Stephen M. Batil

Bio: Stephen M. Batil is an academic researcher from University of Notre Dame. The author has contributed to research in topics: Boundary layer & Airfoil. The author has an hindex of 1, co-authored 1 publications receiving 231 citations.

Experimental Studies of Separation on a Two-Dimensional Airfoil at Low Reynolds Numbers

Abstract: The laminar separation, transition, and turbulent reattachment near the leading edge of a two-dimensional NACA 663 -018 airfoil were investigated using a low-speed, smoke visualization wind tunnel. Lift and drag force measurements were made using an external strain gage balance for a chord Reynolds number range of 40,GOO400,000. An extensive flow visualization study was performed and correlated with the force measurements. Experiments were also conducted with distributed surface roughness at the leading edge and external acoustic excitation to influence the development of the airfoil boundary layer. This study delineates the effects of angle of attack and chord Reynolds number on the separation characteristics and airfoil performance. Nomenclature c = model chord cd = section profile drag coefficient (uncorrected) cf = section lift coefficient (uncorrected) Cp = pressure coefficient / = acoustic frequency, Hz R = reattachment location Rc = Reynolds number based on chord length, U^ civ S = separation location T = location of approximate end of transition £/«, = freestream velocity x/c = nondimensional distance along chord a = angle of attack v - kinematic viscosity

Performance Variations of Leading-Edge Tubercles for Distinct Airfoil Profiles

Abstract: An experimental investigation has been undertaken to determine the influence of sinusoidal leading-edge protrusions on the performance of two NACA airfoils with different aerodynamic characteristics. Force measurements on full-span airfoils with various combinations of tubercle amplitude and wavelength reveal that when compared to the unmodified equivalent, tubercles are more beneficial for the NACA 65-021 airfoil than the NACA 0021 airfoil. It was also found that for both airfoil profiles, reducing the tubercle amplitude leads to a higher maximum lift coefficient and larger stall angle. In the poststall regime, however, the performance with largeramplitude tubercles is more favorable. Reducing the wavelength leads to improvements in all aspects of lift performance, including maximum lift coefficient, stall angle, and poststall characteristics. Nevertheless, there is a certain point at which further reduction in wavelength has a negative impact on performance. The results also suggest that tubercles act in a manner similar to conventional vortex generators.

Wind tunnel tests of wings at Reynolds numbers below 70 000

Abstract: Rectangular planform wings were tested at Reynolds numbers as low as 20 000 in a low turbulence wind tunnel. The lift and drag measurements on a NACA 0012 profile were compared with those for thin flat and cambered plates. For all Reynolds numbers below 70 000 the best profile was a thin plate with a 5% circular arc camber. At all turbulence levels this profile produced the greatest lift-drag ratio, and had the highest lift coefficient at all angles of attack. The 5% camber and all of the thin plates tested were relatively insensitive to either a variation in the Reynolds number, or an increase in the wind tunnel turbulence level, whereas the NACA 0012 was very seriously affected by either, at Reynolds numbers below 50 000.

Experimental Evaluation of Sinusoidal Leading Edges

Abstract: P REVIOUS studies on increasing airfoil lift and improving stall characteristics have addressed various passive and active approaches to modifying the leading and trailing edge shapes. The passive approaches have covered such methods as rippling the trailing edge, applying serrated-edge Gurney flaps, or modifying the leading-edge (LE) profile [1,2]. Other efforts have effectively eliminated the dynamic stall of an NACA 0012 airfoil by perturbing the LE contour as little as 0.5–0.9%of the chord [3]. Levshin et al. [4] demonstrated that sinusoidal LE planforms on an NACA 63-021 airfoil section decreased maximum lift, but extended the stall angle by almost 9 deg. The larger amplitude sinusoids created “softer” stall characteristics by maintaining attached flow at the peaks despite separated flow in the troughs. These tests were performed to simulate the effects of LE tubercles on humpback whale (Megaptera novaeangliae) flippers. Prior work by the authors also reported wind tunnel measurements for idealized scale models of humpback whale flippers [5]. One model had a smooth leading edge and a secondmodel had sinusoidal bumps (tubercles) along the leading edge for the outer 2 3 of the span. It was found that the addition of tubercles to a 3-D idealized flipper increased the maximum lift coefficient while reducing the drag coefficient over a portion of the operational envelope. It is thought that the tubercles on the flipper leading-edge enhance the whale’s ability to maneuver to catch prey [6]. Though the work to date regarding sinusoidal or serrated leading-edge planforms is largely motivated by marine mammal locomotion, the effects of extending the stall point for lifting surfaces at similar Reynolds numbers (Re) may have application to small-UAV (unmanned aerial vehicle) design and the inevitable laminar stall problems [7]. However other relevant applications might benefit from the effects of simulated tubercles such as stall alleviation/separation control on sailboat centerboards or wind turbines, where an expanded operating envelope could improve the overall effectiveness of the blade [8,9]. In the present work, a better understanding is sought of the mechanism of the improvements measured in previous experiments, with a greater applicability in mind. The authors seek to determine whether the performance improvements resulted from enhancements to the sectional characteristics of wings with tubercles (i.e., essentially 2-D effects), or from Reynolds number effects on a tapered planform, or from other 3-D effects such as spanwise stall progression.

Vortex shedding and shear-layer instability of wing at low-Reynolds numbers

Abstract: Flow patterns and characteristics of vortex shedding and shear-layer instability of a NACA 0012 cantilever wing are experimentally studied. Smoke-wire and surface oil-flow techniques are employed to visualize the flow patterns and evolution of vortex shedding. Hot-wire anemometers are used to characterize the frequency domain of the unsteady flow structures. Several characteristic flow modes are classified in the domain of chord Reynolds number and root angle of attack. Effects of the juncture and wing tip are discussed. Vortex shedding can be classified into four characteristic modes. Vortex shedding at low and high angles of attack are found to have different dominant mechanisms. Effects of the juncture and wing tip on the vortex shedding are discussed. Shear-layer instabilities are found to be closely related to the behaviors of the vortex shedding. Behaviors of the shear-layer instabilities can be traced back to the characteristics of the boundary layer on the suction surface of the airfoil.

Effect of acoustic excitation on the flow over a low-Re airfoil

Abstract: Wind-tunnel measurements of lift, drag, and wake velocity spectra were carried out under (tonal) acoustic excitation for a smooth airfoil in the chord-Reynolds-number Re(c) range of 40,000-140,000. The data were supported by smoke-wire flow-visualization pictures. Small-amplitude excitation in a wide, low-frequency range is found to eliminate laminar separation that otherwise degrades the airfoil performance at low Re(c) near the design angle of attack. Excitation at high frequencies eliminates a prestall, periodic shedding of large-scale vortices. Significant improvement in lift is also achieved during poststall, but with large-amplitude excitation. Wind-tunnel resonances strongly influence the results, especially in cases requiring large amplitudes.