A semi-implicit immersed boundary method and its application to viscous mixing

Computational fluid dynamics (CFD) simulations in the context of single-phase mixing remain challenging notably due the presence of a complex rotating geometry within the domain. In this work, we develop a parallel semi-implicit immersed boundary method based on Open∇FOAM, which is applicable to unstructured meshes. This method is first verified on academic test cases before it is applied to single phase mixing. It is then applied to baffled and unbaffled stirred tanks equipped with a pitched blade impeller. The results obtained are compared to experimental data and those predicted with the single rotating frame and sliding mesh techniques. The proposed method is found to be of comparable accuracy in predicting the flow patterns and the torque values while being straightforwardly applicable to complex systems with multiples impellers for which the swept volumes overlap

A conservative coupling algorithm between a compressible flow and a rigid body using an Embedded Boundary method

This paper deals with a new solid-fluid coupling algorithm between a rigid body and an unsteady compressible fluid flow, using an Embedded Boundary method. The coupling with a rigid body is a first step towards the coupling with a Discrete Element method. The flow is computed using a Finite Volume approach on a Cartesian grid. The expression of numerical fluxes does not affect the general coupling algorithm and we use a one-step high-order scheme proposed by Daru and Tenaud [Daru V,Tenaud C., J. Comput. Phys. 2004]. The Embedded Boundary method is used to integrate the presence of a solid boundary in the fluid. The coupling algorithm is totally explicit and ensures exact mass conservation and a balance of momentum and energy between the fluid and the solid. It is shown that the scheme preserves uniform movement of both fluid and solid and introduces no numerical boundary roughness. The effciency of the method is demonstrated on challenging one- and two-dimensional benchmarks

Inertial Coupling Method for particles in an incompressible fluctuating fluid

We develop an inertial coupling method for modeling the dynamics of point-like 'blob' particles immersed in an incompressible fluid, generalizing previous work for compressible fluids. The coupling consistently includes excess (positive or negative) inertia of the particles relative to the displaced fluid, and accounts for thermal fluctuations in the fluid momentum equation. The coupling between the fluid and the blob is based on a no-slip constraint equating the particle velocity with the local average of the fluid velocity, and conserves momentum and energy. We demonstrate that the formulation obeys a fluctuation-dissipation balance, owing to the non-dissipative nature of the no-slip coupling. We develop a spatio-temporal discretization that preserves, as best as possible, these properties of the continuum formulation. In the spatial discretization, the local averaging and spreading operations are accomplished using compact kernels commonly used in immersed boundary methods. We find that the special properties of these kernels make the discrete blob a particle with surprisingly physically-consistent volume, mass, and hydrodynamic properties. We develop a second-order semi-implicit temporal integrator that maintains discrete fluctuation-dissipation balance, and is not limited in stability by viscosity. Furthermore, the temporal scheme requires only constant-coefficient Poisson and Helmholtz linear solvers, enabling a very efficient and simple FFT-based implementation on GPUs. We numerically investigate the performance of the method on several standard test problems...Comment: Contains a number of corrections and an additional Figure 7 (and associated discussion) relative to published versio

Novel computational methods for the study of compliant-wall fluid-structure interaction

Applying compliant-wall coatings to otherwise rigid surfaces can delay the onset of laminar-turbulent transition and offer marked reductions in skin-friction drag and energy consumption, particularly in marine applications.However, the complex dynamics that result from the coupling of fluid and structure give rise to system instabilities that may prevent drag-reductions in engineering applications.A computational method is developed to study such systems and aid the design of compliant-wall technologies

A Ghost Fluid/Level Set Method for boiling flows and liquid evaporation: Application to the Leidenfrost effect.

The development of numerical methods for the direct numerical simulation of two-phase flows with phase change, in the framework of interface capturing or interface tracking methods, is the main topic of this study. We propose a novel numerical method, which allows dealing with both evaporation and boiling at the interface between a liquid and a gas. Indeed, in some specific situations involving very heterogeneous thermodynamic conditions at the interface, the distinction between boiling and evaporation is not always possible. For instance, it can occur for a Leidenfrost droplet; a water drop levitating above a hot plate whose temperature is much higher than the boiling temperature. In this case, boiling occurs in the film of saturated vapor which is entrapped between the bottom of the drop and the plate, whereas the top of the water droplet evaporates in contact of ambient air. The situation can also be ambiguous for a superheated droplet or at the contact line between a liquid and a hot wall whose temperature is higher than the saturation temperature of the liquid. In these situations, the interface temperature can locally reach the saturation temperature (boiling point), for instance near a contact line, and be cooler in other places. Thus, boiling and evaporation can occur simultaneously on different regions of the same liquid interface or occur successively at different times of the history of an evaporating droplet. Standard numerical methods are not able to perform computations in these transient regimes, therefore, we propose in this paper a novel numerical method to achieve this challenging task. Finally, we present several accuracy validations against theoretical solutions and experimental results to strengthen the relevance of this new method

