MFC: An open-source high-order multi-component, multi-phase, and multi-scale compressible flow solver

MFC is an open-source tool for solving multi-component, multi-phase, and bubbly compressible flows. It is capable of efficiently solving a wide range of flows, including droplet atomization, shock–bubble interaction, and bubble dynamics. We present the 5- and 6-equation thermodynamically-consistent diffuse-interface models we use to handle such flows, which are coupled to high-order interface-capturing methods, HLL-type Riemann solvers, and TVD time-integration schemes that are capable of simulating unsteady flows with strong shocks. The numerical methods are implemented in a flexible, modular framework that is amenable to future development. The methods we employ are validated via comparisons to experimental results for shock–bubble, shock–droplet, and shock–water-cylinder interaction problems and verified to be free of spurious oscillations for material-interface advection and gas–liquid Riemann problems. For smooth solutions, such as the advection of an isentropic vortex, the methods are verified to be high-order accurate. Illustrative examples involving shock–bubble-vessel-wall and acoustic–bubble-net interactions are used to demonstrate the full capabilities of MFC

CFD Code Survey for Thrust Chamber Application

In the quest fo find analytical reference codes, responses from a questionnaire are presented which portray the current computational fluid dynamics (CFD) program status and capability at various organizations, characterizing liquid rocket thrust chamber flow fields. Sample cases are identified to examine the ability, operational condition, and accuracy of the codes. To select the best suited programs for accelerated improvements, evaluation criteria are being proposed

Numerical simulation of compressible multiphase flows

The present work is motivated by the pervasive nature of compressible multiphase flow in practical applications. These flows often feature particles (i.e. solid particles, droplets or bubbles) and develop rich dynamics as particles interact with different flow features such as shock waves. These interactions present unique challenges for numerical methods. The underlying primary motivation is to judiciously exploit shock-particle interaction in different flow topology, e.g. in gas-solid and gas-liquid systems, with proper and efficient methods. In the first part, the interaction of shock wave with a particle cloud in dense gas-solid regime is investigated through a particle resolved direct numerical simulation to quantify the unsteadiness and velocity fluctuations, arising from this interaction, in the particle cloud and the wake behind that. This investigation is performed using a Particle-Resolved Direct Numerical Simulation (PR-DNS) by solving the compressible Navier-Stokes equations coupled with a compressible Immersed Boundary Method (IBM), to account for the particles, in the Parallel Adaptive Wavelet-Collocation Method (PAWCM) framework. The PAWCM is a finite difference framework that uses wavelets to dynamically adapt the grid used to represent the solution, which minimizes the overall computational cost and allows larger simulations to be performed. The quantification is performed in three steps. First the simulation of simplified case of the shock interaction with a transverse array of particles is performed to reveal the source of unsteadiness under the wave-wave and wave wake interaction of the neighboring particles and introduce the dilatation effect arise over the particle wake. Then the interaction of the shock wave with the particle cloud is investigated to replicate the experimental canonical multiphase shock tube problem of Wagner et al. (2011). The budget of the vorticity equation explains the sources of strong unsteadiness in the particle cloud that previously was observed by Regele et. al (2014). In the third step the particle cloud is exposed to a compression wave that gradually introduce the flow. A detailed analysis of the velocity fluctuation and kinetic energy in the fluctuating motion is performed for both cases to ascertain the importance of the velocity fluctuations that arise from the strong unsteadiness in the shock induced case. In the second part, a finite difference solver is developed for Parallel adaptive Wavelet Collocation method framework to investigate high-speed compressible gas-liquid flows with surface tension effects. This study is motivated by gaining deeper insight into the process of fuel atomization in a supersonic cross flow of supersonic combustors under the startup conditions. The solver is developed based on the five equation interface capturing scheme by solving compressible multiphase/multicomponent Navier-Stokes equations along with an advection equation for the material interface. An interface capturing scheme is applied to counter the numerical diffusion induced by shock capturing scheme and maintain the immiscibility condition at the material interface. The capillary force is modeled using a continuous surface approach. The gas phase is modeled as an ideal gas and the liquid phase is modeled using a stiffened-gas equation of state. Capability of the model is demonstrated by several one and two dimensional benchmark problem. In the third part a finite volume shock/interface capturing scheme is developed for two phase flows based on the extension of single phase all-speed simple low-dissipation AUSM (SLAU) scheme. SLAU is the latest version of the AUSM-family schemes with a new numerical flux function which features low dissipation without any tunable parameters in low Mach number regimes while maintaining the robustness of AUSM-family fluxes at high Mach numbers with a very simple formulation. To demonstrate the accuracy of the method, it has been tested on the well known two-fluid air/water flow benchmark problems and the results were compared with the two-phase AUSM+ and AUSM+-up schemes. Finally the scheme was applied for the problem of shock particle cloud interaction to solve the phasic averaged governing equations along with the k-ϵ model to attempt modeling the unclosed terms

