{"id":250991,"date":"2023-02-22T11:45:06","date_gmt":"2023-02-22T11:45:06","guid":{"rendered":"\/forum\/forums\/topic\/aqwa-diffraction-analysis-submerged-cylinder-does-behave-wrong\/"},"modified":"2024-02-29T15:23:41","modified_gmt":"2024-02-29T15:23:41","slug":"aqwa-diffraction-analysis-submerged-cylinder-does-behave-wrong","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/aqwa-diffraction-analysis-submerged-cylinder-does-behave-wrong\/","title":{"rendered":"Aqwa Diffraction Analysis: Submerged Cylinder does behave wrong"},"content":{"rendered":"

Hello everyone!<\/span><\/p>\n

I am doing a hydrodynamic diffraction analysis in Aqwa 2022.<\/span><\/p>\n

The body i am researching in is a cylinder, which is floating close underneath the water surface(-0,02m). I defined a speed of 0,75 m\/s and a wave direction of 180°.<\/p>\n

The results are not as expected. I am wondering about the movement of the body at low frequencies. In the RAO RY it has its peak at 0,32 rad\/s with a value of 32,7° which leads to extreme rotation around the y-axis. The movement in z has its peak at the lowest frequency, 0,1 rad\/s with a value of 4,07. Both is far to high and shown in the picures below. I expected both peaks somewhere at 3,77 rad\/s where the wavelength equals the body length. At very low frequencies it should just follow the waterlevel without rotation.<\/p>\n

It follows the case’ setup.<\/p>\n

The technical specifications are L=4m,D=0,3m, m=289 kg (calculated by hand). The Center of Gravity has its position at x=0m; y=0; z=-0,19m. The center of buyancy is at  x=0m; y=0; z=-0,17m.<\/p>\n

First i draw the cylinder as a solid in Ansys Design Modeler, than I converted it to a thin geometry with the wallthickness of 0 m. Next step was shifting the cylinder so that the leading edge is on x=2m; y=0 and the cylinders symmetry axis lies at z=-0,17 and parallel to the global x-axis. \\\\
In Aqwa I included a pointmass with the mass calculated by aqwa due to the strctures position, and manually setting the z Position of the Center of Gravity. Which reads then x=9,36E-5m; y=1,5E-4m, z=-0,19m and 269 kg, which is 20,2 kg to less. Furthermore I added the mass moment of intertia Ixx=3,6 kgm^2,Iyy=Izz=388kgm^2, calculated by assuming the cylinder as a solid and the density of 1025kgm^-3. Meshing has been done by the Aqwa mesher with an element size of 0,1 m gives 418 elements.<\/p>\n

The analysis settings read generate wave grid pressures: Yes; Wave Grid resolution: Fine, Wave grid factor:2; 
Calculate Extreme low frequencies: no; 
Include multi-Directions: No;
Near Field Solution: Programm controlled;
Calculate Full QTF Matrix:No and all output file Options to no;<\/p>\n

wave direction to 180° and a forward speed of 0,75 m\/s.
25 wave frequencies in the interval of 0,1 rad\/s to 6 rad\/s.<\/p>\n

environment constraints: 
Waterdepth: 1000 m
Water density: 1025 kgm^-3<\/p>\n

Analysing Hydrostatic Results (shown in the last picture):<\/p>\n

I guess that the huge movement is related to the low restoring moment at small angles of 0,92 Nm\/° in MX\/MY and that the distance GoG to GoB (BG) is small and negative with a value of -0,01994 m. Why negative? The values for COB and COG are correct. Furthermore I am wondering that it calculated the center of flotation at x=-2,28m, y=-4,49 m. But there is simply no center of flotation because its fully submerged.<\/p>\n

If anybody has an idea what is the solution here I am very thankfull.<\/p>\n

Cheers <\/p>\n

<\/p>\n

 <\/p>\n","protected":false},"template":"","class_list":["post-250991","topic","type-topic","status-closed","hentry"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["275889","34392"],"_bbp_author_ip":["23.192.164.18"]," _bbp_last_reply_id":["0"]," _bbp_likes_count":["0"],"_btv_view_count":["1185"],"_edit_lock":["1677068293:206959"],"_bbp_topic_status":["answered"],"_bbp_topic_id":["250991"],"_bbp_forum_id":["27792"],"_bbp_engagement":["34392","275889"],"_bbp_voice_count":["2"],"_bbp_reply_count":["1"],"_bbp_last_reply_id":["251441"],"_bbp_last_active_id":["251441"],"_bbp_last_active_time":["2023-02-24 16:36:04"]},"test":"lars-glatzeruni-rostock-de"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/250991","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":1,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/250991\/revisions"}],"predecessor-version":[{"id":354593,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/250991\/revisions\/354593"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=250991"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}