Top 10 Reasons to Analyze Your Airfoil

Here are my top ten reasons (no particular order) to analyze your airfoil shape.  

10.  Bricks make terrible airfoil shapes.  Circular shapes do not provide any lift (unless they are spinning).  The airfoil shape makes a difference even if it is not the dominant parameter.


Brick analysis by MultiElemnet Airfoils 5.0: see http://www.hanleyinnovations.com/mefoil.html



9. You can perform airfoil analysis free of cost using XFoil (albeit you cannot analyze the above bricks with this program).  Price is no excuse not to analyze your airfoil.  See: http://web.mit.edu/drela/Public/web/xfoil/

8.  You are forced to learn about Reynolds number and how it can ruin the looks or your airplane (but possibly save your life).  Investigating the behavior of the airfoil shape as a function of Reynolds number can improve the safety of your designs.

7.   Some shapes provide better lift than others at the same angle of attack and speed.  Find out which ones can make or break your project.

Lift Coefficient vs. Angle of Attack. Computed with VisualFoil 5.0

6. Some shapes can provide the required lift (desired loading characteristics) without the increased expense of drag.  This helps you to win races, save fuel and have a good feeling about your design.

Lift Coefficient vs. Drag Coefficient. Computed with VisualFoil 5.0

5.  Some good looking airfoils turn out to provide bad stall behavior. Use analysis to determine the stall angle and maximum lift coefficient of your cross sectional shape.

Lift Coefficient vs. Angle of Attack (showing maximum lift). Computed with VisualFoil 5.0

4.  The moment of truth.   Some airfoils (especially those that provide high lift) often demand in return a huge horizontal stabilizer for longitudinal stability.    You do not want your design to have a high sink rate due to tail drag.

Moment Coefficient vs. Angle of Attack. Computed with VisualFoil 5.0

3.  Speed can slow you down.  It is necessary to know the transonic behavior and the drag divergence Mach number of your airfoil if you design propellers, turbines and jet airplanes.

Drag divergence of an airfoil.  Analysis by VisualFoil Plus:  http://www.hanleyinnovations.com/air_16.html

 

2.  Airfoil analysis inspires you to find more about airfoil characteristics and terms.  You will learn about reflexed airfoils, laminar flow airfoils, high lift airfoils, cambered shapes, split flaps, slotted flaps ....... and how they can be beneficial to your design and project.


1.  VisualFoil 5.0 is a powerful and user-friendly tool for airfoil analysis and design.  It has a built-in library of 1000s of airfoils including the NACA 4, 5 & 6-digit shapes.  Users can also enter custom airfoils as coordinates or .dxf files (line & arc entities).   Hanley Innovations can provide help with setting up your airfoil project, interpretation of the results and provide recommendations.  There is no reason not to produce a successful design.

More information about airfoil analysis and VisualFoil can be found at Hanley Innovations.  Please visit http://www.hanleyinnovations.com/

What are your reasons for airfoil or CFD analysis?

Thanks for reading.

Car Aerodynamics Calculations

Often it is necessary to quickly determine the aerodynamics characteristics of a production vehicle or a new prototype before further testing or construction can move forward.

Hanley Innovations developed Stallion 3D for rapid aerodynamics analysis of three-dimensional solid models.   This software is a quick and inexpensive option to perform aerodynamic conceptual analysis on a 3-dimensional CAD model without the time-consuming ritual of grid generation.  The analysis can be performed on your desktop computer or laptop. 

The following steps can be followed to determine the lift and drag on a road vehicle.

Stallion 3D Main Window

To start the analysis, click on the design menu and select the Import or Edit .STL file option.  This will invoke the .stl file editor show below.   The editor allows you to read in files in the ASCII format and then provide a quick preview of the input.   Stallion 3D can analyze files that are water-tight.  If your file has holes, you might be able to repair them with the free utility called Netfabb-Studio Basic. Netfabb studio can also convert files from a variety of formats to the .stl format.

A car from Turbosquid.com as seen in Stallion 3D.

The .stl input box also allow you to rotate and translate the part.  This allows you to point the leading edge of the model (longitudinal direction) along the x-axis and the lateral direction along the y-axis.

The .stl dialog box can be used to orient the model.

Before exiting the .stl import box, you can also explore the options to scale the model and set the correct dimensions.  The default in Stallion 3D is meters, however, you can select inches, feet and cm.

Scale and dimension the solid model.

Once the part has been imported into Stallion 3D, it can be view in wire frame mode in the Design Editor.  This screen can also be used to add more components (such as wings) to the model.

The design editor window can be used to add wings and other components to the model.

