John T Solomon, Alex Wiley, Rajan Kumar, Farrukh S Alvi 2of amplitude and frequency over a large dynamic source jet. Figure 2b and 2c represents the actuatorrange. This allows their use in subsonic and model and the flow field respectively. The model issupersonic flow control applications where their fabricated in plexi glass material.properties can be adapted according to the specificapplications and flight/operational regimes. In this Source jetpaper we describe the development of a highbandwidth micro-actuator and its implementation incontrolling resonance dominated supersonicimpinging flows.2. MICROACTUATOR- INITIAL DESGIN Cavity An actuator with high amplitude excitation, whosefrequency can be easily tuned over a large bandwidth,is essential for the effective manipulation of highenergy structures of the shear or boundary layer ofhigh speed flows. To realize this goal, we have Unsteady microjets Nozzle Fig. 2b Actuator model Fig 2c Flow field dm hm Source jet The main parameters that govern the properties of the microjet array issuing from the actuator assembly are: a) the distance of cavity from the source jet hm, H=L+hm b) the length of the cylindrical cavity, L and c) the L Cavity source jet pressure `ratio, (NPR)m. The two geometric parameters are indicated in Figure 2a. In the preliminary study, we examined the effect of these parameters on the flow issuing from the microjet actuator to identify the optimal range and combination of these parameters that produce the Unsteady micro desired micro-actuator flow. This has helped us to 400µ nozzles jet array develop a preliminary design approach and scaling laws for such actuators. Figure 2c shows a representative schlieren image Fig. 2a Schematic of the actuator of the flow field associated with the micro-actuator. Large unsteadiness is seen in the source jet at certaindesigned and developed an actuator system that can combinations of geometric and flow parameters thatproduce pulsed supersonic microjets at any desired essentially force and excite the natural resonantrange of frequencies. This micro actuator produces modes of the actuator system at high amplitudes. Thehigh amplitude response by using a very simple flow field image given in Figure 2c corresponds togeometric configuration that leverages the natural (h/d)m=1.3, L/dm=3 and (NPR)m=4.8. The secondaryresonance behavior of various components of this microjets are supersonic, as evident from the shockmicro-fluidic actuator system. cells present in the jet structure. A schematic of the actuator is shown in Figure 2a. 2.1 Experimental detailsAs seen here, the micro-actuator consists of three A 1 mm nozzle (dm), connected to a compressedmain components: a) an under expanded source jet, nitrogen tank is used to generate source jet at variouswhich supplies the air into b) a cylindrical cavity flow conditions. A Kulite probe is placed close to theupon which the source jet impinges, and c) multiple secondary orifices of the actuator to measure themicro nozzles (i.e. microjet orifices) at the bottom of unsteadiness associated with the secondary microjets.the cylindrical cavity, from which the high- The unsteady pressure signals were acquired throughmomentum, unsteady microjets issue. In the present high speed National Instruments digital datadesign, the source jet was issued from a 1mm acquisition cards using LabviewTM. The transducerdiameter (dm) converging nozzle and the micro output was conditioned using a low-pass StanfordTMnozzles array at the bottom of the cavity consists of filter (cut-off frequency = 60 kHz) and sampled atfour 400 µm holes in the pattern shown in Figure 2a. 200 kHz. Standard FFT analysis was used to obtainThe cylindrical cavity has a diameter of 1.6 mm and narrowband pressure spectra. A total of 100 FFT’s oflength L, and is located at a distance hm from the 4096 samples each were averaged in order to obtain
