Pressure fluctuations produced by forward steps immersed in a turbulent boundary layer

Abstract

Experiments have been performed on the disturbance of a high-Reynolds-number turbulent boundary layer by three forward steps with sizes close to 3.8, 15 and 60 % of the boundary layer thickness. Particular attention is focused on the impact of the steps on the fluctuating surface pressure field. Measurements were made from 5 boundary layer thicknesses upstream to 22 boundary layer thicknesses downstream of the step, a distance equivalent to over 600 step heights for the smallest step size. Flow speeds of 30 and $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}60\ \mathrm{m}\ {\mathrm{s}}^{-1}$ were studied, corresponding to boundary layer momentum thickness Reynolds numbers of 15 500 and 26 600 and step size Reynolds numbers from 6640 to 213 000. The steps produce a disturbance to the boundary layer pressure spectrum that scales on step size and decays remarkably slowly with distance downstream. When normalized on step height and free-stream velocity, the disturbance is self-similar and appears to develop almost independently of the enveloping boundary layer. The disturbance is still clearly visible at 150 step heights downstream of the mid-size step. Pressure correlations show the disturbance to be characterized by organized quasiperiodic motions that become visible well downstream of reattachment. The coherence and scale of these motions, as seen in the wall pressure correlations, scales on the step height and thus their visibility relative to the boundary layer grows rapidly as the step size is increased.

Scaling and flow dependencies over forward-facing steps

“…In the same way, authors demonstrated that w > 9 is requiered to comfort the two-dimensional behavior of the flow. It is well known that the h δ ratio and the Reynolds number have significant effects on the reattachment length and turbulent statistics [2], but although the Ffs turbulent flows have been studied in the past, no consensus about these dependencies has been clearly reported in the literature. The constriction ratio C R = S d S u (where S u and S d are respectively the test section upstream and downstream from the step corner) seems to also have a significant effect on the separated flow fields, as reported by Lanzerstorfer and Kuhlmann [18].…”

Outlier Detection for Sensor Systems (ODSS): A MATLAB Macro for Evaluating Microphone Sensor Data Quality

“…For example, the surface of a ship may be mostly smooth except for discontinuities in the hull where plates are joined. Such steps may be acoustically loud and interest lies in the study of the effect of steps on acoustic characteristics of flow [ 11 ] and the design of efficient low-noise vehicles. The program was used to evaluate an experimental study using data collected in the Stability Wind Tunnel at Virginia Tech (Blacksburg, VA, USA) ( Figure 1 ).…”

“…The wind tunnel allows for experiments with anechoic (free from echo) or semi-anechoic conditions. The data evaluated below are part of a study on pressure fluctuations produced by forward steps immersed in a turbulent boundary layer [ 11 ]. Figure 2 shows a schematic of the test section, test wall and anechoic chambers.…”

Large-eddy simulations of turbulent flow in a channel with streamwise periodic constrictions

“…Wall-pressure fluctuations play a key role in a variety of engineering applications, such as flow-induced panel flutter and structural vibration, aircraft cabin noise and hydroacoustics of underwater vehicles (Blake 1970). Many investigations of wall-pressure fluctuations beneath a turbulent boundary layer have been performed in the past several decades, including zero pressure gradient turbulent boundary layer (Bradshaw 1967; Willmarth 1975; Farabee & Casarella 1991); flat-plate turbulent boundary layers with adverse pressure gradient (Mabey 1972; Simpson, Ghodbane & McGrath 1987; Na & Moin 1998 a , b ; Abe 2017); and turbulent flows over a backward- or forward-facing step (Farabee & Casarella 1986; Ji & Wang 2012; Awasthi et al 2014; Doolan & Moreau 2016). Presently, for turbulent flow in a channel with streamwise periodic constrictions, we analyse the pressure fluctuations by relating these to the mean pressure in the separation bubble and the development of the mixing layer.…”