arXiv subjects

Adrián Lozano-Durán

Publications and source records attributed to Adrián Lozano-Durán.

42 records · Page 3Linked to original sources

arXiv2019

DNS-aided explicitly filtered LES

The equations for LES are formally derived by low-pass filtering the NS equations with the effect of the small scales on the larger ones captured by a SGS model. However, it is known that the LES equations usually employed in practical applications are inconsistent with the filter operator when no explicit filter is used. Moreover, even for explicitly filtered LES, some inconsistencies remain in the wall-normal direction due to the constraining effect of the wall. A typically undesirable effect stemming from the inconsistency between LES and the filtered NS (fNS) equations is that DNS data can not be used to aid SGS modeling. Using the proposed form of the incompressible fNS equations for flows over flat walls in which the continuity equation, the filter operator, and the boundary condition at the wall are consistently formulated, we compute a set of plane turbulent channel flow explicitly filtered LES at different grid resolutions with the SGS term computed from a parallel DNS computation starting with an equivalent initial condition. The results show that the SGS term computed from DNS provide a truthful SGS model for short times until the numerical errors compound and the two parallel computations diverge. The effective time where the statistics remain similar is longer for lower-order statistics such as mean velocity profile, as expected. This demonstrates that with the consistent fNS equations, the abundant DNS data can be used to inform SGS model development for explicitly filtered LES. Moreover, the diverging of statitics at longer times shows that the SGS contribution is not necessarily homogeous as most SGS models assume, and a more precise correlation between the instantaneous LES velocity field and SGS model is required to capture the dynamics of turbulence.

physics.comp-ph↗

arXiv2018

Dynamic slip wall model for large-eddy simulation

Wall modelling in large-eddy simulation (LES) is necessary to overcome the prohibitive near-wall resolution requirements in high-Reynolds-number turbulent flows. Most existing wall models rely on assumptions about the state of the boundary layer and require a priori prescription of tunable coefficients. They also impose the predicted wall stress by replacing the no-slip boundary condition at the wall with a Neumann boundary condition in the wall-parallel directions while maintaining the no-transpiration condition in the wall-normal direction. In the present study, we first motivate and analyse the Robin (slip) boundary condition with transpiration (nonzero wall-normal velocity) in the context of wall-modelled LES. The effect of the slip boundary condition on the one-point statistics of the flow is investigated in LES of turbulent channel flow and flat-plate turbulent boundary layer. It is shown that the slip condition provides a framework to compensate for the deficit or excess of mean momentum at the wall. Moreover, the resulting nonzero stress at the wall alleviates the well-known problem of the wall-stress under-estimation by current subgrid-scale (SGS) models. Secondly, we discuss the requirements for the slip condition to be used in conjunction with wall models and derive the equation that connects the slip boundary condition with the stress at the wall. Finally, a dynamic procedure for the slip coefficients is formulated, providing a dynamic slip wall model free of a priori specified coefficients. The performance of the proposed dynamic wall model is tested in a series of LES of turbulent channel flow, non-equilibrium three-dimensional channel flow, and flat-plate turbulent boundary layer. The results show that the dynamic wall model is able to accurately predict one-point turbulence statistics for various flow configurations, Reynolds numbers, and grid resolutions.

physics.flu-dyn↗

arXiv2017

Multiscale analysis of the topological invariants in the logarithmic region of turbulent channels at $Re_τ=932$

The invariants of the velocity gradient tensor, $R$ and $Q$, and their enstrophy and strain components are studied in the logarithmic layer of an incompressible turbulent channel flow. The velocities are filtered in the three spatial directions and the results analyzed at different scales. We show that the $R$--$Q$ plane does not capture the changes undergone by the flow as the filter width increases, and that the enstrophy/enstrophy-production and strain/strain-production planes represent better choices. We also show that the conditional mean trajectories may differ significantly from the instantaneous behavior of the flow since they are the result of an averaging process where the mean is 3-5 times smaller than the corresponding standard deviation. The orbital periods in the $R$--$Q$ plane are shown to be independent of the intensity of the events, and of the same order of magnitude than those in the enstrophy/enstrophy-production and strain/strain-production planes. Our final goal is to test whether the dynamics of the flow are self-similar in the inertial range, and the answer turns out to be no. The mean shear is found to be responsible for the absence of self-similarity and progressively controls the dynamics of the eddies observed as the filter width increases. However, a self-similar behavior emerges when the calculations are repeated for the fluctuating velocity gradient tensor. Finally, the turbulent cascade in terms of vortex stretching is considered by computing the alignment of the vorticity at a given scale with the strain at a different one. These results generally support a non-negligible role of the phenomenological energy-cascade model formulated in terms of vortex stretching.

physics.flu-dyn↗

arXiv2017

An efficient algorithm to compute the genus of discrete surfaces and applications to turbulent flows

A simple and efficient algorithm to numerically compute the genus of surfaces of three-dimensional objects using the Euler characteristic formula is presented. The algorithm applies to objects obtained by thresholding a scalar field in a structured-collocated grid, and does not require any triangulation of the data. This makes the algorithm fast, memory-efficient and suitable for large datasets. Applications to the characterization of complex surfaces in turbulent flows are presented to illustrate the method.

physics.flu-dyn↗

arXiv2016

A statistical state dynamics-based study of the structure and mechanism of large-scale motions in plane Poiseuille flow

The perspective of statistical state dynamics (SSD) has recently been applied to the study of mechanisms underlying turbulence in various physical systems. An example implementation of SSD is the second order closure referred to as stochastic structural stability theory (S3T), which has provided insight into the dynamics of wall turbulence and specifically the emergence and maintenance of the roll/streak structure. This closure eliminates nonlinear interactions among the perturbations has been removed, restricting nonlinearity in the dynamics to that of the mean equation and the interaction between the mean and perturbation covariance. Here, this quasi-linear restriction of the dynamics is used to study the structure and dynamics of turbulence in plane Poiseuille flow at moderately high Reynolds numbers in a closely related dynamical system, referred to as the restricted nonlinear (RNL) system. RNL simulations reveal that the essential features of wall-turbulence dynamics are retained. Remarkably, the RNL system spontaneously limits the support of its turbulence to a small set of streamwise Fourier components giving rise to a naturally minimal representation of its turbulence dynamics. Although greatly simplified, this RNL turbulence exhibits natural-looking structures and statistics. Surprisingly, even when further truncation of the perturbation support to a single streamwise component is imposed the RNL system continues to produce self-sustaining turbulent structure and dynamics. RNL turbulence at the Reynolds numbers studied is dominated by the roll/streak structure in the buffer layer and similar very-large-scale structure (VLSM) in the outer layer. Diagnostics of the structure, spectrum and energetics of RNL and DNS turbulence are used to demonstrate that the roll/streak dynamics supporting the turbulence in the buffer and logarithmic layer is essentially similar in RNL and DNS.

physics.flu-dyn↗

arXiv2012

The 3D structure of turbulent channels up to $\mathrm{Re}_τ=4000$

Several time-resolved fluid dynamics videos of turbulent channels from $\mathrm{Re}_τ=180$ to $\mathrm{Re}_τ=4000$ are presented. The videos show the temporal evolution of sweeps (bluish) and ejections (reddish) in one half of the channel (only the bottom wall is shown). The color changes from dark for points close to the wall, to bright for those reaching the center of the channel. As the Reynolds number increases the scale separation becomes more clear and the complexity of the dynamics observed rises.

physics.flu-dyn↗