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Evaluating the Performance of a NACA 4414 Slotted Airfoil: An Experimental and Numerical Investigation
Improving the flow characteristics over an airfoil has been a much researched topic, and numerous experimental and numerical investigations have been conducted in an effort to mitigate boundary layer separation. Beyond a certain angle of attack (stall angle), the flow over the suction side of the airfoil can no longer adhere to the wall due to the adverse pressure gradient downstream of the airfoil. This detachment of flow results in a sudden reduction in lift and the airfoil has undergone stall. The point of flow detachment is known as the boundary layer separation point. Techniques to delay the separation point can be classified as either active or passive forms. Active forms include wall heating, synthetic jet usage, acoustic excitations, and pulsed blowing, while passive forms include vortex generators, surface roughness, and uniform suction/blowing. The proposed passive airfoil design is unique since the slot does not fully separate the trailing edge from the airfoil (unlike Fowler or Slotted Flaps) thus reducing the mechanical complexity. The slotted airfoil allows for bleeding of a small quantity of air from the pressure side of the airfoil to the suction side which energizes the flow thereby delaying boundary layer separation. This slotted configuration has been designed by utilizing the fundamental concepts studied in the Fluid Mechanics course.
The airfoil considered for this study is a custom NACA 4414 airfoil with a chord length of 100 mm. The numerical setup in FLUENT was validated by running a test with the well-established Clark-Y airfoil and comparing the numerical results to published values. Following this, the flow characteristics such as pressure distribution and velocity contours were studied at various angles of attack for the baseline NACA 4414. The stall angle and separation point were noted. For the NACA 4414 slotted airfoil, the geometric parameters of interest are slot axial location, angle, and width, and 2-D simulations were utilized to identify the optimal combination. The slot located at 58% chord length with an angle of 200 and width of 1 mm provided the best performance. The boundary layer separation was delayed due to the higher pressure stream energizing the suction side flow. The width was then made non-uniform such that the slot tapered by 5 deg. near the suction side. This further energized the flow due to the increase in flow speed at the slot exit. Following this, 3-D simulations were conducted to study flow behavior at slot exit. It was concluded that a fillet of radius 1.5 in. on top and 1 in. on the bottom was required where the slot meets the airfoil to minimize local swirl effects. The airfoil stall was delayed by 20 and the increase in lift at an angle of attack of 140 was approximately 20%. Both the baseline and slotted airfoils were then manufactured using additive manufacturing techniques and tested in-house to analyze the effect of the fixed slot. The pressure ports had to be strategically positioned due to space constraints. The experimental data trends aligned with those from the numerical study.