Abstract
Experiments on the receptivity of two-dimensional boundary layers to acoustic disturbances from two-dimensional roughness strips were performed in a low-turbulence wind tunnel on a at plate model. The freestream was subjected to a plane acoustic wave so that a Stokes Layer (SL) was created on the plate, thus generating a Tollmien-Schlichting (T-S) wave through the receptivity process. An improved technique to measure the T-S component is described based on a retracting two-dimensional roughness, which allowed for phase-locked measurements at the acoustic wave frequency to be made. This improved technique enables both protuberances and cavities to be explored in the range 30�m < jhj < 750�m (equivalent to 0:025 < jhj=��B < 0:630 in relative roughness height to the local unperturbed Blasius boundary layer displacement thickness). These depths are designed to cover both the predicted linear and non-linear response of the T-S excitation. Experimentally, cavities had not previously been explored. Results show that a linear regime is identifiable for both positive and negative roughness heights up to � 150 �m (jhj=��B � 0:126). The departure from the linear behaviour is, however, dependent on the geometry of the surface imperfection. For cavities of signi�cant depth, the non-linear behaviour is found to be milder than in the case of protuberances - this is attributed to the flow physics in the near field of the surface features. Nonetheless, results for positive heights agree well with previous theoretical work which predicted a linear disturbance response for small-height perturbations.
Highlights
Introduction and backgroundA thorough understanding of the transition from laminar to turbulent flow is needed before control strategies can be developed to facilitate natural and hybrid 895 R5-1M
The data analysis procedure is shown in figure 4(a), which focuses on the reconstruction of the T–S components, uTS, normalised by the acoustic wave amplitude in the free stream, uac, for the 600 μm (h = 50.4 %δB∗) roughness height at four measurement stations
The maximum measured velocity fluctuations within the boundary layer at x = 1320 mm downstream of 150-micron protuberance is O(10−3U∞), whilst after having subtracted the smooth component reduces to a signal of O(10−4U∞)
Summary
A thorough understanding of the transition from laminar to turbulent flow is needed before control strategies can be developed to facilitate natural and hybrid. This employs two secondary speakers downstream from the model to counteract possible acoustic reflections This sophisticated approach only requires one boundary layer profile; this comes at the cost of significant complications with (i) the processing of the data due to the modal decomposition, (ii) the need for an active control technique, and (iii) the experimental complexity and costs (several acoustic drivers are required). Another simple and effective technique to measure the T–S component created by the roughness in isolation is to compensate for any other effects by acquiring reference data in the absence of the said roughness (Zhou, Liu & Blackwelder 1994).
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