On the laminar–turbulent transition mechanism on megawatt wind turbine blades operating in atmospheric flow

Among a few field experiments on wind turbines for analyzing laminar–turbulent boundary layer transition, the results obtained from the DAN-AERO and aerodynamic glove projects provide significant findings. The effect of inflow turbulence on boundary layer transition and the possible transition mechanisms on wind turbine blades are discussed and compared to CFD (computational fluid dynamics) simulations of increasing fidelity (Reynolds-averaged Navier–Stokes, RANS; unsteady Reynolds-averaged Navier–Stokes, URANS; and large-eddy simulations, LESs). From the experiments, it is found that the transition scenario changes even over a single revolution with bypass transition taking place under the influence of enhanced upstream turbulence, for example, such as that from wakes, while natural transition is observed in other instances under relatively low inflow turbulence conditions. This change from bypass to natural transition takes place at azimuthal angles directly outside the influence of the wake indicating a quick boundary layer recovery. The importance of a suitable choice of the amplification factor to be used within the eN method of transition detection is evident from both the RANS and URANS simulations. The URANS simulations which simultaneously check for natural and bypass transition match very well with the experiment. The LES predictions with anisotropic inflow turbulence show the shear-sheltering effect and a good agreement between the power spectral density plots from the experiment and simulation is found in case of bypass transition. A condition to easily distinguish the region of transition to turbulence based on the Reynolds shear stress is also observed. Overall, useful insights into the flow phenomena are obtained and a remarkably consistent set of conclusions can be drawn.</p

Towards Improved Scale-Resolving Modeling and Simulations of Turbulent Flows

Scale-resolving simulations are viewed as powerful means for predicting complex turbulent flows, as often encountered in aeronautical applications. However, since turbulent scales span over a considerable range from the smallest Kolmogorov scales to the largest of equivalence to configuration size, scale-resolving computations are often demanding on computational resources and, furthermore, on the underlying numerical methods used in the simulations. Nonetheless, hybrid RANS (Reynolds-Averaged Navier-Stokes)-LES (Large-Eddy Simulation) techniques are considered computationally accurate and affordable for aeronautical industry applications. This thesis explores and develops numerical methods suitable for hybrid RANS-LES. These methods are implemented in the Computational Fluid Dynamics (CFD) solver M-Edge

Liquid metal flows in continuous casting molds: A numerical study of electromagnetic flow control, turbulence and multiphase phenomena

Der Effekt eines externen Magnetfeldes auf die mehrphasige und turbulente Strömung in Stranggußkokillen und deren Wechelspiel führt in den wissenschaftlichen Arbeiten zu widersprüchlichen Aussagen. Die verschiedenen Prozessparameter können innerhalb eines kleinen Varianzbereichs entscheidenden Einfluss auf die Aussage haben, ob ein Magnetfeld begünstigend oder schädigend auf die Qualität des Produkts wirkt. Um wichtige Einflussfaktoren zu identifizieren, werden daher numerische Strömungssimulationen des Prozesses durchgeführt. Dazu wird zunächst ein mehrphasiger und inkompressibler Mehrregionen-CFD-Löser für magnetohydrodynamische Strömungen entwickelt und validiert, um die komplexe Strömung in einer Stranggußkokille mit hoher Genauigkeit simulieren zu können. Darauf aufbauend wird das numerische Setup anhand einer Modellkokille mit aktuellen Messdaten validiert. Durch die neuartige Kombination Lagrange'scher Lösungsmethoden mit angepassten Termen für die Magnetohydrodynamik sowie der skalenaufgelösten magnetohydrodynamischen Turbulenz, können erstmals Aussagen zur optimalen Magnetfeldverteilung im Hinblick auf Strömungsstabilität, Turbulenzmodulation und Blasenverteilung getroffen werden. Mit Hilfe dieses Wissens können neuartige Konzepte elektromagnetischer Bremssysteme für den Stranggußprozess entwickelt werden

Analysis of injection, mixture formation and combustion processes for innovative CNG Engines

