Downwash Vorticies

1. Downwash Vorticies • A Cessna Citation was flown above a cloud bank over Lake Tahoe at approximately 165 knots. The trailing vorticies decended over the fog layer due to downwash, and were made visible by the distortion at the interface. • This photo was taken from the tail gunner's position in a B-25 flying in formation slightly above and ahead of the Cessna. The Cessna is initiating a climb after a slow level flight. • Aircraft Data • Velocity = 165 knots • Wing Area = 29 m^2 • Wing Span = 16 m • Mean Aerodynamic Chord = 2 m • Weight = 8000 kg • Chord Reynolds Number = 1.18E7

2. Bae146 flap vortices. Photo by Jan-Olov Newborg, Bromma Airport, ESSB, RWY30, 1999-02-11 (Jan-Olov Newborg , from Stockholm).

3. Vortex Bursting The vortex formed over the front part of the fuselage is made visible using smoke in this NASA photograph. In the region over the wing, the vortex is seen to grow suddenly in diameter - this process is called vortex bursting. The vortex interacts with the boundary layer on the wing, and vortex bursting can greatly alter the flow pattern. Some of the flow pattern over the wing and fuselage is made visible using tufts, which are small pieces of string or similar material. Since they have very small stiffness or inertia, they give an indication of the instantaneous flow direction in the region of the flow near the surface.

4. Trailing Vortices On a clear day, trailing vortices are often seen in the sky following the passage of an airplane. The vortices are formed because the wing develops lift. That is, the pressure on the top of the wing is lower than on the bottom, and near the tips of the wing this pressure difference causes the air to move around the edge from the bottom surface to the top. This results in a roll-up of the fluid, which then forms the trailing vortex. In the sky, the vortex becomes visible when the air has a high humidity. The velocities inside the vortex can be very high, and the pressure is therefore quite low. Water vapor in the air condenses as water droplets, and the droplets mark the presence of the vortex. In this picture of a cropduster flying near the ground, the vortex is made visible by a red flare placed on the ground. The red smoke is wrapped up by the trailing vortex originating near the tip of the wing. The strength of the vortex is related to the amount of lift generated by the wing, so they become particularly strong in high-lift conditions such as take-off and landing. They also increase in strength with the size of the airplane, since the lift is equal to the weight of the airplane. For a large transport airplane such as a 747, the vortices are strong enough to flip a small airplane if it gets too close. Trailing vortices are therefore the principal reason for the time delay enforced by the FAA between take-offs and landings at airports

5. Another picture from the 1993 Aviation Week & Space Technology Photo Contest. This came second in the Military category, and it was taken by James E. Hobbs, from Lockheed Aircraft service Co., Ontario, California. The plane is ejecting flares during a test of an infrared missile warning and self-protection system installed on a C-130 Hercules. The trailing vortices formed in the wake are clearly visible. The size of these vortices is related to the lift produced by the wings, and the photograph suggests that the aircraft was climbing during this maneuver.

6. This spectacular picture of a B1 bomber flying over a lake (sea?) appears to show the wake generated by the wave field emanating from the airframe

7. Image provided by Jan-Olov Newborg , from Stockholm).

8. A great many studies have been directed towards understanding the flow over wings and, in particular, over a delta wing. Very surprisingly indeed, in our view, there exist no visualizations, from the laboratory, of the far-field development of the trailing vortex pair as it travels downstream, to our knowledge. Our flow visualization, involving novel free-flight gliding of a delta wing in water, shows for the first time, the exquisitely beautiful structure of the turbulent wake, in plan view. The fluorescence dye, illuminated by a laser, shows that the near wake comprises an interaction between the primary streamwise vortex pair with the "braid" wake vortices between the pair. This work is supported by the Office of Naval Research

11. CONDENSATION ON AN RAF TORNADO The photo at the right is another good example of lift generated condensation. In spite of the strong condensation in the low pressure region above the wing, no wingtip vortices are present. Shock disks in the exhaust are also clearly evident.

12. TYPICAL CONDENSATION CLOUD DUE TO LIFT The photo at the right shows a typical cloud generated at low speed due to high lift. The cloud forms a layer over the wing due to the fact that the largest disturbances are at the boundaries, i.e., the equations of motion are elliptic. Note the difference between this pattern and that occurring in a near-sonic flow.

13. VORTICES GENERATED AT ROTORTIPS The primary lift of helicopters is due the rotor blade. Just as any other finite length lifting surface, rotor blades also have wingtip vortices. These are nicely visualized by condensation in the picture at the right. Such vortex-induced condensation is quite common, particularly in jungle or aircraft carrier footage. It is just difficult to see due to the motion of the blades.

14. DELTA WING VORTICES The picture at the right is a Saab Viggen at the 1999 International Air Tattoo. Condensation has visualized vortices on the canards and delta wing. Both types of vortices are of the delta wing type. Further spectacular pictures of delta wing vortices on Saab Viggens can be found on E. Koningsveld's 1999 International Air Tattoo site.

15. WINGTIP VORTICES AND SU-27'S The picture at the right shows two SU-27's apparently in a high-g manuever. The wingtip vortices are clearly visualized by condensation. Lift-generated condensation is also clearly evident on the upper (suction) side of the wings.