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The safe and stable operation of the transformer is the key to ensure the reliable operation of the power grid. Loose windings will produce a vibration acceleration signal with twice the power supply frequency signal, i.e., 100 Hz, as the main spectral component. The realization of transformer winding vibration acceleration detection can effectively determine the operating status of the transformer winding. In this report, a dual-cantilever beam Fiber Bragg Grating (FBG) vibration acceleration sensor for transformer winding with a two-point package structure was designed based on theoretical analysis and finite element simulation of COMSOL Multiphysics software. Through the demodulation of the center wavelength change to achieve the detection of external vibration acceleration and to achieve the vibration acceleration sensitivity of 37.813 pm/g at 100 Hz, linearity up to 99.97%, cross-sensitivity is less than 17.4%. The resonant frequency of the sensor is 134 Hz, and the sensitivity of the vibration acceleration at the resonant frequency point is 425.52 pm/g. The sensor is installed under the transformer to measure the vibration acceleration of the transformer winding. The vibration component under 100 Hz frequency could be obviously captured, and the FFT of the center wavelength captured the amplitude signal to realize the measurement of the vibration acceleration of the transformer winding less than 0.02 g micro-vibration acceleration, which could satisfy the general vibration range of the transformer winding. [ABSTRACT FROM AUTHOR]

A Mach 1.5 non-reactive flow in a cavity-stabilized combustor of a model scramjet is studied via a direct-numerical simulation approach, and the analysis is focused on the interaction among boundary layer, free shear-layer above the cavity and shock wave. It is found that the impingement of the free shear-layer on the aft wall of the cavity leads to strong turbulence kinetic energy, high local pressure, and a fan of compression waves. The compression waves evolve into an oblique shock, which reflects between the upper and lower walls and interacts with the boundary layers attached to the two walls. The analysis of the turbulence production reveals that the amplification of turbulence in the core of the shear-layer and around the reattachment point is mainly due to the shear production, but the deceleration production mechanism presents a significant impact in the regions above the aft wall of the cavity and around the shock interaction points. The very low frequency commonly observed in shock wave/boundary layer interactions is not observed in the present research, which might be due to the low Reynolds number of the studied case. [ABSTRACT FROM AUTHOR]

In this paper, the effect of the secondary flow induced by convergent–divergent riblets in supersonic turbulent boundary layers over a 24° compression ramp at Mach number 2.9 is studied via direct numerical simulation. Two riblet cases with the wavelength Λ being 1.1δ and 1.65δ (δ is the boundary layer thickness) are conducted to examine their impact on the secondary rolling motion, momentum transfer, turbulent fluctuations, flow separation, and unsteady shock motion. As the flow develops over the riblet section, both the size and intensity of the secondary rolling motion tend to increase. For the riblet case with Λ / δ = 1.1 , a single rolling mode is observed within a half wavelength, while a pair of co-rotating vortical structures is obtained for Λ / δ = 1.65. Both rolling patterns lead to an apparent spanwise variation of the flow field. The results reveal that the secondary flow contributes to the increase of both the mean momentum flux and turbulent fluctuations. By using the spanwise averaging, the mean momentum flux contributed from the dispersive stress and compressible effect caused by the secondary flow is identified. Both components appear to enhance the near-wall momentum mixing, and a larger enhancement is observed for Λ / δ = 1.1 , where the intensity of the secondary flow is stronger. Compared to the baseline case, the area of the separation zone at Λ / δ = 1.1 and Λ / δ = 1.65 is decreased by 56% and 38%, respectively. For all the cases, the low-frequency motion near the foot of the shock is observed. In comparison, the frequency of the low-frequency motion for the riblet case is two times higher than that in the baseline case, owing to the reduction of the separation area and length. [ABSTRACT FROM AUTHOR]

