Marco Arienti

Many-body dissipative particle dynamics simulation of liquid/vapor and liquid/solid interactions.

The combination of short-range repulsive and long-range attractive forces in many-body dissipative particle dynamics (MDPD) is examined at a vapor/liquid and liquid/solid interface. Based on the radial distribution of the virial pressure in a drop at equilibrium, a systematic study is carried out to characterize the sensitivity of the surface tension coefficient with respect to the inter-particle interaction parameters. For the first time, the approximately cubic dependence of the surface tension coefficient on the bulk density of the fluid is evidenced. In capillary flow, MDPD solutions are shown to satisfy the condition on the wavelength of an axial disturbance leading to the pinch-off of a cylindrical liquid thread; correctly, no pinch-off occurs below the cutoff wavelength. Moreover, in an example that illustrates the cascade of fluid dynamics behaviors from potential to inertial-viscous to stochastic flow, the dynamics of the jet radius is consistent with the power law predictions of asymptotic analysis. To model interaction with a solid wall, MDPD is augmented by a set of bell-shaped weight functions; hydrophilic and hydrophobic behaviors, including the occurrence of slip in the latter, are reproduced using a modification in the weight function that avoids particle clustering. The dynamics of droplets entering an inverted Y-shaped fracture junction is shown to be correctly captured in simulations parametrized by the Bond number, confirming the flexibility of MDPD in modeling interface-dominated flows.

A Coupled Level Set-Moment of Fluid Method for Incompressible Two-Phase Flows

A coupled level set and moment of fluid method (CLSMOF) is described for computing solutions to incompressible two-phase flows. The local piecewise linear interface reconstruction (the CLSMOF reconstruction) uses information from the level set function, volume of fluid function, and reference centroid, in order to produce a slope and an intercept for the local reconstruction. The level set function is coupled to the volume-of-fluid function and reference centroid by being maintained as the signed distance to the CLSMOF piecewise linear reconstructed interface.The nonlinear terms in the momentum equations are solved using the sharp interface approach recently developed by Raessi and Pitsch (Annual Research Brief, 2009). We have modified the algorithm of Raessi and Pitsch from a staggered grid method to a collocated grid method and we combine their treatment for the nonlinear terms with the variable density, collocated, pressure projection algorithm developed by Kwatra et al. (J. Comput. Phys. 228:4146–4161, 2009). A collocated grid method makes it convenient for using block structured adaptive mesh refinement (AMR) grids. Many 2D and 3D numerical simulations of bubbles, jets, drops, and waves on a block structured adaptive grid are presented in order to demonstrate the capabilities of our new method.

Time-resolved proper orthogonal decomposition of liquid jet dynamics

New insight into the mechanism of liquid jet in crossflow atomization is provided by an analysis technique based on proper orthogonal decomposition and spectral analysis. Data are provided in the form of high-speed videos of the jet near field from experiments over a broad range of injection conditions. For each condition, proper orthogonal modes (POMs) are generated and ordered by intensity variation relative to the time average. The feasibility of jet dynamics reduction by truncation of the POM series to the first few modes is then examined as a function of crossflow velocity for laminar and turbulent liquid injection. At conditions where the jet breaks up into large chunks of liquid, the superposition of specific orthogonal modes is observed to track long waves traveling along the liquid column. The temporal coefficients of these modes can be described as a bandpass spectrum that shifts toward higher frequencies as the crossflow velocity is increased. The dynamic correlation of these modes is quantifie...

Coupled Level-Set/Volume-of-Fluid Method for Simulation of Injector Atomization

This paper presents results of a multiphase computational fluid dynamics code using a coupled level-set/volume-of-fluid method to simulate liquid atomization. This interface-capturing approach combines the mass conservation properties of the volume-of-fluid method with the accurate surface reconstruction properties of the level-set method, and it includes surface tension as a volume force calculated with second-order accuracy. Developed by one of the authors, the multiphase code builds upon the combined level-set/volume-of-fluid methodology to enable bubbly flow, liquid breakup, and phase-change simulations. The extension presented in this paper couples a Lagrangian dispersed phase model for postbreakup tracking of droplets with block-structured adaptive mesh refinement on the Eulerian grid. Under an appropriate set of criteria, the transfer of droplets representation from the Eulerian to the Lagrangian discretization enables the simulation of sprays on larger domains and for longer physical times without...

Towards an Efficient, High-Fidelity Methodology for Liquid Jet Atomization Computations

The aim of this work is to show how adaptive mesh refinement and Lagrangian tracking can be integrated to enable high-fidelity computations of jet atomization and dispersion for industrially relevant configurations. In its present form, the Coupled Level Set and Volume of Fluid (CLSVOF) method for multiphase flow calculations is embedded in a dynamic, block-structured Adaptive Mesh Refinement (AMR) data structure which maximizes grid density at the liquid-gas interface. With the treatment proposed here, small liquid structures formed by atomization can be removed from the Eulerian description, transformed into Lagrangian particles, and advected using, for instance, a simple spherical model. As a result, mesh refinement is not required in the dilute spray region and can be contained to a smaller portion of the computational domain with minimal loss of accuracy. Two validation studies of liquid jet atomization by crossflowing gas and by jet-on-jet impingement are presented to demonstrate this new approach.

