University of Twente Student Theses


Flow visualisation using small bubbles inside opaque structures : An experimental approach

Hoogt, K.J.J. van der (2017) Flow visualisation using small bubbles inside opaque structures : An experimental approach.

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Abstract:Cardiovascular diseases are a major cause of death in the Netherlands. One of these diseases is the aneurysm of the aorta abdominalis (AAA). In this case the vessel diameter is too large, which can cause rupturing of the aorta. When a patient with an AAA have to be treated, this can done by the placement of a stent graft. Here, a surgeon has two options: Open repair or Endovascular repair (EVAR). This topic is discussed in chapter 1, which is the introduction of this thesis. Here, we discuss the pathology, the treatment, and the complications that might occur. Furthermore, some aspects about choosing the right stent graft are discussed. Placing a stent graft can also have an influence on the fluid mechanical behaviour in and around the stent graft. Research is done, in vitro and by computation fluid dynamics (CFD), to gain more knowledge about the fluid mechanical phenomena. Doing in-vitro-studies can give some problems. Stent grafts are not transparent. For analysing flow, structures have to be made transparent in some cases. For example, if Particle Image Velocimetry (PIV) based on optical techniques (lasers) is used. In this thesis, we looked for a method to visualise flow in opaque structures. As first step, optic techniques were used to visualise a laminar flow inside a straight tube. This is discussed in chapter 2 in detail. Visualising flow profiles can be done with proper illumination, a camera system and tracer particles (which are injected into the flow). The diameter of these particles has to be small enough to follow the flow. Following the flow accurately is called tracer fidelity. Combining all these aspects makes flow visualisation possible. In our experiments, flow visualisation was done with the hydrogen bubble technique. This is a technique which is based on the electrolysis of water. By placing a very thin metal wire in a tube and using pulsed voltages, hydrogen bubbles were created. These bubbles formed so-called timelines. Using these timelines, the offset of a parabolic profile was seen in the some obtained images. The results of these experiments were used as first step in our flow experiments. Chapter 3 discusses visualisation of bubbles with X-rays. X-rays can be used for visualisation inside opaque structures. For this part of the research, bubbles were injected via thin needles (placed inside a tube filled with water) and visualised with single shot X-ray images. The results of this technique showed that small microbubbles are difficult to visualise with X-rays. Some of the limitation factors, here, were the resolution of the imaging system and not much contrast between bubbles and water. Because we made snapshots, only the position of bubbles in one frame could be seen. To visualise a motion more frames are needed. In X-ray imaging, this can be accomplished by adding a shutter in front of the X-ray tube. A shutter is able to block X-rays for a predefined period. This means the timing of the shutter is essential to make consecutive images. A shutter was build and placed in front of the X-ray tube. Because this timing was not optimal during the experiments, future research is necessary to optimise this imaging technique further. In the last chapter, other parameters (pulse wave velocity and wall shear stress) are discussed in relation to the measurement principles to obtain data in in-vitro studies. These include aspects like algorithms needed to obtain the pulse transit time (PTT), and some theoretical background regarding to these parameters.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Programme:Electrical Engineering MSc (60353)
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