Shock tubes are devices used to study the behaviour and properties of fluids at speeds, pressure and temperatures not practically attainable in conventional wind tunnels. In their simplest form, they are comprised of a ‘driver’ and a ‘driven’ section at different pressured separated by a diaphragm. The sudden rupture of said diaphragm induces a shockwave which propagates down the driven section of the tube.
Due to circumstances, the MECH3410 shock tube practical was removed last year. The purpose of this thesis was to design its replacement. Initially the desired result was to have a
functioning unit that could be used in a practical without any further alterations. However circumstances only allowed for two test units to be constructed. One was located in the engineering workshop while the other was fabricated at home from hardware store parts and a vacuum cleaner. Both test units were functional proofs of concept, echoing the seminar
presentation’s sentiment of ‘compressible flow on a shoe string’ and going to show that strange phenomena such as shock waves are accessible to the common “enthusiast”.
Test results were in clear agreement with analytical predictions indicating that the losses incurred during the shock propagation were within allowable tolerances, even at low pressure ratios. As such, the students undertaking the compressible flow practical in future years should have little trouble identifying the similarities between the analytical inviscid
predictions and the real-world behave of shock waves. There was a lot of oscillation and noise in the post-shock signals which was expected with minimal shield and other electrical sources in the room.
In addition to the final report, the deliverables include the drafts for all apparatus parts requiring fabrication as well as an estimated budget and supplier list. The budget of $6,595.11
is for a complete unit, inclusive of components that have already been sourced by the university and will be shared by other practicals. As such the actual cost per unit for the university will be $2,534.11 This is also an upper estimation as the trading discounts the university has have not been taken into account. Nor has availability of scrap and offcuts that could likely be used to create some of the require components.
Initial designs were of an open shock tube, which led to loud noise generation due to the shockwave propagating into the surrounding room. The implementation of a silencer was
considered sourcing work by Raghunathan et al (1998) who investigated various designs, determining that a perforated inner tube surrounded by an outer box provided the best noise
reduction (12dB reduction from 171 dB). The final design however employs are partially sealed inlet to stifle the noise while still allowing air through to remove any vacuum and facilitate removal.
Two Motorola pressure transducers placed along the length of the shock tube are used to monitor both the pressure and the propagation of the shock wave. These readings are logged using an NI USB-6221 BNC data acquisition unit.
The shock tube itself is constructed from aluminium which is more robust than the uPVC used in the test units construction. The aluminium is also easier to machine and alter compared to uPVC with the apparatus designed to be modular, screwing together allowing for easy disassembly.
In conclusion, the shock tube apparatus fulfils the needs required of it and should hopefully prove to be an informative and worthwhile inclusion to MECH3410 in future years.