How can we engage in direct modeling for computational fluid dynamics?

Direct modeling for computational fluid dynamics provides an effective methodology to develop multi-scale numerical algorithms for flow simulation in all regimes from rarefied to continuum ones, which will help to improve the conventional CFD methods which are based on numerical partial differential equations.

The study of fluid dynamics needs go beyond the traditional numerical partial differential equations (PDE). The emerging engineering applications, such as air-vehicle design for near space flight and flow and heat transfer in micro-devices, do require further expansion of the concept of gas dynamic equations to a larger domain of physical reality, rather than the traditional distinguishable governing equations, such as the Navier-Stokes and the Boltzmann equations.

At the current stage, the non-equilibrium flow physics, especially in the transition regime, has not been well explored and clearly understood due to the lack of appropriate governing equation and the corresponding numerical tools. Under the current numerical PDE approach, it is hard to develop such a meaningful tool due to the absence of valid PDEs. Theoretically, the flow dynamics should have a continuous spectrum from the kinetic Boltzmann equation to the hydrodynamic Navier-Stokes ones. In order to develop efficient multi-scale and multi-physics numerical methods to cover such a continuous flow dynamics, similar to the modeling process of constructing the governing equations, the numerical algorithm is better based on the first principle of physical modeling.

In Professor Kun Xu's book published by World Scientific, instead of following traditional numerical PDE path, a direct modeling as a principle for the computational fluid dynamics algorithm development is proposed. Since all computations are conducted in a discretized space with limited resolution, the flow physics to be modeled has to be done in the mesh size and time step scales. Here, the computational fluid dynamics is to directly construct a discrete numerical evolution equation, where the mesh size and time step themselves will play dynamic roles in its modeling process.

With the variation of the ratio between mesh size and local particle mean free path, the unified scheme will capture flow physics from the kinetic particle transport and collision to the hydrodynamic wave propagation. Based on the direct modeling, a continuous dynamics of flow motion will be simulated in the unified gas-kinetic scheme. This scheme can be faithfully used to study the unexplored non-equilibrium flows in the transition regime.

More information:
The book retails for US$118 / £78 at all major bookstores. More information on the book can be found at: www.worldscientific.com/worldscibooks/10.1142/9324.

Provided by World Scientific Publishing