A robust interface method for drop formation and breakup simulation at high density ratio using an extrapolated liquid velocity

© 2016 The Authors. A two-phase flow formulation for atomisation modelling is presented, with a Coupled Level Set/Volume Of Fluid (CLSVOF) technique adopted for interface-tracking. In order to achieve stable numerical solution at high density ratios, an extrapolated liquid velocity field is constructed and used in discretisation of the momentum equations. Solution accuracy is also improved when this field is also used in the scalar (VOF and Level Set) advection equations. A divergence-free algorithm is proposed to ensure satisfaction of the continuity condition for the extrapolated liquid velocity. The density and viscosity across the interface are treated sharply as a function of the Level Set to maintain the physical discontinuity. The developed method is shown to accurately predict drop formation in low Re liquid jets and the deformation and breakup morphology of a single droplet in uniform air flow at different Weber numbers (from 3.4 to 96). The mechanism for droplet breakup is determined based on an analysis of the simulation results. The computed Rayleigh–Taylor instability wavelength extracted from the acceleration of the simulated liquid droplet agrees well with experimental measurements and theoretical analysis, confirming that Rayleigh–Taylor instability dominates single drop breakup in the Weber number range studied. Finally, the influence of liquid viscosity on droplet breakup is numerically investigated; the critical Weber number separating deformation and breakup regimes is well predicted at different Ohnesorge numbers in comparison with the experimental data

A coupled level set and volume of fluid method for automotive exterior water management applications

Motivated by the need for practical, high fidelity, simulation of water over surface features of road vehicles a Coupled Level Set Volume of Fluid (CLSVOF) method has been implemented into a general purpose CFD code. It has been implemented such that it can be used with unstructured and non-orthogonal meshes. The interface reconstruction step needed for CLSVOF has been implemented using an iterative ‘clipping and capping’ algorithm for arbitrary cell shapes and a reinitialisation algorithm suitable for unstructured meshes is also presented. Successful verification tests of interface capturing on orthogonal and tetrahedral meshes are presented. Two macroscopic contact angle models have been implemented and the method is seen to give very good agreement with experimental data for a droplet impinging on a flat plate for both orthogonal and non-orthogonal meshes. Finally the flow of a droplet over a round edged channel is simulated in order to demonstrate the ability of the method developed to simulate surface flows over the sort of curved geometry that makes the use of a non-orthogonal grid desirable

An LES turbulent inflow generator using a recycling and rescaling method

The present paper describes a recycling and rescaling method for generating turbulent inflow conditions for Large Eddy Simulation. The method is first validated by simulating a turbulent boundary layer and a turbulent mixing layer. It is demonstrated that, with input specification of mean velocities and turbulence rms levels (normal stresses) only, it can produce realistic and self-consistent turbulence structures. Comparison of shear stress and integral length scale indicates the success of the method in generating turbulent 1-point and 2-point correlations not specified in the input data. With the turbulent inlet conditions generated by this method, the growth rate of the turbulent boundary/mixing layer is properly predicted. Furthermore, the method can be used for the more complex inlet boundary flow types commonly found in industrial applications, which is demonstrated by generating non-equilibrium turbulent inflow and spanwise inhomogeneous inflow. As a final illustration of the benefits brought by this approach, a droplet-laden mixing layer is simulated. The dispersion of droplets in the near-field immediately downstream of the splitter plate trailing edge where the turbulent mixing layer begins is accurately reproduced due to the realistic turbulent structures captured by the recycling/rescaling method

An iterative interface reconstruction method for PLIC in general convex grids as part of a Coupled Level Set Volume of Fluid solver

Reconstructing the interface within a cell, based on volume fraction and normal direction, is a key part of multiphase flow solvers which make use of piecewise linear interface calculation (PLIC) such as the Coupled Level Set Volume of Fluid (CLSVOF) method. In this paper, we present an iterative method for interface reconstruction (IR) in general convex cells based on tetrahedral decomposition. By splitting the cell into tetrahedra prior to IR the volume of the truncated polyhedron can be calculated much more rapidly than using existing clipping and capping methods. In addition the root finding algorithm is designed to take advantage of the nature of the relationship between volume fraction and interface position by using a combination of Newton's and Muller's methods. In stand-alone tests of the IR algorithm on single cells with up to 20 vertices the proposed method was found to be 2 times faster than an implementation of an existing analytical method, while being easy to implement. It was also found to be 3.4–11.8 times faster than existing iterative methods using clipping and capping and combined with Brent's root finding method. Tests were then carried out of the IR method as part of a CLSVOF solver. For a sphere deformed by a prescribed velocity field the proposed method was found to be up to 33% faster than existing iterative methods. For simulations including the solution of the velocity field the maximum speed up was found to be approximately 52% for a case where 12% of cells lie on the interface. Analysis of the full simulation CPU time budget also indicates that while the proposed method has produced a considerable speed-up, further gains due to increasing the efficiency of the IR method are likely to be small as the IR step now represents only a small proportion of the run time

