Turbulence and wall shear stress in unsteady internal flows with rough surfaces

Lead Research Organisation: University of Aberdeen
Department Name: Engineering

Abstract

Knowledge of the fundamental flow physics for steady flow over rough-walls has progressed steadily through experiments, and more recently through advanced numerical simulations using Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS). Well-founded engineering methods exist for calculating friction. In contrast, the study of unsteady flow and friction over rough walls is very limited and is mostly confined to open channel oscillatory flow, largely motivated by application to sediment transport under sea waves. For internal flows (pipe and duct flow), present understanding of unsteady flow and practical engineering models for predicting unsteady friction are limited primarily to smooth wall conditions and this despite the fact that most internal unsteady flows occur over rough boundaries. There are basic differences between the near-wall structure of flow and turbulence in smooth and rough wall flows which make it highly likely that unsteady flow dynamics over rough walls are significantly different from those over smooth walls, and the extent to which results relating to unsteady flow over smooth walls apply to rough wall conditions is unknown. This knowledge gap handicaps applications ranging from the development of advanced methods of leak detection in pipelines and the prevention of sonic booms from railway tunnels to optimising the control of hydro and nuclear power systems. The aim of the proposed research is to advance understanding of turbulence and wall shear stress in unsteady internal flows over rough surfaces, thereby underpinning the development of engineering models through an integrated programme of experimental, numerical and theoretical studies. The numerical simulations using DNS/LES will generate very detailed information on the turbulent flow behaviour, especially in the near-wall region extending below the roughness elements, but only for conditions of low Reynolds number and high relative roughness since computing resources required increase exponentially beyond these conditions. Complementary experiments will be carried out to produce data covering a greater range of flow conditions, more directly relevant to practical applications. Computational and experimental data will be analysed to quantify turbulence dynamics and wall shear stress in unsteady flows over rough surfaces.

Publications

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He K (2016) DNS study of a pipe flow following a step increase in flow rate in International Journal of Heat and Fluid Flow

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He S (2013) Turbulence in transient channel flow in Journal of Fluid Mechanics

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Pokrajac D (2014) River Flow 2014

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Seddighi M (2015) Turbulence in a transient channel flow with a wall of pyramid roughness in Journal of Fluid Mechanics

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Seddighi M (2011) A Comparative Study of Turbulence in Ramp-Up and Ramp-Down Unsteady Flows in Flow, Turbulence and Combustion

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Seddighi M (2011) Influence of a k-type Roughness on the Behaviour of Turbulence in an Unsteady Channel Flow in Journal of Physics: Conference Series

Related Projects

Project Reference Relationship Related To Start End Award Value
EP/G068925/1 01/07/2010 01/03/2011 £340,417
EP/G068925/2 Transfer EP/G068925/1 01/03/2011 31/12/2013 £296,649
 
Description The aim of the research is to advance understanding of turbulence and wall shear stress in unsteady internal flows over rough surfaces, thereby underpinning the development of engineering models. An integrated programme of experimental, numerical and theoretical studies have been conducted which has led to new understanding, new data and new modelling methodologies.

(1) Flows in nature and engineering systems are either laminar or turbulent. A laminar flow is calm and orderly whereas a turbulent flow is random and chaotic, which make them categorically different in terms of the drag on surfaces and mixing capabilities, for example. It is often the central design task to predict and control the location of the transition in fluid systems/devices, including airplanes, turbo-machineries, and wind turbine, to name but a few. Using direct numerical simulation (DNS), we have first the first time shown that the transient turbulent channel flow following a rapid flow increase should be treated as a laminar flow followed by a laminar-turbulent transition. This finding can have profound implications in the understanding, prediction and design of practical flow systems, and can lead to the development of new strategies for turbulence control, all of which are yet to be explored.
(2) We have observed the initial pseudo-laminar flow developed followed by transition in accelerating channels flows in the Fluids Lab at the University of Sheffield, which has consolidated the theory established based on numerical simulations.
(3) We have further shown using numerical simulations and in the Fluids Lab that, when the surface is rough, the transient flow is a roughness-induced laminar-turbulent transition. This transition closely resembles roughness-induced transition in boundary layer flow over an isolated roughness element.
(4) We have shown that a popular turbulence model (the Launder-Sharma low Reynolds number model) implemented in a widely used computational fluid dynamics (CFD) package FLUENT is flawed, and demonstrated that this is likely to be associated with the formulation chosen to be used in the code rather than the numerical methods.
(5) We have systematically assessed the performance of a number of widely used turbulence models against newly generated direct numerical simulations (DNS) of unsteady flows, and have shown that only very few of them can reasonably reproduce the main unsteady characteristics of the flow and turbulence. These include an early model (Launder-Sharma 1972) and a more recent model due to Langtry and Menter (2009).
(6) The validity of assumed frozen-viscosity conditions underpinning an important class of theoretical models of unsteady wall-shear stress in transient flows in pipes and channels is assessed using detailed CFD simulations. As a result, we have shown that that, for smooth-wall flows, models based on a wholly-laminar viscosity distribution might out-perform one of the most commonly used Vardy-Brown method even though their representation of individual analyses of steady or unsteady flows are much poorer. Convolution models based on more realistic distributions of the effective viscosity in the wall region are expected to out-perform either of the above methods.
Exploitation Route A number of journal papers have been published over the duration of the project and more are in preparation. These systematically report our new theory, modelling methodologies, and new understanding, which are likely to benefit fluid dynamists and modellers.
Sectors Aerospace, Defence and Marine,Energy,Environment,Transport

URL http://www.sheffield.ac.uk/heft/uforce
 
Description The ultimate goal of the research is to improve the design of engineering flow systems involving complex transient flows, but the present project is fundamental in nature and the immediate beneficiaries are researchers and academics who work in the field of unsteady flows and those who develop models for them. The research papers which have resulted from this project have started attracting a good number of citations, demonstrating the impact of the research.
First Year Of Impact 2014
Sector Aerospace, Defence and Marine,Energy,Environment,Transport
Impact Types Economic