ACTIVE FLOW CONTROL TECHNIQUE USING PIEZO-FILM ACTUATORS APPLIED TO THE SOUND GENERATION BY A CAVITY. X : location in the streamwise direction

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1 Proceedings of the 3rd ASMENSME Joint Fluids Engineering Cbnference July 18-23,1999, San Francisco, California FEDSM ACIVE FLOW CONROL ECHNIQUE USING PIEZO-FILM ACUAORS APPLIED O HE SOUND GENERAION BY A CAVIY Satoshi Kikuchi Dept. of Machine Intelligence and Syst. Eng. Graduate School of Engineering ohoku University Sendai , Japan kikuchi@fluid.mech.tohoku.ac.jp Yu Fukunishi Dept. of Machine Intelligence and Syst. Eng. Graduate School of Engineering ohoku University Sendai , Japan fushi@fluid.mech.tohoku.ac.jp Key words : Cavity Flow, Flow Control, Active control, Flow Noise ABSRAC An experimental investigation aimed at controlling cavity noise using piezo-film actuators is carried out. wo pieces of thin piezo-film actuators, attached to the plate surface side-byside in the spanwise direction at the upstream edge of the cavity, are used to introduce perturbations with different phases. he flow noise suppression can be accomplished in some cases, as a result of the active flow control. However, it is shown that for the cases where two peaks appear in the velocity fluctuation spectrum, the noise reduction is not successful. INRODUCION he flow noise generated by transportation vehicles, such as high-speed trains, is becoming an important issue. Because the flow noise increases in proportion to the sixth or eighth power of the speed of the flow, the demand to reduce the flow noise has grown with the increase in the speed of transportation vehicles. At a cavity, which can be found at places like gaps between the cars of a train, a shear layer, which starts at the upstream edge of the cavity, rolls up into vortices, hit the downstream edge of the cavity, and generate a pressure fluctuation. his pressure fluctuation affects the beginning of the shear layer forming a feedback loop. Because of this mechanism, the frequency of the roll up will be fixed and the noise from the cavity becomes peaky and large. herefore, the cavity flow have been investigated by many researchers, for example ani et al.( 196 l), Rockwell( 1977) and Howe( 1997). According to Curle (1953, the pressure fluctuation at the surface of a body is a dipole source of sound. Because pressure fluctuation accompanies velocity fluctuations and visa versa, this investigation concentrates in controlling the spatial pattern of the velocity fluctuation. Reduction of the sound generation is sought based on the following idea: If the flow can be controlled so that the velocity fluctuation inside the cavity change its sign along the spanwise direction, the pressure fluctuation at the surface of a body will also be fluctuating with its sign changing in the spanwise direction. In such a situation, the sound waves generated at the neighboring sources will cancel each other out in a far field. Piezo-film actuators are used for the flow control. he piezo-film piece changes its shape when the voltage is applied. he advantage of using the piezo-film for an actuator is, because it is thin enough, it can be simply glued to the surface, and there is no need to machine the plate itself. he piezo-film doesn t generate a large noise when activated. wo pieces of piezo-film are positioned at the upstream edge of the cavity, which is a receptivity-sensitive location. his is necessary, because the amplitude of the piezo-film vibration is very small (about 2 urn). NOMENCLAURE : forcing frequency ; : velocity fluctuation frequency A. : sound frequency H, : cavity depth -L : cavity width St, : Strouhal number (=fflc / Uo) UO : freestream velocity X : location in the streamwise direction Y : location in the direction normal to the wall z : location in the spanwise direction 1 Copyright 0 #### by ASME

