| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系 === 106 === The phenomenon which drastically influences fluid flow and appears the most in natural flow is the phenomena of turbulent flows. The flow phenomena play an integral part in a variety of fields, especially in fluid mechanics. Depending on the situation, one can base on the characteristics of turbulent flow to meet their requirements such as creating more energy or increasing mass transfer and mixing, since there has been the presence of eddies and vortices inside the flow. However, in some cases, one would like to maintain the state of fluid at laminar flow. For instance, the movement of blood flow is deemed to be laminar and researchers want to keep it flowing smoothly like this. The reason is due to the characteristics of fluid flow. When the state of fluid flow becomes turbulent, any disturbances can cause an obstruction of arterial blood; therefore, the supply of blood distributed to each cell is not satisfied with the need of tissue. Consequently, many researchers have investigated laminar-turbulence transition phenomena for a long time in order to learn about the process in detail. From that, one can illustrate some methods to control flow and make it adapt to distinct situations. Nevertheless, the process of a laminar flow becoming turbulent is complicated and still remains a controversy. The first important point to understand is to clarify the first signal, called as “initial instability”, and when it happens. Understanding the initial instability plays a crucial role in laminar-transient transition process and facilitates the awareness of the evolution from a small disturbance into a turbulent one and then manipulates fluid flow.
This study aimed to explore the initial instability in pulsating pipe flow and try to use trip wire to clarify the effects of the method. In addition, the characteristic of disturbance along the tube was also recognized. In the experiments, a hot wire anemometer was employed to measure the speed of fluid. A rotating disc was located at downstream to produce pulsating flow and the three main parameters which represented the pulsating flow condition are Womersley number α, Reu and Rem, respectively. In order to investigate the characteristic of intermittent disturbance component from original signal, a method called Empirical Mode Decomposition (EMD) was considered to extract the disturbance signal into a number of Intrinsic Mode Functions (IMFs). Moreover, the feature of each IMF was analyzed to get information of the disturbance frequency by the Hilbert-Huang Transform. In the analysis, the Rayleigh’s theorem successfully clarified the mechanism of the initial instability in a mixing layer. The theorem showed that the development of flow instability following the existence of inflexion point. The additional parameter which governed the state of fluid flow was trip wire. The factor which influenced the development of boundary thickness and was the one considered whether the change of flow instability was through installing and replacing with a series of different diameters or not. Furthermore, the travelling of disturbance over time was recognized from the cases studied. Finally, the analysis method which was mentioned above was used to analyze the data and have a comparison with the previous cases studied.
Key words: Pulsating pipe flow, Hot-wire measurement, Rayleigh’s inflection point theorem, Trip wire.
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