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Air flow through a tube and Vena Contracta
| The following illustrates a basic airflow fundamental. First, remember air is a fluid, like water or oil. The biggest differences between air and most other fluids are:
1. It is very compressible (oil is basically incompressible)
2. It has a very low viscosity
BUT air does have a viscosity - there is friction between air molecules, and also between air molecules and a surface they flow past.
The diagram below shows what happens when air flows through a straight tube, and is forced to flow through a plate with a hole in it. The plate is called a sharp edged orifice or an orifice plate. Now remember the two biggest causes of flow loss in air flow are:
1. Sudden expansions
2. Sharp bends
Rule #1 applies in this example. |
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Let's go back to my college fluid dynamics class... we did an air flow test studying the Vena Contracta effect where we had a 10 foot long tube, about 1 foot in diameter (see diagram). There was a big turbine fan at one end, forcing air through it. There was a set of tubes (similar to a laminar flow element) to straighten out the air flow and take away any swirling air movement. After that, the tube had a series of vacuum ports where you could check air pressure. As the pressure of a moving fluid drops in the tube, the velocity of that fluid increases, and vice a versa. This is Bernoulli's Principle. After the series of vacuum ports (about 3/4" apart) there was an orifice plate, with about a 6" hole in it. Immediately after the orifice plate, there were more pressure taps. The air pressure would drop right before and for a while after the orifice plate, because the air must speed up here to get through the smaller orifice plate with the same volumetric flow rate as through the tube. The lowest air pressure though, was a few inches AFTER the orifice plate. NOT inside it, and not right before or right after it. The air flowing through the orifice plate recirculates around and pinches off the air flow going past the plate, causing the greatest restriction to be AFTER the orifice plate. The actual flow area of the air is SMALLER than the hole in the orifice plate as a result. This is the Vena Contracta. This correlates to why sudden expansions in air flow cause the greatest flow losses. If you were to fill the area after the orifice plate you would pick up more flow than if you filled the area before it.
I had someone in an e-mail note that they were flowing air through an orifice plate on top of a flow bench bore adapter (no head). The person noted that when they moved the orifice plate over to the edge of the bore adapter, the air flow increased. What happened in that case is because one side of the orifice plate comes closer to matching the bore, you reduce the Vena Contracta effect. You have reduced the sudden expansion on one side of the orifice plate, thus improving air flow.
Shown in the diagram below are 3 examples of the same Vena Contracta effect. All the examples illustrate how air flow is not always intuitive. The diagram first shows how a truck tailgate "net" INCREASES drag force over a standard truck tail gate. All those little holes in the tail gate net act as sharp edged orifices just like in the test above.
If you ever look at dive brakes or air brakes on an airplane, you will see that they usually have holes in them. This was known to increase drag, even back in the 1930's and 40's, as even WW2 aircraft show this. The size of the hole relative to the surface area does have a large effect on the amount of drag generated. If the holes are too large, the drag force will be reduced rather than increased. |
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| Another thing to pick up from this information is that to learn how to better port cylinder heads, learn about air flow in general. To better understand air flow in general, look all around you. A lot of the information I am posting here is talked about in my college Physics books. And again, most of these phenomenon were known about 50 years ago. You can get this information from places other than automotive information sites. Learn about the fundamentals from any good source, even if the information is about making a more efficient 4 cylinder engine (to get better MPG); if you can change a head to get better gas mileage, you can also modify that setup for more power with the same fuel consumption. |
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* Updated 07/14/03 - More technical info coming soon!
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