negative lift of underwater vehicles
 

Hello fluent user,

I have a problem calculating the lift of a vehicle 10 degree angle of attack of the hydroplanes underwater. Fluent is giving the pressure coefficient of lift in negative value and total lift becomes negative as well.According to my study the lift coefficeint should be positive while the vehicle is coming on to the surface. Please give me some advice regarding the above problem as soon as possible. Thanks in advance.

Re: negative lift of underwater vehicles
 

maybe you have to describe your problem setup more detailed. is it 2d or 3d? tet or hex? what about hull hydroplane interaction? what is y+? which turbulence model? domain size? etc.

if it is 2d problem there may be a basic problem setup error. if it is full 3d problem with sub hull and hydroplanes the problem size will be large at least several million cells... kolay gelsin.

Re: negative lift of underwater vehicles
 

Merhaba arkadasim it is 3d problem with tetrahedral mesh. hull hydroplane interaction is conventional type and turbulance model used is k-e RNG with standard wall funtion approach. domain size is almost 1.9 milliom tetrahedral cells. one thing more that angle of attack of hydroplanes is 10 degrees. we do not know about the actual value of y plus value.Boundary conditions we r using as velocity inlet, wall body , outflow or pressure outlet.hoping for your early reply. Tesekur ederim

Re: negative lift of underwater vehicles
 

it is clear that the physical dimesions of the sub hull and the hydroplanes are very different. so i guess for a good meshing you will need multiblock meshing (if you have not used yet) to get enough domain resulation on the bow, hydroplanes etc...

if the hydroplanes on the shoulders of sub, the meshing on the bow section (until the hydroplanes) is very critical because flow conditions on the bow affects the flow on the planes deeply. according to my experiences for sub modeling, hex meshing will be a far better choice then tet. and spalart allmaras model is faster and better then any k-e.

since i have not used fluent for a long time but remember that you can get actual y+ values about the simulation. did you calculate your mesh size requirement for suitable meshing (http://geolab.larc.nasa.gov/APPS/YPlus/) my advice; first make a 2d simýlation with hydroplane cross section to model and understand the flow phonemena and requirements for meshing, turbulence modeling and accuracy... then model the hydroplane in 3d with some part of the bow (or sail, where the planes are) i think, if you dont need a special interests full modeling is not required. i have read a paper for simulating an ohio class sub with full features (hull, rudders, hydroplanes, sail and prop) they have reqired required above 10 million cell domain for getting just some acceptable results (which is not very accurate to validation actually)... iyi çalþmalar, bu arada bu bir okul çalþmasý mý?