Optimal Fluid Dynamics with Ansys CFD

Ansys CFD tools like Fluent or CFX, along with the dif­fer­ent avail­able options for grid gen­er­a­tion, are a popular choice for engi­neers when it comes to eval­u­at­ing the fluid-dynamic behavior of their designs. They provide valuable insight into the per­for­mance measures of interest. But even more, auto­mated opti­miza­tion and design explo­ration work­flows involv­ing CFD can be realized. These sig­nif­i­cantly augment the devel­op­ment process by leading to better designs, short­en­ing devel­op­ment times, and reducing design cycles, as well as increas­ing the knowl­edge about the product’s behavior, i.e., the influ­ence of various para­me­ters on its per­for­mance, early in the design process, when the freedom for making deci­sions is largest.

These auto­mated processes require a suitable CAD tool that can reliably produce the dif­fer­ent geometry variants to be analyzed. In our expe­ri­ence, this is a crucial bot­tle­neck that engi­neers commonly encounter in setting up and running a design explo­ration or opti­miza­tion process, with typical symptoms such as:

• Geometry vari­a­tion with tra­di­tional CAD systems is often tedious or prone to failure, i.e., some or even many of the variants fail to regen­er­ate when changing para­me­ter values.
• It is dif­fi­cult to consider, or even auto­mat­i­cally fulfill, given con­straints (e.g., due to man­u­fac­tur­ing or packaging).
• The quality of the CAD model might not be suitable for sim­u­la­tion (e.g., w.r.t. water­tight­ness or level of detail).
• Too many para­me­ters make opti­miza­tion inefficient.
• Not enough control is given over critical geo­met­ri­cal properties.

For the most part, our design tool CAESES® is a highly spe­cial­ized CAD system that specif­i­cally tackles the afore­men­tioned problems. It is focused on para­met­ric modeling and vari­a­tion of complex – often free-formed – geome­tries for the purpose of design studies and opti­miza­tion. It provides effi­cient para­me­ter­i­za­tion methods that offer a high degree of flex­i­bil­ity while using less para­me­ters, superior robust­ness, com­pre­hen­sive capa­bil­i­ties for inte­grat­ing con­straints, and sim­u­la­tion-ready exports that require no manual pro­cess­ing. Fur­ther­more, it can be fully inte­grated into Ansys Work­bench to be used as a depend­able geometry gen­er­a­tor for design explo­ration and optimization.

Ansys CFD Workflow Integration

Devel­oped with the above-men­tioned concerns in mind, the CAESES® app for Ansys Work­bench gives access to the full capa­bil­ity of the CAESES® para­met­ric geometry modeling and vari­a­tion platform. It enables you to insert any CAESES® geometry model into the user inter­face and the work­flows of Ansys Work­bench. With just a few clicks, you are ready to run large studies, such as design of exper­i­ments and formal opti­miza­tions – every­thing fully automated.

So how does the inter­fac­ing actually work? First, a para­met­ric geometry model is created in the CAESES® user inter­face, includ­ing a set of design vari­ables that control its shape. Also, the geometry export is con­fig­ured: Usually, the ACIS (*.sat) format is used, which allows to transfer addi­tional infor­ma­tion for repeat­edly iden­ti­fy­ing the dif­fer­ent parts of the geometry. Finally, a script file is auto-gen­er­ated, which controls the CAESES® batch process (i.e., opening the model, setting the para­me­ter values and export­ing the updated geometry).

A crucial com­po­nent of this setup is that you are able to assign colors with user-defined names to the indi­vid­ual faces of the model in CAESES®, which are then trans­ferred as iden­ti­fiers to Ansys Work­bench by the geometry file. This is required to automate the meshing pro­ce­dure, where you need to ref­er­ence the dif­fer­ent patches of the model (by using ​“named selec­tions”). The color names are later shown in the Ansys Mesher, Design­Mod­eler, or SpaceClaim.

In Ansys Work­bench, you just have to install the CAESES® app to make CAESES® avail­able as a com­po­nent. Load the script file for your current project through the CAESES® com­po­nent and update it. The gen­er­ated geome­tries are exported and loaded into Ansys Work­bench auto­mat­i­cally, where they can be con­nected to other workflow components.

After the update of the CAESES® com­po­nent in Ansys Work­bench, the design vari­ables of the geometry are auto­mat­i­cally shown in the para­me­ter set. New design can­di­dates can now be gen­er­ated by changing these para­me­ters, either manually or by opti­miza­tion tools (DesignX­plorer, optiS­Lang, etc.).

