A study of the effect of wake passing on turbine blades

SIGLELD:D50632/84 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

What happens to pressure when a flow enters a side branch?

The behaviour of incompressible side-branching flows is examined theoretically at high Reynolds numbers and compared with direct numerical simulation at moderate Reynolds numbers. The theoretical model assumes the branching (daughter) tube is small compared to the main (mother) tube and that the branching angle is small. The theory is applicable to steady and unsteady flows in two or three dimensions, and to a broad range of flow splits between mother and daughter vessels. The first main result of the work is that, in the vicinity of the branch, the flow adjusts to the imposed downstream pressure in the daughter tube through a jump (a rapid change over a short length scale) in flow properties across the daughter entrance. It is shown that, for large pressure drops in the daughter tube, fluid is sucked in at high velocities from the mother and thereby provides a favourable upstream feedback. This counteracts the tendency of the flow to separate from what would otherwise be an adversely shaped upstream wall. Increased divergence of mother and daughter tubes can thus be achieved at high daughter flow rates without separation. The second main result of the work is that the direct numerical simulations confirm the very rapid variation in flow properties and show reasonable agreement with the theory at moderate Reynolds numbers

Computational fluid dynamics benchmark dataset of airflow in tracheas

Computational Fluid Dynamics (CFD) is fast becoming a useful tool to aid clinicians in pre-surgical planning through the ability to provide information that could otherwise be extremely difficult if not impossible to obtain. However, in order to provide clinically relevant metrics, the accuracy of the computational method must be sufficiently high. There are many alternative methods employed in the process of performing CFD simulations within the airways, including different segmentation and meshing strategies, as well as alternative approaches to solving the Navier–Stokes equations. However, as in vivo validation of the simulated flow patterns within the airways is not possible, little exists in the way of validation of the various simulation techniques. The data presented here consists of very highly resolved flow data. The degree of resolution is compared to the highest necessary resolutions of the Kolmogorov length and time scales. Therefore this data is ideally suited to act as a benchmark case to which cheaper computational methods may be compared. A dataset and solution setup for one such more efficient method, large eddy simulation (LES), is also presented

Apports et limitations de la vélocimétrie par résonance magnétique en biomécanique. Mesures dans un embranchement plan symétrique

New contributions and present limitations of the nuclear magnetic resonance velocimetry are illustrated by means of measurements in two plane symmetrical bifurcations, both with sharp apex and hips. The bifurcation angles were equal to 70 and 60 degrees and the area ratios to 2 and 0.83 in models 1 and 2 respectively. Velocity-induced phase-shift are encoded by bipolar magnetic field gradients. The velocity was measured in steady (Reynolds number $< 700$) and in non-zero mean starting-stopping (mean Reynolds number of 150, Stokes number of 30) laminar flows. The phantom experiments have demonstrated the following advantages of the NMR velocimetry: (i) the variations of the velocity components, in space (bend-like type motion in the branches) and in time (cross-sectional peak-velocity migration, secondary flow reversal), can be monitored with on-line quantifications of axial velocity extrema in bidirectional flow, and (ii) flow separation, deposition site for conveyed particle deposition at the vessel wall, can be observed in the 70 degrees bifurcation only. However, our techniques have current limitations: (i) a spatial averaging of the velocity measurements in the excited slice thickness, over which the velocity field varies, and (ii) the inability to synchronize the velocity component measurements between the various modes of operation.La vélocimétrie par résonance magnétique nucléaire (RMN ou RM) repose sur la mesure de la différence de phase de l'aimantation des protons en mouvement macroscopique induite par des gradients de champ magnétique, en général bipolaires. Des capacités nouvelles et les limitations des techniques utilisées par les auteurs sont illustrées par des mesures effectuées dans deux embranchements plans symétriques. Les maquettes 1 et 2, de forte courbure de la paroi externe, ont un angle de 70 et 60 degrés, et un rapport de sections 2 et 0,83 respectivement. Deux types d'écoulements laminaires ont été employés : un écoulement stationnaire (nombre de Reynolds $\rm Re < 700$, nombre de Dean infini) et un écoulement périodique du type débit en créneau (nombre de Reynolds moyen de 150, nombre de Stokes de 30). La vélocimétrie RM permet, dès l'inspection des images, (1) de fournir les variations des composantes axiale et transversale de la vitesse (distribution spatiale due à la courbure et temporelle avec migration du pic de vitesse dans la section et inversion du sens du mouvement secondaire) et la valeur des extrema locaux de la vitesse axiale de l'écoulement bidirectionnel, et (2) de vérifier l'existence de décollements, lieux privilégiés de dépôts de particules solides (décollement visible uniquement dans la maquette 2). Les principaux inconvénients actuels sont (1) l'épaisseur de la coupe du vaisseau trop grande pour mesurer les variations du champ de vitesse tridimensionnel, et (2) l'impossibilité de reconstruire sans interpolation du champ de vitesse en raison de la différence entre les instants d'acquisition des divers modes de mesures