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The scaling relations mapping the turbulence statistics in compressible turbulent boundary layers (TBLs) onto their incompressible counterparts are of fundamental significance for turbulence modelling, such as the Morkovin scaling for velocity fields, while for pressure fluctuation fields, a corresponding scaling relation is currently absent. In this work, the underlying scaling relations of pressure fluctuations about Mach number ($M$) contained in their generation mechanisms are explored by analysing a series of direct numerical simulation data of compressible TBLs over a wide Mach number range $(0.5\leq M \leq 8.0)$. Based on the governing equation of pressure fluctuations, they are decomposed into components according to the properties of source terms. It is notable that the intensity of the compressible component, predominantly originating from the acoustic mode, obeys a monotonic distribution about the Mach number and wall distance; further, the intensity of the rest of the pressure components, which are mainly generated by the vorticity mode, demonstrates a uniform distribution consistent with its incompressible counterpart. Moreover, the coupling between the two components is negligibly weak. Based on the scaling relations, semiempirical models for the fluctuation intensity of both pressure and its components are constructed. Hence, a mapping relation is obtained that the profiles of pressure fluctuation intensities in compressible TBLs can be mapped onto their incompressible counterparts by removing the contribution from the acoustic mode, which can be provided by the model. The intrinsic scaling relation can provide some basic insight for pressure fluctuation modelling. [ABSTRACT FROM AUTHOR]

Wall pressure fluctuations beneath turbulent boundary layers are a fundamental source of aerodynamic noise by exciting the wall structure, with their space-time characteristics serving as the basic ingredient for predicting the wall structural response. To this end, direct numerical simulations of fully developed compressible turbulent boundary layers at Mach numbers of 0.5, 1.2, and 2.0 are conducted to investigate wall pressure fluctuations comprehensively. The effects of Mach number on the single-point statistics of wall pressure fluctuations, such as the root mean square, skewness and flatness factors, probability density function, and frequency spectrum, are assessed to be very weak. Regarding the space-time characteristics, the convection velocity Uc determined by the space-time correlation of wall pressure fluctuations increases slightly with the Mach number, which only reflects the convective behavior of turbulent vortices. On the wavenumber–frequency spectrum, characteristic peaks of both the acoustic wave and convective vortices are identified. At Mach 0.5, the peaks of the fast (U c + c) and slow (U c − c) acoustic waves are unattached to others with c denoting acoustic speed, while only the peak of the fast acoustic wave is distinguishable from the convective peak at Mach 1.2 and 2.0. Due to the aerodynamic heating at supersonic conditions, the thermal effect on acoustic speed should be taken into account in determining the acoustic wavenumber. By introducing a convective Prandtl–Glauert parameter, a refined relation is proposed to provide a more accurate depiction of the acoustic domain in the wavenumber–frequency spectrum. [ABSTRACT FROM AUTHOR]

The efficacy of steady large-amplitude blowing/suction on instability and transition control for a hypersonic flat plate boundary layer with Mach number 5.86 is investigated systematically. The influence of the blowing/suction flux and amplitude on instability is examined through direct numerical simulation and resolvent analysis. When a relatively small flux is used, the two-dimensional instability critical frequency that distinguishes the promotion/suppression mode effect closely aligns with the synchronisation frequency. For the oblique wave, as the spanwise wavenumber increases, the suppression effects would become weaker and the mode suppression bandwidth diminishes/increases in general in the blowing/suction control. Increasing the blowing/suction flux can effectively broaden the frequency bandwidth of disturbance suppression. The influence of amplitude on disturbance suppression is weak in a scenario of constant flux. To gain a deeper insight into disturbance suppression mechanism, momentum potential theory (MPT) and kinetic energy budget analysis are further employed in analysing disturbance evolution with and without control. When the disturbance is suppressed, the blowing induces the transport of certain acoustic components along the compression wave out of the boundary layer, whereas the suction does not. The velocity fluctuations are derived from the momentum fluctuations of the MPT. Compared with the momentum fluctuations, the evolutions indicated by each component's velocity fluctuations greatly facilitate the investigations of the acoustic nature of the second mode. The rapid variation of disturbance amplitude near the blowing is caused by the oscillations of the acoustic component and phase speed differences between vortical and thermal components. Kinetic energy budget analysis is performed to address the non-parallel effect of the boundary layer introduced by blowing/suction, which tends to suppress disturbances near the blowing. Moreover, viscous effects leading to energy dissipation are identified to be stronger in regions where the boundary layer is rapidly thickening. Finally, it is demonstrated that a flat plate boundary layer transition triggered by a random disturbance can be delayed by a blowing/suction combination control. The resolvent analysis further demonstrates that disturbances with frequencies that dominate the early transition stage are dampened in the controlled base flow. [ABSTRACT FROM AUTHOR]

