A study of the flow in an s shaped duct 6

Chapter FLOW CONTROL IN S-SHAPED DUCT 6.1 Introduction In previous Chapters, a detailed study was made to highlight the various vortical structures in S-duct flows In this Chapter, different flow control techniques are studied to investigate their effects It is well known from various studies of S-shaped duct with circular (Bansod and Bradshaw (1972)), rectangular (Guo and Seddon (1982)) and square cross sections (Taylor et al (1982a), Ng et al (2006) and Ng et al (2008))) that such flows are accompanied by the presence of swirl in the bulk flow and near-wall stream-wise vortices The optimum aerodynamic performance of S-shaped ducts (or aircraft air-intake ducts) demands that a relatively uniform flow with minimal swirl exits the duct and a smallest possible pressure loss These requirements naturally lead one to consider the use of traditional flow control devices like vortex generators, blowing jets, vortex generator jets and fences Passive devices like vortex generators placed on the side wall eliminate flow separation if it is present They “locally” mix the high-momentum fluid in the free stream with low-momentum fluid near the wall and thus suppress flow separation Recent trends show that micro-vortex generators, whose heights are of the order of boundary layer thickness, are effective devices in flow separation control These were discussed in the review paper by Lin (2002) In contrast to the passive means of flow control, blowing jets and vortex generator jets are active flow control devices for flow separation control, whereby mass addition near the separation point energizes the low momentum fluid close to the wall to overcome the adverse 69 pressure gradient Other devices like zero-net-mass-flux jets and micro-actuators were used in Sduct for separation control by Mathis et al (2008) and Hamstra et al (2000) respectively To study the effects of flow control in square cross sectioned S-duct, three different flow control methods were studied, namely, vortex generator, variable directional tangential blowing and vortex generator jets Previous flow control studies are focused primarily on the aerodynamic improvements in S-shaped duct or diffuser Less emphasis is placed on whether competing aerodynamic criteria exist in S-duct flow control, especially one with large curvature In this Chapter, this issue is studied by comparing these effects and to weigh the relative merits of vortex generators, tangential blowing and vortex generator jets on the flow in a square crosssectioned S-duct Suppression of flow separation, reduction of total pressure loss, swirl control and flow uniformity at duct exit are the four chosen criteria Quantitative variables like total pressure loss coefficient, swirl coefficient and distortion coefficient are used to ascertain their effectiveness The experimental set-up is explained in the next section 6.2 Experimental Set-Up Chapter of this thesis detailed the experimental set-up and instrumentation used in this flow control work Only a brief outline of the experiment is given here Experiments were conducted in an open loop suction wind tunnel using an interchangeable S-duct as the test section The flow velocities were Um = m/s and 15 m/s thus giving a Re, based on hydraulic diameter (D = 0.15 m), of 4.73x104 and 1.47x105 Inlet boundary layer thickness is 7.5 mm or /D = 0.05 Only Test Section is used here which has an inlet-to-outlet offset of 1.3D with curvature ratio (RC/D) of 1.667 and flow turning angle ( ) of 53.1O as shown in Fig 6.1(a) Test 70 Section is selected from the three test sections because it has the largest turning angle and will thus pose the most challenge to the various flow control devices evaluated In reference to the first turn, the vertical side walls of the duct are labeled as near-side and far-side walls A row of 36 pressure taps located at mid-height of each vertical side wall is used to measure the wall pressure distribution The reference static pressure (PS) is measured from the side wall as indicated in Fig 6.1(b) Also shown in Fig 6.1(b) is the coordinate system used in this experiment Its origin is at the center of the inlet plane of the S-duct, with the positive scoordinate pointing downstream, along the duct centerline The total pressure distribution and cross flow velocities at the S-duct exit were measured with a Pitot-static tube and a rotatable cross wire probe respectively A linear traversing mechanism translates the probes at the exit plane both horizontally (y-direction) and vertically (zdirection) and moves the probes at a spatial interval of mm Total pressure distribution on the entire exit plane was measured while for the cross flow velocity, only the lower half of the exit plane was measured using the cross wire Three type of flow control devices were investigated in this study, namely vane-type vortex generators (VG), tangential blowing, and vortex generators jets (VG jet) Chapter described in detail the set-up of these devices, their respective dimensions, angle of inclination ( vg, blow and jet) and blowing momentum coefficients (Cm , 2Q 2Q = 0.012 and or 2 U m A jet S U m Ablow S 0.045) Smoke wire flow visualization was performed on the inner wall of the first bend of the Sduct to show the presence of flow separation and the effectiveness of flow control devices on suppressing flow separation Paraffin oil was coated on three 0.25 mm diameter smoke wires that stretched across the width of the duct Smoke streaks formed when the smoke wires were heated 71 by electrical current