When we want to apply the Bernoulli Principle, we cannot use the pressure of the fluid, but using the pressure at that particular location?
If yes, how we measure or know the pressure at that particular location? By any calculation from the pressure of the fluid?
If not, the pressure at the surface of the water should be zero in an open tank? Then why we need to use the atmospheric pressure in the calculation by Bernoulli Principle?
The principle of Bernoulli is best explained by looking at a pitot tube. It includes stagnation pressure (or total pressure) = static pressure + dynamic pressure. When you analyse a flow system, you start at a given point (knowing the total pressure) and flollow the stream-line.
https://en.wikipedia.org/wiki/Pitot_tube
when you use a pitot tube you measure the total pressure = hydrostatic pressure + dynamical pressure, here the hydrostatic pressure is P (say the state variable of the fluid, in the common sense) and the dynamic pressure is 0.5*rho*v^2, where rho is the fluid density and v is the fluid velocity. In the Bernoulli equation you have:
P + 0.5*rho*v^2 + rho*g*h = constant !
It is common to take the that equation at two different points, say 1 and 2, and write:
P1 + 0.5*rho*v1^2 + rho*g*h1 = P2 + 0.5*rho*v2^2 + rho*g*h2
The hydrostatic pressure accounts for the pressure when the fluid is at rest (including the atmospheric pressure). It is important to make the distinction between those two kinds of pressure,
I apologize as I still confuse in something.
May I know, are hydrostatic pressure is due to the force and gravity, and dynamic pressure is due to the density and velocity of the fluid? Stagnation pressure is the pressure when fluid velocity is zero?
Then how can we use fluid pressure in the calculation of Bernoulli principle?
The pressure in the Bernoulli principle is not only the fluid pressure?
You have to differentiate different pressure contributions. The equation (valid for steady state, incompressible flow) is
0.5 * rho * vel * vel + rho * g * z + p = const.
i.e. dynamic pressure + hydrostatic pressure + static pressure
The principle is only valid along a streamline! Losses and friction are dealt with by adding loss coefficients to the dynamic pressure:
0.5 * rho * vel * vel * zeta
https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Incompressible_flow_equation
Is these mean the dynamic pressure + hydrostatic pressure is the fluid pressure?
As 0.5 * rho * vel * vel + rho * g * z + p = const., the summation of 'dynamic pressure + hydrostatic pressure + static pressure' will not be the fluid pressure at that location. Am I right?
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