Acoustic excitation of Tollmien–Schlichting waves due to localised surface roughness

Placidi, Marco; Gaster, M.; Atkin, Chris

doi:10.1017/jfm.2020.349

Cited by 7 publications

(12 citation statements)

References 22 publications

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“…These are moving wave bundles with modest streamwise scales, relatively wide spanwise scales, and exponentially increasing amplitudes as they travel downstream [19]. T-S waves are a result of the interaction between the smooth laminar (undisturbed) flow near the surface and disturbances in the flow, mostly due to imperfections or perturbations [20]. As the fluid flows over the surface, these disturbances can grow and evolve into more pronounced oscillations, eventually leading to the transition from a laminar flow to a turbulent flow [21].…”

Section: Aeroacoustic Methodsmentioning

confidence: 99%

Recent Advances in Airfoil Self-Noise Passive Reduction

Amirsalari,

Rocha

2023

Aerospace

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Airflow-induced noise prediction and reduction is one of the priorities for both the energy and aviation industries. This review paper provides valuable insights into flow-induced noise computation, prediction, and optimization methods with state-of-the-art efforts in passive noise reduction on airfoils, blades, and wings. This review covers the combination of several approaches in this field, including analytical, numerical, empirical, semi-empirical, artificial intelligence, and optimization methods. Under passive noise reduction techniques, leading and trailing edge treatments, porous materials, controlled diffusion airfoils, morphing wings, surface treatments, and other unique geometries that researchers developed are among the design modification methods discussed here. This work highlights the benefits of incorporating multiple techniques to achieve the best results concerning the desired application and design. In addition, this work provides an overview of the advantages and disadvantages of each tool, with a particular emphasis on the possible challenges when implementing them. The methods and techniques discussed herein will help increase the acoustic efficiency of aerial structures, making them a beneficial resource for researchers, engineers, and other professionals working in aviation noise reduction.

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Section: Aeroacoustic Methodsmentioning

confidence: 99%

Recent Advances in Airfoil Self-Noise Passive Reduction

Amirsalari,

Rocha

2023

Aerospace

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“…It is also important to note that a certain degree of uncertainty affects the identification of the wall location from experimental data acquired with hot-wire sensors. Here, the measured velocity profile was extrapolated downward towards the wall from the first measurement point in a linear fashion, as in Placidi et al (2020). This procedure is affected by higher uncertainty when the velocity profile is less conventional (e.g.…”

Section: Mean Flow Characterisation and Validationmentioning

confidence: 99%

“…Part of energy within this first frequency range of interest is also been previously shown to be related to the blockage caused by the presence of displacement bodies used in the current experimental setup, acoustic noise, and vibrations. This was extensively discussed in van Bokhorst (2018) and Placidi et al (2020), hence omitted here as these effects are minor and are not considered to drastically influence the transition phenomenon. On the contrary, by looking at the range of interest for the travelling modes previously described in this section, 80 Hz < f < 200 Hz in Fig.…”

Section: Unsteady Disturbancesmentioning

confidence: 99%

“…By examining the effect of sound on the stability of three-dimensional boundary layers, they found that neither the disturbance growth nor the transition front location was affected by the increased sound levels, concluding that threedimensional boundary layers are only very weakly receptive to sound, while this does largely contribute to the initial amplitude of Tollmien-Schlichting (T-S) waves. This topic is further explored in Placidi et al (2020), hence omitted here together with most of the previous works that considered the role of environmental disturbances on the stability of two-dimensional flows, which is not relevant here. Deyhle and Bippes (1996) also conducted experiments in several facilities with freestream turbulence intensity in the range of 0.08% ≤ Tu ≤ 0.57% , where Tu represents the ratio of the root-mean-squared (rms) of the velocity fluctuations to the freestream velocity U ∞ .…”

Section: Introductionmentioning

confidence: 99%

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Effect of environmental disturbances on crossflow instability

et al. 2023

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Wind tunnel experiments on the receptivity of three-dimensional boundary layers were performed in a range of freestream turbulence intensities, Tu, from 0.01%—the lowest level ever achieved in this type of work—up to 0.41%. This work confirms that for $$Tu=0.01\%$$ T u = 0.01 % , and presumably below this level, the transition process is dominated by stationary modes. These are receptive to surface roughness and generate Type-I and Type-II secondary instabilities that eventually cause the transition to turbulence. The saturation amplitude of these stationary waves is highly sensitive to the level of environmental disturbances; the former is here recorded to be the highest in the literature, with the latter being the lowest. Travelling modes are still present; however, their influence on the transition process is marginal. At matched surface roughness levels, when the level of environmental disturbance is enhanced to $$Tu\ge 0.33\%$$ T u ≥ 0.33 % , the travelling modes acquire more importance, strongly influencing the laminar/turbulent transition process, whilst the initial amplitude and growth of the stationary modes are hindered. For this level of Tu, is the interaction of steady and unsteady disturbances that produces highly amplified waves (Type-III), that quickly lead to nonlinear growth and anticipated turbulence. Finally, a simple rule of thumb is proposed, where the transition front was found to move forward by roughly $$10\%$$ 10 % chord for an increase in one order of magnitude in the Tu levels. Graphical abstract

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“…However, the exact physical mechanism is not yet clarified and it cannot be excluded that other aspects could impact the laminar-turbulent transition. In particular, it is well known that acoustic receptivity can drive transition (see for example [45]), and potential discrepancies between acoustic environments (for instance, possible tonal noise produced by glider components) have not been addressed.…”

Section: Comparison Of Flight and Wind Tunnel Inflow Characteristicsmentioning

confidence: 99%

Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

et al. 2021

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Using flight measurements conducted between altitudes of 700 m and 3000 m, this work characterizes atmospheric turbulence and investigates the effects of an increase in turbulence level on the laminar-turbulent transition taking place on the pressure side of a laminar airfoil. Flight conditions ranging from calm to moderately turbulent and natural transition driven by Tollmien-Schlichting waves are considered. The inflow conditions are first characterized and reported using single and two-point statistics. Moreover, it is shown how characteristic parameters can be estimated from the turbulence intensity. Then, the sensitivity of the transition location to an increase of turbulence level is investigated. Flight results show a low sensitivity of the transition location to an increase of turbulence level, when the latter is not associated with significant variations of pressure gradient. Similar investigations are also conducted in a wind tunnel where the turbulence level is increased using an active grid and a significant change of the transition location is observed with increasing turbulence level. The differences in the response of the transition to freestream turbulence level in flight and in the wind tunnel is postulated to be attributable to differences in the probability density distributions of the inflow velocity fluctuations.

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Acoustic excitation of Tollmien–Schlichting waves due to localised surface roughness

Cited by 7 publications

References 22 publications

Recent Advances in Airfoil Self-Noise Passive Reduction

Recent Advances in Airfoil Self-Noise Passive Reduction

Effect of environmental disturbances on crossflow instability

Small-Scale Atmospheric Turbulence and Its Impact on Laminar-to-Turbulent Transition

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