Abouelmagd Abdelsamie

Dissipation element analysis of a turbulent non-premixed jet flame

The objective of the present work is to examine the interaction between turbulent mixing and chemistry by employing the method of dissipation elements in a non-premixed turbulent jet flame. The method of dissipation elements [L. Wang and N. Peters, J. Fluid Mech. 554, 457–475 (2006)] is used to perform a space-filling decomposition of the turbulent jet flow into different regimes conditioned on their location with respect to the reaction zone. Based on the non-local structure of dissipation elements, this decomposition allows us to discern whether points away from stoichiometry are connected through a diffusive layer with the reaction zone. In a next step, a regime based statistical analysis of dissipation elements is carried out by means of data obtained from a direct numerical simulation. Turbulent mixing and chemical reactions depend strongly on the mixture fraction gradient. From a budget between strain and dissipation, the mechanism for the formation and destruction of mean gradients along dissipation elements is inspected. This budget reveals that large gradients in the mixture fraction field occur at a small but finite length scale. Finally, the inner structure of dissipation elements is examined by computing statistics along gradient trajectories of the mixture fraction field. Thereby, the method of dissipation elements provides a statistical characterization of flamelets and novel insight into the interaction between chemistry and turbulence.

Modulation of Isotropic Turbulence by Resolved and Non-resolved Spherical Particles

The main objective of the current study is to investigate the effect of either non-resolved (point particles) or fully resolved particles on a field of isotropic, either stationary or decaying turbulence using direct numerical simulation (DNS), clarifying how those different settings influence the turbulence statistics.

Comparison Between ODT and DNS for Ignition Occurrence in Turbulent Premixed Jet Combustion: Safety-Relevant Applications

Abstract This work investigates the ability of the one-dimensional turbulence model (ODT) to detect, in a predictive manner, occurrence of successful ignition or misfire in a reacting gas mixture subjected to turbulence. Since ODT is computationally very efficient, this significantly aids in the analysis of safety-relevant applications. ODT delivers fast predictions, while still capturing most relevant physicochemical processes controlling ignition. However, ODT contains some empirical parameters that must be set by comparison with reliable reference data. In order to determine these parameters and check the accuracy of resulting ODT predictions, they are compared in this work with reference data from direct numerical simulation (DNS). DNS is recognized as the most accurate numerical tool to investigate ignition in turbulent flows. However, it requires very high computational times, so that it cannot be used for practical safety predictions. It is demonstrated in this article that, thanks to validation and comparison with DNS, ODT realizations can be used to predict correctly the occurrence of ignition in turbulent premixed flames while saving more than 90% of the required computational time, memory and disk space.

Impact of the Collision Model for Fully Resolved Particles Interacting in a Fluid

The impact of the collision model employed when simulating fully resolved particles interacting in a fluid is investigated in the present study. We are using for this purpose a p seudo-spectral in compressib le Direct Numerical Simulation (DNS) code based on the Navier-Stokes equation as well as a Lattice-Boltzmann Method (LBM), developed in our group and coupled with the direct-forcing Immersed Boundary Method (IBM) to describe the particles.Most of the corresponding literature assumes that the collision model does not have a significant impact on the flow field. Additionally, the impact of the collision model on the particle trajectories has not been analyzed in a systematic manner. Thus, by using the DNS solver, four different collision models (velocity barrier, repulsive potential force, lubrication barrier and hard-sphere model) have been employed in order to examine consequences for particle behavior and turbulence structure. It was found that the particle motion and turbulence statistics are qualitatively similar for all models. However, noticeable quantitative differences appear concerning the turbulent dissipation rate.In the LBM section two different types of repulsive-force collision model are selected and their effect on a 2D fluid-particle interaction is investigated. Furthermore, other factors affecting performance of the LB-IBM solver, like the forcing scheme will be discussed.Copyright

DNS of Burning N-Heptane Droplets: Auto-Ignition and Turbulence Modulation Mechanisms

Safety-relevant auto-ignition by n-heptane droplets exiting as a turbulent jet within a combustible gas mixture is investigated under different conditions by means of Direct Numerical Simulations (DNS).

Direct Numerical Simulations of Hotspot-induced Ignition in Homogeneous Hydrogen-air Pre-mixtures and Ignition Spot Tracking

A systematic study relying on Direct Numerical Simulations (DNS) of premixed hydrogen-air mixtures has been performed to investigate the hotspot ignition characteristics and ignition probability under turbulent conditions. An ignition diagram is first obtained under laminar conditions by a parametric study. The impact of turbulence intensity on ignition delays and ignition probability is then quantified in a statistically-significant manner by repeating a large number of independent DNS realizations. By tracking in a Lagrangian frame the ignition spot, the balance between heat diffusion and heat of chemical reaction is observed as function of time. The evolution of each chemical species and radicals at the ignition spot is checked and the mechanism leading to ignition or misfire are analyzed. It is observed that successful ignition is mostly connected to a sufficient build-up of a HO2 pool, ultimately initiating production of OH. Turbulence always delays ignition, and ignition probability goes to zero at sufficiently high turbulence intensity when keeping temperature and size of the initial hotspot constant.

On-The-Fly Tracking of Flame Surfaces for the Visual Analysis of Combustion Processes: On-The-Fly Tracking of Flame Surfaces

The visual analysis of combustion processes is one of the challenges of modern flow visualization. In turbulent combustion research, the behaviour of the flame surface contains important information about the interactions between turbulence and chemistry. The extraction and tracking of this surface is crucial for understanding combustion processes. This is impossible to realize as a post‐process because of the size of the involved datasets, which are too large to be stored on disk. We present an on‐the‐fly method for tracking the flame surface directly during simulation and computing the local tangential surface deformation for arbitrary time intervals. In a massively parallel simulation, the data are distributed over many processes and only a single time step is in memory at any time. To satisfy the demands on parallelism and accuracy posed by this situation, we track the surface with independent micro‐patches and adapt their distribution as needed to maintain numerical stability. With our method, we enable combustion researchers to observe the detailed movement and deformation of the flame surface over extended periods of time and thus gain novel insights into the mechanisms of turbulence–chemistry interactions. We validate our method on analytic ground truth data and show its applicability on two real‐world simulations.

Turbulence Structure Analysis of DNS Data Using DMD and SPOD: Mixing Jet and Channel Flow

The objectives of this work are to analyze and investigate the turbulence structures in a channel flow and in a mixing jet using Dynamic Mode Decomposition (DMD) and Snapshot Proper Orthogonal Decomposition (SPOD). The analyzed data have been generated by Direct Numerical Simulation at high Reynolds numbers. In the channel flow, the occurrence of turbulent superstructures is mainly examined. The jet case is employed to investigate mixing in a turbulent jet flow. In both cases, DMD and SPOD analysis are compared to test their performance concerning the analysis of complex flows and to highlight the complementarity between these two approaches.