Podcast
Questions and Answers
What is the primary goal of performing a mesh independence study?
What is the primary goal of performing a mesh independence study?
What action should be taken if the monitor point values from Step 2 do not match the values from Step 1 within the allowable tolerance?
What action should be taken if the monitor point values from Step 2 do not match the values from Step 1 within the allowable tolerance?
If monitor point values match within 5% in a mesh study, what does it indicate?
If monitor point values match within 5% in a mesh study, what does it indicate?
Why is it preferred to use three levels of mesh during a convergence study?
Why is it preferred to use three levels of mesh during a convergence study?
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How does the accuracy of the mesh and boundary conditions affect the 'converged' solution?
How does the accuracy of the mesh and boundary conditions affect the 'converged' solution?
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Which of the following best describes 'convergence' in computational fluid dynamics?
Which of the following best describes 'convergence' in computational fluid dynamics?
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What is the significance of defining correct boundary conditions in CFD modeling?
What is the significance of defining correct boundary conditions in CFD modeling?
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What does a significant difference in mass flow rate at inlets and outlets generally indicate?
What does a significant difference in mass flow rate at inlets and outlets generally indicate?
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Which method captures large eddies directly and is more accurate than RANS?
Which method captures large eddies directly and is more accurate than RANS?
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In a compressible flow scenario, what is the pressure value represented by P?
In a compressible flow scenario, what is the pressure value represented by P?
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Which boundary condition can be changed after mesh generation?
Which boundary condition can be changed after mesh generation?
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What does symmetry plane in CFD allow for?
What does symmetry plane in CFD allow for?
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Which of the following is NOT a guideline for specifying well-posed boundary conditions?
Which of the following is NOT a guideline for specifying well-posed boundary conditions?
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What type of boundary condition is often used to set a defined velocity inlet?
What type of boundary condition is often used to set a defined velocity inlet?
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What is the primary purpose of performing a Mesh Independence Study?
What is the primary purpose of performing a Mesh Independence Study?
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Which condition must be carefully defined to ensure accurate simulation in CFD?
Which condition must be carefully defined to ensure accurate simulation in CFD?
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What describes periodic boundary conditions in CFD?
What describes periodic boundary conditions in CFD?
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What is necessary for creating profile boundary conditions from experimental data?
What is necessary for creating profile boundary conditions from experimental data?
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Study Notes
CFD Setting Up Physics
- This module covers computer-aided design and engineering (REE470) in Fall 2024.
- Computational fluid dynamics (CFD) is used in the process.
- Inlet and outlet conditions are visualized.
- Velocity, pressure, and other related parameters are monitored.
Lecture Outline
- Recall of convergence, mesh independence, and validation methods.
- Introduction to material properties.
- Cell zone conditions are defined.
- Boundary conditions are explained.
- Mesh interfaces are discussed.
- A summary of the lecture is presented.
Convergence, Mesh Independence Study and Validation
- The precision of the numerical solution relies on accurate mesh and boundary conditions.
- Steady-state simulation should satisfy three conditions:
- Residual RMS error values below 10⁻⁴ or 10⁻⁵.
- Monitor points for values of interest reaching a steady state.
- Imbalances in the domain less than 1%.
- Expected patterns or phenomena (e.g., separation bubble, vortex shedding) in the flow field should also be exhibited.
Convergence, Mesh Independence Study and Validation (Examples)
- Graphs of monitoring residuals illustrate convergence trends.
- Graphs show monitoring a value of interest, demonstrating a steady-state solution.
- Monitoring domain imbalances demonstrates stability.
- Critical values for the study are Residual RMS Error, Monitor points, and Domain imbalance.
Convergence, Mesh Independence Study and Validation (Further Details)
- Values of interest include pressure drop, forces, and mass flow rates.
- These values should converge to a steady state.
- Residual RMS Error values should be at least 10⁻⁴.
- Domain imbalance should be less than 1%.
Convergence, Mesh Independence Study and Validation (Mesh Independence Study)
- A single solution is generated for the given mesh.
- Next, mesh independence is ensured via refinements to the mesh.
- Monitoring points from Step 2 are compared with those from Step 1, ensuring consistency within tolerance (e.g., 5%).
- If values differ, refining the mesh and repeating the process is required.
- Three levels of mesh are generally preferred.
Convergence, Mesh Independence Study and Validation (Step 3)
- If the solution changes with refinement, a mesh-independent solution hasn't been achieved.
