Setting up Reference Values in ANSYS Fluent-In Detail

ANSYS Tutorial #5
For evaluating force or moment coefficients, Reynolds number or some other derived quantities in Fluent, user has to set the reference values which depend upon the geometry and flow conditions. Often times we make mistakes in setting these values and end up getting unrealistic values of theses coefficients even though our solution converges.
In this tutorial, we have discussed what these reference values are and how we should choose them as per our requirement.
Link for Excel File
drive.google.c...
Following websites and links were used in the making of this video
www.engineerin...
www.aeroprobe.c...
www.kth.se
www.nasa.gov/c...
grabcad.com/li...
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[@ernestjesly]

What should be the reference area for analysing the whole aircraft?

[@nicolamenga8943]

Hello. Thank you for the video. I have a question. In the tab of the reference values there are two further spaces: the "Compute from" and the "Reference zone", for which a part of the geometry can be selected. How can I set these information? Thank you.

[@Ansystant]

"Compute from" option is used to calculate reference values automatically based on a selection (e.g. using value of velocity at inlet as reference value). You can also manually enter the values. The option is just there for ease. Reference zone selection becomes important when you are dealing with problems having multiple frames of references. If your problem only has a single FOR, you don't need to select any.

[@nicolamenga8943]

@@Ansystant Ok, so I can set the reference values without to set the "Compute from" information. Before seeing your video I only set the "Compute from" to the "Inlet" (velocity inlet). In fact, I was getting very wrong value of cl and cd. This video is very useful. Thank you.

[@Ansystant]

Glad we could be of help.

[@alvaromfo]

I'm working on a 3D semi tapered wing analysis using Fluent Aero, and I'm setting up the value of the root chord as the reference length, but, since it's not a constant chord wing, should I use the value of the mean aerodynamic chord instead?

[@opalkovdima]

Thank you from Russia

[@서지완학생공과대학기]

Hello In 2D, i stretch depth 1m to 2m. Does frontal area double in size? i want to know relationship between depth and area in 2D reference value. Thanks :)

[@PaoloFerla123]

What are these values for a channel with fluid (liquid water) flowing in it at 293.15 K?

[@cR29777]

What would be Ref. Area for 2D darrieus type VAWT?

[@SunilKumar-er5jq]

not clear about the length for calculating the moment about the centre.

[@alitoori4779]

What Reference area and length value we should use while using wing,pylon and store geometry, our main concern is to find a Pressure coefficient on store. can we use the whole surface area or just to use the top view area of the wing. Thanks

[@Ansystant]

Length is not required for pressure coefficient calculations. As for the reference area of store, user can select any area and calculate the pressure coefficient accordingly. But in the end you have to mention which area you used for your calculations. However we would recommend you to use the frontal area. Generally (not necessarily) area which provides the greatest opposition to flow is chosen.

[@Ansystant]

www.grc.nasa.gov/www/k-12/airplane/drageq.html

[@alitoori4779]

Thank you @Software Delirium

[@carlosalmeida5329]

Hello! For CL, CD and CM of an entire aircraft, should I use the total area seem from the top? Like the wing + tail + fuselage areas. Would it be the same for the length parameter? Thanks!

[@Ansystant]

Yes you can take the entire area. But it's better to use the wing area only. Again, there are no hard and fast rules. However using the same reference values as other researchers produces results that can be easily compared among different cases.

[@Ansystant]

Just make sure you mention the reference values used while presenting your results. Read this article by NASA. It'll be really helpful. www.grc.nasa.gov/www/k-12/airplane/dragco.html

[@carlosalmeida5329]

@@Ansystant Thank you!

[@relaxgaming29]

Hi, the frontal area of vehicle should be taken on 2D or 3D? If taken on 3D, should it be considering the 3D curvature area on the model as well?

[@Ansystant]

Frontal area refers to the maximum area of car when projected on a plane perpendicular to the air flow. The projection will have curvatures. But if by curvatures, you are referring to include the overall body area of car, then no. Only the area projected on a plane if seen from front will be included.

[@alpayacar7066]

what is your suggetion for submarine frontal area or frontal?

[@Ansystant]

You can choose any area as your reference and mention it while reporting the results. Take frontal area as your reference. You can refer to the given link link.springer.com/article/10.1016/S1001-6058(11)60311-9

[@diegofernandofloreztrujill5154]

Muchas gracias 😁

[@radwaattia3259]

thanks a lot i have a question, if i have a variable inlet velocity (i used a udf file) . what is the reference value should be?

