ACTA AERONAUTICAET ASTRONAUTICA SINICA >
Aerodynamic characteristics of airfoil and evolution of laminar separation at different Reynolds numbers
Received date: 2016-04-20
Revised date: 2016-09-19
Online published: 2016-09-21
Supported by
National Natural Science Foundation of China (11672282, 91216116); National Basic Research Program of China (2014CB744100)
Due to the outstanding air viscosity at low Reynolds number, laminar separation is common on airfoil surface, resulting in the deterioration of aerodynamic characteristics compared with those at general Reynolds number. The numerical simulation technology for solving the unsteady compressible Navier-Stokes equations with the preconditioning Roe method and oil flow visualization in the low Reynolds number low turbulence wind tunnel is used to investigate the aerodynamic characteristics and flow structures of the FX63-137 airfoil at different Reynolds numbers. The main conclusions are described in the following. Two clear oil accumulation lines are captured in laminar separation flow through oil flow visualization. Numerical results show that the lines are the primary and secondary separation lines respectively. The qualitative and quantitative results of the two technologies are in good agreement, showing the effectiveness and credibility of the research method in this paper. Airfoil dynamic characteristics and laminar flow separation structures change significantly with the decrease of Reynolds numbers from 500 000 to 20 000. With the sharp increase of drag coefficient and reduction of lift coefficient, the time-average flow structure develops from attachment to classical long laminar separation bubble, and eventually evolves into trailing-edge laminar separation bubble. Correspondingly, two types of separation bubbles are significantly different in unsteady flow. There are different critical Reynolds numbers for the drag coefficient and lift coefficient. The reason is that the drag coefficient increases as the long laminar separation bubble can increase the pressure drag, while the lift coefficient sharply decreases as the appearance of trailing-edge laminar separation bubble can cause the decrease of equivalent airfoil camber. Unsteady results display that it is because of the generation and shedding of periodic vortex on airfoil surface, the lift coefficient has periodic fluctuation. When the main vortex is about to shed, the streamlines are near the trailing edge, and the lift coefficient reaches the peak. When the fluid is rolled up to generate the secondary vortex, the streamlines separate significantly, and the lift coefficient reduces to the valley.
LIU Qiang , LIU Qiang , BAI Peng , LI Feng . Aerodynamic characteristics of airfoil and evolution of laminar separation at different Reynolds numbers[J]. ACTA AERONAUTICAET ASTRONAUTICA SINICA, 2017 , 38(4) : 120338 -120338 . DOI: 10.7527/S1000-6893.2016.0257
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