UA-33602782-1
Turbulence Part 4 - Reviewing how well you have resolved the Boundary Layer
May06

Turbulence Part 4 - Reviewing how well you have resolved the Boundary Layer

In recent posts we have comprehensively discussed inflation meshing requirements for resolving or modeling wall-bounded flow effects due to the turbulent boundary layer. We have identified the y-plus value as the critical parameter for inflation meshing requirements, since it allows us to determine whether our first cell resides within the laminar sub-layer, or the logarithmic region. We can then select the most suitable turbulence model based on this value. Whilst this theoretical knowledge is important regarding composite regions of the turbulent boundary layer and how it relates to y-plus values, it is also useful to conduct a final check during post-processing to ensure we have an adequate number of prism layers to fully capture the turbulent boundary layer profile, based on the turbulence model used (or more precisely, whether we aim to resolve the boundary layer profile, or utilize a wall function approach). In certain cases, slightly larger y-plus values can be tolerated if the boundary layer resolution is sufficient. How can I check in CFD-Post that I have adequately resolved the boundary layer? For the majority of industrial cases, it is recommended to use the two-equation turbulence models, or models which utilize the turbulent viscosity concept and the turbulent viscosity ratio (i.e. the turbulent viscosity over the molecular viscosity). We can make use of this concept to visualize the composite regions of the turbulent boundary layer, and ultimately visualize how well we are resolving the boundary layer profile. Consider the conceptual case-study of the turbulent flow over an arbitrarily curved wall. Prism layers are used for inflation, and tetra elements in the free-stream. Once we have calculated the solution, within CFD-Post we can create an additional variable for the eddy viscosity ratio. Then by plotting this variable on a suitable plane, and superimposing our mesh in the near-wall region, we can visualize the boundary layer resolution.                   Figure 1 provides an example of a reasonable wall function mesh. There is a good cell transition from the prisms to the free stream tetra elements. The y-plus we have prescribed at the first cell indicates we are in the logarithmic composite region of the turbulent boundary region, which is the region largely dominated by inertial forces and thus we have high levels of turbulence. The turbulence gradually dissipates as we approach free stream conditions (where the levels of turbulence are governed by inlet conditions), which is expected. At this stage, we could even reduce the number of cells in the inflation layer as we are clearly capturing the logarithmic region layer before approaching the free stream. Correspondingly, we could aim to reduce the...

Read More
Turbulence Part 3 - Selection of wall functions and Y+ to best capture the Turbulent Boundary Layer
Apr12

Turbulence Part 3 - Selection of wall functions and Y+ to best capture the Turbulent Boundary Layer

In recent posts in our series of Turbulence Modelling posts, we have covered boundary layer theory and touched on some useful meshing and post-processing guidelines to check you are appropriately resolving the boundary layer profile.  Today we will consider three critical questions that are often asked by CFD engineers when developing or refining a CFD simulation:   - Am I using the correct turbulence model for the type of results I am looking for? - Do I have an appropriate Y+ value and a sufficient number of inflation layers? - Am I using the right wall function for my problem? This topic is so important because we know that in turbulent flows the velocity fluctuations within the turbulent boundary layer can be a significant percentage of the mean flow velocity, so it is critical that we capture these effects with accuracy. A Reynolds averaging approach using turbulence models will provides us with an estimate of the increased levels of stress within the boundary layer, termed the Reynolds stresses. In order to appreciate the use of wall functions and the influence of walls on the turbulent flowfield, we should first gain familiarity with the composite regions of the turbulent boundary layer:                 In the laminar sub-layer region (Y+ < 5) inertial forces are less domineering and the flow exhibits laminar characteristics, which is why this is known as the low-Re region. Low-Re turbulent models (e.g. the SST model) aim to resolve this area and therefore require an appropriate mesh resolution to do this with accuracy. This is most critical for flows with a changing pressure gradient where we expect to see separation, as observed below.                       In the law of the wall region, inertial forces strongly dominate over viscous forces and we have a high presence of turbulent stresses (this is known as the high-Re composite region). If using a low-Re model, the whole turbulent boundary layer will be resolved including the log-law region. However, it possible to use semi-empirical expressions known as wall functions to bridge the viscosity-affected region between the wall and the fully-turbulent region.                     The main benefit of this wall function approach lies in the significant reduction in mesh resolution and thus reduction in simulation time. However, the shortcoming lies in numerical results deteriorating under subsequent refinement of the grid in wall normal direction (thus reducing the Y+ value into the buffer layer zone). Continued reduction of Y+ to below 15 can gradually result in unbounded errors in wall shear stress and wall heat transfer (due to the damping functions inherent within the wall...

