Fluid imagining
tags: fluid mechanics Schlieren effect digital imagining
People often associate science with something complex and difficult. One of the ways to understand it is to relate it to our intuition (eg. geometric interpretation of derivative). Based on my experience the most complex phenomena became more obvious once properly visualized. Moreover excellent visualisations are mesmerizing espcecailly when it comes to fluid dynamics. In the following article I'm going to show my efforts to visualize motion of the fluid using Shlieren effect
Shlieren effect is one of the non-invasive, optical methods of imagining fluid motion. It is used for imagining flow of transparent fluids. This effect is based on deflection of light due to refraction which depends on local density of the fluid hence this method is used for flow which is compressible, multiphase or exchange significant amout of heat. In prticular it is applied to visualize high Mach number flow which is often associated with shock waves where sudden pressure and density changes occur.
Figure 1 shows scheme of optical setup needed to obtain schlieren effect. The main element is sperical mirror. I used parabolic mirror from my Newtonian telescope. Parabolic mirror geometry approximates spherical geometry sufficiently well. Point source of light is located at twice the focal length of the mirror at it's axis. Light, as it travels through the fluid, refracts differently while passing through different density areas (blue - low density area refracts light up, red - high density area refracts ligh down). As a consequence beams from different density areas hit the mirror at different spots. These beams became separated once are back at twice the focal length and when one of them is blocked it appears as a dark spot on the final image - it can be projected on a screen or captured by the camera.
Fig.1. Schlieren effect scheme (beam paths are exaggerated).
While classic Schlieren effect uses sharp razor blade to block the beam I decided to use color filters instead. The idea was that refracted beam doesn't have to be stopped - it can be filtered by color foils such that different parts of the beam have different colors. The easiest way to handmade nonuniform density flow was to use heat source (candle). Below video shows gas flow around the candle (red/blue filter was used):
Fig.2. Gas flow around the candle.
I realized this setup is excellet to capture smoke which does not refract the light but just blocks it. Following video shows blowing out the candle, its smoke and then lighting it up again. I used four color filter (red, blue, green and yellow).
Fig.3. Blowing out and lighting up the candle.
I was able to capture an interesting phenomena called Candle Flame Jump. Once candle is blown out flame from the match can be transferred to the candle through the smoke getting out of it. This is shown on followign video (two color filter was used - blue and yellow):
Fig.3. Candle Flame Jump phenomenon.
The smoke rising from blown out candles is actually evaporated wax. After candle is blown a spark can still remain in the wick. When a flame is put in evaporated wax the flame can travel through the vapour and light up the candle again.
In order to make four color filter results more spectacular higher density gradients were needed hence I used a celluloid instead of a candle as it is highly flammable. As expected colors were more vivid this time:
Fig.4. Burning of celluloid.
While above examples were just a result of playing around with the candles Schlieren effect played a huge role in aerodynamics at the moment and nowadays can be used in conjunction with image processing to deliver quantitative results. At the same time it can deliver mesmarizing videos of fluid motions as shown in this article. While my efforts were modest I recommend you to review other people's results here which inspired me and can be considered a masterpiece for sure.