Flow Around Curved Tandem Cylinders

The flow around curved tandem cylinders of equal diameter has been investigated for the first time, by means of direct numerical simulations. A convex configuration was used. The nominal gap ratio was L/D = 3.0 and a Reynolds number of 500 was chosen. Due to the change in effective gap ratio along the cylinder axis, there is a variation of tandem flow regimes, from alternating overshoot/reattachment, via stable reattachment, to co-shedding, in this case called gap shedding. The combination of reattachment and gap shedding gives near-zero drag and vertical forces for the downstream cylinder, whereas the corresponding forces on the upstream cylinder are comparable to single curved cylinders. Meanwhile, the opposite is true for the lift forces. A low-frequency variation of horizontal and vertical forces is seen, and this is attributed to a slow variation of the position where gap shedding commences. Finally, the concept of a critical angle is proposed to describe the transition to gap shedding, for a given combination of nominal gap ratio and Reynolds number.acceptedVersio

Real-Time Echocardiography Guidance for Optimized Apical Standard Views

Measurements of cardiac function such as left ventricular ejection fraction and myocardial strain are typically based on 2-D ultrasound imaging. The reliability of these measurements depends on the correct pose of the transducer such that the 2-D imaging plane properly aligns with the heart for standard measurement views and is thus dependent on the operator's skills. We propose a deep learning tool that suggests transducer movements to help users navigate toward the required standard views while scanning. The tool can simplify echocardiography for less experienced users and improve image standardization for more experienced users. Training data were generated by slicing 3-D ultrasound volumes, which permits simulation of the movements of a 2-D transducer. Neural networks were further trained to calculate the transducer position in a regression fashion. The method was validated and tested on 2-D images from several data sets representative of a prospective clinical setting. The method proposed the adequate transducer movement 75% of the time when averaging over all degrees of freedom and 95% of the time when considering transducer rotation solely. Real-time application examples illustrate the direct relation between the transducer movements, the ultrasound image and the provided feedback.publishedVersio

Kartlegging av radioaktive stoffer i Oslofjorden

Prosjektleder: Anders RuusDSA har ønsket å undersøke om utslipp av radioaktive stoffer fra helsesektoren kan gjenfinnes i miljøet og om I-131 tas opp og/eller oppkonsentrerer i Oslofjorden. Det ble ikke funnet I-131 eller Tc-99m i biota i detekterbare mengder. Dette tyder på lavt opptak, og liten tilknyttet risiko for organismer i Oslofjorden. Lave konsentrasjoner av Tc-99m og I-131 ble målt i noen vannprøver, samt I-131 i noen sediment prøver. Lu-177 ble ikke detektert i noen prøver. Disse observasjonene er ikke i konflikt med strømningsmønsteret i området, men partiklene skal også rekke å sedimentere til bunnen, tidsnok til at deteksjon er mulig (I-131 har halveringstid på 8 døgn). Selv med fordeling av prøvene over fire ulike prøveinnsamlingstidspunkter ble det for lang tid mellom prøvetaking og måling, med så lave konsentrasjoner av kortlevde radionuklider. Med så lave nivåer må man i fremtiden ta sikte på færre prøver og enda kortere tid mellom prøvesamling og måling, kanskje ved å fokusere på et mindre geografisk område. Resultatene gir et innblikk i situasjonen før en forventet økning i bruk av nukleærmedisin ved sykehusene.Direktoratet for strålevern og atomsikkerhet (DSA)publishedVersio

Studying Value-at-Risk in Equity Markets

Master's thesis in Industrial economicsValue-at-Risk, in financial risk management, is a central method for estimating and controlling risk exposure for financial institutions. Challenges with current VaR methods is that its imprecise, especially under times of financial turmoil where precise estimations are most crucial and current methods su er from too optimistic VaR estimations. Addressing these challenges and improving current methods is what serves as motivation for this paper. There are several methods for estimating VaR, and two of the most used methods is historical simulation and Monte Carlo simulation. And a study on how macroeconomic factors can improve these two methods is conducted and investigated for different confidence levels. In this thesis, we investigate and develop methods for how historical VaR and Monte Carlo simulation can be improved by implementing macroeconomic variables, such as VIX, brent price, gold price, and US treasury note. The data analysis is performed on the American stock market index S&P 500 and the Norwegian Oslo Stock Exchange, and a selection of sectors for these two markets, with a span of two decades. With the intent to test how precise the VaR methods estimates are across well diversified and more specialized portfolios. An investigation on how well the VaR methods perform in financial stressing times was done by sectioning the time series into periods, to differentiate between where a market shock has occurred and when the market is in equilibrium. The new versions of historical VaR and Monte Carlo simulation are compared to a benchmark model, historical VaR with 250 days rolling window, to see the advantages of implementing macroeconomic factors to these highly used methods. The conclusion of this thesis yield interesting results regarding how macroeconomic factors a ect VaR estimation and give a contribution to and validate research and studies previously done on VaR. We find that implementing VIX, and the other macros studied, to current VaR methods can improve the estimates, especially concerning optimistic VaR estimates during financial turmoil

