An experimental investigation on the analysis of flow dynamics in collapsible elastic tubes

In this study airflow through collapsible elastic tubes was experimentally investigated. The experimental set-up was designed and constructed as a result of literature survey. In the experiments, Penrose tubes having a wall thickness of 0.55 mm and silicone rubber tubes having a wall thickness of 1 mm, 2 mm and 3mm were used. The inside diameter of the tubes are 25.4 mm and three different tube lengths were tested such as the ratios of tube length to the tube inside diameter would be 10, 7.5 and 5. The elastic tube was placed without any tension in an airtight box known as Starling resistor and it was attached to pieces of metal pipes at both of its ends. Airflow was steadily supplied to the system by means of a screwtype compressor. The steady air flow rate was regulated between 0.0049 m3/s and 0.0214 m3/s. When the internal and external pressure difference of the tube in which air flowed through it was negative on applying pressure into the Starling resistor, the self excited oscillations were observed in most of the cases and flow was therefore unsteady. However the oscillations were not observed in some cases depending on the tube type (the 3 mm thick silicone rubber tube) and flow rate. The time dependent pressure and velocity measurements were stored on the computer by means of a data acquisition system. The measurements of pressure at the inlet and exit of the tube and the velocity at the exit of the tube had been performed at the onset of oscillations. The frequency of the oscillations had been determined by applying Fast Fourier Transform (FFT) to these measurements. The experimentally determined frequency of the oscillations values were between 18 Hz and 35 Hz. It was seen that the properties of the tube (wall-thickness, length, inside diameter, tube material) and magnitude of the flow rate seriously affected pressure and velocity measurements. Therefore it was emphasized that these parameters should be considered in the collapsible tube flow investigations. The experimental results were also compared with the theoretical calculations based on linear stability theory obtained from literature. The frequency value obtained from the calculations was seen to be in the range of measurement results. The flows through collapsible tubes are up-to-date topic and the studies are continuing on them due to the fact that they are basically related to physiological flows and biomechanical applications. Therefore this experimental study which had been presented in detail may be considered to be beneficial for the investigators dealing with this topic. In addition, since water had been used as fluid in most of the previous experimental studies related to the subject, it was considered that it might be useful to test also airflow in collapsible tubes. Consequently, the performed investigation in which this gap was taken into consideration can be said to have significance in this respect.