A Pseudo Direct Numerical Simulation to solve Turbulent CFD Problems

IDELSOHN, SERGIO; NIGRO, NORBERTO M.; LARRETEGUY, AXEL; GIMENEZ, JUAN M.; RYZHAKOV, PAVEL

Austin, TX

Congreso; US National Congress on Computational Mechanics (USNCCM); 2019

Although the Navier-Stokes equations are equivalently applicable to both laminar and turbulent fluid flows, the current computing power generally precludes employing fine meshes that would allow to simulate turbulent flows without introducing empirical approximations. The prediction power of the models is therefore restricted to problems within the margins of the selected empirical approximation. Solving a problem without such approximations on a mesh sufficiently fine so as to represent the whole expected range of eddy sizes is known as ?Direct Numerical Simulation? (DNS). Taking into account that many fluid flow problems of industrial interest are indeed turbulent, it is worthwhile to continue improving the models so that they fit more and more with the physics of the problem. Simulating a CFD problem in a given domain with a ?fine enough? DNS mesh introduces an unmanageable number of unknowns for current computers. The project we are working on involves modelling turbulent flow in a DNS fashion, i.e. without any additional turbulence model. However, we strive to develop an approach considerably more computationally efficient than a classical DNS. The basic idea is as follows. The global (macro) domain, can be subdivided into many equally-shaped small domains, the so-called RVEs, ?Representative Volume Elements? (micro problems). These RVEs, in principle, can be solved individually and independently from each other. The RVEs may even be previously solved off-line for different time-dependant loads. The micro and macro problems require to be coupled. This will be performed via the theory known as ?homogenization?. Another important aspect of the solution process in our approach is the use of a Lagrangian particles at the macro level [1]. The Lagrangian particles will take care of the transport of the macro-velocity and also of the micro-eddies, thus convecting the turbulent energy. This is a very important feature in turbulent flows where turbulence is produced in some high gradient regions and is then convected to other regions.This presentation is a work in progress, aimed more at opening a discussion on the topic, rather than presenting elaborate results.