Types of Flow
A distinction is made between laminar and turbulent flow.
In the case of laminar flow, the hydraulic fluid moves through the pipe in ordered cylindrical layers. The inner layers of liquid move at higher speeds than the outer layers. If the flow velocity of the hydraulic fluid rises above a certain point (known as the critical speed), the fluid particles cease to move in ordered layers. The fluid particles at the centre of the pipe swing out to the side. As a result, the fluid particles affect and hinder one another, causing an eddy to be formed; flow becomes turbulent. As a consequence of this, power is withdrawn from the main flow. A method of calculating the type of flow in a smooth pipe is enabled by the Reynolds’ number (Re). This is dependent on
• the flow velocity of the liquid v (m/s)
• the pipe diameter d (m)
• and the kinetic viscosity ? (m2/s)
The physical variable “kinematic viscosity” is also referred to simply as “viscosity”.
A value for Re calculated with this formula can be interpreted as follows:
• laminar flow: Re 2300
The value 2300 is termed the critical Reynolds’ number (Recrit) for smooth round pipes.
Turbulent flow does not immediately become laminar on falling below (Recrit).
The laminar range is not reached until 1/2 (Recrit).
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