{"id":4858,"date":"2012-01-06T16:27:58","date_gmt":"2012-01-06T05:27:58","guid":{"rendered":"https:\/\/www.computationalfluiddynamics.com.au\/?p=220"},"modified":"2012-01-06T16:27:58","modified_gmt":"2012-01-06T05:27:58","slug":"tips-tricks-inflation-layer-meshing-in-ansys","status":"publish","type":"post","link":"https:\/\/www.leapaust.com.au\/blog\/cfd\/tips-tricks-inflation-layer-meshing-in-ansys\/","title":{"rendered":"Tips & Tricks: Inflation Layer Meshing in ANSYS"},"content":{"rendered":"
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Throughout our first set of Tips & Tricks posts relating to meshing controls and meshing methods,\u00a0we have made mention of\u00a0Inflation Layers a number of times.\u00a0 Let’s take this opportunity to explain exactly why inflation layers are a critical component of a good CFD mesh and how we can create them easily within ANSYS Meshing.<\/p>\n<\/div>\n

In our first posts on Mesh Sizing <\/a>we explained that\u00a0as well as capturing all key features of the geometry (using local sizing and the curvature size function), we also need to have a sufficiently fine mesh to adequately capture regions where the flow will experience rapid change in key variables such as pressure, velocity or temperature.\u00a0\u00a0 This initially led us to a better understanding of how we should apply Global and Local Mesh Controls.<\/p>\n

Now, if you think about moving a probe from the freestream flow towards\u00a0one of the\u00a0walls in your fluid domain, as you approach the wall you will notice that the velocity decreases non-linearly up to a point where the fluid will have zero velocity at the wall.\u00a0 This is what is termed the “no slip” wall condition in CFD.<\/p>\n

If we plot\u00a0a typical\u00a0velocity profile in the near-wall region, we can see that we have a large change in velocity in the wall normal direction and it is important to our CFD simulation that we capture this gradient correctly.\u00a0 To do this, we need to use inflation layer meshing to\u00a0accurately capture the boundary layer region for\u00a0any wall-bounded turbulent flows.\u00a0 The image below\u00a0plots the non-dimensional velocity versus the non-dimensional wall normal distance, with each line from top to bottom demonstrating the difference between a favourable pressure gradient through to adverse\u00a0pressure gradient with flow separation.\u00a0 It is clear that the flow behaviour in the near wall region is fairly complex and needs to be captured appropriately to have any confidence in our CFD results, especially\u00a0if we intend to report\u00a0key engineering\u00a0data such as separation points or pressure drops.<\/p>\n<\/div>\n

\"Near-wall<\/a>
Near-wall velocity profiles<\/figcaption><\/figure>\n

Providing a suitable inflation mesh for the geometry is strongly tied to the choice of the turbulence model, and the flow field we are\u00a0interested\u00a0in capturing. We can elect to resolve the complete profile of the boundary layer of alternatively we can make use of empirical wall functions to reduce the cell count (see our post on turbulence modelling and wall functions). If we refer to the images below,\u00a0on the left hand side we observe that the boundary layer profile is modelled with a reduced cell count, which is characteristic of a wall function approach.\u00a0On the right, the boundary layer profile is resolved all the way to the wall. This will provide a more accurate resolution of the boundary layer. For certain simulations such as flows with strong wall-bounded effects, this resolution is absolutely necessary. For other simulations, the added computational expense is not always\u00a0justifiable.<\/p>\n

\"wall<\/a>
Representation of Wall Function approach vs fully resolving the boundary layer<\/figcaption><\/figure>\n

At this point, we begin to understand that the placement of the first node\u00a0in our near-wall mesh is very important.\u00a0\u00a0We use a non-dimensional distance (based on local cell fluid velocity) from the wall to the first node, which we term the y+.\u00a0 To use a wall function approach for a particular turbulence model with confidence, we need to ensure that our y+ values are within a certain range.\u00a0 This topic is covered in detail here<\/p>\n

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y+ definition<\/figcaption><\/figure>\n

This topic will also be discussed more in a future post, but in general it is considered good practice to include between 10 and 15 inflation layers situated within the boundary layer of your flow to accurately resolve the boundary layer and accurately predict any separation or reattachment points.\u00a0\u00a0So now that we understand the importance of inflation layers to our CFD mesh, how can we set them up?<\/p>\n

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Inflation Layer setup and options<\/figcaption><\/figure>\n

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For a 3D body, we begin by\u00a0choosing the body (typically a fluid domain) that we want to create the inflation mesh within, and then identify the faces that we want to inflate from.<\/p>\n

For a swept body, we begin by selecting the source face, and then the boundaries or edges from which\u00a0we wish to inflate.\u00a0 This is a different approach from other 3D bodies as the source face is meshed first individually, which is then swept through the body.<\/p>\n

Once we have specified these boundaries to be inflated (either surfaces or edges),\u00a0then it is easy to define the number of inflation layers, inflation growth rate and select one of the following methods: Total Thickness, First Layer Thickness, First layer Aspect Ratio, Last Layer Aspect Ratio or the Smooth Transition option (which uses a ratio based on the local surface mesh size).<\/p>\n

As I mentioned earlier, we find that the First Layer thickness option is the best place to start, especially when you are trying to achieve a particular y+ value as this value can be parametrised in ANSYS Workbench so that any changes to the First Layer Thickness parameter will be proportional to the y+ values that you expect in your solution.\u00a0 The first layer thickness that you choose should be used to obtain the correct y+ value for the turbulence model and flow behaviour that you are dealing with, which we will discuss in a future post.<\/p>\n

If you have any questions related to this post or you would like some advice for your own inflation meshing, then please post a comment below or alternatively contact our LEAP ANSYS CFD Support Team.<\/p>\n<\/div>\n

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[vc_row][vc_column][vc_column_text] Throughout our first set of Tips & Tricks posts relating to meshing controls and meshing methods,\u00a0we have made mention of\u00a0Inflation Layers a number of times.\u00a0 Let’s take this opportunity to explain exactly why inflation layers are a critical component of a good CFD mesh and how we can create them easily within ANSYS Meshing.… Read More »Tips & Tricks: Inflation Layer Meshing in ANSYS<\/span><\/a><\/p>\n","protected":false},"author":3,"featured_media":923,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"neve_meta_sidebar":"","neve_meta_container":"","neve_meta_enable_content_width":"","neve_meta_content_width":0,"neve_meta_title_alignment":"","neve_meta_author_avatar":"","neve_post_elements_order":"","neve_meta_disable_header":"","neve_meta_disable_footer":"","neve_meta_disable_title":"","neve_meta_reading_time":"","footnotes":""},"categories":[323],"tags":[181,446],"class_list":["post-4858","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-cfd","tag-ansys-meshing","tag-inflation-layer-meshing"],"_links":{"self":[{"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/posts\/4858","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/comments?post=4858"}],"version-history":[{"count":0,"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/posts\/4858\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/media\/923"}],"wp:attachment":[{"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/media?parent=4858"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/categories?post=4858"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.leapaust.com.au\/blog\/wp-json\/wp\/v2\/tags?post=4858"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}