Development and use of interface-capturing methods for investigation of surfactant-covered drops in electric fields

This thesis investigates the development and use of interface-capturing methods for detailed simulations of surfactant-covered drops in electric fields. A mathematical model is established for the full hydrodynamic behavior of the drops, including both electric forces caused by an applied electric field and forces due to the non-uniform surface tension caused by the presence of surfactants. Equations for the electric field and for the coupled evolution of surfactant on the interface and in the bulk are also considered. Numerical methods suitable for the solution of the mathematical model are investigated. Both the level-set method and the phase-field method are used. For the level-set method, the ghost-fluid method which treats discontinuities across the interface in a sharp manner is considered and compared to the conceptually simpler continuous surface-force method. For the phase-field method, sophisticated numerical approaches including nonlinear multigrid methods on block-structured adaptive grids are used to enable simulations in full 3D. Several physical configurations are examined. It is shown how an electric field can suppress the partial coalescence phenomenon occurring when a drop coalesces with an interface. Is is demonstrated that the presence of a surfactant can considerably slow down a sedimenting drop due to inhibition of internal circulation. Conversely, an electric field speeds up the sedimentation due to stretching which leads to reduced drag. The deformation of a surfactantcovered leaky dielectric drop in an electric field is studied, and rich deformation behavior due to the complex interaction between the electric field and the surfactant is demonstrated. Finally, full 3D simulations of a drop in shear flow are performed, with particular emphasis on the influence of a soluble surfactant. It is shown that the deformation of a drop with soluble surfactant in general lies between that of a clean drop and that of a drop covered with insoluble surfactant. However, for the breakup of a drop, it is shown that for the insoluble case, the drop can break up at a earlier time compared to a clean drop, while for the soluble case, the drop can break up at a later time.PhD i energi- og prosessteknikkPhD in Energy and Process Engineerin

Precursor turbulent inflow dataset for large eddy simulation of a semi-idealized European generic city.

This data article provides high-quality turbulent inflow boundary data with a high spatial and temporal resolution of a very rough atmospheric boundary layer (ABL) wind tunnel, which can be applied as the large eddy simulation (LES) inflow condition on the Michelstadt test cases. A high-quality LES of the WOTAN wind tunnel of the Environmental Wind Tunnel Laboratory (EWTL) was conducted using OpenFOAM software, and data is stored at a plane at 1000 Hz frequency at the end of the roughness elements. This data serves as the turbulent inflow boundary condition, offering computational fluid dynamics (CFD) researchers a cost-effective means to simulate the benchmark Michelstadt test cases for LES validation. This data will be utilized to perform high-quality LES, which are pivotal in bridging the research gap in understanding the intricate nature of wind dynamics in a realistic urban environment.publishedVersio

Pedestrian Wind Comfort Assessment Using Computational Fluid Dynamics Simulations With Varying Number of Wind Directions

The construction of a building inevitably changes the microclimate in its vicinity. Many city authorities request comprehensive wind studies before granting a building permit, which can be obtained through Computational Fluid Dynamics (CFD) simulations. Investigating the wind conditions for 12 wind directions has previously been considered sufficient in most literature and the industry. However, the effect of changing the number of simulated wind directions is still not well understood. This article investigates the influence of the number of simulated wind directions on pedestrian wind comfort maps. A neighborhood in Niigata city, Japan, was chosen as a case study. Simulations are performed in OpenFOAM using a Reynolds-averaged Navier-Stokes model and the realizable k-ϵ turbulence model. The inlet profiles form a homogeneous atmospheric boundary layer with neutral stratified conditions and a logarithmic velocity profile. The pedestrian wind comfort maps are converging toward a final map as more wind directions are included. The area of the maps classified with the same comfort as using 64 wind directions is 79% using 4 wind directions, 92% using 8 wind directions, 96% using 16 wind directions, and 99% using 32 wind directions. A greater understanding of the influence of the number of simulated wind directions included may enable more efficient pedestrian wind comfort studies that recognize the associated uncertainties.publishedVersio

