The sum of the pressure energies, kinetic energies, and the potential energies upon mass gives Bernoulli's principle. However, we have ignored some amount of energy that is lost during the flow due to internal friction between the layers. The several layers in the fluid flow with different velocities which in turn causes friction to arise between the layers thus causing the loss of energy. This property is known as viscosity and some kinetic energy is lost due to viscous force. Hence, Bernoulli’s principle is only valid for nonviscous fluids. In Bernoulli's equation, we do not consider elastic energies and hence we need incompressible fluids. Bernoulli's principle also fails to be applied for the turbulent flow of fluids because the velocity and pressure of the particles alter at different points of time.