The next step is to analyze the model.  Stallion 3D performs a 3-dimensional CFD analysis on an ordinary PC running MS Windows.  The user can choose the model size and settings by clicking the CFD Solver menu followed by the Setup CFD option.   In the first tab, the user can select the CFD problem size (number of nodes) and layout.   The options are for a quick run (less accurate) or a moderately sized problem (more accurate).  In this problem, I choose a Small Size ( less than 240,000) nodes. 

The user can choose the problem size.  A quick run is helpful to test the setup.

The next option is to choose the flow domain dimensions and location.  The flow domain is a big cube. I recommend that the length of the flow domain is about 10 times the maximum length of the part.   This allows quicker convergence and better accuracy for the radiation boundary conditions. 

Set the flow domain dimensions to simulate the physics and facilitate convergence.

For the car problem, I select the lower z-dimension to coincide with the ground and then make the ground a reflection plane (instead of a radiating boundary).  This will simulate the hard road.

Set the lower boundary as reflective (instead of radiative) to simulate the road.

The next step in the solution is to set the flow speed and reference areas and lengths for the lift, drag and moment computations.  

Finally, simply click the CFD Solver menu and select Generate Grid/Solve flow.  The program will automatically generate the grid and launch the flow solver.  For this specific problem, the grid generation took about 20 - 30 minutes on my laptop.  The algorithm generated 132,833 nodes.



Once the grid was generated, the flow solver started to solve the 3-dimensional compressible Euler equations (with the low speed flow option).  The solver computes the lift, moments and pressure drag for the 3-dimensional configuration with good accuracy.  Stallion 3D can also solve the 3-D Navier-Stokes equations for the same configuration.

The flow solver updates 132,833 nodes every iteration using a 4-stage Runge-Kutta method.

 The solution converges in less than 2 hours for this problem on my laptop computer.   The windows below shows the pressure computed on the surface of the car.  This is obtained by clicking on the Visualization menu and selecting the View Solution option.

Pressure on the surface of the car.

 The speed near the surface of the car is a useful design parameter for drag reduction and flow visualization.   Stallion 3D allows the user to see the pressure, Mach number, velocity and temperature on the surface of the part.

Velocity on the surface of the car.

 Another technique to assess the quality of the solution is to look at the streamlines and flow at the ground plane.  The following  graphs show these two images.

Velocity at the ground plane.

Flow streamlines.

 Stallion 3D computes the overall lift, drag and moments acting on the vehicle.  These numbers can be used as design parameters to improve and determine the performance of the vehicle.   The moments can also be used to determine stability with subsequent computations.

Computed lift, drag and moment coefficients based on vehicle frontal area.

Note, the .stl file must be in very good shape for good computation of the forces and moments.  It must not have multiple facets (in the same location) and inverted facets.  If this is the case, the user can approximate the forces on the Cartesian-front (an approximate representative of the car).  This method, however, is not valid for downforce calculation.  A good .stl file is required for downforce calculation when the lower boundary is used as a solid wall.

More information about Stallion 3D can be found at http://www.hanleyinnovations.com/

Thanks for reading.

Can a Turkey Escape Thanksgiving?

When in danger, it is a well known fact that turkeys can muster up very high flight speeds. Some legends have it that they can fly at supersonic speeds. 

Image from the 3-D Studio. Analysis from Stallion 3D.

An often overlooked fact is turkeys are extremely adept at aerodynamics and CFD analysis. They never attempt an unfamiliar regime of flight without careful analysis.

So when told about Thanksgiving, a young bird was not at all happy about its prospects.  It decided to find a way to beat the system.  "Perhaps", it thought,  "I can use my supersonic flight capabilities to escape the humans."

A quick google search revealed that humans have jet aircraft that are also capable of supersonic flight.  What are the consequences (if any) of  supersonic flight and out running a jet?

Stallion 3D Mach number analysis at M = 1.5.

Using it's laptop PC and Stallion 3D, the turkey was able to perform a careful CFD analysis within a few minutes.  A variable of concern was the temperature.

Simulation of Turkey & Jet.  Notice Temperatures....

At Mach 1.5 (a reasonable speed), the analysis showed temperatures of 466 degree K or 380 degrees Fahrenheit on most of the bird's body.

Another quick google search showed that Martha Stewart recommends that you cook your bird at 325 F.

"Hmm.... ", the turkey thought, "Can a turkey ever out run Thanksgiving?" 

"Yes!!!" exclaimed the turkey, answering its own questions, as it applied a gell-like substance (resembling cranberry sauce but with a negligibe coefficient of thermal diffusivity) to the areas indicated by red on the temeprature charts (above picture).

