A study of the flow in an s shaped duct 4

Chapter ♠ SWIRL DEVELOPMENT IN SQUARE CROSS SECTIONED S-SHAPED DUCT 4.1 Introduction As discussed in the Chapter 3, curved duct flows are influenced predominately by two related forces, namely, the centrifugal force and the radial pressure gradient force that exist between the outside and inside walls of the curved duct, resulting in the formation of secondary flow In that chapter, a non-dimensional parameter , defined as the ratio of radial pressure gradient to centrifugal force, was established and its relation with Cp and curvature ratio was shown explicitly The variation of (s/SO) with normalised downstream distance, (s/SO) results in collapsed curves independent of curvature ratio Using the data from square cross-sectioned S-shaped duct, it was also shown that data scatter in the collapsed curved was attributed to differences in inlet condition, the presence of flow separation and stream wise vortices In this Chapter, the swirl development of square cross sectioned S-shaped duct is investigated in more detail, with attention focused on the formation mechanism of these stream-wise vortices in the second bend of the S-duct and to propose a simple flow model to clearly show the vortical flow topology ♠ A major part of this chapter has been published in Experiments in Fluids (2006, Vol 41 Issue: pp 975989) under the title “On swirl development in a square cross-sectioned, S-shaped duct” by Ng YT, Luo SC, Lim TT, and Ho QW 47 4.2 Experimental Set-up and Methodology As described in detail in Chapter 2, the investigations here were conducted at Um = m/s and 15 m/s with corresponding Re = 4.73x104 and 1.47x105, respectively Using three square cross-sectioned, S-shaped ducts (termed TS to TS 3) with curvature ratio RC/D = 2.422, 1.933 and 1.667, and a duct turning angle =33.4O, 43.6O and 53.1O, respectively The inlet boundary layer thickness, measured using a single hot-wire at a flow speed of 15 m/s, was 7.5 mm (or 0.05D) The side wall Cp was measured using the Scanivalve system and the total pressure and cross flow velocity at the S-duct’s exit were measured using Pitot-static tube, and cross-hot wire respectively A single hot wire was also used to measure the turbulence intensity at the S-duct’s exit The probes were mounted on the linear traversing device, and the spatial resolution for all probe measurements is mm In all cases, a sec waiting time was allowed for the flow to stabilise before acquiring data at each new probe position For total pressure and single wire measurement, the entire exit plane was measured while for cross wire measurements, only the lower half plane was surveyed Smoke wire visualization was used to visualize the flow separation phenomenon Vertical wire smoke visualization was used close to the near-side wall in the first bend of the S-duct to visualize the flow near that wall due to swirl development in the first bend Finally, surface flow visualization on the entire bottom wall of the S-duct was also conducted to give an overall picture of swirl development in the duct 4.3 Results and Discussion 4.3.1 Side wall Cp distribution The data for side wall Cp distribution and flow visualization were presented in Chapter 3.3.1, and they are repeated here to aid further discussion Fig 4.1(a) to 4.1(c) shows 48 the side wall CP distribution versus s/D (where s is measured from the center of the duct inlet plane along the curved duct centerline), for the three S-shaped test sections investigated In all cases, the CP distribution on the near and far-side wall displayed a general sinusoidal-like variation, and the pressure difference between the two side walls increases with the duct curvature In addition, all the three pressure distributions of the near-side wall display an inflection point, which indicates the presence of flow separation This was confirmed by smoke wire flow visualization near the separation point as shown in Fig 4.2(a) to (c) for Test Section to 3, respectively This clearly shows that the presence of flow separation led to a local distortion of the CP distribution on the near-side wall resulting in the inflection point The strong adverse pressure gradient due to the high curvature side wall in the duct led to flow separation along the near-side wall The adverse pressure gradient is defined in the present investigation as the difference between the minimum near-side wall pressure and the pressure at the inflection point for each test section Denoting the adverse pressure gradient along the near side wall for the three test sections as ∆C p1 ∆s , ' shown in Fig 4.1(a) to (c) respectively, and