Learning Forum

[fasouras]

Hi

I am having this problem for like 2 months and after countless of youtube tutorials and meshing and fluent guidebook i still cant find what the problem is.

So i have to calculate the cl and cd on 3 different naca profiles ( NACA0012, NACA0015, NACA4412) for 8 different aoa. My analysis is 3d.

The big problem is that WHATEVER i do to the mesh and whichever model i try to solve it the cl gives me a number of somehting like 0.05 which ofc is undeniably low. I change the aoa from the boundary conditions x=vcosθ, y=vsinθ, cl-->x=-sinθ y=cosθ and cd the oppoisite without minus. 

Till now i ve tried different kinds of meshing options different methods but i cant solve it.. 

I have added some photos of the mesh to help you get a glimpse of what i am talking about.

Btw i am using Ansys 19.2 ACADEMIC and the programm is ANSYS FLUENT.

MATERIAL=WATER

SPEED=4m/s

BOUNDARY CONDITIONS= COMPONENTS,ABSOLUTE 

MODEL= K-OMEGA SST

INLET=INLET VELOCITY

OUTLET=PRESSURE OUTLET

AIRFOIL=WALL

NEAR SIDE=SYMMETRY

FAR SIDE= WALL

 

Thank you in advance

Kostas 

 

P.s If you need more photos with other infos just tell me so 

[fasouras]

[Keyur Kanade]

Can you please insert more images of the mesh. Please insert some zoomed images to check the mesh. 

Also did you have a good convergence? Please check. Please share set up images. 

[fasouras]

 

my solution converges..sometimes at 0.05 and some other times at 0.01 depending on the model i use...my theory suggests i have something like 0.7, 0.8

[Amine Ben Hadj Ali]

Just for testing: Are you running direct in Fluent or updating from Workbench?

Moreover: Check if the force report is chaning based on AoA.

[fasouras]

i update from workbench.. why is there a difference? it is changing but not in the desired way..

[Amine Ben Hadj Ali]

Then please whenever you click o the SetupCell check properties and switch initialization to "Solver Controlled" instead of "Program controlled" reset all and try again.

[fasouras]

Hmm ok i ll give it a shot but do you believe it will make a difference? What exactly does it change?

[Amine Ben Hadj Ali]

That would change the way the design points are initialized if you have for each AoA a design point; Perhaps the solver finishes rapidly if it starts from the results of previous DP and that is why results are similar. But if you do not have DPs then it won't help. 

 

So please check the results of the force monitor and not coefficients if they are same: Just eliminate any possible doubts.

U can at first create a 2D case to get things quicker and then extend to 3D whenever you are confident. 

Please share where the boundary conditions are applied: is not clear from screenshots 

[fasouras]

That would change the way the design points are initialized if you have for each AoA a design point; Perhaps the solver finishes rapidly if it starts from the results of previous DP and that is why results are similar. But if you do not have DPs then it won't help. 

 

So please check the results of the force monitor and not coefficients if they are same: Just eliminate any possible doubts.

U can at first create a 2D case to get things quicker and then extend to 3D whenever you are confident. 

Please share where the boundary conditions are applied: is not clear from screenshots 

So just to exclude some things yiu don't think that the mesh has something to do with it

[Amine Ben Hadj Ali]

On first sight not really: off-course one creates a much better mesh.

[Keyur Kanade]

Mesh looks ok. 

Is there any reason you are using spalart allmaras model?

Can you try k-w sst?

[fasouras]

On first sight not really: off-course one creates a much better mesh.

 

can you tell me how? i am really struggling to get a bettrer mesh and i think thats the best i have managed.. i ve been looking all over the internet to make a structured 3d mesh but all the options for a structured mesh were for 2d cases which i have already done it and btw the results were perfect..how can i improve the mesh?

 

 

[fasouras]

Mesh looks ok. 

Is there any reason you are using spalart allmaras model?

Can you try k-w sst?

 

i have tried all the models and for some reason just yesterday k-w model got me a message "reversed flow in 14 faces on pressure-outlet 5." and i 've checked it out and some guy said to change the model..

[fasouras]

That would change the way the design points are initialized if you have for each AoA a design point; Perhaps the solver finishes rapidly if it starts from the results of previous DP and that is why results are similar. But if you do not have DPs then it won't help. 

 

So please check the results of the force monitor and not coefficients if they are same: Just eliminate any possible doubts.

U can at first create a 2D case to get things quicker and then extend to 3D whenever you are confident. 

Please share where the boundary conditions are applied: is not clear from screenshots 

[Amine Ben Hadj Ali]

Please start from 2D case for this hydrofoil: to have things run faster at first stage.

You are referring to the theory: please explain more.

 

Again what does the force report tell you for different AoA?

[fasouras]

Hi 

for some reason and out of the blue i tried to change aoa from bcs and start testing your point and the whole project went crazy and got me a reversed flow message and turbulent viscosity limited message... 

 

[Amine Ben Hadj Ali]

Start from scratch and 2d in the workbench create design points with input parameter based on So a via UDF or create one starting geometry mesh and connect it to different Fluent containers each of them referring to particular AoA

[fasouras]

Start from scratch and 2d in the workbench create design points with input parameter based on So a via UDF or create one starting geometry mesh and connect it to different Fluent containers each of them referring to particular AoA

i have already done it in 2D for 3 different profiles and have already got some decent results..in fact that was my bechelor thesis...and now for the master of engineering thesis i have to expand it to 3d..

UDF...isnt that some kind of code that is being used as an input to lets say inlet to automatically change inlet direction?

[Amine Ben Hadj Ali]

I do not understand now the problem. If you have already have some decent results in 2D planar why the same thing in 3D (and with symmetry boundary condition)??

Or what is the difference between 2D and your actual 3D?

Regarding UDF: to prescribe the velocity component based on AoA which is inputted as scheme RP-variable or an input parameter which might be accessed inWorkbench. If this does not mean anything for you just forget it for now if you do not have time.

[fasouras]

why do you emphasize on the symmetry boundary condition?

So i have to do it cause i would like to apply the boundary layer, check the y+ value and insert in my thesis the contours of the 3d wing. Also it would be awsome to have almost the same results in 3d and 2d..

 

You think a 3d analysis is not needed and a 2d is enough?

[Amine Ben Hadj Ali]

Yes I think so but this is your thesis. 

Back to topic: Try to work properly all the AoA and check the force report.