Alistair Revell

DESider A European Effort on Hybrid RANS-LES Modelling.

The present volume contains results gained from the EU-funded 6th Framework project, DESider (Detached Eddy Simulation for Industrial Aerodynamics). 18 European organisations from industry, research and universities, have collaborated on topics centred around the improvement of hybrid RANS-LES methods, the investigation and validation of these methods in respect of a variety of aerodynamic, aeroelastic and aeroacoustic test cases including novel URANS methods and the new SAS turbulence modelling strategy. The book presents an introduction to the project, exhibits partners methods and approaches, and provides comprehensive reports (i.e. definition as well as results) of all applications treated in the project.

Flow over a Wing with Leading-Edge Undulations

The stall-delaying properties of the humpback whale flipper have been observed and quantified in recent years, through both experimental and numerical studies. In the present work, numerical simulations of an infinite-span wing with an idealized representation of this geometry are reported at a Reynolds number of 1.2×10(to the power of 5). Using large-eddy simulation, an adequate spatial resolution is first established before also examining the spanwise extent of the domain. These results are then analyzed to provide an explanation of the conditions that drive the lift observed beyond the conventional stall angle. The undulating leading-edge geometry gives rise to a spanwise pressure gradient that drives a secondary flow toward the regions of minimum chord. In turn, this leads to the entrainment of higher-momentum fluid into the region behind the maximum chord, which energizes the boundary layer and delays stall. Aside from demonstrating a significant poststall lift, the undulations also have the added benefit of substantially reducing lift fluctuations.

Parallelisation of an interactive lattice-Boltzmann method on an Android-powered mobile device

The role of mobile devices in interactive engineering simulation is discussed.Two parallelisation frameworks for simulation on Android-based mobile devices are presented.Task-based parallelisation performs slightly better than thread-based parallelisation.Implementing the kernel natively improves performance by 20%. Engineering simulation is essential to modern engineering design, although it is often a computationally demanding activity which can require powerful computer systems to conduct a study. Traditionally the remit of large desktop workstations or off-site computational facilities, potential is now emerging for mobile computation, whereby the unique characteristics of portable devices are harnessed to provide a novel means of engineering simulation. Possible use cases include emergency service assistance, teaching environments, augmented reality or indeed any such case where large computational resources are unavailable and a system prediction is needed. This is particularly relevant if the required accuracy of a calculation is relatively low, such as cases where only an intuitive result is required. In such cases the computational resources offered by modern mobile devices may already be adequate. This paper proceeds to discuss further the possibilities that modern mobile devices might offer to engineering simulation and describes some initial developments in this direction. We focus on the development of an interactive fluid flow solver employing the lattice Boltzmann method, and investigate both task-based and thread-based parallel implementations. The latter is more traditional for high performance computing across many cores while the former, native to Android, is more simple to implement and returns a slightly higher performance. The performance of both saturates when the number of threads/tasks equal three on a quad-core device. Execution time is improved by a further 20% by implementing the kernel in C++ and cross-compiling using the Android NDK.

Application of a lattice Boltzmann-immersed boundary method for fluid-filament dynamics and flow sensing

Complex fluid-structure interactions between elastic filaments, or cilia, immersed in viscous flows are commonplace in nature and bear important roles. Some biological systems have evolved to interpret flow-induced motion into signals for the purpose of feedback response. Given the challenges associated with extracting meaningful experimental data at this scale, there has been particular focus on the numerical study of these effects. Porous models have proven useful where cilia arrangements are relatively dense, but for more sparse configurations the dynamic interactions of individual structures play a greater role and direct modelling becomes increasingly necessary. The present study reports efforts towards explicit modelling of regularly spaced wall-mounted cilia using a lattice Boltzmann-immersed boundary method. Both steady and forced unsteady 2D channel flows at different Reynolds numbers are investigated, with and without the presence of a periodic array of elastic inextensible filaments. It is demonstrated that the structure response depends significantly on Reynolds number. For low Reynolds flow, the recirculation vortex aft of successive filaments is small relative to the cilia spacing and does not fully bridge the gap, in which case the structure lags the flow. At higher Reynolds number, when this gap is fully bridged the structure and flow move in phase. The trapping of vortices between cilia is associated with relatively lower wall shear stress. At low to intermediate Reynolds, vortex bridging is incomplete and large deflection is still possible, which is reflected in the tip dynamics and wall shear stress profiles.

