You are very welcome

Thanks!

Glad it was helpful!

Thanks!

Thank you!

Good point! Thanks for the comment!!

Smoke would be great but for a closed loop-wind tunnel the smoke would just go round-and-round. Which means after about a minute, the whole tunnel is filled with smoke and you can't see anything! I have done it a couple of times. The other downside to smoke is, what is the smoke made of: glycol, glycerin, dry ice? Ideally the smoke should be neutrally buoyant so that it follows the flow and doesn't rise or fall based on a difference in density between the smoke and the air. Think about hot steam from a smoke stack, it rises pretty quickly. Another consideration is what happens to the smoke over time. So if it is a Glycol based smoke, like a lot of theatrical fog machines, it will leave everything in the tunnel a little greasy for a while as it accumulates on your test section and models. I wonder about its effect on hot-wires. A strobe light would be cool to try. Especially if it were synced to something periodic such as a vortex shedding event.

@@marquetteshockphysics5518 Yes, good point about the density difference. I hadn't thought that through. That was my thought on the strobe light as well. It could be variably synched to match flow rates at different points. Then these would appear "frozen" compared to the rest of the flown It would make specific flow rates detectable using vision directly. (Sort of how astronomers use software to reverse moving objects to make them look still, and stationary objects to make them look moving. It helps pick out dwarf planets and asteroids against star fields.) If it doesn't work out, you could have a disco party fundraiser to recoup your costs.

sorry do u know how he worked out static pressure? for I do not know total pressure

I guess the Static pressure has been measured by the scanivalves to which the tubes are connected from the pressure ports on the surface via the pressure taps

Hi IllIa Thanks for the comment and good point regarding pressure measured by the ports. For sure if a pressure port is oriented parallel with the wind (facing the incoming flow directly) then it would measure the stagnation pressure. In fact that is how one can align the airfoil to the wind, by insuring the leading edge port is measuring stagnation pressure. -jb

do u know what total pressure is for the experiment??

I do not. I wanted to build a water tunnel but it is still on my to-do list!

A good reference is a book entitled "Theory of wing sections", www.amazon.com/Theory-Wing-Sections-Aeronautical-Engineering/dp/0486605868 The book contains lots and lots of dimensionless data for many of the 4 and 5 digit NACA airfoils.

@@marquetteshockphysics5518 thank you very much. I really appreciate the prompt response

Hi Armel, to get lift you can integrate the pressure profile over the area. The resulting force will act at the center of pressure. For more details seek out reference materials in a textbook or a reliable web-source like www.grc.nasa.gov/www/k-12/airplane/cp.htm To get a moment relative to a given location, it is a similar process: integrate the pressure distribution times a moment arm.

No idea! I suppose you are talking about a wind turbine, whether it be horizontal or vertical (more like a water wheel type). The tunnel cross-section at the test section is 2 feet x 2 feet, so the wheel would have to be small. As a rule of thumb, anything you place in the tunnel should have a projected area less than 10% of the test section area.

There are several ways to get the lift and drag. One way is to measure the pressure on the surface of the airfoil and then integrate that pressure over the area, force=pressure*area. Another way is to measure the velocity profile behind the airfoil and calculate the momentum loss, that will be equivalent to the drag. Then perhaps the best way is to directly measure it with a force balance. If you are interested in learning more you can check out a nice fluids text book, such as Frank White's "Fluid Mechanics" or there are lots of resources on line such as www.grc.nasa.gov/www/k-12/airplane/right1.html

The general rule of thumb for blockage is that the frontal projected area of the test article should not exceed 10% of the total cross-section of the tunnel. The total cross section of the wind tunnel is 2ft x 2ft. Thus the frontal projected area should not exceed 0.4 ft^2. This is of course just a general rule.

XFLR5

Designfoil is a shareware tool you can use for this. It has the NACA airfoil generators in it along with a 2D wind tunnel.

Super interesting!! I am not sure where in the video you are referring to but there are regions of the flow where the pressure isless than atmospheric pressure (ie a vacuum pressure). But the pressure never goes to absolute zero (like a hard vacuum). In regions of the flow where the air is moving faster the pressure is low, lower than atmospheric pressure. So it is a vacuum pressure in the sense that it is slightly lower than atmospheric. This is due to the Bernoulli effect, which for incompressible fluid behavior (as is reasonable to assume here) states that as the pressure and velocity are inversely proposal: if pressure goes up, then velocity goes down and if velocity goes up then pressure goes down.

See the following webpage for the definition of the center of pressure (cp): www.grc.nasa.gov/www/k-12/airplane/cp.html So you could perform numeric integration for cp using something as simple as the trapezoid rule

@@marquetteshockphysics5518 amazing setup sir. I was wodering about putting naca duct in a long wing where the air detaches at the rear end of the wing. I believe that naca ducts on such wing will suck the air from the bottom of the wing. 1) improving the downforce 2) forcing the air to remain attached to the bottom side of the wing making it more efficient. Please let me know what you think and if you could experiment such setup.

