Baseline measurements for testingof improved triple-sensor hotwire
anemometer probe in a momentum
conserving turbulent round jet
F. Gökhan Ergin, CTA Product Manager, Dantec Dynamics, Copenhagen, Denmark. gokhan.ergin@dantecdynamics.com
Clara M. Velte, Dept. Mechanical Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark. cmve@dtu.dk

Experiments

Streamwise velocity

Power Spectra

Figure 3 Jet profile development in downstream direction x. Nozzle located at (0;0)
Figure 6 Left: PSD computation positions at
x=30D. Right: Power spectra for u, v, and w at
30D on the jet centerline. Arrow on the rightindicates the movement of PSD results as themeasurement point moves from a to e.

Figure 1 Jet facility, hot-wire probe and 3-axis
traversing mechanism.
Figure 4 Unscaled (left) and similarity scaled(right) streamwise velocity (U) profiles.

Figure 2 Left: New triple-sensor probe withstraight prongs. Right: Error variation withincidence angle.

Three-component velocity measurements
have been performed in a momentum
conserving turbulent round jet (Fig. 1) using
a CTA system and a new triple-sensor probe
featuring straight prongs (Fig. 2L). The
objective is to test the reliability of the newprobe in a classical canonical flow that lends
itself well to testing physical quantities suchas momentum flux conservation as well ascomparing directly to baseline data. The
probe is positioned in excess of 2500
positions in each measurement plane using acomputer controlled traversing mechanism(Fig. 1). A conventional two-step calibration isperformed where the directional calibrationis conducted following a velocity calibration.
The calculated pitch and yaw factors arecompared to expected values for similarprobes reported in the literature (Tab. 1). An
error analysis is performed using the pitch /yaw - roll mechanism in order to identify theacceptance angle for the probe for the data
reduction scheme (Fig. 2R).
Table 1
Measured
directional
calibration
constants.

Figure 5 Left: Centerline velocity change in x.
Right: Momentum flux conservation in x.

The measurements capture the main wellknown characteristics of the round jet suchas the jet centerline, jet half width, andvirtual origin (Fig. 3). The stream wisevelocity profiles at each downstream position
were plotted in regular and similarity scaling
variables, displaying clear similarity collapseof the mean velocity even as far upstream as
x/D=10 (Fig. 4). Due to the turbulence
dependency upon initial conditions, theprofile could not be quantitatively validated
against other jet measurements, although it
was seen that the measurements in generaldisplayed the expected trends. The jet
expansion and centreline velocity
developments were determined from thedata and a multiple check of the momentum
flux conservation was performed (Fig. 5). The
results indicate a general agreement, eventhough some spurious points existed thatwere hypothesized to stem from thedifferences in measurement plane extent.

Poster SP_0514_v1. Subject to change without notice.
Copyright © 2017. Dantec Dynamics. All Rights Reserved

Power spectra were measured at 30 diameters
downstream of the jet exit on the centerline,quarter-width, half-width, three-quarters widthand at full jet width, demonstrating thevariations in the dynamic statistical moments(Fig. 6L). The measured power spectra capture
the expected characteristics even at large
distances from the jet centreline where theturbulence intensity is exceptionally high (Fig.
6R).

Conclusions & Future work

The new probe design produces accurate
directional calibrations by eliminating theprong effects observed in previous designs.
As a direct result of this, the reconstructed
3D velocity vector is more accurate within theacceptance cone of the probe –approximately 60° for the current prong
configuration and data reduction scheme.
Future work includes (i) the comparison ofdifferent directional calibration schemes forreduced error over a larger acceptanceangle. (ii) The PSD spectra will be comparedto high-precision Laser Doppler Anemometer(LDA) power spectra for validation of thenovel hotwire probe design. (iii) Further,static moments such as mean velocity, rms
values etc. measured with the novel hot-wireprobe will be compared to corresponding
LDA measurements in the same jet facility. In
particular, the comparison will focus on thechallenging case of high turbulence intensity
in the outer parts of the jet where hot-wireprobes have classically had significantdifficulties in producing reliable results.

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