The Effect of Dimples on Aerofoil Performance

This project is a continuation of my ongoing research comparing the lift performance of various dimpled aerofoils to more conventional wings employing flow-controlling devices. As a result of extensive testing, I found benefits in dimpling and became incredibly interested in furthering my research on the topic. My interest was further piqued after my teacher provided me a research paper on dimpled aerofoils.

For this project, I decided to perform tests of various locations of dimples at set angles of attack (AOA) to determine how dimple location affects the lift, maximum lift coefficient, and stall of a set aerofoil. My tests are simulating the wing of an airplane, though dimples could benefit any type of aerofoils.

There have been several studies utilizing computational fluid dynamics (CFD) to simulate certain designs of dimpled aerofoil that found reductions of drag when dimples were installed on aerofoils. Drag reduction is incredibly beneficial for aircraft.

My goal is to have these dimpled aerofoil designs placed on the aircraft responsible for flying in more remote places, likely to communities that are off grid. Living in these areas is incredibly expensive due to the need of flying supplies out. If the dimpled aerofoils have lift benefits alongside drag reduction, then aircraft that use them may be able to operate more efficiently, possibly allowing for reduced costs to live in remote communities.

Most background research has accumulated throughout my years of participating in science fair and resources provided by early judges.

My previously constructed wind tunnel was updated to be more reliable and powerful. The fan that powered the tunnel was replaced with a 2750 CFM fan. Plywood surfaces were smoothed with layer of tape, as were any seams, joints, or screws on the inner surfaces. A honeycomb flow straightener was 3D printed.

The aerofoils were NACA 2412 aerofoils, chosen for their common use. The dimples were based on the studies I could find, where most dimples were two percent the chord length and set up in three staggered rows. The aerofoils were modelled in AutoDesk Fusion and printed on my Bambu Labs P1S 3D printer.

The five aerofoils consisted of one control and four different dimpled aerofoils.

The tests started with checking the air pressure, humidity, and temperature of my testing facility hourly. The air pressure and humidity were converted into a density altitude, with the temperature being modified to keep the density altitude around 5800ft throughout testing.

For tests, the angle of attack was set using a 3D-printed gauge. Following the entrance cone being reattached, the force gauge was checked, and an anemometer was placed in the front of the test section. The fan was then turned on for one minute to reach full speed. After that, my phone was set to record a video of the force gauge screen for another minute. After, the tunnel was turned off for a minimum of two minutes. Then, the video was reviewed and a value was recorded into a table every six seconds for eleven measurements per video.

This was repeated for five aerofoils at six angles of attack, three times per angle.

I found that dimples can benefit an aerofoil at some angles of attack, though they are not always beneficial.

I found that dimples positively benefited the coefficient of lift at low angles of attack (0-5 degrees).

All aerofoils stalled between 10- and 15-degrees angle of attack. At 10 degrees, more centred dimples saw higher lift coefficients, while control sat just behind them and the dimples on the fringes of the aerofoil had a lower lift coefficient. Though, of these dimples, C1-150L performed substantially worse than C4-850L, having barely seen an increase in lift from the previous angle of attack.

Immediately after stall at 15 degrees angle of attack, the control aerofoil had the most lift. However, C1-150L and C4-850L both lost less lift in between angles than the control did.

By 20 degrees, the control aerofoil kept the highest lift coefficient, though by only a small gap.

Finally, at 25 degrees, C1-150L had the least lift, followed by the control aerofoil. The remaining aerofoils saw lift begin to rise again in what I have identified as a case of flow reattachment.

Overall:

Control: A low amount of lift initially. It only has the most lift after stall, though it lost a substantial amount immediately after stalling.

C1-150L: Impressive at level flight and 5 degrees angle of attack but nowhere else. It had the lowest maximum lift coefficient by a substantial margin and did not experience flow reattachment like the other dimpled aerofoils.

C2-383L: If the aerofoils are ordered by most lift, C2-383L appeared most consistently near the top. It has the highest lift coefficient at 10 degrees angle of attack and of six angles of attack, it had the first or second most lift at five of those angles.

C3-616L: It was better than C1-150L overall but performed worse than C2-383L and C4-850L overall.

C4-850L: it performed unremarkably for maximum lift coefficient. It was second-worst by maximum lift coefficient. However, the lift loss after stall was incredibly low compared to the other dimpled aerofoils. Combined with the impressive flow reattachment, it had an interesting performance compared to the other aerofoils.

My tests found a positive relationship between dimpling aerofoils and the lift coefficient. More centered dimples allowed for a higher maximum lift coefficient, with C2-383L seeing the highest lift coefficient.

Conversely, dimples placed towards the rear of the aerofoil reduced lift loss after stall dramatically, though the maximum lift coefficient lowers as the dimples move to the rear.

Additionally, aerofoils C2-383L, C3-616L, and C4-850L showed signs of flow reattachment between 20- and 25-degrees AOA. C2-383L saw the most substantial regain of lift.

Combined with other studies I found which reported drag reduction in dimples placed further back of aerofoil, certain circumstances may find C4-850L would be ideal for usage, even if it produces a lower maximum lift coefficient than C2-383L. Circumstances calling for more lift, would benefit from the C2-383L configuration for its high lift result. C2-383L would also come with reduced drag, though it would be less than what C4-850L would experience.

My current setup had certain limitations that were difficult to address due to the design and circumstances, such as struggles in temperature control and inability to conduct tests on days of similar barometric pressure. While the temperature control struggles can be mitigated by moving the thermostat away from the downstream of the wind tunnel, other issues were difficult to rectify.

Plans for my next project have begun. I have developed plans for a wind tunnel design overhaul, including a lengthened test section, an improved aerofoil mounting system, and a force balance to allow for lift and drag measurement.

I want to extend my heartfelt thanks to everyone who provided any aid to my project. While there are too many to name, a thanks goes out to all those who tolerated being used as sounding boards throughout these months of work. A special thanks goes out to the following people:

My mother, Judith Friesen, for board design aid, editing, and snacks,

My father, David Friesen, for providing information, ideas, editing, and funding,

Dan Fletcher, for detailed methodology ideas and further suggestions and sources,

Josh Gette, for providing sources and methodology information, and,

Lily Sears, for finding sources I had lost some months ago and solutions when I got stuck on an idea.

·       Tay, J., & Lim, T. T. (2018). Drag reduction with teardrop-shaped dimples. In 2018 Flow Control Conference (p. 3528).

·       Ali, H., Rasani, M. R., Harun, Z., & Shahid, M. A. (2024). Passive flow-field control using dimples for improved aerodynamic flow over a wing. Scientific Reports, 14(1), 12918.

·       Saraf, A. K., Singh, M. P., & Chouhan, T. S. (2017). Effect of dimple on aerodynamic behaviour of airfoil. International Journal of Engineering and Technology, 9(3), 2268-2277.

·       NASA. (n.d.-d). Wind tunnel parts. NASA. https://www.grc.nasa.gov/www/k-12/airplane/tunpart.html

·       NASA. (n.d.). Index of wind tunnel slides. NASA. https://www.grc.nasa.gov/WWW/K-12/airplane/shortt.html

·       Density Altitude Calculator - English/Metric, Relative Humidity. (n.d.). Wahiduddin.net. https://wahiduddin.net/calc/calc_da_rh.htm

·       Aerodynamic lift, drag and moment coefficients. AeroToolbox. (2022, September 28). https://aerotoolbox.com/lift-drag-moment-coefficient/

·       Leishman, J. G. (2023). Introduction to aerospace flight vehicles. Embry-Riddle Aeronautical University.

·       Most knowledge was developed through previous projects and research during those projects