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# Vortex Shedding Simulation

### Key Takeaways

• Vortex shedding in aircraft occurs when the air flows through the structural component such as the wings or fuselage acting as a bluff body.

• The frequency of vortex shedding is dependent on the flow behavior, speed, and structural design and significantly impacts the performance of the aircraft.

• Vortex shedding simulation helps in analyzing how the formation of vortices affects the aerodynamic forces and stability, allowing engineers to make the necessary optimization decisions in favor of aircraft efficiency.

Vortex shedding has a significant impact on aircraft performance

What causes a kettle to whistle when the water boils? Why does a  bridge vibrate during high winds? These common phenomena are the result of fluid flowing through a body, creating a whirling motion. This behavior is called vortex shedding. This flow phenomenon can also be applied to complex engineering applications such as the aerospace, energy, or construction industries where the flow may induce noise, vibration, or drag.

In aerospace design, vortex shedding may occur due to the interaction between the aircraft body and the wind flowing across it. The frequency of vortex shedding is directly influenced by how the airfoil is shaped and how it behaves at certain airspeeds. A better way to get insight into the relationship between vortex shedding and aircraft design is through effective CFD simulation

In this article, we will take a look at how vortex shedding simulation with computation fluid dynamics (CFD) can help us understand the flow pattern when encountered through an aircraft body and how careful design consideration is essential to reduce any negative effects.

## Vortex Shedding in Aircraft

Vortex shedding is the flow phenomenon that occurs when the fluid passes through the bluff body, causing the formation of vortices. Vortices are swirling patterns that occur alternately along each side of the object.

In aircraft, wings, fuselage, and other structural components function as a bluff body on the way of the airflow. As the aircraft moves, these components separate the flow, forming vortices. In aircraft wings, this occurs along the trailing edge of the wing, leaving a wake of turbulent air downstream. The wake turbulence results in the formation of a series of vortices behind the aircraft, i.e., vortex shedding.

The frequency of vortex shedding in aircraft is dependent on many factors, listed below.

### Factors Affecting the Frequency of Vortex Shedding

 Surface roughness Surface roughness causes the airflow to separate earlier, forming more unstable and larger vortices. Angle of attack At a low angle of attack, there is no vortex shedding. With an increased angle of attack, the separation starts occurring earlier, leading to an increased frequency of vortex shedding with stronger shedding patterns. Speed The velocity of airflow influences the formation and behavior of vortices. High airspeed means high vortex shedding frequency – stronger interaction between the airflow and structure causes frequent vortex shedding. Flow behavior Laminar flow (low Reynolds number) usually has little to no vortex shedding. Turbulent flow behavior (high Reynolds number) produces numerous smaller vortices of complex patterns at higher frequencies.

During aircraft design, the overall effects of the above-mentioned factors must be carefully studied to avoid significant effects on the safety and performance of the aircraft. One way to do so is with a vortex shedding simulation.

## Vortex Shedding Simulation for Analyzing Aircraft Performance

Vortex shedding has a significant effect on the lift and drag forces induced in the aircraft. The predictions of these aerodynamic forces and their effect on the aircraft performance are simplified by CFD-based vortex shedding simulation. Engineers can simulate the flow behavior around the aircraft to analyze the vortex shedding frequency and evaluate its impact on the “bluff body” components. The result obtained can inform decisions regarding design optimizations of the aerodynamic surfaces so as to minimize the effects of vortex shedding on the aircraft's performance and stability.

The CFD simulation of vortex shedding analysis involves solving the Navier-Stokes equation associated with the fluid motion around the aircraft surface. The pressure distribution over the surface can be analyzed to calculate the lift and drag forces acting on the aircraft. The common observation of the vortex shedding phenomenon in aircraft is increased drag, increased noise, and reduced lift.

Below are a few ways vortex shedding simulation can help in aircraft performance analysis.

### How Vortext Shedding Simulation Supports Aircraft Performance Analysis

 Lift and drag analysis Analyzes flow behavior and pressure distribution over the aircraft surface.Alternating low and high-pressure zones or flow separation reduces the lift.The formation of turbulent eddies increases aerodynamic drag. Stability analysis Analyzes the flow behavior around the aircraft surface to identify stability-affecting vortex shedding effects such as:BuffetingVibrationWake turbulenceRoll instability Noise analysis Identify the areas prone to noise generation due to vortex shedding.Tonal noise – near wings, tails, or engine nacelles.Broadband noise – the wake of the aircraft; is more pronounced during takeoff and landing.

The insight into the above factors affecting aircraft performance allows system designers to make necessary design optimization decisions so as maximize the aerodynamic efficiency and performance of the aircraft.

## Optimize Aircraft Design to Reduce Vortex Shedding Effects

Using CFD tools, the aircraft and flow model can be generated. Through fine discretization and numerical analysis of the governing equation at each cell, it is possible to obtain the flow field where the vortex shedding pattern can be studied. CFD tools can make use of methods like the finite element method (FEM) or finite volume method (FVM) to solve the Navier-Stokes equations for airflow.

The visualization of flow behavior, pressure distribution, and vortex shedding patterns around the bluff body in the simulation model can be taken as a reference to make decisions about the modification needs for the aircraft. With tools such as  Fidelity and Fidelity Pointwise, you can run accurate CFD-based vortex shedding simulations so you can make accurate predictions and optimization decisions.

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# About the Author

With an industry-leading meshing approach and a robust host of solver and post-processing capabilities, Cadence Fidelity provides a comprehensive Computational Fluid Dynamics (CFD) workflow for applications including propulsion, aerodynamics, hydrodynamics, and combustion.

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