Improving the noise reduction capability of airfoil broadband noise through serrated trailing edge design is a challenging task. To address this, we propose a novel porous-serrated trailing edge design where the gaps between the serrations are filled with porous media. Implicit large eddy simulations were conducted at Mach number Ma = 0.1631 and Reynolds number Re = 96 000 under a zero incidence angle. In addition to straight trailing edges and conventional serrated trailing edges, cutting-type porous-serrated (CPS) and insert-type porous-serrated (IPS) trailing edges with different porosities were designed. The flow in the porous media is described by Darcy's law, which is related to the pressure and velocity. The results indicate that the CPS trailing edges offer limited noise reduction compared to conventional serrated trailing edges, while IPS trailing edges achieve a significant noise reduction of approximately 5.21 dB. However, the drag force increases by 8.0% in the IPS case with maximum noise reduction. The composite control mainly affects flow structures near the trailing edges, especially inducing the flow penetration across the porous surface. To investigate the noise reduction mechanism, dynamic mode decomposition was conducted to show that both the CPS and IPS designs promote energy transferring significantly from the energetic mode to the modes at other frequencies, which would partly explain the difference in the noise reduction performance to some extent. Furthermore, the analysis of the wall pressure fluctuations reveals that the reduced convection velocity on the porous surface and enhanced destructive interference between the porous and the solid surfaces in IPS cases could be identified as the key factors contributing to lower noise radiation efficiency. [ABSTRACT FROM AUTHOR]

With the fast development of microelectronic devices and the Internet of Things, triboelectric nanogenerators (TENGs) have been widely explored in self-powered micro/nano electronic devices recently. However, there are still many limitations for direct current (DC) TENGs. This study developed a semiconductor DC tribovoltaic nanogenerator (SDC-TVNG) based on n-type WS2 and p-type silicon (p-Si). Unlike alternating current output in contact-separation mode, the current generated during horizontal sliding friction, regardless of the sliding direction and whether it was positive or negative, was a DC signal from p-Si to n-type WS2. We studied the influence of frictional characteristics on the electrical output and found that the normal pressure and motion frequency had a positive correlation with the size of the DC signal. The maximum short-circuit current was 0.01 μA at a frequency of 1 Hz and a normal pressure of 10 N. The SDC-TVNG achieved a maximum power density of 2.46 μW/m2 at an outer impedance of 40 MΩ. Particularly, the tribovoltaic effect at the semiconductor–liquid–semiconductor interface was explored. Since the filled liquid layer could eliminate the insufficiency and uncertainty of the semiconductor–semiconductor contact friction, the maximum DC output of the semiconductor–liquid–semiconductor DC tribovoltaic nanogenerator (SLSDC-TVNG) was increased to 4.2 μA by a factor of 142 as compared to the current of 0.03 μA without adding the liquid (100 g/L NaCl solution) layer. The maximum power density of the device was 5.09 μW/m2 with a DC of 0.02 μA when the external resistance was 40 MΩ. This increased tribovoltaic nanogenerator with surface states at semiconductor interfaces, a composite system coupled by the liquid-enhancement effect and the tribovoltaic effect, promotes the separation of electron–hole pairs and the enhancement of current output. [ABSTRACT FROM AUTHOR]

Conventional triboelectric nanogenerators (TENGs) usually use polymer as friction layers, because polymer usually has large electronegativity differences and reliable frictional recovery properties. In recent years, inorganic material-based TENG has drawn attention due to its much higher resistance to high-temperature aging, light aging, and fatigue resistance than polymer. However, the performance of TENG with aging has not been fully studied. In this work, pure inorganic TENG based on WS2 and ITO glass was prepared, achieving a short-current (ISC) of 0.3 μA and an open voltage (VOC) of 30 V in the dark. Particularly, the current output of the nanogenerator could be enhanced to 0.61 μA under illumination. In addition, accelerated aging experiments indicated that the inorganic nanogenerator had a good improvement in performance degradation compared to most conventional polymeric nanogenerators. Based on this work, WS2-based inorganic TENGs have stability advantages and may be fully used in a wider range of environments. [ABSTRACT FROM AUTHOR]

In this study, large eddy simulations of two rectangular jets with different aspect ratios and one planar jet with spanwise periodic boundaries are carried out in an attempt to investigate and clarify some key issues regarding the flow dynamics and noise generation mechanisms for the screech tone. The substantial effects of the nozzle configuration on shock structures and noise are first revealed in detail. It is found that when the lateral confinement is weakened, the spacing of the shock cells increases and the jet oscillates more intensively in the minor-axis plane, increasing the noise level and altering the screeching frequency. By analyzing the pressure fluctuations of the shear layer in the wavenumber-frequency space, different kinds of waves in these supersonic jets are examined. Importantly, it is revealed that the guided jet wave should play a dominant role in closing the resonance loops rather than the acoustic wave. Moreover, the energy-containing structures with pertinent frequencies are extracted by employing the reduced-order variational mode decomposition, and some underlying flow dynamics are presented. Especially, a novel mechanism has been identified: the low-frequency stretching motions of shock cells have a significant modulation effect on the screeching amplitude in the planar jet. Furthermore, with the aid of the multi-process acoustic theory, the characteristics of physical noise sources are diagnosed, particularly the source mechanisms related to the screech tone. The general structures and distribution of the kinematic noise source Sβ and entropy noise source Se are presented. Sβ mainly exhibits a vortex-like structure near the nozzle, while Se exhibits a lamellar bilayer structure. The spatiotemporal correlations between the physical sources and far-field noise show that the dominant mechanisms for the screech tone rely on the nozzle configuration and the screech tone tends to be produced by multiple sources in all cases. [ABSTRACT FROM AUTHOR]