49 th International Symposium of Applied Aerodynamics Lille, March Noise signature of a MAV rotor in hover

Transcription

1 49 th International Symposium of Applied Aerodynamics Lille, March 214 Noise signature of a MAV rotor in hover FPnum-214-pagliaroli 49th APPLIED AERODYNAMICS SYMPOSIUM Lille, France, MARCH 214 Tiziano PAGLIAROLI (1), Jean-Marc MOSCHETTA (1), Emmanuel BENARD (1), Cyril NANA (1) (1) Université de Toulouse, ISAE, 1 av. Édouard Belin - BP Toulouse Cedex 4, France Corresponding cyril.nana@isae.fr Abstract: Unmanned Aerial Vehicles are often used as tactical surveillance tools, or for reconnaissance purposes, and in both roles the acoustic furtivity is an essential feature for the success of the mission. Micro Aerial Vehicles (MAVs) are also deployed in civilian roles and are flown in close proximity to populated areas. This leads to mounting pressure on reducing the noise impact of those surveys. Within the community of MAVs, tilt-body configurations have been developed in order to offer a wide range of services as they offer great versatility in the field of urban reconnaissance through the quick switching between horizontal and vertical flights. Tilt-body MAV can fly in both of hover and forward flight. In hover, the inflow velocity is small and the proprotor must provide high thrust to support aircraft weight. By contrast, in forward flight, the inflow velocity is relatively large and the low thrust is just to overcome the drag. The difference in the inflow and thrust requirement between flight modes suggests different blade pitch angle in order to optimize overall flight performance. Moreover as most of MAVs are equipped with electric motors, the main acoustic source is the propeller [1]. As a matter of fact the rotor efficiency maximization and its acoustic signature minimization are contradictory goals [2]. As a result most of this investigation is focused on identifying the best compromise between these goals. The effect of the pitch angle on aeroacoustic signature for small rotor not being documented in literature, the presented work is targeted at providing experimental data on a low Reynolds number rotor in hover mode. The investigations are made on a two bladed rotor without shroud, installed inside an anechoic room. The rotor is instrumented with an embedded balance to measure thrust and torque. The database is completed by near-field measurements made with a single microphone on a moving support. The chord-based Reynolds number ranges from 2, to 5, and the tip Mach number is about,25. In this work is shown that this signature is dominated by broadband noise, despite the tonal component is not negligible. The wall pressure measurements show that the pressure signature is dominated by the broadband component probably generated by separation bubble, showing that the more commonly study of the tonal rotor noise, in the case of small rotor in hovering must be extend to the more relevant broadband. Further a simple approach to reduce the number of variable in optimization problem is herein proposed and discussed. : 1. INTRODUCTION Nowadays mini Unmanned Aerial Vehicles are used for reconnaissance and surveillance missions. These vehicles are applied as tactical surveillance tools, used by soldiers for reconnaissance purposes. muavs are also beginning to see use in a large and ever increasing variety of roles that are in close proximity to populated areas. In the last years the tilt-body muavs have been developed in order to offer a wide range of services. The tilt concept of typical aircraft attracts interest of researchers who are working on muavs. In 28, Shkarayev and Moschetta introduced the efforts on the aerodynamic design of a tiltbody muav named minivertigo, which had a tilt-body configuration [3]. Tilt-body muav can fly in both of hover and forward flight. In hover, the inflow velocity is small and the proprotor must provide high thrust to support aircraft weight. By contrast, in forward flight, the inflow velocity is relatively large and the low thrust is just to overcome the drag [4]. The difference in

