<p>QC 20170517</p>

The cardiovascular system is an important part of the human body since it transports both energy and oxygen to all cells throughout the body. Diseases in this system are often dangerous and cardiovascular diseases are the number one killer in the western world. Common cardiovascular diseases are heart attack and stroke, which origins from obstructed blood flow. It is generally important to understand the causes for these cardiovascular diseases. The main causes for these diseases are atherosclerosis development in the arteries (hardening and abnormal growth). This transform of the arterial wall is believed to be influenced by the mechanical load from the flowing blood on the artery and especially the tangential force the wall shear stress. To retrieve wall shear stress information in arteries invivo is highly interesting due to the coupling to atherosclerosis and indeed a challenge. The goal of this thesis is to develop, describe and evaluate an in-vivo method for subject specific wall shear stress estimations in the human aorta, the largest artery in the human body. The method uses an image based computational fluid dynamics approach in order to estimate the wall shear stress. To retrieve in-vivo geometrical descriptions of the aorta magnetic resonance imaging capabilities is used which creates image material describing the subject specific geometry of the aorta. Magnetic resonance imaging is also used to retrieve subject specific blood velocity information in the aorta. Both aortic geometry and velocity is gained at the same time. Thereafter the image material is interpreted using level-set segmentation in order to get a three-dimensional description of the aorta. Computational fluid dynamics simulations is applied on the subject specific aorta in order to calculate time resolved wall shear stress distribution at the entire aortic wall included in the actual model. This work shows that it is possible to estimate subject specific wall shear stress in the human aorta. The results from a group of healthy volunteers revealed that the arterial geometry is very subject specific and the different wall shear stress distributions have general similarities but the level and local distribution are clearly different. Sensitivity (on wall shear stress) to image modality, the different segmentation methods and different inlet velocity profiles have been tested, which resulted in these general conclusions: The aortic diameter from magnetic resonance imaging became similar to the reference diameter measurement method. The fast semi-automatic level-set segmentation method gave similar geometry and wall shear stress results when compared to a reference segmentation method. Wall shear stress distribution became different when comparing a simplified uniform velocity profile inlet boundary condition with a measured velocity profile. The method proposed in this thesis has the possibility to produce subject specific wall shear stress distribution in the human aorta. The method can be used for further medical research regarding atherosclerosis development and has the possibility for usage in clinical work.

Filamentous fungi are of interest for biotechnologists particularly because of thefungi’s ability of producing commercial products after undergoing certain industrialprocess. Although because of the complicated and intricate internal mechanism ofthe fungi there are certain aspects which need to be studied to maximize the produc-tion output. A team at Chair of Measurement and Control Bioprocess Group at TUBerlin studies the internal behavior of the fungi when they are exposed to certainamount of wall shear stress (WSS) by performing small-scale experimentation. Forthis purpose a backward facing step (BFS) chamber is used.  This thesis work aims to perform Computational Fluid Dynamics (CFD) analysesto study the flow in the BFS chamber and to find appropriate locations to adherethe fungi spores on the chamber’s bottom wall. Commercially available CFD software Star CCM+ has been used for the CFD calcu-lations. The BFS chamber has been divided into two parts namely ’inflow channel’and ’step channel’ and simulations are performed separately. RANS model SST k-ωhas been used to simulate the flow in the inflow channel and Large Eddy Simulation(LES) model has been used in the step channel. The simulation result predicted that the streamwise WSS (WSS x ) is highest (≈ 8Pa) at the primary reattachment location downstream of the step. Due to reverseflow it is observed that WSS x is high (≈ 5 Pa) in the primary separation region.Standard deviation WSS x is highest (≈ 0.35 Pa) in the region around 28 x/h distancedownstream of the step on the bottom wall of the step channel and it is observedthat this is the region where the turbulence kinetic energy (TKE) is also maximumin the mean flow of the step channel. It is observed that there is small WSS x devi-ation in the primary reattachment region as well.                From the study it is concluded that the overall flow in the chamber is laminar withsome level of unsteadiness at few locations. To adhere the fungi spores on the bot-tom wall suitable location will be in the region where maximum variation in WSS xis observed. Keywords: Filamentous fungus, computational fluid dynamics, large eddy simula-tion, wall shear stress, backward facing step, turbulence kinetic energy