Physiological mechanisms of pulmonary hypertension

Pulmonary hypertension is usually related to obstruction of pulmonary blood flow at the level of the pulmonary arteries (eg, pulmonary embolus), pulmonary arterioles (idiopathic pulmonary hypertension), pulmonary veins (pulmonary venoocclusive disease) or mitral valve (mitral stenosis and regurgitation). Pulmonary hypertension is also observed in heart failure due to left ventricle myocardial diseases regardless of the ejection fraction. Pulmonary hypertension is often regarded as a passive response to the obstruction to pulmonary flow. We review established fluid dynamics and physiology and discuss the mechanisms underlying pulmonary hypertension. The important role that the right ventricle plays in the development and maintenance of pulmonary hypertension is discussed. We use principles of thermodynamics and discuss a potential common mechanism for a number of disease states, including pulmonary edema, through adding pressure energy to the pulmonary circulation.

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

Turbulence modelling of strongly detached unsteady flows: The circular cylinder

This paper reports the predictions of three turbulence modelling schemes in the application to the flow around a circular cylinder in a square duct. A URANS scheme known as the Stress-Strain Lag model has been developed in the DESider project, specifically to capture the effects of stress-strain misalignment observed in unsteady mean turbulent flows (Revell, 2006). Coupling of the Lag model with the popular k -ω SST model, to form the so-called SST-Cas model, has been shown to incorporate some of the advantages of a full Reynolds Stress transport Model (RSM), whilst retaining the efficiency and stability benefits of a eddy viscosity model (EVM) (Revell et al., 2006, 2007).

Improved Embedded Approaches

In contrast to the non-zonal, DES-like, hybrid approaches, in which a transition from RANS to LES relies upon a natural instability of separated shear layers in massively separated flows, the zonal RANS-LES (actually, RANS – Wall Modelled LES or RANS-WMLES) hybrids imply the presence of a sharp interface between the flow regions treated by RANS and LES. The location of this interface may be arbitrarily specified by the user based on their understanding of the flow physics, available computational resources or the objectives of the simulation, e.g., a need for unsteady flow characteristics.

Assessment of turbulence models for pulsatile flow inside a heart pump

Computational fluid dynamics (CFD) is applied to study the unsteady flow inside a pulsatile pump left ventricular assist device, in order to assess the sensitivity to a range of commonly used turbulence models. Levels of strain and wall shear stress are directly relevant to the evaluation of risk from haemolysis and thrombosis, and thus understanding the sensitivity to these turbulence models is important in the assessment of uncertainty in CFD predictions. The study focuses on a positive displacement or pulsatile pump, and the CFD model includes valves and moving pusher plate. An unstructured dynamic layering method was employed to capture this cyclic motion, and valves were simulated in their fully open position to mimic the natural scenario, with in/outflow triggered at control planes away from the valves. Six turbulence models have been used, comprising three relevant to the low Reynolds number nature of this flow and three more intended to investigate different transport effects. In the first group, we consider the shear stress transport (SST) model in both its standard and transition-sensitive forms, and the ‘laminar’ model in which no turbulence model is used. In the second group, we compare the one equation Spalart–Almaras model, the standard two equation and the full Reynolds stress model (RSM). Following evaluation of spatial and temporal resolution requirements, results are compared with available experimental data. The model was operated at a systolic duration of 40% of the pumping cycle and a pumping rate of 86 BPM (beats per minute). Contrary to reasonable preconception, the ‘transition’ model, calibrated to incorporate additional physical modelling specifically for these flow conditions, was not noticeably superior to the standard form of the model. Indeed, observations of turbulent viscosity ratio reveal that the transition model initiates a premature increase of turbulence in this flow, when compared with both experimental and higher order numerical results previously reported in the literature. Furthermore, the RSM is indicated to provide the most accurate prediction over much of the flow, due to its ability to more correctly account for three-dimensional effects. Finally, the clinical relevance of the results is reported along with a discussion on the impact of such modelling uncertainties.

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.