Surge vessels are water storage devices that maintain a consistent supply of water to a pipeline system when one or more pumps fail. A surge vessel has compressed air on top and usually water on the bottom. The heat transfer during the expansion processes of the air pocket is of great interest since the heat transfer behavior significantly affects the size of the surge vessels. Thus their thermal behavior is of great importance.
Two approaches are mainly applied to describe this heat transfer process: the classic polytropic method and the rational heat transfer (RHT) model. The polytropic model has been used for years to describe the heat transfer behavior. The RHT model can contain a detailed estimation of all heat transfer terms. This report will specifically examine the two models, highlight the advantages and disadvantages identified in current related research, and clarify the research approaches.
To address this need, the research employs both mathematical calculations and Computational Fluid Dynamics (CFD) simulations. This involves calculating various heat transfer processes and validating the simulation results against experimental data. The LES model is used to simulate the complex process of air pocket expansion within the surge vessel.
The validation process includes verifying the ideal gas model within the software, assessing mesh resolution, and evaluating the chosen models and solvers. A comparison of simulation results with experimental data from a separate case is included, providing a robust validation.
The results reveal detailed insights into the temperature and velocity fields within the surge vessel, highlighting temperature variations at probes and comparing findings with scaled models and mathematical calculations. A comparative analysis of simulation results against experimental data and mathematical calculations is also presented.
The findings of this research offer significant insights into the heat transfer characteristics within surge vessels such as temperature distribution and amount of heat transfer. These contributions are essential for refining surge vessel design.

Laminar to turbulent transition is the process in which a smooth orderly laminar flow becomes turbulent, chaotic and unpredictable. A laminar boundary layer (BL) may become turbulent due to growing disturbances in the flow. If a disturbance is small its behavior is governed by linear stability theory (LST). Early works on LST were focussed on the growth of normal modes in incompressible or ideal gas compressible boundary layers, with the more recent inclusion of high-temperature and/or dense gas effects. Consideration for growth of perturbations other than modal started in 1970s, and while it has received considerable attention in ideal gas boundary layers, the study of non-modal growth in heavily stratified boundary layers remains limited.
In recent years, research in supercritical fluids and their applications has risen substantially, where supercritical CO2 (SCO2) stands out. It has been proposed as a working fluid for power generation turbines, a heat carrier fluid in geothermal, etc. A supercritical fluid near its pseudo-boiling temperature exhibits extremely large variations of physicochemical properties, leading to strongly stratified transcritical boundary layer flows, which may heavily influence its stability. Recently, transcritical boundary layers have been shown to be unstable to a novel mode, not found in ideal gas boundary layers, further motivating the study of the hydrodynamic behaviour of supercritical fluids.
This study investigates the linear stability of SCO2 boundary layers in the region of the Widom line in the phase diagram. Both heating and cooling are considered, in the sub-, super- and transcritical regimes. Regarding the amplification of normal modes, a special focus is given to 3D perturbations and the conditions for which a 3D perturbation is more amplified than a 2D perturbation. A moderate Mach number is found to be necessary in the further destabilization of 3D waves when compared to 2D waves. As indicated by previous works, the Tollmien-Schlichting (TS) mode is found to be preferentially 3D for moderate Mach number in the sub- and supercritical regimes. However, in the transcritical regime, the TS mode is found to be most amplified for stream-wise propagating waves. The novel mode II, found solely in transcritical flows, is preferentially 3D at sufficiently high Mach number, both in the viscous and inviscid regimes. Regarding non-modal stability, the energy growth of sub- and supercritical boundary layers is comparable to that of an ideal gas compressible boundary layer, with optimal energy growth driven by the lift-up mechanism. In the transcritical regimes, the lift-up mechanism also dominates, but energy growth is considerably larger due to the presence of strong thermodynamic gradients within the flow. This results in substantial energy growth associated with density and temperature streaks in the perturbations. When considering purely kinetic effects, the cooled wall case shows higher growth when compared to an ideal gas boundary layer, whereas the heated wall case shows a reduction of the kinetic energy growth. The cooled wall condition results in high mean vorticity far from the wall, leading to high energy growth, while the heated wall condition results in low mean vorticity far from the wall, leading to a reduced energy growth.

