Coanda Effect and Entrainment for Bladeless Fan

Hey friends, Happy Wednesday!

Answer to the question I posed last week

Why do you think the scotch yoke mechanism was used in the beard trimmer and not any other rotary to linear conversion mechanism? The primary reason I believe is that the Scotch Yoke mechanism in a beard trimmer is more compact and has less moving parts.

The 2 concepts we’ll cover this week to understand bladeless fans are Entrainment and Coanda effect. We’ll explore the majority of fluid dynamics used in gadgets and devices over the 2 weeks, including the true reason for how an airplane generates lift. I aim to write my newsletter issues in a way one can follow them while traveling on a bus, having a coffee, waiting for your food, etc. But maybe this one needs a little more attention. Let’s jump in!

Entrainment

Refer to the image on the left for Entrainment (marked 1.), and right for Coanda effect (marked 3.).

A fluid jet of air moving at high speed will have low pressure due to Bernoulli’s principle. Assume all other ideal conditions for this scenario to happen. The air outside this jet is at ambient pressure (environment), which is relatively higher than the low-pressure fluid jet. This pressure difference forces the volume of air immediately outside the fluid jet to be pulled into the stream, thereby increasing the total volume of airflow. This is Entrainment, basically.

Coanda Effect

If a solid surface is placed close to the jet almost in parallel, then the entrainment (and therefore removal) of air from between the solid surface and the jet causes a reduction in air pressure. Both sides of the jet reduce in pressure due to entrainment. But the pressure on the side between the jet and the surface cannot be balanced as rapidly as on the other side of the jet due to the lesser volume of air available. This pressure difference across the jet causes it to deviate towards the surface, and then adhere to it. This bending of fluid flow is Coanda effect.

If the surface is not too sharply curved, the jet bends towards the surface adhering to it as it flows. Such surfaces are called Coanda surfaces. Example - Wings of an aircraft

Question of the week

Show the curved backside surface of your spoon under medium-running tap water, aligned vertically along the stream. What happens to the water stream and can you explain this phenomenon? Share your thoughts by replying to this email/attaching videos of what you find, and we can have a discussion. I’ll answer this in next week’s issue.

Blog post

My blog post S1E3 on Bladeless fan is coming out next week along with a newsletter issue on the true physics behind how an airplane generates lift. I’ll notify you next Wednesday on the same.

I’m thinking of writing about the heart rate monitoring algorithm on a fitness watch for the next episode S1E4. Also, let me know if you’d like to read about a specific gadget/device.

Have a nice rest of the week, and take care!
Until next Wednesday,
Chendur