Happy Thanksgiving from Hanley Innovations.
http://www.hanleyinnovations.com/stallion3d.html

Analyzing "Difficult" Airfoils

Using a computer program to analyze the cross section (airfoil) of a wing, keel, sail combination or race car spoiler gives the design engineer valuable insight into the behavior and efficiency of the device.  Furthermore, airfoil analysis provides a cost effective and quick first step in the conceptual design of the product.   However, using conventional airfoil analysis tools (often based on panel methods) cannot handle a number of "difficult" shapes that are of interest to the modern design engineer.   For example, panel methods have difficulty with truncated or blunt airfoils, airfoils with gurney flaps or wicker bills (often used on high end race cars), ground effect,  thin airfoils (for modeling yacht sails) and airfoils at high angles of attack (for modeling wind energy devices).

Multi-Element wing section in ground effect computed using MultiElement Airfoils.

Computational fluid dynamics (CFD)  based on the Euler or Navier-Stokes equations provides solutions where panel methods fail.  However, even in two-dimensions, general CFD methods can prove to be costly and time-consuming due to the grid generation process for complex geometries.

MultiElement Airfoils 5.0 provides a unique solution based on Euler/Navier-Stokes equations and automatic Cartesian grid generation method.  The software is a turn-key approach to airfoils analysis (with a library of built-in airfoils and the ability to read-in DXF shapes) where solving a challenging system of multiple cross sectional shapes is as easy as analysing a single conventional airfoil.

MultiElement Airfoils is an Euler/Navier-Stokes solver based on

an automatic Cartesian grid method.

A user can enter the shape from the library of airfoils, a custom shape from a file containing the airfoil ordinates or a .dxf file.  In this case, a blunt airfoil shape is shown in the picture below.

Blunt airfoil geometry in MultiElement Airfoils.

The analysis starts by automatically generating a Cartesian grid using the program's menu system.

Automatic Cartesian grid adapts to the emersed boundary.

Next, the user can choose either an Euler equations solution or Navier-Stokes solution.  The picture below shows the Euler equations solution coupled with a boundary layer solver.  The program displays pressure, velocity ratio (seen below), Mach number, Temperature and density.

Flow velocity ratio with streamlines.



Airfoils with Gurney flaps (wicker bills) used in the racing industry can also be easily and accurately analyzed with MultiElement Airfoils 5.0.

Gurney flap grid in MultiElement Airfoils.

The Navier-Stokes solution is chosen for this analysis (the Euler solution was also obtained for comparison).  The pressure field is shown in the below picture.

Gurney flap solution showing pressure and streamlines.

The following picture shows the flow close to the gurney flap.  It features a double separation region downstream of the flap.  On the left side of the screen, a comparison is made between the surface pressure coefficient of the main airfoil and the flap computed using the Euler equations and the Navier-Stokes equations methods.

Streamlines near the trailing edge as computed in MultiElement Airfoils

Another application of MultiElement Airfoils is in analyzing thin airfoil shapes.  This is useful for the design and analysis of the propulsion system for high performance yachts and sailboats.  The picture below shows a model of the jib, mast and main sail as modeled in the software.

Sail geometry modeled in MultiElement Airfoils.

MultiElement Airfoils is ideal for computing the most optimal angles for the sail components.

Flow velocity and streamlines.

The picture below shows the mast and main sail components.  The solution suggests that the sails are not at the best angles.

Flow velocity and streamlines near the mast and main sail.

In conclusion, MultiElement Airfoils 5.0 is a modern tool for the efficient and accurate evaluation of difficult airfoil cross sections.  More information about the program can be found at http://www.hanleyinnovations.com/mefoil.html

Transonic Airfoil Analysis for Jet Aircraft, Propellers and High Speed Blades

Introduction

Whether you design blades for propellers or airfoils for high speed executive jets, as an engineer you must have an efficient tool that is capable of screening and designing custom airfoil shapes over all speeds of operations. An aerodynamic problem that arises (and this is a potential show stopper) is the tendency of the pressure drag to increase by an order of magnitude through the transonic regime of operation. This sudden drag rise depends on such factors as airfoil thickness, camber and angle of attack.  For a propeller design, the efficiency can decrease dramatically after reaching a certain helical tip mach number (this limits forward airplane speed and/or available power).  For a wing design, the performance of the airplane can be severely limited due to airfoil shapes that were poorly designed and tested across the transonic regime of flight.  The figure below shows how the profile drag can suddenly increase on an airfoil past a given speed or Mach Number.  This Mach number is known as the drag divergence Mach number or Mdd.

Profile drag can suddenly increase by a factor of 10.