together with ∆C p ∆s s’= ' and ∆C p3 s/D, which ∆s ' as is the curvilinear distance taken over the two end points in the adverse pressure gradient region, the estimated magnitudes of ∆C p1 ∆s = 0.2053, ' ∆C p ∆s = 0.2878 and ' ∆C p3 = 0.3748 ∆s ' Expectedly, this shows that the increase in curvature leads to a corresponding increase in the magnitude of the adverse pressure gradient In addition to the presence of flow separation, the side wall pressure distribution clearly shows that the pressure difference between the near and far side walls changes sign along the axial flow direction of the S-duct Compared to the works of Anderson et al (1982) and Taylor et al (1982a) on the gentle curvature S-shaped duct of RC/D = 7.0, it is quite well known that swirling flow develops in the first bend and the direction of swirl is largely 49 reversed after the second bend, due to both the change in curvature and the change in sign of the pressure difference between the two side walls Therefore, the present surface pressure measurements are consistent with those reported in the literature In the present study, where the ducts have sharper curvature than those reported in literature, surface oil flow visualization on the bottom floor of the duct and smoke wire flow visualization on the nearside wall of all the three S-duct test sections were conducted to better understand the flow and the swirl development This is discussed in the next section 4.3.2 Surface and Smoke Flow Visualization Fig 4.3(a) to (c) show the surface oil flow patterns at the bottom floor of Test Section to respectively All three test sections show the distinctive feature of a dividing line (where the white powder is accumulated) that emanates from the end of the first bend at the near-side wall, and leaves the duct exit close to the far-side wall of the second bend The presence of this line marks two flow regions denoted as region A and B in the figures On closer inspection of the streaks in region A in Fig 4.3(a) to (c), it is noted that the flow, upon entering the first bend, develops a swirl such that the flow component, next to the bottom floor, moves from the far-side wall to the near-side wall However, on entering the second bend, a reversal of swirl direction begins, which can be seen from the streaks in region B In addition, the streaks in region A in the second bend, moves from the near-side wall to the farside wall The existence of this dividing line, which arises due to the swirling flows of opposite direction, was not previously reported in the literature To complement the surface flow visualization study and provide more insight to swirl development in S-duct flows, smoke wire flow visualization on the near-side wall was also conducted As pointed out in Chapter 2.7, a vertical smoke wire was placed upstream of the inflection plane and close to the near-side wall of the S-duct Due to flow symmetry about the 50 mid-plane of the S-duct (the z/D = 0.0 plane), the flow visualization was conducted for the upper half of the near-side wall only, and the results are shown in Fig 4.4(a) to (c) for Test Section to 3, respectively Of interest is the change in flow direction of the smoke trails especially those near the top wall of each test section In all cases, it can be seen that the smoke trails near the top wall exhibit an initially downward trajectory before turning upward Although not visualized, it can be conjectured that the smoke trail at the bottom half of the test section, will follow an initially upward trajectory before turning downward, essentially a mirror image trend The change in direction of the smoke trails is due to the change in swirl directions, close to the near-side wall, as the smoke is convected from region A into B as labeled in Fig 4.3(a) to (c) In addition to the flow visualization study, which provides a qualitative understanding of the swirl development in a square cross-sectioned S-duct, total pressure distribution and cross flow velocity measurements at the S-duct exit were also conducted to obtain a quantitative appreciation of the flow features The results are discussed in the next section 4.3.3 Total Pressure Distribution and Cross Flow Velocity The data for total pressure distribution and cross flow velocity had been presented in Chapter 3.3.3 and are repeated here for convenience of further discussion and presentation Fig 4.5(a) to (c) show the total pressure distribution (using a Pitot-static tube) and the normalized cross flow velocity (using a cross hot-wire) at the S-duct exit for Test Section to 3, respectively The Reynolds number was Re = 1.47x105 