The experiment was then repeated to observe the effects of these flow control devices on suppressing flow separation To have a quantitative measure of the aerodynamic performance of S-shaped ducts, several non-dimensional variables were used They include the total pressure loss coefficient ( ω ), swirl coefficient (SC) and distortion coefficient (DC) These definitions are also given in Seddon and Goldsmith (1999) and Sullerey et al (2002) The averaged total pressure loss coefficient ( ω ) was determined by calculating the difference in the averaged total pressure at the duct inlet and the averaged total pressure at duct outlet, divided by the dynamic pressure The swirl coefficient (SC) at the duct exit is defined as the maximum cross flow velocity magnitude normalized by the mean flow velocity (Um) The distortion coefficient (DC) is defined as the difference between averaged total pressure at the duct exit and the minimum total pressure at the duct exit divided by the dynamic pressure These non-dimensional variables are stated as, ω= PT , Ave Inlet ρU m SC = Max v DC = − PT , Ave Um Outlet (6.1) (6.2) Exit PT , Ave − PT , Min ρU m (6.3) In the literature, it is common to define the SC and DC for a certain angular sector at the exit plane for circular cross sectioned S-duct flow For example SC60 and DC60 refer to the respective coefficients for a 60O sector at the exit plane of the duct In the present investigation, it 72 is more convenient to consider the entire half plane for a square cross-sectioned S-shaped duct and this provides a better assessment of the aerodynamic performance for such ducts In the next section, the effectiveness of these flow control devices on the flow in Sshaped duct is studied The results indicate that a conflicting nature exist in which flow control devices are successful in suppressing flow separation and in reducing total pressure loss but unable to eliminate stream-wise vortices, to improve flow uniformity, and to attenuate swirling flow at the S-duct exit 6.3 Results and Discussion 6.3.1 Flow separation The effects of the three flow control devices on the side wall CP distribution are shown in Fig 6.2(a)-(c) for VG, tangential blowing and VG jet respectively at Re = 4.73x104 Fig 6.3(a)(c) show the same but at a higher Re = 1.47x105 Cm = 0.012 for all these cases that involve blowing and the CP distribution for the bare duct case which had been shown previously is also shown in each figure for comparison Without any flow control devices, the Cp distribution shows a distinct inflection point on the near-side wall indicating the presence of flow separation Next, the flow control devices are installed upstream of the separation point where the near-side Cp is a minimum This location was chosen because downstream of it the adverse pressure gradient increases and the boundary layer thickens rapidly With these devices installed, the relevant CP distribution no longer has a point of inflection as indicated in the figures Hence it can be concluded that flow separation is suppressed by all three flow control methods at both Reynolds numbers, and supporting evidences from smoke wire flow visualisation at the lower Re = 4.73x104 are shown in Fig 6.4(a)-(d) As seen in Fig 6.4, flow separation is present when no 73 flow control devices are installed (Fig 6.4(a)) and when either VGs, tangential blowing and VG jets is implemented, flow separation is suppressed as shown in Fig 6.4(b)-(d) respectively The CP measurements and flow visualizations show the expected performance of these flow control devices In the next sections, the effects of suppressing flow separation on the total pressure loss in the S-duct will be discussed 6.3.2 Total Pressure Loss Using the definition of Sullerey et al (2002), the averaged total pressure loss coefficient ( ω ) was determined by calculating the difference in the averaged total pressure at the S-duct inlet and the averaged total pressure at the S-duct outlet, divided by the dynamic pressure For each test, the inclination angle for VG’s, tangential blowing and VG jet was varied and the corresponding ω was measured The test was also repeated for two different momentum coefficient (Cm = 0.012 and 0.045) in tangential blowing and VG jet Fig 6.5 and 6.6 summarizes the variation of ω with inclination angle vg, blow, jet for VGs, tangential blowing and VG jets at Re = 4.73x104 and 1.47x105 respectively The total pressure loss coefficient for bare duct and the case for mixed VG configuration are also indicated in the figures The figures generally show that there is a reduction in ω as vg and blow increases, thus implying that the averaged flow rate in the S-duct increases when VGs and tangential blowing are used To further optimize the performance of VGs, the mixed VG configuration (as indicated in Fig 6.5 and 6.6) led to further reduction of ω For tangential blowing, increasing the blowing rate from Cm = 0.012 to Cm = 0.045 results in lower ω at all angles of tangential blowing This trend is reversed for VG jet where increased blowing led to an increase in ω as shown in both figures This is most probably due to the transverse (not tangential) blowing of VG jet into the 74 first bend of the S-duct The increased momentum that enters the S-duct transversely interacts with (or disrupts) the main flow and decreases the kinetic energy in the main flow For effective flow control using VG jets, there seems to be an optimum blowing rate where the jets roll up into stream-wise vortex pairs Flow visualization by injecting smoke created by mixing dry ice with