Simple Flows Last fall, we started discussing fluid dynamics a little bit using straws as a basis for our understanding (see link in sidebar if you're interested). In choosing straws, we were able to pretend our fluid was an ideal fluid4, which means that: 1. The fluid is not viscous 2. The fluid has laminar flow 3. The fluid is non-compressible This was super convenient, and enabled us to actually calculate some silly things like how long a straw superman could use and suchlike. However, air is not an ideal fluid by any stretch of the imagination. It is indeed non-viscous and often can have a laminar flow, but air is highly compressible, meaning the volume that it occupies varies greatly with pressure and temperature changes5.  However, for any specific pressure and temperature, the volume that a given mass of air occupies will be constant. Although we won’t delve deeply into calculations for this introductory webpage, because once we cannot reliably use simplified models the math gets complex, we can still use the principles of physics to determine what the likely results would be. For now, we’ll start with a hallway with equally sized open doors on each end.   Conservation of Energy and Mass Two of the fundamental principles of physics are that neither matter nor energy are created nor destroyed, but can only  change form. This means, that for our hallway, if any mass of air enters from one side (but the pressure and temperature of the hallway are unchanged), an equal mass of air must have left the far side (or a simple flow. Since we didn’t change our pressure or temperature, the volume would have to be the same at both ends. If we know the volume of air moving through the doorway per unit time (called V-dot) and the area of the doorway (Adoor), we could find the speed at which the air is flowing through the door. Now, let’s make it a little more complicated. Say one door is actually a window that is half the size of the door, and air is flowing in the door. The hallway’s temperature and pressure are still constant. Then,  the relation above still holds and we can see that the velocity of air moving out the window is twice that of the velocity entering the room. Of course, this is still a simple flow. We could make it more complicated. One of the first ways would be adding more doors and windows. But, that still wouldn’t be terribly complex: so long as we know what the inlet areas (which doors and windows air is entering) are and the velocity through them, and which openings are outlets, life is still simple. The thing that truly complicates matters is when we change the specific volume of air, that is the amount of space that one mass unit of air occupies. Previous Return to Ventilation Homepage Next

If you want to have a quick refresher on simpler fluid dynamics (or missed the exciting information on straw physics) feel free to hit the image above! The same principle we are using with air in a hallway also governs water! In firefighting, using a nozzle with a smaller tip can increase the exit velocity (and range of the stream) significantly. However, the ratio is not perfect, as the water's flow is more turbulent as the flow rate through the nozzle increases. Image from firebytrade.com For the interested, image links to article discussing nozzle selection and fire streams.