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

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

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

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

Bruk av nettbrett til digitale innleveringer i fag med matematisk notasjon

MNT-fag har utstrakt bruk av matematisk notasjon og diagrammer. For øvingsoppgaver og eksamen i disse fagene blir det fortsatt overveiende brukt penn og papir. Samtidig er det et økende fokus på digitalisering. Digitale innleveringer og eksamen har flere fordeler; blant annet vil det minske administrativt arbeid med lagring og distribusjon av papirbesvarelser og det vil gjøre sensors arbeid enklere da sensur kan utføres digitalt og med økt lesbarhet. Å ha innleveringer på en heldigital form kan også ha en klar pedagogisk fordel. Med de matematiske uttrykkene i en digital form kan utrykkene utnyttes til løsning av likninger i beregningsprogrammer og studentene kan gis tilbakemeldinger underveis i løsningsprosessen. Å skrive inn matematiske uttrykk på en PC er ofte tungvint og krever bruk av spesialisert notasjon som LaTeX eller pek-og-klikk verktøy som Microsoft sin ligningseditor i Word. Det samme gjelder for eksamensoppgaver i for eksempel Inspera. Det finnes også andre løsninger for digitalisering i dag, hovedsakelig basert på scanning av besvarelser. Dette innfrir noen av fordelene til digitale innleveringer, men man mangler fortsatt lesbarheten til en heldigital form, og man har ikke mulighet til å gjøre en analyse av besvarelsen for å få de pedagogiske mulighetene. Det har i de siste årene skjedd en rivende utvikling innenfor nettbrett og håndskriftsgjenkjenning. Nyere nettbrett har svært lav forsinkelse i input, høyoppløste skjermer og avanserte algoritmer for håndflategjenkjenning og håndskriftsgjenkjenning, som kombinert kan gjøre opplevelsen nærmere penn og papir. I dette arbeidet har vi undersøkt i hvor stor grad dagens teknologi er anvendelig i fag med bruk av matematisk notasjon og figurer. Vi valgte ut to fag på masternivå for maskiningeniører og lot en gruppe studenter gjøre øvingsoppgaver med henholdsvis penn og papir, et tekstredigeringsverktøy (Microsoft Word) og et nettbrett (iPad med Apple Pencil). Deretter analyserte vi opplevelsen til studentene gjennom dybdeintervju og evaluering av innsendte øvingsoppgaver. Resultatene viste at studentene opplevde nettbrett som et godt alternativ til penn og papir, og en klar forbedring sammenlignet med tekstredigeringsverktøy, men at gjenkjenningen av matematiske uttrykk fortsatt ikke er god nok til å brukes uten betydelig opplæring og trening.publishedVersio

Configurable convolutional neural networks for real-time pedestrian-level wind prediction in urban environments

Urbanization has underscored the importance of understanding the pedestrian wind environment in urban and architectural design contexts. Pedestrian Wind Comfort (PWC) focuses on the effects of wind on the safety and comfort of pedestrians and cyclists, given the influence of urban structures on the local microclimate. Traditional Computational Fluid Dynamics (CFD) methods used for PWC analysis have limitations in computation, cost, and time. Deep-learning models have the potential to significantly speed up this process. The prevailing state-of-the-art methodologies largely rely on GAN-based models, such as pix2pix, which have exhibited training instability issues. In contrast, our work introduces a convolutional neural network (CNN) approach based on the U-Net architecture, offering a more stable and streamlined solution. The process of generating a wind flow prediction at pedestrian level is reformulated from a 3D CFD simulation into a 2D image-to-image translation task, using the projected building heights as input. Testing on standard consumer hardware shows that our model can efficiently predict wind velocities in urban settings in real time. Further tests on different configurations of the model, combined with a Pareto front analysis, helped identify the trade-off between accuracy and computational efficiency. This CNN-based approach provides a fast and efficient method for PWC analysis, potentially aiding in more efficient urban design processes

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

Effect of different source terms and inflow direction in atmospheric boundary modeling over the complex terrain site of Perdigão

Assessing wind conditions in complex terrain requires computational fluid dynamics (CFD) simulations incorporating an accurate parameterization of forest canopy effects and Coriolis effects. This study investigates how incorporating source terms such as the presence of trees and the Coriolis force can improve flow predictions. Furthermore, the study examines the impact of using different sets of atmospheric boundary layer inflow profiles, including idealized profiles with a logarithmic velocity profile, and a set of fully developed profiles from a pressure-driven precursor simulation. A three-dimensional steady Reynolds-averaged Navier–Stokes (RANS) equations model is set up using OpenFOAM to simulate the flow over a complex terrain site comprising two parallel ridges near Perdigão, Portugal. A 7.5 km×7.5 km terrain of the Perdigão site is constructed from elevation data centered around a 100 m met-mast located on the southwest ridge. A 30 min averaged stationary period is simulated, which corresponds to near-neutral conditions at met-mast Tower 20 located at the southwest ridge. The period corresponds to the wind coming from southwest at 231∘ at 100 m height above ground at Tower 20. Five case setups are simulated using a combination of different source terms, turbulence models and inflow profiles. The prediction capability of these models is analyzed for different groups of towers on the southwest ridge and, on the towers further downstream inside the valley, on the northeast ridge. Including a canopy model improves predictions close to the ground for most of the towers on the southwest ridge and inside the valley. Large uncertainties are seen in field measurement data inside the valley, which is a recirculation zone, and large prediction errors are seen in the wind velocity, wind direction and turbulent kinetic profiles for most of the models. The predictions on the northeast ridge are dependent on the extent of recirculation predicted inside the valley. The inflow wind direction plays an important role in wind profile predictions.publishedVersio