Chapter 17: Problem 6

Why does stream flow tend to become turbulent?

Short Answer

Expert verified

Stream flow tends to become turbulent due to increased flow velocity, changes in channel properties, the presence of obstacles, and the properties of boundary layers. The transition from laminar to turbulent flow is governed by the Reynolds number, which increases with higher flow velocities. Flow disturbances caused by channel irregularities and obstacles, along with high shear stresses in boundary layers, further contribute to the development of turbulence in stream flow.

Step by step solution

Understand turbulence

Turbulence is a disordered, chaotic flow of a fluid characterized by a complex system of eddies and whirls. These flow patterns can lead to mixing, enhanced heat and mass transfer, and increased resistance to flow. The transition from laminar to turbulent flow is governed by the Reynolds number, which is a dimensionless quantity that describes the ratio of inertial forces to viscous forces.

Define Reynolds number

The Reynolds number (Re) is defined as: Re = \(\frac{\rho V L}{\mu}\) Where \( \rho \) is the fluid density, V is the flow velocity, L is the characteristic length (typically the channel width or hydraulic diameter), and \( \mu \) is the dynamic viscosity of the fluid. The Reynolds number helps us determine whether the flow is laminar (smooth and predictable) or turbulent (unpredictable and chaotic). If the Reynolds number is below a certain critical value (around 2000 for pipe flow), the flow is generally laminar, and if it's above this value, the flow is usually turbulent.

Relate flow velocity to turbulence

As the flow velocity increases, so does the Reynolds number. Once it reaches the critical value mentioned earlier, turbulent flow is more likely to occur. This means that higher flow velocities are generally associated with more turbulence in the stream. It's worth noting that turbulence is a self-sustaining phenomenon, so once it starts, it tends to perpetuate itself.

Understand channel properties and obstacles

When a fluid flows through a channel or pipe, it interacts with the channel walls and any irregularities or obstacles present in the flow path. This interaction can cause flow separation and the formation of vortices, both of which contribute to the development of turbulence. In natural stream channels, factors like changes in channel slope or the presence of rocks and debris can create flow disturbances and induce turbulence.

Consider boundary layers

A boundary layer is a thin layer of fluid that forms at the interface between a solid surface and the fluid moving past it. In this layer, velocity gradients and shear stresses are high, making it susceptible to the onset of turbulence. As the flow velocity increases, the boundary layer thickness decreases, leading to higher shear stresses and a greater likelihood of turbulent flow. In conclusion, stream flow tends to become turbulent due to increased flow velocity, changes in channel properties, the presence of obstacles, and the properties of boundary layers. These factors cause disturbances in the flow, leading to the development of chaotic and unpredictable flow patterns.

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Why does stream flow tend to become turbulent?

Short Answer

Step by step solution

Understand turbulence

Define Reynolds number

Relate flow velocity to turbulence

Understand channel properties and obstacles

Consider boundary layers

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