The NASA 2D wall-mounted hump simulated using DDES-SA with the G3D::Flow solver

With the interest in simulating complex geometries using the Delayed Detached Eddy Simulation (DDES) model, an initial step should be taken in verifying the methodologies needed. The DDES model is used with a newly proposed modification to the sub-grid length-scale, the shear-layer-adaptive length-scale, to improve the transition from RANS to LES. The well-known 2D NASA wall-mounted hump test case is simulated. RANS simulations are performed to verify a correct implementation of the turbulence model developed by Spalart and Allmaras (SA). The SA model is important as it will serve as the underlying sub-grid-scale model for the DDES. Furthermore, RANS was used in an initial grid study. Two simulations are performed using the DDES model, where the difference lies in the number of cells and the grid topology. The results show an extended steady shear-layer in the separated region, delaying the transition from RANS to LES, where the cause is suggested to be insufficient grid resolution in the focus region. This influences the prediction of the re-attachment location and the velocity profiles downstream of the hump. However, one of the transient simulations improves the predictions of the re-attachment location and downstream velocity profiles. The other transient simulation is, however, not capable of improving the RANS results due to the delayed breakdown of two-dimensional coherent structures generated at the separation location. The results from the two DDES simulations indicate that the grid-resolution near the separation point needs refinement for a faster transition from RANS to LES. Using an explicit CFD solver for transient simulations of wall-bounded flow configurations, special treatment is needed to make the time-step requirements restricted by flow physics, rather than the numerical stability-limit. To achieve this, the dual-time stepping method has been implemented in to the in-house CFD solver, G3D::Flow. When using the dual-time stepping method, in combination with residual smoothing and low-speed reconditioning, a speed-up of approximately 50 is achieved

Off design simulations of an S-shaped intermediate compressor duct: Experimental validation of DDES and RANS using G3D::Flow

A comparison is made between measurements obtained in an experimental test rig at GKN Aerospace and simulations using the CFD solver G3D::Flow, developed and maintained at Chalmers University of Technology. The geometry represents an Intermediate Compressor Duct (ICD) of an aircraft engine. The aim is to validate the different CFD turbulence closure techniques with experimental data and to compare the CFD methods with each other. The turbulence techniques used are RANS, Unsteady-RANS (URANS) and Delayed-Detached Eddy-Simulation (DDES) models. The one-equation turbulence model, developed by Spalart and Allmaras, is used as the main model for the (U)RANS simulations and as the sub-grid-scale model for the DDES. To save computational resources, wall-functions are used to model the boundary layers. Overall, the CFD simulations are in a good agreement with the measured data, where some differences are observed when considering radial profiles of total pressure, downstream of the ICD. Furthermore, there are instabilities present in the DDES simulations at the inner wall of the ICD. Large instabilities were also observed in the experiments, represented by relatively large uncertainties. This behavior was not captured by the (U)RANS simulations. Additionally, the instantaneous DDES is capable of representing the true flow much better (if sufficiently small scales are resolved), where the (U)RANS results will never exist in reality, limiting the information acquired

Microfaunal primary succession on the volcanic island Surtsey

The island of Surtsey, Iceland, was formed in 1963 by a volcanic eruption. Since then, it has served as a unique natural laboratory for scientists interested in primary succession. In this study we investigated the state of the soil microfauna succession in 1995. We examined locations on the island with different vegetation types (unvegetated soil, soil with one or two plant species, and bird colony soil with a diverse vegetation). We recorded at least 16 nematode taxa and 13 flagellate taxa. Most of these were not reported in previous surveys from Surtsey. On the location with unvegetated soil, ciliates and nematodes were absent and only amoebae and heterotrophic flagellates were found. Most of the protozoan populations we examined were unable to survive salinity levels corresponding to seawater. We therefore conclude that many of soil protozoa populations on Surtsey arrived to the island as airborne cysts brought there from nearby land. However, in the bird colony soil with a high input of salts from the bird droppings, several flagellate species survived and multiplied at seawater salinity. This indicates that the bird colony soil harbours microhabitats where marine flagellate populations have been established