A NUMERICAL STUDY OF A NEW SPRAY APPLICATOR

This study focuses on the design and development of a new spray applicator design utilizing effects of imposed pressure oscillations in conjunction with cavitation collapse energy to create distribution of fine droplets. An oscillating horn placed inside the nozzle performing high frequency oscillations is envisioned to provide the necessary pressure perturbations on the exiting liquid jet, while the nozzle geometry design in configured to amplify cavitation process. Initially, a two-zone approach modeling the nozzle interior and exterior in a separate fashion and later, a coupled strategy is proposed. Parametric studies describing the effect of horn stroke length, frequency, its position inside the nozzle in combination with different nozzle designs and liquid flow rates are explored to identify their contribution in obtaining desired cavitation characteristics. In this regard, incorporation of a backward facing step profile within the nozzle shows strong capability of generating the required cavitation and flow field distribution at the nozzle exit. The velocity modulations occuring at the nozzle exit due to oscillating horn structure result in a wide gamut of liquid structures specific to the imposed oscillation frequency and modulation amplitude. The disintegration characteristics of these modulated liquid jets are studied using a Volume-of-Fluid (VOF) interface capturing approach based on finite volume methodology employing an interface compression scheme. VOF methods are validated against experimental results and then subsequently used to study scaling parameters governing the modulated liquid jets. To perform coupled interior-exterior nozzle computations with cavitation, two new cavitation models are presented: First, a model based on Homogeneous Equilibrium assumptions for tracking cavitation events in a compressible framework is presented. Owing to its inability to simulate incompressible cavitating flows, a new cavitation event tracking model based on a Cavitation-Induced-Momentum-Defect (CIMD) correction approach is formulated utilizing a scalar transport model for vapor volume fraction with relevant transport, diffusion and source terms. Validations of both the models against experimental observations are detailed. Coupled internal-external liquid flow computations from the proposed atomizer design using a VOF-CIMD strategy shows strong potential for rapid drop formation in the presence of cavitation effects. A prototype model of a new spray applicator design is presented

Volume-of-Fluid computational foundation for variable-density, two-phase, supercritical-fluid flows

A two-phase, low-Mach-number flow solver is proposed for compressible liquid and gas with phase change. The interface is tracked using a split Volume-of-Fluid method, which solves the advection of the liquid phase. This split advection method is generalized for the case where the liquid velocity is not divergence-free and both phases exchange mass across the interface, as happens at near-critical and supercritical pressure conditions. In this thermodynamic environment, the dissolution of lighter gas species into the liquid phase is enhanced and vaporization or condensation can occur simultaneously at different locations along the interface. A sharp interface is identified with a Piecewise Linear Interface Construction (PLIC). Mass conservation to machine-error precision is achieved in the limit of incompressible liquid, but not with the liquid compressibility and mass exchange. The numerical cost of solving two-phase, supercritical flows is very high because: a) local phase equilibrium is imposed at each interface cell to determine the interface solution (e.g., temperature); b) a complete thermodynamic model is used to obtain fluid properties; and c) phase-wise values for certain variables (i.e., velocity) are obtained via extrapolation techniques. Furthermore, the Volume-of-Fluid method and the PLIC add extra computational costs. To alleviate this numerical cost, the pressure Poisson equation (PPE) is split into a constant-coefficient implicit part and a variable-coefficient explicit part. Thus, a Fast Fourier Transform (FFT) method can be used to solve the PPE. Various validation tests are performed to show the accuracy and viability of the present approach. Then, the growth of surface instabilities in a binary system composed of liquid n-decane and gaseous oxygen at supercritical pressures for n-decane are analyzed. Other features of supercritical liquid injection are also shown.Comment: 52 pages, 19 figure

