PARAMETRIC STUDY OF JET MIXING ENHANCEMENT BY VORTEX GENERATORS, TABS, AND DEFLECTOR PLATES Mark J. Carletti and Chris B. Rogers Tufts University, Mechanical Engineering Department Medford, Massachusetts 2155 David E. Parekh McDonnell Douglas Corporation Saint Louis, Missouri 63166 ABSTRACT This paper provides a comparison of three passive mixing enhancement techniques employed on an axisymmetric jet. In the first part of this paper, blockage, velocity, and average vorticity data are presented for delta tabs and half delta-wing vortex generators. It is determined that the blockage associated with a half delta-wing vortex generator (h/d=.2, angle of attack=3, and sweep angle=6 ) is approximately 1/3 of the blockage of a delta tab of equivalent projected frontal area blockage (2% of jet exit area). This accounts for the apparent dramatic effect of the delta tab on jet mixing. In a parameter analysis of half delta-wing vortex generators, we found that the centerline velocity decay is a strong function of the generator s height and angle of attack, yet not as sensitive to sweep angle variations The final technique considered in this report is the use of extremely large deflector plates to alter the trajectory and mixing characteristics of an axisymmetric jet. For deflector plates with a length of 1 jet diameter and blockage of approximately 25% of the jet exit area, the peak temperature is decreased by over 5% at a distance of 2 jet diameters. The decrease in temperature is found to be primarily a function of blockage, while the vectoring of the jet varies strongly with the shape and angle of the deflector plate. INTRODUCTION A jet flow is a continuous flow of fluid issuing from an orifice into a medium of lower speed fluid. As the jet fluid travels further away from its origin, it slows down due to the mixing with slower speed ambient fluid. This interaction between the jet and the ambient fluid forms the mixing layer, or shear layer. As the primary jet roll-up structures, or ring vortices, move downstream, they grow in size due to the entrainment of slower moving ambient fluid. The resulting jet decay is proportional to the velocity gradient across the shear layer and is a strong function of the distance downstream of the jet exit normalized by the orifice diameter. In a recent study on the near-field entrainment of round jets, Liepman and Gharib (1992) show that streamwise vorticity drastically alters the mass entrainment of a jet, and the efficiency of this vorticity in entraining fluid increases relative to that of azimuthal vorticity as the jet evolves downstream. There are numerous systems, especially in the aeronautical community, where the ability to enhance the mixing characteristics of a jet will greatly improve their performance. For example, by increasing the rate of mixing between air and fuel, the efficiency of a combustion cycle may be improved. Also, by increasing the rate of mixing with the ambient, the infrared radiance of a plume can be reduced. With regard to jet noise (Csanady, 1966 and Rogers & Parekh, 1994), alteration of the coherent structures of the jet can produce a measurable reduction in far-field noise or can change the directivity or spectral characteristics of the noise field. Research has also shown that an increase in jet mixing is essential to enhancing ejector performance (Carletti et al., 1995). In general enhanced jet mixing techniques are divided into two categories: passive and active. Passive jet mixing techniques may be permanent or deployable, but have no moving parts during operation. They range from alterations in the exit shape of the jet nozzle (Ho & Gutmark, 1987, and Longmire et al., 1991) to the implementation of tooth-like tabs (Ahuja & Brown, 1989, Bradbury & Khadem, 1975, Samimy et al., 1991, Zaman, 1993, and Zaman et al., 1994) and vortex generators (Carletti & Rogers, 1994, Rogers & Parekh, 1994 and Surks et al. 1994) at the exit of the jet. In contrast, active flow control approaches use energized actuators to dynamically manipulate flow phenomena based on open- or closed-loop algorithms. Pulsed jets (Parekh et al., 1996), piezoelectric actuators (Wiltse and Glezer, 1994), and oscillating jets (Raman and Cornelius, 1995) are among the most effective actuators for active mixing enhancement.
