In WWII there was a need to attack the industrial heartland of Germany. An idea that was hit upon is to attack dams in the interior of Germany. This resulted in the Germans not being able to produce as much steel as large amounts of water were needed in the process of creating steel, it would also flood the country side denying ground transportation and food as well as denying water transportation to the Germans. A normal bombing mission was studied but rejected because of strict accuracy requirement which probably would not be able to be met under the level of anti-air fire expected. The next idea was to drop torpedoes into the river to blast the dam. This wasn’t feasible because the Germans had strung torpedo nets across the river.
A man by the name Barnes Wallis came up with the idea of skipping a bomb across the surface of the river, much like a stone. To get this to work one needs to fly low and spin the barrel along its long axis. If one spins the barrel away from the direction of flight an interesting thing happens. But first I must divert to some fluid dynamics for the explanation to be complete.
One of the fundamental theorems of Aerodynamics is the Kutta-Joukowsk theorem. In its final form it is very simple, elegant and powerful. This equation will tell you if your airplane is going to fly or not, how much down force will be exerted by a NASCAR, how a golf ball flies, etc. It states simply that the lift force on an object moving through a flow (think airplane wing or ball flying through the air) is directly related to the density of the fluid (water is more dense then air), the velocity of the object through the fluid, and the flow around the object. This equation is so amazing because no where in it appears the shape of the object. You would think that a baseball might produce lift differently then an airplane wing. It is the same equation for both. Shape comes into flight in that it changes the circulation and drag.
But what does an aerodynamics equation have to do with blowing up a dam? Well the way that this Kutta-Joukowsk theorem is arrived at is by modeling the flow around a rotating cylinder moving through a flow. Sound familiar? Looking at the diagram below, we have a fluid moving over the barrel (doesn’t matter if it is the barrel moving or the fluid) and the barrel is spinning. Lets do some simple qualitative fluid dynamics (trust me it will be fun). We have a spinning barrel and a fluid flow. Because of friction the spinning barrel is making the fluid around it spin also (see Boundary Layer for more information on how this works in more detail). Fluid flow in this case is additive. When we apply additive flow to the area at the bottom of the barrel we see that the velocity of the free stream fluid flow will cancel to some extent with the boundary layer fluid flow around the barrel resulting in low velocity. When there is low velocity there is high pressure. Looking at the top of the barrel the velocities will add creating high velocity and low pressure. This is *exactly* what happens with an airplane wing, and in this case also there will be lift on the barrel, just like with an airplane wing. The reason that we fly around on airplanes with wings instead of rotating barrels is because of the previously mentioned drag forces that would be much increased with a barrel.
A man by the name Barnes Wallis came up with the idea of skipping a bomb across the surface of the river, much like a stone. To get this to work one needs to fly low and spin the barrel along its long axis. If one spins the barrel away from the direction of flight an interesting thing happens. But first I must divert to some fluid dynamics for the explanation to be complete.
One of the fundamental theorems of Aerodynamics is the Kutta-Joukowsk theorem. In its final form it is very simple, elegant and powerful. This equation will tell you if your airplane is going to fly or not, how much down force will be exerted by a NASCAR, how a golf ball flies, etc. It states simply that the lift force on an object moving through a flow (think airplane wing or ball flying through the air) is directly related to the density of the fluid (water is more dense then air), the velocity of the object through the fluid, and the flow around the object. This equation is so amazing because no where in it appears the shape of the object. You would think that a baseball might produce lift differently then an airplane wing. It is the same equation for both. Shape comes into flight in that it changes the circulation and drag.
But what does an aerodynamics equation have to do with blowing up a dam? Well the way that this Kutta-Joukowsk theorem is arrived at is by modeling the flow around a rotating cylinder moving through a flow. Sound familiar? Looking at the diagram below, we have a fluid moving over the barrel (doesn’t matter if it is the barrel moving or the fluid) and the barrel is spinning. Lets do some simple qualitative fluid dynamics (trust me it will be fun). We have a spinning barrel and a fluid flow. Because of friction the spinning barrel is making the fluid around it spin also (see Boundary Layer for more information on how this works in more detail). Fluid flow in this case is additive. When we apply additive flow to the area at the bottom of the barrel we see that the velocity of the free stream fluid flow will cancel to some extent with the boundary layer fluid flow around the barrel resulting in low velocity. When there is low velocity there is high pressure. Looking at the top of the barrel the velocities will add creating high velocity and low pressure. This is *exactly* what happens with an airplane wing, and in this case also there will be lift on the barrel, just like with an airplane wing. The reason that we fly around on airplanes with wings instead of rotating barrels is because of the previously mentioned drag forces that would be much increased with a barrel.
Now we get to the good part. The next diagram shows the barrel sinking through water after it has hit the dam. This is basically the previous diagram rotated 90 degrees counter clockwise. The barrel is sinking so the free stream flow is now coming from the bottom of the barrel. This exerts a net force on the barrel to the left, pushing it into the dam. Now you can have a timer delay on the bomb that allows it to sink and be more effective.
This was done in Operation Chastise of May 17, 1943. Chastise when very well in terms of the strategic objectives being met. This included massive lose of food production, both plant and animal. 53 of the 153 air crew members were killed in action, with 8 of 19 planes lost in the action. Operation Chastise was portrayed in a movie called “Dam Busters” which I found to be very good.
In the intro of “The Century of Warfare” there is also some footage of planes dropping what appear to be supplies. They bounce the barrels off of the sea and they skip into a holding pond.
http://en.wikipedia.org/wiki/Barnes_Wallis
http://en.wikipedia.org/wiki/Kutta-Joukowski_theorem
http://en.wikipedia.org/wiki/Bouncing_bomb
http://en.wikipedia.org/wiki/Additive_function
http://en.wikipedia.org/wiki/Operation_Chastise
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