Bjørn Anders Pettersson Reif

Direct numerical simulation of transitional flow in a patient-specific intracranial aneurysm

In experiments turbulence has previously been shown to occur in intracranial aneurysms. The effects of turbulence induced oscillatory wall stresses could be of great importance in understanding aneurysm rupture. To investigate the effects of turbulence on blood flow in an intracranial aneurysm, we performed a high resolution computational fluid dynamics (CFD) simulation in a patient specific middle cerebral artery (MCA) aneurysm using a realistic, pulsatile inflow velocity. The flow showed transition to turbulence just after peak systole, before relaminarization occurred during diastole. The turbulent structures greatly affected both the frequency of change of wall shear stress (WSS) direction and WSS magnitude, which reached a maximum value of 41.5Pa. The recorded frequencies were predominantly in the range of 1-500Hz. The current study confirms, through properly resolved CFD simulations that turbulence can occur in intracranial aneurysms.

Variational multiscale turbulence modelling in a high order spectral element method

In the variational multiscale (VMS) approach to large eddy simulation (LES), the governing equations are projected onto an a priori scale partitioning of the solution space. This gives an alternative framework for designing and analyzing turbulence models. We describe the implementation of the VMS LES methodology in a high order spectral element method with a nodal basis, and discuss the properties of the proposed scale partitioning. The spectral element code is first validated by doing a direct numerical simulation of fully developed plane channel flow. The performance of the turbulence model is then assessed by several coarse grid simulations of channel flow at different Reynolds numbers.

Flow and swell noise in marine seismic data

Various weather-related mechanisms for noise generation during marine seismic acquisition were addressed from a fluid-dynamic perspective. This was done by analyzing a number of seismic lines recorded on modern streamers during nonoptimal weather conditions. In addition, we examined some of the complex fluid-mechanics processes associated with flow that surrounds seismic streamers. The main findings were that noise in the 0–2-Hz range is mostly the result of direct hydrostatic-pressure fluctuations on the streamer caused by wave motion. For normal swell noise above 2 Hz and for crossflow noise, a significant portion of the observed noise probably comes from dynamic fluctuations caused by the interaction between the streamer and fluid structures in its turbulent boundary layer. This explanation differs from most previous work, which has focused on streamer oscillations, bulge waves inside old fluid-filled seismic streamers, or strumming/tugging as the main source of weather-related noise. Although modern s...

Numerical simulations of the pulsating flow of cerebrospinal fluid flow in the cervical spinal canal of a Chiari patient

The flow of cerebrospinal fluid (CSF) in a patient-specific model of the subarachnoid space in a Chiari I patient was investigated using numerical simulations. The pulsating CSF flow was modeled using a time-varying velocity pulse based on peak velocity measurements (diastole and systole) derived from a selection of patients with Chiari I malformation. The present study introduces the general definition of the Reynolds number to provide a measure of CSF flow instability to give an estimate of the possibility of turbulence occurring in CSF flow. This was motivated by the fact that the combination of pulsating flow and the geometric complexity of the spinal canal may result in local Reynolds numbers that are significantly higher than the commonly used global measure such that flow instabilities may develop into turbulent flow in these regions. The local Reynolds number was used in combination with derived statistics to characterize the flow. The results revealed the existence of both local unstable regions and local regions with velocity fluctuations similar in magnitude to what is observed in fully turbulent flows. The results also indicated that the fluctuations were not self-sustained turbulence, but rather flow instabilities that may develop into turbulence. The case considered was therefore believed to represent a CSF flow close to transition.

Post-blast explosive residue – a review of formation and dispersion theories and experimental research

The presence of undetonated explosive residues following high order detonations is not uncommon, however the mechanism of their formation, or survival, is unknown. The existence of these residues impacts on various scenarios, for example their detection at a bomb scene allows for the identification of the explosive charge used, whilst their persistence during industrial explosions can affect the safety and environmental remediation efforts at these sites. This review article outlines the theoretical constructs regarding the formation of explosive residues during detonation and their subsequent dispersal and deposition in the surrounding media. This includes the chemical and physical aspects of detonation and how they could allow for undetonated particles to remain. The experimental and computational research conducted to date is presented and compared to the theory in order to provide a holistic review of the phenomenon.

Investigation of flow and flow noise around a seismic streamer cable

In marine seismic explorations, flow noise from the turbulent boundary layer that forms around a streamer cable due to its relative motion through water significantly affects the quality of collected data. Understanding this noise generation mechanism is valuable for the development of future seismic streamer cables. We qualitatively characterize the area of turbulent flow surrounding a seismic streamer cable, and relate this characteristic to the statistics of the measured noise signal. The main finding is that the boundary layer thickness around a seismic streamer appears to be about 25 cm in an ocean environment. This is significantly larger than the thickness of 2.5 to 5 cm that has been reported in the literature from laboratory experiments. We attribute this discrepancy to the unsteadiness of the ocean environment. Estimations of the spatial extent of the recorded boundary layer noise indicate that the “optimal” hydrophone separation needs to be about 0.5 m for the noise to be uncorrelated. The sign...

Tracking Legionella in air generated from a biological treatment plant - A case study of the outbreak of legionellosis in Norway-

Two outbreaks of legionellosis occurred in the Sarpsborg/Fredrikstad region southeast of Norway in 2005 and 2008 where more than 60 exposed individuals were infected and 10 case patients died. The air scrubber at Borregaard, a wood-based chemical factory, was identified as the outbreak source. High concentration levels of Legionella species, including the etiological agent L. pneumophila SG1 was found in the aeration ponds, which belongs to Borregaards biological treatment plant. Results showed that these ponds were able to generate Legionella-containing aerosols that were transported by the wind as such aerosols were measured up to 200 meters downwind of the pond. Our studies did not detect L. pneumophila SG1 isolates, only L. pneumophila SG4 during the air sampling measurement campaign. Furthermore, the operational conditions of the air scrubber proved to be harsh for Legionella growth as the outbreak L. pneumophila strains were not able to grow at 45ºC and pH8 (conditions during the outbreaks). These results, together, lead us to suggest that the aeration pond should be regarded as the primary amplifier and disseminator of Legionella and L. pneumophila and thereby most likely being the outbreak source.

Reducing high Reynolds number hydroacoustic noise using superhydrophobic coating

The objective of this study is to assess and quantify the effect of a superhydrophobic surface coating on turbulence-generated flow noise. The study utilizes results obtained from high Reynolds-number full-scale flow noise measurements taken on a commercial seismic streamer and results from low Reynolds-number direct numerical simulations. It is shown that it is possible to significantly reduce both the frictional drag and the levels of the turbulence generated flow noise even at very high Reynolds-numbers. For instance, frequencies below 10 Hz a reduction in the flow noise level of nearly 50% was measured. These results can be attributed to a reduced level of shear stress and change in the kinematic structure of the turbulence, both of which occur in the immediate vicinity of the superhydrophobic surface.

Direct Numerical Simulation of Transitional Flow in a Patient-Specific MCA Aneurysm

Stroke is a leading cause of death in the Western world (1). One reason for stroke is the rupture of aneurysms usually found in or near the Circle of Willis, an arterial network located at the base of the brain. It is estimated that 1–6% of the population harbor aneurysms during their lifetime (2), and that the average risk of rupture is 1–2% annually. The initiation, growth, and rupture of intracranial aneurysms are complex and multi-factorial processes. The precise mechanism however, is still not known. If the wall shear stress (WSS) exceeds a limit of 40 Pa, the endothelial cells are believed to be damaged, and an aneurysm might form (3) (4).Copyright