As a simple retrofit solution to enhance mixing and reduce noise, many studies have focused on the placement of small tabs and vortex generators at the exit of axisymmetric and rectangular nozzles. Both tabs and vortex generators introduce streamwise vorticity to entrain low speed fluid while forcing out higher speed core fluid. The main difference between the two methods is in the vortex structures which are generated. In the case of tabs (which are placed at a 9 AOA to the flow), a pair of counter-rotating vortices is produced by each tab, whereas a half delta-wing vortex generator (typically oriented at an AOA less than 3 ) produces only a single vortex. Extensive research on tab flows has been completed. An early study by Bradbury and Khadem (1975) found that two tabs placed 18 apart at the exit of an axisymmetric jet caused it to bifurcate, increasing the overall mass entrainment. Since this study several studies have reported results for variations in flow field conditions, as well as tab shape, size, number and angle. The following is a summary of published results (Ahuja & Brown, 1989, Bradbury & Khadem, 1975, Samimy et al., 1991, Zaman, 1993, and Zaman et al., 1994): Appreciably faster decay of the centerline velocity for 1,2,4, and 6 tab cases, relative to the reference case. Mixing enhancement greater for supersonic cases than subsonic cases. Heating the jet provides no significant change in the effectiveness of the tabs. Smaller tabs have less effect in distorting the jet. Tab width is more critical than tab height. A tab height greater than the boundary layer thickness is required to be effective. Tab shape (rectangular vs. triangular) has little effect on tab performance. Orientation or angle of the tab is more critical than shape; triangular tab leaning 45 downstream, referred to as a delta tab, has the greatest effect. Half delta-wing vortex generators are similarly effective in jet mixing enhancement. A summary of key conclusions from recent studies (Carletti and Rogers, 1994, Rogers and Parekh, 1994, and Surks et al. 1994) follows: Half delta-wing vortex generators allow for more flexibility in nozzle design in that they only produce a single vortex. Counter-rotating pairs may be produced by having two adjacent generators of opposing sign. Generator orientations producing co-rotating vortices show increased mixing relative to counter-rotating configurations. Improved mixing is achieved for increased generator height and angle of attack to the oncoming flow. Generators smaller than the boundary layer thickness were found to have essentially no effect on the mass entrainment of an axisymmetric jet, at low speeds (Re=5,). This research focuses on the optimization of half delta-wing vortex generators and presents a comparison of their effectiveness relative to tabs and large deflector plates. Although tabs and vortex generators are considered to be a proven technology, much work is needed in the area of optimizing generator parameters. A comparison of the velocity decay and blockage associated with tab (counter-rotating vortex pair) and half delta-wing vortex generator (single vortex) flows is presented for generators of varying size, shape, orientation and location relative to the jet s exit plane. Finally, for applications where thrust loss is not an issue, the mixing and vectoring effects of large deflector plates, tab-like devices that span across the nozzle, are examined for varying angles and spacings from the nozzle exit. EXPERIMENTAL APPARATUS The results presented in this report are a product of research conducted both at the Tufts University Fluid Mechanics Laboratory and the McDonnell Douglas Corporation (MDC) Jet Research Facility. Nozzles implementing both tabs and vortex generators were compared at each of the two facilities. Tests done at Tufts and MDC used the same 2.54 cm diameter nozzles. The generators in these nozzles are permanently brazed into place, and a different nozzle exists for each parameter variation. Additional data was collected at MDC on a 6.35 cm diameter nozzle, which allowed for the variation of generator parameters (see figure 1). This nozzle not only allows for variation in the number and location of generators, but also their size, shape, and angle of attack. As illustrated in figure 1, larger generators were rounded to mate the inner curvature of the nozzle for high angles of attack. All of the tabs and vortex generators were professionally machined to ensure generator uniformity. Tufts Univer sity Water Jet Facility Measurements taken at the water jet facility at Tufts University were acquired on the recirculating jet shown in figure 2, using a twocomponent laser velocimetry (LV) probe. The mass flow rate of the jet is controlled by a 3-hp variable speed pump and can be held steady (±.5%) from 2.5 to 7.5 m/s. The flow is conditioned as it passes through a flow straightener and a series of screens. The flow then passes through a 5th-order polynomial contraction (area ratio of 9:1) and issues out of an axisymmetric nozzle (diameter, D, equals 2.54 cm) into the measurement region (tank is 24D high x 24D wide x 72D long). The nozzles are interchangeable, and a rotating collar allows us to rotate the nozzles 36. The distance from the contraction to the nozzle exit is 5 cm. The flow exits the jet with less than.2% turbulence intensity in the core. Velocity information is collected using a TSI 2-D fiber optic LV probe, a Colorlink Multicolor Receiver, and an IFA 75 Digital Burst Correlator. The water is seeded with titanium-dioxide particles (9% less than 1 microns). The intersection of the beams creates an elliptical control volume with a diameter of approximately 2 microns and a length of about 1 mm. The 2-D probe measures the u and w components of velocity. In order to make vorticity measurements, the v-velocity components are necessary. To acquire this data, the nozzle is rotated exactly 9 and a second profile is taken, giving us the v and u velocity components. A PC computer and LabVIEW software are used to position the probe and record the velocity measurements of the fluid.
