Tag Archives: wind stress

When not the fetch but a funnel shapes the wave field

As you know we are currently preparing for future wave riddles. So this afternoon I went out for a wave hunt again and found something beautiful for you!

The ship coming out of the Kiel-Holtenau locks into the Kiel Canal is making waves, but although those are pretty exciting, too, there are more things going on in the picture above…

Many processes can create waves

In addition to waves made by ships, seagulls, the locks opening and closing, and those waves being shaped by reflection, refraction, and all those other processes, most waves look actually pretty similar, and they are all formed by the same process.

Most waves are wind waves

In almost all situations it’s a safe guess that most of the waves you see are caused by the wind. Either locally, or by storms far away. Of course, the waves might look very different from day to day and location to location. But as a rule of thumb, the stronger the wind, and the longer it has been blowing, and the longer its way over water without any obstacles in its way, the higher the waves.

Usually the length of the fetch shapes the wave field

This uninterrupted stretch that the wind can blow over the water is called the “fetch”. And it explains why you don’t have really large waves on small ponds: if the fetch isn’t long enough, waves just don’t have enough time to build up from when they are generated at the upwind side of the pond until they have reached the downwind side.

Sometimes obstacles shape the wind field

Sometimes though, there are obstacles in the wind field that cause interesting wave phenomena. Below you see that the wind that has been coming across Kiel Canal is interrupted by those pylons. Upwind of the pylons the waves are fairly regular and pretty boring.

But remember your Bernoulli? If the area across a flow decreases, for continuity reasons the flow speed has to increase.

Since air is “flowing” in that sense, too, it’s accelerated where it goes in between and around those pylons since it has to squeeze through a smaller cross section than it had to its deposal further upwind.

The wind field is mirrored in the wave field — well, kind of

Do you see how the faster wind causes all these nice little ripples? Maybe “mirroring” the wind field isn’t quite the right way to express it, but you can definitely see where the wind speeds are different from the rest of the Kiel Canal just by looking at the waves! From there the waves then propagate as sectors of circles outwards and leave the areas of the high wind speed, but they quickly dissipate and vanish again.

Wave watching is awesome. Can you think of anything better to do on a Saturday afternoon? :-)

Influence of stratification on mixing

A wind stress is applied to the surface of a stratified and a non-stratified tank to cause mixing.

This is an experiment that Martin and I ran at the JuniorAkademie this summer, but since I posted soooo much back than (just look for the tag “JuniorAkademie” to get an impression of what we did) I feel it never got the attention it deserves. So here we go again! :-)

We ran two experiments, one after the other.

In the first one, we took a tank full of freshwater, added dye droplets and switched on a hair dryer to force mixing through the wind stress. After about a minute, the tank was fully mixed.

In the second experiment, we created a salt stratification: salt water with approximately 35 psu, and freshwater. We then added the dye droplets. The droplets never penetrated into the salty layer but instead layered in at the interface between the two layers. We then added the wind stress.

After a minute, the surface layer was well mixed, but there was no mixing penetrating into the bottom layer. To fully mix the whole depth, the wind forcing ran for 86 minutes (and I am proud to report that my hair dryer survived this ordeal!).

Mixing in a non-stratified tank (left) and in a stratified tank (right). See the stop watch at the bottom of the panels for an impression of the time scales involved!

This is a great demonstration of how mixing is inhibited by stratification. We had been expecting to see a difference, but we were really surprised that the difference was so large. I started the experiment an hour before a meeting we had to attend, but then obviously couldn’t leave on time, because I could neither stop the experiment (seriously! How could I have stopped?) nor leave the hair dryer running while I wasn’t in the room.

Watch a short movie below and a movie containing the full time lapse even further down!

 

Mixing in a non-stratified and in a stratified tank

A wind stress is applied to the surface to cause mixing.

This is an experiment that I have been wanting to do for a long time, but somehow it never worked out before. But last night Martin and I finally ran it!

We ran two experiments, one after the other.

In the first one, we took a tank full of freshwater, added dye droplets and switched on a hair dryer to force mixing through the wind stress. After about a minute, the tank was fully mixed.

In the second experiment, we created a salt stratification: salt water with approximately 35 psu, and freshwater. We then added the dye droplets. The droplets never penetrated into the salty layer but instead layered in at the interface between the two layers. We then added the wind stress.

After a minute, the surface layer was well mixed, but there was no mixing penetrating into the bottom layer. To fully mix the whole depth, the wind forcing ran for 86 minutes.

Watch a short movie below and a movie containing the full time lapse even further down!