16
Chapter 2
EXPERIMENTAL SET-UP

2.1 Introduction
In this chapter, the details of the experimental set-up used in the present study are
described. A description of the S-duct wind tunnel test rig and the four different S-duct test
sections used in this investigation is described first. Next, all the instrumentation and
equipment used in experiments are presented. These include the measurement of side wall
surface pressure distribution, the use of a Pitot-static tube, a cross hot-wire and a 7-hole cobra
probe to measure the total pressure, and velocity components within the S-duct.
Subsequently, the different flow visualisation techniques used in the present study will be
presented. This is followed by the experimental set-up for flow control study using vortex
generator, tangential blowing and vortex generator jets. Estimated experimental errors are
stated.
The chapter concludes with a comparative study by benchmarking the present
experimental results with published data of Taylor et al. (1982b) and Sugiyama et al. (1994).
The bench marking establishes the accuracy of the present experimental method against
known data.
2.2. S-shaped Duct Wind Tunnel
An open loop suction wind tunnel was fabricated for this project and is shown in Fig.
2.1, with the square cross-sectioned S-duct as the test section. Flow velocities were U
m
= 5
and 15 m/s, thus giving Reynolds numbers, Re (based on the hydraulic diameter of the duct D
17
= 0.15 m) of 4.73x10
4
and 1.47x10
5
, respectively. A honeycomb and three sets of mesh were

, R
C
/D = 1.933 and  =
43.6
O
and R
C
/D = 1.667 and  = 53.1
O
for Test Section 1, 2, and 3 respectively. The entire
test section is fabricated from transparent Perspex sheets, so that flow visualisation can be
conducted and viewed from the side and the top. The coordinate system used in this
experiment is shown in Fig. 2.3 and its origin is at the center of the inlet plane of the S-duct,
with the positive s-coordinate pointing downstream, along the duct centerline.
To compare the experimental results with available literature, a fourth square cross
sectioned S-duct test section (similar in geometry to that of Taylor et al. (1982a)) was tested.
This comparative study is to benchmark the experimental results with those available in the
literature. Referred to as Test Section 4, its geometry is shown in Fig. 2.4. It has hydraulic
diameter, D = 0.15 m, with a curvature ratio of R
C
/D = 7.0 and a turning angle  = 22.5
0
.
Pressure taps were placed at mid height along each side wall of this test section. For this
18
comparative study, the test speed was 4.2 m/s and the Re = 4.0x10
4
. A boundary layer trip
was placed at the inlet of the test section to thicken the boundary layer to 0.15D as reported in
Taylor et al. (1982a). The measured data from this S-duct test section were compared to

) was measured by a ±0.1 psid Setra pressure transducer (Model 239). All data were stored
in a computer via an analogue-to-digital acquisition card (Model DT2838 from Data
Translation). The error in the measurement of the pressure coefficient is about 4%. The
pressure coefficient C
P
is defined as
2
2
1
m
S
U
PP
ρ
−
. Error analysis is enclosed in Appendix D of the
thesis.

2.3.2 Total Pressure Measurement
The total pressure distribution on the exit plane of each test section was measured
using a Pitot-static tube, mounted onto a computer-controlled linear traversing device. The
diameter of the Pitot-static tube measures 3 mm. A ±0.3 psid pressure transducer was used to
measure the total pressure (P
T
), while the ±0.1 psid Setra pressure transducer measured the
reference wall static pressure (P
S
). As shown in Fig. 2.1, the Pitot-static tube (near the centre
of the picture) extends into the test section through a transverse slot that was cut on the top
cover of the test section near the S-duct exit. The linear traversing mechanism translated both

from Dantec) probe was employed. The cross-wires and its probe support were mounted on
the linear traversing device. The diameter of the cross-wire probe is 5 mm. The cross-wires
probe was first orientated to measure the velocity components in the s-y plane at the duct exit
in the first pass. The experiment was then repeated with the cross-wires rotated 90
O
about its
own axis to measure the velocity components in the s-z plane. This enables the cross flow
velocity components in the y-z plane to be determined. Due to the larger probe size, the
velocity measurements could not be carried out within a 5 mm space around the interior side
walls. All cross-wire measurements were confined to a lower half plane of the duct exit, i.e.
from y/D = -0.4667 to 0.4667 and z/D = -0.4667 to 0.0 and at a spatial interval of 5 mm. Like
before, at each new probe position, a 5 sec delay precedes data acquisition. The two cross-
wire signals from the Dantec Constant-Temperature Anemometer (Model 56C01) were low
pass filtered at 1.5 kHz before being digitally sampled at 3 kHz.
The modified sum and difference method as outlined in Bruun (1995) was used to
decompose the effective velocity measured by each wire to the required velocity components.
This method requires the knowledge of the mean yaw angle (
i
_
α
) and the yaw coefficient
21
(
2
i
k
) of each wire which can be obtained through a yaw calibration of the crossed wires. Fig.
2.5(a) shows a typical result from a yaw calibration on the crossed wires at U
m
= 15 m/s. The

= 5 to 20 m/s) to both wires. A best fit curve is applied to the calibration
curve (based on King’s Law,
(
)
n
m
n
iii
UkBAE
2/
2222
sincos
αα
++=
) which account for flow
angle 
1
=
_
α
1
+ 
yaw
for wire 1 and 
2
=
_
α
2
- 








+
+
=








−
−
=
=
i
k
k
AE
AE
E
iii
iii

2*
=−−=− iEkE
i
i
i
yawyaw
αα
θθ
. (2.2)
Eq. (2.2) may be interpreted in the form of y = mx, and by plotting the above equation
as
(
)
1
2*
−
yaw
E
θ
versus
(
)
i
i
yaw
E
αα
θ
222*
sinsin −

