ANSYS FLUENT Airfoil Analysis And Tutorial
ANSYS FLUENTAirfoil Analysis and TutorialENGR083: Fluid Mechanics IITerry Yu5/11/2017
AbstractThe NACA 0012 airfoil was one of the earliest airfoils created. Its mathematically simpleshape and age have meant that it is one of the first choices for validating CFD programs, as thereis a wealth of data on this particular airfoil. Though the NACA 0012 airfoil is not in general usecurrently due to its relatively poor lift to drag ratio, it remains a useful research tool. It still seessome use, however, in control surfaces and helicopter blades, as it is symmetric and thus has asimple correlation between angle of attack and lift.This project validated methods for using ANSYS Fluent and created a tutorial for futureE83 students to also analyze this airfoil. The resulting coefficient of lift plot is remarkablysimilar to one produced by NASA’s Langley Center, while the coefficient of drag plot seems tobe in error. This is likely due to our simple viscosity model.IntroductionANSYS Fluent is part of the ANSYS suite of engineering analysis software, whichincludes Mechanical APDL, BladeGen, and many other software packages capable of thermalanalysis, electromagnetic analysis, and more. Fluent is also widely recognized as one of theindustry standard CFD software packages. Although Solidworks is capable of performingrudimentary CFD analysis, it is only a rudimentary approach as Solidworks is fundamentallyCAD software and is not built with simulation in mind, as ANSYS is. The versatility andcapability of ANSYS Fluent makes it an easy first choice for fluid dynamics analysis.This project seeks to find a basic understanding of ANSYS Fluent and develop a tutorialfor future students to also learn from this software. In a previous project, I analyzed the NACA0012 airfoil structurally to understand its deflection due to aerodynamic forces in a helicopter.Thus, I had the model readily available. However, the model was in 3D form, and I had toanalyze only a cross section of the airfoil. Unfortunately, Fluent’s academic license does notallow for enough meshes to cover the entire 3D airfoil. This was fortuitous, though, as we wereable to corroborate our results with a NASA Langley Center study on the characteristics of theairfoil.After completing our analysis, we created a tutorial to introduce new E83 students toFluent. This tutorial will walk the user through importing the airfoil model and setting up theboundary conditions for meshing and solving the fluids problem. Due to time, we were unable toimplement a tutorial for modeling or for more simple shapes in Fluent, which may have beenmore helpful to novice students.
ProblemThe fundamental problem we are attempting to solve is the modeling of a 2D airfoil crosssection. Given the below figure, how can we determine the lift and drag acting on the airfoil?The picture above shows a box with an airfoil cutout. In order to analyze an airfoil, myimplementation does not implement a wind tunnel and an object suspended in it, although that isa valid approach. Instead, this procedure cuts out an airfoil shape from a bounding box, andmeshes the box. The entire box is then divided into finite elements, and the fluid dynamicsequations are solved. By examining the fluid properties near the airfoil cutout, the lift and dragcan be determined.We have wind tunnel tests and other software validations from the Langley center, so we are ableto compare our lift and drag results to validated results, and see if there were any mistakes in ourprotocol or software implementation.For our airfoil, we are working in the turbulent regime, with Re 6million. Our speed isM 0.15, which converts to about 51 m/s. This region is essentially incompressible, but theNASA website cautions against running incompressible flow, as the results may differ.From determined lift and drag forces, we can calculate the lift and drag coefficients. These areplotted against the angle of attack, and compared to available data. A close match confirms thatour implementation and procedure are correct.