Development and Implementation of High Frequency Pulsed Microactuators for Active Control of Supersonic 3 Impinging Jetstatistically reliable narrowband spectra. Preliminary correlation that captures it; this is discussed in thestudies were conducted for different combinations of following section. More details of the actuatorgeometric and flow parameters of microactuators characterization are available in reference [7, 8]such as L/dm, (h/d)m and (NPR)m. In the present study,L/dm is varied from 1-5, (NPR)m from 1.9 to 5.8 and(h/d)m from 1 to 2. Figure 3a and 3b show therepresentative spectra of secondary jetscorresponding to L/dm = 5. For this case, experimentswere carried out by varying (h/d)m for a fixed (NPR)m= 4.8 and by varying (NPR)m keeping a fixed (h/d)m=1.7.3. ACTUATOR CHARACTERIZATION3.1 Unsteady spectra- Effect of geometry andsource jet flow The pressure spectra shown in Figure 3a and 3bclearly show the presence of high amplitude peaksindicating the presence of highly unsteady flowissuing from the actuators. Here we see that for L/dm Fig. 3b Actuator spectra, L/dm=5, h/d)m=1.7= 5, the control knobs, (h/d)m or (NPR)m variationproduce high amplitude, unsteady microjets in the Figure 4 summarizes the effect of (NPR)m andrange of 6-11 kHz. Equally noteworthy is the trend of (h/d)m shown in Figure 3 but over a large range ofpeak frequency variation, where a very small parametric space.variation of ∆(h/d)m, by ~600µm or a variation of∆(NPR)m~1.5 , leads to a significant shift in the peakfrequency of ~5 kHz. Consequently, there issignificant potential for developing a compact,robust, pulsed, tunable actuator with high mean andunsteady properties. This design approach allows formultiple ‘control knobs’ that can be used to modifythe actuator response in real time, as dictated by theapplication. Fig. 4 Summary of actuator data7 As seen here, for a given actuator design, i.e. fixed L/dm, very small changes in the source jet distance and operating pressure allows one to sweep the output frequencies over a rather large range of ∆factuator = 5-20 kHz. However, this plot also shows a wide range of actuator frequencies can be produced for a given (h/d)m or (NPR)m, by varying L/dm. In order to better collapse the performance, in terms Fig. 3a Actuator spectra, L/dm=5, NPR=4.8 actuator dimensions, we define a new variable H which is defined by H=hm+L, where hm is the As seen in Figure 3, the data from the parametric distance of nozzle exit to the cavity entrance and L isstudy is classified into two sets, one is the data the length of the cavity, as before. This parameter Hderived from the (h/d)m variation (Figure 3a) and theother set reflect the effect of (NPR)m variation (Figure represents the length of the jet column from the3b). This grouping can then be used for micro-nozzle end to the impinging end of the cavity. The actuator frequency is non dimensionalizedunderstanding the overall behavior in terms of these using ideally expanded jet velocity of the underparameters and for deriving a more general
John T Solomon, Alex Wiley, Rajan Kumar, Farrukh S Alvi 4expanded source jet. The non dimensional frequencyis given by Fig. 5 A correlation for actuator design Fig. 6a Unsteady amplitude variation with (h/d)m Stideal = fd m / U ideal (1) As seen in Figure 7a, this entire variation occurs within an (h/d)m range of ~1 to 1.8 and is seen for all In equation (1) f is frequency of the actuator, dm is the cavity lengths examined. This suggests thesource jet diameter and Uideal is the ideally expanded existence of a region where the flow is particularlyjet velocity of the under expanded actuator source jet. unsteady and where the instabilities are amplified, i.e,The new parameter H is plotted against the non a region of instability.dimensional frequency Stideal as shown in Figure 5.Interestingly these new variables collapsed into asingle trend curve as seen in the figure. The collapsedcurve is approximated as an empirical correlation,represented by equation (2) Stideal = 0.4( H / d m ) −1.45 (2) Equation (2) can be used as a guide for designinghigh bandwidth microactuators for variousapplications that demands high bandwidth actuation.3.2 Unsteady amplitude of the actuator For an unsteady actuator