22
Aug

Turbulence. Understanding unsteady loads

TurbSim synthetic turbulence integration with dynamic analysis software ProteusDS.

 

When you think of turbulence, likely the first thing that comes to mind is those anxiety inducing bumps that happen 30,000 feet above sea level in an airplane.

However, turbulence doesn’t just occur in the air, in fact, turbulence occurs in most naturally occurring fluid flow. In tidal energy, characterizing turbulent flow is vital for engineers to determine the loading on structures and turbines. How the system will respond to unsteady flow conditions affects design life and power prediction, both of which are critical for the tidal industry.

 

Ocean engineers and other ocean industry professionals use dynamic analysis as a method to test virtual prototypes in simulated ocean environments to determine the loading from complex environmental conditions. Incorporating both time and spatially varying current models into dynamic analysis has traditionally not been possible in dynamic analysis software packages.

However, Dynamic Systems Analysis has recently demonstrated the ability to simulate turbulent flows to our dynamic analysis software ProteusDS, using the time and spatially varying current functionality.

Turbulent flows can be readily simulated using the stochastic turbulence generation tool, TurbSim. TurbSim is developed by the National Renewable Energy Laboratory (NREL), it produces statistically similar oceanic and riverine turbulence by generating two-dimensional planes of spatially and temporally varying velocity.

The three-dimensional spatially and temporally varying current option in ProteusDS provides the framework to specify the current data at any location in space and time to use in a simulation. Using Taylor’s frozen turbulence hypothesis, a specially designed converter was developed to allow TurbSim’s two-dimensional time-series output to be formatted as input into ProteusDS.

The two-dimensional planes generated by TurbSim change with time. To convert this 2D representation to 3d, the planes are ‘marched downstream’ to create a 3D grid. This gridded representation makes it possible to import data into ProteusDS to simulate the turbulent flow.

Planes are placed at intervals along the X axis using Taylor’s frozen turbulence hypothesis

Planes are placed at intervals along the X axis using Taylor’s frozen turbulence hypothesis

 
What are the benefits of having an integration like this?

 

  • Assessment of fatigue loading
  • Prediction of structural response
  • Development of realistic asymmetric loading scenarios (moorings and structures)
  • Assessment of stability and motion

 
These developments can have an impact on design and analysis across many ocean engineering sectors, including tidal energy, wave energy, and aquaculture. Researchers can now better simulate real world turbulent flows and use them to gain invaluable insights on how turbulence will affect peak loading, energy extraction, fatigue, and structure responses.

Areas like the Bay of Fundy, in Canada, are impacted significantly by tidal turbulence, therefore it is safer to simulate the effects of turbulence rather than using a single mean current to simulate how your platform is going to react.

Designing technology for the complex ocean environment is a constant challenge. Dynamic analysis coupled with turbulence simulation allows engineers to safely innovate and optimise.