In present study, aerodynamics of a NACA4412 wings with aspect ratio of 1 and 3 was considered experimentally at Reynolds numbers of 2.5 × 104, 5 × 104 and 7.5 × 104. Studies for AR = 1 wing showed that stall was delayed and extra (vortex) lift was obtained, because separation bubble got smaller in both chordwise and spanwise axes with effect of wing-tip vortices. Oil-flow experiments at higher angles of attack clarified the reason for vortex lift obtained from AR = 1 wing. However, there was an increase in drag coefficient as well as vortex lift, and stall delayed due to tip vortex. Turbulence intensity distributions pointed out location of the transition to turbulence; Reynolds stress and turbulence kinetic energy distributions indicated shear layer. Furthermore, in experiments of AR = 3 wing, the viscous forces and leading edge vortices were effective at Re = 2.5 × 104 and Re = 5 × 104, but flow over the wing at Re = 7.5 × 104 acted as a 2D flow. After α = 12°, bubble burst and stall consisted abruptly because effectiveness of 3D flow decreased over wing. Strouhal (St) numbers of vortex shedding frequencies in wake of AR = 3 wing had a certain difference from St = 0.17/sinα curve at lower angle of attack (α = 0° − 10°) due to separation bubble, but AR = 1 wings showed that St numbers were near St = 0.17/sinα curve.
The performance of the transition models on three-dimensional (3D) flow of wings with aspect ratios (AR) of 1 and 3 at low Reynolds number was assessed in this study. For experimental work; force measurements, surface oil and smoke-wire flow visualizations were performed over the wings with NACA4412 section at Reynolds numbers of 2.5 × 104, 5 × 104, and 7.5 × 104 and the angles of attack of 8 deg, 12 deg, and 20 deg. Results showed that the AR had significant effects on the 3D flow structure over the wing. According to the experimental and numerical results, the flow over the wing having lower ARs can be defined with wingtip vortices, axial flow, and secondary flow including spiral vortex inside the separated flow. When the angle of attack and Reynolds number was increased, wing-tip vortices were enlarged and interacted with the axial flow. At higher AR, flow separation was dominant, whereas wing-tip vortices suppressed the flow separation over the wing with lower AR. In the numerical results, while there were some inconsistencies in the prediction of lift coefficients, the predictions of drag coefficients for two transition models were noticeably better. The performance of the transition models judged from surface patterns was good, but the k–kL–ω was preferable. Secondary flow including spiral vortices near the surface was predicted accurately by the k–kL–ω. Consequently, in comparison with experiments, the predictions of the k–kL–ω were better than those of the shear stress transport (SST) transition.
Abstract. In this study, the flow over NACA4412 was investigated both numerically and experimentally at a different Reynolds numbers. The experiments were carried out in a low speed wind tunnel with various angles of attack and different Reynolds numbers (25000 and 50000). Airfoil was manufactured using 3D printer with a various aspect ratios (AR = 1 and AR = 3). Smoke-wire and oil flow visualization methods were used to visualize the surface flow patterns. NACA4412 aerofoil was designed by using SOLIDWORKS. The structural grid of numerical model was constructed by ANSYS ICEM CFD meshing software. Furthermore, ANSYS FLUENT TM software was used to perform numerical calculations. The numerical results were compared with experimental results. Bubble formation was shown in CFD streamlines and smoke-wire experiments at z / c = 0.4. Furthermore, bubble shrunk at z / c = 0.2 by reason of the effects of tip vortices in both numerical and experimental studies. Consequently, it was seen that there was a good agreement between numerical and experimental results.
Abstract. In this study, unsteady flow around flexible membrane wing which had aspect ratio of 1 (AR=1) was investigated experimentally at various Reynolds numbers (Re = 25000 and Re = 50000). Smoke-wire technique for flow visualization over the flexible membrane wing was utilized in the experiments. Digital Image Correlation system (DIC) was used for measuring deformation of AR = 1 flexible membrane wing. Instantaneous deformation measurements of membrane wing were combined with the flow field measurements. In low aspect ratio flexible membrane wings, unsteadiness includes tip vortices and vortex shedding, and the combination of tip vortices. In these types of wings, complex unsteady deformations occurred due to vortex shedding. The results showed that the increasing angle of attack results in increase of membrane deformation. Moreover, it was concluded that analysis of the instantaneous deformation revealed chordwise and spanwise, modes which were due to the shedding of leading-edge vortices as well as tip vortices. Consequently, vibrational mode decreased and maximum standard deviation location approached to the trailing edge by reason of increasing angle of attack.
Abstract. In this study, the effect of the aspect ratio on the aerodynamics characteristic of flexible membrane wings with different aspect ratios (AR = 1 and AR = 3) is experimentally investigated at Reynolds number of 25000. Time accurate measurements of membrane deformation using Digital Image Correlation system (DIC) is carried out while normal forces of the wing will be measured by helping a load-cell system and flow on the wing was visualized by means of smoke wire technic. The characteristics of high aspect ratio wings are shown to be affected by leading edge separation bubbles at low Reynolds number. It is concluded that the camber of membrane wing excites the separated shear layer and this situation increases the lift coefficient relatively more as compared to rigid wings. In membrane wings with low aspect ratio, unsteadiness included tip vortices and vortex shedding, and the combination of tip vortices and vortex shedding causes complex unsteady deformations of these membrane wings. The characteristic of high aspect ratio wings was shown to be affected by leading edge separation bubbles at low Reynolds numbers whereas the deformations of flexible wing with low aspect ratio affected by tip vortices and leading edge separation bubbles.
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