Wind field

Another boundary layer experience last week: On my way from work I stopped to take pictures of flags that were outside my university’s main building and that very nicely visualised the wind field (as flags tend to do).

If you just look at the flags, they look weird — they wind field was clearly not changing over time, yet the flags were at a weird angle to each other.

And in the next picture you see why: Because the air had to flow around an obstacle, so stream lines were bunching up.

The next morning, I went past there again and stopped to take more pictures, when a colleague of mine stopped next to me, looking a little puzzled that I was taking pictures of our not especially nice main building.

I explained what I was doing, and we got talking about how you see the world with completely new eyes once you have noticed, or have been shown, something tiny. Isn’t that exciting? :-)

Shear flow

Another early morning crossing this bridge.

IMG_1544And the current and the sun glint were perfect for this kind of photos:IMG_1581They almost look like schlieren photography images in those super old papers, don’t they?
IMG_1587And I find it extremely fascinating how you can see the boundary layer between the flow and the stagnant water, and how wind waves don’t manage to cross that boundary.
IMG_1592See the tiny capillary waves on the right side of the boundary? Those are locally generated because the larger waves on the top left just don’t make it over the strong shear.

You want to watch a movie? Sure!

And another thing I love on those early morning trips? Being completely alone in a pretty park, with dew on the grass and flowers in the sun :-)