Community Question:

I'd like to know, more about the curves getting when we run a Dynamic Extreme wave. Could you mention what represents each one of these curves?

what is the reason for modifying the original curve?

could you give any recommendations for inserting time?

What is the main function of the option "Extend insert window for stream wave" in the card WVINS?

Answer 1  

Different curves in WVRNCF chat as a result of Dynamic Extreme wave analysis are

• Original: This is the random wave surface calculated based on the spectrum (i.e., Pierson-Moskowitz, JONSWAP, ...)
• Surface M: the surface of the insert wave generated based on the new wave theory this helps us to yield more realistic wave surface profiles.
• Slope M: is the time derivative of Surface M.
• Modified: is a mixture of original and surface m. We basically mix the two waves around the insertion time based on the Surface M and Slope M.
• Stream: is the surface of the stream wave we generated by solving the stream wave theory for a given wave height.
• STR Modified: is the final wave we use to calculate the forces on the structure. This is a mix of modified and stream waves around the insert time (Tins-Tp/2,Tins+Tp/2). The way we mix the two waves is based on the paper by Rainey & Camp (2007).  

Answer 2&3  

The reason for using a stream wave insert is to model an extreme event. In this way, the program will give the original wave at the insert time a large crest which allows a constrained wave simulation to be run according to NewWave theory. This method can be used instead of running many simulations in search of a wave of the required height, saving much computing time in the process.”

As for the insertion place the only constraint is to insert it at least one constrained wave period away from t=0. Other than that, the user can look at DNV RP-C205 or other applicable standards.

Answer 4  

This process of mixing two waves does not always create a realistic-looking wave. For example, you see in the next two images the wave without Extend insert stream wave and the wave with Extend insert stream wave.

Without using the Extend insert we can see there are secondary troughs at ~40s and ~60s due to the mixing of the stream wave and the modified wave. Such secondary troughs are numerical artifacts and unrealistic.

Without insertion window expansion

The new "Insert Window Expansion" algorithm achieves a more realistic sea surface profile by adaptively expanding the insertion window. To achieve this, the stream wave function is periodically extended to cover the whole time domain. Next, the algorithm progressively expands the insertion window so:

1. The trough located at Tins − Tp/2 is a local minimum.
2. The adjacent crest is above stationary sea level.

The algorithm fails if the expansion windows since is extended to create two crests.

With insertion window expansion

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