3 ms·

> I work in designing simple "closure models" What is the order of the turbulence closure models used in modern work? 1.5? 2? 2.5?

by sampo 5y ago

archermarks 5y ago
Depends on what you mean by order i suppose. The simplest are algebraic (zero-order) models where we prescribe the anomalous transport as an explicit function of other plasma parameters. More complex ones are coupled systems of hyperbolic PDEs, which are typically first-order in space and time. If you are referring to the order of accuracy of the simulations, that's typically 1st to 4th order, depending on method used.
- johnbcoughlin 5y ago
  I answered the GP based on my understanding of "closure" meaning different things for moment models vs turbulence. I think they're referring to energy partition scaling in saturated turbulence.
  - archermarks 5y ago
    Yes, thank you. It's a closure problem in the former sense.
- sampo 5y ago
  > Depends on what you mean by order i suppose. I mean, in the context of Reynolds-averaged Navier–Stokes equations [1], what sort of turbulence model [2] you are using. But apparently you are not working in this context at all, so maybe my question doesn't make sense. [1] https://en.wikipedia.org/wiki/Reynolds-averaged_Navier%E2%80%93Stokes_equations https://en.wikipedia.org/wiki/Reynolds-averaged_Navier%E2%80... [2] https://en.wikipedia.org/wiki/Turbulence_modeling https://en.wikipedia.org/wiki/Turbulence_modeling
  - archermarks 5y ago
    Appreciate the clarification. I am familiar with RANS modeling to a reasonable extent but not with a lot of the modern developments. We're still kind of at the stage of trying to find something that works as well as Prantdl's mixing length model, or maybe a k-epsilon model. Additionally, while our processes are turbulent, the turbulence differs significantly from classical turbulence. For instance, turbulence in Hall thrusters is likely to be governed by an inverse energy cascade, where small disturbances magnify to larger and larger scales and the dissipation occurs via convection at device-scale instead of by diffusion at kolmogorov scales.
johnbcoughlin 5y ago
"closure" in the kinetics to fluid model context refers to a closed expression for the heat flux or anisotropic pressure in terms of the lower order fluid variables. You aren't really looking at a turbulence closure in the sense you're getting at in this context--for these hyperbolic models it's more about the transport than about energy partition.

> I work in designing simple "closure models" What is the order of the turbulence closure models used in modern work? 1.5? 2? 2.5?

by sampo 5y ago

> I work in designing simple "closure models" What is the order of the turbulence closure models used in modern work? 1.5? 2? 2.5?

archermarks 5y ago

Depends on what you mean by order i suppose. The simplest are algebraic (zero-order) models where we prescribe the anomalous transport as an explicit function of other plasma parameters. More complex ones are coupled systems of hyperbolic PDEs, which are typically first-order in space and time. If you are referring to the order of accuracy of the simulations, that's typically 1st to 4th order, depending on method used.

johnbcoughlin 5y ago
I answered the GP based on my understanding of "closure" meaning different things for moment models vs turbulence. I think they're referring to energy partition scaling in saturated turbulence.
- archermarks 5y ago
  Yes, thank you. It's a closure problem in the former sense.
sampo 5y ago
> Depends on what you mean by order i suppose. I mean, in the context of Reynolds-averaged Navier–Stokes equations [1], what sort of turbulence model [2] you are using. But apparently you are not working in this context at all, so maybe my question doesn't make sense. [1] https://en.wikipedia.org/wiki/Reynolds-averaged_Navier%E2%80%93Stokes_equations https://en.wikipedia.org/wiki/Reynolds-averaged_Navier%E2%80... [2] https://en.wikipedia.org/wiki/Turbulence_modeling https://en.wikipedia.org/wiki/Turbulence_modeling
- archermarks 5y ago
  Appreciate the clarification. I am familiar with RANS modeling to a reasonable extent but not with a lot of the modern developments. We're still kind of at the stage of trying to find something that works as well as Prantdl's mixing length model, or maybe a k-epsilon model. Additionally, while our processes are turbulent, the turbulence differs significantly from classical turbulence. For instance, turbulence in Hall thrusters is likely to be governed by an inverse energy cascade, where small disturbances magnify to larger and larger scales and the dissipation occurs via convection at device-scale instead of by diffusion at kolmogorov scales.

johnbcoughlin 5y ago

"closure" in the kinetics to fluid model context refers to a closed expression for the heat flux or anisotropic pressure in terms of the lower order fluid variables. You aren't really looking at a turbulence closure in the sense you're getting at in this context--for these hyperbolic models it's more about the transport than about energy partition.