2 Y.I : : L, = 40mm +... L, = 50mm p L--- L, = 6Omm Figure 1 Experimental setup 1~'""'""'""'~ (a) cross sectional view ;P l 0 edge (b) top view Figure 2 Device used for flow control EXPERIMENAL SEUP he experimental apparatus is shown in Fig. 1. he flat plate with a cavity is used. he cavity is located 300mm downstream from the leading edge. he cavity depth (Hc) is 5Smm, and the cavity width (Lc) is 4Omm, 50mm or mm. he flow-controlling device using the piezo-film actuators is shown in Fig. 2. wo pieces of the piezo-film actuator are glued to surface at the upstream edge of the cavity. he thickness of each piezo film piece is 110 p and each piece is of a rectangular shape ( 50mm x mm ), with the longer side parallel to the cavity edge. Piezo film changes its shape when an electric voltage is applied. he wiring is done individually to each piece, so that each can be controlled independently. Figure 3 Variation of Strouhal number with freestream velocity wo modes of piezo-film manipulation are compared. One is when the two pieces vibrate at the same phase, and the other is when the two pieces vibrate 180 degrees out of phase. In this paper, the two modes will be called the uniform-phase-mode and the alternate-phase-mode, respectively. A single hot-wire probe, on a three-dimensional traversing mechanism controlled by a computer, is used for the velocity fluctuation measurements. A microphone, which is set up above the cavity, is used for the sound measurements. RESULS How the Strouhal nmber change with freestream velocity and cavity wim he velocity fluctuation is measured at locations downstream of the centers of the piezo-film pieces (x/l, = 0.5, y/he = 0.27, z/s,, = 0.25). he velocity fluctuation signal is analyzed by FF spectrum analysis. Figure 3 shows how the peak frequency varies with the freestream velocity, for the three cases where the cavity widths are 4Omm, 50mm or 6Omm. he peak frequencies are normalized into Strouhal numbers. In the figure, some discrete jumps in the Strouhal number can be observed. he Strouhal number of the data shown can be divided into 4 groups, St, = 1.2, 1.7, 2.2 and 2.7. he difference between each group is 0.5. aking into account the fact that the general traveling speed of vortices in a cavity is half of the freestream velocity, the difference of 0.5 in the Strouhal number is equivalent to the difference of one more vortex in the cavity. So the jump in the Strouhal number can be regarded as a change in the number of vortices, which is an 2 Copyright 0 #### by ASME

3 LI I I I I, 1 (a) L, = 40 mm l-1 I I I I, 1s s.$ WI _ (b) L, = SO mm s I I I I, IS s 301, I I I 40 -(c) L, =mm I I J IS s...a... Uniform-phase-mode ---a--- Alternate-phase-mode -+- No Control -- Q -- Background noise Figure 4 Differences in sound-peak levels by the manipulation methods integer. For example, for the case of freestream velocity of 2Om/s and the cavity width of mm, the Strauhal numbers 1.7, 2.2 and 2.7 correspond the vortex counts of 5, 6 and 7, respectively. he change in the flow noise by the control Figure 4 shows how the cavity flow-noise changes when the flow control is applied. he forcing frequency is chosen as the frequency at which the velocity fluctuation introduced into the flow becomes maximum using the same-amplitude actuating Figure 5 Power spectra of the velocity fluctuations (U, = 25mls) signal. he result of the uniform-phase-mode control, the alternate-phase-mode control, the case without forcing and the background noise are compared. In the uniform-phase-mode, the peak of the flow noise becomes higher in all cases. While, in the altemate-phasemode, for the cavity width of L, = 4Omm, a decrease in the flow noise can be found when the freestream velocity is at 20m/s or 22.5m/s, as shown in Fig. 4(a). But at ljo =25m/s, the flow noise increases in spite of the alternate-phase-mode forcing. When the cavity width is L,=SOmm, it can be found that suppression of the flow noise is accomplished for all flow velocities, as shown in Fig. 4(b). When the cavity width is L, =6Omm, the maximum reduction of 5.1 db, which is the largest noise reduction observed in this research, is achieved at the freestream velocity of Ua =22.5m/s, as shown in Fig.4 (c). But at lower freestream velocities, mainly because little noise has 3 Copyright 0 #### by ASME

