# The Types of Vortex Vibrations

### Key Takeaways

• Vortex vibrations are complex, non-linear fluid-structure interactions in which vortex shedding hydrodynamic forces excite and interact with flexible structures.

• Typically, cylindrical structures that are exposed to fluid flow shed vortices.

• At the lock-in state, the vortex shedding frequency equals the natural frequency of the nth vibration mode.

Vortex vibrations are an interdisciplinary engineering problem

Vortex vibrations are of great practical interest, as they are seen in many engineering fields, including in riser tubes carrying oil from the seabed to the surface and in heat exchanger tubes. Vortex vibrations must be taken into consideration while designing marine vehicles, as the large amplitude vibrations generated affect structures in the ocean. This article will explore vortex vibrations, vortex shedding, lock-in, and the types of vortex vibrations.

## Vortex Vibrations

In fluid flows, there are many kinds of fluid-structure interactions. For example, in heat exchanger tubes or riser tubes carrying oil from the seabed to the surface, fluid interacts with the structure and influences the dynamics of the flow. Vortex vibrations are complex, non-linear fluid-structure interactions in which the vortex shedding hydrodynamic forces excite and interact with the flexible structures.

### Vortex Shedding

When a fluid flows past a bluff body in a direction normal to the axis of the body, boundary layers are developed around the body. The fluid just flows past the body, if the Reynolds number of the fluid flow is low. As the Reynolds number increases, the vortices tend to get separated from the body surface. The vortices roll up at the downstream side of the fluid flow. As the Reynolds number increases, the adverse pressure gradient causes alternate shedding of vortices, called vortex shedding.

Typically, cylindrical structures that are exposed to fluid flow shed vortices. The fluctuating lift and drag forces that arise from shedding may induce structural vibrations. These structural vibrations give rise to vortex vibrations when the vortex shedding frequency is close to the fundamental frequency or natural frequency of the structure.

As the vortices are shed on the surface of the cylindrical structure, it experiences periodic forces that make it undergo vortex vibrations. The vibrations continue as long as the vortices are shed.  At the vortex vibrational phenomenon, the amplitude is so large that it affects the system operation and causes system failure or fatigue failure. Offshore structures are constantly experiencing vortex vibrations and are designed to overcome the resulting fatigue.

### Lock-In

The large amplitude vortex vibration mentioned above can result from the lock-in phenomenon. When the vortex shedding frequency is close to the natural frequency of the structure, it causes the lock-in phenomenon. The vibrations of the body in the fluid flow cause the vortex shedding frequency to shift from the natural shedding frequency to the frequency of the body oscillations. At the lock-in state, the vortex shedding frequency equals the natural frequency of the nth vibration mode. The resulting vibrations are either at the fundamental frequency of the structure or somewhat close to it. The lock-in of the vortex shedding frequency to the natural frequency usually occurs at the region where the reduced average velocity is between 5 and 7.

## Types of Vortex Vibrations

1. In-line vortex vibrations - The symmetric shedding of vortices on the surfaces of the bluff body result in in-line vortex vibrations. These vibrations usually happen at an average reduced velocity of 1.25. There are two instability regions in in-line vortex motion–the 1st  instability region corresponding to reduced velocity from 1 to 2.5 and the 2nd instability region ranging from 2.25 to 2.5. There are no vortex vibrations when the average reduced velocities are below unity.

2. Cross-flow vortex vibrations - When vortex shedding is not symmetric, the vortex vibrations generated are called cross-flow vortex vibrations. These vibrations are common at the higher range of reduced velocity. The amplitude of cross-flow vortex vibrations is greater than in-line vortex vibrations.

3. Hybrid vortex vibrations - Vortex vibrations that are a combination or mixture of n-line and cross-flow vortex vibrations are called hybrid vortex vibrations. The reduced velocity range of hybrid vortex vibrations is greater than in-line motions but less than cross-flow motions.

Vortex vibrations are an interdisciplinary engineering problem. The influence of vortex vibrations is commonly observed in civil, subsea, ocean, electrical, mechanical, and aerospace engineering applications. It is important to address vortex vibrational issues before they cause too much damage.

Cadence’s suite of CFD software can help you model vortex vibrations resulting from fluid-structure interactions. Cadence offers an advanced solver to aid engineers in finding solutions for mathematical models related to vortex shedding and vortex vibrations.