The focus of this work was to simulate the fluid flow within a melt pool geometry, during an additive manufacturing process, implementing the CFD software OpenFOAM version 1806. Two separate models were created and run during this work, the first using a temperature mapping from a finite element (FE) model and the second being a free-standing model with Gaussian distributed laser beam striking down on the top surface. Both models were run with the standard solver icoReactingMultiphaseInterFoam, being a multiphase solver, with phase transition possibilities. Addition of gas particles was carried out during post-processing and these were to visualise the imperfections caused by melting metal alloys in a 3D printing case. During comparison of the movement of the free-standing model, using a moving laser beam, to the mapped temperature FE model, it was revealed that the fluid flow in the molten pool was heavily influenced by the pressure introduced by the laser beam. No streamlines were found that would indicate entrapment of gas particles during solidification. / Fokuset på detta arbete var att simulera vätskeflöde i en smältpool-geometri, under en additiv-tillverkningsprocess. Detta implementerades med hjälp av CFD-mjukvaran OpenFOAM, version 1806. Två separata modeller skapades och simulerades under arbetets gång. Den första modellen kördes med hjälp av en mappning av temperaturfältet från finita-element-modellen (FE-modellen) och den andra modellen var en fristående modell tillsammans med en Gaussisk distribuerad laserstråle riktad ned på översta ytan. Båda simuleringarna använde sig av standardlösaren icoReactingMultiphaseInterFoam, vilket är en multifas-lösare, med möjlighet till fasövergångar. Tillägg av gaspartiklar utfördes under post-processing och dessa var för att visualisera porer som kan uppstå under smältning av metall-legering i en 3D-utskrivningsprocess. Vid jämförelse av den fristående modellen, som implementerade en rörlig laserstråle, till den mappade FE-modellen, uppdagades det att vätskeflödet i smältpoolen influerades starkt av trycket som orsakades av lasern. Inga strömlinjer tydde på en inkapsling av gaspartiklar under stelning.

Airflow management is crucial for the performance of cooling systems in data centers. The amount of energy consumed by data centers is huge and a large amount is related to the cooling. In attempts to develop energy efficient data centers, numerical methods are important for several reasons. Experimental procedures are more expensive and time consuming but when done carefully, experiments provide trustful results that can be used to validate simulations and give additional insights. Numerical methods in their turn have the advantage that they can be applied to proposed designs of data centers before they are built and not only to already existing data centers. In this study, Computational Fluid Dynamics (CFD) is used to study the airflow in data centers. The aim is to use an experimentally validated CFD model to investigate the effects of using different designs in data centers with respect to the performance of the cooling systems. Important parameters such as quality of the computational grid, boundary conditions and choice of turbulence model must be carefully considered in order for the results from simulations to be reliable. In Paper A, a hard floor configuration where the cold air is supplied directly into the data center is compared to a raised floor configuration where the cold air is supplied into an under-floor space instead and enters the data center through perforated tiles in the floor. In Paper B, the performance of different turbulence models are investigated and velocity measurements are used to validate the CFD model. In Paper C, the performance of different cooling systems is further investigated by using an experimentally validated CFD model. The effects of using partial aisle containment in the design of data centers are evaluated for both hard and raised floor configurations. Results show that the flow fields in data centers are very complex with large velocity gradients. The k − ε model fails to predict low velocity regions. Reynolds Stress Model (RSM) and Detached Eddy Simulation (DES) produce very similar results and based on solution times, it is recommended to use RSM to model the turbulent airflow in data centers. Based on a combination of performance metrics where both intake temperatures for the server racks and airflow patterns are considered, the airflow management is significantly improved in raised floor configurations. Using side covers to partially enclose the aisles performs better than using top covers or open aisles.

