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Section 1.15 - Flow Conditions

Flow conditions can be of several types. Flows may be steady (meaning not time varying) or unsteady (meaning that the flow is time varying). An example of unsteady flow is when actuation starts or stops, the flow becomes dependent upon the motion of the actuating piston. A steady flow example is when the piston has reached its operating speed in a very long pipe, the flow in the pipe will no longer vary as a function of time.

Flows may be one-dimensional where the flow parameters (for example: density, velocity, temperature, pressure) vary as a function of one spatial variable (for example, x) and variations in the other two spatial dimensions (i.e., y and z) are negligible by comparison. Flows may be two-dimensional where the flow parameters vary as functions of x and y, for example, while variations in z are small and can be neglected. Flows may be three dimensional where the flow parameters depend upon all three spatial dimensions. Flows may be incompressible, i.e., the flow density does not change with position or time. Flows may be compressible, such as in the case of a gas at high speeds. Flows may be described as creep, laminar or turbulent. Creep flow occurs in highly viscous fluids at very low speeds (fluids that exhibit a high resistance to motion; for example, molasses).   For flow speeds that are higher than in creep flow, whether the flow is laminar or turbulent depends upon the Reynolds number.  Flows that start as laminar flows may transition to turbulent flows, (for example flows around spheres or over wings).

Laminar Flow

Laminar flows occur when fluid particles move along straight, parallel layers. These layers are called laminae, from which laminar flow gets its name. Laminar flows normally occur during steady state conditions. The velocity of each layer may be the same or may change slightly from layer to layer. The fluid particles of each layer do not mix with the fluid particles of other layers. In steady situations, the energy of the fluid is conserved.

Turbulent Flow

Turbulent flow occurs when the particles of fluid move in all directions and fluid mixing occurs. This may occur when the flow becomes unsteady. Turbulent flows can also occur during steady conditions when there are small velocity and pressure variations compared to the mean (or average) flow velocity and pressure, BUT the mean flow velocity and pressure DO NOT vary with time. Mean flow velocity and pressure represent the average velocity and pressure of all the fluid particles in the flow.  In unsteady situations, the energy of the fluid is not conserved. Turbulent flow will cause a pressure drop. Because the fluid particles motion defines the flow, this type of flow is sometimes called particle flow.

Causes for Transition

Flows may transition from laminar to turbulent due to the following reasons:

1. Roughness of the inner surface of the pipe
2. Abrupt changes in pipe directions – e.g., 90 degree bends
3. Pipe size changes – a velocity increase in the fluid due to a cross-sectional area decrease in the pipe can cause the flow to change from laminar to turbulent

The type of flow (whether laminar or turbulent) depends upon the Reynolds number, R_N, where

Here, the Greek letter, rho, represents the density; V is the velocity of the flow; x is the pipe hydraulic diameter (equal to the diameter for a circular pipe); and, the denominator, given by the Greek symbol, mu, represents the fluid viscosity.

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Updated: February 17, 1999