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Research Papers: Techniques and Procedures

# Evaluation of a Thermal-Tuft Probe for Turbulent Separating and Reattaching Flows

[+] Author and Article Information
Quentin Schwaab

Thermo-Fluid for Transport Laboratory,
Department of Mechanical Engineering,
École de technologie supérieure
Montréal, Québec, H3C 1K3, Canada

Julien Weiss

Associate Professor
Thermo-Fluid for Transport Laboratory,
Department of Mechanical Engineering,
École de technologie supérieure,
Montréal, Québec, H3C 1K3, Canada
e-mail: julien.weiss@etsmtl.ca

In theory, the Wheatstone bridge could be balanced with the heater wire turned on as this would take into account heat conduction and radiation towards the sensor wires. However, this would require a perfectly symmetric, vertical plume from the sensor wire, which would be extremely difficult to achieve in the wind tunnel.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received December 2, 2013; final manuscript received May 6, 2014; published online September 10, 2014. Assoc. Editor: Mark R. Duignan.

J. Fluids Eng 137(1), 011401 (Sep 10, 2014) (8 pages) Paper No: FE-13-1694; doi: 10.1115/1.4027642 History: Received December 02, 2013; Revised May 06, 2014

## Abstract

The validation and testing of a thermal-tuft probe is described in detail. The thermal tuft consists of three parallel wires where the middle wire is heated and the two lateral wires act as resistance thermometers, thereby sensing the flow direction. The probe's function principle is validated in an acoustic resonator that generates a nearly sinusoidal velocity perturbation with zero mean. It is shown that the variation in electrical resistance of the sensing wires is a measure of the flow direction. The probe's sensitivity to the heater current in the central wire and to the flow angle is also investigated. The electronic circuit is validated by placing the probe on a mechanical shaker. The output voltage is shown to be consistent with the variation in electrical resistance of the sensing wires. The flow direction can thus simply be measured by recording the probe's output voltage with a single digital data-acquisition channel. Finally, the thermal tuft is evaluated in a low-speed, pressure-driven, turbulent, separation-bubble flow. It is shown that the forward-flow fraction and the intermittent frequency can be measured with an uncertainty of about $±1.5%$. The positions of separation and reattachment in the test section, measured with the thermal tuft, are consistent with flow-visualization experiments reported elsewhere.

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## Figures

Fig. 1

Thermal-tuft probe's design

Fig. 2

Schematic of signal conditioning circuit. V+ = 15V and V- = -15V. S1 and S2 are the sensor wires. P1, P2, and P3 are described in the text. Note that the central wire is independent of this circuit and is consequently not represented.

Fig. 3

Sketch of experimental setup in the acoustic resonator. Distances are expressed in mm. V+ = 6.5V, R1 = R2 = 2213.4 Ω. Circular inset shows a top view of the wires orientation.

Fig. 4

Signals obtained in the acoustic resonator during 3T≃15.59 ms. (a) Acoustic pressure and velocity at the location of the probe, (b) voltages V1* and V2* across sensing wires, and (c) V12* = V1*-V2* and resulting histogram R12.

Fig. 5

Representation of the probe for three specific orientations: (a) perpendicular to the flow direction at θ = 0 deg, (b) at the limit angle θ = θl, and (c) at the critical angle θ = θc

Fig. 6

Signals obtained in the acoustic resonator during 3T≃15.59 ms with different probe orientations between 0 and 90 deg. (a) Acoustic pressure and velocity at the location of the probe, (b) voltage V1* across sensing wire S1, and (c) voltage V2* across sensing wire S2.

Fig. 7

Signals obtained in the acoustic resonator during 3T≃15.59 ms with different heater currents Ic = n*0.1A, n∈[1;9]

Fig. 8

Sketch of experimental setup on the mechanical shaker

Fig. 9

Signals obtained on the shaker during 3T = 150 ms. (a) Input voltage and acceleration of the shaker and (b) Wheatstone bridge imbalance voltage, corresponding histogram, and probe output voltage.

Fig. 10

Sketch of experimental setup in the TFT Boundary-Layer Wind Tunnel. Circles represent measurement positions on the test surface. Origin of x axis is at the entrance of the test section and x = 1.5 m corresponds to the start of the pressure gradient [9].

Fig. 11

Typical digital signals obtained in the turbulent separation bubble. (a) When the heater wire is not supplied with current (Vout,0) and (b) when Ic = 0.8 A (Vout).

Fig. 12

Distribution of the forward-flow fraction γ within the separation bubble measured at y = 1 mm from the wall. The dots represent the experimental data from the thermal tuft. The longitudinal lines (along the x axis) are interpolating the data using piecewise cubic Hermite polynomials. The transversal lines show particular iso-γ lines of 20%, 50%, 80%, 99%, and 100%.

Fig. 13

Distribution of the forward-flow fraction γ (solid line) and the intermittent frequency fc (dashed line) along the wind tunnel centerline z=0

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