Low Drag coefficient! :(

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 June 11, 2009, 03:40 Low Drag coefficient! :( #1 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Hi all! I am trying to do find the Drag on a small underwater marine vehicle. Its .8 m in Length and comprises of two bodies: One ellipsoid on top, and a cylinder below it. The total area is .8 sq-m and the speed at which this vehicle is supposed to travel is 0.4 m/s. Now when I solve for this vehicle using the K-epsilon realizable model for the X component of force on the vehicle, I get really low values for the force and the drag coefficient( 4.4 N and Total coefficient(pressure + viscous) as given by FLUENT is .0054).Could you please help me out with what am I doing wrong? Also, when I had a look at the center of pressure of the object, the solver showed me a value which was like REALLY far away from the vehicle. Would you please please please help me out with this? Thanks a lot. DT

 June 16, 2009, 11:47 #2 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Anyone? Please :|

 June 16, 2009, 11:49 #3 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Never mind the center of pressure problem. I got what was wrong there. But can anyone please help me out with the drag coefficient? What could I be doing wrong? Should I increase the roughness height on the hull? (Its already quite high.. 0.8 mm) Please! Any advice would be more than welcome. Thank you.

 June 17, 2009, 04:04 #4 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 I tried increasing the roughness height, decreasing the roughness height, but the forces dont change by much. Please please help me out. What am I doing wrong? The solution converges everytime, but the forces are not different at all. With a roughness height .0005 mm, the Cd was .067 and with roughness height .005, it remained almost the same!! Please please help me out. What am I doing wrong over here?

 June 17, 2009, 05:31 #5 Senior Member   Max Join Date: Mar 2009 Posts: 133 Rep Power: 8 Hello, just to avoid unnecessary discussion, you know that force and moment coefficients use the reference values in the report/reference panel when evaluated? They should be consistent with your actual physical problem. cheers

 June 17, 2009, 05:49 #6 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Hey Max, Thanks for the reply. Yes, the reference values are in accordance with the problem. Is there anything else that I can look into? The forces the solver reports after convergence, the viscous part in them is VERY low. Only about .5-.7 Newtons. This can not be right. Is there anything else that I should check? Thanks a lot in advance.

 June 17, 2009, 07:11 #7 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 On second thoughts, as the vehicle is totally gonna be submerged totally in water, and its gonna travel at a low velocity, around 0.5 m/s, so that is like a Re of 3*10^5. So shouldnt the drag coefficient be low? As it will only comprise of the viscous part? And by the ITTC line(can I use it?), the drag coefficient comes out to be of this order approximately. Please please correct me if I'm wrong.. Is a drag coefficient of .02 for a body (an ellipsoid and a cylinder) travelling at 0.5 m/s totally submerged in water, OK?

 June 17, 2009, 12:30 #8 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Please please help me out. Someone..also, this is the link to the velocity profile which I get for the vehicle after the solution has converged. http://picasaweb.google.com/lh/photo...eat=directlink As you can see, the wake region is very long..is this normal? And also, do you think I should use a longer flow domain? Could that be causing the problem?

 June 19, 2009, 04:27 #10 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Hi Mystic, Thanks for taking out time to reply to my post. I dont think I can increase the domain size, because as it is, I have 2.1 Million cells and when I go up from this number, FLUENT starts giving me "malloc" faults. And btw, I am running a 3D case on the vehicle. And I think that I'll do this drag simulation for a sphere, as you suggested, because that might help me find out whats wrong. But I think you should have a look at this thing. Autosub: An autonomous unmannedsubmersible for ocean data collection Search for this paper on IEEE. OR I can send it to you if you dont have access. I was going over this paper yesterday, and its given in the paper that AutoSub is a 7 m long AUV, 0.9 m in Diameter, a velocity of 2m/s and its mentioned in the paper that the thruster, 5 bladed, is "able to provide the required 130 Newtons thrust." Dont you think that the Cd for this is pretty low too? Please help me out.. I'm so confused. I know this is an unrealistic value, but what about the AutoSub paper? Thanks in advance. Will wait for your replies..

 June 19, 2009, 10:03 #11 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Hey Mystic, I tried modeling the Sphere like you suggested me to. I looked up on the forum and came across this thread. CFX analysis of a sphere - drag too low So I thought why not model the same problem: For an inch diameter sphere,in airflow at 60m/s, I got a drag force of 0.1 N when it should have been .46 N. ( what did I do wrong? Why the low drag coefficient? BTW, did you have a look at that AutoSub paper? I did, just to be sure. And tit says that the thrust required by that 7 m long AUV,0.9 m in diameter, while traveling at 2 m/s is 130 N. That means the Cd is pretty low for this vehicle too, right? Thanks in advance.

 June 19, 2009, 16:04 #12 New Member   HP Kritzinger Join Date: May 2009 Location: South Africa Posts: 25 Rep Power: 8 You might have to trust your answer as being correct. I had a look at this page: http://en.wikipedia.org/wiki/Drag_coefficient (don't have textbook at hand for the moment). they list many drag coefficients, and all aeroplane bodies are in the range that you first reported, i.e. Cd = 0.02 - 0.1, so a minisub should be there ot thereabouts. The 2million cell limit means you are running 32 bit, i would really urge you to try and get onto a 64 bit platform which would allow for a larger mesh to test sensitivity. hope my previous post did not lead you too far down the wrong avenue.

 June 19, 2009, 16:11 #13 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Are you freaking kidding me? Thank you for your replies. I love it when people actually try and help others over here And yes, I think you are right. The answer, it maybe right. And in the lab I'm in, they dont have 64 bit platforms so theres no way I can have more than 2 million cells right now. I have to run the whole thing on my computer and it almost kills it. :| Any suggestions for the flow around a sphere? I am getting 1/5 th of the answer I should get. Thanks again.

 June 19, 2009, 16:28 #14 New Member   HP Kritzinger Join Date: May 2009 Location: South Africa Posts: 25 Rep Power: 8 did you include roughness on your sphere model? according to my "very reliable" wiki page, for a smooth sphere we have: Cd = 0.1 this equates to F = 0.11N if I use V=60m/s; d = 25.4mm; rho = 1.225 kg/m3; mu = 1.8e-5 Pa.s. values of Cd = 0.4(5) for rough spheres... would give you higher force in line with what you seem to be expecting. you might be right again - i found that SST-kw and reliazible ke gave quite accurate drag results for bluff body work that i did in the past.

 June 19, 2009, 16:46 #15 Member   DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 8 Aah..yes. About that, what range do these values(roughness height, roughness constant) lie in? I mean how do enter the value of the roughness height? Is there any page on the internet which gives the values of these for different materials? Thanks.

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