# What is basically the difference between static pressure and dynamic pressure?

What is basically the difference between static pressure and dynamic pressure? While studying Bernoulli's theorem, I came before these terms. The law says:

When the fluid flows through a small area, its pressure energy decreases & kinetic energy increases and vice versa.

Now that's wierd as I know due to having KE, ie. having momentum, one can impart pressure. Then why distinction ? What is then pressure energy?? In order to understand that I went to wikipedia & quora & others; there I found fluid exerts two pressure: Static & dynamic. But really nothing could be understood more than that. What are they actually?

Everyone seems to be making valid points here. I'll try to answer in a way that tackles your confusion directly.

Now that's wierd as I know due to having KE, ie. having momentum, one can impart pressure. Then why distinction ?

Force exerted by this momentum must be looked at from two different perspectives: perpendicular or parallel to the fluid flow? As one can imagine, the force of moving fluid past you is different from fluid moving directly toward you. A fluid with KE will impart different pressures depending on where you measure it with respect to the direction of flow. In the Bernoulli equation it is implied that dynamic pressure is the quantity that is relevant when there is fluid moving along the surface that a pressure quantity is being measured (note: we are looking at the force on the surface which is perpendicular to flow direction).

Dynamic pressure is simply the pressure you would measure when the fluid has some orthogonal velocity component relative to the pressure probe (i.e. the fluid is moving relative to the pressure gauge). It has the units of pressure. It's measurable and not a fictitious number. To make it more clear, if the pressure gauge was moving at the same velocity as the fluid (no relative velocity between fluid and gauge), then the reading will increase and become identical to what you would call Static Pressure.

So the short answer is that we need the distinction because a pressure gauge would show a high reading when fluid is still and a low reading when it is moving.

To fluid dynamicists, Bernoulli's equation is better known as the 'Energy Equation' since it does indeed account for the energy changes that occur along a fluid path. The energy equation says that the energy is constant along any given streamline. Static or stagnation pressure can exist in the absence of fluid velocity creating a potential energy component. Dynamic pressure exists when there is bulk fluid motion creating a kinetic energy component.

Along the streamline, and affected by the boundaries that contain the fluid flow, energy flows between static and dynamic states, but their sum is constant.

• Sir, can you please explain me how the static pressure arise without flow? – user36790 Feb 17 '15 at 4:27
• In the simplest way to conceive, when a fluid is acted on by external forces. Imagine the gas in an internal combustion engine. At peak compression there is no flow, but the gas has a higher static pressure than before. But there are other ways. Influx of energy (heat) can increase pressure without flow. The sudden arrest of a flow can lead to increased local static pressure. – docscience Feb 18 '15 at 16:55

The quantity $\frac{1}{2}\rho v^2$ is called dynamic pressure for two reasons: because it arises from the motion of the fluid, and because it has the dimensions of a pressure.

It is not really a pressure at all: it is simply a convenient name for the quantity (half the density times the velocity squared), which represents the decrease in the pressure due to the velocity of the fluid.

Dynamic pressure is not really pressure at all. It represents the amount that the pressure would increase if all the fluid's kinetic energy per unit volume could be converted to pressure by blocking the flow (say with a pitot tube). On the other hand, static pressure is another name for just "plain old pressure" exerted by parcels of fluid on adjacent parcels of fluid, or on the walls of the flow channel. For incompressible flow, it is, more precisely, the isotropic part of the stress tensor.

Under common assumptions and ignoring potential energy, static pressure is the expression of the fluid's temperature (internal energy) and dynamic pressure is the expression off the fluid's velocity, so if the fluid is brought to a rest adiabatically, their sum is equal to the stagnation pressure. The stagnation pressure represents the total energy of the fluid.