Some Three-Dimensional Effects of Rotating Stall
SUMMARY A small perturbation analysis of rotating stall in inviscid, incompressible flow is developed from an analysis of Yeh. An isolated blade row is considered having a blade height which is not small compared to the msan radius. A criterion is derived for tie occurrence of rotating stall, the speed of stall propagation and the possible number of stall cells involved. From this the frequency of circumferential flow disturbances can be obtained. An example is given of an application Of the analysis. The main assumptions made are that a large number of small amplitude stall cells are induced at onset of stall, that the radial shift of the streamlines is small and that the absolute exit flow angle is small. Possible explanations are suggested for the observed changes in number of stall cells.
Structure and Kinematics of the Vortex System in Axial Turbomachines
This investigation models the kinematics of the vortex system of an encased axial turbomachine at part load and overload applying analytical methods. So far, the influence of the casing and the tip clearance on the kinematics was solved separately. The vortex system is composed of a hub, bound and tip vortices. For the nominal operating point φφ ≈ φφopt and negligible induction, the tip vortices transform into a screw. For part load operation φφ → 0, the tip vortices wind up to a vortex ring, i.e. the pitch of the screw vanishes. For overload operation φφ → ∞, the vortex system of the turbomachine forms a horseshoe, i.e. the pitch of the screw becomes infinite. Both, hub and tip vortices, are semi-infinite, straight vortex filaments.
Numerical analysis and experimental observation of guidewire motion in a blood vessel model.
We have developed a computer-based system to simulate a guidewire in blood vessels for surgical planning, intra-operative assistance, and to facilitate the design of new guidewires. In this study, we compared simulation results with experimental results for validation of the simulation system. First, we inserted a commercial guidewire into a poly (vinyl alcohol) hydrogel blood vessel model using a two-axis automatic stage and measured the position of the guidewire tip and the contact force between the guidewire and the vessel. The experimental apparatus can be used not only for the validation of numerical analyses, but also as a simulation system. Second, similarly to the experiment, the motion of the guidewire in the blood vessel model was calculated when the proximal part of the guidewire model was pushed and twisted. The model of the guidewire is constructed with viscoelastic springs and segments, and the proximal part of the guidewire model is constrained by the fixed catheter model. Collisions between the guidewire and the vessel are calculated, and the contact forces are determined according to the stiffness of the vessel wall. The same tendency was seen in the trajectories and the contact force of both the experimental and simulated guidewire tips.