Eventos Anais de eventos
COBEM 2021
26th International Congress of Mechanical Engineering
Large-Eddy Simulation of a turbulent incompressible round jet flow
Submission Author:
Livia S. Freire , SP
Co-Authors:
Livia S. Freire
Presenter: Livia S. Freire
doi://10.26678/ABCM.COBEM2021.COB2021-1031
Abstract
In this study we test an alternative numerical code for the Large-Eddy simulation of free round jets. The code solves the filtered incompressible Navier-Stokes equation in rotational form, using a Cartesian grid in a rectangular domain. In the radial directions, the code uses a pseudo-spectral method and periodic boundary conditions, whereas in the axial direction the grid is staggered and a second-order centered finite differences method is used. The fully explicit second-order Adams-Bashforth scheme is used for time advancement. A rough wall boundary condition is imposed at the inlet wall, whereas zero axial derivatives are imposed at the outlet wall. A top-hat, constant in time inlet flow is introduced in a domain with the same fluid at rest. We test a water flow with inlet velocity Uj = 7m/s and diameter D = 4mm, which corresponds to a Reynolds number of 28000. The domain corresponds to 64 and 24D in the axial and radial directions, respectively, covered by 360 x 144 x 144 grid points, which corresponds to an approximately uniform grid spacing of 0.17D in both directions. As a subgrid scale model, we compare the Lagrangian-averaging, scale-dependent dynamic approach with its classical counterpart (planar-averaging, scale invariant dynamic model). The CFL number is kept around 0.1 through the use of a time step equal to 0.000005 seconds, and approximately steady state results are obtained after 200000 time steps (1 second), although self-similar results are achieved much earlier (after 100000 time steps). Comparing the results with the theory of jet flows, we notice that the jet presented a negative virtual origin (outside the domain) and a small transition region. Its self-similar region is already observed in the axial position between 5 and 35D, approximately, in which self-similar profiles of the axial velocity with an exponential decay are observed, in addition to a linear behavior in the inverse of the centerline axial velocity (linear rate of 6.3) and in the jet’s half-width (linear rate of 0.07). Beyond 35D, it is likely that the outlet boundary condition is impacting significantly the jet flow. This code can be used to simulate and test new theories of highly turbulent jets with an affordable computational cost.
Keywords
Large Eddy Simulation, Jet flow
