Neil Ashton

Development of an Alternative Delayed Detached-Eddy Simulation Formulation Based on Elliptic Relaxation

CD = drag coefficient CDDES = empirical parameter Cf = skin-friction coefficient CL = lift coefficient Cp = pressure coefficient Ce1 = model constant for the dissipation equation Ce2 = model constant for the dissipation equation c = chord length f = elliptic operator fd = delayed detached-eddy simulation blending function h = hill height k = turbulent kinetic energy L = turbulent length scale Re = Reynolds number S = deformation tensor Ub = bulk velocity U∞ = freestream velocity y = distance to the nearest wall y = nondimensional wall distance Δ = large-eddy simulation filter width Δt = time step e = turbulent dissipation κ = von Karman constant ν = molecular viscosity νt = turbulent viscosity Ψ = delayed detached-eddy simulation correction term

Grey-Area Mitigation for the Ahmed Car Body Using Embedded DDES

The Ahmed car body represents a generic car geometry which exhibits many of the flow features found in real-life cars despite its simplified geometry. It is a challenging test case for the turbulence modelling community as it combines both 3D separation and the formation of counter-rotating vortices, which interact together to produce a recirculation region behind the car body. It is shown that none of the RANS models tested are able to correctly predict the size of the recirculation region, regardless of modelling level, mesh resolution or the choice of the length scale (i.e. \(\omega \) or \(\varepsilon \)). All of these models under-predict the turbulence levels over the slanted back and as a consequence over-predict the separation region. The DDES simulations (regardless of the underlying URANS model) offer an improved predictive capability compared to the RANS models when the mesh resolution is sufficient. When the mesh resolution is insufficient the DDES models produces worse results than either of the URANS models. In both cases, the grey area problem is demonstrated, wherein a lack of both modelled and resolved turbulence in the initial separated shear layer results in an over-prediction of the separation region. A one-way embedded DDES approach is shown to give the best compromise between accuracy and simulation cost. It accurately predicts the level of resolved turbulence in the initial separated shear layer and thus compared to non-embedded DDES and URANS, the injection of synthetic turbulence upstream of the separation point allows for the correct level of turbulence at the onset of separation. The resulting separation zone is correctly predicted and the grey-area problem is reduced.

Computational hemodynamics of abdominal aortic aneurysms: Three-dimensional ultrasound versus computed tomography.

The current criterion for surgical intervention in abdominal aortic aneurysms, based upon a maximal aortic diameter, is considered conservative due to the high mortality rate in case of rupture. The research community is actively investigating the use of computational mechanics tools combined with patient-specific imaging to help identify more accurate criteria. Widespread uptake of a successful metric will however be limited by the need for computed tomography, which is at present the primary image extraction method on account of the location and complex shape of the aneurysms. The use of three-dimensional ultrasound as the scanning method is more attractive on account of increased availability, reduced cost and reduced risk to patients. The suitability of three-dimensional ultrasound is assessed for this purpose in the present work; computational fluid dynamics simulations were performed on geometries obtained from the same patient using both ultrasound and computed tomography. The influence of different smoothing algorithms is investigated in the geometry preparation stage and Taubin’s low-pass filter was found to best preserve geometry features. Laminar, Newtonian, steady-state simulation analysis identified haemodynamic characteristics to be qualitatively similar in terms of wall shear stress, velocity and vorticity. The study demonstrates the potential for three-dimensional ultrasound to be integrated into a more accessible patient-specific modelling tool able to identify the need for surgical intervention of abdominal aortic aneurysms.

EMBEDDED DDES OF 2D HUMP FLOW

The present work aims to investigate the usage of embedded regions of turbulent flow simulation near to the point of separation; towards an approach whereby discrete regions of turbulent resolving techniques are used within a domain predominantly solved using the RANS equations. The common Delayed Detached Eddy Simulation (DDES) approach is here used to compute the flow around a 2D hump centred within a ‘full domain’ i.e. also incorporating an upstream section. Subsequently the domain length is reduced and the flow is started at two locations close to the separation point by means of unsteady turbulent inlet conditions. The Divergence Free Synthetic Eddy Method (DF-SEM) and its predecessor are tested for their ability to reproduce the original DDES results from the full domain. In the present case we aim to return a minimal disturbance from the full domain solution and thus herein we do not focus on the predictive accuracy of the selected DDES approach. The motivation for this technique is to provide guidance for the optimal reduction of embedded regions of turbulent simulation in complex applications; i.e. including multiple instances of separated flow. Some comments regarding computational expense of the method are also provided.

Real-World Variability in the Prediction of Intracranial Aneurysm Wall Shear Stress: The 2015 International Aneurysm CFD Challenge

PurposeImage-based computational fluid dynamics (CFD) is widely used to predict intracranial aneurysm wall shear stress (WSS), particularly with the goal of improving rupture risk assessment. Nevertheless, concern has been expressed over the variability of predicted WSS and inconsistent associations with rupture. Previous challenges, and studies from individual groups, have focused on individual aspects of the image-based CFD pipeline. The aim of this Challenge was to quantify the total variability of the whole pipeline.Methods3D rotational angiography image volumes of five middle cerebral artery aneurysms were provided to participants, who were free to choose their segmentation methods, boundary conditions, and CFD solver and settings. Participants were asked to fill out a questionnaire about their solution strategies and experience with aneurysm CFD, and provide surface distributions of WSS magnitude, from which we objectively derived a variety of hemodynamic parameters.ResultsA total of 28 datasets were submitted, from 26 teams with varying levels of self-assessed experience. Wide variability of segmentations, CFD model extents, and inflow rates resulted in interquartile ranges of sac average WSS up to 56%, which reduced to < 30% after normalizing by parent artery WSS. Sac-maximum WSS and low shear area were more variable, while rank-ordering of cases by low or high shear showed only modest consensus among teams. Experience was not a significant predictor of variability.ConclusionsWide variability exists in the prediction of intracranial aneurysm WSS. While segmentation and CFD solver techniques may be difficult to standardize across groups, our findings suggest that some of the variability in image-based CFD could be reduced by establishing guidelines for model extents, inflow rates, and blood properties, and by encouraging the reporting of normalized hemodynamic parameters.