Read More
10 key points covered in our CFD Meshing Tips & Tricks webinar
Aug15

10 key points covered in our CFD Meshing Tips & Tricks webinar

A quick thanks to the large number of customers in Australia and New Zealand who attended our July webinar on ANSYS CFD Meshing Tips & Tricks.  We've had many enquiries from people wanting to know more, so we thought we'd break the content down into the 10 key points below.  If you want more information on any specific point, then please contact us directly or post a comment in the field at the bottom of the page.    10 key points to a successful CFD Meshing strategy (taken from the demonstration throughout LEAP's webinar on CFD Meshing Tips & Tricks): Firstly, decide what mesh connectivity your problem requires (conformal / non-conformal) and how this will affect the setup of your geometry - single part, multi-body parts or separate bodies.  Utilise the tools available for geometry clean-up - using DesignModeler or SpaceClaim Direct Modeler - to quickly address any small surfaces, split edges, hard edges that are present in your CAD. Decide whether you can use a patch-conforming meshing approach (preferred for most CFD cases), or need to use a patch-independent approach to tackle dirty CAD geometry. Make use of the preview tools for previewing the surface mesh and inflation layers.  This can be a great time saver for large, complex models! Use the show tool to indicate if there are bodies that are automatically sweepable, and/or faces that can be easily map meshed. Make sure you are using the correct meshing preference - talk to us if you need more information on different meshing requirements for Mechanical, CFD, Explicit Dynamics problems. Understand when it is helpful to use Assembly meshing.  If Assembly meshing is part of your strategy, remember that Fluent should be selected as Solver type which gives you access to the Cut-cell and Cut-tet methods.  CFX users can use this same approach to generate Cut-tet meshes. If you are hex meshing, it is useful to understand how to use Sweep meshing controls most efficiently. The ability to display nodes and edge parametric directions are very handy! Don't forget you have the ability to use Virtual Topology and Pinch commands to cleanup geometry in ANSYS Meshing. If you are dealing with large assemblies and/or non-conformal meshes, the automatic contact detection is a tool you cannot live without. Use it to check connectivity of bodies with the new Body View tool. Don't forget the importance of 'group by none' for CFX users.  If you weren't able to attend the webinar, or did but simply want more information on what was covered, please let us know below or contact LEAP's technical support hotline.  For more information on upcoming events, please visit LEAP's webinar, training and events...

Read More
Tips & Tricks: Global Meshing Controls in ANSYS
Dec05

Tips & Tricks: Global Meshing Controls in ANSYS

Welcome to the first in LEAP's series of CFD Tips & Tricks blogs.  The topics for this first series of blog entries will focus on the selection of efficient and appropriate meshing methods, mesh sizing and mesh controls in ANSYS Workbench Meshing.  Meshing is one of the most important influences on CFD simulation accuracy, although it does not necessarily need to be the most time intensive.   In our experience, our ANSYS CFD customers become more confident and efficient with meshing as they gain a better understanding of the importance of certain mesh settings and the likely effect of these settings on final solution accuracy. Where to start? After defining your project scope and the key variables of interest (either geometric and/or boundary/operational conditions), the first step is to prepare the geometry and create the fluid domain for the CFD simulation.  The aim at this stage is to make sure that the geometry is as clean as possible before we create the fluid domain either in or around the body.   Within the ANSYS geometry products there are semi-automated cleanup options that can remove holes, spikes, sliver surfaces and other CAD related features that will cause meshing issues.  Small details such as fillets or bolt heads are often insignificant details which can be assumed to have no impact on the final solution (especially in terms of a key engineering quantity such as lift, drag, max velocity, max temperature, etc...).  Be careful and use your engineering judgement (in collaboration with your colleagues and team members) to ensure that you do not neglect geometric features which may have an effect on the physics or critical flow features!   To run the simulation we must take the clean geometry and discretise the fluid domain into a number of control volumes (often called cells or elements).   Firstly, we should remember that we want to capture the geometry with sufficient mesh resolution that we are solving for the correct shape of the fluid domain (ie. respecting the curvature of all geometry) and, secondly, we want to use sufficient mesh resolution to capture all key flow physics.  This is especially true in regions where we know upfront that there are likely to be large gradients in key variables (for instance, nonlinear changes in the flow velocity in the boundary layer adjacent to walls; or rapid changes in pressure or temperature within a mixing zone).   With this in mind, when first generating a mesh for a particular design, it helps to visualise and/or draw a schematic of the problem which identifies the expected flow behaviour.  Once you have run a simulation for a particular geometry, or a...

Read More
UA-33602782-1