An efficient ghost-cell immersed boundary method (GCIBM) for simulating turbulent flows in complex geometries is presented. A boundary condition is enforced through a ghost cell method. The reconstruction procedure allows systematic development of numerical schemes for treating the immersed boundary while preserving the overall second-order accuracy of the base solver. Both Dirichlet and Neumann boundary conditions can be treated. The current ghost cell treatment is both suitable for staggered and non-staggered Cartesian grids. The accuracy of the current method is validated using flow past a circular cylinder and large eddy simulation of turbulent flow over a wavy surface. Numerical results are compared with experimental data and boundary-fitted grid results. The method is further extended to an existing ocean model (MITGCM) to simulate geophysical flow over a three-dimensional bump. The method is easily implemented as evidenced by our use of several existing codes.

A Ghost-Cell Immersed Boundary Method for Flow in Complex Geometry

Hybrid LES/RANS methods for the simulation of turbulent flows

Modifications are proposed of two recently developed hybrid CFD strategies, Delayed Detached Eddy Simulation (DDES) and DDES with Improved wall-modeling capability (IDDES). The modifications are aimed at fine-tuning of these approaches to the k-ω SST background RANS model. The first one includes recalibrated empirical constants in the shielding function of the SA-based DDES model which are shown to be suboptimal (not providing the needed level of elimination of the Model Stress Depletion (MSD)) for the SST-based DDES model. For the SST-IDDES variant, in addition to that, a simplification of the original SA–based formulation is proposed, which does not cause any visible degradation of the model performance. Both modifications are extensively tested on a range of attached and separated flows (developed channel, backward-facing step, periodic hills, wall-mounted hump, and hydrofoil with trailing edge separation).

Development of DDES and IDDES Formulations for the k-ω Shear Stress Transport Model

We show that the discrepancies in Reynolds-averaged Navier-Stokes (RANS) modeled Reynolds stresses can be explained by mean flow features. A physics-informed machine learning framework is proposed to improve the predictive capabilities of RANS models by leveraging existing direct numerical simulations databases.

Physics-informed machine learning approach for reconstructing Reynolds stress modeling discrepancies based on DNS data

https://publications.lib.chalmers.se/records/fulltext/218126/local_218126.pdf

Nektar++: An open-source spectral/hp element framework ✩

Flow over periodic hills – Numerical and experimental study in a wide range of Reynolds numbers

The analysis and improvement of an immersed boundary method (IBM) for simulating turbulent flows over complex geometries are presented. Direct forcing is employed. It consists in interpolating boundary conditions from the solid body to the Cartesian mesh on which the computation is performed. Lagrange and least squares high-order interpolations are considered. The direct forcing IBM is implemented in an incompressible finite volume Navier–Stokes solver for direct numerical simulations (DNS) and large eddy simulations (LES) on staggered grids. An algorithm to identify the body and construct the interpolation schemes for arbitrarily complex geometries consisting of triangular elements is presented. A matrix stability analysis of both interpolation schemes demonstrates the superiority of least squares interpolation over Lagrange interpolation in terms of stability. Preservation of time and space accuracy of the original solver is proven with the laminar two-dimensional Taylor–Couette flow. Finally, practicability of the method for simulating complex flows is demonstrated with the computation of the fully turbulent three-dimensional flow in an air-conditioning exhaust pipe. Copyright © 2006 John Wiley & Sons, Ltd.

High‐order stable interpolations for immersed boundary methods

The performance of an hybrid LES–RANS strategy, the Detached Eddy Simulation (DES), as a predictive tool for turbulent channel flow with massive separation is scrutinized. This is undertaken in a collaborative effort involving five different flow solvers used by five different groups to cover a broad range of numerical methods and implementations. This paper concentrates on DES results obtained with a computational mesh of approximately one million cells. The results are compared to those obtained by Large Eddy Simulations (LES) using the standard and the dynamic Smagorinsky models and an alternative hybrid LES–RANS – all computed on the same grid. Data of a highly resolved LES (roughly 13 million cells) are used for reference. Furthermore, the impact of resolution and, therefore, the location of the LES–RANS interface is studied.

/pdf/evaluation-of-detached-eddy-simulations-for-predicting-the-nqb4vln0qt.pdf

Evaluation of Detached Eddy Simulations for predicting the flow over periodic hills

A new extended inner scaling is proposed for the wall layer of wall-bounded flows under the influence of both wall shear stress and streamwise pressure gradient. This scaling avoids problems of the classical wall coordinates close to flow separation and reattachment. Based on the proposed extended velocity and length scales a universal nondimensional family of velocity profiles is derived for the viscous region in the vicinity of a wall that depend on wall distance and a parameter α quantifying the importance of the streamwise pressure gradient with respect to the wall shear stress in the momentum balance. The performance of the proposed extended scaling is investigated in two different flow fields, a separating and reattaching turbulent boundary layer and a turbulent flow over a periodic arrangement of smoothly contoured hills. Both flows are results of highly resolved direct numerical simulation (DNS). The results show that the viscous assumptions are valid up to about two extended wall units. If the profiles are scaled by the extended inner coordinates, they seem to behave in a universal way. This gives rise to the hope that a universal behavior of velocity profiles can be found in the proposed extended inner coordinates even beyond the validity of the extended viscous law of the wall.

Nikolaus Peller

Papers

Flow over periodic hills – Numerical and experimental study in a wide range of Reynolds numbers

High‐order stable interpolations for immersed boundary methods

Evaluation of Detached Eddy Simulations for predicting the flow over periodic hills

Near-wall scaling for turbulent boundary layers with adverse pressure gradient

A Comparative Study of the Turbulent Flow Over a Periodic Arrangement of Smoothly Contoured Hills