen, like methane, ethylene, propane or methane/hydrogen blends. Furthermore, these flames were investigate...
Article
The present study concerns the three-dimensional CFD analysis of hydrogen explosions in APR1400 containment. Initial conditions for combustion simulations are obtained by a cost-effective coupling with a lumped parameter code for preceding mixture distribution analysis. Three severe accident scenarios are examined: Small-Break Loss-Of-Coolant Accid...
Article
For the purpose of nuclear safety analysis, a reactive flow solver has been developed to determine the hazardous potential of large-scale hydrogen explosions. Without using empirical transition criteria, the whole combustion process including deflagration-to-detonation transition (DDT) is computed within a single solver framework. In this paper, we...
Article
The presented work aims to improve CFD explosion modeling for lean hydrogen-air mixtures on under-resolved grids. Validation data is obtained from an entirely closed laboratory scale explosion channel (GraVent facility). Investigated hydrogen-air concentrations range from 6 to 19 vol.-%. Initial conditions are p = 1 atm and T = 293 K. Two highly ti...
Article
Full-text available
An open-access online platform containing data from experiments on deflagration-to-detonation transition conducted at the Institute of Thermodynamics, Technical University of Munich, has been developed and is accessible at http:// www. td. mw. tum. de/ ddt. The database provides researchers working on explosion dynamics with data for theoretical an...
Conference Paper
The presented work aims to improve CFD explosion modeling for lean hydrogen-air mixtures on under-resolved grids. Validation data is obtained from an entirely closed laboratory scale explosion channel (GraVent facility). Investigated hydrogen-air concentrations range from 6 to 19 vol.-%. Initial conditions are p = 1 atm and T = 293 K. Two highly ti...
Conference Paper
For the purpose of nuclear safety analysis, a reactive flow solver has been developed to determine the hazard potential of large-scale hydrogen explosions. Without using empirical transition criteria, the whole combustion process (including DDT) is computed within a single solver framework. In this paper, we present massively parallelized three-dim...
Article
Full-text available
Planar laser-induced fluorescence (PLIF) is considered a standard experimental technique in combustion diagnostics. However, it has only been occasionally applied to explosion experiments with fast combustion regimes. It has been shown that single-shot OH-PLIF with high pulse energies yields clear fluorescence images of fast deflagrations and also...
Article
Full-text available
The influence of transverse concentration gradients on detonation propagation in H2-air mixtures is investigated experimentally in a wide parameter range. Detonation fronts are characterized by means of high- speed shadowgraphy, OH* imaging, pressure measurements and soot foils. Steep concentration gradients at low average H2 concentrations lead to...
Conference Paper
Full-text available
OH-PLIF (Planar Laser-Induced Fluorescence) allows for capturing two-dimensional images of flame fronts by visualizing OH radicals. While it is widely used for diagnostics of scientific and technical flames, its application to explosion experiments is more uncommon and has been mostly limited to single-shot PLIF so far. Low speed PLIF systems (arou...
Conference Paper
Full-text available
Extensive knowledge is available on explosions in homogeneous gas mixtures. Mixtures of H2 and air have been investigated particularly in the context of nuclear reactor safety. However, a major current knowledge gap concerns the influence of mixture inhomogeneity. Spatial concentration gradients are omnipresent in real-world accident scenarios. We...
Conference Paper
Full-text available
The influence of water mist on explosion of H2-air mixtures is studied experimentally in a closed channel with rectangular cross section. The investigated range of H2 concentrations covers slow flames, accelerating flames, transition to detonation and detonations. Water loading ratios relevant for severe accident scenarios in nuclear power plants a...
Article
The influence of water mist on explosion of H2-air mixtures is studied experimentally in a closed channel with rectangular cross section. The investigated range of H2 concentrations covers slow flames, accelerating flames, transition to detonation and detonations. Water loading ratios particularly relevant for severe accident scenarios in nuclear p...
Article
A methodology for the computationally efficient CFD simulation of hydrogen-air explosions (including transition to detonation) in large volumes is presented. The model is validated by means of the largest ever conducted indoor DDT experiments in the RUT facility. A combination of models is proposed with a particular focus on the influence of flame-...
Chapter
We present three-dimensional under-resolved detonation simulations in a large-scale confined geometry (RUT facility at Kurchatov Institute, Moscow). Direct detonation initiation and propagation in a uniform mixture of 25.5 vol.-% hydrogen in air (HySafe experiment HYD 09) is investigated by means of a two-step combustion model originally developed...
Article
Full-text available
A current knowledge gap in hydrogen safety research concerns the influence of mixture non-uniformity on explosion processes. Spacial mixture composition gradients are likely to form in real-world accident scenarios. In this work, the influence of such gradients on flame acceleration is experimentally investigated in an entirely closed channel at la...
Conference Paper
Full-text available
Flame acceleration in a closed duct filled with highly explosive hydrogen-air mixtures is investigated using the OH PLIF technique. The applied laser system comprising a pump laser and a dye laser allows for repetition rates up to 40 kHz. Temporal evolution of the flame surface area during the process of flame acceleration is a key parameter. There...