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Tips & Tricks: Estimating the First Cell Height for correct Y+
Jul01

Tips & Tricks: Estimating the First Cell Height for correct Y+

In previous posts we have stressed the importance of using an appropriate  value in combination with a given turbulence modelling approach. Today we will help you calculate the correct first cell height () based on your desired  value. This is an important first step as the global mesh resolution parameters will also be influenced by this near-wall mesh as well as the Reynolds number. Let's review the two main choices we have in choosing a near-wall modelling strategy: Resolving the Viscous Sublayer Involves the full resolution of the boundary layer and is required where wall-bounded effects are of high priority (adverse pressure gradients, aerodynamic drag, pressure drop, heat transfer, etc.) Wall adjacent grid height must be order  Must use an appropriate low-Re number turbulence model (i.e. Shear Stress Transport) Adopting a Wall Function Grid Involves modelling the boundary layer using a log-law wall function. This approach is suitable for cases where wall-bounded effects are secondary, or the flow undergoes geometry-induced separation (such as many bluff bodies and in modern automotive vehicle design). Wall adjacent grid height should ideally reside in the log-law region where  All turbulence models are applicable (e.g. Shear Stress Transport or k-epsilon with scalable wall functions) During the pre-processing stage, we need to estimate the first cell height ( ) so that our  falls within the desired range. The computed flow-field will dictate the actual  value which in reality will vary along the wall.  In some cases, we may need to locally refine our mesh to achieve the desired  value in all regions.   So how to calculate the First Cell Height for a desired Y+ value?   Firstly, we should calculate the Reynolds number for our model based on the characteristic scales of our geometry such that: , where  and  are the fluid density and viscosity respectively,  is the freestream velocity, and  is the characteristic length (e.g. pipe diameter, body length, etc.). The definition of the  value is such that: The target  value and fluid properties are known a priori, so we need to calculate the frictional velocity , which is defined as: The wall shear stress,  can be calculated from skin friction coefficient, , such that: The ambiguity in calculating  surrounds the value for . Empirical results have been used to provide an estimate to this value:  Flow Type   Empirical Estimate Internal Flows External Flows   We can then input these known values into the above equations to estimate our value for  . When considering simple flows and simple geometry, we might find this correlation is highly accurate.  However, when considering complex geometry, refinement in the boundary layer may be required to ensure the desired  value is achieved.  In these cases, you can choose to re-mesh in ANSYS Meshing or use anisotropic mesh adaption (ie. adaption of local cells only in...

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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...

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Tips & Tricks: Size Controls in ANSYS
Dec07

Tips & Tricks: Size Controls in ANSYS

In our previous post we discussed the importance of geometry preparation, utilising the ANSYS Global Mesh Control settings and the best use of the ANSYS Meshing Advanced Size Functions.  We will now focus on the ability to target specific areas of the mesh for additional refinement and mesh control, which is recommended for intermediate to advanced users.   By using these explicit sizing controls, the resolution of the geometry can be accurately captured and we can ensure that we accurately resolve any high gradient areas in the flow, such as a wake or separation/recirculation zones.   As you would expect, in addition to the automated settings, ANSYS Meshing provides users with as much additional control as they desire: with the option to specify combinations of point controls, edge controls, surface controls and/or body controls.  Each one of these has its own options and can be used to influence the mesh in different ways.   Point controls can be used to control the mesh based on a sphere of influence, whereby any region of the mesh that is contained within the sphere will be given the chosen sizing.  An example of this is shown below.   Point Sizing (using Sphere of Influence)   Edge Sizing Controls can be given to any edge and allow for different types of bias control, whereby the mesh size varies along the edge.  In the image below, we have applied an edge sizing to a single edge.   Basic Edge Sizing The edge sizing feature can use either a specified element size, a number of divisions along the edge or can be controlled by a sphere of influence.  This last option will refine along the edge only where it is intersected by the sphere of influence, and we will show this later on in this blog post.   Within the edge sizing function, you have the ability to set a local growth rate for all cells that grow away from this edge (overriding the global growth rate).  There are also useful options for Edge Bias control, which allows you to define how the mesh is distributed along the length of the edge.  For example, you can force the mesh along the edge to vary from large to small, small to large; or more complicated bias such as large to small in the middle then back to large, or small to large in the middle and then back to small again at the other end of the edge.   So it is clear that there are quite a few options available to control edge meshing, and since it is such an easy tool to use, we'd suggest that you simply have a...

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