Winter is a wonderful time to sit by a fireplace, drink a hot beverage, and listen to the rain pound the windows from outside. Even in this cozy setting the engineering principles of fluid mechanics are hard at work.
Flames rising from burning logs always have a way of mesmerizing me, the way they dance mischievously around and leap for the chimney captivated my curiosity since I was a child. Now that I am an engineering student I can explain what causes them to act the way that they do. There are two basic principles that can explain this. The first is as simple as the fact that hot air rises, the hotter the air the faster it rises. The second is that as things get hotter they emit radiation that we see as light, like red hot metal. If you put these together you find that flames are actually a mix of air and smoke that are so hot that the radiation they emit is in the visible spectrum. The flames dance over large fires because of disturbances in the air and because the air is so hot that it rises fast enough for the air to become turbulent. Yet over a candle the flame can stay relatively steady. This is because the air is rising slowly enough, and with few enough disturbances to remain laminar, or steady.
For me the pounding of the raindrops on the windows bring another level of authenticity to the fireside. That being said I am from San Diego so I don’t get rain often in any season. All of this water stays up in the clouds floating across the land and then finally dumps to the ground below. But what causes the trapped water to finally fall? In damp air there is a lot of water vapor that would like to form together and form droplets but it takes a lot of energy. The energy required to form a droplet is related to the surface area of the droplet so a perfect sphere of water forming is very unlikely, there is simply too much energy required to maintain the surface area of a sphere. However, if there is a speck of dust floating around in the air water can form to it. The water only has to maintain a fraction of a sphere of surface area because the dust covers the rest. As water vapor condensates on these small particles they become heavier and eventually fall out of the bottom of the cloud and bring the water back to the ground.
Finally no cozy fireside day is complete without a hot beverage by your side. These beverages never stay hot for long enough. This is because of the natural convection moving air around your mug. While that may sound a tad complicated, it’s based on the same principle that the flames were above. Hot air rises. This means that the air around your mug was heated by the hot beverage, this hot air rises away from the mug and draws cool air up to take more heat. The cycle continues until you finish the drink or it gets disappointingly cold.
Next time you sit by the fire with your drink think of what you’re seeing and let your curiosity take you into the realm of engineering.