Experimental and Computational Investigation of Subcritical Near-Nozzle Spray Structure and Primary Atomization in the Engine Combustion Network Spray D

[EN] In order to improve understanding of the primary atomization process for diesel-like sprays, a collaborative experimental and computational study was focused on the near-nozzle spray structure for the Engine Combustion Network (ECN) Spray D single-hole injector. These results were presented at the 5th Workshop of the ECN in Detroit, Michigan. Application of x-ray diagnostics to the Spray D standard cold condition enabled quantification of distributions of mass, phase interfacial area, and droplet size in the near-nozzle region from 0.1 to 14 mm from the nozzle exit. Using these data, several modeling frameworks, from Lagrangian-Eulerian to Eulerian-Eulerian and from Reynolds-Averaged Navier-Stokes (RANS) to Direct Numerical Simulation (DNS), were assessed in their ability to capture and explain experimentally observed spray details. Due to its computational efficiency, the Lagrangian-Eulerian approach was able to provide spray predictions across a broad range of conditions. In general, this "engineering-level" simulation was able to reproduce the details of the droplet size distribution throughout the spray after calibration of the spray breakup model constants against the experimental data. Complementary to this approach, higher-fidelity modeling techniques were able to provide detailed insight into the experimental trends. For example, interface-capturing multiphase simulations were able to capture the experimentally observed bimodal behavior in the transverse interfacial area distributions in the near-nozzle region. Further analysis of the spray predictions suggests that peaks in the interfacial area distribution may coincide with regions of finely atomized droplets, whereas local minima may coincide with regions of continuous liquid structures. The results from this study highlight the potential of x-ray diagnostics to reveal salient details of the near-nozzle spray structure and to guide improvements to existing primary atomization modeling approaches.Battistoni, M.; Magnotti, GM.; Genzale, CL.; Arienti, M.; Matusik, KE.; Duke, DJ.; Giraldo-Valderrama, JS.... (2018). Experimental and Computational Investigation of Subcritical Near-Nozzle Spray Structure and Primary Atomization in the Engine Combustion Network Spray D. SAE International Journal of Fuel and Lubricants. 11(4):337-352. https://doi.org/10.4271/2018-01-0277S33735211

A consistent, scalable model for Eulerian spray modeling

Despite great practical interest in how sprays emanate from fuel injectors, the near-nozzle region has remained a challenge for spray modelers. Recently, Eulerian models have shown promise in capturing the fast gas-liquid interactions in the near field. However, with the inclusion of compressibility, it can be difficult to maintain consistency between the hydrodynamic and thermodynamic variables. In order to resolve numerical inconsistencies that occur in segregated solutions of Eulerian spray model equations as well as to provide good scalability and stability, a new construction of a -Y model is introduced. This construction is built around an IMEX-RK3 algorithm which offers accuracy and efficiency. The new algorithm is compared to an existing implementation for speed and is validated against experimental measurements of spray evolution in order to test the accuracy. The predictions of the new construction are slightly more accurate and, when tested on 256 processors, are 34 times faster.Also this research used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. The authors gratefully acknowledge the computing resources provided on the Texas Advanced Computing Center (TACC) at The University of Texas at Austin that have contributed to the research results reported within this paper URL: http://www.tacc.utexas.edu.Pandal-Blanco, A.; Pastor Enguídanos, JM.; García Oliver, JM.; Baldwin, E.; Schmidt, D. (2016). A consistent, scalable model for Eulerian spray modeling. International Journal of Multiphase Flow. 83:162-171. doi:10.1016/j.ijmultiphaseflow.2016.04.003S1621718