1.

Segment V9.1: Space Shuttle Landing
(Related to Textbook Section 9.1 - General External Flow Characteristics)

Flow past complex shaped bodies may exhibit a variety of characteristics.

Although the space shuttle spends most of its mission in the vacuum of space, there are many phenomena associated with flow past it during the final stages of landing. The shuttle needs to be a relatively streamlined object so that it can glide properly. Upon touchdown a drag chute is used to make it more like a blunt object and increase its aerodynamic drag to slow it down. Note the wing tip vorticies (swirls) made visible by the smoke from the skidding tires at the moment of touchdown. (Video courtesy of NASA.)

2.

Segment V9.2: Streamlined and Blunt Bodies
(Related to Textbook Section 9.1.2 - Characteristics of Flow Past an Object)

It is sometimes desirable to make an object as streamlined as possible. In other situations a blunt object is desired. Typically, streamlined objects have less drag than blunt objects.

A kayak is a streamlined object that moves through the water with minimal resistance and disturbance to the fluid. It requires a relatively small propulsive force. The purpose of the paddle is to impart the propulsive force to the kayak. To do so it must generate a relatively large resistance to motion through the water. A paddle is a blunt object. The Reynolds numbers for the paddle and the kayak are on the order of 100,000 to 1,000,000.

3.

Segment V9.3: Laminar/Turbulent Transition
(Related to Textbook Section 9.2.1 - Boundary Layer Structure & Thickness on a Flat Plate)

Near the leading edge of a flat plate, the boundary layer flow is laminar. If the plate is long enough, the flow becomes turbulent, with random, irregular mixing. A similar phenomenon occurs at the interface of two fluids moving with different speeds.

As shown in a computational fluid dynamic simulation, the interface between two fluids moving horizontally with different speeds becomes unstable and waves develop on the surface. Similarly, the rising smoke plume from a cigarette is laminar near the source, becomes wavy at a certain location, and then breaks into turbulence. (CFD video courtesy of ITS Corp.)

4.

Segment V9.4: Snow Drifts
(Related to Textbook Section 9.2.6 - Effects of Pressure Gradient)

At high Reynolds numbers, non-streamlined (blunt) objects have wide, low speed wake regions behind them.

As shown in a computational fluid dynamics simulation, the streamlines for flow past a rectangular block cannot follow the contour of the block. The flow separates at the corners and forms a wide wake. A similar phenomenon occurs for flow past other blunt objects, including bushes. The low velocity wind in the wake region behind the bushes allows the snow to settle out of the air. The result is a large snowdrift behind the object. This is the principle upon which snow fences are designed. (CFD video courtesy of ITS Corp.)

5.

Segment V9.5: Sky Diving Practice
(Related to Textbook Section 9.3.3 - Drag Coefficient Data and Examples)

The drag coefficient for an object can be strongly dependent on the shape of the object. A slight change in shape may produce a considerable change in drag.

When falling at terminal (steady) speed, a sky diver's weight is balanced by the aerodynamic drag. If the air speed in a vertical wind tunnel equals the terminal speed, the sky diver can remain motionless. Any change in shape (bend or straighten arms or legs, rotate a hand, etc.) alters the drag coefficient (and the drag) and causes the sky diver to rise, fall, or rotate until an equilibrium configuration is again produced. (Video courtesy of Flyaway.)

6.

Segment V9.6: Oscillating Sign
(Related to Textbook Section 9.3.3 - Drag Coefficient Data and Examples)

Steady flow past a blunt object may produce an oscillating Karman vortex street wake behind the object. The shedding of vortices from the body exerts a periodic force on the body that, if the frequency is right, can cause the body to oscillate.

As shown in a computational fluid dynamics simulation, flow past a flat plate can produce a well-defined Karman vortex street. Similarly, wind blowing past a rectangular speed limit sign can excite the sign into significant twisting motion, provided the conditions (wind speed and direction, stiffness of the sign support, etc.) are correct. (CFD video courtesy of ITS Corp.)

7.

Segment V9.7: Jet Ski
(Related to Textbook Section 9.3.3 - Drag Coefficient Data and Examples)

The drag on a boat comes from friction drag along the boat's surface and form drag that manifests itself in the form of waves. The energy needed to produce the waves is provided by the boat's motor.

At very low speed, a jet ski plows through the water and produces large waves in its wake. The form (wave) drag is very large. Under such conditions the boat is not an efficient mode of transportation. However, when properly trimmed at higher speed, the jet ski planes across the surface with a minimal wake. Most of the relatively small drag is friction drag; very little energy is lost to wave making.

8.

Segment V9.8: Drag on a Truck
(Related to Textbook Section 9.3.3 - Drag Coefficient Data and Examples)

A significant portion of the power needed to drive a vehicle at highway speeds is lost overcoming aerodynamic drag. Appropriate design of the vehicle can reduce the drag and thus increase the fuel economy.

Because of geometric constraints, little redesign can be done to the rear of a truck to reduce the drag. On the other hand, a simple, well-designed air deflector on the cab can help smooth the airflow past the front of the truck and reduce its drag. However, even with the deflector, there is flow separation at the top leading edge of the trailer portion of the truck. (Video courtesy of the National Research Council of Canada and the Institute for Aerospace Research.)

9.

Segment V9.9: Wing Tip Vorticies
(Related to Textbook Section 9.4.2 - Circulation)

The pressure difference between the upper and lower surfaces of an airfoil causes trailing vorticies to form at the tips of the wing.

The spoiler (wing) on a race car is used to produce a downward force, allowing the car to corner better. The high pressure surface for this negative lift device is the upper surface. The resulting trailing vorticies are made visible by the injection of smoke. Some airplane wings have vertical winglets at the wing tips to help reduce the effect of the trailing vorticies and therefore make the wing more efficient. (Video courtesy of the National Research Council of Canada and the Institute for Aerospace Research.)