Numerical Simulations of the Aerodynamic Characteristics of Circulation Control Wing Sections

1. Numerical Simulations of the Aerodynamic Characteristics of Circulation Control Wing Sections Ph.D Thesis Defense By Yi Liu Advisor: Prof. Lakshmi N.Sankar Supported by NASA Langley Research Center

2. Motivation and Objectives Circulation Control Wing Technology Previous Research Work Mathematical and Numerical Formulation 2D Simulation Results and Discussion Steady Blowing Results Pulse Blowing Results 3D Simulation Results and Discussion Tangential Blowing on a Wing-flap Configuration Spanwise Blowing over a Rounded Wing-tip Conclusions from the 2D Simulations Conclusions from the 3D Simulations Recommendations Outline of Presentation

3. Noise pollution from the large aircraft has become a major problem that needs to be solved. NASA proposed a plan to reduce the noise by a factor of four (20dB) by 2025. A major source of large aircraft airframe noise during take-off and landing is the high-lift system - namely flaps, slats, associated with flap-edges and gaps. The high-lift system also contains many moving parts, which add to the weight of the aircraft, and are costly to build and maintain. These devices for generating high lift are necessary for large aircraft that use existing runways. Motivation and Objectives

4. Boeing 737 Wing/Flap System (Paper by Robert Englar)

5. An alternative to conventional high-lift systems is the Circulation Control Wing (CCW) technology. • The CC wing can generate the same high lift with much less complexity compared to the high-lift system, and many noise sources such as flaps and slats, can also be eliminated by the CC wing. • For example, as shown in previous figure, there are just 0-3 moving elements per wing for a Circulation Control wing with leading edge blowing, compared to 15 moving parts of a conventional Boeing 737 wing with high-lift systems.

6. Circulation Control Wing Concept • Circulation Control Aerodynamics: In this approach a tangential jet is blown over a highly curved aerodynamic surface (the Coanda surface) to increase or modify the aerodynamic forces and moment with few or no moving surfaces. • Figure (Taken from paper by Englar) shows a traditional Circulation Control Airfoil with a rounded trailing edge.

7. Circulation Control Wing Concept • In general, the driving parameter of Circulation Control is the jet momentum coefficient, Cm, which is defined as: • At very low momentum coefficients, the tangential blowing will add energy to the slow moving flow near the surface. This will delay or eliminate the separation, and is called Boundary Layer Control. • When the momentum coefficient is high, the lift of the wing will be significantly increased. This is called Circulation Control. • The lift augmentation, which is defined as CL / Cm, can exceed 80 as shown in previous figure.

8. Advanced CC Airfoil • The advanced CC airfoil, i.e. a circulation hinged flap, was developed by Englar et al to replace the traditional CC airfoil. • This advanced CC airfoil use a small trailing edge flap with a large-radius arc upper surface and a flat low surface. The flap can be deflected 00 < f < 900. • During take-off / landing, the flap is deflected, thus generating very high lift like a traditional rounded trailing edge CC airfoil. • During cruise, f = 00, leading to a conventional airfoil shape with a sharp trailing edge that significantly reduces the drag.

9. Some Applications of the CCW Technology • STOL (short take-off and landing) aircraft: Englar et al (1979) • Advanced Subsonic Aircraft and High Speed Civil Transport (HSCT): Englar et al (1994, 1999) • Circulation Control Rotor (CCR): Wilkerson et al (1973, 1979) • X-wing stopped rotor aircraft: Williams et al (1976) • Ground heavy vehicles, such as large tractors and trailing trucks: Englar (2000) • There are many other potential applications for Circulation Control or Pneumatic Aerodynamic technology, which are summarized in the paper by Englar (2000).

10. Previous Research Work • The early research work about Circulation Control was done in England by Cheeseman (1966) and Kind (1967) et al. • This concept was introduced into United States by Navy researchers in the 1970s. The David Taylor Naval Ship Research and Development Center (DTNSRDC) became a major center for the CC study. • Experiments by Williams and Howe (1970), Englar (1970, 1975), Abramson (1975), Abramson and Rogers (1983) and others in DTNSRDC examined the effect of a wide range of parameters on CC airfoils performance, including the geometric factors such as the thickness, camber, angle of attack, and free-stream conditions such as Mach number. • Englar and Applegate (1984) gave a very good summary of this research work for the years 1969 through 1983. • Recently, many experimental studies have been focused on the CCW applications for the rotary and fixed wing aircraft.

