Laminar Flow vs. Turbulent Flow
Laminar flow is uniform air movement in layers. The term for these layers is lamina and are shown using streamline diagrams. When an object is put into the airstream, it interacts and changes the lamina. As long as the lamina don't mix the flow remains rather steady. If the object disrupts the flow enough to start mixing the lamina, the flow becomes turbulent.
Several factors determine how steady or turbulent the air flow may be.
The main factors are:
1. A change in the velocity of the flow.
2. A disturbance in the flow such as an unusually rough wing surface.
To quanitfy the type of flow scientists use a number called the
Reynolds Number(R). The number is dimensionless (has no units) and divides
flows into three large categories:
1. If R is less than 2100, the flow is laminar
2. if R is between 2100 and 40000, the flow is in transition between
laminar and turbulent.
3. Greater than 40,000 the flow is turbulent.
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This is a high degree of drag.The Reynolds Number is greater than 40,000 |
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This is a moderate amount of drag. The Reynolds Number is between 21,000 and 40,000. |
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This is minimal drag. The Reynolds number is less than 21,000. |
The formula for R is R=(pVL)/u.
For airfoils the variables relate the wing to air variables:
p= fluid density of air
V= velocity
L= chord length (distance from leading to trailing edges of
wing).
u= viscosity of air