hot water into the plenum was performed The vortical structures were illuminated with a light sheet This is shown in Fig 6.7 and in this figure, the free stream flow points out of page It shows that the VG jets roll up into counter-rotating vortex pairs at a low blowing rate This vortical configuration draws the high momentum fluid from the free stream and mixes with the low momentum fluid near the curved side wall thereby suppressing flow separation The discussion above shows that total pressure loss can be reduced (in most cases investigated) when flow control devices are present The reduction in total pressure loss is not due to the added mass injection for the blowing and VG cases The mass flow rate ratio (i.e mass flow rate of air injection to the mass flow rate in the S-duct) of the various cases can be computed for tangential blowing and VG jets at both Reynolds number For both Re, the mass flow rate ratios for tangential blowing at Cm = 0.012 and 0.045 are 0.00858 and 0.01665 respectively Also, the mass flow rate ratios for VG jets at Cm = 0.012 and 0.045 are 0.0197 and 0.0383 respectively Hence, the maximum mass flow rate ratio is about 4% with respect to the bulk flow and for this case, it was shown that VG jets at a high blowing rate "worsen" the total pressure loss in the S-duct due to the transverse injection of VG jets For other cases, the mass flow rate ratio ranges from 0.8% to about 2% The % reduction in the total pressure loss when these devices are installed exceeds these percentages In the next section, the effects flow control devices on the stream-wise vortices and swirl at the S-duct exit are discussed 75 6.3.3 Stream-wise Vortices and Exit Swirl Due to the large number of test cases and for brevity, selected results are used to highlight the salient features of the effects of flow control on the stream-wise vortices and exit swirl The bench mark case is the bare duct where no flow control was used Fig 6.8 and 6.9 show the total pressure distribution and the normalized cross flow velocity vector plot at the S-duct’s exit for the bare duct case at Re = 4.73x104 and 1.47x105 respectively The pictures are drawn such that one is viewing upstream at the S-duct’s exit and the free-stream flow direction points out of page Due to flow symmetry, only the normalized cross flow velocities for the lower half of the S-duct exit are shown The exit total pressure coefficient distributions are fairly symmetrical about the z/D = 0.0 plane There is a relatively lower total pressure region around the four surrounding walls as compared to the core region In addition, the corresponding normalized cross flow velocity plots showed 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 A distinctive swirl is thus present at the S-duct exit plane At the lower Re = 4.73x104, a vortex can be seen clearly close to the near-side wall and symmetry plane while a single vortex is noted at the higher Re = 1.47x105 near the bottom corner The formation mechanism of these stream-wise vortices was explained qualitatively in Chapter and by making reference to the Squire and Winter’s (1951) formula According to Squire and Winter (1951), stream-wise vorticity is generated in the flow through a duct bend of angle ∂u ∂z − ∂w ∂s is rotated by Since the component ∂w (in radians) when the radial vorticity ∂s is small, it was ignored in Squire and Winter’s approximation and they stated that the stream-wise vorticity for a flow after completing a bend of angle (in radians), is equal to the velocity gradient ∂u ∂z in the flow multiplied by 76 It was stated in previous Chapters that the initial formation of these stream-wise vortices was due to the initial swirl generated in the first half of the S-duct This initial swirl altered the velocity gradient ( ∂u ∂z ) close to the near-side wall close to the inflection plane of the S-duct As the flow enters the second bend, this velocity gradient results in a redistribution of stream-wise (or longitudinal) vorticity with the growth and formation of stream-wise vortices On installing flow control devices, changes in the configuration of the stream-wise vortex can be seen in Fig 6.10(a) to (c) for VG, tangential blowing and VG jets respectively for lower Re = 4.73x104 while Fig 6.11(a) to (c) show the same for higher Re = 1.47x105 They are angled at vg = blow = jet = 10O with blowing rate at Cm= 0.012 for all the cases It was shown previously that these devices suppress flow separation and enhance the mixing process Figs 6.10 and 6.11 show that this led to a thinner boundary layer at the near-side wall and a slight improvement in the total pressure distribution especially near the mid height of the near-side wall In addition, the normalized cross flow vector plots indicate that they alter the vortex configuration, position and size of the near-wall stream-wise vortices For example, the single vortex near the symmetry plane for the bare duct case (Fig 6.8) was altered to a pair of counterrotating vortices when VGs were installed (Fig 6.10(a)), to a single large vortex closer to the bottom wall of the duct when tangential blowing was used (Fig 6.10(b)) and to a single smaller vortex when VG jets were used (Fig 6.10(c)) The total pressure loss data presented in the previous section shows that the flow in the Sduct can be further optimized by adopting the “mixed” VG configuration or by blowing at a higher Cm for tangential blowing These results are shown in Fig 6.12 and 6.13 respectively for lower Re = 4.73x104 For mixed VG configuration, Fig 6.12 shows that there is a slight