The Impact of Density Ratio on the Primary Atomization of a Turbulent Liquid Jet in Crossflow

Atomizing liquids by injecting them into crossflows is a common approach in gas turbines and augmentors. Much of our current understanding of the processes resulting in atomization of the jets, the resulting jet penetration and spray drop size distribution have been obtained by performing laboratory experiments at ambient conditions. Yet, operating conditions under which jets in crossflows atomize can be far different from ambient. Hence, several dimensionless groups have been identified that are believed to determine jet penetration and resulting drop size distribution. These are usually the jet and crossflow Weber and Reynolds numbers and the momentum flux ratio. In this paper we aim to answer the question of whether an additional dimensionless group, the liquid to gas density ratio must be matched. To answer this question, we perform detailed simulations of the primary atomization region using the Refined Level Set Grid (RLSG) method to track the motion of the liquid/gas phase interface. We employ a balanced force, interface projected curvature method to ensure high accuracy of the surface tension forces, use a multi-scale approach to transfer broken off, small scale nearly spherical drops into a Lagrangian point particle description allowing for full two-way coupling and continued secondary atomization, and employ a dynamic Smagorinsky large eddy simulation (LES) approach in the single phase regions of the flow to describe turbulence. We present simulation results for a turbulent liquid jet (q = 6.6, We = 330, Re = 14,000) injected into a gaseous crossflow (Re = 740,000) analyzed under ambient conditions (density ratio 816) experimentally by Brown and McDonnel (2006). We compare simulation results obtained using a liquid to gas density ratio of 10 to those obtained using a density ratio of 100, a value typical for gas turbine combustors. The results show that the increase in density ratio results in a notice-able increase in liquid core penetration with reduced bending in the crossflow and spreading in the transverse directions. The post-primary atomization spray, however, penetrates further in both the jet and transverse direction. Results further show that penetration correlations for the windward side trajectory commonly reported in the literature strongly depend on the value of threshold probability used to identify the leading edge. Correlations based on the penetration of the jet’s liquid core center of mass, on the other hand, can provide a less ambiguous measure of jet penetration. Finally, the increase in density ratio results in a decrease in wavelength of the most dominant feature associated with a traveling wave along the jet as determined by proper orthogonal decomposition.Copyright

High-Fidelity Simulation of Atomization and Evaporation in a Liquid Jet in Cross-flow

The physical complexity of liquid fuel atomization and evaporation coupled with the substantial challenges in experimental quantification of these phenomena endow advanced simulation with unique potential for enhancing combustor performance. In this paper we continue our efforts towards the development of a methodology that will enable this kind of simulation and present results that focus on two-way coupling effects and evaporation physics. The Eulerian Coupled Level Set and Volume of Fluid (CLSVOF) method is used to directly capture the breakup of the liquid jet, and Lagrangian, discrete phase, spherically symmetric models are applied to describe the small-scale physics of droplets. Two-way coupling between the Lagrangian droplets and the gaseous phase is enforced that conserves mass momentum and energy. The analysis is performed in the context of a liquid jet in crossflow atomization problem and results are validated against data acquired in a spray rig at high-Weber, high-Reynolds number injection conditions. Results indicate that two-way momentum coupling between droplets and gas is essential for correctly capturing spray penetration, dispersion, and the far-field cross-stream droplet distribution which has a horseshoe shape. In addition, it is seen that when the injected liquid is at the same temperature as the cross-flowing gas, the vapor distribution is similar to that of the droplets. In contrast, when the injected liquid is colder than the cross-flowing gas, then the vapor downstream distribution is center peaked. This is related to the relative sizes of the droplets in the distribution and their respective heat up and evaporation times.

Coarse molecular-dynamics analysis of an order-to-disorder transformation of a krypton monolayer on graphite

The thermally induced order-to-disorder transition of a monolayer of krypton (Kr) atoms adsorbed on a graphite surface is studied based on a coarse molecular-dynamics (CMD) approach for the bracketing and location of the transition onset. A planar order parameter is identified as a coarse variable, psi, that can describe the macroscopic state of the system. Implementation of the CMD method enables the construction of the underlying effective free-energy landscapes from which the transition temperature, T(t), is predicted. The CMD prediction of T(t) is validated by comparison with predictions based on conventional molecular-dynamics (MD) techniques. The conventional MD computations include the temperature dependence of the planar order parameter, the specific heat, the Kr-Kr pair correlation function, the mean square displacement and corresponding diffusion coefficient, as well as the equilibrium probability distribution function of Kr-atom coordinates. Our findings suggest that the thermally induced order-to-disorder transition at the conditions examined in this study appears to be continuous. The CMD implementation provides substantial computational gains over conventional MD.

Liquid Film Formation by an Impinging Jet in a High- Velocity Air Stream

The impingement of a liquid jet on a solid surface, and the development of a shear-driven liquid film is characterized in a planar experiment. Weber number and momentum-flux ratios were chosen to be representative of gas turbine fuel injector operations. High-speed digital imaging was used to visualize the formation of the liquid film from impinging droplets and the development of a continuous, wavy film. Film thickness measurements indicated growth of the film along the length of the impinging surface (streamwise direction), and reduction in the film thickness in the crossstream direction. In general, film thicknesses increased with increasing momentum flux ratio, as more liquid drops reached the filmer surface. Three different mechanisms of atomization from the liquid film were identified.namely droplet splashing, film surface atomization via aerodynamic instability and film breakup at channel exit.

Analysis of Liquid Jet Atomization Dynamics Using Proper Orthogonal Decomposition

Sequences of consecutive line-of-sight images taken at a frequency of 14,035 Hz are analyzed to extract the most relevant dynamic struc tures of near-field unforced liquid jet in gas crossflow. Proper orthogonal decomposition ‐ well established in other areas of fluid dynamics ‐ is applied in this multiphase context to reduce the huge amount of information contained in high-speed videos of jet experiments t o a few characteristic modes. Examples are presented where jet surface features are extrac ted and quantified in time and space for gas Weber numbers between 10 and 300 and momentum flux ratio of 45 and 100.