Chapter 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 Um = and 15 m/s, thus giving Reynolds numbers, Re (based on the hydraulic diameter of the duct D 16 = 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 LO = 0.8 m for all the three ducts under investigation The test section consists of a straight inlet and outlet section of length LS = 0.2 m (or LS/D = 1.333), and the curved S-duct of length LD = 0.4 m (or LD/D = 2.666) For the given D and LD, 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 and Test Section 3, respectively The corresponding curvature ratio (RC/D) and flow turning angle ( ) are RC/D = 2.422 and 43.6O and RC/D = 1.667 and = 33.4O, RC/D = 1.933 and = = 53.1O for Test Section 1, 2, and 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 RC/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 17 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 ± mm And the uncertainty in the flow velocity measurement is about 1% at Um = 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 sec to allow the pressure to stabilize The pressure data 18 from each pressure tap was sampled at 2000 Hz for 10 sec The reference wall static pressure (PS) 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 CP is defined as P − PS Error analysis is enclosed in Appendix D of the ρU m 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 mm A ±0.3 psid pressure transducer was used to measure the total pressure (PT), while the ±0.1 psid Setra pressure transducer measured the reference wall static pressure (PS) 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 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 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 CPT is defined as PT − PS ρU m 19 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 hotwire also traversed from y/D = z/D = -0.4833 to 0.4833 and at a spatial interval of mm At each new position on the exit plane, a 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 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 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 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 mm Like before, at each new probe position, a sec delay precedes data acquisition The two crosswire signals from the Dantec Constant-Temperature Anemometer (Model 56C01) were low pass filtered at 1.5 kHz before being digitally sampled at 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 ( α i ) and the yaw coefficient 20 ( ki2 ) 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 Um = 15 m/s The crossed wires were yawed at angles of yaw = -50O to 50O The mean yaw angle of each wire _ ( α i ) 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 α i for each wire To compute ki2, 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 Um= to 20 m/s) to both wires A best fit curve is applied to the calibration ( curve (based on King’s Law, E = A + B cos α i + k i2 sin α i angle _ = α1+ yaw for wire and _ = α2- yaw ) n/2 n U m ) which account for flow for wire 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 Um = 15 m/s) at different yaw angles of yaw = -50O to 50O At each yaw angle, the following ratio is evaluated: Eθ yaw = *2 2/n Eθ2yaw − A = Eθ2yaw =0 − A cos α i + k i sin α i cos α i + k i sin α i i = or , (2.1) _ which contains k i2 , α i and α i but not B Eq (2.1) can be re-expressed as, ( Eθ*yaw − = − k i )(E *2 θ yaw sin α i − sin α i ) i = or (2.2) Eq (2.2) may be interpreted in the form of y = mx, and by plotting the above equation ( ) ( ) ( ) as Eθ*2 − versus Eθ*yaw sin α i − sin α i and applying a curve fit to the data, − k i2 and yaw 21 therefore k i2 can be evaluated from the gradient Fig 2.5(b) shows a plot of Eq (2.2) where k i2 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, U= [V / f (α ) g (α ) + Ve / f (α ) g1 (α ) , V= [V / f (α ) − Ve1 / f1 (α ) , e1 e2 ] [ g1 (α ) + g (α ) ] [ g1 (α ) + g (α ) ] (2.3) ] (2.4) where U = velocity component parallel to probe axis, V = velocity component normal to probe axis, Vei = effective velocity measured by wire i = or 2, fi = yaw function = (cos α i + k i sin α i )1 / , and cos α i (1 − k i ) gi = yaw function = (cos α i + k i sin α i ) tan α i _ It is assumed that the values of k i2 and α i 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 22 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 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 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 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 23 commence cobra probe’s data acquisition, 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 24 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 Sduct 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 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 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 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 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 25 each other (or in a counter-rotating fashion) at an angle vg = 5O, 10O and 15O, 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 = 20O, with the rest of the VGs remaining at 10O 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 zdirection) 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 define the blowing angle, blow As shown in the side view of Fig 2.10(b), positive blow refers to an angle where the jets are directed towards the top or bottom wall and negative blow refers to angle with jets directed towards the S-duct’s horizontal center-plane In this study, blow varies from -5O, 0O, 5O, 10O, 15O and 20O, each at two different jet velocities of 12.5 m/s and 24.5 m/s and at the test Reynolds numbers At these blowing velocities, the respective jet 26 momentum coefficient, Cm 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 = 5O, 10O, 15O and 20O 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 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 Similar to that of tangential blowing, tubes connected to the plenum direct air to the row of VG slots and the air is injected into the S-duct on the near-side wall The flow rate, Q was measured by a flow meter and was varied (and hence the exit jet velocity) to match the jet momentum coefficient, Cm, of tangential blowing, which is 0.012 and 0.045 2.9 Comparative Study with Existing Literature This section describes a comparative study of the present experimental result with those of Taylor et al (1982a) and Sugiyama et al (1997) for the S-duct test section of RC/D = 7.0, = 22.5O The purpose is to benchmark our results with the existing published data to ensure that the experimental facilities were working well It can be seen from Fig 2.12(a) that the present surface pressure coefficients (CP) compare fairly well with those reported in 27 Taylor et al (1982a) These CP were conducted using a micro-manometer; the same technique used by Taylor et al (1982a) There is a clear general downward and sinusoidallike trend in the Cp distribution Fig 2.12(b) shows the normalized exit velocity distribution, which also compare well with the data from Taylor et al (1982a) and Sugiyama et al (1997) At the near-side wall, a region of low momentum fluid exists in all three cases, and the normalized exit velocity in the present investigation agrees quantitatively with the two 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 28 ... 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... 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, ... 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