9 Generator mating curvature a pitot-static or total temperature probe, which is mounted on a three dimensional computer-controlled traverse. Temperature probe 3 3 Available generator locations +y +z +x Jet coordinate system Air plenum chamber Heater Plenum pressure transducer 6.35 cm jet exit Contraction (36:1 area) 3-D Traverse Figure 1. MDC nozzle design allowing for parameter variation. Flow Meter 6 psi Source or blower control To ensure statistical independence for each point of data taken, between 1, and 3,5 points are averaged over a period of 3-1 seconds depending on the data rate. In the center of the jet the data rate is between 7-1, Hz and for regions of zero velocity (outside the shear layer) the data rate is approximately 1 Hz. MDA Aero acoustic and Aerodynamic Jet Facility Two different nozzles were tested at MDC. The tests were run on the air jet shown in figure 3, which issues into an anechoic chamber. A 5 kg/s, 4 MPa compressed air source with an inline critical flow venturi and 3-kW electric heater provides a continuous source of ambient or heated air. For low speed flows, ranging from to 1 m/s, a dedicated centrifugal blower was used as an alternate to the shared compressed air source. Flow direction Computer-controlled traverse Figure 3. MDC air jet apparatus and probe traverse. Nozzles and Parameter Variations The following definitions of parameter variations are used for all of the nozzles discussed in this report and are consistent with most of the existing literature on vortex generator nozzles. Figure 4 outlines the geometries and parameters for the simple tab, the delta tab, and the half delta-wing vortex generator. A simple tab is a rectangle with a 2:1 aspect ratio oriented at 9 to the oncoming flow. The thickness of all generators is 1.6 mm (1/16 inch). A delta tab has the shape of a 45 right triangle which leans downstream of the nozzle at a 45 angle. The half delta-wing vortex generator is also a right triangle; however, variations in the sweep angle, angle of attack, and height are considered in this report. To compare the effect of these generators, parameters were chosen that would provide an equivalent projected area of blockage at the nozzle exit. A simple tab with h/d=.18, a delta tab with h/d=.16, and a half delta-wing vortex generator with h/d=.2, sweep angle of 6, and an angle of attack of 3, all produce a projected frontal blockage of approximately 2% (±.1%) of the nozzle exit area. Fiber probe IFA 75 Simple Tab Delta Tab Half Delta-Wing Vortex Generator ColorLink Signal processor PC - 486 h/d=2w/d w/d h/d 45 Sweep angle y h/d Figure 2. Tufts recirculating water jet apparatus with 2-D fiber optic LV probe. Temperature and pressure flow conditions are monitored in the 15- cm-diameter plenum. The air exits through a 6.35-cm-diameter, 5th-order-polynomial contraction and a 1-cm-long, constant diameter extension which holds the vortex generators. For mounting smaller 2.54-cm nozzles, the extension is replaced with an additional contraction. Measurements may be taken either with 9 t=.625" AOA Angle of attack 9 45 Figure 4. Generator parameters.
Note that the 2% projected blockages takes into account the reduction of generator base area necessary to mate with the curvature of the nozzle. These generators are the standard configurations in this study, and all results presented are for these parameters unless otherwise mentioned. The effects of the generators on the jet s mixing characteristics are strongly dependent on the number, orientation, and type of generator. An axisymmetric nozzle without generators is referred to as the reference nozzle throughout this report. This report considers nozzle configurations containing, 1, 2, 3, and 4 equally spaced generators, referred to as the reference nozzle, single generator, 18 generator, 12 generator, and 9 generator configurations. Figure 5 illustrates the net effect of various generator configurations on a jet flow. The final part of this paper examines the ability of deflector plates to mix and vector an axisymmetric jet. The deflector plates considered here are triangular (base width =.5 D) or rectangular (base width =.2 D) in shape with a length of 1 jet diameter. Figure 6 illustrates the variable parameters of angle and spacing of the deflector plate. The angle is measured relative to the jet axis, and the spacing is the distance from the exit of the jet to the base of the deflector plate. Reference 12 Symmetric 9 Symmetric MEASUREMENT TECHNIQUES AND ANALYSIS LV Velocity and Vorticity Measurements To ensure an accurate representation of the flow field, the velocity measurements taken with the 2D LV probe in water are an average of 3,5 points taken over 1 seconds. The repeatability of the velocity measurements is approximately ±.2% in the core of the jet, and better than ±2% in the low velocity regions. Grid size for 2D velocity maps was decreased until a change of less than 1% was achieved in mass flow rate calculations. For the data presented in this report, the step size is approximately 1/16th of a jet diameter for the 31x31 grids at 2 jet diameters downstream. Two dimensional velocity maps were also used to measure the vorticity. Equation 1 shows the equation for vorticity in Cartesian coordinates. ( ) j + 1 2 ω = 1 w 2 y v z i + 1 u 2 z w x v x u y k (1) As can be seen from the equation, the u-,v-, and w-components of velocity are needed to measure vorticity in all three directions. Since only a 2D probe is available, we first took a yz-cut measuring the u and w velocities, and then rotated the nozzle exactly 9 (we used precut grooves and the laser for alignment) in the counterclockwise direction and measured the u and w velocities again, now effectively yielding the u and v velocity components. To calculate vorticity, we also need to be able to determine spatial variations in all three directions, x, y, and z. To do this we took two yz-cuts at a distance dx apart (dx=dy=dz). The vorticity measurements presented are calculated using finite differences on 3,9 points, each point being an average of over 1, points. The magnitude of the averaged 3D vorticity vector is calculated by taking the square root of the sum of the squares of the individual components of vorticity calculated in equation 1. 