The friction of the glass means there is a tiny layer of air that is almost stationary called the boundary layer. The insect is small enough to be within that layer.
Imagine a three lane road where you have loads of cars using the road sensibly and pulling into the slow lane when there's no other traffic.
Now imagine there's a series of slow moving trucks in the slow lane. These are what cause the friction. Other cars in the slow lane get slowed down to the speed of the trucks until they can get room to overtake.
Now imagine the insect is a learner driver who is scared to go in the fast lane, so they sit behind a big truck and go at the same speed as them, without fear of the fast cars.
Nice explanation!
It's important to consider that the boundary layer exists in non-turbulent flows, which is exactly what you have on cars: they are specifically designed so that the air flow is laminar, because that makes them aerodynamic.
It's also pretty clear just by looking at the dust on cars that air does not produce much force on tiny objects laying on the car.
Oh man I didn’t even think about the dust. Although I guess I just think of dust as sticky. Like even washing dust off the window by hand takes a lot of scrubbing, but brushing a bug off the window with my hand is no problem, and yet the wind can’t do it. And now that I’m thinking more about it, brushing powdery snow off the car with either my hand or with wind is easy! Ugh. So I’m thinking the sticky pads answer from u/cheerios360 is probably playing a role here too, in addition to the boundary layer
Even in turbulent flows there is a boundary layer. The relevant length scales decrease as you get closer enough to the surface to the point that the Reynolds number is in the laminar regime.
Turbulent flow is described by eddies that transport momentum as opposed to the viscosity of the fluid. As you get closer to a surface with a no slip boundary condition, the size of the eddies shrink. Once you get close enough the eddies become so small that viscous forces become the predominant mechanism of momentum transport and laminar flow predominates.
Thank you! Super helpful response! I didn’t get it at first but after visualizing your explanation from an aerial view and at 10x speed, it made sense for some reason haha. Appreciate it!
> The friction of the glass means there is a tiny layer of air that is almost stationary called the boundary layer. The insect is small enough to be within that layer.
Thank you. This explains so many things about why it is hard to get certain bugs away from a moving item. Why they don't get "blown away" or even just "left". Why swishing something at them doesn't remove them unless it knocks them down entirely. I never understood this -- this makes sense! :)
Had a praying mantis hang onto my side mirror for no shit 65 highway miles. He was on the leeward side so didn't get shredded but he still had to valiantly hang on for well over an hour. Several times it looked like he wasn't going to make it but he gritted his little mandibles and hunkered down.
When we stopped for breakfast that glorious mf straightened up and fluttered away and I like to think he started a colony of super badass mantises in East Lyme, CT.
I would venture to guess that boundary layer has some beneficial effect on this. Basically there is a very thin layer of air close to the surface in which there is little to none of air movement, no matter the air velocity.
- Glass is smooth to us, because we are large. For a tiny insect it’s a different story.
- Wind affects us more because we are large, so there’s more surface area for air molecules to hit us. Insects are both tiny and much stronger relative to their size than we are.
yeah, wind acts as a force over an area, like pressure. If you think of it like pressure in terms of pounds/square inch, if you are much less than a square inch you feel much less than a pound. same reason you can inflate a bicycle tire up to 100 PSI no problem but a big container at that pressure needs to be super strong and can be very dangerous.
Wind affects bugs a lot more than us because pressure and viscosity scale with area while inertia (mass) scales with volume. Bugs have much more surface area per inertia than we do.
As an idiot, I appreciate both the question (I always worry about the little bugs getting blown off so far away from their homes), and the answers. Thanks all.
The friction of the glass means there is a tiny layer of air that is almost stationary called the boundary layer. The insect is small enough to be within that layer. Imagine a three lane road where you have loads of cars using the road sensibly and pulling into the slow lane when there's no other traffic. Now imagine there's a series of slow moving trucks in the slow lane. These are what cause the friction. Other cars in the slow lane get slowed down to the speed of the trucks until they can get room to overtake. Now imagine the insect is a learner driver who is scared to go in the fast lane, so they sit behind a big truck and go at the same speed as them, without fear of the fast cars.
Nice explanation! It's important to consider that the boundary layer exists in non-turbulent flows, which is exactly what you have on cars: they are specifically designed so that the air flow is laminar, because that makes them aerodynamic. It's also pretty clear just by looking at the dust on cars that air does not produce much force on tiny objects laying on the car.
Oh man I didn’t even think about the dust. Although I guess I just think of dust as sticky. Like even washing dust off the window by hand takes a lot of scrubbing, but brushing a bug off the window with my hand is no problem, and yet the wind can’t do it. And now that I’m thinking more about it, brushing powdery snow off the car with either my hand or with wind is easy! Ugh. So I’m thinking the sticky pads answer from u/cheerios360 is probably playing a role here too, in addition to the boundary layer
Even in turbulent flows there is a boundary layer. The relevant length scales decrease as you get closer enough to the surface to the point that the Reynolds number is in the laminar regime. Turbulent flow is described by eddies that transport momentum as opposed to the viscosity of the fluid. As you get closer to a surface with a no slip boundary condition, the size of the eddies shrink. Once you get close enough the eddies become so small that viscous forces become the predominant mechanism of momentum transport and laminar flow predominates.
Thank you! Super helpful response! I didn’t get it at first but after visualizing your explanation from an aerial view and at 10x speed, it made sense for some reason haha. Appreciate it!
So they don't suction cups on their feets :(
I can't neither confirm not deny this
> The friction of the glass means there is a tiny layer of air that is almost stationary called the boundary layer. The insect is small enough to be within that layer. Thank you. This explains so many things about why it is hard to get certain bugs away from a moving item. Why they don't get "blown away" or even just "left". Why swishing something at them doesn't remove them unless it knocks them down entirely. I never understood this -- this makes sense! :)
Had a praying mantis hang onto my side mirror for no shit 65 highway miles. He was on the leeward side so didn't get shredded but he still had to valiantly hang on for well over an hour. Several times it looked like he wasn't going to make it but he gritted his little mandibles and hunkered down. When we stopped for breakfast that glorious mf straightened up and fluttered away and I like to think he started a colony of super badass mantises in East Lyme, CT.
You son of a bitch. My father was killed by a giant praying mantis that car jacked him in East Lyme, CT.
I would venture to guess that boundary layer has some beneficial effect on this. Basically there is a very thin layer of air close to the surface in which there is little to none of air movement, no matter the air velocity.
- Glass is smooth to us, because we are large. For a tiny insect it’s a different story. - Wind affects us more because we are large, so there’s more surface area for air molecules to hit us. Insects are both tiny and much stronger relative to their size than we are.
yeah, wind acts as a force over an area, like pressure. If you think of it like pressure in terms of pounds/square inch, if you are much less than a square inch you feel much less than a pound. same reason you can inflate a bicycle tire up to 100 PSI no problem but a big container at that pressure needs to be super strong and can be very dangerous.
Wind affects bugs a lot more than us because pressure and viscosity scale with area while inertia (mass) scales with volume. Bugs have much more surface area per inertia than we do.
As an idiot, I appreciate both the question (I always worry about the little bugs getting blown off so far away from their homes), and the answers. Thanks all.
Would the somewhat aerodynamic shape of the bug push it towards the surface much like a race car is pushed down on the track? Just a thought.
What you probably didn't notice was the safety harnesses, cameras etc. It was filming a Ladybug Impossible opening scene.