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How do you calculate lift and drag coefficients using CFD simulations?

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Lift and drag coefficients are important parameters for evaluating the aerodynamic performance of aircraft. They measure how much force the air exerts on the aircraft in the direction perpendicular and parallel to the flight direction, respectively. However, measuring these coefficients experimentally can be costly and time-consuming, especially for complex shapes and flow conditions. That's why computational fluid dynamics (CFD) simulations are often used to estimate them numerically. In this article, you will learn how to calculate lift and drag coefficients using CFD simulations in four steps.

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Pooja Sawant

Mechanical Systems Service Engineer at Boeing with expertise in Technical Support and Aeronautics.

1 Step 1: Define the geometry and domain

The first step is to define the geometry of the aircraft and the domain of the fluid flow around it. You can use a CAD software to create or import the geometry of the aircraft, and then use a meshing tool to discretize the domain into smaller elements. The quality and resolution of the mesh will affect the accuracy and speed of the simulation, so you need to choose them carefully. You also need to define the boundary conditions of the domain, such as the inlet and outlet velocities, pressures, and temperatures.

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Pooja Sawant

Mechanical Systems Service Engineer at Boeing with expertise in Technical Support and Aeronautics.
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To calculate lift and drag coefficients using CFD simulations: Set up the simulation with object geometry and fluid properties. Run the simulation to simulate flow around the object. Post-process results to extract pressure and shear stress data. Integrate these data to find lift and drag forces. Divide forces by reference area and dynamic pressure to get lift and drag coefficients.

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Andrew Wang Su

Chief Engineer at MACH Design Build Fly
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Meshing is a critical aspect of the problem and can greatly influence the accuracy of the results. First you need to size your domain such that all the flow features of interest are captured. Then you need to decide whether to use a structured or unstructured mesh, generally structured meshes will be superior in accuracy, though they require more work to create. Refinement should be carried out around areas of interest, like near the aircraft geometry and in the wake. Typically, having a good idea of how the flowfield should look like is helpful here. Adaptive meshing in the solver may be used to supplement this. A mesh convergence study should then be done to determine the mesh refinement required. Prior data will aid in this process.

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2 Step 2: Select the solver and model

The next step is to select the solver and the model for the CFD simulation. The solver is the algorithm that solves the governing equations of fluid flow, such as the Navier-Stokes equations. The model is the set of assumptions and simplifications that describe the physics of the flow, such as the turbulence model, the compressibility model, and the heat transfer model. Depending on the type of flow and the level of detail you want to capture, you need to choose the appropriate solver and model for your simulation.

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3 Step 3: Run the simulation and monitor the convergence

The third step is to run the simulation and monitor the convergence. The convergence is the process of reaching a steady or periodic solution that satisfies the conservation of mass, momentum, and energy. You can monitor the convergence by checking the residuals, which are the errors in the solution, and the forces, which are the components of lift and drag. You want to make sure that the residuals are below a certain threshold, and that the forces are stable and consistent with your expectations.

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4 Step 4: Post-process the results and calculate the coefficients

The final step is to post-process the results and calculate the lift and drag coefficients. You can use a post-processing tool to visualize and analyze the flow field, such as the velocity, pressure, and vorticity contours, and the streamlines and wake patterns. You can also extract the lift and drag forces from the simulation output, and divide them by the dynamic pressure and the reference area of the aircraft to obtain the lift and drag coefficients. You can compare these coefficients with experimental or theoretical values, and evaluate the performance and efficiency of your aircraft design.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Andrew Wang Su

Chief Engineer at MACH Design Build Fly
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You should always ask yourself "Is CFD the appropriate tool for the task?" CFD should primarily be used for detailed design and optimization as well as for validation in some instances (though the CFD itself needs to then be validated by physical experiments). Each run is costly in both time and effort, therefore it should be used strategically. For determining an aircraft's CL and CD, oftentimes a lower order model like Vortex Lattice will suffice early in the design process, for analysis of major trends and preliminary optimization, though its limitations must also be kept in mind. CFD may also require extensive tweaking to converge to an accurate solution, in certain cases iterating with physical testing may prove faster and cheaper.

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How do you calculate lift and drag coefficients using CFD simulations?

1

2

3

4

5

1 Step 1: Define the geometry and domain

2 Step 2: Select the solver and model

3 Step 3: Run the simulation and monitor the convergence

4 Step 4: Post-process the results and calculate the coefficients

5 Here’s what else to consider

Aerospace Engineering

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How do you calculate lift and drag coefficients using CFD simulations?

1

2

3

4

5

1 Step 1: Define the geometry and domain

2 Step 2: Select the solver and model

3 Step 3: Run the simulation and monitor the convergence

4 Step 4: Post-process the results and calculate the coefficients

5 Here’s what else to consider

Aerospace Engineering

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