The present study comprises steady state, twodimensional computational investigations performed on NACA 0012 airfoil to analyze the effect of Gurney flap (GF) on airfoil aerodynamics using
A Gurney flap (GF) is a microtab fitted to the airfoil near the trailing edge on its pressure side as shown in Figure
GF on wing trailing edge.
Jang et al. [
Brown and Filippone [
The earlier numerical investigations by Fripp and Hopkins [
RANS investigations have been carried out recently by Date and Turnock [
Chen et al. [
Considerable experimental and computational efforts are carried out on the effects of GF on airfoil aerodynamics. However, there are no systematic investigations on the effect of various parameters of GF (Reynolds number, height, position, mounting angle, configuration, etc.) on airfoil aerodynamics. Hence the present investigation is undertaken with the main aim of investigating computationally the influence of these parameters on the airfoil performance in a systematic manner. GF was analyzed for six different heights ranging from 0.5% to 4% at the trailing edge perpendicular to the chord. Trailing edge of the wing is generally thin and may not be able to support the flap due to structural factors. Hence, GF with
Airfoil considered in this study is NACA 0012 airfoil with chord length of 1 m. Ctype domain and grid are created in ICEM CFD with farfield boundaries 12.5 chords away from trailing edge in all directions. Figure
Trailing edge of airfoil with various GF geometries.
Without GF
GF with
GF with
GF with
For GF investigation, the grids are generated in ICEM CFD with at least 200,000 nodes as verified by grid dependency studies done by Krishnaswamy et al. [
Fine layer of grid cells around the airfoil boundary generated in ICEM CFD.
For GF investigation at high Reynolds number
Flow field for all the simulations is assumed to be fully turbulent. As for all the cases, Mach number is always less than 0.3, flow is incompressible [
The airfoil boundary is assigned as solidwall with noslip condition while inlet is assigned as velocity inlet and outlet is assigned as pressureoutlet conditions. Density based implicit solving scheme is used with the flow medium being air and Mach number less than 0.3. Hence the fluid is assumed to be incompressible with constant density of 1.225 kg/m^{3} and dynamics viscosity of
First order upwind is used for calculating the transport variables for each turbulence model. Under relaxation factors for all the transport variables are set to 0.8. Solution initialization is computed from velocity inlet followed by FMG initialization with solution steering. Equations are solved until a convergence criterion of 10^{−5} for all the residuals is satisfied.
Results obtained from CFD are compared with the available experimental results from Li et al. [
The variation of lift coefficient with AoA is presented in Figures
Comparison of

Computational 
% increase  Experimental 
% increase 

0.0  1.128  —  1.165  — 
0.5  1.382  22.6  1.299  11.5 
1.0  1.497  8.3  1.446  11.3 
1.5  1.595  6.6  1.528  5.6 
2.0  1.667  4.5  1.580  3.5 
3.0  1.782  6.9  1.646  4.1 
4.0  1.877  5.4  —  — 
Comparison of