[@Ansystant]

Question not clear. But, what I can understand from the statement is that you need to select inlet boundary as the zone for automatically defining reference value of velocity. It is available under drop down of "compute from" available in reference values section. But keep in mind, not all boundary conditions maybe defined automatically. You may need to set length and area manually.

[@ghost-nh8vw]

so basically , reference pressure is that pressure in which fluid comes out of inlet, is it??

[@Ansystant]

Yes, If the inlet is at free stream condition. But there might be cases where the inlet conditions are not free stream conditions, there you will set the free stream values as reference values.

[@ghost-nh8vw]

@@Ansystant thanks a lot!

[@FILYPTC]

great!

[@Astatum]

bizarre video. many thanks.

[@abbasj8780]

I don't think the reference temperature is meant here the wall temperature.

[@christosvasiliou219]

i love you

[@suryavishwa2159]

But still i am getting negative enthalpy value

[@Ansystant]

Getting negative values just means that they are below chosen reference. They are not wrong.

[@suryavishwa2159]

@@Ansystant but those ref. Mai be calculated during iteration

[@atulwithsmile]

Doesn't make sense for temperature at all. Looks like you don't know what to talk about how to set reference temperature. If wall temperature does varies in most cases as you suggested, then what should be chosen as reference temperature, becuase bulk flow and wall temperatures are inlet and bc .

[@karthiksdas3348]

I know this comment was posted 1 year back. Have you found any way to set reference temperature?

[@imranebennageh7592]

Thanks a lot for the Video, it's really helpful. In my case, I designed an aircraft and I'm totally confused about the reference values to calculate Cl and Cd. Should I use the same Area and Length for both coefficients? And which Area and Length should I use? The area seen from TOP or FRONT view (perpendicular to Air Flow) of the aircraft? the Length of wing span or of the aircraft length ? I checked in this link www.grc.nasa.gov/www/k-12/airplane/dragco.html but don't tell much about the accurate references. the mini aircraft is intended to be launched in the air, so the results are not intended to be mentioned in some report but in full action, thus the results shouldn't be relative.

[@Ansystant]

Length is not used in the calculation of Cl and Cd. And yes, you should use the same reference area for both Cl and Cd whatever that maybe. Now comes the choice of this area. The link of NASA website given in this video was to stress that any area can be taken as a reference value. There are no hard and fast rules, just guidelines. Of course, its better to keep the methodology same as other researchers, engineers for the results to be comparable and easy to interpret. In your case, we would recommend you use the total planform area of wings (as seen from above) as reference. Some reference that might be useful: 1) www.researchgate.net/publication/329519759_Aircraft_Drag_Polar_Estimation_Based_on_a_Stochastic_Hierarchical_Model 2) "This area could be any area including tail area, wing area and fuselage cross sectional area (i.e., fuselage cross section), fuselage surface area, and even aircraft topview area. No matter what area is selected, the drag force must be the same. This unique drag comes from the fact that the drag coefficient is a function of the reference area. Therefore, if we select a small reference area, the drag coefficient shall be large, but if we choose a large reference area, the drag coefficient shall be small. In an air vehicle with a small wing area (e.g., high-speed missile), the fuselage cross-sectional area (normal to the flow) is often considered as the reference area. However, in an aircraft with a large wing, the top-view of wing; planform area (in fact gross wing area) is often assumed to be the reference area". Taken from: d1wqtxts1xzle7.cloudfront.net/56500925/Aircarft_drag_modeling-with-cover-page-v2.pdf?Expires=1645890138&Signature=eKNFZw6rTqJ~Ale~2wy7xWRsuYgZCXMn6W~TOTOPMxGfwDUyHwevlAkPy7ioUUTIBwMj42oMLtnqchyQxjgdE6coPEjfWpgU3-n6~xJZri35x-HHyqw8bu5C-dN~Fy5bDlWRYoW0bvLjYQEop66YFdzI3uZvxXHvanmWDcrDEa7E3YxXU5070eQw~qNeuTKj6~STREFG2~WwrpKEtNeRmVw8t6goKsIivKnHHf2XQ42sWS1YLtld43nHriwFMjNBCgw3iMRF8-Eiy6tLjeidpaPcqDar5OM5XZYW041tWZbtQ2-AoC4C6etAESeus45I2b37zVNUh38OkTo5JyZaCA__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA