Fluid Flow

Inviscid flows are an idealization resulting from neglecting the viscous effects in simulating the equations of motion.

The solution of the resulting Euler equations (as opposed to the Navier-Stokes equations) generally results in significant savings of computer resources. Boundary layers and other viscous effects are not resolved. This approximation is only valid for certain physical situations, such as high-Reynolds number compressible aerodynamics.

Viscous flows are typically of high viscosity and can be classified as either laminar or turbulent. Laminar and turbulent flows occur in nature. Both types are described using the Navier-Stokes equations which include the effects of viscosity, thermal conductivity, and mass diffusion.

For more information, see Viscous Flow.

The term laminar refers to a well-ordered flow, free of macroscopic, non-repeating fluctuations. Laminar flows occur in nature when the Reynolds number (the ratio of viscous to inertial forces) is low enough that transition to turbulence does not occur.

In computational simulations, numerical instabilities can arise from simulating laminar flows at Reynolds numbers that are too large. If you are seeking a steady solution, these instabilities can impede convergence. Therefore, laminar flow simulation is appropriate if you already know that the Reynolds number of the problem is sufficiently low.

Transition is a term that refers to the breakdown of laminar flow, through amplification of infinitesimal disturbances, to turbulence. A transitional flow can be defined as one that encompasses this process.

The occurrence of physical instabilities in laminar simulations cannot be relied on as an accurate indication of transition. Furthermore, when using turbulence models in low-Reynolds number simulations, the onset of turbulence in viscous layers cannot be predicted with any reliability by the turbulence model itself.

Strictly speaking, Simcenter STAR-CCM+ has no means of predicting transition. However, a Transition model is available that allows you to mimic the effect of transition by suppressing the turbulence in a certain pre-defined region.

For more information, see Transition.

A flow that is in a state of continuous instability, exhibiting irregular, small-scale, high-frequency fluctuations in both space and time is termed turbulent.

It is strictly possible to simulate turbulent flow directly by resolving all the scales of the flow (termed direct numerical simulation). However, the computer resources that are required are too large for practical flow simulations. Therefore, a suitable turbulence modeling approach must be selected.

For more information, see Turbulence.