system, the amplitude ofunsteadiness is equally important as its frequencyresponse. The total energy in the unsteady componentof the micro-actuator flow can be captured by the rmsof the total pressure measured, Prms, by the Kulite Fig. 6b Unsteady amplitude variation withtotal pressure probe. In the following, we describe actuator nozzle pressure ratiohow the geometric and flow parameters affect theunsteady amplitude of the micro-actuator system. While discussing the pressure spectra of the Figure 6a shows the variation of Prms with (h/d)m micro-actuator flow (Figure 3a) we have seen thefor different cavity lengths. It is observed that Prms emergence of distinct frequency tones when (h/d)m isincreases over a range of smaller values of (h/d)m and in the range ~1.3-1.8. In the preceding discussion, weit remains nearly constant and decreases at larger noted high Prms, levels in the same h/d range. It isvalues of (h/d)m. For example, for L/dm = 1, the Prms is clear that the discrete peaks in the frequency144dB at (h/d)m = 0.75 reaches nearly 168 dB at spectrum, which are indicative of significant(h/d)m=1.1 and remains nearly constant up to unsteadiness, are responsible for the high Prms. The(h/d)m=1.6 and falls down to 158 dB at higher (h/d)m conclusion is that for a fixed NPR, there exists avalues. region of instability within which the variations of
Development and Implementation of High Frequency Pulsed Microactuators for Active Control of Supersonic 5 Impinging Jet(h/d)m or H/dm give rise to high amplitude secondary around the periphery of the nozzle as shown in thejet fluctuations. Furthermore, the unstable Figure 7. These actuator modules are designed tofrequencies can be controlled by selecting the generate microjets pulsing at 4-6 kHz at various NPRappropriate (h/d)m and cavity length. The variation of values of the actuator source jet. The micro actuatorsPrms with nozzle pressure ratio (NPR) is shown in are designed for this frequency range so that they canFigure 6b. In the present experiments, at a fixed value be tuned to match the baseline frequency of the flowof L/dm and (h/d)m (corresponding to large field. In this case, the baseline spectra of theunsteadiness), the NPR is varied from 4 to 5.5. It is impinging jet have a dominant frequency componentobserved that at each L/dm, with increase in (NPR)m near 6 kHz. The design details of the actuator are(> 4.2) there is a sharp increase in OASPL, however available in .its value saturates beyond (NPR)m = 4.6 within the For the present study, a 100psia Kulite (Modelrange tested. XCE-062-100A) was flush mounted in the ground plane at the stagnation point of impingement (r/d=0)4. IMPLIMENTATION OF ACTUATOR to measure unsteady loads on the ground. A second 5psid Kulite (Model XCS-062-5D) was flush The impinging flow field to be tested for the mounted in the lift plate at r/d=2 to measure theeffectiveness of the high bandwidth actuator is unsteady loads experienced by the aircraft. For neargenerated by an ideally expanded supersonic jet, field acoustic measurements, a microphone at r/d=10issued vertically through a Mach 1.5 C-D nozzle as was mounted in plane of the nozzle exit (see Figureshown in the Figure 7. 7). All three measurements were recorded 4 Actuator modules simultaneously. integrated to the lift plate 10d CD Nozzle High bandwidth Actuator integrated Micro phone to the lift plate close to the nozzle Lift Plate exit 10d h/d Fig. 8a Impinging spectra with actuator operating at NPR=5.4 The results shown in Figure 8a and b correspond Ground plane to a nozzle-to-ground distance of h/d=4.5 where a dominant impinging tone is generated at 5.3 kHz in Ground Kulite the baseline flow. The control effects of the actuator, ` operating at (NPR)m=5.4 and 6.5, on the baseline Fig. 7 Schematic of test facility and actuator flow are shown in Figure 8a & b respectively. At integration (NPR)m= 6.5, the microjets are pulsing at 5.3 kHz. It Temperature is controlled using an inline heater is important to note that the impinging tones arewhich maintained a temperature ratio, TR=1.0 (where completely eliminated in both the cases. Also noteTR=stagnation temperature/ ambient temperature) for that a new tone is generated at ~6.7kHz along with itsall experiments. To simulate the presence of an harmonics, which is neither present in the base flowaircraft in hover, a circular plate (referred to as the lift nor with the actuator. This needs to be investigatedplate) of diameter 10d (d is C-D nozzle throat further.diameter, =25.4 mm) is flush mounted with the Although these new tones are of similarnozzle exit. Four actuator modules that can generate amplitude, they are narrower than the impinging16 pulsed microjets were integrated in the lift plate tones resulting in lower energy content. This is
John T Solomon, Alex Wiley, Rajan Kumar, Farrukh S Alvi 6reflected in overall sound pressure level (OASPL) REFERENCESreduction of ~4dB. 1. Lou, H., Alvi, F. S. and Shih, C., “Active and adaptive control of supersonic impinging jets,” AIAA Journal, Vol. 44, No. 1, 2006, pp.58-66. 2. Alvi, F. S., Shih, C., Elavarasan, R., Garg, G. and Krothapalli, A., “Control of supersonic impinging jet flows using supersonic microjets,” AIAA Journal, Vol. 41, No. 7, 2003, pp.1347-1355. 3. Ukeiley, L. Sheehan, M., Coiffet, F., Alvi, F. S., Arunajatesan, S. and Jansen, B.,“ Control of Pressure Loads in Geometrically Complex Cavities,” Journal of Aircraft, 45, No. 3., 2008, 1014-1024. Pre-print published as AIAA Paper 2007-1238. 4. Cattafesta, L. N., Williams, D., Rowely, C. and Alvi, F. S., “Review of Active ControlFig. 8b Impinging spectra with actuator operating of Flow-Induced Cavity Oscillations,” at NPR 6.4 @ 5.3 kHz Progress in Aerospace Sciences, 44, 2008, 479–502.5. CONCLUSIONS AND FUTURE WORK 5. Zhuang, N. Alvi, F. S. and Shih, “Another Look at Supersonic Cavity Flows and Their Design and development of a novel, simple and Control,” AIAA Paper 2005-2803, presentedrobust micro actuator is described in this paper. The at 11th AIAA/CEAS Aeroacousticfirst generation actuator consists of a source microjet, Conference and Exhibit, Monterey, CA,under expanded into a short, cylindrical cavity and June 2005.multiple secondary microjets emanate out of the 6. Zhuang, N. Alvi, F. S., Alkilsar, M. andcavity through multiple micro orifices. The Shih, C., “Aeroacoustic Properties ofremarkable feature of this micro-actuator is its high Supersonic Cavity Flows and Theirmomentum mean flow along with high amplitude and Control,” AIAA Journal, vol. 44, No. 9,a high bandwidth unsteady component. Based on a Sept. 2006, pp. 2118-2128.detailed parametric study and characterization, a 7. Solomon, T. J., Kumar, R. and Alvi, F. S.suitable actuator system was fabricated and its “High Bandwidth Micro-Actuators forperformance was tested in controlling the highly Active flow control” AIAA paper 2008-unsteady impinging jet flow field of a supersonic jet. 3042.The results show that the impinging tones were 8. Solomon, T. J., Kumar, R. and Alvi, F. S.completely eliminated with the activation of these “Development and characterization of highmicro-actuators, but, new peaks at a frequency bandwidth actuator” ASME paper, 2008-different from the actuation frequency and its 3042.harmonics were observed in the spectra. These need 9. Solomon, J.T., Hong, S., Wiley, A., Kumar,to be further explored. The current design actuates R., Annaswami, A.M., and Alvi, F. S. “only 30 % of the circumference of the main jet. Control of supersonic resonant flows usingActuator modules that span a larger spatial extent, high bandwidth Micro actuators” AIAA-around the entire periphery of the main jet may 2009-3742further enhance control effectiveness. Also, we are inthe process of integrating this actuator for controllingthe flow field associated with other high speedapplications.Acknowledgment The development and testing of pulsed actuatorswas primarily supported by the Florida Center forAdvanced Aero-Propulsion (FCAAP). We also wantto thank the Air-force Office of Scientific Research(AFOSR) which has consistently supported our priorwork on the control of impinging jets.