Energy Engineering - CFD for Energy Engineering

Project - Guidelines for the Report (from W. Malalasekera)

Projects

REPORTING/ DOCUMENTATION OF CFD SIMULATION INPUTS AND RESULTS In order to open up CFD simulations to independent scrutiny within industrial organisations it is essential to have a comprehensive and uniform system of reporting. This is also useful as a basis for archiving simulations for future use with the ultimate aim of preserving past learning experiences and spreading best practice throughout groups of users within an organisation. First we suggest a list of items necessary for documentation of user input. INPUT DOCUMENTATION • General description of the problem and purpose of CFD simulation • Code chosen for solution of problem • Computing platform used for run • Schematic diagram of the region of interest with all key dimensions, flow inlets and outlets • Boundary conditions – include comments on/justifications of assumptions made and known areas of approximation or lack of information • Initial conditions for transient flow simulations or field initialisations for steady flow studies • Fluid properties – include comments/justifications of assumptions and data sources • Modelling option selections: (i) laminar/turbulent + turbulence model + near -wall treatment, (ii) combustion model, (iii) other physical models − with comments/justifications on selections • Grid design: temporal mesh, space mesh including one or more diagrams of the grid that are sufficiently clear to illustrate the approach to mesh design – also give written comments on compromises and details of grid -independence study • Solution algorithm choices – particularly non -default choices; note that default settings may change as a CFD code evolves, so a comprehensive summary of all the main selections (first/second -order schemes, multigrid options, segregated/coupled solver etc.) is preferable for long -term archiving • Iterative convergence criteria choices: settings of truncation levels for residuals and choice of additional target quantities for convergence monitoring • Brief summary of particular aspects of simulation design that required special attention to get simulation to work and to get accurate results, also noting unresolved problem issues Next, we list items to assist scrutiny and confidence building in result analysis and reporting. RESULT INTERPRETATION AND REPORTING Alongside options for alphanumeric output, commercial CFD codes have the ability to produce a wide variety of result visualisations, such as: • Velocity vector plots • Streaklines and particle paths • Contour plots of flow variable • Profile plots • Grid display • View manipulation It is important to note that high -quality presentation is not necessarily synonymous with high -quality results. Less experienced users should not be taken in by the power of the post -processing capabilities of a CFD code. Before communicating the findings of a CFD study and drawing conclusions it is essential that the quality of the results is checked thoroughly by verification and validation. Below, we summarise the main elements to be checked and documented: • Verification study: give estimates of numerical errors. For high -accuracy work all key target quantities should be shown to be independent of iterative convergence criterion and mesh. Diagrams of spatial distributions of residuals can help illustrate regions of unacceptably high residuals even if global residuals are sufficiently low to indicate iterative convergence. Identify where compromises were necessary if the results are still grid dependent. • Quantification of input uncertainty: the main problem is generally specification of the boundary conditions; where necessary also consider fluid properties. • Validation study: summarise method used to validate CFD approach; outline how the AIAA building block approach was applied if a complex system is studied; give comments on how improved match with experimental data was achieved by changes to the modelling strategy. • Further confidence in the results can be built by analysis of the results using basic knowledge of fluid dynamics and conservation laws. This might involve consistency checks to identify where results are different from expectations. An obvious check would be a test of global mass, momentum, energy and species conservation by balancing the fluxes in and out of the region of interest with the sum of all sources and sinks inside the domain. We have previously noted that time constraints and computer resources often determine the acceptable degree of convergence of a CFD simulation. This means that global conservation checks will not show exact balance of all the relevant fluxes and rates of creation and destruction. However, a significant departure from global conservation indicates problems. Whilst we have made it clear that the only true quality check is validation, it is advisable to apply a range of common -sense quality tests where new flow problems are investigated. These can be based on a general understanding of fluid mechanics and/or specific knowledge of the application that is being studied. Here we give some items (trivial and profound) that might be checked when the outcome of a CFD simulation is evaluated: • Fluid flows from high to low pressure (in pressure -driven flows) • Static pressure decreases when velocity increases (Bernoulli’s theorem for inviscid flows) • Friction losses cause a decrease of total pressure in the direction of flow (viscous flow) • Entropy must increase in the flow direction in a flow without heat transfer (consequence of second law of thermodynamics) • The speed of a fluid near a stationary wall is smaller than the speed further away from the wall (boundary layer formation) • Flow adopts a fully developed state after a sufficiently long distance in a straight duct with constant cross -section • Boundary layers rapidly separate under the influence of an adverse pressure gradient (pressure increases in the direction of flow outside the boundary layer) • Flows will usually separate at corners • If a flow separates there is always recirculation • A flow emerging into a large expanse of fluid from a small hole generally forms a jet • Pressures are higher at the outside of a bend (or curved streamline) and lower at the inside due to centrifugal forces • Pressure increases with depth in a liquid due to gravity • Heat flows from regions of high to low temperature • Hot fluid rises and cold fluid sinks under the influence of gravity • Turbulence is generated in regions with sheared flow, i.e. where velocity gradients are high It is obviously not possible to give a comprehensive list of items, and we should aim to develop specific checks for the flow problem to be studied based on our knowledge of fluid dynamics, heat transfer etc. and comprehensive research of the background to the problem .