Time resolved pressure measurements in hydropower applications are of great interest. Different parts of the machine experience highly transient flows that influence the function and efficiency of the turbine. This thesis addresses different time dependent pressure phenomena. Assessment of the efficiency of a hydropower plant requires accurate flow measurements. Gibson's method is a pressure time based method to measure the flow rate. To improve the method outside its standard range an experimental investigation is performed in a laboratory setup in parallel with numerical solutions of the governing equations. The results indicate that it is possible to correct the flow measurements outside the limitations of the standard. A draft tube is an integrated part of a hydropower plant with a reaction turbine where the remaining kinetic energy of the flow after the turbine is converted into pressure. An experimental investigation is performed on a model hydropower draft tube at Älvkarleby to establish the details of the pressure recovery in the early part of the draft tube. The objective is to increase the understanding of the pressure behaviour and to contribute with data for CFD (Computational Fluid Dynamics) validations. The results show a high damping of the oscillating parts of the pressure in the axial direction. From earlier investigations done as part of the Turbine-99 workshops, it has been observed that the radial pressure distribution just under the turbine runner show a marked discrepancy between experiments and CFD. The flow in the region is highly time dependent so the behaviour of the Pitot tube used for the pressure measurements is investigated for oscillating flow in a lab setup. A method to derive more accurate data is proposed. / <p>Godkänd; 2006; 20070109 (haneit)</p>

The utilization of rivers for hydropower production leads to problems for fish migration. Migratory fish that swim upstream for reproduction need to overcome obstructions to reach their spawning grounds. On their way upstream they follow high water velocities. Since most of the water in regulated rivers flow through the power plants the fish is often attracted to the turbine outlets. To guide the fish past the power plants, fishways are often used. However the efficiency is often low due to inefficient attraction water. An attraction channel that uses a small fraction of the tailwater, or any free stream, is studied. The channel is open and U-shaped. A local acceleration of the water is created by changing the cross sectional area in the downstream end of the channel. The flow in the channel is measured with LDV in a lab setup to examine the acceleration of the water, and in full-scale to investigate the fish tendency to use the channel. The results show that the velocity out of the channel can be as much as 38 % higher than the approaching flow. The acceleration is detectable downstream the channel up to 18 times the exit water depth of the channel. The results from the field work shows that fish do use the channel and it is important that it is painted dark. / <p>Godkänd; 2006; 20070109 (haneit)</p>

Recent studies have shown that nanoparticles may be more toxic than larger particles of the same material, but the health risks associated with widespread use largely depend on the extent of exposure. When dealing with potentially toxic particles, precautionary measures have to be taken in order to minimize contact. For larger particles, mechanical filtering is commonly used. Nanoparticles, however, are too small to be effectively impeded by these filters and thus alternative methods need to be developed. Experiments are performed where clusters of carbon nanotubes are dropped vertically into a region with an electric field, generated between two parallel plates. The clusters are strongly affected by the field and move swiftly towards the electrodes. In this setup, most clusters simply bounce between the electrodes. By adding an electrically insulating layer to one of the plates, however, the particles get stuck. This implies that electrostatic filtration is an effective means of collecting airborne carbon nanotubes. Nanoparticles may enter human lung regardless if filtration is used or not. To examine the health risks, therefore, knowledge of transport and deposition properties of aerosol particles in lung flows is necessary. This information is also essential in the optimization of targeted drug delivery with pharmaceutical aerosols. In vivo and in vitro studies are cost-intensive and difficult to perform for studying particle deposition in the airways. Hence, numerical simulations constitute a valuable complement. The extent and location of particle deposition depend on particle properties, airway geometry and breathing pattern. To start with, Computational Fluid Dynamics simulations are performed for spherical particles, 15 nanometer to 50 micrometer in diameter, in a multiply bifurcated asymmetric 3D model, representing trachea to the segmental bronchi. Steady, laminar flow is considered for inhalation flow rates of 0.1 and 0.5 l/s. The largest particles are captured near the first bifurcation, whereas smaller microparticles are less efficiently, but more uniformly, deposited. The site of deposition is also affected by geometric asymmetry. The nanoparticles essentially follow the streamlines and travel unaffected through the region modeled. Thus, transport to the distal airways can be assumed extensive. Because of their specific shape, fibers may cause additional harm compared to spherical particles. Asbestos is a well-known example of hazardous fibrous materials. More recently, this has also called for concern on the extended use of nanotubes. A numerical model is developed for fiber transport in the respiratory airways. The coupled equations for fiber rotation and translation are solved using MATLAB. The model is valid for arbitrary Stokes flows at low particle concentrations and for particle sizes from nano- to the micro range. The results suggest that the potential of a fiber to reach the distal airways increases with increased fiber aspect ratio, regardless of particle size. / <p>Godkänd; 2008; 20080428 (ysko)</p>

