Numerical study of the flow over symmetrical airfoils with ground effects

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The paper presents the ground effects on the flow over five NACA symmetrical airfoils by analyzing the coefficients of lift. The unbounded flow over a selected airfoil was first simulated, and the results were validated against the established data in order to validate the method used in obtaining the lift coefficients.

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Nội dung Text: Numerical study of the flow over symmetrical airfoils with ground effects

International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 04, April 2019, pp. 1-8. Article ID: IJMET_10_04_001 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=4 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed NUMERICAL STUDY OF THE FLOW OVER SYMMETRICAL AIRFOILS WITH GROUND EFFECTS Aslam Abdullah, Surendran Suresh, Siti Juita Mastura Mohd Saleh, Mohd Fadhli Zulkafli and Mohammad Fahmi Abdul Ghafir Department of Aeronautical Engineering, Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, Parit Raja, Batu Pahat, Johor, Malaysia ABSTRACT The paper presents the ground effects on the flow over five NACA symmetrical airfoils by analyzing the coefficients of lift. The unbounded flow over a selected airfoil was first simulated, and the results were validated against the established data in order to validate the method used in obtaining the lift coefficients. In the presence of ground, we kept the angle of attack to be constantly zero throughout the computations. Ground clearances were set based on those for aircrafts whose wing cross sections are the airfoils of interest. ANSYS was used for such numerical computations. We illustrate percentage increment of lift that is inversely proportional to the ground clearance. This work sheds insight on the important parameters that need to be taken into account in the operational of an aircraft. Keywords: Aerodynamics, ground clearance, ground effects, lift coefficient, symmetrical airfoils Cite this Article Aslam Abdullah, Surendran Suresh, Siti Juita Mastura Mohd Saleh, Mohd Fadhli Zulkafli and Mohammad Fahmi Abdul Ghafir, Numerical Study of the Flow Over Symmetrical Airfoils with Ground Effects, International Journal of Mechanical Engineering and Technology, 10(4), 2019, pp. 1-8. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=4 1. INTRODUCTION The phenomenon of ground effects becomes apparent in the proximity of solid surface, and is beneficial in the control of normal fixed wing aircrafts during take-off and landing, and of wing- in-ground (WIG) craft during all flight phases. When an aircraft is flown at approximately one wing span above the runway, the interaction between the airflow around the airfoil and the ground surface modifies the fluid velocity’s vertical component. In particular, the flow field structure consisting of the trailing vortices is partially blocked by the ground which in turn decreases the amount of downwash generated by the wing. [1-3]. http://www.iaeme.com/IJMET/index.asp 1 editor@iaeme.com
Numerical Study of the Flow Over Symmetrical Airfoils with Ground Effects The airfoils of interest form the cross sections of reference aircrafts’ wings as shown in Table-1. The specifications of the individual aircraft can be found in [4-8]. Table-1. Airfoils of interest and their corresponding military aircrafts. Airfoil series Aircraft NACA 0003 F-4 Phantom II NACA 0010 Curtiss NACA 0012 S-3 Viking NACA 0018 Bell P-39 Airacobra NACA 0019 Blackburn Firebrand Aerodynamic characteristics of the flow over the airfoils in Table-1 in the presence of ground are numerically investigated by solving the compressible 2D Reynolds Averaged Navier-Stokes (RANS) Equations and Spalart-Allmaras turbulence model using ANSYS. Such flows are simulated for different chosen ground clearances and fixed angle of attack. The influence of ground is investigated based on the lift coefficient profiles. 2. GEOMETRY, GRID AND COMPUTATIONAL DOMAIN The geometries are shown in Fig-1. The only geometrical difference between these airfoils is their thickness. The chord length of each airfoil is normalized. The geometries of interest are relatively simple in comparison to, for instance, multi-element airfoils [9-10]. Meshes in computational domain is shown in Fig-2. Ground clearance is defined by the ratio of distance from ground to trailing edge of airfoil h to chord length c. Inlet and outlet boundary conditions were specified on the outer sides of computational domain (see Fig-3). The no-slip boundary condition on the airfoil was enforced in order to produce better representation of the reversing flow assumption [11]. .1 .0 (a) -.1 .0 .2 .4 .6 .8 1.0 .1 .0 (b) -.1 .0 .2 .4 .6 .8 1.0 .1 .0 (c) -.1 .0 .2 .4 .6 .8 1.0 http://www.iaeme.com/IJMET/index.asp 2 editor@iaeme.com
