The wind and the wall

I have an undergraduate degree in mechanical engineering but I’ve always struggled with thermodynamics. To the uninitiated, this means most of the knowledge specific to mechanical engineering over other branches remains out of my reach. I would struggle even with the simpler concepts, and perhaps one of the simplest among them was pressure.

When a fluid flows through a channel, like water flowing through a pipe, it’s easy to intuit as well as visualise what would happen if it were flowing really fast. For example, you just get that when water flowing like that turns a corner, there’s going to be turbulence at the elbow. In technical parlance, it’s because of the inertia of motion (among other things, perhaps). But I’ve never been able to think like this about pressure, and believed for a long time that the pressure of a fluid should be higher the faster it is flowing.

In my second or third year of college, there was a subject called power-plant engineering, a particularly nasty thing made so because it was essentially the physics of water in different forms flowing through a heat-exchanger, a condenser, a compressor, a turbine, etc. Each of these devices mollified the fluid to perform different services, each of them a step in the arduous process of using coal to generate electricity.

Somewhere in this maze, a volume of steam has to shoot through a pipe. And I would always think – when picturing the scene – that the fluid pressure has to be high because its constituent particles are moving really fast, exerting a lot of force on their surroundings, which in turn would be interpreted as their pressure, right?

It was only two years later, and seven years ago, that I learnt my mistake, when my folks moved to an apartment complex in Bangalore. This building stands adjacent to a much larger one on its right, separated by a distance of about 40 feet, with a wall that rises as high as an apartment on the sixth floor. My folks’ house is on the fourth floor. Effectively, the complex and the wall sandwich a 40-foot-wide, 80-foot-high and 500-foot-long corridor. The whole setup can be surveyed from my folks’ house’s balcony.

When there’s a storm and the wind blows fast, it blows even faster through this corridor because it’s an unobstructed space through which the moving air can build momentum for longer and because its geometry prevents the air from dissipating too much. As a result, the corridor becomes a high-energy wind tunnel, with the wind whistling-roaring through on thunderous nights. When this happens, the curtains against the window on the balcony always billow outwards, not inwards.

This is how I first realised that the pressure outside, in the windy corridor, is lower than it is inside the house. The technical explanation is (deceptively) simple: it’s composed of the Bernoulli principle and the Venturi effect.

The moving wind has some energy that’s the sum of the kinetic energy and the potential energy. The wind’s speed depends on its kinetic energy and its pressure, on its potential energy. Because the total energy is always conserved, an increase in kinetic energy can only be at the expense of the potential energy, and vice versa. This implies that if the wind’s velocity increases, then the corresponding increasing in kinetic energy will subtract from the potential energy, which in turn will reduce the pressure. So much is the Bernoulli principle.

But why does the wind’s velocity increase at all in the corridor? This is the work of the Venturi effect. When a fluid flowing through a wider channel enters a narrower portion, it speeds up. This is because of an elementary accounting principle: the rate at which mass enters a system is equal to the rate at which mass accumulates in the system plus the rate at which it exits the system.

In our case, this system is composed of the area in front of the apartment complex, which is very wide and wherefrom the wind enters the narrower corridor, the other part of the system. Because  the amount of wind exiting the corridor at the other end must equal the rate at which it’s entering the corridor, it speeds up.

So when the wind starts blowing, the Venturi effect accelerates it through the corridor, the Bernoulli principle causes its pressure to drop, and that in turn pulls the curtains out of my window. If only I’d seen this in my college days, that D might just have been a C. Eh.