Numerical Analysis of Dimple Effect on Airfoils for varied AoA

The main objective of aircraft aerodynamics is to enhance the aerodynamic characteristics and manoeuvrability of the aircraft. This enhancement includes the reduction in drag and stall phenomenon. The airfoil, which contains dimples, will have comparatively less drag than the plain airfoil. Introducing dimples on the aircraft wing will create turbulence by creating vortices, which delays the boundary layer separation resulting in decrease of pressure drag and increase in the angle of stall. In addition, wake reduction leads to reduction in acoustic emission. The overall objective of this study is to improve the aircraft manoeuvrability by delaying the flow separation point at stall and thereby reducing the drag by applying the dimple effect over the aircraft wing. This Study includes computational analysis of dimple effect on NACA 0018 airfoil. Dimple shapes of square selected for the analysis; airfoil tested under the inlet velocity of 30m/s at different angle of attack (12 ̊, 14 ̊, 16 ̊, 18 ̊,) for 8mm & 10mm diameter of Dimples. This analysis favours the dimple effect by increasing L/D ratio and thereby providing the maximum aerodynamic efficiency, which provides the enhanced performance for the aircraft.


Introduction
Aircraft drag reduction is the primary objective of improving the manoeuvrability of the aircraft to delay the flow separation and stalling of the airfoil in turn reduce the drag around the airfoil. Extensive search has dedicated to the for the dimple effect around the airfoil. Surface modification of airfoil with dimple delays the separation of boundary layer, thus reducing the pressure drag &increasing the stall angle in turn. Dimples on the airfoil surface delay the separation of boundary layer & delay the separation of the flow stage. [1]. Outward and inward dimples considered for surface modification of dimples. Dimples produce turbulence that delays the separation of the boundary layer and decreases the formation of the wake. The experimental results of the distribution of surface pressure indicate that flow separation occurs and the boundary layer is delayed by dimples. Separation by producing more turbulence over the surface, thereby reducing the development of wake, demonstrates that the dimple change the stall angle.
[2] Dimples act on surface of the wing as protuberances. These protuberances produce vortices that decrease the separation of flow on the wing's suction side. This delay and decreases the growth rate of the chordwise boundary layer.The study in this paper describes the experimental research performed on a symmetrical wing that uses the dimple effect to decrease the drag and postpone theflow separation point over theupper surface wing. By increasing the L / D ratio, the experimental findings support the dimple effect, which also improves an aircraft's manoeuvrability. This also offers the highest aerodynamic efficiency that improves an aircraft's output. [3][4][5][6][7][8] During operating cycles, airfoil blades may undergo a major change in the angle of attack that may lead to a static or dynamic stall in different applications. A distributed dimple configuration has been investigated and compared to a baseline smooth NACA0015 airfoil at low Reynoldnumberit is unclear how elements distributed at the leadingedge will impact aerodynamic efficiency and stall behavior.Results show that the current designimproved the patterns of the separated shear layer under stalled conditions. [9][10][11][12][13][14][15] The aerodynamic characteristics of the built blade presented & corresponding results show that aerodynamic efficiency was mitigated bythe stall after an angle of attack of 150. The evaluation outcome of various dimple configurations for delaying flow separation presented in this section. It observed that dimples on airfoil reduces the drag by delaying the flow separation and increase the Stall AoA.  By explaining about the golf ball, we know the flow around the smooth surface. In the same way, the drag is higher than an airfoil with a smooth airfoil dimple airfoil. Therefore, choosing the form of the dimpleis a vital part of this experiment as well. We considered a square shaped dimple before finalizing the aerofoil shaper, which is a bluff body, and when vput in a flow separation device, it acquires some kinetic turbulence energy to adhere to the wing surface. Therefore, the drag will be minimized.

Fig.4: Dimples of diameter 8mm
The semispherical dimple used as a Flow control over Airfoils using various shaped dimples.

Fig.5: Top view of modelled wing
The above image shows the semi-spherical dimple airfoil, which was designed using CATIA modelling software. Using computer based software such as AUTOCAD and ANSYS, optimizing the models was easy. In this case, the pressure on the upper surface of airfoil is high when compared to the lower surface of the airfoil. It has high lift. In this case, as compared to the lower region of the airfoil, the velocity on the upper surface of the airf oil is high. In this case, the velocity on the upper surface of the airfoil is high when compared to the lower surface of the airfoil. In this case, the pressure distribution is high on the upper surface when compared to the lower surface of the airfoil. The airfoil will have more lift..  In this case, the velocity around the lower surface is high when compare to the lower surface. The drag will be high when compared to the previous case.  In this case, the velocity around the upper surface is low when compared to the lower surface. In this case, the velocity around the upper and the lower surface are same; it will not have any effect In this case, pressures around the upper and lower surface are same In this case, the velocity around the upper and lower surface is same. The drag will start reducing In this case, the pressure on upper surface is low when compared to the lower surface. In this case, the velocity around the upper surface is high when compared to the lower surface. The drag will be high.

Fig.24: Pressure profile With 8mm dimple at 18˚ AoA
In this case, the pressure on the lower surface is high when compared to the lower of the airfoil. The airfoil will have high lift.

Fig.25: Velocity profile With 10mm dimple at 12˚ AoA
In this case, the velocity around the upper surface is high when compared to the lower surface of the airfoil.

Fig.26: Pressure profile With 10mm dimple at 12˚ AoA
In this case, there is no pressure distribution around the airfoil, so the airfoil will not have any effect in reducing the drag.   In this case, the velocity around the upper surface is high when compared to the lower surface.   In this case, the pressure on the lower surface is high when compared to the upper surface. Results show that the drag coefficient of dimple wing varied significantly due to varied dimple geometry. The results indicate that the increase of the dimple depth ratio or surface roughness of the dimple wing can shift the transition to a lower Reynolds number and decrease the drag coefficient in trans critical regime. The results also established a positive linear correlation between relative roughness and drag co efficient. Implementation of dimple over NACA0018 has proven to be more effective in altering various aspects of the flow structure with varied lift and drag forces. Results obtained through the computational are discussed in previous chapter. The following conclusions have been drawn from the work presented here. When the flow along the surface of the airfoil enters a dimple, a small separation bubble is formed in the cavities. The consequence of the bubble formation is the acceleration of the flow between the dimples on the surface of the airfoil and boundary layer undergo a transition from laminar to turbulent. This transition leads to delay of separation of flow from the airfoil causing a substantial reduction of drag force. Comparative study between with and without dimple at constant inlet velocity (i.e. constant Reynolds number) shows that the co-efficient of drag is very low for a dimpled aerofoil results from the generation of separation bubbles inside inward dimples and the delay of separation through the shear layer instability. Modification in terms of dimples creates turbulence in order to delay flow separation, which increases the stall angle at which the aircraft is no longer controllable when air is not flowing over the wing properly