Welcome back to TeamUV.org and let me just start off by saying that we are going to have a busy next couple of days. We (of course) have this Well Read post today, will have a special Veterans day post at 1300 hours (1 p.m./in 3 hours), a post announcing the start of our fundraising campaign tomorrow (Wednesday 1000 hours/10 a.m.), and then we will be back to our regular schedule with a Presentation post by Andrew on Thursday. Whew!
Today, we are going to continue off of my last Well Read post with a discussion of the science behind fireworks. More specifically, today we are going to take a brief look at what determine the actual shape of the burst, whereas last time we took a look at the thrust created during the initial firing of the fireworks. Without getting too much into the chemistry of this subject, we can basically see the compounds within the firework’s shell as consisting of a mixture of liquids and solids (fuel, oxidizer, color-producing compounds, a binder, and possibly other additives) that are used for both the bursting charge and the aesthetic effects. When an aerial firework is initially fired, the reactants combust to produce thrust (as discussed last time), launching the actual firework shell into the air. At the same time as the shell is rocketing up into the air, a fuse (which was lit by the combustion) is burning. Eventually, that fuse will burn down to the firework’s charge, igniting the charge and thus producing the explosion, and triggering the reaction of all the other compounds within the shell. Upon explosion of the entire shell, a bunch of particulate jets are formed and are shot out in many directions. Generally speaking, these jets are what determine the form of the burst that we see when we watch fireworks. These jets are shown below; it is also interesting to note the Karman Vortex Streets visible in the turbulent wakes of the projected particulate, as caused by flow separation (and subsequent recirculation) along the blunt body of the projectiles.
So what governs the size, shape, and behavior of these jets? While a lot is unknown about the exact cause of the instabilities associated with the jets, the main factors that are commonly seen as governing the jet structure are: nature of the particles (composition; liquid, solid, etc.), geometry (shape) of the charge, and mass ratio of explosives to particles (amount of explosive vs. other compounds). These assumptions are reflected in the research video below (Video Credit: D. Frost, Y. Gregoire, S. Goroshin, F. Zhang) which shows different combinations of particle nature, charge geometry, and explosive-particle mass ratio and their effects on jet structure. A cool note is that you will be able to see the shock wave (a pressure discontinuity, or large change in pressure in a very small distance – across the shock wave) created by each blast in the video. The main conclusions of the video (and some possible reasons for these observations) are:
1. Wet mixtures produce more jets (possibly due to smaller molecule size), which disperse sooner (maybe due to liquid surface tension effects).
2. Dry mixtures produce less jets (possibly due to larger particulate), which disperse slower (possibly due to higher momentum due to larger particulate mass and more friction effects due to the generally larger particulate vs. the wetted particulate mixture).
So there you have it: a relatively brief look at what governs the shape of the burst of fireworks! Please remember to check back later today for our salute to the brave men and women who have served this great nation! I’ll leave you with another image of fireworks and the Statue of Liberty as today is a great day for patriotism!