UA-33602782-1
Apr28

## How to Shrink Wrap a biomedical STL file in Fluent Meshing

A How-To guide for Fluent Meshing's new Shrink Wrap tool which provides a powerful, easy-to-use solution for meshing complex STL geometries. This is a step-by-step guide on how to produce a high-quality CFD mesh for an abdominal aorta, imported in medium-resolution STL format (NIH).

Sep26

## Turbulence Part 5 - Overview of Scale-Resolving Simulations (SRS)

An increasing number of industrial CFD users are recognising the need to move away from RANS modelling and resolve a greater spectrum of turbulence (particularly in cases involving large-scale separation, strongly swirling flows, acoustics, etc.). Here we present an overview of Scale Resolving Simulation techniques and important considerations when considering applying SRS to your project.

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...

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...