A study of the flow in an s shaped duct 2

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 vel

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

= 0.15 m) of 4.73x104 and 1.47x105, respectively A honeycomb and three sets of mesh were installed at the inlet of the wind tunnel, followed by a contraction section with a contraction ratio of about 12

Three interchangeable S-duct test sections of different curvatures were used in this investigation Their geometry and dimensions are shown in Fig 2.2 In reference to the first bend, the vertical side walls are labeled as “Near-side” and “Far-side” walls and these names continue onto the second bend despite the reverse in bend direction The internal dimensions

of the square cross-sectioned S-duct measures D = 0.15 m and the overall length of the S-duct test section is kept constant at L O = 0.8 m for all the three ducts under investigation The test

section consists of a straight inlet and outlet section of length L S = 0.2 m (or L S /D = 1.333),

and the curved S-duct of length L D = 0.4 m (or L D /D = 2.666) For the given D and L D , the

offset between the inlet and outlet centerlines of the S-duct are 0.8D, 1.0D and 1.3D for Test

Section 1, Test Section 2 and Test Section 3, respectively The corresponding curvature ratio

(R C /D) and flow turning angle ( ) are R C /D = 2.422 and = 33.4O, R C /D = 1.933 and =

43.6O and R C /D = 1.667 and = 53.1O 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 = 22.50 Pressure taps were placed at mid height along each side wall of this test section For this

comparative study, the test speed was 4.2 m/s and the Re = 4.0x104 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 corresponding data from Taylor et al (1982a) and Sugiyama et al (1997)

To measure the exit flow condition in the S-duct, a transverse slot was cut on the top cover of the test section near the S-duct exit, so that different probes (Pitot-static and hot-wire probes) could be inserted into the wind tunnel A diffuser was installed downstream of the test section and it is 2000 mm in length, with a half angle of 3.5O and an area ratio of 5.585 The exit of the diffuser was connected to an axial fan, using a flexible canvas tube to minimise vibrations from the fan transmitted upstream to the test section The uncertainty in the spatial measurement and probe position is about ± 1 mm And the uncertainty in the flow

velocity measurement is about 1% at U m = 15 m/s The inlet boundary layer thickness for a

flow speed of 15 m/s was 7.5 mm (or 0.05D) Error analysis is given in Appendix D of the

thesis

2.3 Pressure Measurement

2.3.1 Side Wall Pressure Measurement

To measure the side wall pressure (P) distribution on the S-duct test section, 36

pressure taps were installed at mid-height of each vertical side wall The pressure taps were spaced 15 mm apart and their locations are shown in Fig 2.3 All the pressure tubes were connected to two pressure scanners, each with 48 ports and the pressure was measured using two ±0.3 psid pressure transducers (Model PDCR23D from Scanivalve Corp.) Both the pressure transducers were housed within a Scanivalve for computer controlled pressure scanning/measurement Each time the Scanivalve was switched to a new port, data acquisition was delayed for about 5 sec to allow the pressure to stabilize The pressure data

from each pressure tap was sampled at 2000 Hz for 10 sec The reference wall static pressure

(P S) 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

P P

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 horizontally (cross flow) and vertically and moved the Pitot - static tube at a spatial interval

of 5 mm The entire exit plane was surveyed by the probe except for a 2.5 mm gap near the

four surrounding walls due to the probe’s finite size Thus, the probe traversed from y/D =

z/D = -0.4833 to 0.4833 Each time after the probe was moved to a new position on the exit

plane, a 5 sec time delay was allowed for the flow to stabilize before sampling the pressure

data at a rate of 2000 Hz for 10 sec The total pressure coefficient C PT is defined as

2 2

1

m

S T

U

P P

ρ

−

2.4 Single and Cross Hot-Wire Measurements

A single hot-wire (55P11 from Dantec) was used to measure the flow velocity and turbulence intensity at the S-duct exit plane It was mounted onto the linear traversing device The surveyed exit plane was the same as that made by the Pitot-static probe Thus, the

hot-wire also traversed from y/D = z/D = -0.4833 to 0.4833 and at a spatial interval of 5 mm At

each new position on the exit plane, a 5 sec time interval was allowed for flow to stabilize before acquiring the hot-wire signal from the Constant Temperature Anemometer (Model 56C01) The signal was low pass filtered at 1.5 kHz before sampled at 3 kHz for 10 sec, using the said computer and the analogue-to-digital acquisition card

To measure the cross flow velocity at the exit plane, a rotatable cross-wires (55P61 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 90O 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

( 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 crossed wires were yawed at angles of yaw = -50O to 50O The mean yaw angle of each wire

( i

_

α ) is the angle at which it registers the maximum voltage This is because at this angle, the wire under calibration is normal to the flow, resulting in maximum loss of heat through force convection, and hence yields a maximum voltage Multiple yaw calibrations were performed

for this part of the work to obtain an accurate measurement of i

_

α for each wire

To compute k i 2 , Bruun et al (1990) outlined a method as described below

(a) A velocity calibration was carried out at yaw = 0O yaw angle at different flow

velocities (say U m= 5 to 20 m/s) to both wires A best fit curve is applied to the calibration

m

n i i

B A

E2 = + cos2α + 2sin2α /2 ) which account for flow

angle 1 = α_ 1 + yaw for wire 1 and 2 = α_ 2 - yaw for wire 2 to obtain the unknown

coefficients of A and B for the two wires

(b) A yaw calibration was then carried out at fixed flow velocity (say U m = 15 m/s) at different yaw angles of yaw = -50O to 50O At each yaw angle, the following ratio is evaluated:

