On the Flow and Noise of a Two-Dimensional Step Element in a Turbulent Boundary Layer
This paper presents results of a study on the flow and noise generated by two-dimensional step elements attached to a flat plate. The step elements used in this study are equivalent to a forward–backward facing step pair and have length to height ratio of
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Abstract This paper presents results of a study on the flow and noise generated by two-dimensional step elements attached to a flat plate. The step elements used in this study are equivalent to a forward–backward facing step pair and have length to height ratio of l/h = 8, 4 and 2.7. Aerodynamic and acoustic measurements have been taken in an anechoic wind tunnel at the University of Adelaide. Numerical simulations of turbulent boundary layer flow over the step with l/h = 2.7 are also presented. The data given in this paper provide fundamental information on how a sub-boundary layer step element affects the noise and flow field. Keywords Sub-boundary layer step noise
Step flow
1 Introduction Aerodynamic sound is an important source of the noise produced by modern air, land and underwater vehicles. Surface discontinuities due to rivets, ribs, joints, environmental contamination or errors in machining can disturb the turbulent boundary layer that develops over a vehicle and generate increased levels of aerodynamic noise. Given the prevalence of surface discontinuity noise, it is important from a practical design perspective that this noise source is well understood and the baseline case of a step element attached to a flat plate is ideal for such a fundamental investigation. Previous research on step flows has largely focused on aerodynamic aspects of problem (Moss and Baker 1980) and the backward facing step is the most commonly studied configuration (Jacob et al. 2001). Leclerq et al. (2001) have conducted one of the few experimental studies on D. J. Moreau (&) J. L. Coombs C. J. Doolan School of Mechanical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia e-mail: [email protected]
Y. Zhou et al. (eds.), Fluid-Structure-Sound Interactions and Control, Lecture Notes in Mechanical Engineering, DOI: 10.1007/978-3-642-40371-2_33, Ó Springer-Verlag Berlin Heidelberg 2014
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the sound generated by flow over a forward–backward facing step pair with length to height ratio of l/h = 10. This paper presents results of a study on the flow and noise generated by a two-dimensional forward–backward facing step pair with l/h = 8, 4 and 2.7 in low Mach number turbulent flow. Aeroacoustic measurements taken in an anechoic wind tunnel at the University of Adelaide are presented along with a numerical simulation of turbulent boundary layer flow over the step element with l/h = 2.7.
2 Experimental Method Experiments were conducted in the University of Adelaide’s anechoic wind tunnel with rectangular test section of 275 mm (width) 9 75 mm (height) (Moreau et al. 2011). The test model consists of a rectangular step element attached to a flat plate as shown in Fig. 1. Three different step elements are used in this study with height of h = 2.5, 5 and 7.5 mm corresponding to 26, 52 and 77 % of the local unperturbed boundary layer thickness. The step elements are located (one at a time) on the flat plate at a distance of 30 mm downstream of the jet exit plane. Acoustic
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