Science Buddies: "Ask an Expert"

Pls help my project deadline is in just 2 weeks,pls answer this question or suggest me the sources or website adress to know the solution
Q: is it possible to make an sealed container with pressure little greater than atmosphere by just passing the air at high speeds,i think due to the pressure difference it can happen as per bernoulli's principle .

sreeharsha - I don't think I quite understand your question very well as stated. There are many sources that will describe the physics of "lift" of an airfoil as a result of Bernouli's principle. Simply, the shape of an airfoil is such that the path of air over a wing cross section is designed to be longer on the top surface. This increases the air velocity on the top surface of the airfoil resulting in a lower pressure on the top of the airfoil. This is Bernoulli's principle. The bottom surface of the airfoil, or wing, has a higher pressure (atmospheric pressure). This differential is what is causing the upward lift.

I'm unclear as to what the "sealed container" issue is. If you created a sealed container with a traditional airfoil shape, and had an open vent on the top surface of the airfoil, positioned at the "center of lift" you certainly would see a lower pressure inside the vessel. This would be measurable with a sensitive pressure (vacuum) instrument. In essence, your measurement, say in absolute PSI, would be a measure of "lift" in PSI of that airfoil. This is probably what you meant, and would make an interesting experiment. Look into basic aviation explanations of airfoil, lift, drag, attack angle and similar topics for a complete explanation.

Rick Marz

thank you for your answer, but their is no hole on the top surface and the container is sealed completely so that their can be high presssure difference i also posted a file please have a look for better understanding

sreeharsha - The reference to a sealed container still confuses me. Any basic reference to airfoil dynamics and Bernoulli's principle will discuss this region of low pressure on the top surface and higher pressure on the lower surface as the differential components of lift. If you could measure the pressure on both sides of the airfoil you could show the changes as air velocity and angle-of-attack changed. Here are a few starting references. If you attached a file describing your experiment it didn't come through. Feel free to re-post it.

Rick Marz

http://www.aviation-history.com/theory/airfoil.htm
http://en.wikipedia.org/wiki/Airfoil
http://www.av8n.com/how/htm/airfoils.html
http://www.grc.nasa.gov/WWW/K-12/airplane/shape.html

thank you sir,i will make my question simpler if i some how make all the air to flow over the top surface of an aero foil and maintain constant pressure below the bottom surfacestill can i achive lift?

sreeharsha - I would like to see more about the experiment you are conducting. In your last comment you suggested that all airflow acts solely on the top surface. If you could design such a setup it would seem that Bernoulli's principle is not invoked, and you might be looking more at the Newtonian camp explaining lift. My opinion, and just my opinion, is that lift would not be generated in this instance. My reasoning is that, depending on the shape of the airfoil, both positive and negative pressures would be generated on the top surface and would cancel each other out. The constant pressure on the lower surface has no bearing. Here is a link to the Bernoulli-Newton background on lift. I don't think I'm answering your question yet, but would like to see your links to the problem. This is still a puzzle.

Rick Marz

http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html

my idea was simple sir i taught that we can have higher pressure difference if we some how manage to pass all the air on the top surface,but i think its not correct.Sir if you please dont mind i have another question:assume we cut a part of the aeroplane's wing thats aerofoil(hallow inside) of some width and place it on the ground and blow air on the top surface at high velocity then what happens?.....one more question:what are the criterias to be satisfied for a body lift other than this pressure difference concept.thanks for your interest you reply is most awaited thank you