Hanoi University of Science and Technology, No. I. Dai Co Viet Sir., HaiBa Tnmg, Ha Noi, VietNam Received: February 28, 2014; accepted: April 22, 2014
Abstract
Supercritical airfoil and wing have been developed more than 30 years. For supercntical wing design, one of the most important objectives is improving the cruise efficiency. And some other characteristics are also very important for a practicable supercritical wing, such as drag divergence characteristic, buffet boundary, stall behaviors, and so on.
In this study, the aerodynamic characteristics of supercritical airfoil were carried out by numerical simulation (CFD) using ANSYS software and then compared with experimental results using subsonic wind tunnel. The air velocity varied from 5 to 15m/s and attack angle varied from -5 to 30° for both numerical and expenmental studies.
Keywords, CFD, Supercritical airfoil, Low speed. Wind tunnel, Aerodynamic characteristics.
1. Introduction
A concerted effort within the National Aeronautics and Space Admimstration (NASA) during the 1960's and I970's was directed toward developing practical two-dimensional tiirbulent airfoils with good transonic behavior while retaining acceptable low-speed charactenstics and focused on a concept referred to as the supercritical airfoil. This distinctive airfoil shape, based on the concept of local supersonic flow with isentiopic recompression, was characterized by a large leading-edge radius, reduced curvature over the middle region of the upper surface, and substantial aft camber. The high maximum lift and docile stall behavior observed on thick supercritical airfoils generated an interest in developing advanced airfoils for low-speed general aviation application [1-5].
This paper presents a study on the aerodynamic characteristics of supercntical airfoils using the CFD method combined with experimental method. In this study, the Mach number was less than 0.1 and attack angle (AOA) varied from -5 to 30° in order to have more adequate mformation on the supercritical airfoil in a low-level flight mode representing the takeoff and landing stages.
2. Supercritical Wing Model 2.1. Supercritical Wing Dimension
Supercritical wing was a rectangular wing with properties: Profile Whitcom B integral
" Con-esponding author: Tel: (+84) 949.737,767 E-mail- dung,[email protected]
supercritical airfoil - profile largely used in transonic domain; Span length (b) was 0 4 m; Chord length (c) was 0.015 m; and aspect ratio (b/c) was about 27.
2.2. Experimental Set-Up Wind tunnel
Experiments of supercritical wing were conducted at the low speed blow-down wind tunnel.
The wind tunnel employed belongs to Department of Aeronautic and Space Engineering at Hanoi University of Science and Technology (HUST) which is of the open type with a closed test section. The test section is rectangular and has dimensions of 0,4*0.5*l.Om^, The maximum free stieam velocity in the empty test section is 30m/s (M = 0,1) and the turbulence level is slightly less than 1%. The tunnel is operated continuously with a centrifugal blower driven by an 8kW electric motor
Wing model
Supercritical wing was made of Balsa wood (Fig. 1). Aerodynamic characteristics of this wing were first determined from numerical study and then experimental study. The pressure taps were identified from the pressure distribution obtained by numerical stiidy (Fig la). These pressure taps were connected to an extemal digital manometer via stainless and silicon tubes (Fig. lb).
Experiment conditions
Experiments m this study were performed al wind speed of 5-15m/s, where the Reynolds numbei based on wing chord was from 0.15*10^ to 0.45*10^
The free-stream velocity was kept constant withii
±2%. The free-stieara total and dynamic pressures
a. Position of pressure taps Fig.
b. Wing model in wind tunnel . Supercritical wing model
were measured by Pitot tube within ±2%. The temperature of the air was measured within ± 1 % . The test conditions for the current investigation covered a range of attack angle from -5° to 30" withm ±0,5".
Pressure measurements
Static pressures at 20 pressure taps see in Fig.
1 a were measured using Keygence pressure measurement. Each pressure tap was measured one time with waiting time of 5 seconds (average of about 1000 instant values). The standard deviation of measurement enors was within ±O.OOIPa,
From static pressure, the coefficient of pressure Cp was estimated as follow;
where: Pmcasure. Measured static pressure; Pa,: Free- stieam static pressure or ambient pressure in test section; p; Air density; V^: Air free-stream velocity.
Aerodynamic characteristics
Lift force (L), drag force (D) and pitching moment (M) of the delta wing were first calculated by expression (2), Then, these aerodynamic characteristics were dimensionless by expression (3) to estimate coefficient of lift (CL), coefficient of drag (CD) and coefficient of pitching moment (Cm).
