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.

## References

References
1.
Eaton
,
J.
,
Jeans
,
A.
,
Ashjaee
,
J.
, and
Johnston
,
J.
,
1979
, “
A Wall-Flow-Direction Probe for Use in Separating and Reattaching Flows
,”
ASME J. Fluids Eng.
,
101
(
3
), pp.
364
366
.10.1115/1.3448978
2.
Eaton
,
J. K.
,
Westphal
,
R. V.
, and
Johnston
,
J. P.
,
1982
, “
Two New Instruments for Flow Direction and Skin-Friction Measurements in Separated Flows
,”
ISA Trans.
,
21
(
1
), pp.
69
78
.
3.
Eaton
,
J. K.
, and
Johnston
,
J. P.
,
1982
, “
Low Frequency Unsteadiness of a Reattaching Turbulent Shear Layer
,”
Turbulent Shear Flows 3
, Vol.
2
,
Springer, Berlin
,
Heidelberg
, pp.
162
170
.
4.
Driver
,
D. M.
,
Seegmiller
,
H. L.
, and
Marvin
,
J. G.
,
1987
, “
Time-Dependent Behavior of a Reattaching Shear Layer
,”
AIAA J.
,
25
(
7
), pp.
914
919
.10.2514/3.9722
5.
,
B.
, and
Simpson
,
R.
,
1982
, “
Evaluation of a Wall-Flow Direction Probe for Measurements in Separated Flows
,”
ASME J. Fluids Eng.
,
104
(
2
), pp.
162
166
.10.1115/1.3241800
6.
Chaehoi
,
A.
,
Mailly
,
F.
,
Latorre
,
L.
, and
Nouet
,
P.
,
2006
, “
Experimental and Finite-Element Study of Convective Accelerometer on CMOS
,”
Sensors Actuators A
,
132
(
1
), pp.
78
84
.10.1016/j.sna.2006.04.057
7.
Spazzini
,
P.
,
Iuso
,
G.
,
Onorato
,
M.
, and
Zurlo
,
N.
,
1999
, “
Design, Test and Validation of a Probe for Time-Resolved Measurement of Skin Friction
,”
Meas. Sci. Tech.
,
10
(
7
), pp.
631
639
.10.1088/0957-0233/10/7/309
8.
Buder
,
U.
,
Petz
,
R.
,
Kittel
,
M.
,
Nitsche
,
W.
, and
Obermeier
,
E.
,
2008
, “
Aeromems Polyimide Based Wall Double Hot-Wire Sensors for Flow Separation Detection
,”
Sensors Actuators A
,
142
(
1
), pp.
130
137
.10.1016/j.sna.2007.04.058
9.
Mohammed-Taifour
,
A.
,
Schwaab
,
Q.
,
Pioton
,
J.
, and
Weiss
,
J.
,
2014
, “
Design, Construction, and Validation of a New Wind Tunnel for the Study of Pressure-Driven Separating and Reattaching Flows
,”
AIAA
Paper No. 2014-1307.10.2514/6.2014-1307
10.
Simpson
,
R.
,
1981
, “
A Review of Some Phenomena in Turbulent Flow Separation
,”
ASME J. Fluids Eng.
,
103
(
4
), pp.
520
533
.10.1115/1.3241761
11.
Lotton
,
P.
,
Blanc-Benon
,
P.
,
Bruneau
,
M.
,
Gusev
,
V.
,
Duffourd
,
S.
,
Mironov
,
M.
, and
Poignand
,
G.
,
2009
, “
Transient Temperature Profile Inside Thermoacoustic Refrigerators
,”
Int. J. Heat Mass Transf.
,
52
, pp.
4986
4996
.10.1016/j.ijheatmasstransfer.2009.03.075
12.
Patrick
,
W. P.
,
1987
, “
Flowfield Measurements in a Separated and Reattached Flat Plate Turbulent Boundary Layer
,” NASA Contractor Report No. 4052.
13.
Simpson
,
R. L.
,
1996
, “
Aspects of Turbulent Boundary-Layer Separation
,”
Prog. Aerosp. Sci.
,
32
, pp.
457
521
.10.1016/0376-0421(95)00012-7