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 wingin-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].
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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
NACA 0003
NACA 0010
NACA 0012
NACA 0018
NACA 0019
Aircraft
F-4 Phantom II
Curtiss
S-3 Viking
Bell P-39 Airacobra
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
(a)
.0
-.1
.0
.2
.4
.6
.8
1.0
.0
.2
.4
.6
.8
1.0
.0
.2
.4
.6
.8
1.0
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2
.1
(b)
.0
-.1
.1
(c)
.0
-.1
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Aslam Abdullah, Surendran Suresh, Siti Juita Mastura Mohd Saleh, Mohd Fadhli Zulkafli and
Mohammad Fahmi Abdul Ghafir
.1
(d)
.0
-.1
.0
.2
.4
.6
.8
1.0
.0
.2
.4
.6
.8
1.0
.1
(e)
.0
-.1
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%.
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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
values
0
0.21
0.50
0.92
0.93
α
0o
2o
5o
10o
o
12 (αstall)
Calculated
values
0
0.19047
0.45904
0.84213
0.94303
|Error| (%)
0
9
8
8
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
Cl
0.6
0.4
0.2
0
0
3
6
α
9
12
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.
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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.
(b)
6.0
(a)
2.0
1.6
4.0
Cl (x 10-4)
Cl (x 10-4)
5.0
3.0
2.0
.0
.0
1.5
2
h/c
2.5
3
(d)
1.5
2
1.5
2
h/c
2.5
3
2.5
3
5.0
3.5
3.0
4.0
Cl (x 10-4)
2.5
Cl (x 10-4)
.8
.4
1.0
(c)
1.2
2.0
1.5
1.0
3.0
2.0
1.0
.5
.0
.0
1.5
2
h/c
2.5
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5
h/c
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Numerical Study of the Flow Over Symmetrical Airfoils with Ground Effects
(e)
5.0
Cl (x 10-4)
4.0
3.0
2.0
1.0
.0
1.5
2
2.5
h/c
3
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
NACA 0010
5.0
Cl (x 10-4)
NACA 0012
4.0
NACA 0018
NACA 0019
3.0
2.0
1.0
.0
1.5
2
h/c
2.5
3
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.
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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
0003
0010
0012
0018
0019
h/c
3.0
2.5
2.0
1.5
3.0
2.5
2.0
1.5
3.0
2.5
2.0
1.5
3.0
2.5
2.0
1.5
3.0
2.5
2.0
1.5
Cl (x 10-4)
1.79 (ref)
2.3
3.06
5.51
1.11(ref)
1.27
1.32
1.84
1.19(ref)
1.28
2.65
3.28
1.87(ref)
2.3
3.91
3.91
2.01(ref)
2.25
2.65
4.46
∆ Cl (%)
28
71
208
-14
19
66
-8
123
176
-23
109
109
-12
32
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.
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Numerical Study of the Flow Over Symmetrical Airfoils with Ground Effects
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