Cellulose material is processed to pulp suspensions and MDF boards in order to produce products such as papers, magazines, laminate floors or door skins. A critical stage of these processes is when the cellulose fiber networks are compressed to specific densities and when most of the fluid originally positioned between and inside the fibers is forced to leave the network. The fiber network is then exposed to a drag force generated by the flow. The magnitude of this force is dependent upon how easy the fluid can flow through the network, which is commonly described by its permeability. In addition to the permeability, which relates to the drag on each fiber, there is a solid network force. The response to this force from the fiber network is often termed as the compressibility of it. Hence, to be able to model and predict the compression stage in cellulose material related processes these two material properties must be known. In this thesis two equipments to measure the permeability of MDF networks and pulp suspensions are evaluated and a neat model for a part of the MDF- compression stage is developed. A reference material consisting of spherical particles and relevant fiber networks are used as test objects for the equipments enabling a comparison to theoretical models and existing experimental results. The outcome is that correct enough permeability data are obtained with respective equipment as long as Reynolds number is sufficiently low. The equipments are then used to study different materials showing, for instance, that highly compressed MDF-networks are strongly anisotropic as to permeability and that the tested hardwood pulps have an overall higher permeability compared to the softwood pulps investigated. It was also found that the permeability of the pulps was not influenced by different mechanical treatments of the fiber network, as long as the geometrical dimensions of the fibers were constants. / <p>Godkänd; 2006; 20070109 (haneit)</p>

Noise control and sound quality analysis are important, since noise has been registered to be a predominant factor in stress and a source of great annoyance. Traffic noise is a problem and a major part of this noise comes from heavy vehicles. The only legislative requirement for heavy-duty trucks regarding noise emissions, is that the noise level does not exceed an Aweighted sound pressure level of 80 dB. The specification of an A-weighted sound pressure level is, however, not an adequate description of psychoacoustic annoyance and therefore work towards defining a better description of loudness is one of the principal fields of acoustics today. Sound radiation from trucks is speed-related. At medium and high speeds, the overall noise level is comprised mainly of the tyre noise, whereas at low speed and during acceleration, exhaust noise and noise from the engine and transmission structure are predominant. In front of the truck, the noise from the engine and especially that from the timing transmission cover, the torsional damper and the oil sump, comprises a greater proportion of the total noise. The aim of sound quality analysis of diesel engines is to find cost-efficient methods of reducing sound radiation and of changing the character of the sound in order to minimise annoyance. This thesis concerns the development of experimental methods for analysing the sound quality of diesel engines, and focuses on measurement of acoustic intensity, multivariate data analysis, structural modification and subjective assessment of engine noise. The applicability of the FFT-based sound intensity method is evaluated. It is found that the intensity measurements may be influenced by high reactivity, interference due to partlycoherent sources, difficulties in performing the spatial average, real-time limitations and engine speed variations. Scanning the intensity probe, preferably by a robot, is necessary when measuring within narrow bands to avoid interference problems. Scanning achieves more reliable estimates of sound power and intensity vectors. Experimental design and the multivariate techniques, principal components analysis (PCA) and partial least squares (PLS) were utilised to facilitate interpretation of intensity measurements. The results show that PCA and PLS enable independent phenomena in the sound field to be extracted and which can thereby be visualised by principal spectra and principal radiating patterns. The characteristics of sound radiation are determined by designed experiments, sound intensity measurements and operational deflection shape estimations. These methods enable the effects on sound radiation of structure modifications to be predicted. An annoyance index for in-line 6-cylinder diesel engines in stationary running conditions was developed using multivariate statistics. The index is based on engine sounds resulting from structure modifications and changes in fuel. The annoyance level was measured during listening tests of sound stimuli recorded in stereo and reproduced by loudspeakers under anechoic conditions. The different sound stimuli were ranked using paired comparisons or the method of successive intervals. It was found that 94% of the variance of annoyance can be explained by a model based on loudness (Sone), sharpness (Acum) and harmonic ratio (rumble). Impulsiveness, roughness and tonality were other important criteria used in the study and which were found to have a relationship with specific speed ranges. The annoyance was minimised by an increase in stiffness in the lower part of the engine achieved by using a ladder frame in combination with a bearing beam. / Godkänd; 1996; 20061214 (biem)