arXiv subjects

Johan Larsson

Publications and source records attributed to Johan Larsson.

25 records · Page 2Linked to original sources

arXiv2019

Towards systematic grid selection in LES: iterative identification of the coarse-graining length scale by minimizing the solution sensitivity

The accuracy of a large eddy simulation (LES) is determined by the accuracy of the model used to describe the effect of unresolved scales, the numerical errors of the resolved scales, and the optimality of the length scale that separates resolved from unresolved scales (the filter-width, or the coarse-graining length scale). This paper is focused entirely on the last of these, proposing a systematic algorithm for identifying the "optimal" spatial distribution of the coarse-graining length scale and its aspect ratio. The core idea is that the "optimal" coarse-graining length scale for LES is the largest length scale for which the LES solution is minimally sensitive to it. This idea is formulated based on an error indicator that measures the sensitivity of the solution and a criterion that determines how that error indicator should vary in space and direction to minimize the overall sensitivity of the solution. The solution to this optimization problem is that the cell-integrated error indicator should be equi-distributed; a corollary is that one cannot link the accuracy in LES to quantities that are not cell-integrated, including the common belief that LES is accurate whenever 80-90\% of the energy is resolved. The full method is tested on the wall-resolved LES of turbulent channel flow and the flow over a backward-facing step, with final length scale fields (or filter-width fields, or grids) that are close to what is considered "best practice" in the LES literature. Finally, the derivation of the error indicator offers an alternative explanation for the success of the dynamic procedure.

physics.comp-ph↗

arXiv2016

Toward real-time high-fidelity simulation using integral boundary layer modeling

One of the greatest challenges to using large-eddy simulations (LES) in engineering applications is the large number of grid points required near walls. To mitigate this issue, researchers often couple LES with a simplified model of the flow close to the wall, known as the wall model. One feature common to most wall models is that the first few (about 3) grid points must be located below the inviscid log-layer, and the grid must have near-isotropic resolution near the wall. Hence, wall-modeled LES may still require a large number of grid points in both the wall-normal and span-wise directions. Because of these requirements, wall-modeled LES is still unfeasible in many applications. We present a new formulation of wall-modeled LES that is being developed to address this issue. In this formulation, LES is used to solve only for the features of the velocity field that can be adequately represented on the LES grid. The effects of the unresolved features are captured by imposing an integral balance of kinetic energy in the near-wall region. This integral energy balance translates into a dynamic partial differential equation defined on the walls, which is coupled to the LES equations. We discuss details of the new formulation and present results obtained in laminar channel flows.

physics.flu-dyn↗

arXiv2016

Heat transfer and wall temperature effects in shock wave turbulent boundary layer interactions

Direct numerical simulations are carried out to investigate the effect of the wall temperature on the behavior of oblique shock-wave/turbulent boundary layer interactions at freestream Mach number $2.28$ and shock angle of the wedge generator $φ= 8^{\circ}$. Five values of the wall-to-recovery-temperature ratio ($T_w/T_r$) are considered, corresponding to cold, adiabatic and hot wall thermal conditions. We show that the main effect of cooling is to decrease the characteristic scales of the interaction in terms of upstream influence and extent of the separation bubble. The opposite behavior is observed in the case of heating, that produces a marked dilatation of the interaction region. The distribution of the Stanton number shows that a strong amplification of the heat transfer occurs across the interaction, and the maximum values of thermal and dynamic loads are found in the case of cold wall. The analysis reveals that the fluctuating heat flux exhibits a strong intermittent behavior, characterized by scattered spots with extremely high values compared to the mean. Furthermore, the analogy between momentum and heat transfer, typical of compressible, wall-bounded, equilibrium turbulent flows does not apply for most part of the interaction domain. The pre-multiplied spectra of the wall heat flux do not show any evidence of the influence of the low-frequency shock motion, and the primary mechanism for the generation of peak heating is found to be linked with the turbulence amplification in the interaction region.

physics.flu-dyn↗

arXiv2015

A parametrized non-equilibrium wall-model for large-eddy simulations

Wall-models are essential for enabling large-eddy simulations (LESs) of realistic problems at high Reynolds numbers. The present study is focused on approaches that directly model the wall shear stress, specifically on filling the gap between models based on wall-normal ordinary differential equations (ODEs) that assume equilibrium and models based on full partial differential equations (PDEs) that do not. We develop ideas for how to incorporate non-equilibrium effects (most importantly, strong pressure-gradient effects) in the wall-model while still solving only wall-normal ODEs. We test these ideas using two reference databases: an adverse pressure-gradient turbulent boundary-layer and a shock/boundary-layer interaction problem, both of which lead to separation and re-attachment of the turbulent boundary layer.

physics.flu-dyn↗

arXiv2015

Exploiting Active Subspaces to Quantify Uncertainty in the Numerical Simulation of the HyShot II Scramjet

We present a computational analysis of the reactive flow in a hypersonic scramjet engine with focus on effects of uncertainties in the operating conditions. We employ a novel methodology based on active subspaces to characterize the effects of the input uncertainty on the scramjet performance. The active subspace identifies one-dimensional structure in the map from simulation inputs to quantity of interest that allows us to reparameterize the operating conditions; instead of seven physical parameters, we can use a single derived active variable. This dimension reduction enables otherwise infeasible uncertainty quantification, considering the simulation cost of roughly 9500 CPU-hours per run. For two values of the fuel injection rate, we use a total of 68 simulations to (i) identify the parameters that contribute the most to the variation in the output quantity of interest, (ii) estimate upper and lower bounds on the quantity of interest, (iii) classify sets of operating conditions as safe or unsafe corresponding to a threshold on the output quantity of interest, and (iv) estimate a cumulative distribution function for the quantity of interest.

math.NA↗

arXiv2014

The prospect of using LES and DES in engineering design, and the research required to get there

In this paper we try to look into the future to divine how large eddy and detached eddy simulations (LES and DES, respectively) will be used in the engineering design process about 20-30 years from now. Some key challenges specific to the engineering design process are identified, and some of the critical outstanding problems and promising research directions are discussed.

physics.flu-dyn↗

arXiv2013

The Shock and the Turbulence: the story of an interaction

Video highlighting multiple features of the interaction between isotropic turbulence and a nominally planar shock wave, normal to the mean flow direction. The visualizations showcased in this video were obtained from a Direct Numerical Simulation.

physics.flu-dyn↗