Evaluation of the influence of aeration on the discharge capacity and flow induced vibrations of Piano Key Weir spillways

Lombaard, Jaretha (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: A Piano Key Weir (PKW) spillway is a popular option for increasing the discharge capacity of existing dams due to its high discharge efficiency compared to linear and traditional labyrinth weirs, as well as its small structural footprint. Thinner elements of a PKW, such as the sidewalls, may be susceptible to flow induced vibrations (FIVs) and subsequent structural fatigue and/or cause significant noise disturbance to the environment when the PKW is spilling. Previous PKW hydraulic model studies have observed that vibrations that occur in the PKW structure disappear by artificially aerating the air cavity behind the overflow nappe. A physical hydraulic model study of PKWs was conducted at Stellenbosch University to evaluate the influence of artificial aeration on the discharge capacity, the overall hydraulic behaviour and the FIVs, specifically nappe oscillations. Two physical hydraulic models were constructed – one large-scale half key model, constructed from structural steel, and one smaller scale multi-key model, constructed from timber. Both models were constructed with the option of introducing air to the air cavity behind the overflow nappe via two different sizes of aeration pipes The study found that PKWs are indeed self-aerating, especially for higher headwater levels. Artificially aerating the air cavity behind the nappe slightly increases the discharge efficiency at lower headwater levels and has an overall stabilising effect on the pressure fluctuations in this air cavity. Pressure fluctuations in the air cavity occur over all ranges of headwater levels tested and increased in magnitude with the increase in headwater level. At lower headwater levels, these pressure fluctuations and FIVs were mainly caused by vortex shedding which were induced by the upstream flow conditions around the upstream overhang of the PKW. These vortices presented itself as swirls of flow that were passed over the sidewall. At higher headwater levels, the fluctuations could be attributed to the flow dynamics in the outlet key as it became drowned. In both cases, aeration had a limited effect on the pressure fluctuations. Nappe oscillations, as a type of FIV, only occurred at very low headwater levels. These oscillations reduced in intensity when the nappe air cavity was aerated but disappeared when the nappe was broken or split. As indicated by other studies, the latter observation confirmed that the air cavity only functions as an amplifier of the pressure fluctuations. The sidewall vibrations appeared to be excited by the air pressure fluctuations as a result of the aforementioned vortex shedding. Although there was some correlation between the excitation frequencies and natural frequencies of the sidewall, the vibration amplitudes were relatively low and none of the mode shapes were such that resonant behaviour of the PKW sidewall would occur. Artificial aeration had no effect on the vibration frequencies of the PKW sidewall or the intensity thereof for the flow ranges tested.

AFRIKAANSE OPSOMMING: ‘n Klavierklawerkeerwal (KKK) oorloop is ‘n gewilde keuse om die oorloopkapasiteit van bestaande damme te verhoog weens hul hoë deurstromingdoeltreffendheid in vergelyking met lineêre of tradisionele saagtandkeerwalle, asook hul klein strukturele voetspoorarea. Dunner elemente van ‘n KKK, soos die symure, mag vatbaar wees vir vloei-geïnduseerde vibrasies (VIV) en daaropvolgende strukturele vermoeidheid en/of omgewingsgeraasversteuring wanneer die KKK oorloop. Vorige KKK fisiese hidrouliese modelstudies het waargeneem dat vibrasies wat in die KKK-struktuur voorkom verdwyn wanneer die lugholte agter die oorloopnappe kunsmatig belug word. Fisiese hidrouliese modelstudies is by Stellenbosch Universiteit uitgevoer om die invloed van kunsmatige belugting op die oorloopkapasiteit, die algemene hidrouliese gedrag en die VIV te evalueer. Twee fisiese hidrouliese modelle is gebou – een grootskaal halwe klawer model, gebou uit strukturele metaal, en een kleinskaal multi-klawer model, gebou met hout. Beide modelle is gebou met die keuse om lug toe te dien na die lugholte agter die oorloopnappe met behulp van belugtingspype. Die studie het bevind dat KKKe vanself belug word, veral vir hoë oorloopwatervlakke. Die deurlatingsdoeltreffendheid neem effens toe as die lugholte agter die nappe kunsmatig belug word onder lae vloeitoestande. Kunsmatige belugting het ook ‘n algemene stabiliseringseffek op die lugdrukvariasie in hierdie lugholte. Die lugdruk in die lugholte fluktueer vir alle getoetsde oorloopwatervlakke en neem toe met die toename in die oorloopwatervlak. Vir lae oorloopwatervlakke word die drukfluktuasies meestal veroorsaak deur werwelstorting wat ook ‘n tipe VIV is. Hierdie werwelstortings word veroorsaak deur die vloei toestande rondom die stroomopoorhang van die KKK en kom voor as dikker kolkende vloeistrome wat bo-oor die symuur se kruin vloei. Vir hoër oorloopwatervlakke kan die drukfluktuasies toegeskryf word aan die vloei dinamika in die uitlaatklawer soos wat dit oorstroom word. In beide gevalle het kunsmatige belugting ‘n beperkte invloed op die drukfluktuasies gehad. Nappe-ossillasies, as 'n VIV, kom slegs voor tydens baie lae oorloopwatervlakke. Die intensiteit van hierdie ossillasies verlaag wanneer die lugholte belug word en verdwyn heeltemal wanneer die nappe gebreek of gesplit word. Soos aangedui in ander studies, bevestig die laasgenoemde waarneming dat die lugholte slegs as 'n versterker van die ossillasies funksioneer. Dit blyk dat die symuurvibrasies opgewek word deur die drukfluktuasies as gevolg van die werwelafwerping wat ontstaan stroomop van die KKK. Alhoewel daar 'n mate van korrelasie bestaan tussen die opwekkingsfrekwensies en natuurlike frekwensies van die symuur, is die vibrasie-amplitudes relatief laag en word geen van die vibrasie modus vorms in so 'n mate bevoordeel dat dit die resonante gedrag van die KKK-symuur sal aktiveer nie. Kunsmatige belugting het geen effek op die vibrasiefrekwensies van die KKK-symuur of die intensiteit daarvan vir die deurstromings wat getoets is nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107891
This item appears in the following collections: