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About 19 important definitions of Hydrodynamics

Hydrodynamics: Hydrodynamics is a branch of physics that deals with the motion of fluids and the forces acting on solid bodies immersed in fluids and in motion relative to them.

Also, the branch of science concerned with forces acting on or exerted by fluids (especially liquids)

Perfect fluid: In physics, a perfect fluid is a fluid that can be completely characterized by its rest frame mass density ρm and isotropic pressure .

Perfect fluids are idealized models in which these possibilities are neglected. Specifically, perfect fluids have no shear stresses, viscosity, or heat conduction.

Ideal Fluid: An Ideal Fluid is a fluid that has no viscosity. It is incompressible in nature. Practically, no ideal fluid exists.

Or, An ideal fluid is a fluid that has several properties including the fact that it is:

  • Incompressible – the density is constant
  • Irrotational – the flow is smooth, no turbulence
  • Nonviscous –(Inviscid) fluid has no internal friction ( η = 0)

Ideal fluid do not actually exist in nature, but sometimes used for fluid flow problems.

We use ideal fluid in 2 equations :

1.Continuity equation

2.Bernoulli equation

Viscous fluid: A viscous fluid is one which resists movement or the movement of an object through the fluid. All fluids, liquid, gas, or plasma, have some measure of viscosity which can be compared using mathematical formulas or direct measurements of movement. Though all fluids have viscosity, a viscous fluid, in the everyday sense of the term, is one that has a high level of viscosity. These types of fluid may move slowly or not at all, depending on how viscous they are.

Inviscid fluid: An inviscid fluid is a continuous fluid substance which can exert no shearing stress however small.

Shear stress: A shear stress, denoted τ (Greek: tau), is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section.

Normal stress:  Arises from the force vector component perpendicular to the material cross section on which it acts.


No-slip : No-slip is the fact that a fluid velocity tends to “0” at the solid-fluide interface, noslip there is no fluid-surface friction at all.

Compressible flow: Compressible flow (gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density.

Incompressible flow:  In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric flow) refers to a flow in which the material density is constant within a fluid parcel—an infinitesimal volume that moves with the flow velocity. An equivalent statement that implies incompressibility is that the divergence of the flow velocity is zero.

Kinematics: Kinematicsis the branch of mechanics that deals with pure motion, without reference to the masses or forces involved in it.

Dynamics: Dynamics is the branch of mechanics that deals with the motion and equilibrium of systems under the action of forces, usually from outside the system.

Steady flow: If the flow parameters, such as velocity, pressure, density and discharge do not vary with time or are independent of time then the flow is steady.

Unsteady flow: If the flow parameters , such as velocity, pressure, density and discharge vary with time then the flow is categorized as unsteady.

Uniform flow: The fluid flow is a uniform flow if the flow parameters remain constant with distance along the flow path.

Or, flow of a fluid in which each particle moves along its line of flow with constant speed and in which the cross section of each stream tube remains unchanged.

Non-uniform flow : The fluid flow is non-uniform if the flow parameters vary and are different at different points on the flow path.

Or, Fluid flow which does not have the same velocity at all points in a medium, at a given instant.

Steady uniform flow: Conditions do not change with position in the stream or with time. An example is the flow of water in a pipe of constant diameter at constant velocity.

Steady non-uniform flow: Conditions change from point to point in the stream but do not change with time. An example is flow in a tapering pipe with constant velocity at the inlet – velocity will change as you move along the length of the pipe toward the exit.

Unsteady uniform flow: At a given instant in time the conditions at every point are the same, but will change with time. An example is a pipe of constant diameter connected to a pump pumping at a constant rate which is then switched off.

Unsteady non-uniform flow:Every condition of the flow may change from point to point and with time at every point. For example waves in a channel

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