2 the inflow and thrust requirement between the two flight modes suggests different blade pitch angle, instead muavs are usually equipped with fixed pitch propellers of small diameters [1,2]. So the research activity herein proposed is focused on a small rotor using a collective pitch. Most of muav are also equipped with electric motors that contribute to simplify operations and significantly reduce their noise signature [2]. This aspect is of primary importance because the muavs acoustic signature has a significant effect on their detection. Since the main acoustic source of electric motor-equipped muavs is the propeller, there is a renewed interest in reducing the noise produced by small rotor [2,5,6,7]. The reduction of the propeller s noise requires special attention in the design process, determining the performance and the efficiency of the propulsion system. As a matter of fact the rotor efficiency maximization and its acoustic signature minimization are contradictory goals. As a result most of this investigation is focused on identifying the best compromise between these goals. For example Gur and Rosen [1] proposed as cost function the SPL of the propeller or the power extracted from the battery, separately, indeed the cost function herein proposed takes into account both the efficiency of the rotor and its acoustic performance. In the past some authors provided an aeroacoustic study of the rotor like Succi s [8] or Miller and Sullivan s [9] work for reducing the acoustic signature maintaining propeller s efficiency constant. More recently Leslie introduced a method to reduce the propeller s broadband noise component by tripping the laminar boundary layer in order to accelerate its transition from laminar to turbulent boundary layer [7,1]. Sinibaldi and Marino provided some experimental tests on the traditional and acoustic optimized propeller, keeping the thrust constant [12,5]. To our knowledge, the effect of the pitch angle on aeroacoustic signature for small rotor, suitable for muav propulsion, is not available in literature. In fact, commonly little attention is given to the noise of a small propeller running at low values of the advance ratio, which correspond to poor efficiency values. In particular at fixed point, where a sizeable part of the disk is stalled and the effect of the recirculation bubble on the aerodynamic and acoustic fields is significant. For these reasons the present experimental investigation has been organized as follows: a description of the blade geometry, test bench and instrumentation is given in section 2, while the results provided are discussed in section EXPERIMENTAL SETUP AND RESULTS The blade radius is equal to.124 m and the maximum thickness of the blade profile is 2 mm and the twist angle is equal to zero. The blade profile is constant in spanwise direction and demonstrating satisfactory matching with NACA1234. The blades are mounted on a collective pitch to vary the pitch angle from to 21 Deg. A picture of this system is shown in Figure 1. The rotational regime of the rotor is varied from 28 to 7 rpm. All aeroacoustic tests are provided in an anechoic chamber with the size of 4x4.5x8 m and a low frequency cut off equal to 9 Hz, such chamber is located at Office National d'études et de Recherches Aérospatiales (ONERA). A sketch of the layout of the experimental setup can be seen in Figure 2. Figure 1.. Reference system is superimposed on rotor and the microphone location is sketched. Figure 2. Render of the aeroacoustic test bench designed to provide the rotor signature measurement. The acoustic measurement equipment consists in a Brüel and Kjær (B & K) ½ microphone mounted on an aluminum rotating support. The microphone is calibrated using a B & K 423 pistonphone generating a sound pressure level of 94 db at 1 Hz. This microphone is located at some different distance to the rotor and can be rotated using Newport ESP 31 controller in order to obtain the directivity pattern automatically. For the

3 Figure of Merit sake of brevity, only the measurement of the noise generated at the rotor tip are herein considered (y/r =1.1, x/r =.5 see Figure 1). The microphone is connected to a B & K Nexus 269 signal conditioner allowing for the amplification and anti-aliasing filtering of the signals at 1 khz. The sampling frequency of acoustic instruments is 8 KHz using the NI 9234 digital card to acquire the signals. A resolution equal to 1 Hz is adopted to guarantee a sharp definition of the acoustic discrete tones radiated by rotor. The rotor is driven by an electric brushless motor model AXI 288/24. The rotational frequency of the motor is measured by means of an optical photodiode computing its frequency response. The thrust and torque are measured with an aerodynamic balance. 3. RESULTS AND DISCUSSION The thrust, the torque and the pressure fluctuations generated at the blade tip were measured depending on the pitch and the rotational regime for a rotor with collective pitch system. The test matrix is composed of over 5 measurement conditions gathered in Table 1. Table 1. Values assumed rotational regime ( and pitch angle (. (rpm) Deg) The values of rotational regime, pitch angle and thrust, corresponding to hovering condition, are gathered in Table 2. Despite in the following we refer to 5 fixed point flight conditions, it is noticeable that the thrust values are weakly different, this aspect is due to the accuracy of the digital speed controller of the rotational regime. Commonly rotor efficiency is represented in term of Figure of Merit (FM ) which is equivalent to static thrust efficiency and defined as the ratio of ideal power required to hover and the actual power required [11]: FM C 3/ 2 T. (1) 2C p Table 2. Values assumed by pitch angle, thrust and thrust coefficient for 5 different rotational regimes corresponding to fixed point flight condition. Conf. rpm) (Deg) T (cn) C T ,E ,E ,9E ,9E ,3E-3 As noticeable in Figure 3, the most efficient configuration is related to C T * equal to that correspond to a pitch angle of 11 Deg. The maximum value of FM achieved is relatively low; this aspect is due to the high value of the AOA on the blade tip sections. As a matter of fact at fixed point flight, a sizable part of the disk is stalled [5]. Usually the acoustic performance of the rotor is evaluated in term of sound pressure level (SPL). The SPL depends on the blade radius and thickness, the number of blades and the rotational regime. Some preliminary considerations must be given in order to reduce the number of variables and to define an acoustic performance coefficient. Generally two components on the rotor noise are distinguished: tonal and broadband noise component. According to Marino [12] the pressure fluctuations related to tonal noise can be normalized defining a pressure reference as follows: p ref R. (2) =474, rpm =51, rpm =564, rpm =66, rpm =654, rpm Thrust Coefficient Figure 3. Figure of merit realized for the 5 rotational regimes investigated. This pressure leads a satisfactory collapse of the noise signature [12]. Furthermore the aeroacoustic performance can be defined through the computation of a non-dimensional coefficient herein proposed:

