Bernoulli Principle -- Reversed [closed]

Question

The Bernoulli Principle states that as the velocity of a fluid increases, the pressure of that fluid decreases. However what if it were to be reversed: As the velocity of the fluid decreases, the pressure increases; is this possible?

I am looking at pressure differentials and how pressure is 'high' on the lower surface of the aerofoil. But is the pressure truly 'higher' or is it of normal pressure of the surroundings of the aerofoil?

So this is where the question comes in: If the pressure of the airflow is greater ont the lower surface of an aerofoil, then doesn't that mean that the airflow is slower? Or is the 'high pressure differential' just a word to help differenciate the concept of pressure on an aerofoil, and truly the pressure on the lower surface of the wing is normal?

Answer 1

As said on Wikipedia, the principle says:

$$\frac{v^2}{2} + gz + \frac{p}{\rho} = \text{constant}$$

i.e., assuming no changes in the gravity potential energy or density:

• an increase in velocity comes with a decrease in pressure
• a decrease in velocity comes with an increase in pressure

It's all part of the same principle, there is no "reverse" needed.

Answer 2

So this is where the question comes in: If the pressure of the airflow is greater ont the lower surface of an aerofoil, then doesn't that mean that the airflow is slower?

Yes! But there is nothing technically 'reverse' in this. The Bernoulli Principle states that these parameters are linked, and if you increase one, the other decreases, and vice versa.

(Note that the pressure you are talking about is static pressure. The Principle effectively states that static pressure gets 'exchanged' for dynamic pressure.)

I am looking at pressure differentials and how pressure is 'high' on the lower surface of the aerofoil. But is the pressure truly 'higher' or is it of normal pressure of the surroundings of the aerofoil?

It is higher than at the top: that's how you get lift. But this is a very good thing to ponder. There is a very common misconception that the pressure below the wing is 'high'. This is not generally true. Oftentimes, esp. on airfoils with significant curvature of the lower surface (such as supercritical airfoils) the pressure on the bottom will be low, i.e. lower than ambient - but just 'less low' than above the wing.

Have a look at this question: What is the pressure on the pressure side in relation to the freestream? , and especially on this picture from it:

See this blue part? This is low pressure.