Air Blowing Applications
When the naturally induced airflow over a moving aerodynamic device, such as a wing, just isn't enough to satisfy design requirements then strategic air or gas blowing is always an option to enhance the device's performance.
Air being blown relative to an airfoil or wing, also called a boundary layer control system or blown flap, is a well known means to re-energize a slowing boundary layer toward the rear of an airfoil. The extra energy prevents flow separation allowing a higher angle of attack and therefore extra lift compared to the un-blown trailing edge.
By blowing or sucking air at a strategic location on the top (suction) surface of an airfoil it is possible to maintain a laminar boundary layer for an extended region along the airfoil, thus delaying the onset of a turbulent boundary layer and the increased skin friction drag that accompanies it.
Increasing the air speed over an entire airfoil by blowing will increase the lift (or downforce) due to the Coanda effect – ever wondered why the engine exhausts on a Formula 1 racecar are directed at the rear wing? This technique is also exploited by wing in ground-effect vehicles such as the Caspian Sea Monster or more formally an Ekranoplan.
The front jet engines on an Ekranoplan can direct their exhaust nozzles toward the main wing, thus providing lift independently of forward speed to aid in lifting the giant vehicles clear of the water.
A hovercraft uses a dedicated fan that blows air vertically down to produce a cushion of air for it to float on. An additional horizontal fan blows air horizontally to propel the hovercraft forward on its frictionless air cushion.
By literally turning the hovercraft concept on its head i.e., sucking air from under a vehicle, you create what is known in motor racing circles as downforce. Downforce is an essential ingredient for a successful racecar. The first car to employ a fan to create downforce was the Chaparral 2J 'sucker car' in 1970. An added benefit of the air sucked from below the car and blown behind it was a reduction in the car's drag, thus improving its performance even more.
Another car that exploited a fan to create downforce was the F1 Brabham BT46 'fan car' of 1978, which resoundingly won its first race and never raced again. Most forms of motor racing prohibit movable aerodynamic devices, such as fans, in an attempt to keep speeds within relatively safe limits. However, passive shaping to create downforce through diffusers is permitted in many formulas.
Thrust vectoring is a technique employed by the Harrier airplane to produce vertical/short take-offs and landings. Instead of a dedicated fan blowing vertically, as with a hovercraft, the Harrier can direct its jet engine exhaust nozzles vertically to counter its weight during takeoff and then once airborne can redirect the same nozzles horizontally for forward propulsion.
The Joint Strike Fighter (F-35 Lightning II) that will replace the Harrier will be able to vector its main exhaust nozzle and also use a dedicated lift fan, similar to a hovercraft, to mimic the Harrier's vertical takeoff and landing capability.
A novel use of blowing occurs in the Shkval torpedo, helping it achieve in excess of 370 km/h in water. It releases (or blows) a portion of its rocket exhaust gases out of its nose to aid a supercavitation effect that reduces its drag to the extent that it can travel at high speeds under water.
The Shkval torpedo essentially flies within a gas and water vapor bubble becoming an underwater missile.
Simulation of air blowing or sucking is relatively straightforward using a panel method such as our Panel Flow add-on for Caedium. An inlet can simulate blowing and an outlet can simulate sucking. We look forward to adding your new application of air blowing or sucking to the list above.
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