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Simulations of fluid flow turbulence

Researchers from Monash University are simulating turbulent fluid flows, which affect airplane aerodynamics and design. The research uses direct numerical simulations (DNS) of turbulent flows to understand more about what is happening in the thin layer of air or water close to a solid surface, known as the turbulent boundary layer, which is responsible for drag on airplanes, cars, trains and ships.

Professor Julio Soria explains that “We’re simulating every aspect of the turbulent flow, from the large scales right down to the smallest scales where the energy is being dissipated into internal energy. Every detail is being computed.”

Instantaneous vortex structures of contiguous boundary layers, from one of Professor Soria's simulations.

Instantaneous vortex structures of contiguous boundary layers, from one of Professor Soria’s simulations.

The simulation process allows them to measure variables that are very hard to measure experimentally.

Things like pressure and vorticity are almost impossible to measure at the required precision with current instruments on a physical model, but “in a direct numerical simulation, we can measure all these and more at all the relevant scales, we actually get all of that information.”

This is because DNS provides a perfect representation of fluid physics, one where the data at every point is measurable. Every physical feature can be accurately reproduced digitally, right down to things like the fine scale structure of turbulence, and where energy is dissipated. In this way mechanisms to reduce drag can be investigated.

The NCI supercomputer has enabled the team to run the largest DNS in the world for an adverse pressure gradient turbulent boundary layer. Using 25 million CPU hours to run these simulations and collect data fields for statistical and structural analysis, the computer needs to keep track of over 30 billion individual points throughout the calculation.

As Professor Soria says, “We are simulating every aspect of a turbulent flow, because we have to know every detail in order to understand the mystery of turbulence.”

Professor Soria runs DNS at NCI and physical experiments in wind and water tunnels, each one helping to inform the limitations of the other. Understanding the boundary conditions to impose on the equations for example, is one of those limitations. “We’ve got to cut the simulation off somewhere in space, where you cut it off is quite important, and the experiment guides us in this aspect.”

The team at Monash is looking forward to continuing this work into the future. “As the computers get more powerful, this allows us to push the resolution even higher, it also allows us to bring in real geometric complexity.”

For now, they are focusing on fully understanding the details of the physics involved in turbulent flows, because “this gives us a lot more insight into the dials that we need to turn in order to manipulate the flow to our desire and control it.”

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