As the power of the PC increases, engineers now have access to tools to screen blades that operate poorly in the transonic regime of flight.  VisualFoil Plus has the capability to analyze airfoils in subsonic, transonic and supersonic flows. The above figure shows the pressure drag rise for a NACA 2412 airfoil as predicted using VisualFoil Plus.  The NACA 2412 is a popular airfoil for the wings of general aviation aircraft.  It is also used in some propeller designs.  Currently, propeller companies design and test propeller blades based on modern (often proprietary) airfoil shapes. 

 VisualFoil Plus can be used to design an airfoil that pushes the drag rise towards the right side of the graph resulting in a significant performance boost.  The software features automatic grid generation, a built-in airfoil library and tools to run the analysis from an external program (to facilitate custom airfoil design and analysis).

Transonic Flow Past an Airfoil.

To find out more about testing airfoils in high speed, please read the following article "High Speed Blade Screening" (.pdf file). 

More information about VisualFoil Plus can be found at http://www.hanleyinnovations.com/air_16.html.

Designed to Start: Developing Successful Vertical Axis Wind Turbines

Small vertical axis wind turbines (VAWT) are very attractive sources of clean wind energy for individual residences, small communities, localized power supplies (such as cell phone towers) and other applications.  A major problem for custom designed and production vertical axis turbines is in starting the rotation process.  This is especially true for low wind conditions .  This blog entry provides a new solution where the designer can develop working designs directly on their personal computer and reduce both the cost of building prototypes and development times.  This results in a less expensive line of products that will be an affordable source of clean energy for a growing client base.

The Design Process
Wind turbines and other devices are efficiently designed using a modern computer aided design (CAD) program such as SolidWorks or Autodesk Inventor.  However, for the engineer to know if the designs work or not, they must be built and tested in a wind tunnel or the physical environment.  This is a time consuming and costly step. 

A faster and cost effective solution is to test the design directly on the computer (ideally the same one where the design was developed).  An aerodynamics analysis software program such as Stallion 3D that can test the turbine designs directly in their digital form (as produced by the CAD program).

The figure below shows a vertical axis wind turbine that was imported into Stallion 3D using the STL format.  This file format is available export option in most three dimensional CAD software tools.

Analyze the Turbine

The next step is to determine whether or not the design will spin under desired wind conditions.  Stallion 3D allows the engineer to enter various wind conditions and angles to test the design.  This is done by simply entering the wind condition in a dialog box as shown in the picture below.

Of course Stallion 3D is not the only software package that can perform an aerodynamics analysis on a CAD file.  However, Stallion 3D works on an ordinary personal computer or laptop that runs Windows XP, Vista or 7.  It also eliminates the time consuming and expensive process of grid generation that is inherent in other computational fluid dynamics (CFD) method.  Since cost and time reduction is the goal of this new design  process, using Stallion 3D with its automatic grid generation tool is the logical choice.  The picture below shows the pressure solution and a crosssection of the computational grid that comprises 150,000 cells.  The grid and solution for this case were obtained in less than two  hours.

Designed to Start

To design a starting machine, the engineer must test the design for various wind velocity conditions (speed and direction).   Stallion 3D computes the flow field, forces and moments for any value of the wind direction.  The picture below shows separated flow on the lower blade at 45 degree wind direction and 15 miles per hour.

The software also computes the aerodynamics moments generated by the wind turbine and compares it to the starting torque of the power generator.  If the aerodynamic moments are greater than the starting torque, then the generator will turn for the specified wind conditions and we will have success.  If not, the engineer can make modifications in the CAD program and repeat the process without the expensive step of metal cutting.  The picture below shows the forces and moment that are computed by Stallion 3D to determine the torque and structural requirements of the wind turbine tower.

Conclusion
Stallion 3D is a 3-dimensional aerodynamics software product developed by Hanley Innovations.  The product is intended as a tool for engineers, designers and students that will allow them to solve problems on their personal computers (without the need for external assistance).  It is a component of their professional toolbox and bag-of-tricks that allow them to be innovation and productive at work and contribute to their professional advancement.

More information about Stallion 3D and other professional aerodynamics software tools can be found at http://www.hanleyinnovations.com/product.html

Smaller, Lighter Wings???

Eagles in flight give us a hint on how to make smaller, ligher wings; shorter keels; or less intrusive wind turbine blades.   The key is the tip feathers.  I wanted to know how effective these feathers might be so I used MultiSurface Aerodynamics to perform a simple test.  The figure below shows the results of this test.

In the experiment, I tested three wings.  The first wing had an aspect ratio of 4; the second had an aspect ratio of 6; the third had an aspect ratio of 4 along with wing sails (tip feathers).  The above figure shows that the wing sails elevated the performance of the wing with an aspect ratio of 4 to that of the wing with an aspect ratio of 6.

The MultiSurface Aerodynamics analysis provides proof that taking hints from nature can improve engineering structures.