Article

# Thermodynamic theory of incompressible hydrodynamics.

ETH-Zürich, Institute of Energy Technology, CH-8092 Zürich, Switzerland.

Physical Review Letters (Impact Factor: 7.73). 04/2005; 94(8):080602. DOI: 10.1103/PhysRevLett.94.080602 Source: PubMed

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**ABSTRACT:**The incompressible Navier-Stokes equations currently represent the primary model for describing stratified turbulent fluid flows at low Mach number. The validity of the incompressible assumption, however, has so far only been rigorously established for adiabatic motions. Here, we show from first principl es that the use of available energetics and thermodynamics considerations applied to a turbulent mixing event associated with stratified shear flow instability r efutes the widespread idea that the incompressible assumption is also valid when diabatic irreversible effects are important. The main consequence is that dynamics and thermodynamics are strongly coupled in stratified turbulence. This departs strongly from the currently accepted wisdom, and calls for a complete revisiting of the physical processes governing stratified turbulence at low Mach numbers.12/2007; - [Show abstract] [Hide abstract]

**ABSTRACT:**An alternative artificial compressibility (AC) scheme is proposed to allow the explicit simulation of the incompressible Navier-Stokes (INS) equations. Traditional AC schemes rely on an artificial equation of state that gives the pressure as a function of the density, which is known to enforce isentropic behavior. This behavior is nonideal, especially in viscously dominated flows. An alternative, the entropically damped artificial compressibility (EDAC) method, is proposed that employs a thermodynamic constraint to damp the pressure oscillations inherent to AC methods. The EDAC method converges to the INS in the low-Mach limit, and is consistent in both the low- and high-Reynolds-number limits, unlike standard AC schemes. The proposed EDAC method is discretized using a simple finite-difference scheme and is compared with traditional AC schemes as well as the lattice-Boltzmann method for steady lid-driven cavity flow and a transient traveling-wave problem. The EDAC method is shown to be beneficial in damping pressure and velocity-divergence oscillations when performing transient simulations. The EDAC method follows a similar derivation to the kinetically reduced local Navier-Stokes (KRLNS) method [Borok et al., Phys. Rev. E 76, 066704 (2007)]; however, the EDAC method does not rely on the grand potential as the thermodynamic variable, but instead uses the more common pressure-velocity system. Additionally, a term neglected in the KRLNS is identified that is important for accurately approximating the INS equations.Physical Review E 01/2013; 87(1-1):013309. · 2.31 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Engineering applications of computational fluid dynamics typically require specification of the boundary conditions at the inlet and at the outlet. It is known that the accuracy and stability of simulations is greatly influenced by the boundary conditions even at moderate Reynolds numbers. In this paper, we derive a new outflow boundary condition for the lattice Boltzmann simulations from non-equilibrium thermodynamics and Grad's moment closure. The proposed boundary condition is validated with a three-dimensional simulation of a backward facing step flow. Results demonstrate that the new outlet condition significantly extends the simulation capability of the lattice Boltzmann method.EPL (Europhysics Letters) 01/2007; 74(2):215. · 2.26 Impact Factor

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