18 Simple Tab 18 Delta Tab 9 Delta Tab Figure 5. Effect of generator configurations. Djet Side View Spacing/Djet x θ=9 Frontal View Figure 6. Deflector plate parameters. y z Pitot and Temperature Probe Measurements The centerline velocity data presented in the results and discussion section of this report was acquired at MDC using a pitotstatic pressure probe. The ambient pressure and plenum pressure are continuously monitored throughout the run to compensate for any fluctuations in flow velocity that might occur. Each point is an average of 5 samples acquired at a rate of 5 Hz. The temperature measurements were taken using an aspirated total temperature probe. This probe is similar to a pitot probe, but it has a thermocouple mounted inside the tube before the bend. Holes located downstream of the thermocouple allowed the air to continuously flow over the thermocouple. To eliminate the effects of probe hysteresis, each temperature measurement is recorded and compared to a second measurement taken in the same location. If this value is within a.2% tolerance, it is accepted and the probe moves to the next location. If it is not within this tolerance, the probe will keep taking measurements in that location (15 seconds apart) until this condition is met. Again, to compensate for any drifting in the temperature of the air exiting the jet, each point is normalized by the corresponding plenum conditions at the time it is taken.
Blockage Estimates The blockage data presented in this paper is acquired using the low speed blower fan at MDC. Since a flow meter suitable for this flow was not available at the time of the tests, the relative blockage is approximated by comparing the change in exit velocity resulting from each of the nozzle configurations for constant blower control setting, which corresponds to a constant fan speed. This approach can determine which configurations have more or less blockage than others, but cannot quantitatively specify the difference in blockage since the mass flow rate is not exactly constant with variations in nozzle configuration at a fixed blower setting. Tests were run at two different velocities, 4 m/s and 8 m/s. The flow speed is very steady, varying by no more than ±.5% during a specific run. The relative blockage of each nozzle is estimated by measuring the increase in exit velocity (calculated through monitoring the plenum pressure) relative to the reference nozzle as follows: ( ) V nozzle V reference Blockage = V reference (2) Each point of blockage data presented is an average of 5 points taken over a three minute time period. The repeatability of the runs is within the steadiness of the flow and is well within ±.5%. RESULTS AND DISCUSSION Vorticity Measurements for Tabs and VGs The basic difference between tabs and vortex generators is in the generation of streamwise vorticity. As mentioned earlier, tabs produce a counter-rotating pair of vortices, whereas a half deltawing vortex generator produces a single vortex. In order to examine the production, or reorientation of vorticity, nozzles with single generators were tested and compared to a reference nozzle (an axisymmetric nozzle with no generators). Figure 7 shows the u-velocity contours in the yz-plane at 2 jet diameters downstream of the jet exit. For all cases the velocity at each point is normalized by the maximum exit velocity of that data Reference Standard V.G. (AOA=3 ) set. As expected, the reference nozzle shows an axisymmetric velocity profile, spreading in concentric circles of uniform velocity. In the single generator cases, the generator is located to the left-hand side of the plot (the 9 o clock position of the jet), and marked by indentations into the potential core. Each generator entrains low speed ambient fluid, displacing the jet core, thus causing an increase in spread rate. To compare the relative effectiveness of the two generators, their size is scaled to provide a projected area blockage of 2%. The standard half delta-wing vortex generator (h/d=.2, AOA=3, and Sweep=6 ) has the same projected blockage as a standard delta tab (h/d=.16). It is apparent that the delta tab has more of an effect in the distortion of the shear layer. As is discussed later in this paper, the larger effect of the delta tab on the flow is associated with a flow blockage of approximately three times that of the half delta-wing vortex generator. If we increase the blockage of the vortex generator, by increasing its angle of attack from 3 to 6, we can produce a similar effect to that of the tab. Qualitatively, there is very little difference between these two profiles. Using the method described in the previous section, vorticity data for these nozzles was acquired. Figure 8 shows a plot of the magnitude of the 3-D averaged vorticity vector at 2 jet diameters downstream. The shapes of the average vorticity contours are similar to the u-velocity contours shown in figure 7. This is expected as the potential core in each case is a near zero vorticity region and vorticity is strongest in regions of high shear, where the velocity gradient is the steepest. The reference nozzle shows concentric contours of averaged vorticity, essentially marking the azimuthal