DiscoverHover CURRICULUM GUIDE #18
VISCOSITY AND TURBULENCE

© 2005 World Hovercraft Organization

NAME DATE

Viscosity in fluid flow is analogous to friction in the motion of solid bodies. When we slide one solid body over another, in order to keep the body moving we must provide an external force to overcome the frictional force. To understand fluid motion, we can consider two parallel plates, a, ass illustrated in Figure 18-1. A force F is applied to the upper plate so that it moves with a constant speed vtop relative to the lower plate, which is at rest. The force F opposes the viscous drag on the upper plate to maintain the velocity of the fluid. The fluid in between can be imagined to be divided into layers parallel to the plates. Viscosity acts not only between the fluid and the upper plate, but also between each layer of fluid and the layers on either side of it. The speed of each layer differs from the one below it by an amount Δv. Note that we assumed that the top layer of fluid has the same velocity v as the top plate and that the bottom layer of fluid is at rest.


Figure 18-1: Velocity profile of fluid between two plates
Image ©2007 DiscoverHover

The units of viscosity are lb-s/ft2 and N-s/m2. The equivalent cgs unit is called the poise, and is a dyne-s/cm2. The unit is named after French physician Jean-Louis-Marie Poiseuille.

Jean-Louis-Marie Poiseuille
1799-1869

Poiseuille, educated in physics and mathematics, developed an improved method for measuring blood pressure. Poiseuille’s interests were the forces that affect the blood flow in small blood vessels. He performed meticulous tests on the resistance of flow of liquids through capillary tubes and published a paper in 1846 on his experimental research. He discovered that the rate of flow through a tube increases proportionately to the pressure applied and to the fourth power of the tube diameter.

Flow of a fluid can be laminar or turbulent. Layers of a fluid flowing over one another at different speeds with virtually no mixing between layers is known as laminar flow. Turbulent flow is characterized by irregular movement of particles of fluid.

The water beneath a hovercraft becomes turbulent and unsteady due to the fact that the hovercraft hovers on a cushion of air. The air inside the cushion is under pressure, which causes the water underneath to become turbulent. However, a hovercraft can travel over turbulent water with relative ease, even when the water beneath it is unsteady due to outside forces, as illustrated in the photo below:

Figure 18-3: Hovercraft traveling over turbulent water

Image ©2005 DiscoverHover

Quiz Question:

  1. Given that the skirt of the hovercraft is open at the bottom, would a hovercraft move more easily over water or over a more viscous liquid, such as oil?

Answers are in the Answer Key.

 
 
©2005 World Hovercraft Organization
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