LES of turbulent liquid jet primary breakup in turbulent coaxial air flow

A robust two-phase flow Large Eddy Simulation (LES) algorithm has been developed and applied to predict the primary breakup of an axisymmetric water jet injected into a surrounding coaxial air flow. The high liquid/gas density and viscosity ratios are known to represent a significant challenge in numerical modelling of the primary breakup process. In the current LES methodology, an extrapolated liquid velocity field was used to minimise discretisation errors, whilst maintaining sharp treatment of fluid properties across the interface. The proposed numerical approach showed excellent robustness and high accuracy in predicting coaxial liquid jet primary breakup. Since strong turbulence structures will develop inside the injector at high Reynolds numbers and affect the subsequent primary breakup, the Rescaling and Recycling Method (RM) was implemented to facilitate generation of appropriate unsteady LES inlet conditions for both phases. The influence of inflowing liquid and gas turbulent structures on the initial interface instability was investigated. It is shown that liquid turbulent eddies play the dominant role in the initial development of liquid jet surface disturbance and distortion for the flow conditions considered. When turbulent inflows were specified by the RM technique, the predicted core breakup lengths at different air/water velocities agreed closely with experimental data. © 2013 The Authors

LES of unsteady vortex aerodynamics in complex geometry gas-turbine fuel injectors

LES of unsteady vortex aerodynamics in complex geometry gas-turbine fuel injector

Coupled level-set volume of fluid simulations of water flowing over a simplified drainage channel with and without air coflow

The motivation for this paper is to predict the flow of water over exterior surfaces of road vehicles. We present simulations of liquid flows on solid surfaces under the influence of gravity with and without the addition of aerodynamic forces on the liquid. This is done using an implementation of a Coupled Level Set Volume of Fluid method (CLSVOF) multiphase approach implemented in the open source OpenFOAM CFD code. This is a high fidelity interface-resolving method that solves for the velocity field in both phases without restrictions on the flow regime. In the current paper the suitability of the approach to Exterior Water Management (EWM) is demonstrated using the representative test cases of a continuous liquid rivulet flowing along an inclined surface with a channel located downstream perpendicular to the oncoming flow. Experimental work has been carried out to record the motion of the rivulet in this case and also to measure the contact angle of the liquid with the solid surface. The measurements of the liquid/solid characteristics such as equilibrium and dynamic contact angles are described along with the analytical expression for contact angle vs. capillary number used in the CFD code. The results from the simulations are compared to experimental measurements. The simulations are carried out with air co-flows of 0, 0.5 and 10 m/s. The simulations are seen to reproduce physical phenomena such as the liquid pinning at sharp corners and the longitudinal stretching of the rivulet with higher air velocity

Large eddy simulation of liquid-jet primary breakup in air crossflow

A robust two-phase-flow large-eddy-simulation methodology is applied to simulate the primary breakup of an axisymmetric liquid jet injected into an air crossflow at atmospheric pressure. The coupled level-set and volume-offluid method is implemented for accurate interface tracking. To deal with high liquid/gas density ratio, an extrapolated liquid-velocity field is created and used for momentum-equation discretization in the vicinity of the interface. Based on the local level-set value, sharp jumps in fluid density and viscosity are assumed across the interface. By simulating the nonturbulent inflow of a liquid jet into a nonturbulent gaseous crossflow, regular surface waves are observed in the large-eddy-simulation predictions on the upstreamside of the liquid jet, with the wavelength agreeing well with experimental measurements. The predicted wavelength decreases as the gaseous Weber number increases, implying that the surface waves arise from a Rayleigh–Taylor-type instability. The simulated velocity field shows that, as the instability grows, gaseous vortices develop in the wave troughs, further enhancing the breakup of the liquid core. The turbulent inflow of a liquid jet into a turbulent gas crossflow is also simulated, and the effect of turbulent eddies on the liquid-jet primary breakup is examined. The rescaling/recycling method for large-eddysimulation inlet-condition generation is implemented to generate realistic (i.e., physically correlated) turbulent inflows. It is found that it is the liquid rather than the gaseous turbulence that determines the initial liquid-jet instability and interface characteristics; further downstream, the turbulent liquid jet disintegrates more chaotically than the nonturbulent jet due to strong aerodynamic and turbulence effects. When appropriate turbulent inflows are specified, the liquid-jet penetration into the air crossflow (and the subsequent spread of the spray) is correctly predicted by the current large-eddy-simulation methodology, which displays good numerical robustness and accuracy for high liquid/gas density-ratio two-phase systems

Comparison of unsteady Reynolds averaged Navier–Stokes and large eddy simulation computational fluid dynamics methodologies for air swirl fuel injectors

It is well documented that various large-scale quasiperiodic flow structures, such as a precessing vortex core (PVC) and multiple vortex helical instabilities, are present in the swirling flows typical of air swirl fuel injectors. Prediction of these phenomena requires time-resolved computational methods. The focus of the present work was to compare the performance and cost implications of two computational fluid dynamics (CFD) methodologies—unsteady Reynolds averaged Navier–Stokes (URANS) using a k- model and large eddy simulation (LES) for such flows. The test case was a single stream radial swirler geometry, which has been the subject of extensive experimental investigation. Both approaches captured the gross (time-mean) features of strongly swirling confined flows in reasonable agreement with experiment. The temporal dynamics of the quadruple vortex pattern emanating from within the swirler and observed experimentally were successfully predicted by LES, but not by URANS. Spectral analysis of two flow configurations (with and without a central jet) revealed various coherent frequencies embedded within the broadband turbulent frequency range. LES reproduced these characteristics, in excellent agreement with experimental data, whereas URANS predicted the presence of coherent motions but at incorrect amplitudes and frequencies. For the no-jet case, LES-predicted spectral data indicated the occurrence of a PVC, which was also observed experimentally for this flow condition; the URANS solution failed to reproduce this measured trend. On the evidence of this study, although k- based URANS offers considerable computational savings, its inability to capture the temporal characteristics of the flows studied here sufficiently accurately suggests that only LES-based CFD, which captures the stochastic nature of the turbulence much more faithfully, is to be recommended for fuel injector flows