Fluid Dynamics Lesson 6

Fluid Dynamics is the study of the factors that affect fluids in motion. As a fluid flows, the forces of attraction between the molecules cause internal friction, or resistance to the flow.

The Fluid’s viscosity is a measure of the resistance to flow The Fluid’s viscosity is a measure of the resistance to flow. Fluids with high viscosity, such as honey do not flow readily. A fluid with low viscosity such as water has low internal resistance to flow.

As temperature increases, the viscosity of a fluid generally decreases because the particles of the liquid have more energy and flow more easily. However, gases generally increase in viscosity as the particles collide more often making it more difficult for them to flow in one direction.

Laminar and Turbulent Flow As fluids flow, their particles interact with their surroundings and experience external friction. In a pipe the water flowing close to the sides is almost not moving while the water in the middle of the pipe can be moving quickly

If the speed of a flow is slow and the adjacent regions flow smoothly over one another, the flow is called laminar flow. This term can also be applied to air flow over a smooth object. Laminar flow is difficult to achieve because as the fluid flows past the object, the flow becomes irregular, resulting in whirls called eddies.

Eddies are common in turbulent flow, a fluid flow with a disturbance that resists the fluid’s motion. Turbulent flow decreases kinetic energy as some of the energy is converted into thermal and sound energy. As velocity increases turbulence also increase.

Streamlining The forces that act on an object’s motion through a fluid are called drag. Drag is a form of frictional resistance. To reduce drag, a technique called streamlining is used. Streamlining is the process of reducing turbulence by altering the design, which includes shape and surface features, of an object that moves rapidly relative to a fluid. Streamlined flow is the same as laminar flow.

Streamlining is found in nature in animals that move quickly though water or the air. Scientist study animals streamlining and try to model it in technology

Examples car design, shark skin bathing suits and golf balls. Wind tunnels and water tank tests are used on models help to improve the factors that affect fluid flow, thus reducing unwanted turbulence.

 Some advancements that have been applied to cars are in the following: Air dams, tapered light covers, tapered hood, concealed windshield wipers, low profile windshield, flush pillars, flush door handles, flush wheel openings, aerodynamic mirrors, tapered rear.

Drag Coefficients  Scientists communicate the amount of drag something has with a number referred to as the Drag coefficient (Cd). The lower the drag coefficient the less drag there is. Example: A highly streamlined airplane wing is around Cd = 0.050 while an open parachute designed for maximum drag is around Cd = 1.35.

In the 1930’s when gas was inexpensive, most cars had a drag coefficient of Cd = 0.70, today most cars around Cd = 0.40 and some are as low as 0.15

Shapes and Drag

Questions For each of the following liquids, state whether the viscosity is high or low: Skim milk Liquid honey Whipping cream Is the fluid flow more likely to be laminar in a pipe with a smooth interior or a corroded interior? Explain

Imagine you are driving a motorcycle traveling around 60 km/h Imagine you are driving a motorcycle traveling around 60 km/h. You extend your left arm to signal a turn, and you feel drag caused by air resistance. Sketch your hand in the position in which it feels maximum drag Sketch your hand in the position in which it experiences maximum streamlining Draw a diagram of an egg so that its drag coefficient is lowest when it is moving to the right.

In which sports, besides speed skating and ski jumping, do the athletes try to reduce the drag coefficient? Small cars and motorcycles can get better gasoline consumption by following a transport truck relatively closely. Explain why the gasoline consumption improves Explain the dangers of this practice.

Describe the types of features used on each of the following vehicles to reduce drag. A sport bike Locomotive Bobsled