^ = X(^»,.,,--^,„,,)^,s
^ = t{^io..i-P..,..)^
C, = - M
(2)
(3) where:
Piow,i and Pup,i: Measured lower and upper static pressure at i'^ position; N: number of measure pressures (N=45); AS,: area conesponding to the i""
pressiue tap; x,: Cartesian coordinate follow x- direction.
3.3. Numerical Model
The ntimerical model was generated by the ANSYS Design Modeler and then meshed by the ANSYS Meshing software (Fig. 2).
In order to combine the numerical results with experimental results, the computational domain had the same dimension with the test section of the wind tunnel (0.4*0.5*I.0m^).
Fig. 2. Computational mesh
3.1. Velocity 10 m/s & AOA 10"
Lower numerical pressure of supercritical airfoil was higher in absolute value than that on the upper side (Fig. 3a). This remark was the same between numerical and experimental results (Fig. 3b).
The numerical aerodynamic charactenstics were good accord ,;With experimental results within
±20%. The main cause of the error was;
• For experiments; Fabricating model, prepared the presstue holes, experimental conditions, fluctuation from the experimental devices and model itself...
• For simulation: Quality of mesh, order of selected model ... Calculating time depended on the mesh. The mesh was fin, the mn time was long. The mesh was poor, the run time was short but the obtained results were missed information. The model for discrete differential equation should be P', 2"'' or 3'^'' order. Risuig level of order made increasing the complexity of problem.
a. Numerical result b. Compare EXP & CFD results Fig. 3. Pressure distribution - velocity 1 Om/s & AOA 10"
a. PressLure distribution - Velocity 5 m/s b . Pressure distribution - Velocity 10 m/s
c. Pressure distnbution - Velocity 15 m/s d. Aerodynamic characteristics
;. 4. Influence of velocity Table 2. Aerodynamic characteristics -velocity lOm/s
& AOA 10"
a
CD L/D C„
EXP 1.257 0.838 1.501 0.414
CFD 1.348 1.022 1.319 0.430
Error (%) 6.735 18.048 13.805 3.667 3.2. Influence of velocity-AOA 10"
For air velocity from 5 to 15 m/s, lower pressure of supercritical airfoil was higher in absolute value than that on the upper side for both in numerical and experimental results (Fig. 4a, b , c). It
seemed that the pressure distribution of bofli experimental and numerical study were the same tendency with e n o r maximmn 20%.
Lift, drag and moment coefficient of supercritical airfoil were nearly uniform at air velocity from 5 to I5m/s for both experimental study and numerical study (Fig. 4d). The range of velocity in this study had a weak influence to aerodynamic characteristics of the airfoil.
3.3. Influence of attack angle - velocity lOm/s In regard the Fig. 5, the pressure distribution and aerodynamic characteristics of both experimental and numerical stiidy were the same tendency with error maximum 20%.
a. Pressure disfribution - AOA = -5" b. Pressure distribution - AOA = 0°
c. Pressure distribution - AOA = 10" d. Pressure distribution - AOA = 20"
e. Pressure distribution - AOA = 30" f. Aerodynamic characteristics Fig. 6. Influence of attack angle - velocity 10 m/s
Lift, drag and moment coefficient of supercritical increased with attack angle for both experimental study and numerical study (Fig. 6 f).
Coefficient of lift increased with attack angle from -5 to 25". It means that the stall angle of this airfoil was higher than 25". That was the reason why this airfoil was chose for high velocity and modern plane, especially, in tiansonic/supersonic flow, 4. Conclusions
Through experimental and numencal studies for supercritical airfoil, the raain conclusions were summarized as follows:
- Both numerical and experimental results had the same tendency within ±20%. The main cause of the errors due to quality of mesh, order of computational mode! for numerical study; and sensor, experimental conditions, support ,. for experimental study,
- The air velocity had a weak influence to aerodynaimc characteristics of supercritical airfoil,
- The attack angle had a strong influence to aerodynamic characteristics of the object. The lift and drag coefficient increased with attack angle.
Matrix of Family-Related Airfoils; NASA Technical Paper 2969, 1990.
[3] Kasim B., Carl P. T.; Supercritical airfoil design for future HALE concepts; 4V Aerospace Sciences Meeting and Exhibit, Reno, Nevada, 2003.
[4] Zhang Y., Chen H., Zhang M., Zhang M., Liu T., Fu S.; Supercritical wing design and optimization for transonic civil airplane;
Proceeding of the 13"' Asian Congress of Fluid Mechanics, Dhaka, Bangladesh, 2010.
[5] Shantanu K.; CFD analysis of supercritical airfoil over simple airfoil; Thesis of University of Petroleum and Energy Studies, 2011.