11. Previous Research Work • Acoustic studies for CC wings are very limited. Salikuddin, Brown and Ahuja (1987), Howe (2002) and Munro(2002) are the only known work on CCW. • Early numerical research by Davork et al (1979, 1983), based on potential methods did not achieve enough accuracy for CC airfoil design purpose. • Recently numerical studies based on the Navier-Stokes equations, such as Berman (1985), Pulliam (1985), Viegas et al (1986), and Shrewbury (1985, 1986, 1989) etc, have demonstrated that Navier-stokes simulations can provide good estimates of the lift, pressure distribution, and pitch moments of CC airfoils provided the turbulence model is accurate enough to give a reasonable good estimate of the jet separation point from the Coanda surface.

12. Previous Research Work • Other characteristics of CC airfoils, such as dynamic stall (Shrewbury 1990), jet stall (Linton 1994), and unsteady effects (Liu and Sun 1996) etc, have also been studied by Navier-Stokes methods. • A limited number of numerical studies have also been done for the advanced hinged flap CC airfoil by Englar and Smith et al (1993). • Studies by Wygnansky et al (1996,2000), Lorber et al (2000), Wake et al (2001), and Hassan (1998) etc, have been done on the use of synthetic jets (massless jets) to control the boundary layer and eliminate flow separation. However, studies on using pulsed jets to achieve high lift with relative less mass flow rate compared to a steady jet are very limited (Olyer 1972). • The use of advanced turbulence models (Slomski et al 2002), Large-eddy Simulation (Yang and Voke 2001) and Direct Numerical Simulation (Li and Liu 2003) to model the CC airfoil numerically have also been reported in last two years.

13. Research Objectives • Computational modeling of advanced dual radius CCW configuration • Assessment of the use of pulse jets to achieve desired high lift values, at lower mass flow rates • Evaluation of Circulation Control for the elimination or modification of flap edge vortices and tip vortices

14. Three-dimensional compressible unsteady Reynolds Averaged Navier-Stokes equations are solved in a strong conservation form on curvilinear coordinates. This solver can be used in both a 2D mode and a 3D mode in this study for different applications. The scheme is second or fourth order accurate in space and first order accurate in time. Baldwin-Lomax and Spalart-Allmaras one-equation turbulence models have been used. The jet slot location, slot size, blowing velocity and direction of blowing can easily be varied in the analysis. Mathematical and Numerical Formulation

15. Initial and Boundary Conditions • Initial flow conditions are set to free stream values inside the flow field. • Boundary Conditions • Outer Boundary • Solid Surface Boundary • Wake Cut Boundary • Jet Slot Exit Boundary

16. The driving parameter for jet blowing is the momentum coefficient, Cm, defined as follows: Jet Slot Boundary Conditions is the mass flow rate of jet flow Where • The Cm, orientation of the jet and the total temperature of jet are specified in the analysis. • Other quantities such as pressure and density are found by extrapolation and /or Ideal Gas Law. • The total jet pressure can also be specified as the boundary condition instead of the momentum coefficient.

17. Code Validation • The figures are the Cp distribution at two span locations of a small aspect-ratio wing made of NACA 0012 airfoil sections. • The results are in good agreement with the measured data (from Bragg and Spring 1987) except near the tip region where increased grid resolution is needed.

18. Lift distribution along span for NACA 0012 wing. • Coarse Grid (121*21*41); Fine Grid (151*51*51)

19. 2D Steady Blowing Results • Steady blowing performance at different Cm values, and at different angles of attack • Effects of parameters that influence the momentum coefficient: • Free-steam velocity effects with fixed Cm • Jet slot height effects with fixed Cm • Other considerations for the CC airfoil: • Comparison with the unblown baseline case • Steady blowing at a given total jet pressure • Comparison with conventional high-lift systems • Leading edge blowing

20. The CCW Airfoil

21. The Computational Grid

22. Flow Conditions • P = 14.2 psia = 0.9324 atm •  = 0.00225 slugs/ft3 = 1.1596 kg/m3 • V = 94.3 ft/sec = 28.743 m/s • M = 0.0836, Re = 0.395 * 106 • Chord of the Airfoil : C = 8” = 0.20 m • Jet Slot Height : h = 0.015” = 0.0004 m  0.2 % Chord • Jet slot is located at x/c = 88.75% on the upper side of the airfoil. • These values closely match the test conditions.

