Vortex shedding and its implications
Role of resonant frequency and vortex-induced vibration in failure analysis
The use of CFD tools to analyze vortex-shedding behavior
Example of vortex shedding simulation
A vortex forms due to velocity discrepancy, resulting in fluid moving spirally. You may have observed this phenomenon in the form of tornadoes or whirlpools in basins. In aerodynamics, air forms a similar pattern when it passes through an airfoil. A similar phenomenon can be true in hydrodynamics for water flowing through a pipe.
Vortex shedding occurs when the fluid flows over the bluff body and vortices form behind it. The nature of the flow can cause irregular separation of the fluid and induce vibration in the object, leading to failure. Thus, it is important to reduce vortex shedding. With CFD tools, it is possible to simulate vortex shedding, analyze its behavior, and optimize a design accordingly.
Understanding Vortex Shedding
To explain this phenomenon, multiple studies have been carried out to analyze the behavior of structures like the industrial chimney stack as the wind flows across it. When tall structures like towers or chimneys made of steel confront the wind blowing at a considerable velocity, a low-pressure zone forms at the downside of the structure. Vortex shedding occurs as the wind flow continues beyond the structural barrier, leaving a vibrating effect. This vibration can be responsible for serious damage and failure of the structure.
The same concept is true for pipe flow where the components like thermowell or joints (t-joints or corner joints) act as bluff bodies. This is where the flow path changes, causing vortex shedding. When the frequency of the vortex shedding matches the natural frequency of the piping system, it is likely to vibrate and fail.
Fatigue in Piping Systems Due to Vortex Shedding
Continuous oscillation or vortex-induced vibration (VIV) can make the piping system prone to multiple stress cycles. As the fatigue accumulates due to stress, cracks can appear and the eventual result is the failure of the entire system. Identifying when the failure happens can be difficult given the possibility of vortex shedding at a wide range of Reynolds numbers.
However, it is possible to identify the frequency at which the vortex shedding takes place. The value is dependent on the Strouhal number – the factor that describes the oscillating mechanism of the fluid flow. Mathematically, the frequency can be expressed as:
Fs is the vortex shedding frequency or Strouhal frequency
S is the Strouhal number
Vo is the velocity of the fluid flow
D is the diameter of the cylindrical body
Reduce Vortex Shedding With Efficient Fluid System Analysis
Vortex shedding can be detrimental to the operation of the fluid flow system at or near the resonant frequency. Therefore, exploring ways to reduce vortex shedding is critical. Thorough vortex shedding analysis can help in the assessment of:
Bluff body structure and its influence on vortex formation
Fluid system analysis – mechanical properties of the structure material and fluid characteristics (temperature, pressure, velocity)
Stress developed in the system during fluid-structure interactions
Resonant frequency and VIV evaluation
Analysis can be better performed with the help of CFD tools. Not only can these tools help simulate vortex shedding behavior, but they also provide insight into flow dynamics by solving the Navier-Stokes equation associated with the fluid.
To reduce vortex shedding in the pipe flow system as mentioned above, it is recommended to provide more piping support to counter the vibration. Many experiments have also found the addition of a helical groove to the bluff body (such as the thermowell) beneficial in minimizing the effects of vortex shedding.
CFD Tools Help You Design a Reliable Flow System
Cadence’s CFD tools facilitate solving complex flow problems as the flow passes through the bluff body. These tools can numerically simulate complex vortex shedding behavior and solve for the pressure, velocity, and frequency correction – all key calculations required to reduce vortex shedding for the optimization of the fluid system.
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