4 Aeroacoustic Performance 2 AP C T p ref / p, (3) where σ p represents the standard deviation of the pressure signal. The aeroacoustic performance coefficient (AP) is written as the ratio between the squared thrust coefficient and an overall amplitude pressure coefficient. The AP calculated for several rotational speeds and pitch angles is shown in Figure 4. As expected the coefficient distribution is invariant with rotational speed, whereas it depends considerably on the pitch angle, in analogy with the figure of merit Thrust Coefficient Figure 4. Aeroacoustic performance coefficient distribution provided by varying the rotational regime of the rotor. So this coefficient can be considered as counterpart of the FM in order to characterize the acoustic performance of the rotor. A comparison between the FM and the AP highlights that the maximum thrust efficiency and the low acoustic amplitude are achieved for C * T and C T equal to and respectively. According to the literature, thrust and acoustic optimization of the rotors are contradictory goals and subjected to optimization strategy. To found a compromise between these goals an objective function is proposed in the following form: F FM, AP a FM CT b APC T (4) Finally the constrained optimization problem, in fixed point flight can be written: MAX f ( CT ), s. t. T( CT, ) W a b 1. / 2, =474, rpm =51, rpm =564, rpm =66, rpm =654, rpm (5) Solving the system in Eq.(5), the compromise between rotor efficiency and acoustic performance is carried out for C T * equals to In order to clarify the physical meaning of the results achieved by means of the proposed optimization procedure, a preliminary discussion of the statistical property of the pressure fluctuations is given. In Figure 5 an example of the time history of a signal acquired to the blade tip is shown. As can be observed the broadband noise component embedded in the signal is dominant, while the tonal component is barely distinguishable. Physically, we may suppose that since in hovering, a sizeable part of the disk is stalled, the occurring recirculation bubble is a significant broadband noise source [13,14]. Furthermore in Figure 6 examples of the PDFs of the signature are reported in log-linear scale and reduced form, in order to have zero mean and unitary standard deviation. PDFs right tail is quite larger than the Gaussian distribution (included as reference in Figure 6). Despite the lack of local aerodynamic data, the present behavior can suggest a large scale separation region. Pagliaroli observes a similar intermittent behavior in the pressure fluctuations generated in close proximity of a recirculation bubble subjected to flapping and wandering dynamic [15]. p/p ref 5 x Rotor Revolution Figure 5. Example of the pressure fluctuations time history normalized with respect to pressure reference and referred to a rotor revolution time window. In Figure 7a) the sound pressure level computed for the 5 configurations under investigation is displayed. The minimum SPL measured is related to configuration 3 at which corresponds a C T equal , in agreement with the value that maximizes the AP. So the classical approach to calculate the best acoustic performance is in agreement with the novel approach proposed. To explain the physical meaning of this result, namely the reduction of the SPL in configuration 3, a comparison between configuration 1 and 3 is made. At case 1 corresponds the worst

