圆柱体振动

Reducing the deleterious effect of Vortex Induced Vibrations (VIV) in marine risers is animportant task for ocean engineers and many competing factors exist in the design ofVIV suppression devices. This thesis explores the experimental minimization of the dragforce and the disruption of v0l1ex formation by utilizing VIV suppression devices.Two series of tests are conducted-both utilizing separate testing designs. The first testsare the flexible cylinder experiments, detailed in Chapter 2, which determine the dragforce and vibration amplitude of numerous, original testing configurations. The secondseries of tests are the rigid cylinder, PIV experiments, detailed in Chapter 3. These testsmeasure both the drag force on the cylinder and the oscillating component of the liftforce, the latter of which is a good indication of v011ex formation. The Chapter 3 testsalso image the test section wake-providing helpful insight into the physical process ofvortex formation.In brief, this thesis presents a detailed description and results of the two series of originalVIV suppression tests. Many original configurations are tested, and the results arecontained herein.

The focus of this study is an experimental apparatus thatserves as a model for studying blood flow in a total artificiallung (TAL), a prototype device intended to serve as a bridge tolung transplantation or that supports pulmonary function duringthe treatment of severe respiratory failure. The TAL consistsof hollow cylindrical fibers that oxygen-rich air flowsthrough and oxygen-poor blood flows around. Because gasdiffusivity in the TAL is very small, a convection mechanismdominates the gas transport, which is why we focus on thevelocity around the fibers (modeled as a 0.05-cm-in-diameterand 5-cm-long cylinder). We designed a low-speed watertunnel to study the flow mechanism around the cylinder,across which the flow is generated by a linear actuator thatallows different flow patterns to mimic the flow in a TAL. Wetested the flow in the test section by numerical simulation andby the particle image velocimetry method to study the flowprofile. The results show a uniform flow near the centerline ofthe water tunnel where the cylinder is placed. This decreasesthe effects of free-stream turbulence in the shear layers andreduces the uncertainty in determining the flow patternsaround the cylinder. Knowledge gained from the flow aroundone cylinder (fiber) is beneficial for understanding vortexformation around multiple cylinders. We present a summaryof vortex formation behind a cylinder for Reynolds numbers(Re) of 1, 3, and 5 and Stokes numbers (Ns) of 0.18 to 0.37 results show that higher Re and Ns favor vortex formation.These findings regarding the parameter range for vortex formationmay provide principles for designing artificial lungs toenhance convective mixing. We anticipate that the pulsatileflow circuit presented here can be used to mimic the flow notonly in TALs but in other physiological systems.

In order to avoid model support interference, the flow over cylinders of varying fineness ratio (length/diameter) aligned with the free stream were examined using a magnetic suspension and balance system. Thedrag coefficient variation of a right circular cylinder was obtained for a wide range of fineness ratios. ParticleImaging Velocimetry (PIV) was used to examine the flow field, particularly the behavior of the leading-edgeseparation shear layer and its effect on the wake. Reynolds numbers based on the cylinder diameter rangedfrom 5x104 to 1.1x105, while the major portion of the experiment was conducted at the ReD= 1.0x105. For themoderately large fineness ratio, the shear layer reattaches with subsequent growth of the boundary layer,whereas over shorter cylinders, the shear layer remains detached. Differences in the wake recirculation regionand the immediate wake patterns are clarified in terms of both the mean velocity and turbulent flow fields,including longitudinal vortical structures in the cross-flow plane of the wake. The minimum dragcorresponded to the fineness ratio where the separated shear layer reattached at the trailing edge of thecylinder. The base pressure was obtained with a telemetry technique. Pressure fields and aerodynamic forcefluctuations are also discussed.