Natural gas is a promising alternative fuel for internal combustion engines application due to its low carbon content and high knock resistance. The work presented in this thesis deals with the fluid dynamics, experimental study and optimization of different technologies aimed at exploiting the potentials of such fuel at best. The first section of the work is aimed at the combustion chamber optimization with the focus on the combustion stability. The engine considered in the study is a prototype specifically dedicated to CNG. It features a variable valve actuation system and has been released with different and very high compression ratios ranging from 12 to 14. An innovative experimental methodology based on hot wire anemometry (HWA) purposely developed by Centro Ricerche Fiat (CRF) has been adopted for the characterization of the steady-state tumble. The HWA method has been validated against the well-known Ricardo method and is used as a basis for the development and validation of a numerical “virtual flow bench”. The numerical model has been used to gain a deeper insight into the fluid dynamic phenomena and to replace the experimental campaign considering a head variant and quantifying its tumbling and volumetric performances. A transient 3D CFD analysis for the complete engine cycle has been performed in order to evaluate the effect on the combustion process of different compression ratios and head designs.The results showed that the HWA technique represents a factual alternative to the integral technique for the tumble characterization. The “Virtual flow box” model turned out to be accurate enough to evaluate the main flow motions induced by the head design and to be a valid tool complementary to the experimental method. Finally, the transient model used in combination with the ECFM-3z combustion model is fairly accurate for the comparative analysis between different engine designs and/or valve actuations. Despite the main findings of the flow model activity, importance should also be placed onto advanced technologies for natural gas engines such as direct injection. Thus, the second section is aimed at the numerical study of a natural gas direct injection engine. The numerical complexity caused by the high pressure ratio at nozzle exit has been faced using an accurate mesh procedure able to correctly capture the formation of shocks structures. Moreover, the actual needle geometry and the realistic needle movement has been taken into account in order to correctly simulate the opening and closing transient. The final mix and turbulence level has been evaluated comparing two engine prototypes and considering several injection strategies. Finally, a qualitative validation of the computational model has been performed comparing the simulation results with the available experimental data obtained through the PLIF procedure on an equivalent optical engine. The CFD model resulted to be accurate in the prediction of the mixing quality and it shows to be a reliable tool for the analysis of the main mixing mechanism and so for the assessment of the best injection strategy

Numerical analysis for active control of drag over flat plate using sinusoidal travelling wave method

The drag is present due to flow around bodies such as vehicles, aircraft, bullet trains or ships etc. It plays a significant role in vehicle performance, rate of fuel consumption and stability. This drag depends mainly on flow, fluid and surface properties. Different methods are used for drag reduction like controlling surface roughness and injecting long-chain polymers etc. In the current research, a method based on the generation of a spanwise mean velocity gradient in a flat plate boundary layer by applying a sinusoidal travelling wave in the spanwise direction before the transition to turbulence occurrence is used for the drag reduction purpose. A solver is developed using the open-source CFD software OpenFOAM libraries. It consists of routines for generating synthetic isotropic homogeneous turbulence at the inlet plane of the channel flow and solving the Navier Stokes equations using the Monotonically Integrated Large Eddy Simulation method (MILES). The implemented inlet boundary condition showed improvements in the predictions of turbulence structures within a streamwise distance of approximately 5 times the half channel height (δ) from the inlet plane, a shorter distance than the other similar previous boundary conditions can predict. These improvements resulted in a considerable reduction in overall channel length required for numerical simulations and hence reduction in the associated computation costs. In addition, a prediction method for getting the required friction Reynolds number for setting up any simulation case is been developed and verified. The MILES solver and the inlet boundary condition are used together to generate the bypass transition through the channel flow. A travelling sinusoidal wave in the spanwise direction is applied by the effect of blowing and suction from the bottom wall of the channel before the bypass transition occurrence. This travelling wave leads to a drop in the value of the skin friction coefficient which means a drop in the drag. This may be due to the increase of the mixing effect of the eddies and the induced streamwise travelling wave. The bypass transition onset also appears to be delayed when an inclined sinusoidal wave is applied in the streamwise direction. However, grid independence was not thoroughly tested in this study

Aircraft Noise

Noise generated by aircraft continues to be a pressing issue for society, as an increasing number of people residing in close proximity to airports make noise complaints on a regular basis. The reduction in aircraft noise is therefore a very important engineering task that would require the careful identification of different acoustic sources around the airplane, the understanding of noise source behavior and ranking along flight trajectories, sophisticated measurement techniques, and robust and accurate numerical tools aimed at predicting the generation of noise, the propagation through the atmosphere, and the resulting noise impact along approach and departure flights. For an overall assessment of the situation, it has to be assessed along entire flight trajectories rather than assessing limited operating conditions only. Furthermore, it is highly recommended to apply multiple acoustic metrics and account for different and widespread observer locations along the flight. Only then can the overall situation be adequately captured. Obviously, this is a highly multidisciplinary effort and no single discipline can address this problem. This reprint includes selected research studies with that multidisciplinary context that deal with numerical or experimental investigations that range from the investigation of specific noise sources to the assessment of noise generated by the overall aircraft in operation. Both basic and applied research studies involving the modelling and simulation of aircraft noise are included

