Antoni Blazewicz

Interaction between a vortex wake and an immersed rectangular plate

Air jet experiments and water flow visualization experiments to investigate the relationships between the characteristics of flow over a tandem array of two thick plates, force fluctuations on the downstream plate, and sound generated by the array are described. The leading edge of the upstream, vortex-generating, plate was elliptical and the trailing edge square with sharp corners; the downstream plate was of rectangular cross section. Flow visualization shows that as the gap between the plates is increased, the flow within the gap undergoes transition from one with stationary vortices at small gaps (the “trapped vortex” regime) to one with vortex shedding from the trailing edge of the upstream plate at large gaps (the “vortex street” regime). With progressive increase in gap, the flow changes progressively, over a range of gap widths, through a transitional regime in which weak convected vortices are formed at the downstream end of the gap; then, at a critical value of gap-to-thickness ratio G, vortex formation shifts abruptly upstream to a position close to the trailing edge of the upstream plate, and the vortex street regime is suddenly established. In the vortex street regime, the formation length and spacing of the vortices vary quasi-periodically with G, in accord with the number of vortices occurring within the gap. Radiated acoustic power at vortex frequencies is found to be proportional to the 5.7th power of flow velocity, indicating that the sound source is essentially dipole in character and supporting a previous conclusion that force fluctuations generated on the downstream plate by its interaction with vortices formed in the gap between the two plates constitute the main sound source. The radiated power reaches a maximum at a value of G slightly less than the critical transition value and varies quasi-periodically with G in the vortex street regime according to the number of vortices in the gap.

Detection of Cracks Using Piezoelectric Wafer Active Sensors

The capability of piezoelectric wafer active sensors to identifying cracks which are common to metallic structural elements subjected to fatigue loading were explored. A number of laboratory tests were performed to investigate Lamb wave transmission and reception characteristics as well as the sensitivity of the transmitted and reflected signals to the presence of a through-the-thickness crack of various length and orientation in a thin plate. Based on the laboratory tests an optimum strategy for crack detection with the pulse-echo method was investigated. The method can be used for large areas scanning with a small amount of sensors and is favoured for embedded and leave-in-place sensor applications.

Effect of flowing medium for a simply supported pipe subjected to impulse loading

Abstract The primary aim of this study is to investigate the effects of a flowing medium on the transient response of a simply supported pipe subjected to dynamically applied loading. The importance of this study is manifested in numerous applications such as oil and gas transportation where dynamic loading can be the result of an accident. The classical Bernoulli-Euler beam theory is adopted to describe the dynamic behaviour of an elastic pipe and a new governing equation of a long pipe transporting gas or liquid is derived. This governing equation incorporates the effects of inertia, centrifugal and Coriolis forces due to the flowing medium. This equation can be normalised to demonstrate that only two non-dimensional parameters govern the static and dynamic response of the system incorporating a pipe and flowing medium. The transient response of this system is investigated based on a standard perturbation approach. Further, a numerical method utilising the finite difference method is developed and applied to investigate the dynamic response of a simply supported pipe. It is demonstrated that the previous dynamic models which largely ignore the internal flow effects and interactions between the flow and structure normally produce a large error and are inapplicable to the analysis of many practical situations. One interesting effect identified in the numerical study is that at certain flow ratio the system becomes dynamically unstable and any, even very small, external perturbation leads to a growing unstable dynamic behaviour. Such behaviour, which is called pipe whip, is well known to everyone who waters a garden using a flexible long hose.