Droplet Dynamics Under Extreme Ambient Conditions

This open access book presents the main results of the Collaborative Research Center SFB-TRR 75, which spanned the period from 2010 to 2022. Scientists from a variety of disciplines, ranging from thermodynamics, fluid mechanics, and electrical engineering to chemistry, mathematics, computer science, and visualization, worked together toward the overarching goal of SFB-TRR 75, to gain a deep physical understanding of fundamental droplet processes, especially those that occur under extreme ambient conditions. These are, for example, near critical thermodynamic conditions, processes at very low temperatures, under the influence of strong electric fields, or in situations with extreme gradients of boundary conditions. The fundamental understanding is a prerequisite for the prediction and optimisation of engineering systems with droplets and sprays, as well as for the prediction of droplet-related phenomena in nature. The book includes results from experimental investigations as well as new analytical and numerical descriptions on different spatial and temporal scales. The contents of the book have been organised according to methodological fundamentals, phenomena associated with free single drops, drop clusters and sprays, and drop and spray phenomena involving wall interactions

Visual analysis of droplet dynamics in large-scale multiphase spray simulations

We present a data-driven visual analysis approach for the in-depth exploration of large numbers of droplets. Understanding droplet dynamics in sprays is of interest across many scientific fields for both simulation scientists and engineers. In this paper, we analyze large-scale direct numerical simulation datasets of the two-phase flow of non-Newtonian jets. Our interactive visual analysis approach comprises various dedicated exploration modalities that are supplemented by directly linking to ParaView. This hybrid setup supports a detailed investigation of droplets, both in the spatial domain and in terms of physical quantities . Considering a large variety of extracted physical quantities for each droplet enables investigating different aspects of interest in our data. To get an overview of different types of characteristic behaviors, we cluster massive numbers of droplets to analyze different types of occurring behaviors via domain-specific pre-aggregation, as well as different methods and parameters. Extraordinary temporal patterns are of high interest, especially to investigate edge cases and detect potential simulation issues. For this, we use a neural network-based approach to predict the development of these physical quantities and identify irregularly advected droplets

Droplet Dynamics Under Extreme Ambient Conditions

This open access book presents the main results of the Collaborative Research Center SFB-TRR 75, which spanned the period from 2010 to 2022. Scientists from a variety of disciplines, ranging from thermodynamics, fluid mechanics, and electrical engineering to chemistry, mathematics, computer science, and visualization, worked together toward the overarching goal of SFB-TRR 75, to gain a deep physical understanding of fundamental droplet processes, especially those that occur under extreme ambient conditions. These are, for example, near critical thermodynamic conditions, processes at very low temperatures, under the influence of strong electric fields, or in situations with extreme gradients of boundary conditions. The fundamental understanding is a prerequisite for the prediction and optimisation of engineering systems with droplets and sprays, as well as for the prediction of droplet-related phenomena in nature. The book includes results from experimental investigations as well as new analytical and numerical descriptions on different spatial and temporal scales. The contents of the book have been organised according to methodological fundamentals, phenomena associated with free single drops, drop clusters and sprays, and drop and spray phenomena involving wall interactions

Numerical Simulation of Particle Collision and Agglomeration in Turbulent Channel Flows

The study described in this thesis concerns the simulation of dispersed and dense particle-laden turbulent channel flows. The research primarily investigates the role of gravity; in terms of its contribution to particle collision, agglomeration and re-distribution. Large eddy simulation is employed to predict the fluid-phase, with solutions coupled with a Lagrangian particle tracking routine to model the particle-phase. In order to establish the validity of the preferred numerical method, results generated from the single-phase and the dilute particle-phase predictions were compared with those based on DNS, with good agreements found. Results obtained for horizontal zero gravity channel flows, show effects of particle size, particle concentration and turbulence on colliding and agglomerating particles. All variables were shown to strongly impact on collision and agglomeration, with the number of events reaching maximum towards the channel walls due to increased particle concentrations and turbulence levels in these regions. The collision and agglomeration is, however, shown to enhance exponentially with the inclusion of gravity and accentuated on the lower wall of the channel. An extension of the investigation into vertical channels of upward and downward flow configurations, also demonstrated the significance of the gravity force on particle collision and agglomeration. The effect of the particles on the flow is small, owing to the low mass-loading. Agglomeration is found to be most favourable for flows of low turbulence; and unlike collisions, dominantly forms in the channel centre. The investigation presented is a novel contribution to literature that provides a fundamental improvement on the understanding of turbulent fluid-particle flows. Particularly, it extends the existing knowledge on cohesive particle behaviours in turbulent flows by examining the effect of gravity on such flows. The contribution finds relevance in many engineering and industrial flow processes and should aide the design of better flow processes