increase in total pressure coefficient near the z/D = 0.0 plane when compared to the corresponding plot in 77 Fig 6.10(a) where VGs of constant angle vg = 10O were used In addition, there is a slight reduction in size for counter-rotating vortex pair at the corner It would seem to indicate that the VGs placed at 20O near the corner while keeping the rest at 10O led to further mixing near the corners of the S-duct which results in a relatively higher axial flow velocity near that region as compared to VGs placed at constant 10O incidence For Fig 6.13, tangential blowing at a higher Cm = 0.045 results in further increases in total pressure coefficient near the corners of the S-duct as depicted by the general absence of contour lines near that wall and corner The stream-wise vortices are generally deflected towards the top and bottom wall of the duct The above data show conclusively that the use of flow control devices will change the configuration, size and position of the stream-wise vortices, depending on the devices’ operating parameters In addition, these devices affect the cross flow magnitude at the S-duct exit To quantify the swirl flow at these Reynolds numbers, a plot of the normalized v-velocity (horizontal velocity) at y/D = 0.0 (midway vertical plane) on the S-duct exit is shown in Fig 6.14 and 6.15 for the low and high Re cases respectively The figures show the variation of the velocity’s profile with z/D Also, the swirl coefficient (SC) is stated for each case in these figures When vg = blow = jet = 10O and the tangential blowing and VG jet has Cm = 0.012 and 0.045, the effects of using vortex generators, tangential blowing and VG jets on the swirl flow can be clearly seen in Fig 6.14 and 6.15 by comparing them to the bare duct case where these devices were not implemented With the use of flow control devices, the magnitude of the cross flow mostly increases as indicated in the figures When flow control devices are not present, the cross flow velocity is distinctively lower in magnitude especially in the region nearer to the horizontal plane of symmetry (z/D = 0.0) The installation of any flow control device therefore also increases the magnitude of SC when compared to the bare duct case 78 In the previous section, it was shown that flow control devices are effective in suppressing flow separation and reducing total pressure loss However, it is shown here that this is accompanied by an increase in swirl which is generally considered as a detrimental effect The increase in swirl magnitude is related to the absence of the inflection point in the near side wall Cp distribution which in turn increased the radial pressure difference between the side walls on the S-duct when flow control devices are present This increase in radial pressure difference can be appreciated visually in the Cp plots of Fig 6.2(a)-(c) and Fig 6.3 (a)-(c) for the respective Re cases, where the use of flow control devices results in an increased radial pressure difference which drives a stronger swirling flow in both the first and second bend of the S-duct To show this increase in radial pressure difference, the variation of Cp, defined as the difference in Cp between the far-side and near-side wall, along the S-duct’s normalized centerline distance, s/D is plotted For brevity, the variation of tangential blowing and VG jets with Cp is shown for three cases, namely VG at blow= jet vg=10 O , =10O and Cm = 0.012 and compare these to the bare duct case when these devices are absent for Re = 4.73x104 (in Figs 6.16(a)-(c)) and Re = 1.47x105(in Figs 6.17(a)-(c)) As noted in these figures, there is an increase in magnitude of Cp in the first and second bend of the S-duct when VG, tangential blowing and VG jets are implemented Since a larger radial pressure difference between the side walls drives a larger cross flow in the S-duct, this explains the increased swirl at the exit plane that was measured when flow control devices were used 6.4 Chapter Conclusion The effectiveness of VGs, tangential blowing and VG jets to control flow separation, reduce total pressure loss in a constant area, square cross sectioned S-duct were investigated The 79 three methods were shown to be effective in suppressing flow separation and reducing total pressure loss but seem to be ineffective in attenuating the swirl magnitude in the S-duct In fact, the three methods increase the swirl in the S-duct With these devices implemented, the radial pressure difference between the side walls increases, resulting in a stronger swirl in the S-duct Using total pressure and cross flow measurements, it was shown that this increase in radial pressure difference was due to the elimination of flow separation in the first bend and the changes in vortical configuration and position of stream-wise vortices on the outer wall of the second bend in the S-duct The data therefore indicates competing parameters for improving the performance of flow in S-ducts 80 ... mass injection for the blowing and VG cases The mass flow rate ratio (i.e mass flow rate of air injection to the mass flow rate in the S- duct) of the various cases can be computed for tangential... implemented, the radial pressure difference between the side walls increases, resulting in a stronger swirl in the S- duct Using total pressure and cross flow measurements, it was shown that this increase... for all the cases It was shown previously that these devices suppress flow separation and enhance the mixing process Figs 6. 10 and 6. 11 show that this led to a thinner boundary layer at the near-side