358
CARL II. GIRSON kT AL.
0,. = / 3 @ ~ ( v / c ) " ~ k -
' where is another universal constant taken to be
about 2. x3 is found from the assumption of local isotropy and the formula
6 0 s ; k2QTdk. Clearly c3 and x, will be difficult to obtain since they
demand very high frequency response and signal to noise ratios for velocity
and temperature detection.
2.3. Electronic Circirits
A variety of circuits have been employed in our efforts to bring turbulence
signals out of the noise. Casual observation of an oscilloscope trace of the
anemometer output suggests there is no turbulence at all when the instrument is suspended in the mixed layer, since the velocity is generally dominated by the surface wave orbital motion. If & = 1 m and
c = 10- cm2/sec3 we find the turbulence velocity ( t L R ) " 3 = 0.2 cm/sec
compared to 10-100 cm/scc orbital velocity. Only by high pass filtering and
increasing the gain would it be possible to observe such ti low level signal.
During the February 1972 FLIP measurements a Model 1010 Thermosystems constant temperature anemometer bridge was used with a 120-m
probe cable. This arrangement was successful, although noise pickup in the
long cable could not be avoided and reduced frequency response to achieve
balance was required. For the November 1972 towed measurements off
San Diego a constant temperature anemometer bridge was designed by Tom
Deaton of our group using compact integratd circuit components which
could be conveniently housed in a pressure case and submerged to reduce
cable pickup. Figure 2 shows power spectra of hot film derivative signals
from the microcircuit constant temperature anemometer bridge and the
Model 1010 used alternately with the same probe at overheat ratio of 1 . 1
on the axis of the wake of a sphere in a water tunnel at Reynolds number of
about 60,000. It was found that the measured power spectra and distribution
function for the two circuits were identical, except that the frequency response and noise level of the microcircuit anemometer were slightly better
than those of the commercial circuit.
Also included in the submerged electronics package were an ac bridge to
measure small scale temperature fluctuations with hot film probes used as
resistance thermometers, circuitry for the Marine Advisors ducted current
meter. as well as cable driving circuitry to transmit the three signals up the
four conductors with less than -80dB cross talk; all designed by Tom
Deaton. The circuit arrangement is shown in Fig. 3. The purpose of the
ducted current meter was to provide continuous calibration of the hot film
anemometer by matching power spectra for frequencies less than about
1.5 Hz which is the maximum response of the ducted current meter.
CARL II. GIRSON kT AL.
0,. = / 3 @ ~ ( v / c ) " ~ k -
' where is another universal constant taken to be
about 2. x3 is found from the assumption of local isotropy and the formula
6 0 s ; k2QTdk. Clearly c3 and x, will be difficult to obtain since they
demand very high frequency response and signal to noise ratios for velocity
and temperature detection.
2.3. Electronic Circirits
A variety of circuits have been employed in our efforts to bring turbulence
signals out of the noise. Casual observation of an oscilloscope trace of the
anemometer output suggests there is no turbulence at all when the instrument is suspended in the mixed layer, since the velocity is generally dominated by the surface wave orbital motion. If & = 1 m and
c = 10- cm2/sec3 we find the turbulence velocity ( t L R ) " 3 = 0.2 cm/sec
compared to 10-100 cm/scc orbital velocity. Only by high pass filtering and
increasing the gain would it be possible to observe such ti low level signal.
During the February 1972 FLIP measurements a Model 1010 Thermosystems constant temperature anemometer bridge was used with a 120-m
probe cable. This arrangement was successful, although noise pickup in the
long cable could not be avoided and reduced frequency response to achieve
balance was required. For the November 1972 towed measurements off
San Diego a constant temperature anemometer bridge was designed by Tom
Deaton of our group using compact integratd circuit components which
could be conveniently housed in a pressure case and submerged to reduce
cable pickup. Figure 2 shows power spectra of hot film derivative signals
from the microcircuit constant temperature anemometer bridge and the
Model 1010 used alternately with the same probe at overheat ratio of 1 . 1
on the axis of the wake of a sphere in a water tunnel at Reynolds number of
about 60,000. It was found that the measured power spectra and distribution
function for the two circuits were identical, except that the frequency response and noise level of the microcircuit anemometer were slightly better
than those of the commercial circuit.
Also included in the submerged electronics package were an ac bridge to
measure small scale temperature fluctuations with hot film probes used as
resistance thermometers, circuitry for the Marine Advisors ducted current
meter. as well as cable driving circuitry to transmit the three signals up the
four conductors with less than -80dB cross talk; all designed by Tom
Deaton. The circuit arrangement is shown in Fig. 3. The purpose of the
ducted current meter was to provide continuous calibration of the hot film
anemometer by matching power spectra for frequencies less than about
1.5 Hz which is the maximum response of the ducted current meter.