Hi Sreejith,

i personally think it would be more convenient to measure lift and drag with the help of a balance. Computing the lift from pressure measurement could be inaccurate. Computing the drag from pressure measurements only is not possible, since viscous stresses at the surface of the airfoil will have a large contribution.

For a visualization of the pressure distribution yould could use pressure sensitive paint. Thus you would have an infinite resolution.

The size of your airfoil in the wind tunnel depends on two factors:

1) scalability: If you have the same Reynoldsnumbers for the model airfoils and the real airfoil, the velocity of the wind channels has to higher that in the real case if the model is smaller than the real geometry:

Re_model=Re_original =>    U_m * L_m/nu = U_o*L_o/nu =>    U_m=L_o/L_m

This can cause a problem: If in the original case a low Mach number, say Ma_o=0.1 prevails, this Mach number rises due to scaling, i.e. Ma_m=Ma_o  * L_o/L_m.

If the Mach number is higher than in the originial case, your wind tunnel experiment will show compressibilty effects while the original flow is incomressible.

Thus, make the Model geometry as close to the original geometry scale as possible, optimal:  L_m=L_o. Some people say, that compressibility effects in external flow will arise for Ma>0.3.

2) blockage of the wind tunnel cross section: If your model becomes to large, it might interfere with the wind tunnel. From this point of view, your model airfoil should be as small as possible.

Altogether you should find a compromise for the geometry scale, e.g. the wingspan could be around 0.2 m which would be a third of the diameter of the windtunnel, if Ma is smaller that 0.3.

Was this helpful?

Best regards

Bastian

Hi Sreejith, Bastian is right. Just let me add the following to his explanations:

The validity of wind tunnel results obtained in testing of scale models is based on the fluid mechanics Principle of Dynamic Similarity (White, F.M., Fluid Mechanics, 6 ed., Boston, USA: McGraw-Hill, 2003, p. 866) which states the following: "If two physical phenomena can be described using the same formulation (same equations and same initial and boundary conditions), the solutions for one of the phenomena are valid for the other one."

For the motion of air around and obstacle, this Principle guarantees that the non-dimensional results measured in a wind tunnel for a scale model will be the same as for the real, full-scale obstacle if the following conditions are fulfilled:

• Geometric similarity: all relevant aerodynamic features of the real obstacle must be accurately replicated in the scale model
• The Mach number must be the same for the model and obstacle
• The Reynolds number must also be the same
• Heat transfer (thermal effects) and diffusion can be neglected

Dilemma: If the fluid properties for both the tests and real case are the same (i.e., same speed of sound (T), ρ and μ), and the scale model is smaller or larger than the real obstacle, it is obvious that it is not possible to have simultaneously the same M and Re as in the real problem. If we cannot replicate simultaneously both parameters, it is then necessary to choose which one will be kept similar to the real case. Generally, we choose the parameter with a stronger influence on the flow. For instance, roughly speaking M is dominant for supersonic flows and Re for subsonic flows. Thus, for example, in high-subsonic testing, we would strive to work with the same Re as in the real case, and would pay less attention to M. Effectively, this means that we are not complying fully with the requirements of the Principle of Dynamic Similarity, and thus the test results show some error respect to the behavior in the real case. Many researchers have characterized the Re and M scaling effects to be able to correct for this error when extrapolating the wind tunnel results to the real case.

So, as Bastian says, if the fluid properties for both your tests and real case are the same, you should try to work with a model as similar as possible in dimensions the real obstacle, bearing in mind also these recommendations (Barlow, J.B., Rae, W.H., Pope, A., Low-speed Wind Tunnel Testing, 3 ed., New York, USA: Wiley & Sons, 1999, p. 713):

• Blocking coefficient (ratio of frontal area of model to frontal area of test chamber): should be less than 0.1 
• Wingspan (or width) of the model: should be less than 4/5 (80%) of test chamber width

Hope this information is helpful, too,

Jose

Sreejith,

I also agree with folks' above suggestions.  Just a few more details would get you closer to a more complete answer.

You did not mention if you were planning on testing a wall-to-wall model (2d) or a 3d model.  If 3d, the 80% wingspan/tunnel span ratio is somewhat of a maximum guideline where corrections will still get you a reasonable answer.  Several mitigating factors would preclude a more conservative span ratio, such as a lower aspect ratio wing (with more three-dimensionality in the flow and larger (proportionally) wingtip vortices, lower Reynolds number etc.

For area blockage, the textbooks generally state the 10% model area/test section area ratio as a max upper limit for a closed jet test section.  With 10% you'll be correcting quite a bit and some corrections work better than others under different flow/tunnel conditions, so it is better to reduce the magnitude of the necessary corrections. As a rule, as suggested by Eswara above, this number is better kept much smaller and I would tend to agree with the 5% guideline for a streamlined body. 

Since you mention detection of flow separation angle of attack as one of the things you'd like to at least qualitatively evaluate, you should pay special attention to solid blockage, wall effects and in particular, model mount upwash effects which can all serve to change the effective angle of attack of the wing due to local changes in flow angularity.

Selection of model dimensions, as with so many things in life, is all about compromise.  In general, you want a larger model to get more easily measurable forces and to more closely match real-life flight conditions for Reynolds number, etc, but you have to balance these desires against solid and wake blockage effects, as well as wall effects.  Depending upon the driving physics of the test, different compromises can be made for each and every test.

Now if I could just eat that really big piece of chocolate cake without gaining another Kilo........

Than you all for your valuable suggestions...i am trying to incorporate your suggestions...