Oil Droplet Impact Dynamics in Aero-Engine Bearing Chambers - Correlations derived from Direct Numerical Simulations

The understanding of the two-phase flow in bearing chambers is crucial for an efficient and reliable operation of future aero-engines. This scientific report investigates the oil droplet impact dynamics for typical impingement regimes relevant to bearing chambers. The application of a Direct Numerical Simulation based on the Volume-of-Fluid method and coupled with an adaptive mesh refinement technique allowed to gain new insights into these dynamics to date not reported in scientific literature

Oil droplet impact dynamics in aero-engine bearing chambers: correlations derived from direct numerical simulations

Bearing Chambers in Aero-Engines are located near the rolling-element type of bearings which support the shafts and accomodate the resulting thrust loads. One of the main task of the bearing chambers is, beside an efficient scavenging of the lubricating oil, the cooling of the hot compartments. A very complex two-phase air-oil flow takes usually place in these bearing chambers consisting of oil droplet-laden air flows and shear-driven liquid wall films. The interaction of the droplets with the wall films is significantly influencing the wall heat transfer and the cooling performance of these systems. For this reason, a detailed characterization and modelling of the mass and momentum exchange between droplets and wall films for the unique impingement parameter range in bearing chambers is inevitable. This scientific report investigates the oil droplet impact dynamics for typical impingement regimes relevant to aero-engine bearing chambers. The application of a Direct Numerical Simulation (DNS) technique based on the Volume-of-Fluid (VOF) method and coupled with a gradient-based adaptive mesh refinement (AMR) technique allowed to characterize the drop impact dynamics during various single micro- and millimeter drop impacts onto thin and thick films. With the help of a special numerical treatment, a self-perturbing mechanism is installed that leads to the correct resolution of the crown disintegration process. The numerical methodology was thoroughly validated using the experimental results of millimeter sized drop impacts onto deep liquid pools. These results were developed with an enhanced back-illuminated high-speed imaging and Particle Tracking Velocimetry (PTV) technique. New insights into the cavity penetration, the crown’s breakup dynamics and the secondary droplet characteristics following a single drop impact have been developed with the help of the isolated variation of different parameters of influence. Particularly the influence of the Froude number, the impingement angle, and the cavity-wall interaction delivered results to date not reported in scientific literature. Beside the advances in fundamental physics describing the drop impact dynamics with the help of the numerical and experimental results, a set of correlations could also be derived. From these correlations, a drop-film interaction model was formulated that is suitable for the parameter range found in bearing chambers

Oil droplet impact dynamics in aero-engine bearing chambers: correlations derived from direct numerical simulations

Bearing Chambers in Aero-Engines are located near the rolling-element type of bearings which support the shafts and accomodate the resulting thrust loads. One of the main task of the bearing chambers is, beside an efficient scavenging of the lubricating oil, the cooling of the hot compartments. A very complex two-phase air-oil flow takes usually place in these bearing chambers consisting of oil droplet-laden air flows and shear-driven liquid wall films. The interaction of the droplets with the wall films is significantly influencing the wall heat transfer and the cooling performance of these systems. For this reason, a detailed characterization and modelling of the mass and momentum exchange between droplets and wall films for the unique impingement parameter range in bearing chambers is inevitable. This scientific report investigates the oil droplet impact dynamics for typical impingement regimes relevant to aero-engine bearing chambers. The application of a Direct Numerical Simulation (DNS) technique based on the Volume-of-Fluid (VOF) method and coupled with a gradient-based adaptive mesh refinement (AMR) technique allowed to characterize the drop impact dynamics during various single micro- and millimeter drop impacts onto thin and thick films. With the help of a special numerical treatment, a self-perturbing mechanism is installed that leads to the correct resolution of the crown disintegration process. The numerical methodology was thoroughly validated using the experimental results of millimeter sized drop impacts onto deep liquid pools. These results were developed with an enhanced back-illuminated high-speed imaging and Particle Tracking Velocimetry (PTV) technique. New insights into the cavity penetration, the crown’s breakup dynamics and the secondary droplet characteristics following a single drop impact have been developed with the help of the isolated variation of different parameters of influence. Particularly the influence of the Froude number, the impingement angle, and the cavity-wall interaction delivered results to date not reported in scientific literature. Beside the advances in fundamental physics describing the drop impact dynamics with the help of the numerical and experimental results, a set of correlations could also be derived. From these correlations, a drop-film interaction model was formulated that is suitable for the parameter range found in bearing chambers