- Refinement and repetition are required until a mesh-independent solution is reached. Use the smallest possible mesh size to reduce simulation time.
Convergence, Mesh Independence Study and Validation (Example)
- Monitor values are plotted against the number of cells.
- Differences in calculated values with increasing mesh resolution can be identified, and a mesh-independent result using the smallest mesh size is presented.
Convergence, Mesh Independence Study and Validation (Further Discussion)
- Analysis of global values like average temperature at the outlet for accurate results and further analysis, and use of 6 million cells.
- Comparison of profiles at specific points (e.g., velocity or temperature.).
- Tables can be used to compare different levels of mesh with respect to their accuracy.
Validation with Experimental Measurements
- Validation is the process of determining a model's accuracy in representing the real world.
- Analytical solutions, experimental measurements, or literature computations from similar studies can be used for validation.
- Large eddies are critical for momentum transfer and turbulent mixing in fluids.
- The RANS approach models these.
Introduction
- Defining material properties, cell zones, and boundary conditions, using Fluent, is crucial.
- Accuracy of simulations relies on properly defining these aspects.
- Learning aims include defining material properties, boundary condition types, defining mesh interfaces and cell zones.
ANSYS Fluent Workflow
- This covers steps in setting up, simulating, and post-processing in ANSYS Fluent.
- Domain setting, physics setup, solving, and post-processing are described.
- Instructions will match the arrangement in COMSOL.
Setting Up Physics: Other Group Boxes
- There's no need to change solver settings unless a simulation is transient or involves high-speed compressible flow.
- Commonly used models include energy, heat transfer, radiation, viscous turbulence, multiphase, species, and discrete phase models.
Setting Up Physics: Create/Edit Materials
- Fluent offers a database of predefined fluids, solids, and mixtures.
- User-defined materials, accessed and modified via the standard materials panel, can be created.
Fluid Density
- In incompressible flows, density is constant.
- Ideal gas properties require specifying density as a function of temperature.
- Polynomial functions or laws for compressible flows are used to define density as a function of pressure and temperature.
Operating Pressure
- Operating pressure is an absolute pressure datum with other relative pressures measured from it.
- It is used to avoid round-off errors in simulations where dynamic pressure differences are small compared to the absolute pressure.
Cell Zones and Boundary Zones
- Cells in a mesh are grouped into zones.
- Each cell is bounded by faces that form face zones.
- Some of these faces form boundary zones, those faces on the boundaries of the model.
Setting Up Physics: Cell Zones
- Cell zones help assign materials to different regions (fluid, solid, porous, etc.) of the domain.
- Cell zone conditions are defined to specify properties for different cells.
Setting Up Physics: Cell Zones (Additional)
- A cell zone is a group of cells for which all active equations are solved.
- A fluid cell zone can be more simply understood as a group of cells for which all fluid equations are solved.
- A solid zone is a group of cells for which only the heat conduction equation is solved, flow equations are not solved.
Setting Up Physics: Boundaries
- Boundaries are defined and filtered by type (inlets, outlets, walls, etc.)
- Boundary types are pre-defined in the preprocessing phase.
- Boundary conditions can be changed after mesh generation for most, except for periodic BCs.
Defining Boundary Conditions
- Boundary conditions define dependent variables on domain boundaries.
- Examples include mass, momentum, and energy fluxes.
- Critical factors for boundary conditions include geometry, location, available data, and model constraints.
Available Boundary Condition Types
- External boundaries encompass different flow scenarios (pressure inlet, pressure outlet, velocity inlet, and outflow).
- Internal boundaries define components (fans, interior, porous jump, radiators, walls).
General Guidelines for Boundaries in CFD
- Flow direction at inflow/outflow boundaries should ideally be normal to the boundary.
- Large gradients normal to boundaries should be avoided if possible.
- Minimize grid skewness for accuracy of simulations.
Symmetry Planes
- Symmetry planes have zero normal velocity and zero normal gradients.
- No explicit inputs are required.
- The geometry and flow must be symmetric around the plane.
Specifying Well Posed Boundary Conditions
- Boundaries near recirculation zones should be placed downstream to allow the correct flow development.
- Location choices have effects on turbulence, temperature, and species-related boundary conditions.
Specifying Well Posed Boundary Conditions (Inlet/Outlet)
- Combination of velocity inlet and static pressure outlet is generally most robust, mass flow in and static pressure out are robust.