Large Eddy Simulation (LES) is a mathematical technique for performing simulations of turbulent flows, such as those found in the Earth’s atmosphere. Compared to traditional numerical weather and climate models, LES is more accurate in representing turbulent processes and cloud dynamics. The computational burden of LES, however, have histor- ically limited its application to relatively small domain sizes. In this work, part of the DALES atmospheric LES model was ported to Graphics Processing Units (GPUs) using the OpenACC programming model. GPUs, originally designed for accelerating computations related to 3D computer graphics, excel at parallel computations, which are abundant in LES models. The performance of the GPU port of DALES was measured on an NVIDIA RTX 3090 in a desktop workstation and an NVIDIA A100 in the Snellius supercomputer and compared to the existing CPU implementation. For the BOMEX intercomparison case, a speedup of 11.6 was achieved versus 8 CPU cores on the desktop system, while on Snellius a speedup of 3.9 was observed compared to 128 CPU cores. Furthermore, the existing MPI parallelization of DALES was adapted such that multiple GPUs can be used simultaneously. This thesis represents a step towards the enhancement of the scalability of DALES, enabling simulations on larger domains at higher resolutions. While a substantial acceleration of DALES was achieved, further efforts are needed to port more components of the model to the GPU to facilitate the simulation of increasingly realistic meteorological phenomena.

Amidst the rise in fossil fuel consumption and the global energy crisis, there is a growing demand for clean, environmentally friendly, and sustainable energy solutions in industrial processes. One promising approach is to substitute conventional working fluids in power cycles with supercritical fluids. Over recent decades, substantial research effort has been made in investigating supercritical fluids. Among these, supercritical carbon dioxide (sCO2) has emerged as a practical and alternative solution. Its low critical pressure and temperature, high thermal efficiency, and operational flexibility have garnered widespread interest across various energy applications, particularly in heat exchangers and gas-powered cycles. However, the complex flow dynamics and heat transfer characteristics of sCO2 near its critical point have become the subject of intense research. When sCO2 flows through a heated channel, strong density gradients can generate dominant buoyancy forces that can significantly affect the supercritical fluid structure, mixing and transport properties. Understanding buoyancy-affected flows is highly crucial as it can lead to flow stratification and heat transfer deterioration or enhancement in the channel.

The role of buoyancy forces in flow stratification is quite substantial and can stabilize or destabilize the stratified structure by inducing or dampening instabilities. While numerous computational simulations have been performed to understand the mechanism of buoyancy-affected stratification in ideal fluids, whilst, a notable research gap exists in the literature on supercritical fluid stratified flow. Therefore, the present study aims to investigate the influence of buoyancy on the sCO2 flow stratification in a channel, considering heating from the top and bottom walls. In this context, a Direct Numerical Simulation (DNS) is conducted using the open-source CFD package ”OpenFOAM” to simulate pressure-driven sCO2 channel flow under constant wall heat flux. A buoyant pimple foam solver was adapted to simulate transient supercritical flow. To gain good accuracy in the thermophysical properties, a custom library was prepared to interpolate supercritical fluid properties at simulation run-time. To assess the influence of buoyancy in the heated channel flow of sCO2, a developing flow profile is initiated at a constant pressure of 80 bar. Stratification is achieved by imposing a heat flux at the wall boundary spanning in the heating range of 5 − 15 kW /m2, resulting in density and temperature variation across the fluid. By varying the heat flux at the wall boundary, we analyze the resulting variation in the flow field (temperature, velocity, and pressure distribution), the dynamics of heat transfer, and the influence of buoyancy by the non-dimensional parameter Richardson number in the stratification of supercritical carbon dioxide. The results show that the effect of heating on the developing boundary layer in sCO2 channel flow is substantial. With increasing heat flux, the flow is accelerated near the heated wall while decelerating in the bulk. A strong non-linear variation in the temperature and density distribution is observed in the wall-normal direction. Moreover, as the heat flux increases, the wall shear stress decreases due to strong property variation, while the Richardson number and Reynolds number increases, and the heat transfer coefficient decreases.

