Learning Forum

[Gumang]

nHi Guysni am a student . I am new to Ansys and using workbench 18.2nI want to perform free-free modal analysis for structures submerged in water (basic geometry :a plate or a cylinder). In this case one need to consider the hydrodynamic added mass while performing modal analysis. Could you guys please suggest me some ways to do this in Ansys. nthank you n

[Ashish Khemka]

nnYou can perform a modal acoustic analysis:nnnDefine a beam and model the model block around it which can be assigned the fluid properties. In the analysis system above define fluid-structure interaction between the beam surfaces and fluid. Also, please refer to the following link:nnnRegards,nAshish Khemka

[Gumang]

Hello Ashish, nThank you very much. I will be learning this analysis system. and will let you know if I succeed nKind regards nUmang Gupta n

[Gumang]

Hallo, ni didnt succeed. Couldn't figure out how to setup/ define FSI between the plate and the Fluid. I don't see an Option for it. nI found a document for immersed modal analysis from Ansys.n

[Ashish Khemka]

Hi,nnYou can change the scoping method to geometry and then select a common face.nnRegards,nAshish Khemkan

[Gumang]

, nthank you it worked using the modal acoustic analysis system. nI get the damped frequency in the solution. ni have another question though - When you do 'free free modal analysis' using modal analysis system . for the first 6 modes the natural frequency is 0. nwhy aren't the first 6 frequencies not 0, when i do free free modal analysis of submerged structures using Modal Acoustic analysis system? nis it a sign that I am doing something wrong ?.Regards nUmang n

[Gumang]

(forgot to add this )nI figured out that- if I increase the size of the fluid Block around the structure. The lowest frequency increases.nand this confuses me, because it should be other way around. nif there is more fluid around the structure. the frequency should get smaller. and after some point it should converge to some value.

[Ashish Khemka]

nnCan you check if the frequency is complex in nature?.Regards,nAshish Khemkan

[Gumang]

nthe frequency is not complex. I followed these steps. n results : nRegardsnUmang n

[Erik Kostson]

Hello,nnYou said: The lowest frequency increases.nand this confuses me, because it should be other way around.nnThat is not correct. The added mass say in a ship vibrating in shallow waters is much larger than in deep waters (this is a complex effect), so the added mass is larger (thus lower freq.) when the waters are shallow. Talking about a ship, thus let's relate the FEA - the ship would be the structure we analyse, then the seabed is the end of the acoustic domain (which represents the water) - so the smaller the acoustic domain, thus the closer the ship is to the seabed (end of domain) the lower the freq. of the ship vibrations.

[Gumang]

hey,nthanks for this information. n

[Erik Kostson]

No worries - happy that it was of some help.nnHere is a link also to a relevant paper explaining this perhapsnnSo the deep water vibrations are about 1.4 -3 times higher than shallow water vibrations for shipsn

[Gumang]

nI think these damped frequencies are from the fluid box and are not the natural frequencies of the plate. nThe mode shapes of the plate that I get here are just the deformation at these natural frequencies of the fluid box?.