This study experimentally investigates aerodynamic characteristics and flow fields of a smooth owl-like airfoil without serrations and velvet structures. This biologically inspired airfoil design is intended to serve as the main-wing for low-Reynolds-number aircrafts such as micro air vehicles. Reynolds number dependency on aerodynamics is also evaluated at low Reynolds numbers. The results of the study show that the owl-like airfoil has high lift performance with a nonlinear lift increase due to the presence of a separation bubble on the suction side. A distinctive flow feature of the owl airfoil is a separation bubble on the pressure side at low angles of attack. The separation bubble switches location from the pressure side to the suction side as the angle of attack increases and is continuously present on the surface within a wide range of angles of attack. The Reynolds number dependency on the lift curves is insignificant, although differences in the drag curves are especially pronounced at high angles of attack. Eventually, we obtain the geometric feature of the owl-like airfoil to increase aerodynamic performance at low Reynolds numbers.
Biomimetics includes the concepts and principles acquired from nature for application in science and engineering such as in aerodynamic and fluid control devices. The serrated wing of the barn owl is a representative example. Many studies have been reported that using a serrated design on the leading edge of an aircraft wing reduces aerodynamic noise [
With a view to the main-wing airfoil design such as MAVs based on the biomimetics, the aim of the present work is to experimentally determine the Reynolds number dependence on the aerodynamic performance of an owl-like airfoil at low Reynolds numbers and to investigate the flow fields that lead to aerodynamic change using flow visualization.
The geometry of the test model airfoil in this study and that of the owl airfoil given by Liu et al. [
airfoil (denoted by the red dash line) in
A series of experiments were performed using an open-circuit low-speed wind tunnel at Kyushu University. The rectangular cross section of the test section was 180 mm × 360 mm. The test section was covered on every side with an acrylic sidewall. Turbulence intensity was approximately 0.3% at 5.0 m/s. Lift and drag measurements were made by vertically mounting the airfoil model on the test section and attaching a three-component microforce balance system (LMC-3501-5N, NISSHOELECTRIC-WORKS) at the bottom of the test section. Each related load of the balance system was set at 5 N for the lift and drag forces, and 0.5 Nm for the momentum. The uncertain accuracy of the aerodynamic force measurements is estimated at approximately 0.2%, based on our micro-force calibration tests. Lift and drag forces were calibrated beforehand using standard weights. The test model was installed in the test section with a gap of 0.5 mm between the sidewall and the model tip in accordance with Rae and Pope [
The same setup for the test model was used to perform the smoke visualization
experiments using the smoke wire technique. Two nichrome wires with 0.1 mm diameter were horizontally set up at the inlet of the test section and coated with liquid paraffin as the working substance. The wires were located 75 mm forward of the leading edge of the airfoil and perpendicular to the leading edge. An alternating electric voltage from a Volt slider (S-130-10, YAMABISHI ELECTRICS) was applied across the wires to generate smoke. A green laser with a 2 W output (SDL-532-2000T, Scitec Instruments Ltd.) was used as a light source. The smoke-line was recorded using a high-speed video camera (Miro C110, Phantom) at a frame rate of 400 frames per second with an exposure time of 2400 μs, and image resolution of 1280 × 1024 pixels.
Aerodynamic force measurements were conducted in the Reynolds number range 23,000 - 60,000. The Reynolds number was based on the chord length of the test model. Since the cruise Reynolds number of the Mars airplane currently under consideration by the Japan Aerospace Exploration Agency is Re = 23,000, we recognized that value as a standard Reynolds number. The corresponding free-stream velocities for each Re are shown in
With respect to lift measurements, the experimental results in
Reynolds Number | 23,000 | 30,000 | 40,000 | 50,000 | 60,000 |
---|---|---|---|---|---|
Flow Velocity (m/s) | 4.7 | 6.0 | 8.0 | 10.0 | 12.0 |
linearly at low angles of attack from α = −5 deg to 4 deg. A conventional symmetric airfoil, such as the NACA0012, at low angles of attack (around Re = 20,000) has a lift curve slope that is much lower than 2 πα due to trailing-edge separation on its suction side and attached flow on its pressure side [
to eventually flatten out or dip at α > 6 deg. At high angles of attack above α = 9 deg, the plots diverge again and the maximum lift coefficient increases with Reynolds number. The relation between Reynolds number and the maximum lift to drag ratio (L/Dmax) is illustrated in
Visualization of the flow fields on the suction side of the airfoil by a smoke wire method at Re = 23,000 from α = 1 deg to 13 deg is shown in
edge. Although the separation point remained fixed at the leading edge, the reattachment point gradually moved to the trailing edge, and the separation bubble stretched [
Angle of attack (α) [deg] | 1 ≤ α ≤ 4 | 5 ≤ α ≤ 11 | 12 ≤ α |
---|---|---|---|
Flow field | Trailing-edge separation | Formation of separation bubble | Leading-edge separation |
The flow fields on the pressure side of the airfoil at Re = 23,000 are shown in
At α = 5 deg, the position of the separation bubble switched from the pressure side to the suction side. This tendency is very similar to the property observed in a circular arc airfoil [
In this study, we performed aerodynamic measurements and flow visualizations of the smooth owl-like airfoil without serrations and velvet structures at low Reynolds numbers using a low-speed wind tunnel. The Reynolds number dependency
on the aerodynamic performance was also evaluated for Re = 23,000 - 60,000.
Compared with a conventional symmetric airfoil such as the NACA0012 airfoil, the owl airfoil had much higher lift performance at Re = 23,000 with a nonlinear lift increase from α = 4 deg to α = 6 deg due to the formation of the separation bubble on the suction side. A distinctive flow feature was a separation bubble formed on the pressure side at low angles of attack. The presence or absence of the separation bubble on the pressure side was largely attributed to differences in aerodynamics between the NACA0012 airfoil and the owl airfoil. Furthermore, the position of the separation bubble switched from the pressure side to the suction side at α = 5 deg, and was continuously present on the airfoil surface at wide-ranging angles of attack. In particular, the separation bubble on the pressure side contributed greatly to lift increase while curbing drag as the positive pressure region inside the separation bubble on the pressure side at the strong undercamber position reduced the drag at low angles of attack. The Reynolds number dependency on the lift curves was not large, although differences in the drag curves became more pronounced with higher angles of attack at α > 5 deg.
In general, increase in pressure drag is associated with a laminar separation or formation of a separation bubble on the pressure side. Hence, to reduce drag, these types of flow phenomena would need to be suppressed. However, the owl airfoil exhibited high lift to drag ratios using its strong undercamber, especially from x/c = 0.2 to 0.6, and thereby taking advantage of the separation bubble on the pressure side. Information from these experiments will prove to be useful to aerodynamic studies of low-Reynolds-number biomimetic wings and flapping wings for MAVs.
We would like to sincerely thank Mitsuru Ishii for providing the test model. We would like to thank Mitsuru Nakauchi and Yuuki Ohkubo, who are current students at Kyushu University, for their contribution to this research.
Anyoji, M., Wakui, S., Hamada, D. and Aono, H. (2018) Experimental Study of Owl-Like Airfoil Aerodynamics at Low Reynolds Numbers. Journal of Flow Control, Measurement & Visualization, 6, 185-197. https://doi.org/10.4236/jfcmv.2018.63015
b: Span of a real owl wing
c: Chord length [m]
CD: Drag coefficient
CL: Lift coefficient
D: Drag [N]
L: Lift [N]
Re: Reynolds number
t: Test model thickness [m]
x: Chordwise coordinate
y: Vertical coordinate
z: Spanwise coordinate
α: Angle of attack [deg]