Creating a new object type?

[adebarthez-ux]

Hello,

I am currently using Leeway model to simulate the backward trajectory of cetaceans carcasses. To start my simulation I am using the object type 1, Person in Water. However I don't think it's enough precise for me to use an human type to simulate a cetacean carcasse.

I was wondering, is it possible to create a new object type with my own drift factors, specific dimensions and immersion of the new object ?

Thank you for your help!

[knutfrode]

Hi,
Leeway is an empirical model, where 9 coefficients are tuned for each object type based on ~hundreds of repeated drift-experiments, where drift, currents and winds were measured.
If you had corresponding coefficients, these could be inserted in this file:
https://github.com/OpenDrift/opendrift/blob/master/opendrift/models/OBJECTPROP.DAT
Or better, the Leeway module could be updated to accept user-defined coefficients as input - this would be a simple task.

However, the actual determination of the "correct" coefficients would be a difficult task. For analytical calculations based on physical parameters, you would need to make a lot of crude assumptions - as the air-water-waves interface is very complex.
And it is also not a deterministic answer, as also carcasses would come in different sizes, shapes (aspect ratio), densities and conditions. Thus some of the coefficients in the Leeway module are designed to capture such variability.

In practice, I guess the best approach is to find a Leeway object with drift properties corresponding as closely as possible to carcasses.
The main physical property would be the immersion ratio, which would mainly affect the first of the Leeway coefficients (~wind_drift_factor). For person-in-water this is as low as 1% (little windage) whereas for a light sailbout it is as high as 7% (mostly wind-driven).
https://github.com/OpenDrift/opendrift/blob/master/opendrift/models/OBJECTPROP.DAT#L3
https://github.com/OpenDrift/opendrift/blob/master/opendrift/models/OBJECTPROP.DAT#L186

I would think that person-in-water would be a reasonable choice for a carcasse, as I guess the immersion ratios would be similar.

[adebarthez-ux]

Hi,
As you said, I think I do not have the ressources to determinate the correct coefficients... ! So to choose a Leeway object seems to be the best solution.

Thank you very much for the detailed reply!