Josef Hasslberger

• Dr. habil.
• Research associate at University of the Bundeswehr Munich

This study is concerned with Large Eddy Simulation of liquid jet atomization using a two-way coupled Eulerian-Lagrangian multiscale approach. The proposed framework combines Volume-of-Fluid interface capturing with Lagrangian Particle Tracking. The former is used to compute the core jet and large liquid elements in the near-nozzle region, whereas t...

The study of suspended binary droplet collisions is an active research topic that has gathered interest due to its complexity and its industrial applications such as bioprinting. For many techniques that rely on the collision of two droplets, the quality of the outcome depends on the mixing process that begins when the droplets come in contact with...

In this study, a series of carrier-phase direct numerical simulations are conducted on spherical expanding premixed hydrogen/air flames with liquid water addition. An Eulerian–Lagrangian approach with two-way coupling is employed to describe the liquid–gas interaction. The impacts of preferential diffusion, the equivalence ratio, water loading, and...

While direct numerical simulation (DNS) of multiphase flows has been the focus of many research investigations in recent years, large eddy simulation (LES) of multiphase flows remains a challenge. There is no standardized set of governing equations for multiphase LES. Different approaches and formulations have been discussed in the literature, each...

The combustion of hydrogen and carbon-monoxide mixtures, so-called syngas, plays an increasingly important role in the safety context of non-fossil energy generation, more specifically in the risk management of incidents in process engineering plants for ammonia synthesis and in nuclear power plants. In order to characterize and simulate syngas/air...

The present work analyses the effect of water vapour addition on NOx\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text {NO}_\text {x}}$$\end{document} emissions of...

The effects of water injection on flame surface topology and local flame propagation characteristics have been analyzed for statistically planar turbulent n-heptane spray flames with an overall (i.e., liquid + gaseous) equivalence ratio of unity using carrier-phase direct numerical simulations. Most fuel droplets have been found to evaporate as the...

The study of suspended binary droplet collisions is an active research topic that has gathered interest due to its complexity and its industrial applications such as bioprinting. For many techniques that rely on the collision of two droplets, the quality of the outcome depends on the mixing process that begins when the droplets come in contact with...

This study proposes two different strategies to enforce accurate volume conservation in volume‐of‐fluid (VOF)‐based simulations of turbulent bubble‐laden flows on coarse grids. It is demonstrated that, without a correction, minimal volume errors on a time‐step level, caused by the under‐resolution of the interface, can accumulate to significant dev...

Turbulent length scales provide important information for analysing and modelling turbulent flows and also for assessing the accuracy of simulations. However, while they are (relatively) easy accessible from Direct Numerical Simulation (DNS) data their accurate determination from Large Eddy Simulation (LES) data is less straightforward and affected...

Turbulent bubble-laden flows are an integral part of numerous technical applications. Due to the computational cost of Direct Numerical Simulation, Large Eddy Simulation (LES) comes into focus for the design of such devices. One of the central challenges for two-phase LES is the under-resolution of the interface dynamics, which leads to a variety o...

Emulsions are suspensions of immiscible liquids, in which the dispersed liquid is present in the carrier liquid in the form of poly-disperse droplets. In order to obtain and maintain an emulsion, a constant energy input in the form of kinetic energy is required to deform and break up droplets. Both processes increase the total droplet surface area,...

The collision of binary droplets plays a key role in several industrial, chemical and biological processes. In these processes, the quality of the desired outcome is strongly dependent on the mixing of the liquid droplets as they collide in mid-air. In this work, multiphase direct numerical simulations based on the volume-of-fluid method have been...

Following the relative success of mixed models in the Large Eddy Simulation of complex turbulent flow configurations, an alternative formulation is suggested here which incorporates the concept of a local subgrid activity sensor. The general idea of mixed models is to combine the advantages of structural models (superior alignment properties), usua...

A new benchmark case for the evaluation of direct numerical simulation (DNS) and large-eddy simulation (LES) models and methods is presented in this study. The known Taylor–Green vortex is modified by replacing the periodic boundary conditions in one direction with a no-slip boundary. A passive scalar is added and transported from the wall into the...

Gas-liquid two-phase flow is a critical component of many industrial processes, including air lift pumps in the oil industry, chemical reactors in the process industry, and nuclear reactors in the power industry. Preventing the release of radioactive particles in the form of aerosols is of paramount importance in the event of a nuclear accident. To...

Carrier-phase direct numerical simulations (DNS) of n-heptane/air combustion with water droplet injection are reported in this paper. The influence of water droplet injection has been analyzed for statistically planar gaseous premixed flames and spray flames where the fuel is supplied in the form of monodisperse fuel droplets. The effects of water...

The reactive Richtmyer–Meshkov instability (RMI) exhibits strong wrinkling of a reactive flame front after an interaction with a shock wave. High levels of deformation and wrinkling can cause the flame surface to intersect with itself, leading to the events of flame self interactions (FSI). As FSI can have a significant influence on the development...

The study of suspended binary droplet collisions is an active research topic that has gathered interest due to its complexity and its industrial applications such as bioprinting. For many techniques that rely on the collision of two droplets, the quality of the outcome depends on the mixing process that begins when the droplets come in contact with...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

Turbulent bubbly flows play an essential role in a large number of technical applications, e.g. for chemical reactors in the process industry.

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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-...

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...

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...