cfd simulation of an uav

prshet · May 30, 2011, 02:24

i havent tried it with the transient case as i got pretty good results with the steady case itself. ya in case ur inlet velocity increases (higher mach no.), then its better to switch to transient case.

Josh · May 31, 2011, 01:23

A UAV would likely experience low-Reynolds-number flow for which unmodified traditional turbulence models, e.g., k-w, have mostly been fruitless.

Have you tried the SST gamma-theta transition model to capture the transition to turbulence? What's your Reynolds number based on chord?

icemaniac178 · May 31, 2011, 05:39

dear josh,
yes i have tried using the SST gamma-theta transition model but unfortunately it does not give me any good stalling angle prediction. my lift ceofficient keep on increase up to 35 degrees. it is sound unrealistic right? my reynolds number is only 146000.

mr presht,
i am currently switching into transient analysis but i was so hard for me (begginner). even my analysis always force stop by some fatal error. now i am trying to play around with the timestep value. any suggestions to improve my transient analysis?

Josh · May 31, 2011, 06:10

Yes, that does sound unreasonably large, but I don't know anything about the model. Perhaps this UAV can fly at 35 degrees without stalling!

What are your angle increments? Just because the lift is increasing doesn't mean it hasn't stalled. For example, the NACA 0012 at Re = 51k stalls at approximately 12 degrees, but the lift begins increasing again from approx. 22 degrees to 40 degrees. Is it possible that you "skipped" over the stall angle, e.g., simulated the flow at 10, 20, and 30 degrees when stall occurs at 15 degrees?

icemaniac178 · May 31, 2011, 20:26

between 0 to 15 degrees, the angle increment was set at 5 degrees per increment. These analysis were well converged after the independent and sensitivity study.Above 15 degree, the increment was set at 1 degree per increment. starts from 17 degree, it shows some lack of convergence and i can see some separation phenomenon on the wing. i suspect the wing will be stall maybe around 19 to 22 degrees but it never happen. the lift keep on increase up to 35 degrees.

Josh · May 31, 2011, 22:00

Strange. Well, let us know how your other results turn out.

icemaniac178 · June 2, 2011, 23:02

mr josh and mr presht,
how do you refine the near wall mesh? did you use the inflation layer?
i used to do inflation layer on my wing model but it had end up with folded mesh problem during iterations/ staggering since i`m also dealing with FSI too. i`ve gone through some tutorials in mesh deformation and FSI but i`m still facing the folded mesh problem. maybe my model was too flexible i think. any suggestions?

Josh · June 2, 2011, 23:22

Yes, my near-wall mesh is a structured inflation layer. Unstructured boundary layer grids are undesirable as they can cause numerical inefficiency and instability. My boundary layer mesh had y+ < 1, which is required for the transitional flow model, and an expansion/inflation ratio of 1.06. Very refined.

I've only ever performed one moving-mesh simulation, but the airfoil was rigid, not flexible, so my inflation layer remained intact. I wish I could help with moving meshes, but I cannot.

icemaniac178 · June 2, 2011, 23:39

mr josh,
what application do you used for developing your mesh? is it the Ansys Mesh, ICEM CFD or others? currently i just used the Ansys mesh since it was better integration in ANSYS-CFX FSI. i cannot find any example to develop a FSI analysis by importing the mesh from other application.

Josh · June 3, 2011, 00:27

I use ICEM. It's a steep learning curve, but once you understand the basics it is much more versatile than ANSYS Mesh.

icemaniac178 · June 3, 2011, 03:08

is it possible to do two-way FSI by importing the mesh from ICEM CFD? i have post a question about this before but someone said it was not an easy setup since i have to combine with APDL application.

Josh · June 3, 2011, 07:14

I have no idea. Sorry!

Lance · June 3, 2011, 08:01

Quote:

Originally Posted by icemaniac178

is it possible to do two-way FSI by importing the mesh from ICEM CFD? i have post a question about this before but someone said it was not an easy setup since i have to combine with APDL application.

If you use Workbench you can import the solid mesh (from ICEM) through the FE-modeler and then connect it to a transient structural box.

icemaniac178 · June 3, 2011, 23:24

mr lance,
do you have any example on this matter?
i think it is very usefull for others too.

icemaniac178 · June 7, 2011, 03:07

Quote:

Originally Posted by Lance

If you use Workbench you can import the solid mesh (from ICEM) through the FE-modeler and then connect it to a transient structural box.

i have tried using the workbench to import the solid mesh (from ICEM) through FE-modeler but it was not successful. i think i dont have any good reference to refer to. do anybody have any tutorials on this?

Lance · June 7, 2011, 03:23

Quote:

Originally Posted by icemaniac178

i have tried using the workbench to import the solid mesh (from ICEM) through FE-modeler but it was not successful. i think i dont have any good reference to refer to. do anybody have any tutorials on this?

