Another neat experiment in the collection I’ve recently been talking about is measuring pressure at different points on a wing profile. It’s not terribly surprising that – as long as the wing is oriented in the correct way in the flow – pressure is high below the wing and low above it. Kinda the whole point of having a wing profile. Yet, it’s nice to actually measure it.
Measuring pressure at different points on a wing profile
And yes – next time I set up that manometer I’m gonna make sure that it’s a little easier to get a good reading!
Another one of those awesome hydrodynamics toys: A Pitot tube!
This is what it looks like:
Pitot tube. The tip pointing downward in this picture would point right into the flow.
What you can’t see here is the little hole at the tip of the tube that is pointing downwards in the picture. What the Pitot tube measures is the pressure difference between that hole (the stagnation pressure since it’s the stagnation point) and the vents some 3.5 cm above (the static pressure), from which you can calculate the dynamic pressure, hence air speed of a plane (if the Pitot tube was mounted on said plane) or, in our case, the speed of air flow from a fan relative to a stationary Pitot tube.
Again, I’m sadly too lazy to calculate anything, but you can take the measurements from the movie below and do it yourself if you so desire! :-)
On Monday I posted about playing with Venturi tubes. Guess what: We are going to play more today! Because today the Venturi tubes are connected to a “proper” manometer:
Venturi tube connected to manometer
Now, if I wasn’t so lazy this would be a great opportunity to get good readings of the pressure differences caused by different flow rates. However, I’ll just let the images speak for themselves. Enjoy!
A Venturi tube is one of the things one hears about in hydrodynamics class all the time, but one never gets to see them for real. And even though I just said on Friday that the thing that I found most fascinating in the aerodynamics collection I got to borrow recently was to see how the flow reversed downstream of a paddle I might have to take that back, because the hands-down most exciting thing was to play with a Venturi tube!
So what is all the fuss about? This is what a Venturi tube looks like:
Venturi tube in the no-flow state
Basically, it is a tube, open at both ends, that gets thinner in the middle and wider again. All the rest you see in the picture is props: The mouth of the fan in the top right, and then three U-tubes filled with dyed water below the Venturi tube.
The Venturi tube is so famous because it nicely demonstrates the Venturi effect, namely the reduction in pressure that occurs when a flow is accelerated. In the case of the Venturi tube, the flow is accelerated in the thin section of the tube, where – for continuity reasons – it has to go faster than in the wider sections. So what happens when we turn on the fan?
Venturi tube. Pressure decreases in the thin section of the tube, visible by the red water being “sucked up” in the middle U-tube
Yep! The levels in the three U-tubes change. And most importantly, the pressure for the middle U-tube drops, as demonstrated by the red water being “sucked up” on the side of the U-tube that is connected to the Venturi tube.
One of the things that fascinated me most when playing with the huge fan we used to look at the flow downstream of a paddle was how the flow direction reverses.
Unfortunately (alas, it was to be expected) we didn’t really see this on the paper towel stream line test I did the other day.
But here is another way to visualize it: using a propeller!
Propeller in air flow
Depending on the direction the air flows at the propeller, its direction changes. So as we move it towards and away from the paddle, when the flow direction changes, so does the direction of rotation of the propeller, too.
Whenever I’m in a canoe or kayak, I love watching the two eddies that form behind the paddle when you pull it through the water. It looks kinda like this:
Flow around a paddle
Instead of pulling a paddle through more or less stagnant water, we could also use a stationary paddle in a flow. And that is the setup I want to discuss today: A stationary, round paddle perpendicular to an air flow.
A very cool feature of the paddle – which we know has to exist from the sketch above – is shown below: There is a point somewhere downstream of the paddle, where the direction of the air flow changes and a return flow towards the paddle starts. You can see that the threads on the stick I am placing in the return flow go partly towards, partly away from the paddle. So clearly the stick is in the right spot!
Visualizing the flow field behind a paddle with a threaded stick
Another visualization that my dad came up with below: Threads are pulled back towards the paddle in the return flow.
Visualizing the return flow behind a paddle with threads
Doesn’t it look awesome?
Visualizing the return flow behind a paddle with threads
Another way to visualize the change in flow direction is to take a rotor and move it from far downstream of the paddle towards the paddle and back.
Visualizing the change in flow direction by moving a rotor towards and away from a paddle blocking an air stream
All of this is shown in the movie:
Don’t you wish you had all this stuff to play with? :-)
(And do you now understand why I was so excited about the diving duck? :-))
Recently, someone at my university told me about a case of experiments connected to aerodynamics* that they occasionally use for demonstrations and outreach. Obviously, I asked if I might possibly borrow the case, and fast forward: my dad and I spent a whole weekend playing.
I’m gonna go through all the experiments over the next couple of posts, so let’s get started!
The first experiment has a slight ring of the balls balancing on water jets. I’m a little torn on which one I like better. The experiment below looks a little more like magic, because the air jet is invisible. But the balls are balancing on water jets. Water! Tough choice!
Ball balancing on air jet
So this is what happens: The ball sits on the edge of the jet. The jet speeds up where it flows around the ball, and according to Bernoulli the pressure sinks and the ball is being pushed into the jet by the air pressure from outside the jet.
In the movie below you see how the ball can balance quite stably if left alone in the position it finds for itself, and how it reacts to the air flow being disturbed.