Fluid-structure interaction has a significant impact on vortex shedding and its effect on marine structural performance.
Vortex shedding applications are ideal for flow measurement, energy harvesting, and seakeeping in marine environments.
With CFD modeling of fluid and marine structures, fluid behavior can be identified, which can be used to calculate vortex shedding frequency. The frequency analysis helps discern the ideal design modifications that foster the efficiency of marine design.
Marine engineering systems often operate under harsh environments, which include exposure to dynamic loading and corrosive media. A wide range of structural issues stems from this exposure, which impacts the performance, safety, and stability of ships and other offshore structures. Structural damage can be further exacerbated by uncontrolled vortex shedding caused by pressure fluctuations and vibration during the interaction of the fluid with components like the hull or propeller.
However, the effects of vortex shedding are not limited to the negative impact they hav on marine design. The study of vortex shedding applications also fosters the development of different design modification and optimization strategies. In this article, we will discuss how the understanding of fluid-structure interactions can help us improve the performance of several vortex shedding applications in marine engineering.
Vortex Shedding in Marine Structures
When a marine structure such as a ship or submarine travels through fluid, the interaction leads to the formation of a low-pressure area towards the downstream side of the bluff body, such as the hull or propeller. A high-pressure area also forms towards the upstream side. The difference in pressure at the two sides creates vortices that shed alternatively on either side of the body. This vortex shedding has a wide range of effects on marine structures.
Effects of Vortex Shedding on Marine Structures
Vortex Shedding Applications
Many marine structures make use of the following vortex shedding applications for efficiency in operation.
Flow measurement: A flow meter is installed in the fluid stream, which measures the frequency of the vortices using sensors located near the bluff body. The accuracy in the measurement of the flow velocity is extremely important, as the frequency of the vortex shedding is directly proportional to the flow rate.
The flow measurement allows the engineers to control the flow rate by ensuring adherence to the design specifications. However, it is important to note that the accuracy of flow measurement may be reduced for turbulent or unsteady flow given the low predictability of the vortex shedding frequency.
Energy harvesting: By installing a piezoelectric harvester at the surface of the bluff body, the kinetic energy of the flow can be converted into an electric charge. The mechanical stress developed in the piezoelectric material due to vortex shedding can be used to generate electric power, which can be used on board.
Stability and seakeeping: Ships experience significant pitch or roll due to turbulent waves, creating difficulty in keeping the course as well as inducing discomfort to passengers. One way to mitigate this is by making use of vortex shedding to induce controlled vibration. The addition of structures like fins induces vortex with controlled vibration so the ships and waves have smooth interaction for improved stability and seakeeping.
Vortex Shedding Effects in Marine Structures
Drag: Vortices are shed when fluid flows around a vessel, creating wake turbulence behind it. This increases drag and compromises the performance of the structure. However, with design modifications, a controlled vortex (i.e., alternating areas of low and high pressure) can be induced for smaller wake turbulence, which helps in reducing the drag.
Cavitation: Vortex shedding causes the formation of regions of low and high pressure. At the low-pressure region, cavitation bubbles form as the pressure drops below the vapor pressure. As these bubbles move to the high-pressure region, they collapse and create a shock wave, causing significant damage to the marine structure. The addition of hydrodynamic components or design modifications for flow rate or pressure optimization can help in reducing cavitation damage.
Noise and vibration: Noise and vibration in marine structures come from vortex shedding, which creates areas of high and low pressure. The pressure difference creates an unsteady fluid force, which causes mechanical vibration in the structure and the surrounding fluid. Efficient optimization of marine design can help reduce vibration and noise transmissions.
The effectiveness of the above-mentioned vortex shedding applications and the design optimization requirements can be better analyzed with the help of computational fluid dynamics (CFD).
Implementing CFD for Optimal Marine Design
In marine engineering, CFD allows system designers to study the fluid-structure interaction so that necessary design modifications can be applied to optimize performance. CFD simulation allows engineers to model the fluid flow around ships to identify the areas of turbulence or high drag due to vortex shedding. This enables the identification of modification strategies such as the addition of flow-altering devices or changing the geometry of the structure.
CFD simulation also helps in evaluating phenomena such as:
- Vortex-induced vibration (VIV): Vibration induced due to vortex shedding, which induces structural fatigue.
- Vortex-induced motion (VIM): Motion induced due to vortex shedding, which causes the structure to oscillate, pitch, or roll, which may lead to structural failure.
CFD simulation can model complex flows to provide a detailed analysis of the areas of high stress and vibration. This information can be used to make design changes to reduce the risk of VIV and VIM, thus enhancing the safety and durability of the structure.
Cadence’s CFD tools such as Fidelity will help you evaluate the temperature and pressure gradient and understand the fluid behavior in the flow field so you can control the vortex shedding frequency and intensity. This can be done through effective design modifications and optimizations so you can enhance the efficiency of your marine engineering and vortex shedding applications.