1
2
22
2
2
1
11
αα
αααα
gg
gfVgfV
U
ee
+
+
=
, (2.3)
[ ] [ ]
)()(
)(/)(/
2
2
1
1
1
11
2
22
αα
αα

iii
ii
k
k
α
αα
α
tan
)sin(cos
)1(cos
2
2
2
2
2
+
−
.
It is assumed that the values of
2
i
k
and
i
_
α
remain constant for each wire during the
experiment. However, an in-situ calibration of the cross-wire probe based on the King’s Law
was carried out for each wire at the start of every experiment to obtain the effective velocity
measured by each wire. The uncertainty in the flow velocity measurement using hot-wire is

the linear traverse for motion control, the Scanivalve for stepping of pressure ports or to
24
commence cobra probe’s data acquisition, 2 channels of digital-to-analogue TTL signal
outputs are available to trigger these devices. Fig. 2.7(a) to (c) show the schematic wiring
diagram used in the measurement of side wall static pressure, cross wire measurement and
cobra probe measurement respectively.
The drawings in Fig. 2.7(a) and (b) show that the data acquisition card is installed in
the Control Computer (as indicated). The inputs from the acquisition card receive voltage
signals from the respective pressure transducers and CTA, while the external devices like the
Scanivalve (Fig. 2.7(a)) and linear traverse system (Fig. 2.7(b)) are controlled by TTL trigger
signal outputs from the same data acquisition card. All data are stored in the hard disk of the
Control Computer. A slightly different wiring diagram is used in Fig. 2.7(c) for cobra probe
measurement. The Control Computer is solely responsible for controlling the linear traverse
and the commencement of data acquisition for the cobra probe via TTL trigger signals. This
Control Computer does not receive any data inputs from the cobra probe. All pressure data
from the cobra probe are stored and processed into velocity data using the Aeroprobe
Computer.

2.7 Surface and Smoke Flow Visualization
Smoke wire flow visualization was conducted on the inner wall of the first bend of the
S-duct to show and observe the presence of flow separation. Fig. 2.8(a) shows the
approximate location of the three smoke wires in the first bend of the S-duct. The flow
separation position was first determined from the measured side wall pressure distribution. In
the vicinity of the separation point, three 0.25 mm diameter smoke wires were threaded
through the pressure taps on the near and far side wall of the duct. The smoke wires stretched
across the width of the duct and were pull taut using weights. Paraffin oil was coated on the
wires with a small brush and beads of oil formed on the wire. With the application of an
25
electrical current, smoke streaks formed which enable flow separation phenomenon to be
seen.

O
and 15
O
, as shown in
Fig. 2.9(a). A different vortex generator configuration is also shown at the bottom of Fig.
2.9(a) where the last pair of VGs at both ends of the row are angled at higher

vg
= 20
O
, with
the rest of the VGs remaining at 10
O
. This mixed VG configuration is to enhance further
mixing at the corner of the S-duct. The row of VGs was then placed vertically (in the z-
direction) on the near-side wall and just upstream of the separation point as determined from
the near-side wall pressure distribution. The placement of VG is shown in the 3D drawing of
Fig. 2.9(b). The most appropriate location was considered to be just prior to the region of
high adverse pressure gradients where the subsequent boundary layer rapidly grows
downstream.

2.8.2 Tangential Blowing
Fig. 2.10(a) shows a schematic set-up for tangential blowing on the near-side wall of
the S-duct. A frequency controlled blower supplies air into a plenum and the flow rate (Q)
was measured by a flow meter. To reduce flow unsteadiness, the air passes through a
honeycomb in the plenum before entering the 14 blowing tubes connected to the near-side
wall in the first bend. Each tube measures 1.5 mm and 1.37 mm in external and internal
diameter respectively and is bent at right angles to direct air to blow tangentially along the
near-side wall. Each blowing tube is also rotatable to blow air at an angle to the main flow.
The two photographs in Fig. 2.10(b) show the top and side view of the blowing tubes and

m
for each tube is 0.012 and 0.045. The small magnitude of the jet
momentum coefficient is due to the small blowing ports used here.

2.8.3 Vortex Generator Jets

Fig. 2.11 shows the set-up for vortex generator jets on the near-side wall. CNC wire
cut was used to fabricate the outlet angled slots of the vortex generator jets and these slots are
angled at

jet
= 5
O
, 10
O
, 15
O
and 20
O
to the flow. The row of outlet angled slots is cut from a
thin copper plate which measures 150 mm x 25mm x 0.1 mm (length x width x thickness)
and 7 pairs of VG slots are cut from each plate. The length and width of each slot is 15 mm
and 0.5 mm respectively. On the near-side wall of the S-duct, square cut-outs are removed
from the side wall and the vertical location of each cut-out correspond to the location of the
angled slot on the copper plate. The copper plate (with outlet angled slots at a particular

jet
)
is then placed over the square cut-outs. The angled slots of the VG jets are placed just
upstream of the flow separation point, as determined from the surface pressure measurement.

published studies. In Fig. 2.12(c), a comparison of the normalized cross flow velocity profile
at the duct exit with corresponding data from Taylor et al. (1982a) and Sugiyama et al.
(1997) also shows fairly good agreement. The shape of the cross flow velocity profile and
their magnitudes are comparable.
This comparative study demonstrates that the experimental technique used in this
investigation re-produces existing data well and gives confidence of its accuracy. Based on
the same experimental technique, the swirl development in the three high curvature, square
cross sectioned S-ducts were studied and the results will be discussed in the subsequent
chapters.