Sensitivity of urban morphology and the number of CFD simulated wind directions on pedestrian wind comfort and safety assessments

Pedestrian wind comfort and safety are critical in urban design, especially as cities densify and climate change impacts intensify. While Computational Fluid Dynamics (CFD) simulations are increasingly used in these assessments, a universally agreed-upon number of wind directions for accurate results has yet to be established. This study addresses this gap by exploring the influence of urban morphology, characterized by different degrees of urbanization and building layouts, on the required number of wind directions for accurate assessments. Extensive CFD simulations were conducted in five geometrically diverse locations: Bryne, Oslo, London, Singapore, and New York. Pedestrian wind comfort maps were generated, and velocity amplification factor (VAF) dynamics were analyzed. The results suggest that urban complexity does not significantly affect the required number of wind directions for reliable assessments. The study provides practical guidance for selecting the number of wind directions based on the study’s focus: a minimum of 8 for basic assessments, 24 for high-accuracy assessments, and at least 36 for safety-focused assessments. This research significantly contributes to urban planning and design, empowering stakeholders with valuable insights for shaping resilient and comfortable urban environments.publishedVersio

Investigation of Rotor Efficiency with Varying Rotor Pitch Angle for a Coaxial Drone

Coaxial rotor systems are appealing for multirotor drones, as they increase thrust without increasing the vehicle’s footprint. However, the thrust of a coaxial rotor system is reduced compared to having the rotors in line. It is of interest to increase the efficiency of coaxial systems, both to extend mission time and to enable new mission capabilities. While some parameters of a coaxial system have been explored, such as the rotor-to-rotor distance, the influence of rotor pitch is less understood. This work investigates how adjusting the pitch of the lower rotor relative to that of the upper one impacts the overall efficiency of the system. A methodology based on blade element momentum theory is extended to coaxial rotor systems, and in addition blade-resolved simulations using computational fluid dynamics are performed. A coaxial rotor system for a medium-sized drone with a rotor diameter of 71.12 cm is used for the study. Experiments are performed using a thrust stand to validate the methods. The results show that there exists a peak in total rotor efficiency (thrust-to-power ratio), and that the efficiency can be increased by 2% to 5% by increasing the pitch of the lower rotor. The work contributes to furthering our understanding of coaxial rotor systems, and the results can potentially lead to more efficient drones with increased mission time.publishedVersio

Influence of fluid viscosity hierarchy on the reverse-circulation displacement efficiency

Reverse-circulation cementing is an alternative strategy for well cementing where the cementing fluids are injected directly into the annulus from the surface. This cementing strategy can reduce downhole circulation pressures compared to conventional circulation cementing and potentially eliminate the need for retarders in the cement slurry. In reverse-circulation operations, the fluid hierarchy will normally involve density-unstable combinations along the annulus. Since the annular geometry prevents the mechanical separation of fluids, reverse-circulation cementing is associated with a risk of slurry contamination and mixing during placement. Although reverse-circulation cementing has been known for several decades and is used for cementing of both onshore and offshore wells, it remains unclear whether conventional circulation job design guidelines apply to reverse-cementing or indeed how fluid properties should be optimized for such operations. The purpose of the current study is to contribute to the understanding of buoyant annular displacements, with a particular focus on the role of viscosity hierarchy on the annular displacement in vertical and near-vertical annuli. We present a combined experimental and numerical study of density-unstable downward displacements in a downscaled, narrow concentric annulus. A transparent annulus flow loop was used to conduct downward displacements. A high-speed camera and a mirror arrangement were used to track the displacement. Numerical simulations of the experiments and selected other cases were performed using the open-source OpenFOAM computation framework. We study Newtonian and mildly shear-thinning fluids, and our study aims to determine whether it is more efficient to use a displacing fluid with higher viscosity or lower viscosity than the displaced fluid while maintaining a constant average viscosity for the fluid pair. The experimental and numerical results, which are in good qualitative agreement, demonstrate that the viscosity hierarchy of the fluids significantly affects the displacement flow features. Our results show that a more viscous displaced fluid leads to faster growth of the instabilities and, as a result, less efficient displacement. Oppositely, we observe less tendency for finger growth and a more diffusive mixing region for more viscous displacing fluids. The effect of the viscosity hierarchy can get stronger by increasing the inclination of the annulus and the viscosity difference between the fluids from 0.006 to about 0.02 Pa s. The findings can assist in the selection of fluid properties for future reverse-circulation displacement operations.publishedVersio

