Numerical study on reduction of aerodynamic noise around an airfoil with biomimetic structures
Introduction
Flow control as a scientific discipline and a technological curiosity is perhaps more intensely pursued by scientists and engineers than any other area in fluid mechanics [1]. Fundamental principles are related to controlling structures of boundary layers or wakes to achieve transition delay, separation postponement, lift increase, skin-friction and pressure drag reduction, turbulence augmentation, heat transfer enhancement, or noise suppression. Flow control involves passive or active devices that exert a beneficial change on the flow field. Passive flow control may be easily applied to engineering as flow is modified without external energy expenditure. A considerable amount of related research has then been performed to modify the structures of the boundary layer and wake flow. Bionics promotes the development of flow control methods and techniques [2]. Biomimetic flow control has recently received increased focus as it imitates biological surfaces and special structures of living organisms to improve the effectiveness of flow control. Anders [3] and Choi [4] provided exhaustive reviews of early and recent biomimetic flow control research, respectively, presenting several examples of successful biomimetic flow controls.
Biomimetic flow control includes biomimetic boundary layer control and biomimetic wake flow control. Boundary layer control utilizing riblets with different cross-sections has received considerable attention after Walsh reported drag reduction benefits through the use of riblets [5], [6], [7]. Researchers were intrigued with the large variety of surface structures on the skin of fast sharks [8], [9], [10]. These are naturally optimised riblet geometries that achieve significant drag reduction performance improvements. The optimised riblets, which act passively, can reduce drag by up to 10% in turbulent flow [11], [12]. Riblet surfaces also act to reduce the boundary layer noise [13], [14]. An alternative approach of passive flow control, widely utilized to reduce form-drag [15] and self-noise, is wake control. Shapes and structures of living organisms have shown adaptability to the flow fields around them, inspiring novel structures of vortex generators. Tubercles on the leading edges of the humpback whale pectoral flippers, for example, inspired Fish and Battle [16] to propose a novel hydrodynamic design. The numerical study performed by Watts and Fish [17] confirmed that leading edge tubercles can reduce the lift of a wing by 4.8%, the drag by 10.9%, and increase the lift to drag ratio by 17.6%. Force measurements at a Reynolds number of 220,000, performed by Bolzon et al. [18], indicate that for angles of attack ranging from 1° to 8°, tubercles reduce lift and drag by 4–6% and by 7–9.5%, respectively, resulting in a 2–6% increase in the lift-to-drag ratio and a 3% increase in the maximum lift-to-drag ratio. Recent work [19], [20], [21] also confirms the morphological features of marine mammals for flow control may be utilized in the biomimetic design of engineered structures for increased power production and increased efficiency. A typical example of biomimetic passive wake control is trailing edge serrations inspired by feathered structures. Gruschka et al. [22] conducted a series of tests at the University of Tennessee Space Institute to determine the level and characteristics of noise produced by owls during flight, especially during gliding approaches. Bachmann et al. [23], [24] compared barn owl feathers to pigeon feathers, deducing that the adaption of barn owl wings to silent flight is due to the wing geometry, where the wing feathers have a velvet-like upper surface and also have leading edge serrations and trailing edge fringes. Recent numerical and experimental investigations confirm that leading-edge waves [25], [26], [27], [28], [29], trailing edge serrations [30], [31], [32], [33], [34] and also the combination of the two [35] show performance related to drag or noise reduction. These previous work have confirmed the feasibility of noise reduction by flow control.
In this study, we proposed a biomimetic airfoil based on the biomimetic flow control concept with leading edge waves, trailing edge serrations, surface ridges and each section reflecting the conventional NACA 0012 airfoil profile. The main purpose was to find a new structure of flow control to reduce the airfoil noise without changing its aerodynamic performance. It is well known that airfoil self-noise is due to the interaction between an airfoil blade and the turbulence produced in its own boundary layer and near wake. For zero angle of attack, at low Reynolds number based on the chord length, largely laminar boundary layers develop, whose instabilities result in regular vortex shedding and associated noise from the trailing edge [36]. The vortex shedding noise includes a main tone and a set of regular spaced side peaks due to a ladder-type structure for the dependency on the free stream velocity [37], whose mechanism was focused on by many researchers, and summarized by Doolan [38]. A recent experimental study found an improved understanding that the effective tone noise generation requires the incoming T-S waves to be amplified by a separated boundary layer first [39]. Therefore, if the proposed biomimetic structures can control vortex shedding mode, it would be feasible to reduce the airfoil noise. In order to verify the validation of noise reduction by the proposed biomimetic structures, three-dimensional numerical simulations were conducted, utilizing the large eddy simulation (LES) and the aeroacoustic analogy of Ffowcs-Williams and Hawkings [40]. These were used for comparing aerodynamic and acoustic performances of the NACA 0012 and biomimetic airfoils at an angle of attack 0°. The difference of the wake flow structure between the two airfoils is addressed in order to explain the noise reduction properties of the biomimetic airfoil.
Section snippets
Descriptions of the biomimetic model
The biomimetic airfoil was created utilizing a loft feature with detailed dimensions illustrated in Fig. 1. Two outlines are the NACA 0012 airfoil profile with a fixed chord, c, of 100.9 mm, a span of 100 mm and a guide curve, which has an arc with the radius of 7.1 mm. The overall length of the biomimetic airfoil in flow direction is 2.1 mm larger than the chord length of NACA 0012 airfoil. However, compared with NACA 0012 airfoil, the maximum thickness of biomimetic airfoil does not change. The
Analysis of aerodynamic performance
Fig. 6 shows time histories of the aerodynamic force coefficients of the biomimetic and NACA 0012 airfoils at 0.25–0.27 s, with a running average over 10 time steps. From Fig. 6, it is seen that the curves of the drag and lift coefficient for NACA 0012 airfoil fluctuate periodically. Further, one can find there is a small pulse in the valley of each period, which is due to the instability of shear layer. For the biomimetic airfoil, the drag coefficient curve slightly fluctuates at random along a
Conclusions
A biomimetic airfoil is proposed in this study with leading edge waves, trailing edge serrations and surface ridges based on the NACA 0012 airfoil profile. Large eddy simulations were performed to compare the aeroacoustic performance between the two airfoils at the zero angle of attack. At Rc=1.0×105, the biomimetic structures are shown to significantly reduce vortex shedding noise around the airfoil at the observer points. Overall noise reduction at audible range can be reduced by 13.1–13.9 dB.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51575227, 51206058), the Joint Funds of the National Natural Science Foundation of China (Grant No. U1134109).
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