vorticity. The maximum value of the streamwise vorticity component for the reference nozzle is less than 15% of the total averaged vorticity at 2 jet diameters. A single region of high vorticity is present in the case of the half delta-wing vortex generator. However, in the last two cases of the standard delta tab and the 6 vortex generator, the two contours of.8s -1 mark the centers of the pair of counter rotating vortices shed by the respective generators. The half delta-wing vortex generator appears to transition from producing a single vortex to a pair of vortices, effectively becoming more tab-like. In comparison to the reference nozzle, the core of the other cases is displaced slightly to the right, however, only the half of the jet appears to be affected by a single generator. Standard V.G. (AOA=6 ) Standard D.Tab.5.1.3.5.5.1.1.1.5.5 -.5.7.95 -.5 -.5 -.5.95.95.95 -.5.5 -.5.5 -.5.5 Figure 7. Comparison of the normalized axial velocity (U/U[x=]) at x/d=2. -.5.5
.5 Reference.4 Standard V.G. (AOA=3 ) Standard V.G. (AOA=6 ).6.8.8.5.5.5 Standard D.Tab -.5 -.5.4.6 -.5 -.5 -.5.5 -.5.5 -.5.5 -.5.5 Figure 8. Average vorticity comparison for at x/d=2. Effect of Vortex Generator Parameter Variations It is apparent that as we are able to block or redirect more of the flow, we will have a greater impact on the spreading and mixing characteristics of the jet. In most aeronautical applications, thrust loss needs to be minimal while a maximum mixing benefit is desired. The following plots show the effect of varying the size, shape and angle of attack of half delta-wing vortex generators on the centerline velocity decay. Although the centerline velocity is not directly related to mixing in all cases, for configurations that do not vector the flow considerably, it provides us with some insight as to the effect of these parameters on mixing. It is evident that the centerline velocity decay has the potential to be very misleading if the jet were to bifurcate or vector significantly. Figure 9 shows the effect of vortex generator height variation for the 9 symmetric configuration (4 generator case) from h/d=.1-.3, on the centerline velocity decay of an axisymmetric jet. The x-axis is the normalized distance downstream of the jet exit, and the y-axis is the normalized centerline Mach number (M(x)/M(x=)). The velocity of the reference nozzle (265 m/s), without generators, begins to decay at approximately 5 jet diameters, which agrees well with the results of Ho and Gutmark (1987). Although the height of the generators is varied, the angle of attack and sweep angles are held at standard values of 3 and 6, respectively. The standard generator (h/d=.2) decreases the centerline velocity by 2% in the first 6 jet diameters. The effect of the generators is directly proportional to the height, and thus the blockage produced by the generators. The streamwise vorticity generated forces high speed jet fluid out, while entraining low speed ambient fluid. For larger generators this impact is greater. The next plot, figure 1, shows another variation of generator height for the 12 symmetric, 3 generator case. For this height variation, the angle of attack (5 ) and sweep angle (6 ) were held constant. At this high angle of attack the velocity decay is greater than for the 3 AOA. The h/d=.2 generator has a 22% decrease in centerline velocity over the reference nozzle within the first 4 jet diameters, and a 5% decrease over the exit velocity at 8 jet diameters, about twice as fast as without generators. Even the tiny generators, h/d=.5, have a strong effect, decreasing the centerline velocity by 2% within the first 6.5 jet diameters. Beyond 1 jet diameters there is only a 3% difference in the centerline velocity decay attained by doubling the height of the generators from.1 to.2. At larger angles of attack the generators have a greater impact in the near field region of the jet, but the mixing benefits decrease further downstream. However, many applications involving thrust augmentation and noise reduction are mainly concerned with the near field region. Figure 9. Height variation for 9 symmetric nozzle. As the angle of attack of the half delta-wing vortex generator increases to 9, it must transition from producing a single vortex to producing a pair of counter-rotating vortices. Since the vorticity is altered, the mixing mechanism changes. Figure 11 shows the decay of the centerline velocity for the 12 symmetric nozzle, varying the angle of attack of the generators. The blockage of the generators should increase with an increase in AOA, and correspondingly, the mixing should also increase. The
diameters, the 3 and 75 cases are identical, while the 15, 45, and 6 cases also pair up. The 3 sweep angle, which has relatively low frontal blockage, has the greatest effect on the centerline velocity decay. The 75 sweep angle generator has a strong presence in the near field, but is no more effective than the 15 sweep angle in the far field. The grouping of these curves suggests that the variation of the sweep angle does not have nearly as strong an effect on the mixing as the height or angle of attack of the generator. This is similar to tab results in that the shape of the tab (triangular or rectangular) is not nearly as important its size and orientation. Figure 1. Height variation for 12 symmetric nozzle with 5 AOA. Figure 12. Sweep angle variation for 18 symmetric configuration (standard h/d and AOA). Figure 11. Angle of attack variation for 12 symmetric configuration (h/d=.1). centerline velocity decays faster with increasing angle of attack. Beyond 4, however, the centerline velocity is essentially the same (±2%). Since the size of the vortex produced by each generator increases with angle of