I am not familiar with NASA ducts. But there is a wiki page:en.wikipedia.org/wiki/NACA_duct separationopeningbleeding-off pressure, aeffectivenessopportunemaneuversseparation or transition could be suppressed or enhancedNeat idea! application

Good question! Not sure I could answer that. I think of it more like there are two simple theories that do a good job of describing the same complicated physics. Both theories have their advantages and disadvantages but neither can describe everything that is happening: laminar-turbulent boundary layer transitions, wing-tip vortex rollup, flow separation. So in a way they are both just models (ideas) of what is happening to the flow along the wing. Sorry that got a little existential for a moment... :)

Even in a "pure" Newtonian sense the lift doesn't only come from the bottom surface / high-pressure. The air that travels over the top surface is deflected downwards as well too which by Newton's second law means it also contributes to the lift force.

@@marquetteshockphysics5518 If Bernoulli's equation is just an equation that calculates the conservation of energy, why is it so often assumed that the increase speed over the top of the airfoil then must have lower pressure, instead of the other way 'round. If the high pressure at the leading edge and slightly the lower surface (assuming an angle of attack), wouldn't this create a low pressure behind this pressure buildup (which also causes the updraft), leading to the fluid speeding up? I don't understand how if a vacuum on the top of the wing lifts the entire gross weight of an aircraft, fuel, passengers, and cargo, how could, say, a fabric coated wing be held together by glue. Seems more intuitive to me that the low pressure causes a high speed, which spills off the trailing edge downward, pushing the wing up. No?

​@@BrentDanley Bernoulli just tells you the numbers. It doesn't explain why it happens. In fact, it is the lower pressure which pulls the air in (accelerates it). Your intuition about vacuum is correct. Take a container with a vacuum inside. There is (literally) nothing inside which "pulls" on the wall. Even just a tiny bit of air inside will always PUSH against the wall. But the outside (atmospheric) pressure will press a lot more against the walls. But there is nothing inside to hold back. Same with the wing. The lower the pressure on the upper side, the more the pressure on the under side can push the wing up. (At least as far as i understand)

@@BrentDanley re: "why is it so often assumed that the increase speed over the top of the airfoil then must have lower pressure" Correct me if I'm wrong, but his plotted data near the end of this video **shows** just that, increased velocity and __lower__ pressure ...

What is nice about reporting non-dimensional numbers is that two data sets (collected at two different free stream velocities) can be compared. If the non-dimensional data matches for these two data sets then we know we have dimensional similarity. Thus we need not data data for every free stream velocity we might be interested in.

@@marquetteshockphysics5518 sir, the idea you're expressing is that while comparing data between 2 models of a body, it is much more convenient to predict Aerodynamic forces if we make use of non-dimensional numbers correct?

@@akarshshetty9262 Absolutely. Making use of non-dimensional data not only reduces the amount of data we have to take but it can also indicate when there are major changes in the physics. For example, if comparing two data sets it might be easier to spot when separation of boundary layer transition occurs.

@@marquetteshockphysics5518 yeah! Sir, here you hav made use of pressure ports to measure the normal pressure loads... Is there any way to measure the Tangential Shear loads experimentally other than methods like CFD? Edit1: *other than non-experimental methods like CFD

@@akarshshetty9262 Sure. Look up hot-film anemometers. You can purchase or make these. They are attached directly to the surface of the body.

Good point!

Hi Danko, yikes that does seem small! I just went down in the lab and remeasured the chord length of that airfoil to be 6 inches (0.1524 m).

Good point!! Thanks for pointing that out!! :)

john.borg@mu.edu

Yes, I know mixed units.... what can you do? I love them both!! :)

@@marquetteshockphysics5518 I can cry....

Cпасибо за комментарии! Крыло меняет направление импульса ветра. Это то, что создает подъемную силу на крыле. Пузырьки гелия показывают, как меняется направление ветра.

@@marquetteshockphysics5518 Пузырьки показывают что воздух который находится над крылом в реальном полете будет лететь за крылом, т.к. над крылом создается разрежение, а основной закон физики и природы звучит так: "Вещество всегда перемещается из области высокого давления в область низкого." Поэтому в реальном полете никакого обтекания крыла потоком не будет. Воздух вокруг крыла летит вместе с крылом в том же направлении что и крыло и с той же скоростью что и крыло из за того что имеет ВЯЗКОСТЬ. А воздух над крылом летит еще быстрей чем крыло в том же направлении что и крыло. А из этого следует, что всю теоретическую аэродинамику, с ее обтеканием и потоком и ускорением потока над крылом, можно спустить в унитаз как лженауку!