23. Lift Coefficient vs. Cm Angle of Attack 0 degrees, Integral Flap at 30 degrees

24. Lift Coefficient vs. Angle of Attack

25. The Stream lines over CC airfoil, Cm = 0.1657,  = 60

26. Free-stream Velocity Effects with Fixed Cm Cm = 0.1657, h = 0.015 in. and V, exp = 94.3 ft/sec

27. Jet Slot Height Effects with Fixed Cm The Efficiency vs. Jet Slot Height, V = 94.3 ft/sec

28. Jet Slot Height Effects with Fixed Cm The Mass Flow Rate vs. Jet Slot Height, V = 94.3 ft/sec

29. Comparison with the Unblown Case

30. The Stream Lines for the Blowing Case

31. Dominant Vortex Shedding Frequency Scott’s measurement =1600 Hz The FFT of the Lift Coefficient Variation with Time Dominate Vortex at 1080 Hz Acoustic Measurement at 1600 Hz Frequency (Hz) Frequency (Hz)

32. Steady Blowing at Given Total Jet Pressure

33. Lift Coefficient vs. Jet Momentum Coefficient

34. Comparison with a Conventional High-lift System Airfoil • The figures show the high-lift systems configuration with a 300 fowler flap and the body-fitted grid. • The results are obtained with a 2-D multi-block version of the present method.

35. For the multi-element airfoil, high lift is achieved by changing the angle of attack; For the CCW airfoil, high lift is achieved by changing the blowing coefficient while the angle of attack is fixed at 0 degrees.

36. Leading Edge Blowing • At high angles of attack, the leading edge separation and stall can occur for the CC airfoil, due to the large pressure gradients. • The stall angle is decreased quickly with the increase of the jet momentum coefficient of the trailing edge blowing. • Leading edge Coanda blowing can eliminate this and increase the stall angle. • In reality, because CCW airfoils can achieve very high lift even at zero angle of attack with a small amount of blowing, there is no real need for operation at high angles of attack unless maneuver requires it.

38. 2D Pulsed JetResults • Pulsed jet studies were done to answer: • ---- Can pulsed jets be used to achieve desired increases • in the lift coefficient at lower mass flow rates relative • to a steady jet? • ----What is the optimum wave shape for the pulsed jet, • ie, how should it vary with time? • ---- What are the effects of the pulsed jet frequency • on the lift coefficient? ( ) ( ) = + C t C C F f , t m m m , 0 , 0 • Sinusoidal and Square wave form variations were considered. Sinusoidal forms were found ineffective.

39. Square Wave Pulsed Jet, Frequency = 40 Hz

42. Average Lift Coefficient Vs. Frequency For Pulsed Jet 2 1.6 Pulsed Jet at 400 Hz requires only 73% of the steady jet mass flow 1.2 rate while achieves 95% of the Cl steady jet lift. 0.8 0.4 Pulsed Jet, Ave. Cmu=0.04 Steady Jet, Cmu=0.04 0 0 40 80 120 160 200 240 280 320 360 400 Frequency (Hz) 0 2.828 1.414 Strouhal Number ( f * Chord / Vinf)

43. Effect of Frequency at Fixed Cm • High Frequencies were more effective. • This is explained as follows: • When the jet is turned off, the beneficial Coanda effect persists for several chord lengths of travel. • If a new cycle starts soon, the Coanda effect quickly reestablishes itself.

44. Time History of the Lift Coefficient Frequency = 40 Hz

45. Time History of the Lift Coefficient Frequency = 200 Hz

46. Pulsed Jet Frequency = 120 Hz

47. Pulsed Jet Frequency = 400 Hz

48. Strouhal Number Effects • The non-dimensional frequency, Strouhal number is defined as : Where, Lref is the chord of airfoil, and U is the free-stream velocity. • Three Cases have been studied: • Case 1: Strouhal number was not fixed; U and Lref were fixed • Case 2: Strouhal number and Lref were fixed; U was not fixed • Case 3: Strouhal number and U were fixed; Lref was not fixed • Strouhal number = 1.41 for Case 2 and 3

49. Lift Coefficient vs. Frequency U  = 118.6 ft/sec U = 94.3 ft/sec Lref = 16 in. Lref = 8 in. Lref = 4 in. U  = 47.15 ft/sec

50. 3D Streamwise Tangential Blowing Symmetry BC 15 C 5 C 5 C  Small blowing to suppress vortex shedding C This region is modeled as shown in next figure 2-D BC The Wing-Flap Configuration with Tangential Blowing