attack, perhaps the vortices are so large that they are inhibiting each other s growth, thus limiting their impact. Another possibility is that vortex bursting has been shown to occur for large angles of attack (Kegelman & Roos, 1989). This reduction in the low pressure core would imply less effective mixing, and hence less effect on the centerline decay. Again, the centerline decay does not tell the entire story. The final parameter variation is sweep angle. Due to limitations in the number of generators manufactured for our test, the sweep angle variations are in the 18 configuration. The results for the variation of sweep angle are presented in figure 12. The trends are not as predictable as for the variation of generator height and angle of attack. Sweep angles of 3 and 75 have the quickest effect on the centerline decay. Within the first 5 jet Generator Blockage Comparison Figure 13 shows the relative increase in flow blockage due to the presence of vortex generators. Blockage data for simple tabs (h/d=.18), delta tabs (h/d=.16), and half delta-wing vortex generators (h/d=.2, AOA=3, sweep angle = 6 ) of equivalent projected frontal blockage of between 2-2.2% per generator is presented. A projected frontal blockage of 2.2% is plotted for omparison. Data is provided for two different velocities (4m/s and 8m/s), corresponding to Reynolds numbers based on jet diameter of 125, and 25, respectively. There is less than a 1% variation in the normalized increases at the two velocities. As expected, the blockage increases fairly linearly as the number of generators is increased. Notice that for both simple tabs and delta tabs the actual flow blockage exceeds the predicted projected area blockage by as much as 5%, whereas the blockage related to the half delta-wing vortex generators is approximately half the value of the projected area blockage. Physically this makes sense as the half delta-wing vortex generator is at an angle of attack to the flow, allowing it to turn the flow, similar to a turning vane, instead of blocking the flow. This blockage information must be taken into account in determining the mixing effectiveness of different generator nozzles. For approximately the same blockage, three half delta-wing vortex generators could be used instead of a single delta tab.
Figure 14 shows the variation of flow blockage for a half deltawing vortex generator of standard height and sweep angle for varying angle of attack to the oncoming flow. For the AOA cases of 2 and 3, the actual blockage is much less than the predicted blockage. For the AOA cases of 4 and 5, however, the actual blockage is well above the predicted values. For the higher angle of attack cases, the blockage is similar to that of the tab flows shown in figure 13. This suggests that between AOA of 3 and 4, or as the generators become more tab-like, there is a transition from generator-like behavior to tab-like behavior. Perhaps the vortex generator is now producing a pair of vortices instead of a single vortex. Actual Blockage (%).14.12.1.8.6.4.2. 1 2 3 4 Number of Generators Simple Tabs(Re=125,) Simple Tabs(Re=25,) Delta Tabs(Re=125,) Delta Tabs(Re=25,) Vortex Generators(Re=125,) Vortex Generators(Re=25,) Projected Frontal Blockage Figure 13. Flow blockage comparison for equivalent frontal area generators..14.12.1.8.6.4.2. 2 4 6 Generator Angle of Attack (degrees) Vortex Generators (Re=125,) Vortex Generators (Re=25,) Projected Frontal Blockage used, we could not run this variation; however, its effects can be interpolated from figure 15..5 -.5 h/d =.12 -.5.5.5 -.5 h/d =.23.1.1.95 -.5.5.95 Figure 15. Symmetric 12 configuration (3 vortex generators). Deflector Plates The deflector plate concept is an extension of previous research done with vortex generator or tab flows. Most mixing enhancement techniques involving vortex generators are greatly limited by thrust loss considerations. To minimize these losses, the size of the generators are very small, and the dynamics of vortex generation is critical to the flow conditions being considered. In applications where thrust loss is not critical, there may be no reason to limit the size of the generator. For these applications we have focused on a simple method enhancing the mixing characteristic of an axisymmetric jet. Figure 16 shows the dramatic effect of a triangular deflector plate of length = 1D, and a base width of.5d, on the mixing characteristics and trajectory of an axisymmetric jet at 8m/s (see figure 6 for nozzle configuration). The plot shows transverse variation of the normalized temperature of the jet (initially at 11 C) in the horizontal plane at 2 jet diameters. Hinged at the jet Figure 14. Flow blockage comparison for 3 H.D.W.V.G. at varying angle of attack. For half delta-wing vortex generators oriented at an angle of attack of 3 or less, their blockage is as much as three times less than a delta tab of equivalent frontal blockage. Figure 15 shows the normalized u-velocity contours for two 12 symmetric vortex generator configurations. The first case consists of three equally spaced half delta-wing vortex generators with a height of.12 jet diameters, and angle of attack of 3, and a sweep angle of 6. The second case shows a larger version, h/d=.23, of the same 12 configuration. The larger generators produce a dramatic distortion of the shear layer. An increase in mixing is evident as the size of the 95% normalized velocity contour has decreased significantly as compared to any of the other cases. In order to match the blockage of a single delta tab, the height of the three generators should be approximately.2 jet diameters. For the 2.54 cm brazed nozzles Figure 16. Triangular deflector plate angle variation (horizontal temperature profiles).
