Modelling and Simulation of Intermittent Flows

Lead Research Organisation: University of Leeds
Department Name: Process, Environmental and Material Eng


In recent years, the use of Reynolds-Averged Navier-Stokes (RANS) based CFD within industry has led to a sea change in the design and development approaches used across all aspects of engineering, allowing more efficient design, reducing development costs, and bringing products to market more rapidly and economically than was previously possible. Because of computer run time constraints, RANS approaches will remain the principal method for representing the effects of turbulence for engineering application for the foreseeable future. However, the turbulence models embodied within such approaches are still dependent on the flow configuration being examined, requiring the use of experimental data in their specification, and case-by-case adjustment to accurately predict particular types of flow. Turbulent shear flows with free boundaries display an intermittent character, such that the fluctuations of velocity rapidly change from rotational to irrotational, and vice versa. Intermittency is important in many practical flows, for example, in boundary layer transition which remains a major challenge because of its relevance to the design of modern compressors and turbines, and airfoils. Intermittency is also important in the safe and efficient operation of many combustion devices, which are dependent on ignition occurring in flows with significant intermittency, as well as in hazard and risk assessments concerning the potential ignition of flammable releases on chemical and process plant. In all these applications, the enhanced performance of these devices, and improved safety, relies on a detailed understanding of turbulent flow and ignition processes, and hence intermittency. The majority of engineering turbulence models in use today were derived for fully developed flows, and hence cannot be expected to accurately predict free shear flows where the outer regions are contaminated with irrotational flow. The development of turbulence models that accommodate intermittency effects is therefore also of fundamental importance. Additionally, the inclusion of such effects helps to generalize existing turbulence models, and provide more accurate predictions of the velocity and scalar fields of interest to the applications noted above. The necessity for such work has also been identified by both the EROFTAC Special Interest Group on Transition Modelling, and the Isaac Newton Institute for Mathematical Sciences Programme on Turbulence, particularly in relation to the need for greater universality in engineering models of turbulence, and the improved modelling of the influence of intermittency, particularly on scalar fields. This proposal concerns the use of an hierarchical approach to the development of more accurate engineering models of intermittent turbulent flows through the co-ordinated application of direct numerical simulation and large eddy simulation to inform the formulation and validation of second-moment turbulence closures that are more fundamentally based that existing approaches. The work proposed will also improve our overall understanding of such flows, the universality of engineering models of turbulence, and will validate the models developed for application to a wide range of practical flows.


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Description Direct numerical simulation (Cranfield) and large eddy simulation (Leeds) were applied to study external intermittency in low Reynolds number turbulent round and plane jets. Significant differences were observed between predictions of the two techniques, indicating a requirement for improved sub-grid scale modelling for use with LES and, potentially, separate intermittency scaling allowance for the scalar and velocity fields. At high Reynolds number, LES of round jets showed good agreement with experimental data, with predicted intermittencies in close accord. Nevertheless, improvements are warranted, and the use of improved sub-grid scale modelling could again be beneficial. A finite-volume method coupled to a novel adaptive mesh refinement (AMR) numerical solution technique was also used (Leeds) as the basis of a Reynolds-averaged Navier-Stokes model of intermittent flows, based on solutions of the transported probability density function equation for scalar variables obtained using AMR applied in both physical and compositional space. Comparisons with data and LES for high Reynolds number round jet flows demonstrated good agreement, with the inclusion of intermittency effects improving the accuracy of predictions obtained over standard turbulence modelling approaches, thereby broadening the applicability and improving the generality of these models. Overall, the project delivered an improved understanding of intermittency in turbulent flows, and more accurate, validated engineering models of such flows for use in practical application.
Exploitation Route The understanding and techniques developed are of strategic value to others working in the UK academic sector in the fields of chemical, mechanical and process engineering, and combustion equipment, turbo-machinery, gas turbine and aircraft design, and at a more generic level to those concerned with turbulence and turbulent reacting flow modelling for a wide range of applications. This is particularly the case for applications where assessments of safety, efficiency and environmental impact are of importance. The methods developed are also available for use by industry in establishing the characteristics of intermittent flows. This will aid combustion equipment, turbo-machinery, gas turbine and aircraft manufacturers and operators in designing efficient and safe devices. It will also assist the designers and operators of chemical and process plant in permitting accurate safety and risk assessments. The engineering models developed have been applied and extended in subsequent EC FP7 Cooperation Work Programme grants: Quantitative Failure Consequence Hazard Assessment for Next Generation CO2 Pipelines (CO2PipeHaz) and Techno-Economic Assessment of CO2 Quality Effect on its Storage and Transport (CO2QUEST). They are also being applied within the National Grid COOLTRANS project. The latter programme of research was initiated by National Grid to address knowledge gaps relating to the safe design and operation of onshore pipelines for transporting dense phase CO2 from industrial emitters in the UK to storage sites offshore.
Sectors Aerospace, Defence and Marine,Chemicals,Energy,Environment

Description The models developed were used in making predictions of relevance to pipeline transport hazards associated with carbon capture and storage schemes. These predictions were used in turn by National Grid to validate their own to models employed during the Front-End Engineering Design (FEED) studies carried out as part of the UK's CCS Commercialisation Competition.
First Year Of Impact 2013
Sector Chemicals,Energy,Environment
Impact Types Societal,Economic