Apart from the pre­vi­ously described general purpose app, two more CAESES® apps for Ansys Work­bench are avail­able: the CAESES® Tur­bo­Grid app that is ded­i­cated to bladed tur­bo­ma­chin­ery geome­tries and uses the pro­pri­etary Tur­bo­Grid format for the transfer of the geometry, as well as an inter­face app, which couples Ansys Work­bench to CAESES® and allows running opti­miza­tions in the CAESES® opti­miza­tion environment.

Ansys CFD Opti­miza­tion Case Study: Gas Turbine Stage

A popular appli­ca­tion for both Ansys CFX and CAESES® is tur­bo­ma­chin­ery. CFX is one of the leading CFD software solu­tions for tur­bo­ma­chin­ery appli­ca­tions and provides stream­lined work­flows for setup and post­pro­cess­ing, as well as suitable com­pu­ta­tional models. CAESES®, on the other hand, comes with ded­i­cated para­met­ric modeling envi­ron­ment for all tur­bo­ma­chin­ery com­po­nents, like axial and radial blades, casings, and volutes.

The subject of this inves­ti­ga­tion is an axial turbine stage con­sist­ing of a stator with 30 vanes and a rotor with 59 blades. Fol­low­ing boundary con­di­tions are applied:

• 25,000 rpm
• 400 kPa total pressure at the inlet
• 1,000 K inlet temperature
• 262.4 kPa static pressure at the outlet
• A mass flow of approx­i­mately 6.3 kg/​s at the outlet

The objec­tives of the opti­miza­tion are to maximize the isen­tropic effi­ciency, as well as the power.

Geometry Para­me­ter­i­za­tion

The geometry para­me­ter­i­za­tion is based on a para­met­ric 2D airfoil def­i­n­i­tion, which gets trans­formed to a stream surface and obtains its cor­re­spond­ing set of para­me­ter values from radial dis­tri­b­u­tion func­tions. Using this approach, the exact 3D airfoil shape can be derived for any arbi­trary radial position and the blade surface can be gen­er­ated as a con­tin­u­ous sweep. The design para­me­ters are then applied to the shape of the radial dis­tri­b­u­tion func­tions, as opposed to discrete airfoil sections, allowing for a flexible but effi­cient vari­a­tion of the geometry. Addi­tion­ally, the shape of the (leading edge) stacking curve can be con­trolled for further variability.

7 design para­me­ters are defined for rotor blade and stator vane, respec­tively, for a total of 14 opti­miza­tion vari­ables. These control:

• Stagger angle
• Leading edge radius
• Leading edge wedge angle
• Leading edge metal angle
• Trailing edge metal angle
• Cir­cum­fer­en­tial bow (stator) or lean (rotor)
• Axial bow

Workflow Inte­gra­tion

The ded­i­cated CAESES Tur­bo­Grid app is used to inte­grate CAESES in the Work­bench process. This uses custom exports to transfer the bladed geometry to Tur­bo­Grid as either sec­tional data or surface geometry. The exports take care of all nec­es­sary prepa­ra­tion steps to provide files that can be inter­preted in the auto­mated process by Tur­bo­Grid. Sec­tional data include point data for a given number of blade sections, as well as hub and shroud contour, while the surface geometry also includes boundary/​feature curves and is exported in ICEM Tetin format.

Two separate Tur­bo­Grid instances are used for stator vane and rotor blade and the combined case is solved in CFX, with a com­pu­ta­tion time of 5 – 10 minutes per variant.

Opti­miza­tion Process and Results

While, strictly speaking, a formal opti­miza­tion was not carried out, the inves­ti­ga­tion con­sisted of a thorough design space explo­ration with 268 design using a Latin Hyper­cube Sampling process through the Ansys DesignX­plorer. The results formed a rather well defined ​“Pareto” frontier, from which improved designs could easily be iden­ti­fied. Fur­ther­more, the results showed a most pro­nounced cor­re­la­tion of the objec­tive func­tions with the trailing edge metal and stagger angles for both rotor and stator.

This design study resulted not only in two sig­nif­i­cantly improved design can­di­dates, but also demon­strated the efficacy of the sim­u­la­tion-driven design approach using Ansys CFD in com­bi­na­tion with a powerful geometry pro­cess­ing, as provided by CAESES®.

More Infor­ma­tion

Watch the webinar record­ing about this case study here.

See this overview for the pos­si­bil­i­ties and capa­bil­i­ties that CAESES® offers for tur­bo­ma­chin­ery design.

Ques­tions?

Please do not hesitate to get in touch with us if you have ques­tions in the context of your specific appli­ca­tion. We look forward to dis­cussing it together with you!