exit, the angle of the plate is varied from (reference case) to 9 (maximum). For the triangular deflector plate at 9, it is apparent that the jet is initially bifurcated producing over a 5% decrease in the normalized maximum temperature. For this case the flow is vectored by the deflector plate such that the peak temperatures shown in figure 16 are 3.15 jet diameters below the centerline at 2 jet diameters downstream, or approximately 9 downward vectoring. As expected, the effect of the deflector plate decreases as its angle is decreased. Although there is a substantial increase in mixing, there is very little lateral spreading of the jet. Figure 17 illustrates the downward vectoring produced by the triangular deflector plates. This figure shows the normalized temperature as a function of vertical position. For all angles tested a downward vectoring is present. The amount of vectoring increases with increasing angle up to 6, and is the same for 9. From flow visualization data not presented in this paper, it appears as though there is a maximum vectoring angle between 7 and 85. plates other than in vectoring angle. The 12 symmetric nozzle of vortex generators is the limiting case as the generators touch in the middle. This case produces a 38% decrease in the baseline temperature. The tab case shown is a 18 simple tab configuration. The blockage of this nozzle has not yet been calculated, but should be much less than the other nozzles considered. Its associated temperature reduction is only 28%. As mentioned earlier a fair assessment of these techniques must incorporate a normalization of the mixing by the effective blockage and thrust losses associated with the nozzle. Figure 18. Temperature reduction comparison for deflector plates, tabs and vortex generators. Figure 17. Triangular deflector plate angle variation (vertical temperature profile). The spacing of the deflector plate from the exit plane of the jet is also a crucial factor in altering the mixing and vectoring characteristics of the flow. Holding the angle fixed at 9 and simply moving the hinge point of the plate downstream.25 D decreases the mixing benefit from 56% (at spacing of D) to 28%, with an associated vectoring of 8. This is similar to results reported by Zaman (1993), showing the decrease in effectiveness of delta tabs as they were moved downstream. Variations in the shape of the deflector plate from triangular, to trapezoidal, and to rectangular showed that the temperature decrease is a strong function of flow blockage and fairly independent of shape (see figure 18). The variation of shape from triangular to rectangular allows for any vectoring angle between and 9 to be achieved. This final plot shows the comparative mixing effectiveness of the deflector plates, three large (h/d=.5) half delta-wing vortex generators, and two simple tabs (h/d=.18). As expected the large deflector plates show the most substantial decrease (56%) in temperature over the reference nozzle There is little difference between the rectangular and triangular CONCLUSIONS This research examines three similar, yet distinct, passive techniques used to enhance and control the mixing characteristics of subsonic, axisymmetric jets. This study considers mixing mechanisms of both small and large mixing enhancement devices. For applications in which effective area and thrust losses are a major consideration, tabs and half-delta-wing vortex generators are considered. For applications which are insensitive to thrust loss or blockage limitations, large deflector plates are considered. The first part of this report provides direct comparison of the blockage, average vorticity, and axial velocity decay of tabs and half delta-wing vortex generators of equivalent projected area. We found that the actual blockage of the half delta-wing vortex generator is approximately one third of the blockage associated with a delta tab of equivalent projected frontal blockage. Earlier comparisons of these techniques did not take this difference into account in examining their relative mixing effectiveness. The maximum magnitude of the average vorticity generated by both a single tab and a single half delta-wing vortex generator is found to be 2-25% higher than the average vorticity of the reference nozzle (axisymmetric jet without generators) at a distance of 2 jet diameters downstream. A parametric analysis of the half delta-wing vortex generators shows that the centerline velocity decay is highly dependent on the height and angle of attack of the generator, yet is not as sensitive to sweep angle, or shape variations.