2

or 1 sin

cos

sin cos

2 2 2

2 2 2

/ 2

2 0

2

+

+

=

−

−

=

=

i k

k A

E

A E

E

i i

i

i i

i n

yaw

yaw

α α

θ

θ

which contains 2

i

_

α but not B Eq (2.1) can be re-expressed as,

yaw

Eq (2.2) may be interpreted in the form of y = mx, and by plotting the above equation

as (Eθ2yaw −1) versus (Eθyaw2 sin2αi −sin2αi) and applying a curve fit to the data, (1−k i2) and

therefore 2

i

k can be evaluated from the gradient Fig 2.5(b) shows a plot of Eq (2.2) where

2

i

k was evaluated to be 0.0708 and 0.0808

With these known parameters, the velocity components can be obtained from the modified sum and difference method (Bruun (1995)) based on the following equations,

) ( ) (

) ( ) ( / )

( ) ( /

2 2 1 1

1 1 2 2 2 2 2 1 1 1

α α

α α

α α

g g

g f

V g

f V

+

+

) ( ) (

) ( / )

( /

2 2 1 1

1 1 1 2

2 2

α α

α α

g g

f V f

V

+

−

where

U = velocity component parallel to probe axis,

V = velocity component normal to probe axis,

V ei = effective velocity measured by wire i = 1 or 2,

f i = yaw function =(cos2αi +k i2sin2αi)1 / 2, and

i i

i

i i

k

α α

α

tan ) sin (cos

) 1 ( cos

2 2 2

2 2

+

−

It is assumed that the values of 2

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 due to the cumulative error from the measurement of velocity from the Pitot-static tube and from other sources, and the error was estimated to be about 5% through multiple and repeated readings

2.5 Seven-hole Multi (Cobra) Probe

While crossed hot-wires were used to measure the cross flow velocity on the exit plane of the S-duct., a seven-hole cobra probe, mounted on the linear traverse system, was used to measure the same on the interior planes of the S-duct This measurement system from the Aeroprobe Corp allows the measurement of the three dimensional velocity components

in the interior planes of the S-duct Fig 2.6(a) shows a detailed view of the seven-hole probe with the associated probes axes, while Fig 2.6(b) shows the complete cobra probe system The diameter of the cobra probe is 5 mm Besides the probe, the system consists of a pressure module (containing seven 0.3 psi pressure transducers), a DAQ board and an acquisition computer The pressure on the probe is measured by the pressure module via 7 tubes The analogue pressure signals are then digitized by the DAQ system board and transmitted to the acquisition computer for storage into hard disk A software called Aeroprobe supplied by the manufacturer controls the data acquisition process It allows the user to set important parameters like the number of data points, the data acquisition rate, and external triggering modes In this study, 10000 data points are acquired at 1000 Hz for each probe position To synchronize the movement of the probe with data acquisition, an external trigger is applied to start data sampling at every probe position This external trigger is generated by another computer that controls the linear traverse system A more detailed explanation on the data acquisition and control process is given in the next section

2.6 Data Acquisition and Control System

The data acquisition system consists of a computer equipped with a DT 2838 Data Translation Card Voltage signals from pressure transducers and CTAs are connected to one

of the 8 channels for analogue-to-digital inputs To control external devices (or events) like the linear traverse for motion control, the Scanivalve for stepping of pressure ports or to

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

electrical current, smoke streaks formed which enable flow separation phenomenon to be seen

Smoke wire flow visualization was also conducted close to the near-side wall of the S-duct to visualize the flow near to that wall due to swirl development In this case, the smoke wire was stretched vertically and pulled taut using weights The vertical smoke wire was placed at about 2 mm away from the near side wall and upstream of the separation point, as determined earlier from the side wall pressure distribution The approximate location of the vertical smoke wire is shown in Fig 2.8(b) Both smoke wire flow visualization experiments were conducted at 5 m/s, in order to coincide with the lower of the two wind speeds investigated

Surface flow visualization on the bottom wall (or floor) of the S-duct was conducted

to observe the flow pattern there A thin, black plastic sheet was cut to fit the S-shaped duct and placed on the bottom wall of each duct Mixture of turpentine and white powder was applied evenly onto the plastic sheet using a sponge With the test speed set at 15 m/s, it took approximately 30 minutes for the mixture to dry completely The plastic sheet was subsequently removed from the wind tunnel, and the surface flow patterns were captured using a still camera

2.8 Flow Control Devices

2.8.1 Vortex Generators

The vane-type vortex generators (VG) used in this study were made from 1 mm thick aluminum sheet, and they are triangular in shape with a sharp leading edge The general

dimensions are shown in Fig 2.9(a) The height (h) of the VG was 5 mm and this

corresponds approximately to the boundary layer thickness of the flow at the duct inlet A row of 10 VGs were placed equi-distant to each other and pairs of VGs were angled towards