Fluid Dynamics Technology Tips
Reliable Turbomachinery Blade Development with Aeromechanical Simulation
Engineers need advanced simulation tools to enable them to meet customer demands for more-efficient and reliable high-performance machines. Engineers must accurately predict aerodynamic performance across an increasingly wide range of speeds and operating conditions, and they also must guarantee reliability in the design. For example, they need to ensure that blade vibration will be damped across the operating range and that cyclic unsteady loading will not impact design life. Watch the video to see how reliable turbomachinery blades can be developed using ANSYS solutions.
ANSYS provides a complete workflow aimed at helping engineers design reliable turbomachinery blading.
- Automated, rapid and high-quality 3-D hexahedral blade row (rotor and stator) meshing
- Accurate CFD simulations to determine key performance indicators, like total pressure ratio and isentropic efficiency along the entire speed line
- Efficient transient blade row (TBR) models to simulate transient full 360- degree blade phenomena by simulating only a limited sector of the system
- Turbomachinery-specific workflow to ensure that all blade natural frequencies and modes of vibration are aerodynamically damped. This is made possible by determining frequencies and modes in ANSYS Mechanical, then importing them as blade deformation into ANSYS CFX, performing a transient CFD simulation under these deformation conditions and assessing the stability of the blade (for example, will the flow damp or excite the vibration?).
- Turbomachinery-specific workflow to determine stresses caused by unsteady flow pressure fluctuations on the blade. This is made possible by determining the unsteady flow pressure loads on the blade using TBR methods in ANSYS CFX. The information is then mapped to the blade geometry in ANSYS Mechanical and stresses are determined.
Previous Tech Tips
Engineers often deal with very large and complex geometries represented by CAD files or surface mesh files, which can have imperfections (holes or gaps that need to be closed before a fluid volume can be extracted). Resolution requirements can lead to large computational element meshes. Manually fixing all geometry imperfections requires a large amount of manual operations (and time); creating large meshes can be computationally time consuming.
ANSYS Fluent meshing has all the key technology needed to mesh complex or dirty geometry quickly: CAD import, hole and gap fixing, high-quality surface mesh creation and fast volume mesh creation. Fluent meshing has many advantages:
- Versatility: Either CAD or surface mesh can be imported.
- Ease of use: Size functions, which capture model features, can be displayed to provide feedback that feature capturing is adequate. The user can save size functions and re-use them directly whenever needed.
- Built-in intelligence: Before volume meshing is done, diagnostic tools find and fix problems in assemblies (gaps or holes), face connectivities (faces overlapping or intersecting); they also improve surface mesh quality.
- Accuracy: Improved wrapping tools capture geometry features; diagnostics tools determine how well the geometry features were captured. Various tools are available to further improve wrapping quality and accuracy when needed.
- Local surface remeshing tools locally improve surface mesh quality when needed, without having to remesh the entire geometry surface.
- Faster volume meshing (up to three-times speedup in prism layer generation)
- Excellent scalability of parallel meshing when generating tet/prism meshes. Performance is case dependent, but 92 percent scalability has been observed on a 42 million-cell mesh when using eight cores.
Learn more about ANSYS Fluent Meshing by viewing this webinar or the rest of the video series:
Multiphysics simulation is critical when predicting performance of a design in which many physics interact. ANSYS has a wide array of multiphysics solutions to gain insight into the issues: for example, fluid structure interactions (FSI) capabilities for thermal as well as fluid force/structure displacement studies for weakly or strongly coupled systems (one-way or two-way). Multiphysics interactions in electric motors include electromagnetic losses and thermal management:
- The magnetic field in the magnetic material heats the material. The heat loss can be predicted by a low frequency simulation tool (in this case, ANSYS Maxwell).
- This heat is diffused and the system is cooled by air, water or both. This thermal management simulation can be simulated by a CFD tool (ANSYS Fluent).
- Thermal management performance impacts the magnetic material temperature, which in turns impacts electric motor performance.
If you conduct single-physics simulation, you can only assume (but not be certain) what the magnetic material temperature is â€” which can lead to errors in predicting electric motor performance. In this example, it is as much as 18 percent error.
Managing such simulations can be complex if you do not have tools to connect them. ANSYS offers a state-of-the-art solution to this challenge:
- 2-D or 3-D? ANSYS Maxwell low-frequency simulation can be performed in either 2-D or 3-D. Whatever strategy you pursue, you can map heat losses to the 3-D CFD simulation in ANSYS Fluent.