Large Eddy Simulations of Flow around a Circular Cylinder in the Vicinity of a Wall at Reynold Number of 13100

The flow around a circular cylinder in the vicinity of a rigid wall at Reynolds number 13100 is simulated using Large Eddy Simulations (LES) with Smagorinsky subgrid scale model. The main purpose of the present work is to investigate the flow behavior and the forces exerted on the cylinder near a rigid wall. The simulations with gap-to-diameter ratios (G/D) of 0.2, 0.6 and 1 are carried out in order to investigate the modifications of the flow field and vortex shedding due to the presence of the wall. Influence of the incoming boundary layer profile is investigated through two simulations with the logarithmic boundary layer inlet profile of thicknesses 0.48D and 1.6D and a simulation with a uniform inlet profile. The velocity field in the cylinder wake as well as the hydrodynamic values and the pressure distribution on the cylinder surface are used to understand the physics of the flow and to separate the influences of the wall proximity and the shear layers interaction. The results are compared to the experimental results obtained by Particle Image Velocimetry (PIV) and point-type pressure measurements.acceptedVersio

Influences of upstream extensions on flow around a curved cylinder

In simulations of flow around a concave curved cylinder, i.e. free-stream aligned with the plane of curvature and directed towards the inner face of the curvature, one cannot avoid interactions between the cylinder and the inlet boundary. To get rid of the effects brought about by this interaction, we consider different lengths of upstream straight extensions at the lower end of the curved cylinder (0D, 5D and 10D, where D is the cylinder diameter), referred to as horizontal extensions. In this study, we directly solve the time-dependent three-dimensional Navier–Stokes equations. Results reveal that the appended horizontal extension allows the boundary layer to develop, so that the velocity profile at the curved cylinder inception is significantly different from the case where no horizontal extension is considered. The laminar boundary layer is thinner than that in the flat plate flow, which is given by Blasius’ solution. The results from 5D and 10D extensions show a clear convergent tendency. We therefore suggest that a horizontal extension is essential for concave curved cylinder flow simulation, and 10D would be a preferred choice

Turbulent wake behind a concave curved cylinder

We present a detailed study of the turbulent wake behind a quarter-of-ring curved cylinder at Reynolds number Re = 3900 (based on cylinder diamter and incoming ow velocity), by means of Direct Numerical Simulation (DNS). The con guration is referred to as a concave curved cylinder with incoming ow aligned with the plane of curvature and towards the inner face of the cylinder. Wake ows behind this con guration are known to be complex, but have so far only been studied at low Re. This is the rst DNS investigation of the turbulent wake behind the concave con guration, from which we reveal new and interesting wake dynamics, and present in-depth physical interpretations. Similar as in the low-Re cases, the turbulent wake behind a concave curved cylinder is a multi-regime and multi-frequency ow. However, in addition to the co-existing ow regimes reported at lower Re, we observe a new transitional ow regime at Re = 3900. The ow eld in this transitional regime is not dominated by von K arm an-type vortex shedding, but by periodic asymmetric helical vortices. Such vortex pairs exist also in some other wake ows, but are then non-periodic. Inspections reveal that the periodic motion of the asymmetric helical vortices is induced by vortex shedding in its neighboring oblique shedding regime. The oblique shedding regime is in turn in uenced by the transitional regime, resulting in a uni ed and remarkably low dominating frequency in both ow regimes. Due to this synchronized frequency, the new wake dynamics in the transitional regime might easily be overlooked. In the near-wake, two distinct peaks are observed in the time-averaged axial velocity distribution along the curved cylinder span, while only one peak was observed at lower Re. The presence of the additional peak is ascribed to a strong favorable base pressure gradient along the cylinder span. It is noteworthy that the axially-directed base ow exceeded the incoming velocity behind a substantial part of the quarter-of-ring and even persisted upwards along the straight vertical extension. As a by-product of our study, we nd that a straight vertical extension of 16 cylinder diameters is required in order to avoid any adverse e ects from the upper boundary of the ow domain

Influence of spanwise no-slip boundary conditions on the flow around a cylinder

First we present high resolution large eddy simulations (LES) for the flow past a circular cylinder at Reynolds number 3900 to prove the quality of the simulations. A thorough grid convergence study is presented, and the agreement in the mean flow statistics between our simulations and the references is excellent. Then we apply a no-slip boundary condition at the spanwise boundaries of the cylinder, with aspect ratios of 6, 12 and 24. This results in large changes in the shear layers and wake topology, even for an aspect ratio of 24. Even though the boundary layers along the side walls are only about 0.4 diameters thick, they nevertheless manage to stabilize the shear layers all the way through the channel, thus effectively delaying the roll-up and transition to a turbulent wake