In these Figures, the s-y-z coordinate axes are adopted, and Fig 4.5 is plotted with the positive s-axis (the exit flow direction) pointed out of page, positive y-axis pointed to the right and positive z-axis pointed upwards On a more general note, the figures show that in all three test sections, the exit total pressure distributions are fairly symmetrical about the z/D = 0.0 plane and the regions close 51 to the four surrounding walls have relatively lower total pressure (and exit velocity) than those in the core Furthermore, the normalized cross flow vector plots showed generally that in the central region of the duct, the main bulk of the cross flow is from the far-side wall to the near-side wall while the cross flow near the bottom floor of the test section is in the opposite direction The cross flow thus sets up a swirl at the duct exit, in the lower half of the duct exit plane With the combined normalized cross flow vector plots in Fig 4.5(a) to (c) and the surface oil flow visualization in Fig 4.3(a) to (c), the swirl developments in Test Section to can be studied in more detail By comparing the respective results from Figs 4.3 and 4.5, one can deduce that the location of flow reversal on the bottom floor of the S-duct (in Fig 4.5) corresponds approximately to the location of the dividing line near the S-duct exit (in Fig 4.3) The oil streaks in region B of Fig 4.3(a) to (c) indicate the existence of a cross flow velocity component near the bottom wall of the S-duct, which is directed from the near-side wall to the far-side wall This observation is supported by the cross-wires measurements shown in Fig 4.5(a) to (c) Since the oil streaks in region A are directed from the far-side wall to the near-side wall, it can thus be said that close to the bottom floor of the test section, the flow in region A and B meet and subsequently separate along the dividing line Hence, this dividing line is actually a flow separation line, resulted from the “meeting” of swirling flows of opposite direction The overall flow behavior can be seen more clearly in a 3-D flow model presented in Fig 4.6 for the lower half of a typical square cross-sectioned, S-shaped duct It is expected that the upper half would behave in a similar but mirror image way It can be seen in Fig 4.6 in the flow model that the flow develops a swirl (in Region A) after passing through the first bend of the S-duct, while in the second bend of the duct, the swirl of the opposite direction (in Region B) develops near the inflection plane of the duct, close to the near-side wall 52 While the swirl in Region B grows in size and strength due to the S-duct’s curvature in the second bend and the increasing transverse pressure gradient, the swirl in region A undergoes a corresponding decrease in size as it approaches the duct exit The two swirling flows of opposite direction meet along a dividing or separation line as shown in Fig 4.6, which is consistent with the surface oil flow visualization presented in Fig 4.3(a) to (c) In addition, the 3-D flow model is in agreement with the vertical smoke wire flow visualization presented in Fig 4.4 As noted and discussed previously, the smoke trails close to the near-side wall on the lower half of the S-duct followed an initially upward trajectory before turning downward as the smoke crosses the dividing line This observation is also reflected in the flow model in Fig 4.6(b), and is labeled as C, near the entrance to the inflection plane of the duct and close to the near side wall Just upstream of C (in the first bend of the S-duct), the flow carries the smoke in an upward trajectory away from the bottom wall However, upon entering the second bend and crossing the dividing line close to the near-side wall, the smoke trails follow a downward path towards the bottom wall Hence, the change in the direction of the smoke trails is a manifestation of the change in swirl direction From Fig 4.5(a) to (c), the normalized cross flow vector plots show that stream-wise vortices are present close to the near-side wall of the S-duct, and they exist either as a counter rotating vortex pair or as just a single vortex For illustration purposes, the presence of stream-wise vortices along the nearside wall of the second bend is also illustrated in Fig 4.6 as a counter-rotating vortex pair The formation mechanism for these stream-wise vortices is quite different from that of the swirl development discussed earlier It should be noted that these stream-wise vortical structures were also observed in the experimental work of Anderson et al.(1982), Taylor et al (1982a) and the numerical work of Sugiyama et al.