In the second part of our study, we considered large deflector plates for applications where thrust loss considerations are minimal or where flow vectoring is desired. A deflector plate is shown to have a dramatic affect on the evolution of the jet. Large deflector plates, blocking 25% of the jet flow, use brute force to spread and deflect the jet, decreasing the maximum temperature of the jet by over 5% at 2 jet diameters. The temperature decrease due to the deflector plate is shown to be a strong function of flow blockage and fairly independent of shape. The vectoring of the jet, however, is highly dependent on shape, and is found to increase with an increase in the angle between the plate and the jet axis. ACKNOWLEDGMENTS The principal investigators of this research are appreciative to both the McDonnell Douglas Corporation and Tufts University for their joint sponsorship of this work. The authors would like to thank our MDC colleagues Val Kibens, David Smith, and Frank Spaid for their insightful comments throughout the course of this work. We would also like to thank Chip Jones and Sherri Yap for their efforts in acquiring data for this project over the past year. We are grateful to Joseph Kroutil and Michael Meers at MDC, and Vincent Miraglia and James Hoffman at Tufts University, for their contributions towards the design and running of the test facilities. Parekh, D. E., Kibens, V., Glezer, A., Wiltse, J. M., and Smith, D. M., 1996, Innovative Jet Flow Control: Mixing Enhancement Experiments, AIAA Paper 96-38. Raman, G. & Cornelius, D., 1995 Jet Mixing Control Using Excitation from Miniature Oscillating Jets, AIAA Journal, Vol. 33, No. 2, pp. 365-368. Rogers, C. B. & Parekh, D. E., 1994 Mixing Enhancement by and Noise Characteristics of Streamwise Vortices in an Air Jet, AIAA Journal, Vol. 32, No. 3, pp. 464-471. Samimy, M., Reeder, M. & Zaman, K., 1991, Supersonic Jet Mixing Enhancement by Vortex Generators, in AIAA/SAE/ASME/ASEE 27th Joint Propulsion Conference, Paper No. 91-2263. Surks, P., Rogers, C. B. & Parekh, D. E. 1994, The Effect of Enhanced Streamwise on the Mixing and Acoustic Characteristics of an Air Jet, AIAA Journal October. Wiltse, J. M., and Glezer, A., 1994, Manipulation of Free Shear Layers Using Piezoelectric Actuators, Journal of Fluid Mechanics, Vol. 249, pp. 261-285. Zaman, K. B. M. Q., 1993, Streamwise Vorticity Generation and Mixing Enhancement in Free Jets by Delta-Tabs, AIAA Shear Flow Conference, Paper No. 93-3253. Zaman, K.B.M.Q., Samimy, M.. & Reeder, M. F., 1994, Control of an Axisymmetric Jet Using Vortex Generators, Physics of Fluids, Feb., vol. 6 (2), 778-793. REFERENCES Ahuja, K. K.& Brown, W. H., 1989, Shear Flow Control by Mechanical Tabs, In AIAA 2nd Shear Flow Conference, 89-994, Tempe, Arizona. Bradbury, L. J. S. & Khadem, A. H., 1975, The Distortion of a Jet by Tabs, Journal of Fluid Mechanics, 7(part 4), 81-813. Carletti, M. & C.B. Rogers, 1994, The Effect of Streamwise Vorticity of Jet Behavior, ASME Fluids Engineering Division Summer Meeting, Lake Tahoe, Nevada, June 19-23, Turbulence Control Session. Carletti, M., Rogers C.B. & Parekh, D.E., 1995, Use of Streamwise Vorticity to Increase Mass Entrainment in a Cylindrical Ejector, AIAA Journal, Vol. 33, No. 9, 1641. Csanady, G.T., 1966, The Effect of Mean Velocity Variations on Jet Noise, Journal of Fluid Mechanics, 26, 186-197. Ho, C.-M., & Gutmark, E., 1987, Vortex Induction and Mass Entrainment in a Small-Aspect-Ratio Elliptic Jet, Journal of Fluid Mechanics, 179, 383-45. Kegelman, J. T. & Roos, F. W., 1989, Effects of Leading- Edge Shape and Vortex Burst on the Flowfield of a 7-Degree-Sweep Delta Wing, AIAA Aerospace Sciences Meeting, Reno, Nevada, Paper No. 89-86. Liepman, D. & Gharib, M., 1992, The Role of Streamwise Vorticity in the near-field entrainment of round jets, Journal of Fluid Mechanics, vol. 245, 643-668. Longmire, E. K., Eaton, J. K. & Elkins, C. J., 1991, Control of Jet Structures by Crown-Shaped Nozzle Attachments, In 29th Aerospace Sciences Meeting, Reno, Nevada. Mehta, R. & Bell, J., 1989, An Experimental Study of Forced Streamwise Vortical Structures in a Plane Mixing Layer, JIAA Stanford University.