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Types of Flow
Types of flow
Fluid flows are classified as:
 Steady and unsteady flow
 Uniform and nonuniform flow
 Laminar and turbulent flow
 Compressible and incompressible flow
 Rotational and irrotational flow
 Ideal and real flow
 One, two and threedimensional flow
Steady and unsteady flow
Steady flow is that type of flow in which fluid parameters (velocity, pressure, density etc.) at any point in the flow field do not change with time. This means that the fluid particles passing through a fixed point have the same flow parameters like velocity, pressure, surface tension etc. The parameters may be different at the different crosssection of the flow passage.
Mathematically, a steady flow is defined as
Unsteady flow is that type of flow in which fluid parameters (velocity, pressure, density etc.) at a point changes with time.
Mathematically, an unsteady flow is defined as
Uniform and Nonuniform flow
Uniform flow is defined as that type of flow in which the velocity of flow of a fluid at any instant does not change with respect to space. In other words, it is the flow in which the velocity of flow remains constant throughout the flow field at any given time.
Mathematically, for uniform flow
where ∂ Ʋ = change in velocity
∂ s = length of flow in a direction, s.
Nonuniform flow is defined as that type of flow in which the velocity of flow changes with respect to space at any given time.
In other words, it is the flow in which the velocity of flow is different for a different section in the path of flow.
Mathematically, for nonuniform flow
Laminar and Turbulent flow
Laminar flow is defined as that type of flow in which each fluid particle has a definite path and paths of individual particles do not cross each other.
Laminar flow is also called streamline or viscous flow. This type of flow occurs in smooth pipes having the low velocity of flow. It also occurs in liquids having high viscosity.
Turbulent flow is defined as that type of flow in which each fluid particle does not have a definite path and the paths of individual particles cross each other.
In other words, it is the flow in which fluid particles move in a zigzag path.
When a fluid is flowing in a pipe, the type of flow is determined by a nondimensional number, called Reynold’s number.
For laminar flow, Reynold number ˂ 2000
For turbulent flow, Reynold number ˃ 4000
Compressible and Incompressible flow
The flow in which the density of fluid changes, due to pressure and temperature variations, from point to point during the flow is called compressible flow.
In other words, it is the flow in which the density of a fluid is not constant during the flow.
Mathematically, for compressible flow
P ≠ constant
The flow in which the density of fluid does not change during the flow is called incompressible flow.
In other words, it is the flow in which the density of a fluid is constant during the flow.
Mathematically, for incompressible flow
P = constant
Liquids are generally incompressible which means that pressure and temperature changes have a very little effect on their volume. Gases are compressible fluids.
Rotational and Irrotational flow
Rotational flow is that type of flow in which fluid particles also rotate about their own axes while flowing along a streamline.
Irrotational flow is that type of flow in which fluid particles do not rotate about their own axes while flowing.
Ideal and Real flow
An ideal flow is the flow of a nonviscous fluid. In the ideal flow, no shear stress exists between two adjacent layers or between the fluid layer and boundary, only normal stresses can exist in ideal flows.
The flow of real (viscous) fluids is called real flow. In real flow, shear stress exists between to adjacent fluid layers. These stresses oppose the sliding of one layer over another.
One, Two and Threedimensional flow
One dimensional flow is the flow in which parameters (velocity, pressure, density, viscosity and temperature) vary only in one direction and the flow is a function of only one coordinate Axis and time. The flow field is represented by streamlines which are straight and parallel.
Mathematically, for onedimensional flow
Twodimensional flow is the flow in which fluid parameters vary along two directions and the flow is the function of two rectangular space coordinates (x and yaxis) and time. The flow field is represented by streamlines which are curves.
Mathematically, for twodimensional flow
Threedimensional flow is the flow in which flow parameters change in all the three directions and the flow is the functions of three mutually perpendicular coordinate Axis (x, y, zaxis) and time. The streamlines are space curves.
Mathematically, for threedimensional flow
Book Fluid Mechanics(Hydraulics), Writer A.K. Upadhyay