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National Advisory Committee for Aeronautics, Technical Notes - A Suggested Method for Measuring Turbulence

The desirability of a quantitative measure of turbulence
is emphasized, and a possible method of attack on the problem
is discussed. Data are presented to show that the hot—wire
anemometer has possibilities as an instrument for measuring -
turbulence. An apparatus consisting essentially of two hot
wires. one parallel to the air flow and one at right angles
to it, is suggested. Further study of the possibilities of
such an apparatus is recommended.

The desirability of being able to make a quantitative
measurement of turbulence in a moving air stream has long
been felt. Even an instrument for quickly indicating the
presence or absence of turbulence without attempting to meas—
ure it would be of considerable value in experimental aero_dy-
namics.

Since turbulent motion of a fluid is very complex, it
would seem well nigh impossible to develOp a si_ngle instrument
which would measure all aspects of a given turbulent motion.

For purposes of this discussion it is useful to divide
the aspects of a turbulent motion into two classifications,
namely:

1) Amplitude, which may be taken to be some
characteristic dimension which determines—the scale
of the motion. It may be the average distance of
particle motion across the general flow direction,
or the average spread or size of vortices, or the
amount of their path departure from the average
path of flow.

2) intensity, which refers to the dynamic prop— 1
erty of the motion. It_may be represented by the
departure of the legal velocity from the avegagg
velocity-in respect to both direction and magnitude
or by the energy of such departure.

Smoke or other type of suspended particle seems to be
the best available indicator of turbulence amplitude. On the
other hand, no suitable apparatus appears to have been devel—
oped to serve as an indicator of turbulence intensity. It is
to this aspect of the problem therefore, that this discussion
is principally directed.