Sweeping-jet active flow control actuation effects on boundary layer separation on airfoil at ultralow Reynolds

Airfoils incur large drag penalty and a drop of lift production when operated at high angles of attack. Active flow control has a potential for improving the aerodynamic efficiency of airfoils at off-design operating conditions. Several simulations with Nektar++ CFD have been performed with intent to show the ability of sweeping jet active flow control method to improve performances of an airfoil at low Reynolds conditions. A sweeping jet is an output of a fluidic oscillator with no mobile parts which is able to make the fluid that goes through it to oscillate at its nozzle in a very stable way. This device has been previously successfully tested on airfoils with a deflected flap, with the intention of attach the boundary layer until the trailing edge. In this study, single sweeping jet on a 2D NACA0012 airfoil has been mathematically parameterized, with multiple degrees of freedom whose effect on the solution has been studied. In order to do so, a structured mesh containing the airfoil has been firstly studied and then modified with the aim of placing the jet on the airfoil’s upper surface. Baseline calculations show that at Re=1000 and with an angle of attack of 9o, the boundary layer detaches at 35% of the chord approximately. This study focuses on the effects on the boundary layer of the parameters of the sweeping-jet: the position, momentum coefficient, oscillation amplitude, frequency and opening angle have been modified. The sweeping jet has been studied in two configurations: the first one pointing normal to the airfoil’s upper surface and oscillating in the XY plane, and the second one near tangential to the upper surface of the airfoil, that is, the jet’s principal direction is almost the stream-wise direction. It oscillates on z-axis, but due to the fact that most experiments have been carried out in 2D, the frequency behaviour will be simulated represented by making the output velocity oscillate on its magnitude. The tangential configuration has been found to give the sweeping jet the authority to control flow by reattaching the boundary layer and significantly increase the lift coefficient of the airfoil. Also, it has been found a strong relationship between the jet’s position over the upper surface of the airfoil and the trajectory of the fluid, which is determinant on the aerodynamic characteristics of a wing. Finally, a quasi-3D study has been carried out, due to the importance found on previous studies of the span-wise distance between two consecutive jets on a finite wing. Taking advantage of the homogeneity of the z direction in this experiment, in order to analyze the stability to span-wise-dependent perturbations a unimodal Fourier expansion of varying wave-number has been used in the span-wise direction, which reduces the computational cost significantly. Also, this has allowed the authors to study the stability of the system depending on the span dimension.Els perfils alars pateixen un greu augment d’arrosegament i pèrdua de sustentació quan operen a grans angles d’atac. El control actiu t é un gran potencial de millorar l’efici ència aerodinàmica als perfils ’off-design’ que operen aquestes condicions.En aquest estudi s’- han dut a terme m´ ultiples simulacions mitjanc¸ant el software CFD Nektar++ per a demostrar l’habilitat que t é el sweeping jet per millorar les condicions a un Reynolds baix. El sweeping jet és un possible output d’un oscil lador fluídic sense parts m` obils que és capac¸ d’aconseguir que el fluid que passa através d’ell oscil li a la tovera d’una manera estable. Aquest dispositiu ja ha estat provat pr èvia i reeixidament en perfils amb fald ó (Flaps), amb la clara intenci ó de sostenir el fluxe fins a la vora final d’aquest (trailing edge). En aquest estudi, un únic jet en un perfil 2D NACA0012 s’ha parametritzat, amb certes variables com a graus de llibertat per tal d’estudiar-ne el seu efecte. En primer lloc s’ha treballat una malla estructurada la qual s’ha reestructurat posteriorment amb l’objectiu d’incloure el jet sobre del perfil. Les simulacions demostren com amb el perfil cru sense jet, a Re=1000 i un angle d’atac a 9o, la capa límit es desprén al 35% de la corda aproximadament. Els principals paràmetres modificats i testejats del jet han estat: la posici ´ o, el coeficient de moment, l’amplitud d’oscil laci ´ o, la freq ¨uència i l’angle d’apertura de la sortida del jet. El sweeping jet s’ha estudiat en dues configuracions: la primera apuntant perpendicular a l’extradós i oscil lant al pl à XY, i la segona quasi-tangencialment a l’extradós, és a dir, que la seva direcci ó principal és similar a la direcci ó stream-wise. En base a aquests resultats, hem obtingut que el jet tangencial permet tenir un control sobre el fluxe fent-lo reenganxarse un altre cop al perfil i augmentant significativament la sustentaci´ o. També s’ha trobat una forta relaci ó entre la posici ó del jet sobre el perfil amb la traject ` oria del fluid, el qualés determinant per a les característiques aerodinàmiques del perfil alar. Finalment, s’ha dut a terme un estudi en quasi-3D degut a l’import ància trobada en altres articles amb respecte a la dist ància d’amplada entre dos jets consecutius en un ala finita. Aprofitant la homogeneitat en la direcci ó Z del nostre estudi, s’ha utilitzat una expansió de Fourier unimodal de nombre d’ona variable en la direcci ó span-wise per tal d’estudiar l’evoluci ó dels paràmetres en direcci ó Z, el qual fa reduir el temps d’obtenci ó de resultats. Aix` o, a més, ha possibilitat als autors la capacitat d’estudi d’estabilitat de l’ala en funci ó de la seva amplada