Tips on modelling non-Newtonian fluid viscosity
Many fluids we encounter in industry do not strain linearly with respect to viscous shear and are thus considered non-Newtonian. This post explores how to model non-Newtonian viscosity of fluids in ANSYS CFD, using blood as an example.
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).
Using CFD to predict flow-generated noise and other aeroacoustic effects
Flow-generated noise can have significantly adverse effects on our everyday lives. Product designers and engineers at the world’s most innovative and successful companies have recognised this fact, and are increasingly using CFD to incorporate noise mitigation strategies into their product design process.
How can I drive Fluent UDF Parameters directly from ANSYS Workbench?
Our support team is tasked with helping our customers to extract maximum value from their CFD simulations, and we are always striving to help customers who work at the bleeding edge of CFD. A common question is: How can I drive Fluent UDF parameters directly from ANSYS Workbench? The ANSYS Fluent User Defined Function (UDF) framework gives Fluent users an almost unlimited ability to modify the physics solved in their simulation model. Customisation can extend from simple properties such as boundary condition profiles, through to complex particle-fluid interaction laws. The ANSYS Workbench interface provides the infrastructure to specify parameters that can be used to drive any simulation inputs (such as geometric dimensions or boundary condition values). By coupling the functionality of Workbench Parameters with Fluent UDF's, we can realize the ability to perform parametric studies on any parameter imaginable. The process for coupling Workbench Parameters into Fluent UDF's is as follows: In an open Fluent Window; first initialise the scheme variables in Fluent. In the TUI, type the following line: (rp-var-define ‘leap 0.0 ‘real #f) Repeat the above line of code for every variable needed by replacing “leap” with a representative variable name. This name will be used to call the variable in the UDF. In the Fluent window, go to "Define">"Parameters". In the displayed window, go to "More" > "Use In Scheme Procedure". Click the "Select" button next to the Input Parameter box at the top and click “New Parameter” to create a new parameter. Name the parameter with a representative name. This will be the name of the parameter referenced in Workbench. Click OK on the two windows until you get back to the “Use Input Parameter in Scheme Procedure” window. In the Scheme Procedure box, type the following Scheme code: (lambda (param) (rpsetvar ‘leap param)) Click the “Define” button to link the WB parameter to the UDF accessable scheme parameter. Do the same for any subsequent parameters, choosing unique and representative names for each. The parameters are now setup. To access the value in the UDF, use the following function in the source code: RP_Get_Real("leap") Where the argument in the Scheme name of the variable as defined in the rpsetvar command. Now that the associations are properly setup, you can change the value of the parameter in Workbench, causing Fluent to notify that there has been an upstream change. When you click OK, the parameter value will be pushed through to the Scheme variable. After the Scheme variable has been updated, any UDFs will have access to the new value when the RP_Get_Real("leap") function is called. NOTE: For best performance, assign the value returned from the...
Tips & Tricks: Calculating the Mean Age of Air for HVAC simulations in ANSYS CFD
Engineers who are tasked with designing heating, ventilation and air-conditioning (HVAC) systems for buildings will need to assess the indoor air quality to ensure optimum health and comfort for occupants and meet minimum regulatory requirements. Generally a HVAC CFD analysis will take into account variables such as air temperature, relative humidity, air species concentrations and velocity. Additionally, CFD engineers can use ANSYS CFD to solve for the local "mean age of air" (MAA) to assess the air quality within an indoor environment. By examining MAA across the habitable space within a building, engineers can quantify the air change effectiveness (ACE) of their ventilation system and confirm that their design meets NABERS/GreenStar regulations. How can I plot the Mean Age of Air in ANSYS CFD-Post? Usually designers are interested in local distributions, therefore it is useful to plot the ACE as the ratio between the nominal time constant and the age of air or its inverse. Within ANSYS CFD-Post, it is possible to calculate the ACE at a specified height with a simple expression defined using CEL. This allows us to quantify the area occupied by air with an age exceeding the mean value across a reference surface (typically located at breathing height, ie. 1m above floor). According to many regulatory standards, the area exceeding the target value must be less than 5%. The setup for the Normalised Age of Air variable (expressed as Age of Air/Nominal Time Constant). Note that when solving using FLUENT, a variable called "Scalar 0" will be present in the variable list. Its dimensions can be displayed by defining a new expression such as AgeofAir = Scalar 0 [s] and then creating a new variable which by definition is equal to AgeofAir. Clipping the Age of Air to a specific range of values This is useful to quantify the size of any pockets of air that may exceed our target MAA value (ie. the value prescribed by regulations). In order to visualize the air which is older than this nominal value, we create an iso-clip for a value greater than the target value, applied to a plane located at a specified height above the floor (typically 1 metre). The result should look similar to the one shown in the image below. The "holes" are regions where the MAA values are above or below the threshold specified in your iso-clip setup (in this case, anything below 1200 secs and above 1300 secs). An iso-clip for the Age of Air Variable. It is then straightforward to create an expression that quantifies the % area of the iso-clip to the full area of the plane. Typically engineers aim to ensure that this does not exceed...
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.
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