axial drag

Good afternoon,
I have questions concerning the calculation of axial drag in Hydrodyn. I may have missed it but I could not find in the user manual the formula used for the theory. I guess it is is implemented as in “Definition of the Semisubmersible Floating System for Phase II of OC4” put in the pdf joined to this post.

1. In this joined formula, I am wondering concerning the velocity of the fluid particle (noted w in the pdf), how is it calculated for a pure Morison approach if it is not by a potential flow theory?

2. Then whatever the velocity is calculated, is the calculation performed in the absence of the structure ? Or it is calculated at small distance from the object (because velocity is supposed to be null on the structure if we are in non sliding conditions?)

3. My last question concerns what is called the reference volume (Vr in the pdf). Is it defined as a spheric domain around the plate as shown in the joined document ?

Many thanks in advance for any help.
Best regards.
Florence Haudin.
axial drag.pdf (775 KB)

Hi Florence,

The document you attached shows a specific instance (simplification) of the general hydrodynamic axial drag formula used by HydroDyn. Here are my answers to your questions:

1 / 2) The wave kinematics (fluid particle velocities, accelerations, and dynamic pressures) used by HydroDyn’s strip theory solution are based on undisturbed wave kinematics (in the absence of the structure), regardless of whether potential-flow is enabled. The wave kinematics generated internally within HydroDyn can be regular (periodic) or irregular (stochastic) and long-crested (unidirectional) or short-crested (with wave energy spread across a range of directions). The wave kinematics can also be generated externally and used within HydroDyn. Internally, HydroDyn generates waves analytically for finite depth using first-order (linear Airy) or first- plus second-order wave theory (Sharma and Dean)with the option to include directional spreading. The second-order implementations include time-domain calculations of difference- (mean- and slow-drift-) and sum-frequency terms.

1. Yes, that is correct, but each side of the plate (joint in HydroDyn) is based on the associated semisphere.

Perhaps you already saw this, but a related question about the hydrodynamic axial effects of HydroDyn was recently discussed on the forum–see: http://forums.nrel.gov/t/question-about-axial-drag-in-morison-f90/2259/1.

Best regards,