Basic of Airfoils, Aerodynamics its Application and CFD Modeling

Understanding the Basics of Airfoils: Lift, Drag, and Flight

• Aerofoils, also known as airfoils or wings, are fundamental components of aircraft and various other aerodynamic applications.
• They play a pivotal role in generating lift, controlling flight, and shaping the performance of vehicles that move through a fluid medium, such as air or water.
• Let’s delve into the basics of airfoils, exploring their anatomy, aerodynamic principles, and how they enable flight.

 

 

Anatomy of an Airfoil:

• An airfoil, also known as an airfoil or wing profile, is a carefully designed shape that plays a pivotal role in generating lift and controlling flight in aircraft and other aerodynamic applications.
• Let’s take a closer look at the various components that make up the anatomy of an airfoil and how each element contributes to the science of flight.

Leading Edge:

• The leading edge is the foremost point of the airfoil, the part that faces the oncoming airflow.
• It is responsible for initiating the flow separation around the airfoil, which is crucial for generating lift.

Trailing-Edge:

• Located at the rear end of the airfoil, the trailing edge is where the airflow recombines after passing over and under the wing.
• It’s the point where the two flows converge and continue downstream.

Chord Line:

• The chord line is a straight line connecting the leading edge to the trailing edge of the airfoil.
• It serves as a reference for determining angles of attack and other aerodynamic parameters.

 Camber

• Camber refers to the curvature of the airfoil’s upper and lower surfaces. It is the main factor influencing the generation of lift.
• A cambered airfoil has a curved upper surface and a flatter lower surface, creating pressure differences that result in lift when the airfoil moves through the air.

Upper Surface:

• The upper surface of the airfoil is characterized by its curvature and often contributes to the generation of lower pressure due to the increased airflow velocity over this region.

 Lower Surface:

• The lower surface of the airfoil is relatively flatter compared to the upper surface.
• This design helps maintain a higher pressure, which, in conjunction with the lower pressure above, contributes to lift generation.

 The angle of Attack:

• The angle of attack is the angle between the chord line of the airfoil and the oncoming airflow.
• Adjusting the angle of attack allows pilots to control the lift and drag forces acting on the airfoil, which is crucial for takeoff, landing, and maneuvering.

 Thickness:

• The thickness of the airfoil is the distance between its upper and lower surfaces.
• It plays a role in determining the amount of lift that can be generated.
• Thicker airfoils tend to produce more lift but might also result in higher drag.

 Camber Line:

• The camber line is a curved line equidistant from the upper and lower surfaces of the airfoil.
• It is used as a reference to quantify the amount of camber present in the airfoil’s design.

Aerodynamic Principles:

The behavior of airfoils is governed by fundamental principles of fluid dynamics, particularly Bernoulli’s principle and Newton’s third law of motion.

Bernoulli’s Principle:

• According to Bernoulli’s principle, as the velocity of a fluid (or air) increases, its pressure decreases.
• The curved upper surface of an airfoil causes air to move faster over it, creating an area of lower pressure.
• Meanwhile, the flatter lower surface maintains higher pressure.
• This pressure difference generates lift, allowing an aircraft to rise against the force of gravity.

 Newton’s Third Law:

• As air flows over the curved surface of the airfoil, it exerts a downward force on the air.
• By Newton’s third law of motion, the air exerts an equal and opposite force (lift) on the airfoil.
• This interaction propels the aircraft upward and sustains flight.

Flight and Control:

• Aerofoils enable aircraft to achieve controlled flight by manipulating lift and drag forces:

Lift Force:

• The upward force generated by the airfoil’s shape and the resulting pressure difference between the upper and lower surfaces allows an aircraft to become airborne and stay aloft.

Drag Force

• Drag is the resistance experienced by an aircraft as it moves through the air.
• Airfoils also contribute to drag due to air resistance against their surfaces.
• By adjusting the angle of attack (the angle between the chord line of the airfoil and the oncoming airflow), pilots can control the lift and drag forces, facilitating takeoff, landing, and maneuvers.

 

Applications of Airfoils

• Aviation:
  • The most prominent application of airfoils is in aviation.
  • From small propeller-driven aircraft to massive commercial jets, aerofoils enable controlled flight by generating lift and providing stability.
  • They also play a role in maneuverability and drag reduction.
• Wind Energy:
  • Aerofoils are utilized in wind turbine blades to capture the kinetic energy of the wind and convert it into electrical energy.
  • The shape of the blades is optimized for efficient energy extraction.
• Automotive Industry:
  • Some high-performance cars and racing vehicles employ airfoils to enhance downforce, improving stability and traction at high speeds.
• Marine Industry:
  • Hydrofoils, a type of underwater airfoil, are used to lift boats and ships partially out of the water, reducing drag and increasing speed.
• Sports Equipment:
  • Sports like surfing, snowboarding, and kiteboarding use airfoil principles to design equipment that maximizes lift and control.

Steps in CFD Analysis of Airfoil Profile

• Computational Fluid Dynamics (CFD) analysis of an airfoil profile involves simulating the flow of air around the airfoil to evaluate its aerodynamic performance.
• Here are the general steps involved in conducting a CFD analysis of an airfoil profile:

Problem Definition:

• Clearly define the objectives of your analysis.
• Are you studying lift, drag, stall behavior, or some other aspect of the airfoil’s performance?

