Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 14/06/2012 11:30:01

Title: Does the Bernoulli prinicple apply at all velocities?
Post by: thedoc on 14/06/2012 11:30:01

Faraone family  asked the Naked Scientists:
   
With respect to Bernoulli's principle : is it fair to say that  this is in effect  at any speed?
 
Can the level of lift be calculated  at ridiculously slow speeds such as when towed or when taxiing?  

Is the relationship between speed of aircraft and lift attained a direct one or does lift rise exponentially  as speed rises from stop to takeoff ( apprx  200 mph) to  cruising speeds ?  

Rob Faraone ( Singapore )

What do you think?

Title: Re: Does the Bernoulli prinicple apply at all velocities?
Post by: Phractality on 14/06/2012 18:29:15

At low speeds in non-turbulent fluid, the simple formula holds. As you approach the speed of sound, it becomes more complex. Applying the simple formula at supersonic speeds yields negative pressures.
In liquids, high velocity results in cavitation. The liquid may boil, which makes the formula invalid. Similarly, low pressure may cause water vapor to condense, which also makes the formula invalid.