Elsevier

European Journal of Mechanics - B/Fluids

Volume 83, September–October 2020, Pages 99-113
European Journal of Mechanics - B/Fluids

Flow/noise control of a rod–airfoil configuration using “natural rod-base blowing”: Numerical experiments

https://doi.org/10.1016/j.euromechflu.2020.04.007Get rights and content

Abstract

When the vortices shedding from an upstream rod impinge upon the surface of a downstream airfoil, the resultant vortex–body interaction noise can be significant. The use of “natural rod-base blowing” to reduce this interaction noise is investigated numerically using a 3D hybrid computational aeroacoustics approach. The natural blowing is generated through an internal slot that interconnects the stagnation and base region of the rod. Numerical simulation is performed for a straight blowing at blowing rates (BRs) between 6.4% and 16.3% and an oblique blowing with slot–incidence angles (θ) between 0° and 15°, respectively. Far-field noise evaluation demonstrates that the natural rod-base blowing under tested BRs can reduce significantly the noise emission, whereas loses its efficiencies gradually with the increase of angle θ. The most effective case is the straight blowing (θ=0°) at BR13.6% where the far-field tonal noise associated with the steady-periodic von Karman vortex shedding is annihilated. The changes in transient flow structures indicate that the natural blowing gradually attenuates the von Karman vortex shedding when BR increases, resulting in a mitigation of vortex–body​ interaction on the airfoil leading surface and hence a reduction of the unsteady lift. At BR=13.6%, as the angle θ reaches 15° from below, a new von Karman vortex street reforms in the flow field, leading to another tonal noise. Linear stability analysis based on a local concept of absolute/convective instability suggests that the absolutely unstable region in the near-wake of the rod elongates first and then shrinks with the increase of BR, while the local absolute growth rate decreases monotonously.

Introduction

Vortex shedding from an upstream bluff body interacting with a downstream solid obstacle gives rise to a host of noise issues. This vortex–body interaction noise had been demonstrated to be more than 10 dB above the rod-only vortex shedding tonal noise ([1], [2]), and is of main concern in several aeronautical and industrial facilities, e.g., aircraft, helicopter rotors, gas turbine and household fans. Any control method, whether it is active or passive, that can suppress the on-coming vortices or mitigate the aerodynamic interaction on surface would be a potential way to reduce the interaction noise. Method examples include splitter plate ([3], [4]), plasma actuator ([5], [6]), rotating cylinder ([7]), leading edge blowing/suction ([8]), soft/perforated leading edge ([2], [9], [10]) or serrated/waving leading edge ([11], [12]) etc. As one of the well-known flow control methods, artificial base blowing had also been applied to reduce the aerodynamic interaction noise. Angland et al. [13] applied air blowing through the perforated surface of an upstream cylinder to reduce the broadband fluid–surface interaction noise of a cylinder-H beam configuration. Winkler et al. [14] utilized successfully different blowing geometries and blowing mass rate at the suction side of an upstream airfoil to reduce the tonal and broadband interaction noise between two tandem airfoils by up to 6 dB. Sutliff et al. [15] applied trailing edge blowing to reduce the rotor–stator interaction noise of a low speed fan. The general noise control mechanism is that the blowing increases the base pressure and prevents the interaction of top and bottom shear layers, hence reducing the turbulent intensity or suppressing vortex shedding. However, this method may not be efficient due to the reverse flow in the near wake of a bluff body. Schumm et al. [16] showed that, towards larger Reynolds number, a BR of about 12% must be needed to suppress vortex shedding behind a blunt cylinder/plate. At high Reynolds number, this makes the external blowing control difficult and/or expensive for a practical application.

Contrary to the artificial base blowing mentioned above, ([17]) first attempted to investigate the effect of a vent (a kind of natural base blowing) by directing the main flow through a 2D slot along the cylinder diameter into the base. This involves interconnecting the stagnation and the base region through an internal slot. The pressure difference between sets up naturally a jet from the base, causing injection of mass, momentum and energy into the near wake. As a result, the near-wake structure including vortex shedding frequency and vortex formation process had been strongly affected, leading to substantial changes in base and total drag. Compared to the external blowing, there is no extra energy involved and the control performance is dependent purely on the pressure difference between the stagnation and the base region. Since the study of Igarashi [17], this natural base blowing had been received attentions from researchers and applied successfully to suppress the vortex shedding and reduce the drag and vibration of 2D and 3D bodies, such as spheres ([18], [19]) and bluff bodies ([20], [21]).