4 80 2 lx0 z 80 :: & if2 3 g 40 I I I I,. *. *,.. (a) uniform-phase-mode I I I I I I I I & WI I I I I, I I I I, I I (b) alternate-phase-mode [Hz fs WI Figure 6 he effect of the forcing modes on the power spectrum of velocity fluctuation (L,=GOmm, x/l,=o.33, y/h,=o.27, ~/S,,~=0.25, f,=525hz) been generated at the cavity from the beginning, little difference in the noise level can be found. It should be noted that, for the freestream velocity of 25m/s, the flow noise reduction is not successful. he conditions where the noise reduction cannot not be achieved For the case of U, =25mJs with cavity widths of L, =40mm and 6Omm, it was obvious that our method was not working. So these cases are investigated in detail. Figure 5 compares the power spectra of the velocity fluctuations at U, =25m/s for the cavity widths of 4Omm, 50mm and mm. It can be found that 10 wtnout torcmg torcmg wnhout torcmg forcmg forcing 870Hz 119SHz forcing 870Hz 119SHz 870Hz component 1195Hz corn ponent I without forcing El uniform-phase-mode m alternate-phase-mode Figure 7. he effect of the forcing methods on the peak-sound level in each case where noise suppression has been unsuccessful ( L, =40mm and 6Omm), two peaks can be found in the spectrum. hough, in the successful case of L, =5Omm, only one peak is found. Next, the effect of the control at these two-peak-conditions is examined. Figure 6 shows how the power spectrum of velocity fluctuation changes in conjunction with the control modes for the two peak (525Hz and 630Hz) case. he controlling frequency is at the lower peak frequency of 525Hz. From the figure, it can be found that 630Hz peak disappears and only the peak with the frequency of the controlling signal survives. his result indicates that the energy has been concentrated to the controlling frequency as a result of the flow manipulation. Figure 7 displays how the heights of the two sound frequency peaks (870Hz and 1195Hz) change as a result of the flow control. From the figure, it is clear that the sound energy, which has been shared by two peaks, is concentrated to the driving frequency of the piezo-actuators. At the same time, the uniform-phase-mode control strengthens the peak sound while the alternate-phase-mode control weakens it. So, the behavior of the sound peaks, shown in Fig. 7, can be understood, as a result of a combination of these two effects. he relationship between the flow noise and the flow field In order to clear out the relationship between the flow noise and the flow field, the ensemble-averaged velocity fluctuation 4 Copyright 0 ####I by ASME

5 0.61 t( 0:O (a) Lc =50mm, U, =22.5m/s, f, =820Hz patterns of the alternate-phase-mode control case are drawn in the x-z plane contour maps and are shown in Fig. 8. Figures 8(a) and (b) show the cases where the flow noise suppression is successful, while Figs. 8(c) and (d) are the cases where the flow noises are not reduced. In the successful cases, flow fields with opposite signed velocity fluctuations, or 180 degrees out of phase, can be found residing side by side in the spanwise direction. On the other hand, in the unsuccessful cases, the velocity fields become two-dimensional in the spanwise direction. his result clearly shows the significance of the velocity fluctuation phase control to obtain reduction of noise generation at the cavity. 0:O 0:2 0:4 0: (b) LC =mm, U, =22.5m/s, f, =870Hz 0:o 0: :6 018 I!0 (c) LC =40mm, U, =15.0m/s, f, -465Hz 0:o 0:2 0: (d) LC =40mm, U, =17.5m/s, f, =575Hz CONCLUSIONS Experimental investigation has shown that the suppression of the flow noise from a cavity could be achieved by piezo-flhn actuators manipulated 180 degrees out of phase. he maximum flow noise reduction achieved in the experiment was 5.ldB and in that case, the flow noise was suppressed nearly to the level of the background noise. It was found that noise reduction was not successful for the cases where two peaks appeared in the velocity fluctuation spectrum. he velocity fluctuation field patterns of successfully controlled cases and not were compared. It was shown that our idea of achieving noise reduction by controlling the velocity fluctuation phase was not mistaken. REFERENCES Curle, N.,. 1955, he Influence of Solid Boundaries upon Aerodynamic Sound, Proceedings of the Royal Society of London, A231, p-p Curie, N.,. 1953, he Mechanics of Edge one, Proceedings of the Royal Society of London, A216, p-p Howe, M. S., 1997, Edge, Cavity and Aperture ones at Very Low Mach Numbers, Journal of Fluid Mechanics, Vol. 330, p-p Rockwell, D., 1977, Prediction of Oscillation Frequencies for Unstable Flow Past Cavities, Journal of Fluids Engineering, ransactions of the ASME, Series I, Vo1.99, p-p ani, I., et al., 1961, Experimental Investigation of Flow Separation Associated with a Step or a Groove, Aeronautical Research institute, University of okyo Report, N0.364, p-p Contour interval 2.0% Figure 8 he contour maps of ensemble averaged velocity fluctuation (x-z plane, alternate-phase-ode forcing) Copyright 0 #### by ASME

INFLUENCE OF CAPTIVE STORES ON THE UNSTEADY PRESSURE DISTRIBUTION WITHIN A RECTANGULAR CAVITY

25 TH INTERNATIONAL CONGRESS OF THE AEROSPACE SCIENCES INFLUENCE OF CAPTIVE STORES ON THE UNSTEADY PRESSURE DISTRIBUTION WITHIN A RECTANGULAR CAVITY D.M. Orchard, B.H.K. Lee and F.C. Tang Aerodynamics