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Bernoulli's principle is a key concept in fluid dynamics that relates pressure, density, speed and height. Bernoulli's principle states that an increase in the speed of a parcel of fluid occurs simultaneously with a decrease in either the pressure or the height above a datum. [1]:
When density and gravity are approximately constant (that is, for relatively small changes in height), simply multiplying height difference, gravity, and density will yield a good approximation of pressure difference. If the pressure at one point in a liquid with uniform density ρ is known to be P 0, then the pressure at another point is P 1:
Density is related to pressure by the ideal gas laws. Therefore, density will also decrease exponentially with height from a sea-level value of ρ 0 roughly equal to 1.2 kg⋅m −3. At an altitude over 100 km, the atmosphere is no longer well-mixed, and each chemical species has its own scale height.
The potential density of a fluid parcel at pressure is the density that the parcel would acquire if adiabatically brought to a reference pressure , often 1 bar (100 kPa). Whereas density changes with changing pressure, potential density of a fluid parcel is conserved as the pressure experienced by the parcel changes (provided no mixing with ...
The equation to calculate the pressure inside a fluid in equilibrium is: + = where f is the force density exerted by some outer field on the fluid, and σ is the Cauchy stress tensor. In this case the stress tensor is proportional to the identity tensor:
The mass flow rate for a compressible fluid will increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a de Laval nozzle. Increasing source temperature will also increase the local sonic velocity, thus ...
For an ideal gas (see gas laws), the stability criterion for an air column is that potential temperature increases monotonically with height. To understand this, consider dry convection in the atmosphere, where the vertical variation in pressure is substantial and adiabatic temperature change is important: As a parcel of air moves upward, the ...
where P is the pressure of the gas, V is the volume of the gas, and k is a constant for a particular temperature and amount of gas.. Boyle's law states that when the temperature of a given mass of confined gas is constant, the product of its pressure and volume is also constant.