On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high-Reynolds-number flow over an Ahmed body
Artikel i vetenskaplig tidskrift, 2014

We investigate a hierarchy of eddy-viscosity terms in proper orthogonal decomposition (POD) Galerkin models to account for a large fraction of unresolved fluctuation energy. These Galerkin methods are applied to large eddy simulation (LES) data for a flow around a vehicle-like bluff body called an Ahmed body. This flow has three challenges for any reduced-order model: a high Reynolds number, coherent structures with broadband frequency dynamics, and meta-stable asymmetric base flow states. The Galerkin models are found to be most accurate with modal eddy viscosities as proposed by Rempfer & Fasel (J. Fluid Mech., vol. 260, 1994a, pp. 351–375; J. Fluid Mech. vol. 275, 1994b, pp. 257–283). Robustness of the model solution with respect to initial conditions, eddy-viscosity values and model order is achieved only for state-dependent eddy viscosities as proposed by Noack, Morzyński & Tadmor (Reduced-Order Modelling for Flow Control, CISM Courses and Lectures, vol. 528, 2011). Only the POD system with state-dependent modal eddy viscosities can address all challenges of the flow characteristics. All parameters are analytically derived from the Navier–Stokes-based balance equations with the available data. We arrive at simple general guidelines for robust and accurate POD models which can be expected to hold for a large class of turbulent flows.

turbulence simulation


low-dimensional models


Jan Östh

Chalmers, Tillämpad mekanik, Strömningslära

Bernd Noack

Universite de Poitiers

Sinisa Krajnovic

Chalmers, Tillämpad mekanik, Strömningslära

Diogo Barros


Universite de Poitiers

Jacquees Borée

Universite de Poitiers

Journal of Fluid Mechanics

0022-1120 (ISSN) 1469-7645 (eISSN)

Vol. 747 518-544




C3SE (Chalmers Centre for Computational Science and Engineering)


Annan fysik

Strömningsmekanik och akustik