TheoryFirst, we must determine the regime that our analysis is in. Our reference data is for Re 6million, which is far into the turbulent region.𝑅𝑒 𝜌𝑣𝐿𝜇where 𝜌 density, 𝑣 velocity, 𝜇 dynamic viscosity, and L is the characteristic length, which forour airfoil is the chord.With known viscosity, density, and speed, we can calculate the length. The values we used were:kgm3m𝑣 51s𝜌 1.185𝜇 1.7894 x 10 5kgm s𝐿 1.719 mTo determine the coefficients of drag and lift, we use the following equations from the NASAwebsite1.where L and D are the lift and drag, 𝜌𝑟𝑒𝑓 is the free stream density, 𝑈𝑟𝑒𝑓 is the free streamvelocity, and A is the reference area (in 2-D it is reference length, or area per unit span).It should be noted that these differ from the equations derived in class.Our solver has the energy equation enabled, which enables compressible flow.To model compressible flow, we use the ideal gas law to determine the density at any nrunning.html
Fluent models turbulence with the Reynolds average Navier-Stokes Equations (RANS). This isone of the most common approaches to computing flows. Other methods exist to model flow,such as Detached Eddy Simulation or Large Eddy simulation. However, while more accurate,RANS is far less computationally intensive and yields close enough results for the scales that weare working at.Below is the full Reynolds averaged momentum equation. At each iteration, this equation issolved for all the mesh quadrilaterals. The Reynolds stress tensor describes unknowns introducedby modeling and must be related to average flow quantities.To calculate the Reynolds stress tensor, we must use eddy viscosity models. The Boussinesqhypothesis states that Reynold stresses can be modeled with an eddy viscosity 𝜇 𝑇 . This isreasonable for simple turbulent shear flows such as boundary layers, round jets, mixing layers,channel flows, but for complex flows we need to fully solve the Reynolds Stress tensor, which iscomputationally intense.The eddy viscosity 𝜇 𝑇 above must be resolved to complete the RANS equations.Below are our choices of models. There are many methods of calculating eddy viscosity, and wehave chosen the Spalart-Allmaras equation for its simplicity.RANS based modelsEddy viscosity models
Spalart-Allmaras is the computationally simplest model. It is economical for large meshes, butperforms poorly for 3D flows, free shear flows, and flows with strong separation. It is suitablefor mildly complex (quasi-2D) external/internal flows and boundary layer flows under pressuregradient (e.g. airfoils, wings, airplane fuselages, missiles, ship hulls), which is perfect for oursimulations.The other equations are more suitable to complex flows, such as turbomachinery, flows withhigh separation, flows with high strain rates, or flows with high swirl rates. The Spalart-Allmarasequations are adequate for our purposes, but any more complex analysis than simple shapes in auniform flow.The Spalart-Allmaras equations can be characterized aswhere eddy viscosity is obtained fromBy solving this equation, Fluent can solve for the eddy viscosity. With the eddy viscosity, theReynolds stress tensor is known, and once the stress tensor is known, the entire ReynoldsAveraged Navier Stokes equations can be solved.
ResultsAt the proscribed Re 6 million, we obtained the following graph for the lift coefficient:This was overlaid with the NASA coefficient of lift chart. Our values are in orange.This shows very good agreement, with only some discontinuities in the stall region.
The coefficient of drag vs coefficient of lift graph was also plotted:
This was also overlaid with the NASA coefficient of lift chart. Our values are in green.This shows fairly poor agreement with the NASA values. Although we managed the samegeneral shape, we are off by a fair amount.
Following are sample contours and streamlines from a 10 degree angle case.
Below are some sample streamlines and contours from the 25 degree case, where the airfoil isstalled.
Discussion and Future WorkOur model was validated for lift. It shows very close agreement to the NASA Langley Center’sdata, which includes wind tunnel and software testing. We have successfully modeled an airfoil’slift in ANSYS Fluent to a very high degree of accuracy.However, our drag results had some disagreements with the NASA results. Although wemaintained the same overall shape, we started producing an enormous amount of drag at higherangles of attacks. A possible cause of this is the farfield boundary effect, which can have effectson lift and drag at high lift conditions. This effect is unique to compressible flows and is a quirkof numerical analysis, where boundary conditions are incorrectly set for compressible flows andthe solver is not aware of the farfield equilibrium conditions for the boundaries, and leads towaves developing in the mesh. Further investigation is required into the settings necessary tocorrect for this in ANSYS Fluent.We have successfully developed a tutorial to complete the lift and drag analysis on an airfoil inFluent. This tutorial starts the user at a model and walks them through configuring the fluiddynamics solver.Future work includes implementing simpler problems into the Fluent tutorial, such as flow over aflat plate or flow over a cylinder. These simulations are relatively short, so a more complexmodeling scheme such as full Reynolds Stress tensor solving could be used. The bounding boxcould be even further extended, and farfield boundary conditions could be investigated to ensurethey are not causing the drag to be erroneous.References1. “2DN00: 2D NACA 0012 Airfoil Validation Case.”https://turbmodels.larc.nasa.gov/naca0012 val.html, NASA Langley TurbulenceModeling Resource, 2015.2. “The Spalart-Allmaras Turbulence Model.” https://turbmodels.larc.nasa.gov/spalart.html,NASA Langley Turbulence Modeling Resource, 2016.3. “Implementing Turbulence Models into the Compressible RANS ementrans.html, 2016.4. “Modeling Turbulent Flow”http://www.southampton.ac.uk/ nwb/lectures/GoodPracticeCFD/Articles/Turbulence Notes Fluent-v6.3.06.pdf , ANSYS.
