Inaug.-diss.--Göttingen.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1993. / "February 1993." "Doctoral dissertation." Cover title. At head of title: Woods Hole Oceanographic Institution, Massachusetts Institute of Technology, Joint Program in Oceanography/Applied Ocean Science and Engineering. Includes bibliographical references (p. 237-247).

Proefschrift--Leyden. / "Errata."

This thesis describes a numerical simulation and some flow visualization of vortex shedding from sharp edges in normal oscillatory flow. The modelling of vortex shedding from sharp edges has been done using a discrete vortex method; the separated shear layer issuing from the separation point is represented by a system of discrete two dimensional vortices. In Chapter Two, a finite wedge is modelled by considering the flow near the edge as the inner region of an oscillatory flow around an infinite wedge. This can be done if the Keulegan-Carpenter number is low, i.e. if the vortex hedding takes place mostly in the vicinity of the edge and is independent of shedding from any other edge(s). The mathematical formulation of this problem, although based on the combination of recent work of other researchers, represents a somewhat different approach when examined in detail. Each new vortex, called the nascent vortex, is introduced into the flow at a position not fixed in advance. Its position is dependent on the edge angle, the time step used in the numerical simulation and the influence of all the other vortices in the field. The expression describing the position of the nascent vortex can be derived as a natural development of the formulation. Therefore, it is not necessary to use empirical formulae to define the initial position of the nascent vortex and/or to fix this position for all time throughout the numerical simulation. Lamb vortices are used in the present study to delay the onset of instability in the numerical calculations. This results in very stable computations. Numerical modelling results concerning vortex induced forces are presented in Chapter Three. These results are then compared to those obtained numerically and experimentally by other researchers. Flow visualization experiments of vortex shedding from finite sharp wedges in an oscillatory flow are described in Chapter Four. The flow was produced using a sloshing tank, and visualized by hydrogen bubbles produced by the electrolysis of water. All results were recorded on video tape and photographs of flow visualizations have been produced through the use of a mirco-computer based frame grabber. The kinematics of the numerical modelling are compared to those obtained from flow visualizations. An application of the model to the roll decay of a simplified geometry of a single chine west coast trawler is presented in Chapter Five. No firm conclusions regarding the accuracy of the numerical prediction of roll decay can be drawn due to the gross simplification of the vessel section. However, the results do indicate that, with the absence of other forms of roll damping, vortex induced forces alone was able to cause roll extinction in the vessel. Therefore, it can be said that the prediction of roll extinction given by the present model is of an acceptable order of magnitude when compared to experimental roll decay results from previous work done in this department. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

No description available.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Approved for public release; distribution is unlimited / Motivation to expand the understanding of a helicopter rotor descending into the vortex ring state (VRS) stems from the aircraft mishaps that have plagued the helicopter community. The V-22 has become the most recent victim of encounters with VRS. The onset of VRS is associated with the collapse of the helical vortex wake in the plane of the rotor. The resulting wake disturbances develop an irregular and aperiodic flow. Rotor blade interaction with the disturbed vortices causes large variations in the blade spanwise aerodynamic load distribution. Harmonic analysis of the loading indicates that higher harmonic content becomes prevalent in this state. The dynamic flow similarities achieved in a water tunnel are used to explore flow visualization and conduct vibration analysis of a rotor system operating in the VRS. A scaled rotor system was operated in the NPS Aeronautical Engineering Department's water tunnel. Sensors were used to gather thrust and vibration power spectrum data when operating in VRS. Experimental results correlate with full scale flight data and show a significant increase in the vibration levels of the even multiples of the blade passage frequency. The relative strength of these higher harmonics can be used as an indicator of impending VRS encounters. / Major, United States Marine Corps

Approved for public release, distribution unlimited / An investigation of vortex shedding downstream of a cascade of compressor stator blades, at off-design inlet-flow angles of 35, 33 and 31 degrees and Reynolds numbers, based on chord length, of 625,000, 750,000 and 800,000 is reported. The objective of the study was to characterize the flow and vortex shedding through blade surface pressure measurements and hot-wire anemometry. Vortex shedding was determined to be a leading edge phenomenon as periodic shedding was only detected on the pressure side of the wake. The relationship between vortex shedding frequency and Reynolds number was nearly linear. The vortex shedding frequency at three incidence angles was observed to be quite similar at lower Reynolds number (i.e. 450,000 and below) but developed into a larger scatter at higher Reynolds number. Similarly, the Strouhal numbers were observed to be fairly consistent (0.22 to 0.24) at low Reynolds number and more scattered (0.18 to 0.25) with increasing Reynolds number. The result obtained was comparable to the experimental results obtained by Roshko[Ref. 14], for vortex shedding behind a circular cylinder. / Major, Republic of Singapore Air Force

No description available.