Numerical studies of fluid-particle dynamics in human respiratory system

 This thesis investigates particle inhalation and its deposition in the human respiratory system for therapeutic and toxicology studies. Computational Fluid Dynamics (CFD) techniques including the Lagrangian approach to simulate gas-particle flows based on the domain airflow are used. The Lagrangian approach is used as it tracks each individual particle and determines its fate (e.g deposition location, or escape from computational domain). This has advantages over a Eulerian approach for respiratory inhalation flows as the volume fraction of the second phase can be neglected and a disperse phase for one-way coupling can be used. However, the very first step is to simulate and detail airflow structures. For the external airflow structures, the heat released from the human body has a significant effect on the airflow micro-environment around it in an indoor environment, which suggests that the transport and inhalation characteristics of aerosol particulates may also be affected since they are entrained by the air and their movement is dependent on the airflow field. Emphasis was put on the effect of human body heat on particle tracks. It was found that body heat causes a significant rising airflow on the downstream side of the body, which transports particles from a lower level into the breathing zone. The importance of body heat decreases with increasing indoor wind speed. Since the rising airflow exists only on the downstream side of an occupant, the occupant-wind orientation plays an important role in particle inhalation. The effect of body heat has to be taken into account when an occupant had his or her back to the wind, and the effect of body heat could be neglected when the occupant is facing the wind. A CFD model that integrates the three aspects of contaminant exposure by including the external room, human occupant with realistic facial features, and the internal nasal-trachea airway is presented. The results from the simulations visualize the flow patterns at different contaminant concentrations. As the particles are inhaled, they are transported through the respiratory airways, where some are deposited onto surrounding mucus walls while others may navigate through the complex geometry and even reach the lung airways, causing deleterious health effects. The studies in this thesis demonstrated that the transport and deposition of micron sized particles are dominated by its inertial property while submicron and nano sized particles are influenced by diffusion mechanisms. Studies based on an isolated model of the human nasal cavity or tracheobronchial airway tree rely on idealised inlet boundary condition imposed at the nostril or where, were a blunt, parabolic or uniform profile is applied. It is apparent that an integrated model made up of: i) room and ventilation, ii) aspiration efficiency, iii) and particle deposition efficiencies in the respiratory airway is needed. This leads to a more complete and holistic set of results, which can greatly contribute towards new knowledge in identifying preventative measures for health risk exposure assessment. With regards to the internal airflow structures and particle inhalation, ultrafine particle deposition sites in the human nasal cavity regions often omit the paranasal sinus regions. Because of the highly diffusive nature of nanoparticles, it is conjectured that deposition by diffusion may occur in the paranasal sinuses, which may affect the residual deposition fraction that leaves the nasal cavity. Thus a nasal-sinus model was created for analysis. In general there was little flow passing through the paranasal sinuses. However, flow patterns revealed that some streamlines reached the upper nasal cavity near the olfactory regions. These flow paths promote particle deposition in the sphenoid and ethmoid sinuses. Some differences were discovered in the deposition fractions and patterns for 5 and 10nm particles between the nasal-sinus and the nasal cavity models. This difference is amplified when the flow rate is decreased and at a flow rate of 4L/min the maximum difference was 17%. It is suggested that future evaluations of nanoparticle deposition should consider some deposition occurring in the paranasal sinuses especially if flow rates are of concern. Inhaled particles with pharmacological agents (e.g. histamine, methacholine) are introduced into the nasal cavity for targeted delivery. Effective nasal drug delivery is highly dependent on the delivery of the drug from the nasal spray device. Atomization of liquid spray occurs through the internal atomizer that can produce many forms of spray patterns and two of these, hollow-cone and full-cone sprays, are evaluated in this study to determine which spray pattern produced greater deposition in the middle regions of the nasal cavity. Past studies of spray particle deposition have ignored the device within the nasal cavity. Experimental measurements from a Particle Droplet Image Analyzer (PDIA) were taken in order to gain confidence to validate the initial particle conditions for the computational models.. Subsequent airflow patterns and its effects on particle deposition, with and without a spray device, are compared. Contours and streamlines of the flow field revealed that the presence of a spray device in the nasal vestibule produced higher levels of disturbed flow, which helped the dispersion of the sprayed particles. Particle deposition was found to be high in the anterior regions of the nasal cavity due to its inertia. Evaluation of the two spray types found that hollow spray cones produced more deposition in the middle regions of the nasal cavity

Gas mixing in anaerobic digestion

Mesophilic anaerobic digestion is one of the most used and successful technologies to treat the sludges resulting from wastewater treatment. However, traditional approaches to digester design are firmly rooted in empiricism and rule of thumb rather than science. Mixing is an energy-intensive operation, and therefore the need to lower the wastewater process carbon footprint requires searching how to lower the input mixing energy without compromising–and indeed enhancing–biogas production. In particular, the literature on gas mixing is still particularly poor. For the first time, an Euler-Lagrangian CFD model was developed for gas mixing in anaerobic digestion. The model was validated against laboratory experiments with PIV and PEPT techniques. Full-scale simulations reproducing a real digester were performed with the validated model, and different scenarios were reproduced. Shear rate distribution was used as a parameter to assess the most appropriate value of input mixing power. The simulations also low-viscosity flow patterns for the first time. This phenomenon is intrinsically linked to the non-Newtonian nature of sludge, and leads to short-circuited mixing. Switching biogas injection between two different nozzle series was found to be an effective strategy to mitigate the issue of the low-viscosity flow patterns. Final recommendations on input mixing power and switching time were given to improve mixing efficiency in the full-scale design taken into consideration. A journal paper published in Water Research and a conference paper presented at the Fifteenth International Conference on Civil, Structural and Environmental Engineering Computing (Civil-Comp) were produced. Two other papers are currently in preparation