Aslam Abdullah, Surendran Suresh, Siti Juita Mastura Mohd Saleh, Mohd Fadhli Zulkafli and Mohammad Fahmi Abdul Ghafir .1 .0 (d) -.1 .0 .2 .4 .6 .8 1.0 .1 .0 (e) -.1 .0 .2 .4 .6 .8 1.0 Figure-1. Geometries (a) NACA 0003, (b) NACA 0010, (c) NACA 0012, (d) NACA 0018 (e) NACA 0019 Figure-2. Meshing in the presence of ground Figure-3. Airfoil moving close to the ground surface 3. METHOD VALIDATION In order to validate the accuracy of the numerical experiment method, the aerodynamic characteristics of NACA 0015 in unbounded flow were calculated. Velocity field around NACA 0015 airfoil at stall angle of attack αstall is shown in Fig-4. Note that the airfoil is in unbounded flow. The results in Table-2 show good agreement with the established simulation data [12] used as reference, where the error is below 10%. http://www.iaeme.com/IJMET/index.asp 3 editor@iaeme.com
Numerical Study of the Flow Over Symmetrical Airfoils with Ground Effects Figure-4. Contours of pressure (top figure) and velocity (bottom figure) in unbounded flow passing the airfoil of reference. Note that the stagnation point is at the pressure side of the airfoil. Table-2. Reference and calculated values of lift coefficient and corresponding error at different angles of attack. Reference Calculated α |Error| (%) values values 0o 0 0 0 2o 0.21 0.19047 9 5o 0.50 0.45904 8 10o 0.92 0.84213 8 12 (αstall) o 0.93 0.94303 1 The Cl profile obtained from both the simulation in this study and the reference [12] is presented in Figure-5. In general, the deviation increases with respect to α. Since the maximum error is small (i.e. less than 10%), the method is used to obtain the results for all the airfoils in Fig-1. 1 0.8 0.6 Cl 0.4 0.2 0 0 3 6 9 12 α Figure-5. Graph of lift coefficient Cl vs angle of attack α. numerical calculation. Reference value 4. DETERMINATION OF GROUND DISTANCE Based on the specifications of aircrafts in Table-1 whose wings’ cross sections are made of the airfoils of interest, we list the individual average ground clerance (h/c)av as given in Table-3. These values are based on the actual airfoil dimension and the height of the trailing edge from the runway surface when the aircraft touches down or it is about to take-off. The vertical size of of the undercarriage which includes the wheels is thus one of main factors in the determination of the height. Based on (h/c)av in Table-3, we set the standard h/c = 1.5, 2.0, 2.5, 3.0. http://www.iaeme.com/IJMET/index.asp 4 editor@iaeme.com
Aslam Abdullah, Surendran Suresh, Siti Juita Mastura Mohd Saleh, Mohd Fadhli Zulkafli and Mohammad Fahmi Abdul Ghafir Table-3. Ground clearance. Airfoils Chord length, m (h/c)av NACA 0003 1.94 1.72 NACA 0010 2.32 2.79 NACA 0012 2.43 3.40 NACA 0018 1.91 3.05 NACA 0019 2.28 3.71 5. RESULTS OF CALCULATION The airfoils are in ground proximity at zero angle of attack (i.e. α = 0). It is known that the velocity is relatively low in particular at the pressure side when the airfoil approaches the ground. This is explained by the airfoils being floated on a cushion of high-pressure air region above the ground surface. Furthermore, the stagnation point are located to the lower side of the symmetrical airfoils due to the ground effect. The presence of ground effects are determined by changes in the value of lift coefficients shown in Fig-6. 6.0 (b) 2.0 (a) 5.0 1.6 4.0 Cl (x 10-4) Cl (x 10-4) 1.2 3.0 .8 2.0 1.0 .4 .0 .0 1.5 2 2.5 3 1.5 2 2.5 3 h/c h/c (d) 5.0 (c) 3.5 3.0 4.0 2.5 Cl (x 10-4) 3.0 Cl (x 10-4) 2.0 1.5 2.0 1.0 1.0 .5 .0 .0 1.5 2 2.5 3 1.5 2 2.5 3 h/c h/c http://www.iaeme.com/IJMET/index.asp 5 editor@iaeme.com
Numerical Study of the Flow Over Symmetrical Airfoils with Ground Effects (e) 5.0 4.0 Cl (x 10-4) 3.0 2.0 1.0 .0 1.5 2 2.5 3 h/c Figure-6. Profiles of lift coefficient Cl against ground clearance, h/c. (a) NACA 0003. (b) NACA 0010. (c) NACA 0012. (d) NACA 0018. (e) NACA 0019. Dash line and circles in each sub-figure represent Cl in the presence of ground and for unbounded flow, respectively. The combined profiles of lift coefficient against ground clearance in Fig-7 show that Cl is inversely proportional to h/c, in general. It is interesting no note that at (h/c)max, maximum lift Lmax acts on the thickest airfoil NACA 0019 as expected (i.e. similar to the case of unbounded flow), yet at (h/c)min , Lmax acts on the thinnest NACA 0003. This illustrates relatively high positive effects of ground surface on flow over thin airfoils. 6.0 NACA 0003 5.0 NACA 0010 NACA 0012 4.0 NACA 0018 Cl (x 10-4) NACA 0019 3.0 2.0 1.0 .0 1.5 2 2.5 3 h/c Figure-7. Combined profiles of lift coefficient Cl against ground clearance, h/c for the flow over the symmetrical airfoils of interest The percentage of increment in lift with the reduction in ground distance is tabulated in Table-4. In general, ∆Cl is inversely proportional to h/c except in the case of NACA 0018 where ∆Cl has the same value (i.e . ∆Cl = 109%) for h/c = 1.5 and h/c = 2.0. Again, here we can see that the maximum effect of the ground surface is realized upon the thinnest airfoil in consideration, where ∆Cl = 208% which is the maximum for the minimum h/c = 1.5. Note that the percentage takes Cl at h/c = 3.0 as reference lift coefficient for all airfoils. http://www.iaeme.com/IJMET/index.asp 6 editor@iaeme.com
