As we all know now a Golf ball with Dimples will fly further than a Golf ball without. What most of us don’t know is why?
There are two main reasons:
- The Dimpled Surface reduces drag.
- The Dimpled Surface improves lift.
First a quick lesson in Hydrodynamics (Aerodynamics).
There are two types of flow around an object: laminar and turbulent.
Laminar flow has less drag, but it is also prone to a phenomenon called "separation." Once separation of a laminar boundary layer occurs, drag rises dramatically because of eddies that form in the gap. Turbulent flow has more drag initially but also better adhesion, and therefore is less prone to separation. Therefore,
If the shape of an object is such that separation occurs easily, it is better to turbulate the boundary layer in order to increase adhesion and reduce eddies (which means a significant reduction in drag). Dimples on golf balls turbulate the boundary layer.
So we at 3DFINScience decided to test these principles and apply them to a Surfboard fin.
With advanced Computer Flow Dynamic testing 3DFINScience compared two exactly the same fins, the Dimpled fin and the same fin design without Dimples.
The tests were conducted by flow dynamics expert Darrin Stephens (formally of CSIRO) and the results were surprising.
A Surfboard Fin with a smooth surface is much like a golf ball with a smooth surface, it has a Lamina flow over the surface. This works quiet well when going straight and at lower speeds, but when you start to turn and reach higher speeds the fluid starts to separate from the foil or fins surface. What this does is creates a separation bubble on the surface of the fin, this occurs when the fluid detaches from the surface of the Fin, resulting in increased drag, reducing the lift and ultimately stalling of the fin.
The smooth surface fin also has known phenomena occur called wing tip vortices.
The pressure imbalance that produces lift creates a problem at the Fins tips. The higher-pressure below a fin spills up over the fin tip into the area of lower-pressure above. The fin’s forward motion spins this upward spill of air into a long spiral, like a small tornado, that trails off the wing tip. These wing tip vortices create a form of pressure drag called vortex drag.
A Surfboard Fin with Dimples much like a golf Ball creates a Turbulent Flow.
Turbulent flow has more adhesion so when you start to turn the Dimpled foil surface delays the flow separation, reducing cavitations (separation bubble) allowing the foil to maintain performance. When the Surfer is turning at high speeds the turbulent boundary layer helps the flow overcome an adverse pressure gradient and allows the flow to remain attached to the surface longer than it would otherwise. This reduces drag, increases lift and improves overall performance of the fin design.
The Dimples also reduce and almost eliminate the wing tip vortices, by reducing the separation of flow at the tip this also increases the lift and overall performance of the fin.
In a broad form, such a Surfboard fin having a foiled surface with Dimples can maximize the known effects Dimples have on flow dynamics.
The 3DFINS Dimple Fin design with Dimples located on the fins curved surfaces, utilizes the turbulent flow created by the Dimples, this turbulent flow draws the fluid back over the foil surface, reducing cavitations and drag during extreme changes of direction (high angles of attack) or when subject to high speeds. The improved performance of a foil with Dimples allows the surfer to increase the lift which in turn creates more drive, allows the surfer to maintain speed and hold through turns, overall increasing performance of the Surfboard.
CFD test have shown the Dimpled Fin Design improves Lift and Drag by up to 15%, which translates to More Drive, More Speed, More Hold, More Fun.