An interface capturing method for liquid-gas flows at low-Mach number

Multiphase, compressible and viscous flows are of crucial importance in a wide range of scientific and engineering problems. Despite the large effort paid in the last decades to develop accurate and efficient numerical techniques to address this kind of problems, current models need to be further improved to address realistic applications. In this context, we propose a numerical approach to the simulation of multiphase, viscous flows where a compressible and an incompressible phase interact in the low-Mach number regime. In this frame, acoustics is neglected but large density variations of the compressible phase can be accounted for as well as heat transfer, convection and diffusion processes. The problem is addressed in a fully Eulerian framework exploiting a low-Mach number asymptotic expansion of the Navier-Stokes equations. A Volume of Fluid approach (VOF) is used to capture the liquid-gas interface, built on top of a massive parallel solver, second order accurate both in time and space. The second-order-pressure term is treated implicitly and the resulting pressure equation is solved with the eigenexpansion method employing a robust and novel formulation. We provide a detailed and complete description of the theoretical approach together with information about the numerical technique and implementation details. Results of benchmarking tests are provided for five different test cases

Numerical simulation of a non-reactive turbulent flow inside a cyclonic industrial boiler using LES and URANS

A numerical simulation of a non-reactive turbulent flow inside a cyclonic industrial CO boiler was investigated in order to understand the swirling formation, the fluid behavior in different locations inside the domain and the distribution of chemical species. As 80% of the energy matrix in Brazil is generated by combustion processes and government regulations about NOx emissions are becoming more restrict, enhancing combustion efficiency in a CO boiler with a turbulent swirling flow to reduce pollutant emissions has become an engineering research topic. Enhancing mixing processes through turbulent swirling flows might reduce thermal NOx formation. Computational fluid dynamics simulations were realized using the in-house MFSim code with the turbulent closure models LES, URANS Standard k − ", URANS Standard k − " Modified and URANS Realizable k − ". A theoretical basis about turbulence, LES and URANS closure models, mixing and swirling flows was provided. A state of art comprising different authors pointed out that some works with URANS Standard k − " demonstrated a premature solid-body rotation formation due to its eddy viscosity assumption and that swirling flows may reduce pollutant emissions by improving mixing of reactants and decreasing flame temperature. Validations concerning multi-component mixing flows and Immersed Boundary method were presented. From the results, LES and URANS Standard k − " presented similar velocity field results, capable of capturing the swirling formation. When analyzing three URANS closure models, a turbulent kinetic energy graph illustrated that it is relevant to observe the modeled part and the value obtained from velocity field fluctuations. The modified model presented low turbulent viscosity values and an LES-like behavior, with similar results to the standard model. The realizable model presented distant results comparing to the other models studied and there was no reverse flow in its swirling core. Adding different chemical species did not modify the velocity field and the highest mixing level was obtained in the most intense turbulent swirling region, close to the inlets. The data provided may assist in the comprehension of swirling formation, mixture processes inside a boiler and temperature control to reduce pollutant emissionsCAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorDissertação (Mestrado)Uma simulação numérica de um escoamento turbulento não reativo em uma caldeira industrial ciclônica de CO foi investigada a fim de se compreender a formação de um escoamento rotativo, o comportamento do fluido em diferentes locais dentro do domínio e a distribuição de espécies químicas. Como 80% da matriz energética no Brasil é gerada por processos de combustão e as regulamentações governamentais sobre as emissões de NOx estão se tornando mais restritas, o aumento da eficiência da combustão em uma caldeira de CO com escoamento turbulento ciclônico para reduzir as emissões de poluentes tornou-se um tema de pesquisa de engenharia. Melhorar os processos de mistura por meio de escoamentos turbulentos rotativos pode reduzir a formação térmica de NOx. Simulações de dinâmica dos fluidos computacional foram realizadas usando o código MFSim com os modelos de fechamento turbulento LES, URANS Standard k − ", URANS Standard k − " Modificado e URANS Realizable k − ". Foi fornecida uma base teórica sobre turbulência, modelos de fechamento LES e URANS, escoamentos com mistura e escoamentos rotativos. Um estado da arte compreendendo diferentes autores apontou que alguns trabalhos com URANS Standard k − " demonstraram uma formação de rotação de corpo sólido prematura devido à sua suposição de viscosidade turbulenta e que escoamentos rotativos podem reduzir as emissões de poluentes, melhorando a mistura de reagentes e diminuindo a temperatura da chama. Foram apresentadas as validações relativas aos escoamentos com mistura de multicomponentes e ao método da Fronteira Imersa. Dos resultados, LES e URANS Standard k − " apresentaram campos de velocidade semelhantes, capazes de capturar a formação de escoamento rotativo. Ao analisar três modelos de fechamento URANS, um gráfico de energia cinética turbulenta ilustrou que é relevante observar a parte modelada e o valor obtido a partir das flutuações do campo de velocidade. O modelo modificado apresentou baixos valores de viscosidade turbulenta e comportamento semelhante a LES, com resultados similares ao modelo Standard. O modelo realizável apresentou resultados distantes em comparação com os outros modelos estudados e não houve escoamento reverso em seu núcleo giratório. A adição de diferentes espécies químicas não modificou o campo de velocidade e o maior nível de mistura foi obtido na região de turbulência mais intensa, próxima às entradas. Os dados fornecidos podem auxiliar na compreensão da formação de escoamento rotativo, processos de mistura dentro de uma caldeira e controle de temperatura para reduzir as emissões de poluentes