Open Entry Windsock Velocities

[Bob Shapiro]

Will a Venturi tube with the entry open to a moving fluid act the same as one that has a closed entry with a known pressure/velocity?

If I have a windsock in a 10 mph breeze, is the velocity inside the entry point (big diameter) 10 mph? Will the wind velocity at the small end (the exit) be the same, less, or greater than at the big end?

Let's put some numbers to it:
*Entry wind = 10 mph
*Entry diameter = 20 inches
*Exit diameter = 10 inches
*Windsock length = 50 inches

Where I'm going with this is that I have a couple of ideas which need a good stream of air (faster than a typical breeze) and I'd like to use a funnel setup rather than a fan to create it.

If it's possible, how can I calculate the output given the input wind speed and the windsock dimensions? (Would a non-flexible tube give better results?)

Thanks!

 

[Valerie Park]

Yes, a Venturi tube with an open entry will act the same as one with a closed entry with a known pressure/velocity. The velocity at the entry point of your windsock in a 10 mph breeze will be 10 mph, and the velocity at the exit (small end) will be greater than at the entry point due to the Venturi effect. To calculate the exit velocity given the input wind speed and windsock dimensions, you can use Bernoulli's equation, which states that the sum of the pressure, kinetic energy, and potential energy in a system remains constant. The equation can be rearranged to calculate the velocity at the exit point of a Venturi tube, given the velocity at the entry point and the ratio of the diameters of the entry and exit points. In this case, the equation would be: V2 = V1 * ((D2/D1)^2)where V1 is the velocity at the entry point (10 mph), D1 is the entry diameter (20 inches), V2 is the velocity at the exit point, and D2 is the exit diameter (10 inches). Plugging these numbers into the equation, we get V2 = 10 mph * ((10/20)^2), or V2 = 5 mph. Therefore, the velocity at the exit point of your windsock will be 5 mph. A non-flexible tube may give slightly better results, as it will not be affected by any fluctuations in the wind speed.