- Steady or unsteady? The time scale of the thermal management simulation is much larger than the electric motor time scale (seconds vs. milliseconds). Simply running a fully coupled unsteady multiphysics simulation is time prohibitive. The solution is to run an unsteady Maxwell low-frequency simulation, time average the heat losses, and perform a steady CFD simulation in ANSYS Fluent.
- How do you easily manage the multiphysics simulation? Passing the information from one physics solver to the other then interpolating the results when needed can be daunting. ANSYS Workbench solves these challenges by automating these complex tasks. Multiphysics simulations can be created simply by dragging and dropping the key simulation tools in the ANSYS Workbench environment window.
Look through these materials for additional information:
Do Not Remesh. Morph Instead.
When you study and simulate variations of a given geometry, don’t spend time meshing each and every geometrical variation. Create only the initial mesh, then you can morph it to all geometrical variation thanks to RBF-Morph, a solution from an ANSYS partner.
- Mesh morphing is fast: Even a mesh with 10 million elements takes seconds to render.
- Mesh morphing recognizes parameters: You can use RBF-Morph with ANSYS DesignXplorer to automatically investigate a range of design parameters or perform design optimization studies.
- Mesh morphing is practical: With RBF-Morph, you can export any of the modified mesh geometry back to CAD.
These materials provide details about mesh morphing and RBF-Morph:
Imagine that you have to design a car and minimize its drag, or engineer a piping system and minimize pressure drop. In both cases, the actual shape of the design is the most important factor. When setting parameters for simulation, usually you define the shape and run parametric variations, often with the help of optimization tools. While this is a good approach, it has many limitations:
- Design shapes can be extremely complex, governed by hundreds of parameters (or more). It is impossible to consider all of them. How do you make sure that you select the relevant parameters?
- Even if you select one set of key design shape parameters, you still have a very large number of designs to evaluate. Simulating all these can be extremely time consuming.
For these reasons, you need a smart shape optimization tool — one that:
- Automatically identifies the section of the design (shape) that needs to be modified.
- Automatically guides shape optimization by determining how to modify the shape directly from simulation results, without the need for trial-and-error simulation run after run
- Quickly performs design shape optimization, requiring the minimum amount of simulations, and performing those simulations as fast as possible
What is the smart shape optimization tool?
The ANSYS smart shape optimization tool is called adjoint technology. The tool is actually a solver that uses CFD simulation results to find an optimal solution based on stated goals (reduce drag, maximize lift over draft ratioreduce pressure drop, etc.). But it doesn’t stop there: It also computes how to specifically modify the design. Because it is a solver, it has many advantages:
- It directly computes which section of the design needs to be modified and how. You do not need to define any parameters.
- It directly determines a better-performing shape as well as the associated performance improvement, all without needing another CFD simulation.
- It can, in a minimal number of simulations, determine the optimal shape. At each iteration, design performance increases until the optimal design is reached.
Why is this smart shape optimization tool so fast?
Because the adjoint solver directly determines what section of the shape to modify and how to do it, it reaches the optimal geometry faster. Because the adjoint solver works hand-in-hand with mesh morphing technologies, you do not need to redefine the geometry nor recreate the computational mesh; rather you simply morph the mesh to the new shape. In summary, this solution is fast because:
- The adjoint solver determines directly how to improve performance, so there is no time wasted by trials-end-error processes.
- A mesh morpher automatically adjusts the design shape and computational mesh following the adjoint solver recommendations, saving even more time.
The following materials provide details about the adjoint solver technology.
Turbulence is unarguably the most challenging area in fluid dynamics. It is the most limiting factor in accurate computer simulation of engineering flows. Turbulence flow constitutes a classic multiscale problem, one that is far beyond human intuitive understanding ― as well as beyond resolution capabilities of the most powerful modern parallel computers (for any foreseeable future).
Nobel-prize winning physicist Richard Feynman once described turbulence as the "most important unresolved problem in classical physics." An even more pronounced quote is associated to Werner Heisenberg: "When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first."
No single model or modeling approach can solve all types of turbulent flow, so different types of turbulence models have been developed in the past decades. So choosing the right turbulence model to match the application is critical to accuracy and computational resource optimization. ANSYS is a technology leader in this area, offering a wide range of the most advanced model formulations, including WMLES, ELES and transition models.
The following materials provide details about turbulence modeling.