RESEARCH· ·1970·

GEO'LOGICAL SURVEY RESEARCH 1970
COMPARISON OF A PROPELLER FLOWMETER WITH A
HOT -FILM ANEMOMETER IN MEASURING TURBULENCE
IN MOVABLE-BOUNDARY OPEN-CHANNEL FLOWS
By J. P. BENNETT and R. S. McOUIVEY,
Fort Collins, Colo.
Abstract.-The hot-film anemometer may be used in measuring
turbulence in large-scale movable-boundary open-channel flows
if a correction is made for the drift of the velocity-voltage
relation of the sensor. The propeller flowmeter may be used for
making turbulence measurements in such flows if corrections
are made for the frequency response and spatial averaging of the
propeller. The true power spectral density, S(w), of the turbulence
velocity may be obtained from S(w) =Soo(w) IH(w) l- 2 '17(w)- 1 ,
where S 00 (w) is the power spectral density of the propeller meter,
IH(w) I is the amplitude of the meter system function, and '17(w)
is the efficiency of spatial resolution. Measurements of turbulence
intensity and power spectral density were made with both hotfilm
anemometer and propeller flowmeter in large-scale openchannel
flows. The power spectral densities from the instruments
are similar in shape except at the higher frequencies where
effects of propeller spatial averaging appear. The efficiency of the
propeller flowmeter as a device for measuring turbulence increases
as the scale of the turbulence increases.
In hydraulic-engineering research the present great
need is for an instrument that can measure the characteristics
of turbulence in flowing water in all situations.
To obtain the statistical parameters describing
turbulence, one should use a measuring system which
has ( 1) a frequency response adequate to follow the
velocity fluctuations instantaneously (2) a sensor
smaller than the areal extent o£ the smallest velocity
fluctuations containing significant energy, and (3) a
readily obtainable and stable relation between the system
output and the actual velocity. If the system does
not satisfy these requirements in some respect, suitable
corrections .mus.t be applied to the measurements.
Two instruments which can be used to determine the
characteristics of turbulence in water are the hot-film
anemometer and the propeller flowmeter. Neither of
these is, however, completely satisfactory for making
turbulence measurements under the conditions encountered
in natural flows. The hot-film anemometer is unstable
when used in water containing suspended or dissolved
impurities, and the propeller flowmeter has too
low a frequency response and covers too great an area
to respond completely to the turbulence-velocity fluctuations
at a point.
This paper is presented to show how, despite their
· shortcomings, the hot-film anemometer and the propeller
flowmeter can be used to measure the characteristics
of turbulence in water and to compare measurements
made with them in large-scale movable-boundary
open-channel flow.
INSTRUMENTATION SYSTEMS
The transformation of the output of a measuring
system to obtain turbulence statistics requires an application
of the theories of the transient response of systems;
however, before these theories can be applied,
one must understand the systems to be used. The hotfilm
anemometer and the propeller flowmeter measuring
systems can best be understood by discussing the
princi pies of operation of the sensors and the equipment
necessary to convert sensor behavior into an
analyzable output.
The hot-film anemometer is similar to the hot-wire
anemometer used in airflow measurements. The velocity
sensor is ·electrically heated, and the heat is convected
a·way by the flow, the rate of convection being proportional
to the flow velocity. In water, a constant-temperature
anemometer is generally used. In the constanttemperature
anemometer, the circuitry is designed to
keep the temperature of the velocity sensor constant by
keeping its resistance constant. This is accomplished
by varying the voltage across the sensor so that the
B254
U.S. GEOL. SURVEY PROF. PAPER 70o-B, PAGES B254-B262