Despite successful applications on improving aerodynamic efficiency, the natural base blowing control technique has not been studied for noise reduction purposes. The particular motivation of the work reported here, therefore, is to study the effects of natural base blowing on the vortex–body​ interaction noise of a bluff body which represents an idealized model of the components of an aircraft or other vehicles. A rod–airfoil configuration consisting of an airfoil embedded in the near-wake of a cylindrical rod is selected since this canonical benchmark model reveals the vortex–body interaction noise generation mechanism ([22], [1]). A natural base blowing with different ratios of h/d (h: width of the slot; d: rod diameter), corresponding to different blowing rates(BRs), and slot–incidence angles is investigated numerically using a 3D hybrid computational aeroacoustics approach with Shear Stress Transport Scale-Adaptive Simulation model (SST-SAS) for unsteady near-field flow simulation and Ffowcs-Williams and Hawkings (FW–H) formula for noise propagation. SST-SAS had been demonstrated by Stamou and Papadonikolaki [23] in a 3D cylinder case to be an efficient, accurate and fast turbulence modeling approach, especially in our multi-element rod–airfoil study where using Large-Eddy-Simulation (LES) at the expense of large computational costs is not required.

In this paper, we first outline the flow setup configuration and the numerical methodology in Section 2. To determine whether or not such a numerical scheme can provide accurately the aerodynamic and noise characteristics, in Section 3 we carry out a numerical calculation using the commercial CFD software-Fluent to simulate the flow and noise features of the rod–airfoil baseline and then compare them with the experimental and numerical results in the available literatures. In Section 4, the performances of the base blowing are then assessed by its ability of suppressing the vortex shedding, mitigating pressure fluctuations/lift on the airfoil and reducing the far-field noise. The concept of absolute and convective instability and how the base blowing changes the stability of the rod–airfoil flow are also detailed in this section. Finally, Section 5 gives the concluding remarks.

Section snippets

rod–airfoil baseline

We consider a 3D rod–airfoil flow configuration to investigate numerically the vortex–body interaction. A 2D sketch of the rod–airfoil setup and of the non-dimensional coordinate system is shown in Fig. 1. A symmetric NACA0012 airfoil of chord length c=100 mm and a cylindrical rod of diameter d=10 mm are placed in tandem along the x-streamwise direction. The downstream separation between the trailing edge (TE) of the rod and the leading edge (LE) of the airfoil is equal to one chord length.

Validation of numerical results with experiments and LES simulations

We verify the SST-SAS simulation of the rod–airfoil flow field with the available experimental/LES results from ([1]) and other researchers (([30], [31], [32], [33]), etc.) before applying it to investigate the effects of base blowing on the vortex–body interaction noise. Flow velocity profiles and vortex shedding frequency are examined first. The noise prediction using FW–H analogy is then carried out based on this unsteady flow simulation, and the noise levels and directivities are compared

Effects of natural rod-base blowing

In this section, we first investigate in Section 4.1 the effects of rod-base blowing on the far-field acoustic since the mitigation of vortex–body interaction noise is the major concern in our study. Then, in Section 4.2 we analyze the noise control mechanism by exploring the effects of base blowing on the aerodynamics. Section 4.3 provides an analytical solution to solve the stability of the rod–airfoil flow, on which how the rod-base blowing has effects.

Concluding remarks

This paper addresses a technique of “natural rod-base blowing” as a method for vortex–body​ interaction noise reduction of a rod–airfoil tandem model with the free stream velocity of U0=72m∕s, a Reynolds number based on the airfoil chord length c is (Rec=) 4.8 × 105. The base blowing is generated through an internal slot that connects the stagnation and the rod base region. The pressure difference between naturally sets up a jet from the base, injecting of mass, momentum into the near wake of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The investigation is made possible through the joint supports provided by National Natural Science Foundation of China (Grant No: 11972022), the State Key Laboratory of Aerodynamics (Grant No: SKLA20180202) and the Key Laboratory for Aerodynamic Noise Control (Grant No: ANCL20170101), China Aerodynamics Research & Development Center , which are gratefully acknowledged.

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