(1997) (in Japanese) Using an S-duct with curvature ratio RC/D = 7.0, duct turning angle = 22.5O and Re = 4.0x104, they found only a single vortex at the near-side 53 wall of the second bend and showed that the axial vorticity developed in the first bend is locally reinforced at the near-side wall of the second bend Anderson et al (1982) and Taylor et al (1982a) explained the formation mechanism of such vortices by considering the Squire and Winter formula (Squire and Winter (1951) and Scorer (1997)) for the production of axial vorticity from radial vorticity in turning flows It was shown by Squire and Winter (1951) that, for flows in a bend, a redistribution of vorticity takes place, such that the axial vorticity in the direction of flow is equal to the velocity gradient (or ∂u ∂z ) in the approaching stream multiplied by twice the angle of deflection That is, ∂w ∂v ∂u − = −2ε ∂r ∂z ∂z (4.1) where u ,v and w = velocity components, r = the radial co-ordinate directed towards the centre of curvature (same as y axis at the exit plane of the duct), z = the vertical co-ordinate and = the turning angle of a bend measured in radians The derivation of the Squire and Winter formula is shown in Appendix A and B of this thesis To test whether the Squire and Winter formula is applicable in the present work, which was conducted at a Re = 1.47x105 and with S-shaped ducts of sharper curvature, ∂u ∂z was deduced from the velocity measurements from a single hot wire and this is shown in Fig 4.7(a) and (b) In Fig 4.5(a) and (b), it can be seen that the center of the counterrotating vortex pair occurs approximately along the line y/D = -0.4 and from Fig 4.7(a) and (b), the rate of change of normalized ∂u ∂z shows two sign changes as z/D increases for Test Section and In other words, with increasing z/D along the line y/D = -0.4, normalized ∂u ∂z is initially positive, and followed by a negative value before turning positive again as z/D increases In reference to Eq 4.1, the sign changes in ∂u ∂z results in axial vorticity of 54 the opposite sign which is consistent with the counter-rotating vortices seen in Fig 4.5(a) and (b) The same argument can be applied to Test section in Fig 4.7(a) and (b) where normalized ∂u ∂z follows a positive and then a negative trend Here, with only one sign change, only single vortex is seen in Fig 4.5(c) The explanation based on Squire and Winter formula suggests that the initial swirl developed in the first bend of the duct leads to a redistribution of the stream-wise velocity (or isotachs) close to the near-side wall of the second bend This in turn leads to velocity gradients, which results in the formation of stream-wise vortices To further study the influence of swirl on the stream-wise vortices near the wall, measurements of exit-plane total pressure distribution, normalized stream-wise and cross flow velocities were repeated for Test section 1, and at a lower Re = 4.73x104 (or Um = m/s) These results are shown in Fig 4.8(a) to (c) respectively for the three above mentioned test sections Fig 4.9(a) and (b) shows the changes in normalized ∂u ∂z for the three test sections Fig 4.8(a) to (c) illustrate that the stream-wise vortices are also present for the low Re case, in the form of a pair of counter-rotating vortices for Test section and and a single vortex for Test Section Therefore, for these test sections, the vortex configuration remains the same as that in the higher Re cases However, the positions of the vortex cores in these low Re cases are located closer to the duct centre plane than in the higher Re cases If the Squire and Winter formula is applied one at a time to Fig 4.8(a) along y/D = - 0.35 for Test Section 1, to Fig 8(b) along y/D = -0.35 for Test Section 2, and to Fig 4.8(c) along y/D = 0.3 for Test Section 3, one notes that the variation of normalized ∂u ∂z for Test Section and undergoes two sign changes as z/D increases, while for Test Section only a single sign change is present, as shown in Fig 4.9(a) to (b) The above illustration thus shows that the Squire and Winter formula is applicable at both the Reynolds numbers and provides a qualitative explanation of the formation mechanism of the stream-wise vortices near the wall 55 In addition, the use of the formula allows one to predict the vortex configuration of these stream-wise vortices based only on the sign changes in velocity gradient 4.3.4 Exit Turbulence Intensity The turbulence intensity at the S-duct exit was measured for Test section 1, and for Re = 1.47x105 and Re = 4.73 x 104 The normalized turbulence intensity (u’/Um) distribution