Swimming is a sport where an incremental gain in performance can lead to either a medal or leaving the swimmer empty-handed. In the front crawl, water is driven backwards by the limbs where most propulsive forces come from the hands. Two main stroke patterns can be observed, the ”straight I pull” is commonly taught to generate forces with drag, and the ”curved S-pull” applies lift as well. This investigation uses a new industrial 6-DOF robot arm to study angle-dependent forces, stroke patterns and hand configuration variables. The forearm and hand are analysed using force measurements and PIV to obtain quantitative information, while a parameterisation of the forearm and hand is used to obtain qualitative analysis. The flow is analysed qualitatively using two analytical methods, but it is discovered that these methods can only be applied to a limited range of cases to predict the angle-dependent lift force. Numerical investigations into the parameterised arm model reveal that forces generated were similar in magnitude to those reported in the literature for a pseudo-transient simulation. Quantitative research shows that lift only surpasses drag at extreme angles of attack immediately after a rapid start. This provides clear evidence that drag-based propulsion is preferred for front-crawl swimming. We utilize PIV technology to visualise the flow field around two cases where the lift was highly prevalent. The results reveal various aspects, including the nature of the observed lift peak. Finally, the study compares three distinct strokes: one utilizing a straight drag-based approach and two utilizing a sinusoidal path, resembling a curved pull. The findings indicate that the sinusoidal path’s lift-to-drag ratio remains relatively consistent compared to steady-state, constant-angle experiments. Additionally, the results suggest that the straight stroke is optimal for propulsion, while one of the sinusoidal strokes might be more energy-efficient. An additional aim of this research is to determine the suitability of an industrial 6-DOF robot arm for stroke experiments. It has been discovered that some experiments need a longer path and should preferably be conducted in a wind or water tunnel.

Urban microclimate significantly affects people’s experiences and activities in urban environments by a series of phenomena, among which urban flow is an important factor to be considered. Computational Fluid Dynamics (CFD) method has become a popular tool for studying urban airflow because of its low cost compared with experiment methods. However, flows over urban areas exhibit turbulence nature of being three-dimensional, unsteady, and multi-scale. Additionally, the large computational domain that should be covered and the inherent inhomogeneity of the urban structures make it challenging to do full-scale modelings. Large Eddy Simulation (LES), with the development of computing power, becomes a promising tool to study such flows.

In the campus of Delft University of Technology (TU Delft), a crossroad near the EWI building (the main building of Faculty of Electrical Engineering, Mathematics and Computer Science) is constantly complained for its strong wind. This research tackles such problem using LES, and takes TU Delft campus area itself as case study. The development of this research is composed of three stages.

In the first stage, Vreman eddy viscosity model is implemented into Canonical Navier-Stokes (CaNS), a massively-parallel Navier Stokes solver developed by Costa, (2018). Based on a structured three dimensional Cartesian grid, the subgrid scale (SGS) eddy viscosity model is inserted into the Navier Stokes equation by adding an extra diffusion term. The inserted diffusion term is discretized with second-order difference scheme along with interpolation of the velocity field due to the staggered grid arrangement. The implementation is validated with a turbulent channel flow with friction Reynolds number 𝑅𝑒𝜏 = 360. The good agreement is found and the discrepancy is small.

In the second stage, the solver employs a direct-forcing Immersed Boundary Method (IBM) and is further validated with the flow over periodic cube arrays. Signed-Distance Field (SDF), as a convenient tool, is generated and functions as read-in data for IBM. The IBM processes the effect of the boundary as an added force on the fluid points at the interface. The stair-step approach approximates the structure boundary with the cuboid cells faces. The results match well with the wind tunnel test data from Castro et al., (2006) and a previous LES study by Tomas et al., (2016).

In the last stage, the validated solver is applied to a scaled-down TU Delft campus model. The simulation setup is designed by considering the achievability of a possible future wind tunnel measurement. Three grids are used for a grid convergence analysis by comparing the total IBM force, mean velocity, and Reynolds stress at certain locations. The flow converges with the finest grid with grid number 𝑁𝑥 × 𝑁𝑦 × 𝑁𝑧 = 960 × 880 × 240. Around the EWI building, a high-speed region is found at the cross road location. Behind the building, a wake area is observed, and a clear shear layer is on the top of the building.