Aslam Abdullah, Surendran Suresh, Siti Juita Mastura Mohd Saleh, Mohd Fadhli Zulkafli and Mohammad Fahmi Abdul Ghafir Table-4. Influence of h/c on Cl at α = 0° NACA h/c Cl (x 10-4) ∆ Cl (%) 3.0 1.79 (ref) 2.5 2.3 28 0003 2.0 3.06 71 1.5 5.51 208 3.0 1.11(ref) -- 2.5 1.27 14 0010 2.0 1.32 19 1.5 1.84 66 3.0 1.19(ref) -- 2.5 1.28 8 0012 2.0 2.65 123 1.5 3.28 176 3.0 1.87(ref) -- 2.5 2.3 23 0018 2.0 3.91 109 1.5 3.91 109 3.0 2.01(ref) -- 2.5 2.25 12 0019 2.0 2.65 32 1.5 4.46 122 6. CONCLUSIONS The study confirms the presence of ground proximity effects on the flow over five symmetrical NACA airfoils even in the case of α = 0°. Since in both theory and practice there is no lift generated at α = 0° in unbounded flow, we highlight that additional lift is generated by the flow over the symmetrical airfoils in the presence of ground. Furthermore, the lift coefficient increases when the airfoils get closer to the ground. It is also found that the amount of the additional lift has no proportional relationship with the airfoil thickness. Carefulness is necessary in selecting the symmetrical airfoils for optimum amount of additional lift for an aircraft. The results indicate that it is important to take the ground clearance h/c into consideration in the operation during take-off and landing for normal aircrafts, and during all flight profiles of wing-in-ground (WIG) craft where h/c is relatively very small. Since the angle of attack α = 0° is used in the calculations, no additional complexity involved in the flow pattern except the aspect of turbulence. One of a more complex case would involve the ground effects on the separated flow with vortex shedding. This can be studied and compared to the unbounded flow as in [13] where α = αstall. ACKNOWLEDGMENT The authors wish to express their sincere gratitude and gratefully acknowledge the financial support received from Universiti Tun Hussein Onn Malaysia under the Tier 1 research grant, Vot H147. http://www.iaeme.com/IJMET/index.asp 7 editor@iaeme.com
Numerical Study of the Flow Over Symmetrical Airfoils with Ground Effects REFERENCES [1] Al-Kaiem, H. H. and Kartigesh, A. K. 2011. An investigation on the aerodynamic characteristic of 2-d airfoil in ground collision. Journal of Engineering Science and Technology. 6(3): 369 – 381. [2] Mahmood, H. 2015, A CFD study on the ground effect, B. Eng. Thesis. Imperial College London. [3] Dole. 2000. Flight theory and aerodynamics. New York: John Wiley & Sons. [4] Davis, P. E. 2016. USN McDonnell Douglas F-4 Phantom II. Osprey Publishing. [5] Bowers, P. M. 1979. Curtiss aircraft, 1907-1947. Naval Inst Press. [6] Elward, B. 2012. S-3 Viking in action. Squadron/Signal Publication. [7] Lednicer, D. A. 2000. Aerodynamics of the Bell P-39 Airacobra and P-63 Kingcobra. SAE Technical Paper. Warrendale, Pennsylvania: SAE International. [8] Brown, E. 2008. From the cockpit no. 8: Blackburn Firebrand. Ad Hoc Publications, Stowmarket. [9] Abdullah, A., Yazi, M. N., Ghafir, M. F. A., Mohd, S. and Rahim, M. Z. 2017. Ground proximity effect on the flow over NACA 4412 multi-element airfoil in clean configuration. IOP Conf. Series: Journal of Physics: Conf. Series. 914: 1-8. [10] Abdullah, A., Kamsani, M. A. and Abdullah, K. 2017. Effect of ground proximity on the flow over STOL CH750 multi-element airfoil. IOP Conf. Series: Materials Science and Engineering. 243: 1-8. [11] Ariff, M., Salim, S. M. and Cheah, S. C. 2009. Wall y+ approach for dealing with turbulent flows over a surface mounted cube: part 2 - high Reynolds number. Seventh International Conference on CFD in the Minerals and Process Industries CSIRO, Melbourne, Australia. 1-6. [12] Kekina, P. and Suvanjumrat, C. 2017. A Comparative study on turbulence models for simulation of flow past NACA 0015 airfoil using openFOAM. MATEC Web of Conferences. 95: 1-4. [13] Sikien, E. R., Abdullah, A., Zulkafli, M. F. and Rahim, M. Z. 2018. The effects of vortex shedding on the aerodynamic performance of airfoils. ARPN J. Eng. Appl. Sci. 13: 8527- 8530. http://www.iaeme.com/IJMET/index.asp 8 editor@iaeme.com