Appendix: Raw DataAoa02468101214161820Lift 4055861.76117.445639.5526079.1014988.397Drag 370.139854-2-4-6-8-10-12-14-16-18-20Lift 76117.445639.5526079.1014988.397Drag .0277930.0393370.0590040.124870.2187370.139854Aoa
Appendix: Fluent TutorialThis is a short tutorial in running the airfoil analysis with ANSYS Workbench with the NACA0012 airfoil.There are four provided files, blade only.agdb, blade 2.iges, airfoil single example.wbpj, andwing analysis aggregate.wbpj, The blade 2.iges file contains the base 3D geometry for theblade. The wing analysis aggregate.wbpj is simply a bunch of models with connected to aparameter set which allows for running all the angles of attacks at once. This is useful when oneneeds to run the entire model, but for the purposes of this tutorial we will only run one system ata time.This tutorial will focus on constructing a model from the blade only.agdb file. Double click thisfile to open it.Once opened, ANSYS will likely throw an error saying that the blade 2.iges file is missing. Thiscan be ignored for now; we will locate it later. The following screen should appear:
This is the home screen for ANSYS Workbench, which incorporates all the ANSYS products.From the left, double click on Fluid Flow (Fluent). A new system will appear. Drag the boxlabeled “Geometry” in the A system to the box labeled “Geometry” in the B system, which hasthe question mark. They have now been linked. The screen should now look like:The large blocks are known as systems, and incorporate all the steps necessary to simulate themodel. The relevant system is labeled “Fluid Flow-Fluent”, as we will be working with Fluenttoday.1.2.3.4.5.6.Geometry: This is the configuration of all the geometries necessary to define the model.Mesh: This creates the mesh on which Fluent’s finite volume method acts.Setup: This defines the physical constants, boundary conditions, and solver method.Solution: This is where the solver is initiated.Results: The results of the simulation can be viewed here.Parameters: Fluent will output the lift and drag values here.To start, save this project. Remember to save often! First, we will inspect the geometry andadjust the angle as needed. Double click the Geometry box in “A” (the master geometry) tolaunch the DesignModeler.
First, we will import the blade 2.iges file. First click on Import as highlighted, and then click onthe source. Some ellipsis will appear. Locate the blade 2.iges file and click it to import.The modeling has been done already. To move around in the window, use the middle mousebutton to rotate and CTRL middle mouse to translate. The axes on the bottom right will reset theview when clicked.Next on the left, click on Rotate. (The subscripts are purely for naming incrementing.) In thedetails section, the angle of the blade can be adjusted. When at a satisfactory angle, clickGENERATE to save the changes and create the final model. For this example, we will use -10degrees. Note that negative angle here corresponds to positive lift.
Now, return to the ANSYS Workbench screen. If properly executed, there will be green checkmarks next to the Geometry label. We will now launch the Mesh by double clicking it.On the left, right click on Mesh, which should have a yellow thunderbolt as the mesh hasn’t beengenerated yet. Go to Insert Refinement. The yellow Geometry on the bottom right will turnyellow, which is a prompt to select a geometry. Select the geometry, which will turn green. Clickon Refinement, and at the details pane in the bottom right set the refinement to 2.We next need to create the named selections so the solver has boundary conditions. Switch toedge selection at the top.Right click the left edge (the inlet) and select Create Named Selection at the bottom. Call thisInlet. Repeat this for the right edge (Outlet), the upper and bottom edges (Wall-use CTRL-clickto select multiple edges before right clicking), and the cutout of the airfoil (Airfoil-again, selectboth top and bottom edge).
Click GENERATE after this is done. The mesh will now generate. To see the mesh, press Meshon the left pane. The meshing window can be closed.Return to the ANSYS workbench screen. It is recommended that you save at this point. Rightclick on the Mesh box and click Update. The Mesh box should have a green checkmark.Next click Setup. This will launch Fluent.
On the left, under Models, turn Energy on, and set Viscous to Spalart-Allmaras.Under Materials Fluids Air, set the density to Ideal Gas Law. This enables compressibility.Under Boundary Conditions Inlet, set to Velocity-Inlet. For this example use 51 m/s. EnsureOutlet is set to Pressure-Outlet.Next, we will set our output parameters. Under Parameters and Customization Parameters Output Parameters, click Create Lift. Click Airfoil for wall zone on the right. Then, selectSave Output Parameter, and create a new parameter called Lift. Click OK, and OK again.For drag, under Parameters and Customization Parameters Output Parameters, clickCreate Drag. Click Airfoil for wall zone on the right. Then, select Save Output Parameter,and create a new parameter called Drag.For the parameters, the force vectors should be correct, but you can reference the DesignModeler to be sure.At this step, we can start preparing the solver. Open the Solution Tab.Under Solution Methods, set the scheme to Coupled.Under Run Calculation, set the Number to Iterations to 200, adjusting if necessary.To run the method, you can double click on Run Calculation, or go to the Workbench and rightclick Update on Results, which will run everything up to and including Results.
From Results, the contours and streamlines can be inspected.Click on Parameter Set to view the lift and drag.To make the process easier, one can define the angle of attack as a parameter. This precludesgoing into the modeler and changing it every time. Simply check the box next to the angle box.Then, the angle can be changed from the parameter set.kgThe density of air used by the ideal gas model is 1.182m3, the chord length is 1.7894m, and theperimeter of the airfoil shape is 3.665m.The airfoil single example.wbpj contains a project that should be the end result of theseinstructions.
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