Sudden expansion used as a diffuser

Abstract

Because of my interest in aviation I wanted to select a project title related to aircraft, either directly or indirectly. After some attempts an opportunity came up while discussing some titles with a lecturer. Hence began the idea of investigating a sudden expansion with boundary layer control. As can be seen later, this is applied on aircraft engines, where a sudden expansion is used as a diffuser. This project was discussed with the fluid-mechanics lecturer at the University, and he approved the idea with a lot of enthusiasm. Now it was time to begin planning. The first step was to consult previous research papers on subjects relating to sudden expansions. At the library of the University there are many papers contained in the Journal of Fluids Engineering published by the American Society of Mechanical Engineers. While browsing through these journals terminology used in boundary layer theory was encountered. Also there were many useful diagrams which gave pictures of the fluid behaviour after a sudden expansion. In particular two papers treated the subject more directly and ideas began to flow in. These ideas were noted so that they were not forgotten. Other literature was found in books of fluid mechanics. By the time this literature research was carried out, the actual work on the project could begin. The first thing that I did was to visit the fluid mechanics laboratory and see what equipment was available. Afterwards I checked what was available at the stores of the University and what was available at the University's mechanical engineering work shop. Then I was able to plan seriously what to do, how to do it and where to do it. With all this in mind, calculations on the required parameters were carried out keeping in mind the limitations of the library and the amount of coverage of the subject during the course. This implies that certain parameters, example the pressure recovery during boundary layer suction, could not be predicted, and so it was only up to the experiments to tell us the pressure recovery during, say, suction. It was decided to plan things step by step, that is, the first step was to carry out a number of experiments and then plan for the next steps according to the results obtained in the previous steps. This procedure had many advantages: - if a defect appears in the previous step, it would not be repeated in the subsequent one; - management of the experiments will be more efficient for the subsequent steps due to the experience gained in the previous steps. Hence due to the above advantages, material, time and costs would be saved. One disadvantage was that if a lot of time was taken on one step, it might have brought everything to a standstill unless some back-up plan was available. The only thing that, under these circumstances, could have been done was to abandon this step and replan. After all this, work on the apparatus began and then, when finished, it was tested at the fluids laboratory. This is explained in more detail in this report.