This approximation is actually quite accurate as an extreme amount of pressure is required to cause a significant density variation. A good analogy to this concept is a ferris wheel. Passengers on a ferris wheel experience irrotation whereas the wheel itself rotates. Irrotational flow is characterized by the fact that a test body does not rotate about an axis through its own centre of mass. Compare honey and water.
Viscosity is a measure of how resistive a fluid is to flow.
A fish moving through a nonviscous fluid will therefore experience no drag force just as a skater sliding across a frictionless surface is not hindered by resistance. Although fluids are practically incompressible, this is not entirely accurate. V is the total volume This equation is quite simple to understand. The opposite of irrotational flow, this case describes a situation where the particles of a fluid essentially spin. A fundamental characteristic of fluids is deformation.