Theoretical and computational analysis of unsteady flow over an asymmetric airfoil
Various methods are employed for studying steady and unsteady flows over an asymmetric SD7003 airfoil. First, linear stability methods are used for the investigation of airfoil flow stability. Second, flows over the stationary airfoil with steady and unsteady freestream conditions are computed by the large eddy simulation (LES) method.The laminar separation bubble (LSB) region of the airfoil flow at Reynolds number of 60,000 and angle of attack (AoA) of 4° is analyzed by the local and global stability methods. Both methods correctly predict that the flow is unstable and transitions to turbulence at these conditions. The maximum growth rates predicted by the two methods are in agreement. The LES results for several steady freestream flows are shown to be consistent with the available experimental and numerical data, and then three types of unsteady freestream flows are simulated by the LES method. In the first type, the AoA is fixed at 4°, while the freestream velocity magnitude varies harmonically with a mean Reynolds number of 60,000, reduced frequency from ð/8 to 2ð, and amplitudes of 0.183 and 0.366 of the mean velocity. In the second type of unsteady flows, the freestream velocity magnitude is fixed at Reynolds number of 60,000, but the AoA varies harmonically with a mean value of 4°, reduced frequency from ð/8 to 2ð, and amplitudes of 4° and 8°. In the third type of unsteady flow, a wind gust model is employed. For the flows with oscillating freestream velocity magnitude, the effect of freestream unsteadiness on the aerodynamic forces, vorticity field, velocity fluctuations, and boundary layer separation and reattachment are studied. The vorticity plots show that the size of the recirculation region changes slightly during a freestream cycle. The mean lift and drag coefficients are nearly the same as those obtained for the steady mean freestream flow. However, there is a phase shift between the aerodynamic forces and the freestream velocity which is mainly affected by the reduced frequency. Higher reduced frequencies and freestream amplitudes cause "wider" oscillations in the lift and drag forces, separation and reattachment locations. For the case with the highest frequency and the largest amplitude, the spanwise velocity fluctuations are much lower than those of the streamwise and normal velocities, while the fluctuation in the streamwise velocity is slightly more than that of the normal velocity. For the flows with oscillating freestream AoA, higher reduced frequencies and AoA amplitudes cause wider oscillations in the lift and drag forces. The amplitude of the lift force is, however, about one order of magnitude larger than that of the drag force. Compared to the steady mean freestream flow, there is little change in the mean lift, while the mean drag is reduced due to Katzmayr effect. The size of the recirculation region changes significantly during a freestream cycle at low reduced frequencies and high AoA amplitudes. Higher reduced frequency decreases the amplitude of separation point oscillations, but higher AoA amplitude increases it. Significant oscillation in the surface friction occurs near the trailing edge at high reduced frequencies and AoA amplitudes, resulting from strong vortex shedding at the trailing edge. Compared to the steady mean freestream flow, the mean separation point moves downstream and the mean reattachment point moves upstream when the freestream velocity magnitude or AoA oscillates.The wind gust simulation shows a rapid increase in the aerodynamic loads on the airfoil at the beginning of the gust. It is also shown that small fluctuations in the gust can cause significant fluctuations in the aerodynamic forces.
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Electronic Theses & Dissertations
 Copyright Status
 In Copyright
 Material Type

Theses
 Authors

Qin, Shiwei
 Thesis Advisors

Jaberi, Farhad
Zhuang, Mei
 Committee Members

Jaberi, Farhad
Zhuang, Mei
Diaz, Alejandro
Zhou, Zhengfang
Koochesfahani, Manoochehr
 Date
 2012
 Program of Study

Mechanical Engineering
 Degree Level

Doctoral
 Language

English
 Pages
 xxi, 138 pages
 ISBN

9781267833419
1267833416
 Permalink
 https://doi.org/doi:10.25335/M58Q15