This works for me:
Right click on the Fe modeler box, add input mesh, browse, select ICEM Input (*.uns), connect the FE-box to Transient structural at the Model part.

icemaniac178 · August 15, 2011, 05:33

finally i have found the stall condition. its all about mesh. when the mesh is right then the stall can be captured. thank you to everybody who helped me...CFX forumers are really helpful..

prassana.dude · August 17, 2011, 04:33

Mr. Prasad Shet the work that u r carrying is very less. U need to work more on UAV's

August 17, 2011, 04:33	To SHET	#38
prassana.dude New Member Prasad Joshi Join Date: Jul 2010 Location: Pune Posts: 2 Rep Power: 0	Mr. Prasad Shet the work that u r carrying is very less. U need to work more on UAV's

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May 30, 2011, 02:24		#21
prshet New Member anonumous Join Date: Mar 2010 Posts: 12 Rep Power: 16	i havent tried it with the transient case as i got pretty good results with the steady case itself. ya in case ur inlet velocity increases (higher mach no.), then its better to switch to transient case.

May 31, 2011, 01:23		#22
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	A UAV would likely experience low-Reynolds-number flow for which unmodified traditional turbulence models, e.g., k-w, have mostly been fruitless. Have you tried the SST gamma-theta transition model to capture the transition to turbulence? What's your Reynolds number based on chord?

May 31, 2011, 05:39		#23
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	dear josh, yes i have tried using the SST gamma-theta transition model but unfortunately it does not give me any good stalling angle prediction. my lift ceofficient keep on increase up to 35 degrees. it is sound unrealistic right? my reynolds number is only 146000. mr presht, i am currently switching into transient analysis but i was so hard for me (begginner). even my analysis always force stop by some fatal error. now i am trying to play around with the timestep value. any suggestions to improve my transient analysis?

May 31, 2011, 06:10		#24
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	Yes, that does sound unreasonably large, but I don't know anything about the model. Perhaps this UAV can fly at 35 degrees without stalling! What are your angle increments? Just because the lift is increasing doesn't mean it hasn't stalled. For example, the NACA 0012 at Re = 51k stalls at approximately 12 degrees, but the lift begins increasing again from approx. 22 degrees to 40 degrees. Is it possible that you "skipped" over the stall angle, e.g., simulated the flow at 10, 20, and 30 degrees when stall occurs at 15 degrees?

May 31, 2011, 20:26		#25
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	between 0 to 15 degrees, the angle increment was set at 5 degrees per increment. These analysis were well converged after the independent and sensitivity study.Above 15 degree, the increment was set at 1 degree per increment. starts from 17 degree, it shows some lack of convergence and i can see some separation phenomenon on the wing. i suspect the wing will be stall maybe around 19 to 22 degrees but it never happen. the lift keep on increase up to 35 degrees.

May 31, 2011, 22:00		#26
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	Strange. Well, let us know how your other results turn out.

June 2, 2011, 23:02		#27
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	mr josh and mr presht, how do you refine the near wall mesh? did you use the inflation layer? i used to do inflation layer on my wing model but it had end up with folded mesh problem during iterations/ staggering since i`m also dealing with FSI too. i`ve gone through some tutorials in mesh deformation and FSI but i`m still facing the folded mesh problem. maybe my model was too flexible i think. any suggestions?

June 2, 2011, 23:22		#28
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	Yes, my near-wall mesh is a structured inflation layer. Unstructured boundary layer grids are undesirable as they can cause numerical inefficiency and instability. My boundary layer mesh had y+ < 1, which is required for the transitional flow model, and an expansion/inflation ratio of 1.06. Very refined. I've only ever performed one moving-mesh simulation, but the airfoil was rigid, not flexible, so my inflation layer remained intact. I wish I could help with moving meshes, but I cannot.

June 2, 2011, 23:39		#29
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	mr josh, what application do you used for developing your mesh? is it the Ansys Mesh, ICEM CFD or others? currently i just used the Ansys mesh since it was better integration in ANSYS-CFX FSI. i cannot find any example to develop a FSI analysis by importing the mesh from other application.

June 3, 2011, 00:27		#30
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	I use ICEM. It's a steep learning curve, but once you understand the basics it is much more versatile than ANSYS Mesh.

June 3, 2011, 03:08		#31
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	is it possible to do two-way FSI by importing the mesh from ICEM CFD? i have post a question about this before but someone said it was not an easy setup since i have to combine with APDL application.

June 3, 2011, 07:14		#32
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	I have no idea. Sorry!

June 3, 2011, 23:24		#34
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	mr lance, do you have any example on this matter? i think it is very usefull for others too.

August 15, 2011, 05:33		#37
icemaniac178 Member iswadi Join Date: Feb 2010 Posts: 44 Rep Power: 16	finally i have found the stall condition. its all about mesh. when the mesh is right then the stall can be captured. thank you to everybody who helped me...CFX forumers are really helpful..