Similar way, the potential energy by this. So, if you it, instead of understanding or deriving along the streamlines, the same concept we can visualize it, if a virtual fluid balls is moving from one location to two locations, since it is a virtual fluid balls, again I am to talk about these, where we consider it is not a one fluid flow ball movements, we consider there are n number of fluid balls are there. They are having a pressure exiting one by others. Because of that, there will be a flow energy, which we quantify into pressure into area into delta x. That is what by mg, that weight of the fluid, that is what will give is on this. So, we can say it, any fluid balls if you consider it, the flow energy per weight, the kinetic energy per weight, and the potential energy per weight, that is what is custom. So, this is the difference between a simple ball and the virtual fluid ball.

So, what I am telling is that, whenever you apply the Bernoulli equation, you should draw the streamlines. You should visualize how the fluid moves. If I consider a balls are moving, a virtual fluid balls are moving it. If I draw the streamlines, I can apply the Bernoulli equations. I should know, the pressure variability, the pressure at the two points or the pressure and velocity.

Let us have a very quick, what are the limitations of Bernoulli equations. Bernoulli equations can be applied for unsteady flow, but the simplified derivations what you use it, those are for per steady flow. That means, there is no time component is there. And remember it, this equation is most frequently used, also misused equations, okay. There is two solutions are available for us, I can say it is one mass conservations and energy conservations.

Applications of Bernoulli equations is too easy, people do so often it is misused. Another is that the incompressible flow, we discussed a lot of times. It is just we have to most of the fluid flow problems in civil engineering and mechanical engineering and others place where flow Mach number is less than 0.3, we can consider is a incompressible flow because the density variation will be less than 5%, which is we can neglect it.

The most important assumption is that the frictionless flow, that means it cannot be applied near to the solid. Because as you know it, whenever the fluid goes through near the solid, if there is a solid fixed surface, there will be the velocity gradient, there will be the shear stress acting on that. Viscous effect will come to pictures. So, those reasons we cannot apply it.

So, whenever you apply the Bernoulli equations, you have to first look it whether frictional effect is significant or not significant. And it is applied along a streamline. That means, before applying that, we should draw a streamline, then we apply the Bernoulli equation at the two points, or you should justify the flow is irrotational, okay. That means flow is not rotational. There is no vorticity.