Question

streamline flow which quantity remain s constant

Answer 1

lIt is not necessary that the density of the fluid remain constant along a streamline. Assuming that the density is constant makes the derivation and application of the Bernoulli’s principle to most liquids easier, since most liquids are incompressible fluids (An incompressible fluid is a fluid whose density does not change despite the variation of pressure on the fluid particles at different points on the streamline). To avoid considering friction losses during fluid flow, it is also sometimes assumed that the fluid is inviscid, i.e. has zero viscosity.

For instances involving compressible flow (gases are a common example of compressible fluids), where densities do change along the streamline, Bernoulli’s principle can be modified suitably. But, this might not be tackled in your syllabus, I think. However, the concept of the Mach number is easier to apply in cases related to gases, which I believe is a part of your syllabus.

Keep in mind that the Bernoulli’s equation/principle is based on the conservation of energy, which is an inviolable law of physics, holding true compressible and incompressible flow