Virtual skeleton methodology for athlete posture modification in CFD simulations

This study focuses on the aerodynamic influence of athlete posture in sports aerodynamics. To analyze a specific posture, wind tunnel measurements and computer simulations are commonly employed. For computer simulations, the growing trend is to use 3D scanning to create accurate representations of an athlete’s geometry. However, this process becomes cumbersome and time-consuming when multiple positions need to be scanned. This work presents a methodology to use a virtual skeleton to perform modifications of an athlete’s posture. This is an efficient approach that can be applied directly to a scanned geometry model, and that allows easy modification and use in optimization procedures. The methodology is applied to two different cases; small adjustment of arm position for a time-trial cyclist, and large alteration of a standing alpine skier into a tucked position. Computational fluid dynamics simulations show that similar results are obtained for aerodynamic drag using the proposed methodology as with geometry models obtained from 3D scanning. Less than 1% difference in drag area was found for the cyclist, and less than 2% difference for the skier. These findings show the method’s potential for efficient use in sports aerodynamics studies.publishedVersio

Modelling of hydrocarbon gas and liquid leaks from pressurized process systems

The hydrocarbon leaks from process systems potentially lead to hazardous consequences with regard to human safety, environmental pollution and valuable assets. The hydrocarbon leaks may be gas leaks, liquid leaks or multiphase leaks. The gas leaks have the highest potential of damage due to explosion accidents. both gas and oil leaks can create long-lasting fires threatening personnel safety and structural integrity of process plants and offshore platforms. One common method for limiting the consequences associated with a process emergency is the rapid depressurization or blowdown of pressurized process systems. There is experimental evidence that the assumption of thermodynamic equilibrium is not appropriate during rapid depressurization, since the two phases show an independent temperature evolution. The current work proposes a model for the simulation of the blowdown of vessels containing two-phase (gas–liquid) hydrocarbon fluids, considering partial phase equilibrium between phases. Two phases may be present either already at the beginning of the blowdown process (for instance in gas–liquid separators) or as the liquid is formed from flashing of the vapour due to the cooling induced by pressure decrease. In addition, the transient behaviour of hydrocarbon leaks from pressurized process systems during depressurization is also included in the model providing the inputs for risk assessments. The model is based on a compositional approach, and it takes into account coupled effects of internal heat and mass transfer processes, as well as heat transfer with the vessel wall and the external environment. The vapour liquid equilibria calculations are performed using dynamic link library provided by the comprehensive pressure volume temperature and physical properties package ‘Multiflash’. Numerical simulations show a generally good agreement with experimental measurements.publishedVersio

Computational investigation of the aerodynamic performance of reversible airfoils for a bidirectional tidal turbine

A reversible airfoil is an airfoil that has equal performance when the flow is reversed. Such airfoils are relevant for many different applications, including use in ventilation fans, helicopter rotors, wind turbines and tidal turbines. Compared to traditional airfoils, reversible airfoils have different performance characteristics and have been less explored in the scientific literature. This work investigates the aerodynamic performance of some selected reversible airfoils using computational fluid dynamics. The selected airfoils are based on existing NACA 6 profiles and a profile using B-spline parameterization. The results show reduced performance for the reversible airfoils compared to a unidirectional airfoil. Of the investigated airfoils, the B-spline airfoil has the highest performance, with a maximum aerodynamic efficiency which is 87 % of the unidirectional design.publishedVersio