Computational 
% decrease  Experimental 
% decrease 

0.0  1.595  —  1.610  — 
2.0  1.598  −0.2  1.610  0.0 
4.0  1.588  0.6  1.602  0.5 
6.0  1.569  1.2  1.560  2.6 
10.0  1.531  2.4  —  — 
15.0  1.478  3.5  —  — 
20.0  1.374  7.0  —  — 
Variation of lift coefficient with angle of attack at different GF heights. Closed symbol + dashed line: Computational results. Open symbol + solid line: Experimental results [
Variation of drag coefficient with angle of attack at different GF heights. Closed symbol + dashed line: Computational results. Open symbol + solid line: Experimental results [
The variation of drag coefficient with AoA is presented in Figure
Experimental drag values are nearly constant for initial AoAs and drastic rise in drag is shown near stall. But these trends are not correctly predicted computationally, where increment in
Distribution of static pressure coefficient on the airfoil surface obtained experimentally and computationally with 2% height GF is compared for AoAs of 10° in Figure
Static pressure distributions for different angles of attack for 2% GF.
Static pressure distribution at an AoA = 10° for different GF heights is shown in Figure
Static pressure distributions for different GF heights at AoA = 10^{°}.
Flow near the trailing edge of airfoil with and without GF at an AoA of 10° is compared in Figure
Pathlines and turbulence intensities for different GF heights at AoA = 10^{°}.
Without Gurney flap
Figures
Variation of lift coefficient with angle of attack for GF positions. Open symbols + solid line: Computational results. Closed symbols + dashed line: Experimental results [
Variation of drag coefficient with angle of attack. Open symbols + solid line: Computational results. Closed symbols + dashed line: Experimental results [
From the above equation, the optimum GF height for the present configuration is found to be 1.8%.
For the sake of clarity, the values of
Maximum
The lift enhancing capability clearly decreases as the GF is moved away from trailing edge. For
For a given
Distribution of static pressure coefficient on the airfoil surface obtained computationally with 1.5% height GF at AoA = 12° is shown in Figure
Static pressure distributions for different GF positions at AoA = 12^{°}.
Flow near the trailing edge of airfoil without and with 1.5% height GF at different Gurney flap positions at an AoA of 12° is shown in Figure
Pathlines and turbulence intensities for different positions with 1.5% height GF at AoA = 12^{°}.
Without Gurney flap
Experimental and computational values of
Comparison of

Computational 
% decrease in 
Experimental 
% decrease in 

120  1.554  2.61  —  — 
105  1.568  1.73  —  — 
90  1.595  —  1.611  — 
75  1.592  0.23  —  — 
60  1.573  1.39  1.580  1.87 
45  1.541  3.39  1.544  4.10 
30  1.467  8.00  —  — 
Variation of lift coefficient with angle of attack for GF mounting angles. Open symbols + solid line: Computational results. Closed symbols + dashed line: Experimental results [
Variation of drag coefficient with angle of attack for GF mounting angles. Open symbols + solid line: Computational results. Closed symbols + dashed line: Experimental results [
For initial angles of attack,
Distribution of static pressure coefficient on the airfoil surface obtained computationally with 1.5% height GF at an AoA = 14° is shown in Figure
Static pressure distributions for different GF mounting angles at AoA = 14^{°}.
Flow near the trailing edge of airfoil without and with 1.5% height GF at different Gurney flap mounting angles at an AoA of 12° is shown in Figure
Pathlines and turbulence intensities for different mounting angles with 1.5% height GF at AoA = 12^{°}.
Without Gurney flap
Φ = 45^{°}
Φ =90^{°}
Φ =120^{°}
From the present computational investigations, the following major conclusions are drawn.
The agreement between computed and experimental values of lift coefficient is very good up to stall angle. Near and above stall angle, the lift coefficient continues to increase. An improved turbulence may provide better results near and above stall angle.
The agreement between computed and experimental static pressure distribution on the airfoil surfaces is good even near GF.
Lift enhancement is achieved for greater heights but at the expense of increased drag. The rate of lift increment decreases for greater heights and drag increases rapidly for
Lift decreases when GF is moved upstream the trailing edge. Moving GF upstream decreases the effective area of pressure difference on the airfoil; hence GF should be positioned within 10% distance from the trailing edge.
Decreasing the mounting angle decreases the drag but lift is also decreased.
Angle of attack (Deg.)
Drag coefficient
Computational fluid dynamics
Airfoil chord (
Lift coefficient
Static pressure coefficient
Experimental
Full multigrid
Gurney flap
GF height in percentage of chord length
Lift to drag ratio
Reynolds averaged Navier stokes
Reynolds number = V Ch/
Renormalization group
Distance in percentage of chord length from trailing edge
Turbulent kinetic energy
Free stream velocity (
Rate of dissipation of turbulent kinetic energy (
Mounting angle with the axial chord (Deg.)
Turbulent kinetic energy (
Kinematic viscosity (
The authors declare that there is no conflict of interests regarding the publication of this paper.
The first author would like to thank his parent institute, PEC University of Technology, and Dr. T. Sundararajan, Professor and Head, Department of Mechanical Engineering, IIT Madras, for allowing him to undertake this project in third year of his undergraduate studies at IIT Madras.