RZR Drag, Lift and Inviscid Flow
By Paul Lew - Director of Technology and Innovation, Reynolds Cycling
Textured Aerodynamic Surfaces
Surface treatment of bicycle wheels intended to reduce drag by the creation of an inviscid (air with no viscosity/ no resistance) layer of air is also confusing and misleading. Applying blunt body theory to streamlined shapes is invalid. A streamlined body refers to an airfoil or flat plate shape. A blunt body refers to primarily round (spherical) shapes. Attention to the surface texture of aerodynamic components is not new. For example wings and airfoils (streamlined bodies) are typically smooth, while golf balls (blunt bodies) are typically dimpled. Wings and airfoils are typically smooth because the smooth surface improves laminar flow and, as a result, increases lift and/or reduces drag.
Laminar flow, also known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers.
In fluid dynamics, laminar flow is a flow regime characterized by high momentum diffusion, low momentum convection, pressure and velocity independent from time. It is the opposite of turbulent flow. In nonscientific terms laminar flow is “smooth,” while turbulent flow is “rough.”
For a streamlined body, such as an airfoil, turbulence typically reduces lift and increases drag.
Airflow over a bicycle wheel cannot be laminar because the wheel is rotating, and turbulence is created as a result of the dynamic nature of its rotation. The airflow over a bicycle wheel is always turbulent unless the wheel is not rotating. Because the airflow on the surface of a bicycle wheel is turbulent, adding a surface treatment that encourages turbulence will have no effect on decreasing drag, and little effect on increasing drag. A rough surface treatment on a bicycle wheel, which is much more of a streamlined body, than a blunt body will increase drag, and has been proven to do so if the wheel is not rotating. When the wheel rotates the effect of the increased drag due to the rough surface is largely lost in the turbulence generated from the rotation of the wheel. Rough surfaces and dimples do have an effect on streamlined objects, they increase drag. The decreased drag, and high-performance from wheels & rims with dimples or other surface textures is not the result of the surface treatment, but is a result of a low drag well-designed rim cross-section shape.
If dimples reduce the drag of golf balls, doesn’t it make sense that they would also
decrease drag on bicycle wheels?
Dimples on golf balls work to reduce drag because the object is blunt, not streamlined, and because the dimples that actually do the work to reduce the drag are the dimples on a leading patch described by an annular ring on the front of the golf ball.
The dimples on the side of the golf ball actually increase drag, but the net gain from the effect of the dimples on the leading patch off-sets the additional drag from the dimples on the sides of the ball. Because golf balls spin, there is no way to predict what region of the surface of the ball will be the leading patch. Dimpling the entire ball ensures that the leading patch will always have dimples presented to the high-pressure (leading) region. In a perfect world, a golf ball would not rotate and the dimples would only be present in one region defined by a small circle on the leading portion of the ball. The reason for adding dimples to the front of the ball is to create turbulence which travels rearward and fills in the vacuous region trailing the ball. This is blunt body theory, which is not valid for stream-lined objects.
A dimpled or textured streamlined surface applied to the leading edge of the tube in a round-tube bicycle would potentially produce lower drag numbers than a tube without dimples or texture. This cross-sectional shape is a blunt body, and therefore would benefit from dimples or texture in the same way a golf ball benefits from dimples. An airfoil shaped tube is a streamlined body, and dimples or texture applied to the tube in any location would increase the drag of the airfoil.
Bicycle wheel manufacturers who claim that a textured surface reduces drag claim that they are creating an inviscid layer of air. Inviscid means a layer of air that has no viscosity (no resistance) so that the air above it can flow over the surface with less friction due to the fact that the inviscid layer creates a slippery surface. As described above, a rotating wheel by nature creates an inviscid layer of air. In order for the layer of air on the surface of a wheel to not be inviscid, the wheel must not rotate. This further invalidates the notion that dimpled or textured surfaces on a rim improve the performance.
Read The Entire Technical Document: Click here to download the PDF
