Computational Modelling Group

Assessment of the performance of novel RANS and hybrid turbulence models on the flow around a cylinder

Manuel Diaz Brito

The turbulent flow around a circular cylinder is a widely studied problem in fluid dynamics. At a certain characteristic Reynolds numbers the development of a turbulent wake occurs simultaneously with separation of the laminar boundary layer. The mechanisms defining this critical flow state are very complex to predict computationally. In this project the suitability of novel non-linear eddy viscosity closures and a hybrid Flow Simulation Methodology formulation to face these massively separated flows is studied. The flow predicting capabilities of the baseline EASM, ?-?-EASM and FSM-?-?-EASM tested are contrasted with the industrial renowned k-?-SST turbulence model. In the visualisation of the results it is evident that the ?-?-EASM has greater flexibility estimating the components of the Reynolds stresses with respect to the baseline EASM and the k-?-SST. Although dome differences are observed, the prediction of the critical flow around a cylinder is not accurately achieved by any of these RANS models, but the FSM-?-?-EASM shows great resemblance with the validation data, demonstrating capabilities of resolving very complex flow phenomena with minimum user input if the computational grid is fine enough. In order to demonstrate even greater advantages of non-linear models it was postulated that the addition of a streamwise impinging vortex hitting the leading edge of the cylinder would make the flow field fully three-dimensional. First attempts were tried in this route but time constraints limited the ultimate scope of the present work.

Centre for Doctoral Training in Next Generation Computational Modelling

Hans Fangohr, Ian Hawke, Peter Horak (Investigators), Susanne Ufermann Fangohr, Thorsten Wittemeier, Kieran Selvon, Alvaro Perez-Diaz, David Lusher, Ashley Setter, Emanuele Zappia, Hossam Ragheb, Ryan Pepper, Stephen Gow, Jan Kamenik, Paul Chambers, Robert Entwistle, Rory Brown, Joshua Greenhalgh, James Harrison, Jonathon Waters, Ioannis Begleris, Craig Rafter

The £10million Centre for Doctoral Training was launched in November 2013 and is jointly funded by EPSRC, the University of Southampton, and its partners.

The NGCM brings together world-class simulation modelling research activities from across the University of Southampton and hosts a 4-year doctoral training programme that is the first of its kind in the UK.

Complexity in Modelling Electric Marine Propulsive Devices

Suleiman Sharkh, Neil Bressloff, Hans Fangohr (Investigators), Aleksander Dubas

This project involves the simulation of turbulent flow around a marine rim-driven thruster and the complex interaction of flow features involved through computational fluid dynamics. Following this, the optimisation of design parameters using computational fluid dynamics to calculate the objective function is performed and surrogate modelling utilised to estimate optimum design configuration.

DIPLOS - Dispersion of Localised Releases in a Street Network

Trevor Thomas, Ian Castro (Investigators)

The security threat level from international terrorism, introduced by the UK Security Service, has been classified as either "severe" or "critical" for much of its six-year history, and currently remains as "substantial" (source: MI5 website). Part of the risk posed by terrorist threats involves potential releases of air-borne chemical, biological, radiological or nuclear (CBRN) material into highly populated urbanised areas. Smoke from industrial accidents within or in the vicinity of urban areas also pose risks to health and can cause widespread disruption to businesses, public services and residents. The Buncefield depot fire of 2005 resulted in the evacuation of hundreds of homes and closure of more than 200 schools and public buildings for two days; consequences would have been much more severe if prevailing meteorological conditions had promoted mixing or entrainment of the smoke plume into the urban canopy. In both these scenarios it is crucial to be able to model, quickly and reliably, dispersion from localised sources through an urban street network in the short range, where the threat to human health is greatest. However, this is precisely where current operational models are least reliable because our understanding and ability to model short-range dispersion processes is limited. The contribution that DIPLOS will make is:

1. to fill in the gaps in fundamental knowledge and understanding of key dispersion processes,
2. to enable these processes to be parametrized for use in operational models,
3. to implement them into an operational model, evaluate the improvement and apply the model to a case study in central London

Most of the existing research on urban dispersion has focused on air quality aspects, with sources being extensive and distributed in space. Scientifically, this research is novel in focusing on localized releases within urban areas, and on dispersion processes at short range. Through a combination of fundamental studies using wind tunnel experiments and high resolution supercomputer simulations, extensive data analysis and development of theoretical and numerical models, DIPLOS will contribute to addressing this difficult and important problem from both a scientific research and a practical, operational perspective.

Evaluation of Vortex Shedding effects on Slender Structures using Large-Eddy Simulation

Zheng-Tong Xie, Ian Castro (Investigators), Steven Daniels

Wind-induced vortex shedding on buildings is a main concern for the engineer, as this can lead to severe structural failures, or at the very least fatigue concerns. Wind tunnel testing of this effect is somewhat limited with the generation of turbulent flow, making the use of numerical techniques more appealing. Using Iridis3&4, Computational Fluid Dynamics has been employed to simulate the turbulent wind flow around tall buildings and bridge decks. The research proposes novel numerical techniques for the analysis of vortex induced effects on these structures for an effective use in industry.

