Behrens, Tim

Efterhånden som vindmøller fremstilles med større og større rotordiameter, giver distribuerede aerodynamiske kontrolflader lovende resultater i forbindelse med reducering af laster. Der bliver derfor fokuseret på sådanne flader i forskning både fra industriel og akademisk hold. Af særlig interesse er bagkantsflapper. Den instationære strømning omkring sådanne flapper bliver typisk undersøgt ved hjælp af lineariserede instationære aerodynamiske modeller eller ved at løse de todimensionelle RANS (Reynolds Averaged Navier-Stokes) ligninger. Den sidstnævnte metode bliver typisk anvendt i forbindelse med bevægelige, eller interpolerede geometrikonforme beregningsnet. En mere fleksibel metode vil give mulighed for at undersøge strømningsforholdene omkring komplekse bevægelige flap geometrier i større omfang. Omsvøbte randbetingelser giver denne fleksibilitet, da geometrien kan repræsenteres ved hjælp af ekstra kildeled i de styrende ligninger. Denne metode giver mulighed for at simulere arbitrære geometrier i stationære beregningsnet, der ikke nødvendigvis er tilpasset geometrien. En elementær implementering af omsvøbte randbetingelser er tidligere blevet indføjet i CFD løseren Ellipsys. I nærværende arbejde er de nødvendige værktøjer til håndtering af bagkantsflapper i to og tre dimensioner blevet frembragt i nævnte kode. Validering af metoden er foretaget på en cirkulær geometri i en kartesisk beregningsnettopologi, såvel som i en geometrikonform topologi. En kombineret metode er blevet udviklet, hvori selve flappen modelleres ved hjælp af omsvøbte randbetingelser, hvorimod resten af vingeprofilet modelleres ved hjælp af geometrikonforme beregningsnet. Denne metode er valideret ved sammenligning med vindtunnelforsøg og en forbedring i forhold til en kordelline (Thin airfoil) vingeprofil metode er eftervist. En metode til kontrol af laster i en varierende anstrømning bliver præsenteret for en leddelt flap med uafhængige aktueringsniveauer. Det bliver demonstreret at den totale flap udbøjning kan opdeles i to uafhængige regimer uden at evnen til at kontrollere lasterne reduceres. Resultaterne indikerer at brug af to forskellige flap aktuatorer er fordelagtig i situationer med en kompleks anstrømning. Fuld skala vindmølle målinger præsenteres, og det indikeres at flap rod momentet kan bruges som input til en kontrolmetode. En ny metode, hvor dette moment benyttes som sensorinput til en last reducerende kontrolmetode bliver præsenteret. Beregninger påviser metodens anvendelighed og robusthed. Metoden påvises at kunne anvendes for 3D vingeprofiler med flere bagkantsflapper, adskilt af mellemrum. Strømningen i og omkring slipstrømmen af en udbøjet flap ved et Reynoldstal på 1.63 MIO er undersøgt under instationære forhold. En kontrol metode for to uafhængie flapper i en vinges længderetning er implementeret og præliminære last reduktioner er påvist. Det er påvist at omsvøbte randbetingelser er en anvendelig og alsidig metode til undersøgelse af bevægelige bagkantsflapper. Resultaterne af nærværende arbejde vil indgå i fremtidige undersøgelser af de instationære strømningsforhold omkring sådanne indretninger.As the rotor diameter of wind turbines increases, turbine blades with distributed aerodynamic control surfaces promise significant load reductions. Therefore, they are coming into focus in relation to research in academia and industry. Trailing edge flaps are of particular interest in terms of control surfaces. The unsteady flow around such flaps is usually investigated by applying linearized unsteady aerodynamic models or by solving the two dimensional unsteady Reynolds averaged Navier-Stokes equations. The latter method is usually applied in combination with moving or interpolated body conforming meshes. A more flexible method would open up an opportunity to investigate the flow features of complex moving flap geometries in great detail. The immersed boundary method offers this flexibility, as the geometry is represented through the introduction of additional forcing terms in the governing equations. This approach allows for simulation of arbitrary geometries in fixed meshes that do not need to conform to the body geometry. The flow solver EllipSys has previously been extended with a base implementation of an immersed boundary method. The present work developed the necessary tools to handle trailing edge flap geometries in two and three dimensions. Validation cases were presented for the circular cylinder in a Cartesian mesh topology as well as in a topology similar to a standard body fitted mesh. To simulate trailing edge flaps, a hybrid approach was developed that modeled only the moving flap as an immersed boundary, while the rest of the airfoil was represented by a conventional body-fitted mesh. The results from the hybrid approach were validated against published wind tunnel measurements and improvement over a thin-airfoil based flow model was proven. A load alleviation control in a changing inflow was presented for a divided flap action, i.e. a segmented flap with independent actuation rates. It has been demonstrated that the total flap deflection can be divided into two separate deflections without deteriorating control authority. The results suggested that the combined use of two independent flap actuators was beneficial when dealing with complex inflows. Full scale turbine measurements were presented and indicated that the flap hinge moment provided suitable input for load control. A novel way of using the hinge moment of a moving flap for load alleviation control was presented. Simulations demonstrated the feasibility and robustness of the approach. The hybrid immersed boundary approach proved to be able to handle 3D airfoil sections with span-wise flap gaps. The flow around and in the wake of a deflected flap at a Reynolds number of 1.63mio was investigated for steady inflow conditions. A control for two span-wise independent flaps was implemented and first load reductions could be achieved. The hybrid method has demonstrated to be a versatile tool in the research of moving trailing edge flaps. The results shall serve as the basis for future investigations of the unsteady flow field around trailing edge flaps

Online Research Database In Technology

Simulation of Moving Trailing Edge Flaps on a Wind Turbine Blade using a Navier-Stokes based Immersed Boundary Method

http://orbit.dtu.dk/ws/files/6668326/tibeh%20phd%20thesis.pdf

Simulation of Moving Trailing Edge Flaps on a Wind Turbine Blade using a Navier-Stokes based Immersed Boundary Method

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