Learning Forum

[Greg Ioannou]

Hello,may i ask if it is possible to draw a heave plate(with thickness and mass) like we do for line bodies?Or we only can draw it in Spaceclaim or draw heave discs(with no thickness and mass).In reference manual it says that it is possible to add mass via defining a PMAS element and attach it to the disk element(6.3.5).How can we do that?Finally ,when we add a heave plate at spaceclaim,how can we define for it a particular density?

Thank you a lot!!!

[Ingrid Machecler]

 

 

Hello, 

An approach would be to defined your plate in SpaceClaim with a given thickness (at least the size of a mesh elements). Then, you can add a series of disks (right click in the geometry section to add them) which will introduce the drag. Make sure you keep your added mass very small so you don’t count it twice.

Alternatively, you could create a wide, thin rectangular cross section line body. This introduces the mass, added mass and drag for the heave plate, and graphically looks like it should, as long as the heave plate is rectangular.
A similar reply has been posted on this discussion thread: https://forum.ansys.com/forums/topic/ansys-aqwa-how-to-add-insert-fins-for-damping-at-our-marine-structure/

I hope this helps.

 

 

[Greg Ioannou]

Thank you a lot for your answer!I asked just because it would be easier for example to draw some line bodies at the same height of the structure....and then give the chance to draw a circle with a particular thickness.Whatever!

Thanks a lot!

[Greg Ioannou]

 

 

 

 

 

I tried all the ways of making it.I tried the way to create a wide rectangular cross section body.But the line body needs to get through the main body and errors appear again like those below.I want a plate to SURROUND the cylinder body,so i could only make line bodies inside the main body too.

So,errors appear and i can do nothing:

 

1)I tried to draw at spaceclaim ,but i had errors again 1.000element violates the condition(smaller thickness of the plate than the element size,almost half of it).I also insert heave disks and i made point masses for them,but i …i want some thickness .My favourite way would be adding a rectangular plate(as it cant happen for a circular plate) via line bodies,but it didnt work.Any idea or help for it???

2)When we insert heave disks…and we add point mass(element) for them(aqwa reference p.70 says about that) ,do we have to insert the moment for inertia for a disk or for a circular ring(the diameter of the heave plate bigger than that of the cylinder)?And when we insert series of them trying to give thickness,what distance should they be for having better results(0.1 or less?) and should they have the same drag coefficient or only the top and bottom disks have?

Thank you a lot!!!

 

[Ingrid Machecler]

Good morning, 

1) the thickness of the plate should at least be the size of an element for the meshing to happen correctly. 

2) you will need to add a point mass & moment of inertia to account for the mass properties of what would be the plate. Then the hydrodynamic properties (drag, added mass) would be modelled through your various Morison elements so there is no need to set a thickness value for them. You ring can be modelled with overlapping discs. You would need to ensure is that the sum of the discs area is equal to the real world area of the skirt. 

I guess you don't need to implement both solutions: it could be one or the other.

I hope this helps.

Kind regards,

Ingrid

[Greg Ioannou]

 

 

1)

Hello again.I draw a cylinder with plates(underwater) at spaceclaim and i added disks from aqwa with D, Cd and Ca.I put Ca=0.0000101(very small)  and…what values of Cd should i put?Must i double the default values first and use for examples values from 2.28 and more? Do we assume that those disks are immersed at both sides ?Did i do well that i used really small added mass coefficients for all disks??

2)The values of Cd do they depend on the waves and the  depth where  the heave disks are?As they depend of the square of the water  particles velocity,how do we know which exact values of Cd to use?Or do we just use hypothetical values of Cd?

Thanks a lot!!!

 

 

 

 

 

 

 

 

 

 

 

 

[Greg Ioannou]

 

If only some1 Aqwa expert answers at least my above 1st question.As the heave disks are immersed,but inside the heave plate(from spaceclaim),what values of drag coefficients should we put?Do we have to double the drag coefficients as the disks are immersed  or not?And did i do right that i used negligible added mass coefficients for the heave disks?The second question above concerns hydrodynamics and fluid mechanics…not Aqwa so,i dont expect a specific answer there…I would appreciate though,if some1 answers the first question...

Thanks a lot!!

 

[Ingrid Machecler]

Hello,

It seems that you have a plate AND the disks :it's one or the other.
If you want to use disks, you don't need the plate. Just one layer of disks spread around the tube (like a flower) to model the behaviour of a the skirt. Just ensure that the area of the plate equals the area of the surface covered by the disks. The default values for one disk are: Ca=1, Cd=1.4. As your disks are submerged and act in both normal directions, you will need to double the value for Cd.

I hope this helps.

Ingrid

[Greg Ioannou]

 

 

 

 

 

 

 

I thought you meant heave plate at spaceclaim for volume and mass PLUS heave disks all at once  for giving drag to the structure,with very small added mass for the heave disks…I misunderstood.

1)Hello again and thank you for your answer.I think it is Cd=1.14 default value at 2023 R1 which i use.I wanted to add mass,volume( and thickness) to my structure,so i added the heave plate at spaceclaim(it adds volume,without it no volume is added) ,not only the disks…as alone itself the heave plate(WITHOUT heave disks) gives terrible responses.I thought at the above messages you meant that we should use them both together.Is it false to use them together?Finally,should i just use a point mass(with the mass of the plate in real world) with the moments of inertia + series of disks(so…heave plate =heave disks+point mass with moments of inertia WITHOUT heave plate  at spaceclaim….) ??

2)If we use heave disks(series of heave disks),should i use the same  added mass coefficient for all the heave disks???I mean the in between disks must they have the same added mass coefficient with the aside 2 disks,or it must be very small for them???And Cd mustnt be different at different depths of our structure as the draught of a spar is big?What do you mean when you say,”when the disks act in both normal directions”?The middle disks of the one series should they have the same drag coeff.with the apex disks?

3)I inserted a heave plate at spaceclaim to give more volume and not having gaps between the heave disks...as those gaps(distances between the series of the disks) may affect adversely the responses.How should i have no gaps as i made the heave disks having distances between each other 0.05m???

Thank you again!!