The Rivers in the air (3)

Lo esencial es invisible a los ojos ( El Principito. Saint Exupery)

What is essential is invisible to the eyes

In this third post about rivers in the air we will talk about the other fundamental element in the evaporation of water: air. In the case of rivers in the air, it is about air that is in motion, forming a strong and continuous wind that, as we saw in one of the videos, reached 74 km/h (20.6 m/s).

We encounter the problem that these very strong winds occur very occasionally and in places that are not predetermined in principle. These winds are even less frequent during the summers of warm areas such as the Mediterranean.

Here the winds that, on the contrary, occur with great regularity are breezes. Breezes are very common local winds throughout the world that are produced by the difference in temperature between 2 separate areas, for example, between the sea and the interior. During late spring, summer and early autumn the sea maintains a relatively stable temperature of around 20°C or 26°C while inland areas often warm up to over 40°C to 45oC. During the day the breezes blow from the sea towards the land and at night the direction is reversed and the night breezes, also called "terral" or land breezes, blow from the land towards the sea.

In the area where we operate, that is to say in the province of Granada (Spain) between the Sierra Nevada mountains and the coast, the breezes converge towards the highest peaks of the Sierra every day.

The breezes blow every day bringing air coming from the sea from 10 or 11 am reaching maximum wind speeds between 4 m/s and 8 m/s at 2 pm and 5 pm in the afternoon, coinciding with the period of maximum daily temperature. These are gentle winds with speeds far from the 20 m/s required to divert the current of a small river.

During the night, the direction is reversed and the terral, a dry wind coming from the mountains, blows towards the sea during the night. This occurs practically every day during the mentioned season except very occasionally, when the dominant winds associated with other meteorological phenomena exceed speeds of 15 m/s, cancelling out the breezes.

The breezes we measured on the coast during the summer, at the foot of the beach, blew at a speed of between 4 and 8 metres per second. After measuring the temperature and relative humidity, we calculated that the water load that the breeze brings from the sea is between 8g and 12 g of water for each kilogram of dry air with a humidity saturation of around 80%. In conclusion, for our project on the rivers of air we have to work with the available winds, that is, with the breezes. These have one major drawback and two major advantages. The drawback is that they are gentle winds of the order of 4 to 8 m/s and they will never reach the 75 km/h (20.5 m/s) necessary to overcome the force of gravity on the water brought by a river, as we have seen in the previous videos. On the other hand, their regularity allows us to predict where, when and in which direction they will blow. This is a great advantage. The other great advantage is that, as breezes are a worldwide phenomenon that occurs in practically all latitudes and on all coasts, if our system works in Andalusia it will to work in many other places.

A bit of aerodynamics

The breezes that come from the sea are a horizontal laminar wind with constant speed. This means that at each point the speed is the same and the current lines are parallel to each other as in image a) below.

Let's see what happens when they approach an obstacle arranged perpendicularly such as a building, a mountain or a cliff.

We can model the movement of the air around this obstacle, and use the theory of potential fluid movement since the following conditions are largely met:

1-the movement is two-dimensional. If the obstacle is perpendicular to the wind, the movement in any vertical plane perpendicular to the obstacle will be the same.

2-the movement is stationary, that is, for a given point it does not vary with time

3-the movement is incompressible, that is, the variations in density due to the change in speed (Bernouilli) are so small that they can be ignored. This is valid for all motions with velocities substantially lower than the speed of sound (340 m/s).

4-The motion is irrotational. This is valid as long as the wind flow does not become turbulent as occurs after the boundary layer detachment.

If there was no boudary layer detachement the flow would be like the figure below

Under these conditions, aerodynamics shows us that the flow around the left corner has the following streamlines

We observe how the wind, initially horizontal, is deflected and becomes almost vertical in the area of the upper corner of the obstacle. It is also here where theory tells us that the speeds are maximum. These maximum vertical speeds are even greater than the initial horizontal wind speed of the breeze. This is therefore our area of interest to form a river of air since the wind speed is vertical or almost vertical and it is also the area where we will find the maximum speed.

In reality, from the upper corner, the boundary layer detaches and the theory of potential fluids is no longer valid since a turbulent movement is formed. This can be clearly observed in wind tunnel images such as this one.

Note the different zones:

a) with the original horizontal laminar wind (at the left)

b) the upward deflection when approaching the vertical obstacle until the wind isvertical, and

c) the detachment of the boundary layer around the corner and finally the subsequent turbulent motion.

Turbulent motions are chaotic and it is not possible to solve their equations of motion and define the precise streamlines they form. However, turbulent motions are very interesting for our rivers of air project since water droplets immersed in a turbulent flow evaporate much faster than when they are arranged in a laminar flow since turbulence constantly renews the air around the surface of the droplets.

All this is consistent when compared to the images of the air rivers shown so far, where it is clearly observed that the air rivers are formed precisely in this area at the top left corner of the cliff or ravine where the winds are vertical and maximum and that in cases where the water current flows below this area the wind speed being lower is not able to bend the river upwards in the air. You can also observe in these videos the detachment of the boundary layer and the subsequent turbulent movement.

With all this information in mind I went in search of ascending winds formed from the breeze. Since the winds are invisible I built a very simple tool to be able to visualize them. It is a 2 or 3 m long cane to which I attached a light red ribbon. The light ribbon is easily dragged by relatively weak winds. In the other hand I carry an anemometer to be able to measure the speed of the winds.

In this photo you can clearly see the presence of a vertical wind. It is formed precisely in the area of an upper corner of a ravine in the area of the Alpujarras of Granada.

Another very special place where ascending winds are constantly formed while the sea breeze blows is the Rules dam, the Beznar dam or probably almost any other dam in the Mediterranean who runs parallel to the coastline, that is, perpendicular to the speed of the breezes.

In these dams, ascending winds are formed regularly and with speeds between 6 and 10 meters per second, perhaps more. In a next video I will show the dynamics of the winds in the upper corner of the Rules dam where you can observe the ascending winds close to the upper corner and the effect of the detachment of the boundary layer so that the strong ascending wind suddenly disappears when we move a couple of meters away from the corner.

In the next post, The rivers in the air (4) I will explain how the presence of these winds, once detected and measured, can help us form a river in the air despite the fact that they are relatively soft winds.