Fluid Dynamics Optimisation of Rim-Drive Thrusters and Ducted Hydrokinetic Generators

Aleksander Dubas, Suleiman Sharkh (Investigators)

This is a Knowledge Transfer Partnership project is a collaboration between the University of Southampton and TSL Technology Ltd. to develop computational fluid dynamics software design tools for modelling and optimising the design of propeller thrusters and water turbine generators.

Fluid Loads and Motions of Damaged Ships

Dominic Hudson, Ming-yi Tan (Investigators), Christian Wood, James Underwood, Adam Sobey

An area of research currently of interest in the marine industry is the effect of damage on ship structures. Research into the behaviour of damaged ships began in the mid nineties as a result of Ro-Ro disasters (e.g. Estonia in 1994). Due to the way the Estonia sank early research mainly focused on transient behaviour immediately after the damage takes place, the prediction of capsize, and of large lateral motions. Further research efforts, headed by the UK MoD, began following an incident where HMS Nottingham ran aground tearing a 50m hole from bow to bridge, flooding five compartments and almost causing the ship to sink just off Lord Howe Island in 2002. This project intends to answer the following questions:
“For a given amount of underwater damage (e.g. collision or torpedo/mine hit), what will be the progressive damage spread if the ship travels at ‘x’ knots? OR for a given amount of underwater damage, what is the maximum speed at which the ship can travel without causing additional damage?”

Fluid Structure Interactions of Yacht Sails

Stephen Turnock (Investigator), Daniele Trimarchi

The research is the main subject of the PhD topic. It regards the application of fluid structure interaction techniques to the domain of yacht sails simulation

Hybrid RANS/LES methods

Richard Sandberg (Investigator), Markus Weinmann

Novel hybrid RANS/LES methods are developed for more accurate and efficient simulation of flow over complex geometries.

Turbulence and tidal turbines

William Batten (Investigator), Tom Blackmore, Luke Blunden

The PhD research is focused on understanding the effects of turbulence on tidal turbines. The problem has been simplified using grid generated turbulence and actuator disc representations of tidal turbines.

Unsteady Aerodynamics of Wings in Extreme Conditions

Charles Badoe (Investigator), Neil Sandham, Zheng-Tong Xie

Sizing of civil aircraft is dictated by extreme loads experienced at the limits of flight envelope, for example during gust, turbulence or low speed manoeuvre. The project aims to understand the unsteady aerodynamic behaviour of wings in extreme conditions involving heaving motions near stall.

Vertical turbulence structures in the benthic boundary layer as related to suspended sediments

Hachem Kassem (Investigator), Charlie Thompson

There is a genuine need for better, more robust modelling of suspended sediment transport in the coastal zone, both to understand its morphological evolution and it's impact on biogeochemical cycling, ecosystems services and to guide engineering applications such as dredging and defence schemes against erosion and flooding.
The suspension of sediment in turbulent flows is a complex case of fluid-particle interaction, governed by shear stresses (momentum exchanges) at the bed and within the benthic boundary layer (BBL). The intermittent transfer of momentum is a manifestation of coherent turbulent vortex structures within the flow. The passage of such structures (or clusters of) is often related to perturbations of bottom sediment, which may be entrained and maintained in suspension if sufficient turbulent energy is provided. The first part of my PhD investigated the temporal and scale relationships between wave–generated boundary layer turbulence and event–driven sediment transport in oscillatory flow in the nearshore. This involved complex statistical, spectral, quadrant and wavelet analysis of high frequency nearshore measurements of turbulence and suspended sediments (medium sand), collected as part of the EU-funded Barrier Dynamics Experiment II (BARDEX II). The following step aims to develop a 3D numerical model in OpenFOAM which would reproduce the fine scale turbulence structures observed over a fixed rippled bed in oscillatory flow. The 3D velocity field, turbulent components, correlations (stresses) and quadrant structures will then be linked to observed sediment resuspension events. The model will be validated against a set of laboratory experiments undertaken at the Fast Flow Facility at HR Wallingford.

Wind Turbine Blade Flow in Abnormal Environments

Zheng-Tong Xie (Investigator), Yusik Kim

Large wind turbines are being installed throughout UK and often in regions with complex meteorology and/or topography (e.g. involving wind gusts, turbulence, icing), which affect turbine performance (energy output, noise emission etc), life expectancy and safety. It is very expensive to conduct experiments to study such problems. This proposal suggests, firstly, an LES study of low-Re flows around an oscillating airfoil, to investigate the transition, separation, vortex shedding and dynamic stall behaviour. Secondly, a combined LES-RANS approach (with, e.g., a transitional RANS model in the near wall region) will be carefully designed (using our recently developed efficient turbulence generator at the interface between LES and RANS) and validated against low-Re results.