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Application Examples

Keita Fujiyama (Software Cradle, Software Engineering Dept.)
Development of the Cavitating Flow Analysis Function

Using the SC/Tetra cavitating flow analysis function helps design engineers accurately predict possible cavitation issues while designing fluid machinery. Cavitation is a highly unsteady phenomenon that can cause many problems. These include device disorientation, aerodynamic noise, and surface erosion. Performing transient cavitating flow analyses early in the design process and implementing appropriate countermeasures are critical for achieving equipment performance and durability. Keita Fujiyama from Software Cradle, Software Engineering Department, explains the background and development of the cavitating flow analysis function.

Keita Fujiyama
Software Cradle, Software Engineering Dept

The demand for cavitation analysis capabilities in marine applications is growing?

Yes. Transient cavitation occurs on propeller due to the ship wake. This can cause substantial pressure fluctuations on the vessel. Knowing the extent of cavitation and how the resultant pressure fluctuations will affect the ship structure and passenger comfort during the design phase of the vessel is imperative. But conducting experiments that account for the ship body and propellers is costly and time consuming. This is why CFD offers so much potential for this particular field.

Fig 4 Analysis model of marine propeller (Click to enlarge)

Fig 5 Predicting the occurrence of transient cavitation on propeller turbine surfaces (Click to enlarge)

Fig 6 Pressure fluctuation on the surface of the ship (Click to enlarge)

Now, let us examine the analysis example*5 where flows around the ship and propeller are simulated together in a single model (Fig 4). We selected an analysis field according to the conditions used in experiments. Analysis models were generated based on the ship body and propellers used in scale models.

For validation, we estimated the cavitation patterns. Fig 5 indicates the area where cavitation occurs (isosurface with 10% void fraction). The calculated transient changes in cavitation caused by the ship wake followed the same pattern as the results from the experiments. Fig 6 shows the comparison between the amplitude of pressure fluctuations on the ship body for CFD and the experimental results. As the CFD results indicate, the amplitude of the pressure variation can be predicted not only for the blade passing frequency but also for its higher harmonic frequencies. I believe this shows that fluid analysis can be highly effective for estimating cavitation and/or pressure fluctuations during the design phase.

How many mesh elements are required?

Fig 7 Predicting cavitation occurring within the eddy near the turbine edge (Click to enlarge)

The maximum number of mesh elements used in CFD analyses used to be only a few million. But as software and hardware capabilities have improved, accurate and efficient analyses can be performed using approximately one hundred million mesh elements. Fig 7 illustrates the cavitation that occurs within the eddy near the marine propeller edge*6. The left turbine in Fig 7 shows long cavitation extending from the turbine edge. To perform this analysis, 75 million mesh elements were used. The mesh adaptation function in SC/Tetra was used to refine mesh elements around the eddy near the blade edge.

The blade on the right side in Fig 7 shows the cavitation predicted when using 130 million mesh elements. The mesh adaptation analysis function was not used, and the cavitation occurring within the eddy near blade edge was not predicted. In conclusion, the accuracy of cavitation analysis is dependent not only on the number of mesh elements used, but also on how adequate the mesh is for the specific phenomenon.

How is an analysis set up?

A cavitation analysis is set up in two steps. First, create an initial flow field by performing an incompressible analysis without cavitation. Using these results, restart as a compressible transient cavitation analysis.

The analysis becomes unstable when the pressure is extremely low or amount of cavitation is large. Any customers having convergence problems with cavitation analysis should contact the Software Cradle technical support team, as we have expertiese with substantial experience working with cavitation analyses. The support engineers can answer questions and provide specific advice.

What are the benefits of using SC/Tetra and are there any special considerations to keep in mind?

Setting appropriate condition and generating a sufficiently large and high quality mesh are required as in any CFD analysis. It is also important to validate the CFD calculations against experiment results. I think it is still too early to declare that CFD analyses can completely replace experiments.

The greatest benefit of CFD simulation is being able to visualize the occurrence of cavitation, which is difficult to actually observe experimentally. Visually observing the phenomena will enhance both learning and understanding, which will be beneficial for satisfying each customer's engineering objectives.

Certainly cavitation analyses have become easier and more effective as the analysis process is better established. Being able to authentically predict cavitation means that engineers can estimate pressure fluctuations and erosion risks. This will generate significant benefits for design engineers.

Any message to the customers?

Our goal is to provide further benefits to the customers by continuing to improve cavitation analysis capabilities. We will continue to introduce new technologies and improve the SC/Tetra cavitation analysis.

(*5)Fujiyama, K., "Investigation of Ship Hull Pressure Fluctuation induced by Cavitation on Propeller using Computational Fluid Dynamics", Proc. of the 17th Cavitation Symposium, 2014
(*6)Fujiyama, K., Kim, C., and Hitomi, D., "Performance and Cavitation Evaluation of Marine Propeller using Numerical Simulations", Proc. of smp'11 Workshop on Cavitation and Propeller Performance, 2011

*All product and service names mentioned are registered trademarks or trademarks of their respective companies.
*Contents and specifications of products are as of April 1, 2015 and subject to change without notice. We shall not be held liable for any errors in figures and pictures, or any typographical errors.



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