Flow induced self-oscillations cause acoustic pressure oscillations of large amplitude in pipe flows. If Reynolds number is treated as a parameter, these floinduced oscillations occur only at discrete and critical values of Reynolds number. However, for a small range of Reynolds numbers around such a critical value, such periodic oscillations may appear intermittently. If intermittency, which is a precursor to these self-oscillations, can be detected, prediction of an impending instability may be possible.
In experiments done by Vineeth and Sujith (Int. J. Aeroacoustics, 2016) on flow in a duct orifice arrangement, where flow enters through the duct inlet, and leaves into the atmosphere through the orifice exit, “whistling” was observed at a Reynolds number of 4200 (based on the orifice thickness and flow speed within the orifice), where large amplitude pressure oscillations were observed. At slightly lower Reynolds numbers, bursts of relatively smaller amplitudes of pressure oscillations were observed to appear intermittently.
For a similar configuration, Large Eddy Simulations (LES) have been carried out with explicit filtering as a sub grid scale model here. Both whistling and intermittency are observed in the simulations. As air flows from the duct into the orifice, it turns sharply around the corner at the duct orifice interface. Due to this sharp turn, flow separation occurs, and hence, a shear layer is formed at the mouth of the orifice. The mechanism of whistling is found to be this shear layer within the orifice flapping about and hitting the trailing edge of the orifice periodically, thus causing the shear layer to break and roll up into a vortex. At Reynolds numbers where intermittency is observed, the shear layer is found to very mildly come in contact with the edges of the orifice walls, thus disturbing it.
In the simulations, time series data of pressure are recorded at various probe locations. In a given time series, if scale invariance behaviour exists, it can be quantified by measuring the Hurst exponent. The numerical value of the Hurst exponent is an index of “long range or short range dependence” in a time series. Hurst exponent is measured in the time series data obtained. It is found to drop to zero as the flow approaches the state of a self-sustained oscillation, since the growth rates of all the long term as well as short term trends in the time series vanish. A loss of multifractality in the time series is also observed as the flow approaches whistling.
As a part of the this thesis, new, split high resolution schemes of high order are designed following the Hixon Turmel Proposal.
| Identifer | oai:union.ndltd.org:IISc/oai:etd.iisc.ernet.in:2005/3595 |
| Date | January 2017 |
| Creators | Kamin, Manu |
| Contributors | Mathew, Joseph |
| Source Sets | India Institute of Science |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Relation | G28210 |
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