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Denne afhandling omhandler stilstandssvingninger af vindmøllevinger. Stilstandssvingningerne kan opdeles i hvirvelinducerede (vortex-induced) og separationsinducerede (stall-induced) svingninger. I denne afhandling er det undersøgt hvorvidt stilstandssvingninger af vindmøllevinger er af den hvirvelinducerede, af den separationsinducerede eller begge typer svingninger. Afhandlingen indeholder analyzer baseret på både ingeniørmæssige modeller og CFD (Computational Fluid Dynamics)-baserede modeller. Der er blevet opstillet todimensionelle ingeniørmodeller af en elastisk ophængt profilsektion med tre strukturelle frihedsgrader. Målet med disse modeller er at undersøge effekten af en tidsforsinkelse i det aerodynamiske respons på de aeroelastiske stabilitetsgrænser. Motivationen for denne undersøgelse er at aerodynamikken i de eksisterende ”state-of-the-art” aeroelastiske beregningsmodeller effektivt er kvasistationær ved fuldt separerede strømninger. Konklusionen fra denne del af afhandlingens arbejde er at selv en relativt lille tidsforsinkelse i det aerodynamiske respons kan forøge den aerodynamiske dæmpning, og derved influere de aeroelastiske stabilitetsgrænser relativt til det der forekommer med et kvasistationært aerodynamisk respons. Ud over arbejdet med ingeniørmodellerne er der foretaget instationære to- og tre-dimensionelle CFD beregninger med såvel ikke bevæget, foreskrevet bevægelse og elastisk profilophængning. 2D og 3D CFD beregninger på et DU96-W-180 vingeprofil med foreskrevet harmonisk kantvis bevægelse indikerer hvirvelinducerede svingninger ved 90 graders indfaldsvinkel ved en frekvens tæt på hvirvelafkastningsfrekvensen fra 2D CFD beregninger med ikke-bevæget profilophængning. Der er fundet væsentlige uoverensstemmelser mellem 2D og 3D CFD beregninger omkring 25 graders indfaldsvinkel. 3D beregningerne foretaget i dette arbejde indikerer at muligheden for hvirvelinducerede svingninger på vindmøllevinger ikke kan afvises, hvorimod den fornødne fristrømshastighed for at separationsinducerede svingninger skulle kunne opstå er så høj at denne type svingninger baseret på disse beregninger forekommer usandsynlige. Analyze af dynamisk lift og drag fra 2D og 3D CFD beregninger foretaget omkring 25 graders angrebsvinkel viste loops med middelhældninger af forskelligt fortegn. Dette indikerer at yderligere undersøgelser er nødvendige, og at der skal udvises forsigtighed ved anvendelse af simple modeller i forbindelsemed aeroelastiske simuleringer af vindmøller ved stilstand. Wind turbine blade vibrations at standstill conditions were investigated in the present work. These included vortex-induced and stall-induced vibrations. Thus, it was investigated whether the stand still vibrations are vortex-induced, stall-induced or a combination of both types. The work comprised analyzes based on engineering models and Computational Fluid Dynamics. Twodimensional,three-degree-of-freedom, elastically-mounted-airfoil engineering models were created. These models aimed at investigating the effect of temporal lag in the aerodynamic response of an airfoil on theaeroelastic stability limits. The motivation for it was that the standard aerodynamics existing in state-of-the-art aeroelastic codes is effectively quasi-steady in deep stall. If such an assumption was incorrect, these codes could predict stall-induced vibrations inaccurately. The main conclusion drawn from these analyzes was that even a relatively low amount of temporal lag in the aerodynamic response may significantly increase the aerodynamic damping and therefore influence the aeroelastic stability limits, relative to quasisteady aerodynamic response. Two- and three-dimensional CFDcomputations included non-moving, prescribed-motion and elasticallymounted airfoil suspensions. 2D and 3D prescribed-motion CFD computations performed on a DU96-W-180 airfoil predicted vortex-induced vibrations at 90 degrees angle of attack at the frequency close to the stationary vortex shedding frequency predicted by 2D CFD computations. Significant discrepancies were observed between 2D and 3D computations around 25 degrees angle of attack. 3D computations predicted occurrence of vortex-induced vibrations while the wind speed necessary for the occurrence of stall-induced vibrations was predicted too high to occur in normal conditions. Analysis of the dynamic lift and drag resulting from 2D and 3D CFD computations carried out around 25 degrees angle of attack showed loops with the slopes of opposite signs indicating that further investigations are needed and that simple models in connection with aeroelastic simulations might not be sufficient to accurately predict vibrations at standstill conditions. |
