The art of mixing: metamorphosis and shock waves – USC Viterbi
Have you ever experimented with food coloring? This can make cooking a lot more fun, and provides a great example of how two fluids can mix well – or not a lot at all.
Add a small drop of water and you might see it slowly dissolving into the larger liquid. Add a few more drops and you might see a wave of color spreading out, with the colored droplets spreading out and separating to diffuse more fully. Add a spoon and start stirring quickly, and you will probably find that the water completely changes color, just as you want it to.
Researchers at USC Viterbi School of Engineering, led by Ivan Bermejo-Moreno, assistant professor of aerospace and mechanical engineering, studied a similar phenomenon with high velocity gases, with a view to a more efficient mixing for support supersonic jet engines. In the study, published in Physics of Fluids, USC Viterbi Ph.D. Jonas Buchmeier, as well as Xiangyu Gao (USC Viterbi Ph.D. ’20) and a former M.Sc. student Alexander Bußmann (Technical University of Munich), developed a new tracking method that focused on the fundamentals of how mixing occurs. The study makes it possible to understand, for example, how the injected fuel interacts with the surrounding oxidants (air) in the engine to make it work optimally, or how interstellar gases mix after a supernova explosion to form new stars. . The method focuses on the geometric and physical properties of the turbulent swirling motions of gases and how they change shape over time when they mix.
Scramjet engines – super-fast experimental engines with no moving parts – have already set the air speed record for jets at Mach 9.6, allowing a potential trip from Sydney to London in around 90 minutes.
“The dynamics of these individual flow structures and the geometric changes they undergo have not been tracked over time,” said Bermejo-Moreno, “because we previously didn’t have the computational techniques to. do it ; especially in a turbulent propulsion system (such as in a jet engine). Now, we can look at thousands or hundreds of thousands of these flow structures simultaneously and track for each how the shape of the structure changes and how it mixes and interacts with surrounding structures.
The value, said Bermejo-Moreno, is that once you can identify the most useful patterns for speeding up the mixing process, you can replicate those specific conditions, because you can see the evolution of structures (fuel and oxidant, for example) at all times.
“In a supersonic combustion engine, you want the fuel mixing to happen as quickly as possible so that the fuel is completely used up before it comes out of the engine,” he said. “To do that, we have to understand how the mixing happens at different times. “
Shape change and shock waves
When fuel is injected into a rocket or Scramjet engine, it begins a diffusion process, Bermejo-Moreno said.
“The injection process will usually break down the fuel into small, almost spherical structures, which are then transported and mixed by the turbulent air flow inside the engine. Turbulence will continue to break down fuel structures and change their shapes. “
The figure above shows an “ideal” case, where the fuel is far from the walls of the engine, and essentially there are no boundaries. But in a real scenario, the walls of the engine will also have an impact on the mixture. The new study focuses on isolating the effects of shock waves as a key component of fuel compression, volume contraction and fragmentation, Bermejo-Moreno said. A shock wave is a disturbance that travels faster than the speed of sound and causes a sudden and discontinuous change in the pressure, temperature, and density of the medium impacting it. In this case, a shock wave flattens the shape of the fuel structures and creates more surface area for the fuel to be broken up by the turbulence inside the engine.
Understanding the effects of compression – via a shock wave, for example – on turbulent mixing processes is very important for advancing air-breathing super and hypersonic propulsion systems.
These systems are characterized by a forced flow of air into the engine, heated and released by an exhaust. Such systems also have compressed time requirements for mixing to occur. Knowing exactly how the injected fuel is broken down can help researchers identify exactly which conditions favor the most beneficial mixing scenario for such engines to operate efficiently.
Previous research by Bermejo-Moreno identified shock waves as a beneficial force to accelerate fuel mixing, but this research did not benefit from the tracking methodology algorithm put in place in the new study. Although several events can be tracked manually, trying to find an accurate representation and recommendation of how the fuel will mix under different conditions relies on a sample large enough to achieve a similar result.
This new tracking methodology provides a clearer picture of the structural change in fuel injected from moment to moment, better educating aerospace engineers on how to reproduce the conditions that will benefit supersonic and hypersonic engines the most.
“Once you have this tracking algorithm, you can apply it to any flow to get a graph that encapsulates the interactions of all the structures found in the flow over time,” Bermejo-Moreno said. “You can query the chart and look for patterns that change similarly over time. You can see how often these patterns repeat and collect statistics on the physical processes involved by saying, for example, “This is common behavior in the process of breaking up the injected fuel. “
Bermejo-Moreno said the impact of a shock wave is especially strong in cases with larger spherical structures rather than smaller spherical structures, as larger spheres are more susceptible to “splitting events” where the fuel breaks into more and more pieces.
“If you think of larger structures,” he said, “you think they will take longer to diffuse, but the turbulent mixing they experience will benefit more from the shock interactions, which will divide them. faster in smaller structures. “
If you think back to the case of food coloring, the more small drops of food coloring there are, the easier it is for the coloring to dissolve in water and combine with it to form a new solution.
“If you can have a better mixture, it will help improve the performance of your propulsion systems,” Bermejo-Moreno said.
Inform future recommendations
Bermejo-Moreno said the next steps are to study what happens when you get closer to the engine walls and into the mix layers – two streams of fluid moving at two different speeds. “We will be following the turbulence structures over time to understand how viscous shear affects mixing processes from a structural dynamics perspective,” he said.
For now, Bermejo-Moreno has said that there are additional factors that will ultimately impact propulsion performance that will be taken into account before providing real-world recommendations, but this is a step forward.
This research was funded by the Army Research Bureau (Grant W911NF20100096, Program Director Matthew Munson). Some of the contributing research was conducted during the 2018 summer program at the Center for Turbulence Research (Stanford University). Computing resources to support this work were provided by USC’s Center for Advanced Research Computing (CARC) and an INCITE grant from the Department of Energy to Argonne’s leadership computing facilities, Argonne National Laboratories.
Posted on November 17, 2021
Last updated on November 17, 2021