Air Jet Flow Control On Pitching Aerofoil

Dynamic stall occurs when a wing or rotor blade exceeds the stall angle, leading to sudden flow separation and the formation of a strong vortex near the leading edge. This results in sharp increases in lift, drag, and pitching moments. It is commonly observed in systems such as helicopter rotors and wind turbines, where rapid motion or oscillations create highly transient flow conditions, impacting performance and contributing to structural fatigue.

In helicopters, dynamic stall significantly limits the safe flight envelope, causing low-speed flow separation on retreating rotor blades during high angles of attack. This process generates a strong stall vortex, resulting in momentary high lift, drag forces, and pitching moments that affect both performance and rotor durability. This experimental study explores the use of air jet vortex generators (AJVGs) to control dynamic stall in low-speed conditions.

AJVGs are devices designed to control boundary layer separation and delay stall by injecting air into the flow over an aerofoil. Initial studies by Wallis in 1952^[1] demonstrated the potential of air jets for turbulent mixing and re-energizing the boundary layer. Subsequent research has identified optimal jet and pitch angles that effectively enhance mixing. Pulsed AJVGs have been found to be more efficient than steady blowing, providing better stall suppression with reduced mass flow requirements^[2]. Studies by Seifert et al.^[3] and McManus et al.^[4] highlighted the importance of parameters such as the jet-to-freestream velocity ratio (VR), pulsing frequency (F+), and duty cycle, which influence the generation of vortical structures and the effectiveness of AJVGs in suppressing boundary layer separation.

This work presents experimental datasets using both steady and pulsed AJVGs to suppress dynamic stall on a sinusoidal pitching RAE9645 aerofoil. Tests at a Reynolds number of 1 million and reduced pitching frequencies between 0.01 and 0.10 evaluated the effects of jet momentum coefficient, duty cycle, and pulsing frequency on dynamic stall suppression. Reduced pitching frequency (k) or non-dimensional aerofoil pitching frequency is a non-dimensional parameter used in unsteady aerodynamics, defined as:

$$k=\frac{\omega c}{2U}$$

where ω is the angular frequency, c is the chord length, and U is the freestream velocity. It represents the ratio of unsteady motion effects to convection effects, with higher values indicating stronger unsteady aerodynamic influence.

All results from these measurements can be found in Prince et al. ^[5], which offers a detailed description of the test conditions and wind tunnel setup, along with access to the full dataset.

Model geometry

The test model developed for the experiments utilized the RAE9645 aerofoil profile (figure 1), representative of a modern helicopter rotor section, with a chord length of 0.5m and an effective span of 1.1m, between the end plates. Oval end plates were fitted to maintain quasi-2D spanwise flow behavior. The model was constructed from molded fiberglass upper and lower surface sections with internal insert sections for structural support. The internal pressure-regulated plenum chamber, constructed from high-pressure plastic pipe, fed air jets at the same pressure at both ends.

The model featured an array of 20 air jet orifices located at 10% chord on the upper surface, spaced 45mm apart. Each orifice had a circular cross-sectional area of 18mm² and was pitched at 30° from the surface tangent, skewed at 60° with respect to the free-stream direction. These angles were chosen to induce co-rotating streamwise vortices over the aerofoil upper surface, in line with the optimal settings for generating maximum vorticity in the downstream boundary layer. The geometry and spacing of the air jet vortex generators (AJVGs) were designed according to established guidelines from previous researchers.

The internal AJVG actuator and ducting system consisted of the plenum pipe, small tubes connecting to pulsed air injectors, and specially designed jet nozzle modules. The air jet nozzles were designed as plastic inserts to link the air injectors with the surface orifices, ensuring the correct pitch and skew angle while maintaining a smooth upper surface profile.

Measurement Locations and Techniques

The model was instrumented with 39 dynamic pressure transducers, flush-mounted in surface pressure tappings positioned at center-span along the chordline. These transducers, rated for a 34kPa range, were strategically located as shown in Figure 1. Additional 10 transducers were mounted on the upper surface, a spanwise distance of 8.5mm from the main array, to verify measurements. The resulting pressure distributions were integrated to calculate aerodynamic forces and moments, with surface skin friction neglected. Plenum chamber pressure was measured using a differential pressure transducer mounted centrally inside the pipe wall. The instantaneous angle of attack of the aerofoil model was measured using an angular displacement transducer connected to the model’s rotating shaft. The signal from the transducer was split to initiate data sampling at a preset angle, and to provide feedback to the hydraulic actuator controller.

The experiments were conducted in the University of Glasgow Handley-Page low-speed, closed-circuit wind tunnel. The model was mounted vertically in the octagonal working section, with oscillatory motion achieved using a hydraulic linear actuator. The actuator was driven by a 7.0MPa supply pressure and controlled via a MOOG 76 series 450 servo-valve and UNIDYNE servo controller. The model’s angular displacement was measured with an accuracy of ±0.1°, providing precise control over the pitching motion.