5 S pp, Pa 2 s S pp, Pa 2 s PDF Sound Pressure Level, db acoustic performance, whereas for the case 3 a minimum in the generated SPL is achieved. the flat plate. A depth analysis of this aspect will be provided in the near future =15 Deg =2 Deg Gaussian 8 =2 Deg 1-4 =15 Deg (p-<p>)/ p Figure 6. Semi-log plot of probability density functions of the pressure fluctuation normalized with respect to the standard deviation (σ p ), dashed line represents a reference Gaussian curve having zero mean and unitary standard deviation Rotational speed, rpm a) =564, rpm =15 Deg =474, rpm =2 Deg The spectra are yield in linear-linear scale to highlight the tonal noise and log-linear scale to analyze the broadband component. The frequency axis is represented in term of harmonics of the blade passing frequency (HarmBPF ) that are defined as: HarmBPF 2f /( B). (6) With reference to Figure 7b) the minimum SPL, generated by configuration 3, is not ascribed to the tonal component of the signal, as a matter of fact the tonal component increases with the rotational speed, but is related to broadband noise decreasing that is more relevant ( see Figure 7c) ). As a consequence, to optimize a small rotor in hovering, from an aeroacoustic point of view, means to take into account the balance of these two components. It is interesting to highlight that the spectrum reported in Figure 7c), referred to as =2 Deg, exhibits two slopes, in the high frequency region, typical of the wall pressure fluctuations in the turbulent boundary layer (TBL). However it is acquired in close proximity of the tip. In fact the first slope 1 is observed in the overlap region, corresponding to Bradshaw s inactive motion [16] and is attributed to activity of large eddies in the loglayer of the TBL, instead the second one 5 is considered by Blake as typical of the innerlayer turbulence [17], i.e. very close to the wall. Thus, the present tip-pressure spectra are quite similar to several canonical flows of Harmonics of the BPF b) Harmonics of the BPF c) Figure 7. Sound pressure level produced by the 5 different rotational regime studied a); comparison between power spectral density computed for β=15 and 2 Deg b) and c). 4. CONCLUSIONS =564, rpm =15 Deg =474, rpm =2 Deg -1 The aeroacoustic behavior of a rotor by varying the pitch angle has been investigated through an experimental analysis carried out using microphones and balance measurements. The optimization strategy -5

6 herein proposed appears interesting in order to reduce de number of variables in the multiphysics problem as rotor optimization; in particular the independency of the acoustic performance coefficient to rotational regime seems useful for this kind of optimization. The classical analysis of the rotor noise computing the SPL leads to the same conclusion of the optimization approach proposed, confirming the validity of the strategy. The wall pressure measurements show that the pressure signature is dominated by the broadband component probably generated by the separation bubble, showing that the more commonly study of the tonal rotor noise, in the case of small rotor in hovering must be extended to the more relevant broadband. Pressure measurements introducing a number of blades as additional parameter will be conducted by the authors in the near future to clarify the role of the blade number in the aeroacoustic field for small scale rotor. 5. REFERENCES 1. Gur, O., Rosen, A., Design of a quiet propeller for an electric mini unmanned air vehicle, J. of propulsion and power, 25(3), , Gur, O., Rosen, A., Optimizing Electric Propulsion Systems for Unmanned Aerial Vehicles, J. of Aircraft, 46(4), , Shkarayev, S., Moschetta, J.-M., Bataille, B., Aerodynamic design of micro air vehicles for vertical flight, J. of Aircraft, 45, , Lv, P., Prothin, S., Mohd-Zawawi, F., Benard, E., Morlier, J., Moschetta, J.-M., Study of a flexible blade for optimized proprotor, ERCOFTAC, Mykonos, Greece, June 17-21, Sinibaldi, G., Marino, L., Experimental analysis on the noise of propellers for small UAV, App. Acoustics, 74, 79 88, Succi, G. P., Design of the quiet efficient propellers, SAE Business Aircraft Meeting, Soc. of Automotive Engineers Paper, SAE79-584, Miller, J. C., Sullivan P. J., Noise constrains effecting optimal propeller designs, SAE General Aviation Aircraft Meeting and Exposition, Wichita, KS, Soc. of Automotive Engineers, Paper , Leslie, A., Wong, K. C., Auld, D., Experimental analysis of the radiated noise from small propeller, Int. Cong. on Acoustics, ICA, Sydney, Australia, Leishman, J. G., Principles of Helicopter Aerodynamics, 2nd ed., Cambridge, New York, 2, Chap Marino, L., Experimental analysis of UAV-propellers noise, 16th AIAA/CEAS, Stockholm, Glegg, S., Baxter, S.M, Glendinning A., The prediction of broadband noise from wind turbines, J. Sound Vib., 118(2), , Brooks, TF, Schlinker, RH., Progress in rotor broadband noise research, Vertica, 7, , Pagliaroli T., Aeroacoustic, acoustic and fluid dynamic characterization of rectangular partial enclosures, Ph.D. Dissertation, Mech and Indust. Dept., Rome Univ., Rome, Bradshaw, P., Inactive motion and pressure fluctuations in turbulent boundary layer, J. Fluid. Mech., 3, , Blake, W. K., Mechanics of Flow- Induced Sound and Vibration, App. Math. and Mechanics, Volume 17-I and Volume 17-II, JanakiRam, D. S., Scruggs, B. W., Noise and detectability characteristics of small-scale remotely piloted vehicle propellers, J. of Aircraft, 19, Leslie, A., Wong, K. C., Auld, D., Broadband noise reduction from a min-uav propeller through boundary layer tripping, Acoustics and Sustainability, Geelong, Victoria, Australia, 28.