• Version
• 170 Downloads
• 542.87 KB File Size
• 1 File Count
• November 4, 2016 Create Date
• November 4, 2016 Last Updated

National Advisory Committee for Aeronautics, Technical Notes - The Effect of Wind Tunnel Turbulence Upon the Forces Measured on Models

Reasons for inguiry.— The tests were undertaken to find
the effect of turbulence in the air stream upon the lift and drag
forces measured on models in the four—foot wind tunnel at the
Massachusetts Institute of Technology.

Maximum lifts and minimum drags were measured
on G5ttingen—387 and R.A.F-—15 airfoils, minimum drag
on a streamlined strut, and the static pressure gradients
for different conditions of turbulence.

Results and further developments.— The results show that
the scale of the turbulence (as defined in this report) has a
marhed effect upon the measured forces on models tested in the
tunnel as well as on the pressure gradient, and it is recommended
that further investigation of the phenomena be made with the aid
of smoke and small wind vanes.

In order to obtain some idea of the effect of turbulence of
flow in a wind tunnel on the forces measured upon modelsi it was
decided to encourage such turbulence by the introduction of wire
screens directly behind the honeycomb. The screens consisted of
wire netting of various degrees of mesh stretched across the mouth_
of the tunnel immediately downstream from the honeycomb. Three
such screens were used successively, one having a mesh of 5 per V,
inch, another, 10 per inch, and a third, 20 per inch. Whatever
turbulence existed in the tunnel before the introduction of a
screen would undoubtedly be modified in either quality or quantie
ty by the screens and connected information concerning its ef—
fects would, thereby, be obtained. As a means of detecting any
such change in the condition of the turbulence, forces were meas—
ured upon two airfoils (R.A.F.15 and thtingen 387) 18" span and _““:
3" chord and on a streamlined strut 18" long and 1 1/4" wide and
of fineness ratio 3. These tests were carried out at 40, 35, I
and 30 miles per hour, except for the ten—mesh screen where it
was not possible to obtain.40 M.P.H- and for the twenty—mesh
screen where it was found impossible to obtain more than 30 M.P.H.,
owing to the throttling effect of these finer screens-
The Results.