The designs to be evaluated are based upon a symmetrical airfoil, as follows:
� The airfoil with 0.06 m clearance to the ground.
� The airfoil with 0.1 m clearance to the ground.
� The airfoil with 0.2 m clearance to the ground.
� The airfoil with 0.3 m clearance to the ground.
� The airfoil with 0.4 m clearance to the ground.
All cases were run with the same mass flow.
Data:
Flow speed = 50 m/s
Mass flow = 122 kg/s (g/sec)
Viscosity = 1.7894 E-05 kg/m-s
Air density = 1.225 kg/m3
Re = 3.41 E 06
Assumptions and Limitations:
CFD models were built with various assumptions:
� The flows were assumed to be incompressible.
� The solutions were computed for steady state.
� The geometry of the inlet, outlet and the fluid was sufficient to provide a reasonable computational domain.
� A Pressure Outlet (Outflow) boundary was used at the outlet.
Analysis Model:
Surface meshes of about 1250 cells were created using GAMBIT to generate the computational domain. The meshed surfaces were imported into FLUENT to solve the flow equations.
Analysis Methods:
In this analysis the segregated solver along with the Spalart-Allmaras turbulent model was utilized. The solution algorithms were run with second order spatial accuracy. Since only the flow characteristics of incompressible flow were desired, the energy equation was disabled.
Results and Discussion:
Fluent post processor aids in the visualization of the behavior of the flow patterns around the symmetrical airfoil varying the ground clearance. Five different heights were under study in order to understand the dependence and the compromise between drag, lift and the ground clearance. (Figures 3 to 7)show the static pressure distribution of the symmetrical airfoil with different clearances. All figures are plotted under the range of values of the worst scenario in order to make an easily visual comparison between them.
After the five cases were analyzed, and based on the computational results, it�s evident that the geometric symmetry between the top and lower part of the airfoil do not significantly affect the main air stream in terms of air drag, we can see in graph 1 that the values of drag can be considered as constant compared to the values of lift.
We can say that above 0.2 m the flow pattern is not disturbed by the ground. For heights lower than 0.2 m, the interaction with the ground is clearly shown by the increase of down force. This negative lift is generated by the air flow being accelerated through the gap between the ground and the airfoil, the increase in velocity of the air flow produces a low pressure area under the body.
This behavior can make us think that the closer the airfoil to the ground the better negative lift achieved, well, that is true under certain values of ground clearance, examining graph 1 we can see that this is true, between a distance of 0.4 and 0.1 m, after 0.2 m the slope of the graph is dramatically changed showing the sensibility of the down force in this range, but any distance below 0.1 m the slope shifts and the lift values increases, this means that less down force is achieved.
This change in the behavior of lift is due to the choking phenomena, that states that when the clearance is reduced that far, the viscous effects of the air flow on the ground and on the bottom of the airfoil interact restricting the flow. This restriction of the flow prevents the high acceleration and pressure drop tending to produce positive lift.
(In figure 8), we can see how a high pressure area (the red color area) is generated on the lower part of the airfoil, this increase in pressure means a decrease in velocity that is why the airfoil is experimenting positive lift in the range of values below 0.1 m. This critical height is totally dependant on the thickness of the boundary layer.
