Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

cally relevant flows. In particular, since the presence of complex boundaries is mimicked by body forces, the simulations are, in
fact, carried out on ’simple grids’, thus taking advantage of the efficiency and accuracy of optimized solution procedures. IB/DNS
simulations of complex-geometry turbulent-flows can be carried out with 1-2 million gridpoints on a PC with CPU times ranging
between a few hours and a few days. In this paper we will show some examples of these simulations for an impeller-stirred cylindrical
tank, and the results will be compared with experiments and RANS simulations.
Author
Turbulent Flow; Direct Numerical Simulation; Large Eddy Simulation; Reynolds Averaging; Boundary Conditions; Navier-
Stokes Equation; Rotor Blades (Turbomachinery)
20010022654 Stanford Univ., Center for Turbulence Research, Stanford, CA USA
Unsteady 3D RANS Simulations Using the v(exp 2)-f Model
Iaccarino, Gianluca, Stanford Univ., USA; Durbin, Paul, Stanford Univ., USA; Annual Research Briefs - 2000: Center for Turbulence
Research; December 2000, pp. 263-269; In English; See also 20010022631; No Copyright; Avail: CASI; A02, Hardcopy;
A03, Microfiche
Recent increased computational power has led to interest in the simulation of time-dependent flows for problems ranging
from noise prediction to fluid/structure interactions. The computational cost and the resolution requirements are mainly related
to the inviscid flow structures induced by geometry and wall layers; nevertheless, turbulence plays a crucial role in establishing
such flow structures. Several approaches can be used to numerically simulate the behavior of unsteady flows: in the simplest, the
Navier-Stokes equations are ensemble-averaged, converting turbulent fluctuations into Reynolds stresses (Reynolds-Averaged
Navier-Stokes equations, RANS), while leaving the large scale, rotational motions to be resolved as unsteady phenomena. The
large eddy simulation (LES) approach, on the other hand, employs a spatial averaging over a scale sufficient to remove scales not
resolved by the particular grid being used. The subgrid scale turbulence is then modeled. A practical difference is in the degree
of mesh resolution required: LES resolves the larger eddies of the turbulence itself, whereas the unsteady RANS approach models
the turbulence and resolves only unsteady, mean flow structures - primarily larger than the turbulent eddies. Consequently, LES
typically requires much higher grid resolution, at least locally, and is therefore more costly. On the other hand, LES only models
the subgrid turbulence structures and is universally applicable to the extent that a universal grid-independent subgrid model can
be established. In addition, LES resolves the complete range of scales of random motion, up to the cut-off frequency, while
unsteady RANS aims to capture a single frequency (e.g. corresponding to coherent shedding) and to model the random motions
using standard turbulence closures. Therefore, LES requires very long integration time to build a statistically-averaged solution;
on the other hand, a few shedding periods are usually enough to obtain accurate phase-averaged solution with RANS, thus limiting
its overall computational cost. The objective of this work is to apply the RANS approach with the v(exp 2) - f turbulence model
to the solution of the flow around a surface mounted cube. A complete and reliable experimental database is available and, in addition,
LES simulations have been carried out with considerable success. This problem was one of the test case presented at the
’Workshop on Large Eddy Simulation of Flows Past Bluff Bodies’; several LES calculations were compared, showing a high
degree of accuracy. RANS results were also included for comparison; they were obtained using simple turbulence models derived
from k-epsilon and generally gave worse agreement with the measurements than did LES.
Author
Navier-Stokes Equation; Reynolds Averaging; Turbulent Flow; Turbulence Models; Applications Programs (Computers)
20010022655 Stanford Univ., Center for Turbulence Research, Stanford, CA USA
Prediction of the Turbulent Flow in a Diffuser with Commercial CFD Codes
Iaccarino, Gianluca, Stanford Univ., USA; Annual Research Briefs - 2000: Center for Turbulence Research; December 2000, pp.
271-277; In English; See also 20010022631; No Copyright; Avail: CASI; A02, Hardcopy; A03, Microfiche
There have been a few attempts in the literature to compare the performance of commercial computational fluid dynamics
(CFD) codes, for instance, laminar arid turbulent test cases have been proposed to several CFD code vendors by the Coordinating
Group for Computational Fluid Dynamics of the Fluids Engineering Division of ASME. A series of five benchmark problems
were calculated, with all the mesh generation and simulations performed by the vendors themselves; only two of the problems
required turbulent simulations. The first of these benchmarks is the flow around a square cylinder: the flow is unsteady and all
of the codes predicted the measured Strouhal number reasonably well. However, poor accuracy was obtained in the details of the
wake flow field. It was also noted that, depending on the code used (and assuming grid-converged results), the same k-epsilon
model predicted very different results (from 2% to 16% accuracy in the Strouhal number, for example). The reasons for this difference
include different grids, non-demonstrated grid convergence, different implementation of the models, and different boundary
conditions. It must be pointed out that the predictions for this problem are strongly affected by the treatment of the stagnation point
region: as reported in Durbin, the k-epsilon models predict a spurious high level of turbulent kinetic energy near the stagnation
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