Direct Numerical Simulation of Turbulent Flows

To study the details of a turbulent flow, it is sometimes more informative to accurately simulate the flow with a computer than to try to observe it in the laboratory. Direct numerical simulation involves the numerical solution of the equations that govern fluid flows. It is a research tool that provides us with an extremely detailed description of the flow field. Expand the images below to see the fine structure that occurs when the red and blue fluids mix and react in a turbulent flow. The image on the left shows where the reaction is occurring, while the one on the right shows how the two fluids have mixed. They are taken from a DNS with 512x512x1024 grid points run at the Arctic Region Supercomputing Center.

The images below are slices from 256³ DNSs in which a blob of fuel reacts with oxidant in a one-step, irreversible reaction. The flow is incompressible, and the fluid properties are constant. The plots show that the reaction zones are thin, even when the reaction is isothermal, and become thinner when the reaction rate depends on temperature. Because the reaction zones are so thin, modeling techiques involving averaging, which are common for non-reacting turbulence, cannot be used.

Mixture Fraction
The pink fuel and blue oxidant mix. The reaction will occur only where the two are mixed. Isothermal Reaction Rate
An isothermal reaction occurs where the fluids have mixed. The reaction zone is about as thick as the mixing zone.

Temperature
If the reaction releases heat, the temperature is highest where all the fuel has been consumed. Temp. Dependent Reaction Rate
Many species react faster at higher temperature. The reaction zones are very thin, since the reaction needs fuel, oxidizer, and high temperature to occur.

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Mixture Fraction The pink fuel and blue oxidant mix. The reaction will occur only where the two are mixed.	Isothermal Reaction Rate An isothermal reaction occurs where the fluids have mixed. The reaction zone is about as thick as the mixing zone.
Temperature If the reaction releases heat, the temperature is highest where all the fuel has been consumed.	Temp. Dependent Reaction Rate Many species react faster at higher temperature. The reaction zones are very thin, since the reaction needs fuel, oxidizer, and high temperature to occur.