Let’s be honest; everyone, even if not a STEM fellow, has heard about aerodynamics and also about acoustics. Probably most people even have a intuitive understanding of the governing laws of both fields; but when you type ‘Aeroacoustic’ in your Microsoft Word you get accused of committing a typo. What?
Yes.
Well, first things first. Either to satisfy your curiosity or to shine my way through your alt+f4 keys, I feel obligated to let you know that I’m a mechanical engineering student currently working on my master thesis in the thermotechnology industry.
And this isn’t just me trying to buy my credibility; I am actually very enthusiastic about Aeroacoustics and I truly believe more information should be posted about it. Out there. In the entire World. So that I wouldn’t need to explain it every time I mention my work to someone.
So what’s it about?
Aeroacoustics is concerned with the interactions between fluids and acoustic fields. It is as simple as it gets! Any variation in the fluid dynamics changes the way it interacts with near solid surfaces, thus altering the sound reflection. And it also works the other way around. Ever wondered about the whistling sharp objects make when whipped through the air? If not, you’re lame.
Jk.
Let me explain it!
The birth of a sound wave!
As you know, everything is made of molecules. The molecules in fluids, specially gases, are simply further apart from each other than the ones in solids.
So when you swing a solid object in the air, it will press the air molecules next to it and dislocate them slightly. The adjacent molecules, not in the first layer of “contact” with the object, will stay in their place for a fraction of a small time step, just being pressed by the moving molecules. These two layers become compressed against each other and in the next moment, the second layer becomes energized enough to compress the third layer of air molecules. This way, a gap is created between the first two layers which is called a rarefaction. A pattern of particles’ compression and rarefaction takes place in the fluid medium and this, my friend, is how you create a sound wave.
Sound waves have a specific frequency. The frequency of a wave shows you how many cycles of compression and rarefaction you can stuff into a certain period of time. If we’re talking hertz, it’s how many of them in one second.
Technically, sound is just a vibration of air particles.
So if you swing a stick around like a crazy person, the probability of generating sound waves that reach human perceptible frequencies is indeed very high. Better even, you can dress up as a Samurai and tell everyone you’re doing Tachikaze. That’s a Japanese word for a sword swooosh. Fancy.
Human hearing perception is stated to be comprehended between 20 to 20000 hertz with some variations depending mostly on age. Do you work out? You can skip arm day. I dare you to reach 70 decibels of Tachikaze.
But that’s not all of it!
This is just the way a sound wave can be formed.
To a great extent, noise can be seen as a purely subjective phenomenon. For a conservative, poncy person, noise can be a punk teen’s alarm clock. But each year, regulations are more and more strict regarding noise emissions in all kinds of industry.
And noise control is complex. Sound in our everyday lives has many distinct sources which generate specific wave frequencies that overlap into one big, messy acoustic spectrum.
Try finding your way around that. [Spoiler alert: FFT]
Better yet. Imagine you are analysing sound on a rotating surface. For example, a fan. Or a turbine. Imagine one blade of it. While rotating, it moves the closest air molecules in all possible directions. And these molecules, strongly energized in all directions, interact with the second layer of molecules around them. A pressure gradient composed of compressions and rarefactions begins.
Then these particles, sooner or later, interact with
a. the neighbour blade,
b. the next best surface or
c. different areas of the flow with different velocity and pressure distributions.
And since no material is perfect, surfaces induce friction and shear forces in the fluid. This complex pattern of pressure gradients due to the waves’ propagation and this flow A meets flow B and fluid meets solid games generate multiple, complex swirls in the fluid domain which are called vortexes.
And guess what: vortexes generate turbulence. And turbulence in aeroacoustics means noise!
Do you enjoy quiet, soft air conditioning in your fancy apartment? Don’t you dream about flying your drone without annoying the crap out of everyone?
I think you understand where I’m getting at.
Now add in temperature gradients, Reynolds stresses, convection, air density variations and some tricky, unsolvable Navier Stokes equations. Good thing we live in the time of decent computing power.
But that’s a whole different story.
This was a four-minute read but I am happy if you learned something new. That was my goal after all.