plots are shown in Fig 4.10 for Test Section to at Re = 1.47x105 and in Fig 4.11 for Re = 4.73x104 The turbulence intensity in each figure is normalized by their respective flow velocities The two figures showed that the core flow has relatively lower turbulence intensity (about 3%) than the near wall, low momentum flow region (typical values of 7% to 12% and above) Comparing Fig 4.10 with Fig 4.5 and Fig 4.11 with Fig 4.8, it is clear that the near wall vortical regions correspond to regions of relatively high turbulence intensity Given the relatively high turbulence intensity, one may question the applicability of the Squire and Winter formula on explaining the formation mechanism of the near wall longitudinal vortices This can be addressed by looking at the data from Taylor et al (1982a) In their work, Taylor et al (1982a) performed an investigation at a low Re = 790 (laminar case) and at comparatively higher Re = 4.0x104 (or turbulent case) and reported that the turbulence intensity for Re = 4.0x104 near the wall is 13% This is in good agreement with the present data for Re = 4.73x104 (typically about 12% and above) The Squire and Winter formula was applied to the two stated cases and was shown to be applicable to both laminar and turbulent flow in Taylor et al.’s work Even though the turbulence intensity in their work is relatively high near the wall (about 13%) for Re = 4.0x104, the Squire and Winter formula can still explain the formation of the stream-wise vortices In the present investigation, the Squire and Winter formula also correctly predicted the number of streamwise vortices Hence it can be argued that the Squire and Winter formula can be extended to 56 the present flow cases Taylor et al (1982a) explained that the primary factor which affects the swirl development and vortical flow structure in the S-duct is actually the inlet boundary layer thickness From their result and discussion, it seems that turbulence intensity has less influence on the applicability of the Squire and Winter formula In the next Chapter, the inlet boundary layer thickness is varied to study its effects on the configuration of these streamwise vortices In addition, the cross flow on multiple, internal planes of the S-duct will be presented to verify the flow model proposed in this Chapter 4.4 Chapter Conclusion The flow features in square cross sectioned S-shaped ducts of large curvature and at Re = 4.73x104 and 1.47x105 have been investigated experimentally in detail Surface pressure measurements and flow visualization indicated the presence of flow separation at the nearside wall of the first bend The swirl flow, which developed in the first bend, was largely attenuated in magnitude in the second bend due to the formation of swirl flow of the opposite direction Surface oil flow visualization conducted on the bottom wall showed the existence of a clear dividing or separation line, with a similar separation line expected to occur on the top wall due to symmetry of the S-duct test section This separation line is caused by the flow of opposite swirl which meet, and subsequently separate along this line In addition, streamwise vortices were found to form, either as a counter-rotating vortex pair or just a single vortex, along the near-side wall at the second bend Their formation follows the mechanism suggested by Squire and Winter (1951), and can be traced to the redistribution of stream-wise isotachs which led to velocity gradient ∂u ∂z The redistribution of isotachs is shown to be a predominately invicid process In addition, a qualitative flow model has been proposed here In the next Chapter, cross flow velocity and total pressure measurements at multiple planes inside the S-duct will 57 provide a clearer picture of the proposed flow model Furthermore, the effects of inlet boundary layer thickness will be investigated for square cross sectioned S-duct flows, which will provide further explanation on the formation of stream-wise vortices along the curved wall of the second bend 58 ... boundary layer thickness, measured using a single hot-wire at a flow speed of 15 m /s, was 7.5 mm (or 0.05D) The side wall Cp was measured using the Scanivalve system and the total pressure and... radial co-ordinate directed towards the centre of curvature (same as y axis at the exit plane of the duct) , z = the vertical co-ordinate and = the turning angle of a bend measured in radians The. .. cross sectioned S- shaped ducts of large curvature and at Re = 4. 73x1 04 and 1 .47 x105 have been investigated experimentally in detail Surface pressure measurements and flow visualization indicated