The role of forcing and eddy viscosity variation on the log-layer mismatch observed in wall-modeled large-eddy simulations

We investigate the role of eddy viscosity variation and the effect of zonal enforcement of the mass flow rate on the log-layer mismatch problem observed in turbulent channel flows. An analysis of the mean momentum balance shows that it lacks a degree-of-freedom (DOF) when eddy viscosity is large, and the mean velocity conforms to an incorrect profile. Zonal enforcement of the target flow rate introduces an additional degree-of-freedom to the mean momentum balance, similar to an external stochastic forcing term, leading to a significant reduction in the log-layer mismatch. We simulate turbulent channel flows at friction Reynolds numbers of 2000 and 5200 on coarse meshes that do not resolve the viscous sublayer. The second-order turbulence statistics agree well with the direct numerical simulation benchmark data when results are normalized by the velocity scale extracted from the filtered velocity field. Zonal enforcement of the flow rate also led to significant improvements in skin friction coefficients

Binarized-octree generation for Cartesian adaptive mesh refinement around immersed geometries

We revisit the generation of balanced octrees for adaptive mesh refinement (AMR) of Cartesian domains with immersed complex geometries. In a recent short note (Hasbestan and Senocak, 2017) [42], we showed that the data locality of the Z-order curve in a hashed linear-octree generation method may not be perfect because of potential collisions in the hash table. Building on that observation, we propose a binarized-octree generation method that complies with the Z-order curve exactly. Similar to a hashed linear-octree generation method, we use Morton encoding to index the nodes of an octree, but use a red-black tree in place of the hash table. Red-black tree is a special kind of a binary tree, which we use for insertion and deletion of elements during mesh adaptation. By strictly working with the bitwise representation of an octree, we remove computer hardware limitations on the depth of adaptation on a single processor. Additionally, we introduce a geometry encoding technique for rapidly tagging a solid geometry for mesh refinement. Our results for several geometries with different levels of adaptations show that the binarized-octree generation method outperforms the linear-octree generation method in terms of runtime performance at the expense of only a slight increase in memory usage. The current AMR capability, rebl-AMR, is available as open-source software

Generalized conductance sum rule in atomic break junctions

When an atomic-size break junction is mechanically stretched, the total conductance of the contact remains approximately constant over a wide range of elongations, although at the same time the transmissions of the individual channels (valence orbitals of the junction atom) undergo strong variations. We propose a microscopic explanation of this phenomenon, based on Coulomb correlation effects between electrons in valence orbitals of the junction atom. The resulting approximate conductance quantization is closely related to the Friedel sum rule.Comment: 4 pages, 1 figure, appears in Proceedings of the NATO Advanced Research Workshop ``Size dependent magnetic scattering'', Pecs, Hungary, May 28 - June 1, 200

Large-eddy simulation of spectral coherence in a wind turbine wake

This work is mainly dedicated to the study of the characteristics of spectral coherence of turbulence fluctuations in wind turbine wakes. A computational fluid dynamics (CFD) code has been implemented using a large-eddy simulation (LES) approach, which is thought to be conceptually more suitable for studying the turbulence evolution in a wind turbine wake. Comparisons with experimental data from the Nørrekær Enge II Windfarm, in Denmark, and with an analytical model proposed by Panofsky and Dutton have been performed, and the results are found to be in reasonable agreement with both

Thermo-Mixed Hydrodynamics of Piston Compression Ring Conjunction

The final publication is available at http://link.springer.com.A new method, comprising Navier-Stokes equations, Rayleigh-Plesset volume fraction equation, an analytical control-volume thermal mixed approach and asperity interactions is reported. The method is employed for prediction of lubricant flow and assessment of friction in the compression ring-cylinder liner conjunction. The results are compared with Reynolds-based laminar flow with Elrod cavitation algorithm. Good conformance is observed for medium load intensity part of the engine cycle. At lighter loads and higher sliding velocity, the new method shows more complex fluid flow, possessing layered flow characteristics on account of pressure and temperature gradient into the depth of the lubricant film, which leads to a cavitation region with vapour content at varied volume fractions. Predictions also conform well to experimental measurements reported by other authors

Computations of unsteady cavitation with a pressure-based method

A computational approach based on the conservative form of the Favre-averaged Navier-Stokes equations, transport equation-based turbulent cavitation models and a pressure-based operator-splitting algorithm is applied to study turbulent cavitating flows through convergent-divergent nozzles. The implications of the compressibility effect, reflected via the speed of sound definition in the two-phase mixture, are assessed with two modeling approaches. Depending on the geometric confinement of the nozzle, compressibility model, and cavitation numbers, auto-oscillations and quasi-steady behaviors are observed. Detailed flow structures and cavitation dynamics are highlighted, and implications of the cavitation model discussed

A Parallel Adaptive Mesh Refinement Software for Complex Geometry Flow Simulations

We present a parallel adaptive mesh refinement method with unstructured connectivity. The AMR package is being developed as an open-source project to support immersed boundary flow simulations around complex geometry. In the current work, we explain the parallel implementation of 2:1 balanced implicit-octree generation. A Cartesian processor topology is used to generate the initial mesh for a given geometry. Each processor then constructs its own local tree. One-sided communication features of the MPI-II library is used to carry out the information exchange with other processors. Several complex geometries are used to demonstrate the versatility of the AMR package

Interfacial dynamics-based modelling of turbulent cavitating flows, part-2: Time-dependent computations

The interfacial dynamics-based cavitation model, developed in Part-1, is further employed for unsteady flow computations. The pressure-based operator-splitting algorithm (PISO) is extended to handle the time-dependent cavitating flows with particular focus on the coupling of the cavitation and turbulence models, and the large density ratio associated with cavitation. Furthermore, the compressibility effect is important for unsteady cavitating flows because in a water-vapour mixture, depending on the composition, the speed of sound inside the cavity can vary by an order of magnitude. The implications of the issue of the speed of the sound are assessed with alternative modelling approaches. Depending on the geometric confinement of the nozzle, compressibility model and cavitation numbers, either auto-oscillation or quasi-steady behaviour is observed. The adverse pressure gradient in the closure region is stronger at the maximum cavity size. One can also observe that the mass transfer process contributes to the cavitation dynamics. Compared to the steady flow computations, the velocity and vapour volume fraction distributions within the cavity are noticeably improved with time-dependent computations. © 2004 John Wiley & Sons, Ltd

Evaluation of cavitation models for Navier-Stokes computations

The predictive capability of three transport equation-based cavitation models is evaluated for attached turbulent, cavitating flows. To help shed light on the theoretical justification of these models, an analysis of the mass and normal-momentum conservation at a liquid-vapor interface is presented. The test problems include flows over an axisymmetric cylindrical body and a planar hydrofoil at different cavitation and Reynolds numbers. Proper grid distribution for high Reynolds number cavitating flows is emphasized. Although all three models give satisfactory predictions in overall pressure distributions, differences are observed in the closure region of the cavity, resulting from the differences in compressibility characteristics handled by each model