Harry E.A. Van den Akker

Mixing in Shallow Cumulus Clouds Studied by Lagrangian Particle Tracking

Mixing between shallow cumulus clouds and their environment is studied using large-eddy simulations. The origin of in-cloud air is studied by two distinct methods: 1) by analyzing conserved variable mixing diagrams (Paluch diagrams) and 2) by tracing back cloud-air parcels represented by massless Lagrangian particles that follow the flow. The obtained Paluch diagrams are found to be similar to many results in the literature, but the source of entrained air found by particle tracking deviates from the source inferred from the Paluch analysis. Whereas the classical Paluch analysis seems to provide some evidence for cloud-top mixing, particle tracking shows that virtually all mixing occurs laterally. Particle trajectories averaged over the entire cloud ensemble also clearly indicate the absence of significant cloud-top mixing in shallow cumulus clouds.

A statistical approach to the life cycle analysis of cumulus clouds selected in a virtual reality environment

In this study, a new method is developed to investigate the entire life cycle of shallow cumuli in large eddy simulations. Although trained observers have no problem in distinguishing the different life stages of a cloud, this process proves difficult to automate, because cloud-splitting and cloud-merging events complicate the distinction between a single system divided in several cloudy parts and two independent systems that collided. Because the human perception is well equipped to capture and to make sense of these time-dependent three-dimensional features, a combination of automated constraints and human inspection in a three-dimensional virtual reality environment is used to select clouds that are exemplary in their behavior throughout their entire life span. Three specific cases (ARM, BOMEX, and BOMEX without large-scale forcings) are analyzed in this way, and the considerable number of selected clouds warrants reliable statistics of cloud properties conditioned on the phase in their life cycle. The most dominant feature in this statistical life cycle analysis is the pulsating growth that is present throughout the entire lifetime of the cloud, independent of the case and of the large-scale forcings. The pulses are a self-sustained phenomenon, driven by a balance between buoyancy and horizontal convergence of dry air. The convective inhibition just above the cloud base plays a crucial role as a barrier for the cloud to overcome in its infancy stage, and as a buffer region later on, ensuring a steady supply of buoyancy into the cloud.

The Details of Turbulent Mixing Process and their Simulation

Abstract This chapter is devoted to turbulent mixing processes carried out in - mainly - stirred vessels. It reviews first a number of turbulent flow characteristics as far as relevant to a wide variety of single-phase and two-phase mixing processes and, secondly and most importantly, the details of the advanced Computational Fluid Dynamics (CFD) techniques required for simulating such processes with a large degree of confidence. The processes considered comprise blending, solids suspension, dissolution, precipitation, crystallization, chemical reactions, and dispersing gases and immiscible liquids. The emphasis in this chapter is on the fruitful application of Large Eddy Simulations for reproducing the local and transient flow conditions in which these processes are carried out and on which their performance depends. In addition, examples are given of using Direct Numerical Simulations of flow and transport phenomena in small periodic boxes with the view to find out about relevant details of the local processes. Finally, substantial attention is paid throughout this chapter to the attractiveness and success of exploiting lattice-Boltzmann techniques for the more advanced CFD approaches.

On multiple stability of mixed-convection flows in a chemical vapor deposition reactor

Abstract The laminar mixed convection flow and heat transfer in a Chemical Vapor Deposition reactor are studied through numerical simulation. It is found that the non-linear interaction between forced and free convection may lead to the existence of multiple stable flows. Arclength continuation techniques, implemented within the framework of the finite volume discretisation, have been applied to determine the causes for this multiplicity. It is shown that the relevant dimensionless groups are Gr/Re and Re×Pr. When both these groups are sufficiently large, multiple stable flows may exist.

Coherent structures in multiphase flows

Abstract The occurrence of coherent structures in multiphase flows is widespread: quantitative experimental data with respect to their size, their velocities, and their frequency in both bubble columns and fluidized beds are presented; these data have been collected by such different techniques as Laser—Doppler Velocimetry, Partice Image Velocimetry, Optical Bubble Probes, and Gamma-Ray Densitometry. In addition, results are discussed from Computational Fluid Dynamics simulations as to particle swarms in the freeboard beyond bubbling fluidized beds. The question is raised whether there is a common explanation for the appearance of coherent structures in such different systems. The similarity of multiphase flows and natural convection phenomena is discussed, while some features of turbulent multiphase flows resemble those in shear-induced turbulence in single-phase flows. Phase locking, on the basis of interplaying time scales, and an analogy with the vortices in a Karman vortex street are suggested as possible mechanisms behind the occurrence of coherent structures.

On turbulent flows in cold-wall CVD reactors

With the increase in diameter of the wafers and the tendency to increase operating pressure of chemical vapor deposition reactors, the flow in these reactors may turn turbulent as a result of buoyancy. The effect of turbulence on the CVD process in a single wafer reactor has been studied numerically with large eddy simulations. It is found that the free-convection induced turbulence increases the heat flux, whereas the conditions are uniform in a large part of the reactor as a result of turbulent mixing, in principle offering the possibilities for a high, uniform deposition rate.

Mixed convection in radial flow between horizontal plates — I. Numerical simulations

Abstract A forced radially outward flow with secondary, buoyancy induced convection has been studied numerically in an axisymmetric geometry, consisting of two differentially heated, horizontal, coaxial, circular plates with a diameter of 25 times their mutual spacing. A forced laminar flow is supplied through the centre of the upper plate. The onset of thermal instability, leading to axisymmetric and three-dimensional rolls, has been determined as a function of the Reynolds, Prandtl and Rayleigh numbers.

Mixed convection in radial flow between horizontal plates—II. Experiments

Abstract Mixed convection of a forced radially outward flow with secondary, buoyancy induced convection has been studied experimentally in an axisymmetric geometry, consisting of two differentially heated, horizontal, coaxial, circular plates with a diameter of 50 cm and a mutual distance of 2 cm. Through the centre of the upper plate, a laminar forced air flow is supplied. Particle image velocimetry, flow visualisation and local temperature measurements have been used to study the onset of thermal instability as a function of the inflow and temperature difference, and to validate the numerical results obtained in Part I of this paper.

Thermohydrodynamics of an evaporating droplet studied using a multiphase lattice Boltzmann method

In this paper, the thermohydrodynamics of an evaporating droplet is investigated by using a single-component pseudopotential lattice Boltzmann model. The phase change is applied to the model by adding source terms to the thermal lattice Boltzmann equation in such a way that the macroscopic energy equation of multiphase flows is recovered. In order to gain an exhaustive understanding of the complex hydrodynamics during evaporation, a single droplet is selected as a case study. At first, some tests for a stationary (non-)evaporating droplet are carried out to validate the method. Then the model is used to study the thermohydrodynamics of a falling evaporating droplet. The results show that the model is capable of reproducing the flow dynamics and transport phenomena of a stationary evaporating droplet quite well. Of course, a moving droplet evaporates faster than a stationary one due to the convective transport. Our study shows that our single-component model for simulating a moving evaporating droplet is limited to low Reynolds numbers.

Parallel Fluid Flow Simulations by Means of a Lattice-Boltzmann Scheme

A parallel implementation of a lattice-Boltzmann scheme for direct numerical flow simulation is presented. Because of the locality of its operation the scheme is well suited for parallelism. Simulations of a sample flow (the flow past a circular cylinder at Reynolds numbers between 0.3 and 120) show good agreement with experimental data reported in literature. The parallel simulations efficiently employ the computational resources, at least for computer platforms with up to eight nodes.