Geometry of Airfoil :

• Create a 3D model or 2D sketch of the airfoil profile.
• You can either create the geometry within ANSYS using DesignModeler or import it from an external CAD software.
• If you’re creating the airfoil from scratch, you can use splines, arcs, and lines to define the shape.
• Import Airfoil Coordinates (if applicable):
  • If you have airfoil coordinates in a file (typically in .dat or .txt format)
  • You can import profiles into ANSYS using DesignModeler if you have input data.

• Blade Generation (TurboGrid):
  • If you’re creating a blade for a turbomachinery simulation, you can use ANSYS TurboGrid to generate the blade geometry.
  • TurboGrid allows you to specify the number of blades, stacking, and other parameters to create a 3D blade geometry from your 2D airfoil profile.

Mesh Generation:

• Generate a mesh that discretizes the geometry into smaller elements
  •  triangles or quadrilaterals in 2D or tetrahedra/hexahedra in 3D).
• The mesh density and quality are critical for accurate results.

• Grid Generation for  3D Model:
  • If you’re working with a full 3D blade geometry, you’ll need to generate a grid (mesh) for the entire blade, including the hub and shroud regions.
  • TurboGrid can also be used for this purpose.

Solver Set up

• General Set up:
  • Configure the solver settings.
  • Select the appropriate solver (e.g., steady-state or transient),
• Boundary Conditions:
  • Define boundary conditions for the CFD simulation. Specify the airfoil’s surface as a no-slip wall, inlet velocity, outlet pressure, and any other relevant conditions.
• Fluid Properties:
  • Define the properties of the fluid, such as density, viscosity, and temperature.
• Solver Selection ANSYS FLUENT or CFX or OpenFOAM:
  • Select an appropriate CFD solver for your analysis. Common solvers include finite volume, finite element, or finite difference methods.
  • The choice of solver may depend on the specific analysis goals and available software.
  • Submit the simulation job to commercial solvers like ANSYS Fluent or CFX.
• Turbulence Model.
  •  Turbulence model (e.g., k-epsilon, SST k-omega with low Reynolds correction)need to select for turbulent flow over an airfoil
  • Make sure to set up convergence criteria and time steps if you’re running a transient simulation.
• Numerical Settings:
  • Configure solver-specific numerical settings, such as convergence criteria, time step size (for transient simulations), and turbulence modeling if applicable.
• Initial Conditions:
  • Set the initial conditions, which may include the initial velocity field or other relevant parameters.
• Solution Calculation:
  • Run the CFD simulation using the defined settings.
  • This step involves solving the governing equations of fluid flow (e.g., Navier-Stokes equations) numerically over the defined mesh.
  • If the analysis reveals areas for improvement or further questions, iterate through the steps to refine your understanding of the airfoil’s behavior.

Post-processing:

• Use the post-processing capabilities of ANSYS Fluent or CFX to analyze the results.
• Plot velocity contours, pressure distributions, lift and drag coefficients, and other relevant data
• Analyze the simulation results. Common post-processing tasks include visualizing flow patterns, calculating lift and drag coefficients, assessing pressure distributions, and studying vortices.
• Validation and Verification:
  • Compare your simulation results with experimental data or benchmark cases to validate the accuracy of your simulation.
  • Ensure that the model is capturing the physical phenomena accurately.
• Grid Sensitivity Analysis
  •   Perform sensitivity analyses to understand how changes in parameters (e.g., angle of attack, airfoil shape) affect the airfoil’s performance.
• Optimization (optional):
  • If necessary, use the results to optimize the airfoil design for specific performance criteria.
•  Reporting CFD results:
  • Report writing the entire CFD analysis process, including the methodology, assumptions, and results.
  • Prepare a comprehensive report summarizing your findings.

Conclusion

• Airfoils are the foundation of flight, essential for achieving lift and control in aviation and many other applications.
• Their design, based on principles of fluid dynamics, enables us to conquer the skies and explore new frontiers.
• Airfoild has applications in aircraft wings, wind turbine blades, or hydrofoils on boats,
• NACA aerofoils have made a significant impact on aeronautical engineering for the design of aircraft design.
• Their systematic approach to aerofoil design, characterized by a structured naming convention, has provided engineers with a valuable toolkit for tailoring aerofoil performance to specific flight conditions.
• For all different speeds of flights ( subsonic, transonic, or supersonic), NACA aerofoils have paved the way for more efficient and effective aircraft designs across a wide range of applications.
• CFD analysis of airfoil profiles can be complex and computationally intensive, so it’s important to have access to appropriate software, hardware, and expertise to carry out a meaningful analysis. Additionally, proper validation and verification are crucial to ensure the accuracy of your results

 

References of Airfoil Database

1. Standford Univerity,  Airfoil series for NACA series
2.  MIT, Fundamental of Airfoil
3. Database of airfoil, Airfoil Database Tools
4. NASA, Airfoil Database
5. Steps for CFD modeling of 2D airfoil profile in FLUENT