CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   Main CFD Forum (http://www.cfd-online.com/Forums/main/)
-   -   Low reynolds number flow (http://www.cfd-online.com/Forums/main/84673-low-reynolds-number-flow.html)

pranavr February 5, 2011 13:06

Low reynolds number flow
 
What are the equations aplicable in the case of a low reynolds number flow as in the case of a flow past an airplane made of paper

truffaldino February 7, 2011 16:50

You should solve incompressible Navier-Stokes

aqib February 8, 2011 05:29

depends upon the problem and your area of interest....
Reynolds Averaged Navier Stroke's Equation(RANS).... is to be the best to catered flow separation

jvr February 16, 2011 06:35

hi,
i want to calculate the Reynolds number of exhaust gas with a given temperature(c) and massflow(g/s).
What is the fastest way?
thanks in advance

aqib February 17, 2011 01:51

i think as per my knowledge, your flow is compressible and their should be some laminar separation....
the best model to catered that is Menter Transition model of 4 four equations....

Shenren_CN February 21, 2011 10:53

I'm afraid low-Re # does not imply low mach number.

You can get shock wave for inviscid fluid, where the Re # is zero.


Quote:

Originally Posted by truffaldino (Post 294057)
You should solve incompressible Navier-Stokes


truffaldino February 22, 2011 11:42

Quote:

Originally Posted by Shenren_CN (Post 296284)
I'm afraid low-Re # does not imply low mach number.

You can get shock wave for inviscid fluid, where the Re # is zero.


This thread is about aerodynamics of a small (paper) airplane.
Have you seen shok waves past paper airplane?

BTW, for inviscid fluid RE=infinity

Shenren_CN February 22, 2011 12:04

Hi truffaldino,
my apology... I didn't read it carefully.
And you are right, Re # is infinity.
hope my advisor is not reading this...

Quote:

Originally Posted by truffaldino (Post 296445)
This thread is about aerodynamics of a small (paper) airplane.
Have you seen shok waves past paper airplane?

BTW, for inviscid fluid RE=infinity


Vasiliy February 22, 2011 13:36

Hi All,
My opinion that transition model is not necessary in this case.
As I understand air flows around paper plane. So velocity is small and we can use incompressible Navier Stokes equation.
About turbulence models. I think SA model is good choise in this case. Flow is not very difficult.
I am not sure about transition and separation on paper plane.

truffaldino February 22, 2011 16:05

I had quite extensive experience with model gliders, they fly at about the same order of RE as paper planes and there is almost always separation and transition there.

Vasiliy February 23, 2011 02:00

Hi truffaldino,

It looks like nice case. It would be interesting to have such model and experiment results. Could you provide it.

Thanks

truffaldino February 23, 2011 03:33

Hi Vasiliy,

Information about low reynolds number airfoil design and its peculiarities (due to separation/transition bubbles) is quite extensive and easily acessible over the web. As an example you could take a look at Martin Heppelere website, and in particular

http://www.mh-aerotools.de/airfoils/

Data on very slow airfoils for Discus Launch Gliders (down to RE=40K) can be found here

http://www.charlesriverrc.org/articl...t-airfoils.htm

People from RC glider commmunity mainly use Mark Drela's xfoil and also XFLR5 software. These are nice programs that are free and increadibly easy to use: Xfoil analyses airfoils, while XFLR5 analyses an airplane as a whole.

These programs are based on viscous-inviscid interaction method with boundary layer integral methods. They use e^n transition model and some turbulence model.

One can also have a rough idea about presence of separation at low Re for 2D airfoils even without using the above software: one can use Thwaites integral BL method. It is very simple and takes a couple of hours to write a program in mathlab or maple.

About experimental data: there has been an extencive testing by UIUC Applied Aerodynamics group, and measured polars were available free of charge on the web. Now they are selling them as reference books: see e.g.

http://www.ae.illinois.edu/m-selig/uiuc_lsat.html

but some of them are still scattered over the internet free of charge

Truffaldino


All times are GMT -4. The time now is 07:50.