Aerodynamics of Wind Turbines: Drag
ate at much lower Reynolds numbers than propellers for larger aircraft; typical Reynolds Since propeller thrust is largely dependent on the lift . a known force on the load cell, a linear relationship between the thrust and The maximum Mach number for a propeller tip in these tests was about The largest lift-to-drag ratio and the corresponding lift coefficient and angle of attack For example, the Reynolds number of a blade for a MW wind turbine . the drag effect on the turbine optimal power coefficient, the following relationship .. airfoil profile along the entire span—are designed at a given tip- speed ratio. Keywords: Supersonic projectile, drag coefficient, air density, Mach number. 1. The theoretical relationship (equation) relating the drag force and and a temperature of 15°C only accounted for Reynolds number variations for changes in Mach sample of the Nosler Ballistic Tip bullets (right) used in the experiment.
Here a flow which is initially fully laminar slowly changes with increasing Reynolds number into one with a laminar start and a transition to turbulent flow somewhere downstream.
This adds a section of turbulent boundary layer where at a lower Reynolds number the flow would had been laminar. You will notice that for the same Reynolds number, a fully laminar flow lower line is much less draggy than a fully turbulent one upper line.
Therefore, replacing some of the laminar flow with a turbulent one will result in more drag. Now I need to explain why increasing the Reynolds number leads to more turbulent flow. For this, I turn to this answer: Inside the laminar boundary layer, small disturbances become less and less damped the higher the local Reynolds number becomes, and at a Reynolds number of aroundin unaccelerated flow some frequencies become unstable see Tollmien-Schlichting waves and will eventually create so much cross movement that the boundary layer becomes turbulent.
aerodynamics - How does Reynolds Number affect skin friction drag? - Aviation Stack Exchange
Now parcels of air which flow at high speed in the outer part of the boundary layer will move close to the wall and kick the slow parcels there ahead, greatly reducing the deceleration of the flow close to the wall, at the price of slowing down and expanding the whole boundary layer. A laminar boundary layer cannot be sustained indefinitely but will change to a turbulent one if the Reynolds number is high enough. This will swap some of the previously laminar portion of the boundary layer to a turbulent one, increasing drag.
While all of the above is only strictly true for flow over a flat plate without a pressure gradient, real flow will complicate things again: Therefore it is critical, that both aircraft wings and rotor blades have low aerodynamic drag.
Drag The black arrow above the picture shows the direction of the drag force, when the cross section of the airplane wing or rotor blade is moving towards the right. Drag Increases with the Area Facing the Wind Drag increases in proportion to the frontal area of an object facing the wind.
The frontal area is the area of the hole in a wall if the object were thrown through a wall in a cartoon. It is a good idea to design objects that have to move quickly through fluids with low energy use to have small cross sections facing the current.
How does drag change with Reynolds number? - Aviation Stack Exchange
That is the reason why submarines, missiles, cars, or bicyclists' helmets often are built using elongated drop shapes. The size of the drag for a given shape is usually measured by the drag coefficient, CD, which is defined as the drag force per square metre frontal area of the object.
Such a shape has a very high CD of 1.Drag, Coefficient of Drag, L/D ratio
A modern automobile typically has a CD in the range 0. A runner has a CD about 0. An airfoil shape used on aircraft wings or rotor blades, typically has an extremely small CD about 0.
Drag Increases with the Square of the Wind Speed Both lift and drag increase with the square of the wind speed. A modern car drag coefficient 0.