A scramjetwhich features an engine that uses an engines forward motion to compress incoming air, which flows at supersonic speeds. PHOTO/NASA, Tony Landis.

Ivan Bermejo-Moreno likes his coffee with a touch of turbulence. But instead of mixingcoffee and cream with a spoon, when it comes to hypersonic jet planes planes that can flyfive times faster than sound he likes to mix oxygen from the air and jet fuel using something a bit stronger: shock waves.

Similar principles govern fluid mixing in aircraft engines, where oxygen from the air has tomix with fuel to help propel it at a certain speed. USC researchers in the USC Viterbi Department of Aerospace andMechanical Engineering, including Xiangyu Gao, aUSC Viterbi Ph.D. student who recently defended his dissertation, and his doctoral advisor,Assistant Professor Ivan Bermejo-Moreno,are studying how to achieve efficient mixing at high speeds. Bettermixing allows supersonic combustion enginesin which airflow is greater than the speed ofsoundto remain shorter in length while enabling vehicles to move hypersonically. One approach to achieve thisis to use shock waves.

A shockwave is characterized by an abrupt change in pressure, temperature and density ofa medium and moves faster than the local speed of sound. Without applying a shock wave, mixing will occur, as in the example with coffee and cream, but it will take much longer,Bermejo-Moreno said. Shock waves amplify turbulencesimilar to a spoon in the coffeeexampleand the more turbulence you have,the more rapidly mixing can occur.

The researchers recently published a study in the Journal of Fluid Mechanics, which sharesconditions in which such rapid mixingwhich supports faster, more efficient vehiclescan occur. Once a shock wavea sudden and strong disturbance in a mediumis produced, thespeed of the fluid passing through it will be drastically reduced, also allowing more time formixing. This puts the fuel and air in a better condition for combustion, and will increase the temperature, making it easier to auto-ignite, the researchers said.

In conditions where mixing can be handled efficiently enough to support hypersonic vehicles, there are numerous implications, including commercial applications for the exploration of space.

Said Bermejo-Moreno: Imagine instead of a rocket you have something lighter and smaller that could take us all the way to Mars. The combination of scramjets and rotating detonation engines, both based on shockwaves and turbulence, may one day do just that.

The research team also includes Johan Larsson, associate professor of mechanical engineeringat the University of Maryland. The researchers conducted this study performing massivelyparallel numerical simulations on the supercomputers at USCs High PerformanceComputing Center and at Argonne National Laboratory.

Fundamental Building Blocks of Flow

The study isolated the physics the researchers were interested in exploring by using afundamental geometric set upessentially a boxand removing variables related tosurface friction on the nature of fluid or air flow. In the study, the flow would come in fromone side of the box and encounter a shockwave created by carefully controlling thepressure inside of the box. Then it exits through the opposite side of the box, Bermejo-Moreno said.

In this way, we isolated the interaction between turbulent flows and shockwaves, Bermejo-Moreno said. While people have studied the pure interaction of turbulence and shockwaves in the past, the researchers said only a few studies have focused on mixing in this configuration. Shockwaves are generated by the large (supersonic) speed of the air as it encounters air inlets, Bermejo-Moreno said. Geometric deflections, like corners, are usually enough to produce shock waves.

The researchers studied a greater range of parameters than in prior studies, as well, including variations in incoming speeds of air flow. The researchers also looked at different levels of turbulence.

To visualize turbulence, consider a faucet, Bermejo-Moreno said. When the faucet is barely on, the flow is slow, transparent and smoothknown as laminar. But as you keep opening the faucet up, the velocity of the water increases. The water stream becomes blurry and no longer transparentits what you would call turbulent. The same thing happens in the air and in mixtures of air and fuel we discuss in hypersonic vehicles.

The researchers said that they are most interested in turbulent flows, because they are most representative of what is actually happening in reality. Just like when you add milk to your coffee and do not stir it up, without a shock wave, which increases turbulence, mixing will occur but it will take much longer. In the study, the researchers found that while some quantities related to mixing levels will saturate after a certain amplification of turbulence, others will keep increasing, suggesting mixing continues to improve as turbulence increases.

Next the researchers hope to look at additional geometries and see how these impact mixing. In the future, one of the elements we want to investigate is how different shapes of turbulent structuresknown as eddiesimpact mixing. For instance, how a tube-like structure might impact the transport and mixing of fuel and air differently than a sheet-like structure. If you know the type of turbulent structures that are dominant in mixing, then you might want to produce more of these structures, Bermejo-Moreno said.

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Shock Waves Might Offer the Jolt Needed to Reach Mars - USC Viterbi | School of Engineering - USC Viterbi School of Engineering