Investigation into flow-induced vibrations of piano key weirs

Denys, Frank (2019-04)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: This thesis considers the transient behaviour of piano key weirs (PKW) from a hydrodynamic and structural perspective. Their geometry makes them very rigid structures, however, there are thinner components of a PKW which may be prone to the onset of vibrations, specifically fluid or flow induced vibrations (FIV). Such vibrations, if strong enough and of sufficient duration may induce material fatigue in high stress zones of the structure. This is especially the case in steel PKWs whose slenderness and low mass may make them susceptible to vibration as detailed in this study. A number of hydrodynamic sources of excitation were found to occur at PKWs. These include the shedding of vortices from a free shear layer which develops at the entrance of the PKW’s inlet key; the unstable fluctuation of the air cavity under the overflow nappe; as well as the oscillation of the nappe itself. The vortex shedding, and the pressure fluctuations it causes on the upstream face of the weir, was the focus of the study as it was found to be a very stable flow feature. In contrast to literature, changes in discharge and the width of the inlet key had no effect on the character of these pressure oscillations. The manner in which these fluctuations manifest on the lateral sidewall of the PKW were examined in light of the modal shapes of the structure. It was found that there was an overlap between the excitation frequencies and the natural frequencies of the structure, thus there is potential for resonance to occur. However, there was a general lack of correlation in the excitations, which reduced their ability to activate any of the mode shapes despite the overlap in frequency. This means that the vibration response of the structure is limited. The study consisted of two physical models as well as a series of numerical models of the hydrodynamic (CFD) and structural (FEM) environment. These separate numerical models were calibrated against their physical counterparts and then combined in a staggered coupled fluid structure interaction (FSI) model. This integrated numerical model was able to emulate many of the vibration features observed in the physical model, including the effect of added mass and damping. Analysis of the generated stress and strain data revealed that, although fluid induced vibrations do occur at the PKW’s sidewall, the magnitude of the structural response is limited in most cases and is thus unlikely to lead to material fatigue. Nevertheless, alternative construction materials such as stainless steel or PVC may still experience such problems and would thus need to account for them in their design. The findings from this dissertation reveal that Piano Key Weirs are indeed prone to fluid induced vibrations due to their geometry and their thin nature. However, good design should be able to limit the negative effect these vibrations have on the operations and longevity of the structure.

AFRIKAANSE OPSOMMING: Hierdie proefskrif beskou die varierende gedrag van klavierklawer keerwalle (KKK) vanuit ’n hidrodinamiese en strukturele oogpunt. Hul vorm maak hulle baie rigied, maar daar is dunner dele van ’n KKK wat moontlik vatbaar is vir die ontstaan van vibrasies, meer spesifiek vloeigedrewe vibrasies. Sodanige vibrasies, indien sterk genoeg en van voldoende tydsduur, kan lei tot die ontwikkeling van materiële vermoeidheid in hoë spanningsones van die struktuur. Dit is veral die geval in staal KKKs waarvan die slankheid en lae massa dit vatbaar maak vir vibrasie soos in hierdie studie uiteengesit. ’n Aantal hidrodinamiese bronne van opwekking is gevind by KKKs. Dit sluit in die afwerp van werwels vanaf ’n vrye afskuiflaag wat by die ingang van die KKK se inlaat klawer ontwikkel; die onstabiele fluktuasie van die lugholte onder die oorloopnappe; sowel as die ossillasie van die nappe self. Die werwelafwerping, en die drukvariasies wat dit op die stroomopkant van die keerwal veroorsaak, was die fokus van die studie aangesien dit gevind is dat dit ’n baie stabiele vloeiverskynsel is. In teenstelling met literatuur het veranderinge in die deurstroming en die inlaatklawerwydte geen invloed gehad op die eienskappe van hierdie drukossillasies nie. Die wyse waarop hierdie drukvariasies op die laterale symuur van die KKK verskyn, is ondersoek in die lig van die modale vorms van die struktuur. Daar is bevind dat daar ’n oorvleueling was tussen die aanwendingsfrekwensies en die natuurlike frekwensies van die struktuur, dus bestaan daar ’n moontlikheid vir resonansie om plaas te vind. Daar was egter ’n algemene gebrek aan korrelasie in die aanwendings, wat hul vermoë om enige van die modusse te aktiveer verminder het ten spyte van die oorvleueling in frekwensie. Dit beteken dat die vibrasie reaksie van die struktuur beperk is. Die studie is uitgevoer met twee fisiese modelle sowel as ’n reeks numeriese modelle van die hidrodinamiese en strukturele omgewing. Hierdie afsonderlike numeriese modelle is gekalibreer teen hul fisiese eweknieë en is dan gekombineer in ’n gekoppelde vloei-struktuur interaksie model. Hierdie geïntegreerde numeriese model was in staat om baie van die vibrasieeienskappe wat in die fisiese model waargeneem is, te simuleer, insluitend die invloed van toegevoegde massa en demping. Ontleding van die gegenereerde stres- en spanningdata het getoon dat, hoewel vloeigedrewe vibrasies by die KKK se symuur voorkom, die grootte van die strukturele reaksie in die meeste gevalle beperk is en dit dus onwaarskynlik sal lei tot materiële vermoeidheid. Nietemin, alternatiewe konstruksiemateriale soos vlekvrye staal of PVC kan nog steeds sulke probleme ervaar wat dus in ag geneem moet word in hul ontwerp. Die bevindings van hierdie proefskrif toon dat klavierklawer keerwalle wel geneig is tot vloeigedrewe vibrasies as gevolg van hul vorm en hul dun aard. Goeie ontwerp kan egter die negatiewe invloed van hierdie vibrasies op die bedryf en duursaamheid van die struktuur beperk

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105731
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