The accuracy of surface pressure measurements was estimated to be ±0.02 at a freestream airspeed of 30m/s, with integrated normal forces and pitching moments within 5% of those measured by a force balance at zero angle of attack, improving to 1% at high angles of attack.

Experimental Facility

All tests were performed in the University of Glasgow Handley-Page low-speed, closed-circuit wind tunnel. The test section dimensions are 1.52m high x 2.13m wide, 3m long, 4:1 contraction ratio, maximum speed 55 m/s, free stream turbulence around 0.4%.

Flow Conditions

Mach number (M_$\infty$) = 0.1
Inlet velocity = 30 m/s
Free-stream turbulence $\approx$ 0.4 %
Chord length (c ) = 0.5 m
Reynolds number based on chord, Re_c = 1.0 x 10⁶
Mean angle of attack during pitching = 6^o – 20^o
Pulsed air-jet frequency = 10-200 Hz
Pitching frequency = 6.03 rad/sec – 18.09 rad/sec(0.96 Hz – 2.88 Hz)
Blowing pressure = 138 kPa – 310 kPa (20 psi – 45 psi)
Duty Cycle = 0.4-0.75

Datasets

(i) Effect of varying blowing pressure

For both Sinusoidal Pitching motion and static aerofoil

Other variables -1 : Pitching frequency =12.37 rad/s (1.97 Hz); Blowing frequency = 71Hz; Duty Cycle = 0.5
Other variables -2 : Pitching frequency =18.09 rad/s (2.88 Hz); Blowing frequency = 71Hz; Duty Cycle = 0.5

Other variables -3 : Blowing frequency = 71Hz; Duty Cycle = 0.5

(ii) Effect of varying Duty Cycle (DC)

Other variables -4 : Pitching frequency =12.37 rad/s (1.97 Hz); Blowing frequency = 71Hz; Blowing pressure = 172 kPa (25 psi); Mean pitching angle = 16°

Other variables -5 : Blowing frequency = 71Hz; Blowing pressure = 172 kPa (25 psi);

(iii) Effect of Steady blowing

For various pressure (steady Blowing)

Other variables -6 : Pitching frequency =12.37 rad/s (1.97 Hz); Blowing frequency = 0Hz

Other variables -7 : Blowing frequency = 0Hz

For various mean angle (steady Blowing)

Other variables -8 : Pitching frequency =12.37 rad/s (1.97 Hz); Blowing frequency = 0Hz

For various pitching frequency (steady Blowing)

Other variables -9 : Blowing frequency = 0Hz; Mean pitching angle = 16°

(iv) Effect of varying blowing (pulsed air jet) frequency

Other variables -10 : Pitching frequency =12.37 rad/s (1.97 Hz); Mean pitching angle = 16°; Duty Cycle = 0.5
Other variables -11 : Duty Cycle = 0.5

(v) Effect of varying Pitching frequency

Other variables -12 : Blowing pressure = 172 kPa (25 psi); Blowing frequency = 71Hz; Mean pitching angle = 16°; Duty Cycle = 0.5

(vi) Effect of varying Mean angle of Pitching

Other variables -13 : Blowing pressure = 172 kPa (25 psi); Blowing frequency = 71Hz; Duty Cycle = 0.5

Open Access

This metadata is provided under the Creative Commons Attribution-NonCommercial 4.0 International License (https://creativecommons.org/licenses/by-nc/4.0/). This license allows for unrestricted use, distribution, and reproduction in any medium, provided that proper credit is given to the original author(s) and the source. Also provide a link to the license, and indicate if any changes were made. Furthermore, this license does not allow the use of this material for commercial purposes.

References

1. Wallis, R. A., 1952. The Use of Air Jets for Boundary Layer Control. Aeronautical Research Labs, Aero. Note 110, Melbourne, Australia.URL

2. McManus, K., Ducharme, A., Goldey, C. and Magill, J., 1996. Pulsed jet actuators for suppressing flow separation. In 34th Aerospace Sciences Meeting and Exhibit (p. 442).

3. Seifert, A., Bachar, T., Koss, D., Shepshelovich, M. and Wygnanski, I., 1993. Oscillatory blowing: a tool to delay boundary-layer separation. AIAA journal, 31(11), pp.2052-2060.

4. McManus, K., Joshi, P., Legner, H. and Davis, S., 1995, June. Active control of aerodynamic stall using pulsed jet actuators. In Fluid Dynamics Conference (p. 2187).

5. Prince, S., Green, R., Coton, F. and Wang, Y., 2019. The effect of steady and pulsed air jet vortex generator blowing on an airfoil section model undergoing sinusoidal pitching. Journal of the American Helicopter Society, 64(3), pp.1-14.