- For total and static pressure inlet with outflow, the combination is sensitive to initial guesses.
- Avoid using total pressure inlet with outflow type.
Specifying Well Posed Boundary Conditions
- Boundaries near recirculation zones should be placed downstream.
- Boundaries must be selected carefully for accurate results.
Specifying Well Posed Boundary Conditions (Domain Size)
- Computational domain should be large enough for accurate solutions, especially with high pressure gradients, where pressure gradients are large, the domain needs to be large.
Defining Boundary Conditions (Further Discussion)
- Boundary conditions affect the behavior of the solution, especially with convergence.
- Accurate choices depend on geometry and available data.
Periodic Boundary Condition
- Simplifies the simulation by reducing overall mesh size.
- Periodic conditions can be translational or rotational for specific geometries.
Summary
- Cell zones assign materials.
- Fluent has a material database.
- Boundary zones define BCs at internal/external boundaries, and mesh interfaces.
- Convergence and accurate results depend on correct boundaries.
Appendix (Tips & Tricks)
- Skewness is defined using angle deviation and relates to cell quality.
- Normalized skewness values help evaluate cell quality.
- Optimal cell size and cell quality are relevant to the mesh.
Appendix: Options for Defining Common Properties
- Options for defining density, thermal conductivity, viscosity, and specific heat are presented.
- These properties are defined as constant, dependent on temperature, through kinetic theory, and user-defined.
Case Setup Replication
- Use TUI commands or the file interface for case setup replication to transfer settings from a 2D case to a 3D case by changing the mesh.
Cell Zone Definition - Fluid
- Selecting a fluid material is crucial in defining the fluid zone contents.
- This is essential for multiple species or multiphase flows.
Cell Zone Definition - Solid
- Identifying the solid material is the essential requirement.
- Volumetric heat source rate can be defined.
Profile Data and Solution Data Interpolation
- Fluent allows interpolation of variable data at face and cell zones using profile and interpolation files.
- Profile files contain point data for different variables on zones.
- Interpolation files hold discrete data for variables on cell zones.
Profile Boundary Conditions (e.g. Velocity Inlet)
- Boundary conditions are applied in panels.
- Profiles are selected for input to the boundary conditions.
- Details on these conditions are in the user guide.
Boundary Zone: Velocity Inlet
- Velocity inlet is suited for incompressible flows, but not recommended for compressible flows, and uses UDFs, magnitudes and direction for input.
Boundary Zone: Pressure Inlet
- Pressure inlets allow static pressure or total pressure input.
- Pressure inlet is suitable for both compressible and incompressible flows.
- Turbulence data and total temperature are crucial for the correct treatment.
Boundary Zone: Mass Flow Inlet
- Mass flow inlets define total mass flow rate or mass flux.
- Suitable for both compressible and incompressible flows.
- Difficult to converge.
Boundary Zone: Pressure Outlet
- Outlet conditions are important for specifying the flow's exit with gauge pressure.
- Backflow information can be important and helpful for inlet and outlet definitions.
Boundary Zone: Outflow
- Outflow boundaries don't need pressure or velocity input but are appropriate only for fully defined exits.
- This boundary is unsuitable for unsteady flows with variable density or backflow.
Modelling Multiple Exits
- Pressure outlet or outflow boundaries can be assigned to model flows with multiple exits.
- Mass flow rate fraction from Flow Rate Weighting, varies from 0 to 1.
Boundary Zone: Wall
- No-slip conditions are applied at walls in viscous flows.
- Wall roughness can be defined for turbulent flows.
Axis Boundaries
- Axis boundaries are assigned at the center line for axi-symmetric 2D problems and are not used for 3D cases.
Other Inlet/Outlet Boundary Conditions
- Pressure far field and exhaust fan/outlet vent are discussed.
- Modeling for these specific elements using appropriate boundaries is described.
Periodic Boundaries
- Periodic boundaries are used to reduce mesh size and are based on rotational or translational periodicity.
Internal Face Boundaries
- Internal faces enable step changes in flow properties, crucial for implementing various physical models (fans, radiators, porous jump models).
Specifying Well-Posed Boundary Conditions (Turbulence)
- Turbulence intensity and viscosity ratio are crucial for specifying turbulence for complex flows.
- Good estimates are useful for preliminary simulations.
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