In this paper, a section of convergent–divergent (C–D) riblets is applied upstream of a compression ramp in a supersonic turbulent boundary layer in a Mach 2.9 flow with a Reynolds number of R e θ = 2240. Direct numerical simulations are undertaken to examine the impact of C–D riblets on the shock wave/boundary layer interaction and the feasibility of using them to mitigate flow separation. Over the riblet section, a large-scale secondary roll mode is produced by C–D riblets with the downwelling motion occurring around the diverging region and upwelling motion near the converging region. This consequently leads to a spanwise heterogeneity in mean quantities and turbulent structures over the riblet section and also in the interaction zone. Compared with the baseline case, the area of the separation zone for the riblet case experiences a dramatic local reduction of 92% in the diverging region, owing to the downwelling motion that injects the high-momentum fluid toward the wall and the near-wall spanwise velocity that transports the low-momentum fluid away. The enhanced upwelling motion around the converging region induced by C–D riblets, on the one hand, contributes to the decrease of the near-wall momentum and subsequently the increase of the local separation area. On the other hand, the upwelling motion effectively reduces the incoming Mach number upstream of the compression corner. This appears to reduce the strength of the separation shock, leading to a more gradual compression of the incoming flow that helps ease the enlargement of the separation area nearby. Overall, the area of the mean flow separation is reduced by 56%, indicating an effective flow separation control by the C–D riblets. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of a turbulent channel flow developing over convergent–divergent (C–D) riblets are performed at a Reynolds number of Reb = 2800, based on the half channel height δ and the bulk velocity. To gain an in-depth understanding of the origin of the drag generated by C–D riblets, a drag decomposition method is derived from kinetic energy principle for a turbulent channel flow with wall roughness. C–D riblets with a wavelength, Λ, ranging from 0.25 δ to 1.5 δ , are examined to understand the influence of secondary flow motions on the drag. It is found that as Λ increases, the intensity of the secondary flow motion increases first and then decreases, peaking at Λ / δ = 1. At Λ / δ ≥ 1 , some heterogeneity appears in the spanwise direction for the turbulent kinetic energy (TKE) and vortical structures, with the strongest enhancement occurring around regions of upwelling. All the riblet cases examined here exhibit an increased drag compared to the smooth wall case. From the energy dissipation/production point of view, such a drag increase is dominated by the TKE production and the viscous dissipation wake component. While the drag contribution from the TKE production shear component decreases as Λ increases, the drag contribution from the wake component of both the TKE production and viscous dissipation follows the same trend as the intensity of the secondary flow motion. From the work point of view, the drag increase in the riblet case at Λ / δ = 0.25 comes mainly from the work of the Reynolds shear stresses, whereas at Λ / δ ≥ 1 , the drag augmentation is dominated by the work of the dispersive stresses. At Λ / δ = 0.5 , both components play an important role in the increase in the drag, which also exhibits a peak. [ABSTRACT FROM AUTHOR]

Robust detection of acoustically quiet, slow-moving, small unmanned aerial vehicles is challenging. A biologically inspired vision approach applied to the acoustic detection of unmanned aerial vehicles is proposed and demonstrated. The early vision system of insects significantly enhances signal-to-noise ratios in complex, cluttered, and low-light (noisy) scenes. Traditional time-frequency analysis allows acoustic signals to be visualized as images using spectrograms and correlograms. The signals of interest in these representations of acoustic signals, such as linearly related harmonics or broadband correlation peaks, essentially offer equivalence to meaningful image patterns immersed in noise. By applying a model of the photoreceptor stage of the hoverfly vision system, it is shown that the acoustic patterns can be enhanced and noise greatly suppressed. Compared with traditional narrowband and broadband techniques, the bio-inspired processing can extend the maximum detectable distance of the small and medium-sized unmanned aerial vehicles by between 30% and 50%, while simultaneously increasing the accuracy of flight parameter and trajectory estimations. [ABSTRACT FROM AUTHOR]

A planar duct flow configuration with a cross-flow injected from a longitudinal slit close to the upper wall of the duct is studied by using a direct numerical simulation approach to explore the underlying flow mechanism in relation to the tip-leakage vortex (TLV), which is one of the most important flow phenomena in turbomachinery. Major characteristics of TLV in a rotor of turbomachinery are identified in the current flow model. The analysis of mean and instantaneous flow fields reveals that the interaction between the main (axial) flow and jet (cross) flow is the primary source of the generation of the TLV. The evolution of the TLV is then investigated, and a vortex breakup phenomenon is identified. The evolution of TLV can be divided into three phases, i.e., the formation phase, the breakup phase, and the diffusion phase. Mean streamlines and turbulence kinetic energy (TKE) budgets are analyzed, showing that the high TKE central spot in the formation phase is due to the interaction between highly swirling vortex filaments and mean velocity gradient. In the outer part of the TLV, the TKE is mainly produced in the shear-layer and transported toward the center by the turbulence transport. [ABSTRACT FROM AUTHOR]

A validated method based on Haidinger fringes has been proposed to measure the optical constants for thin-walled glass cells. The method can significantly enhance measurement accuracy by utilizing the reflected spectrum to determine the optical constants of the sample. The reflected light off the inner and the outer surfaces of the cell combined together and formed the interferential pattern (Haidinger fringes) detected by the photodiodes. Taking into account that the sample in our experiment is a sealed cell, we deliberately set the incident angle to a non-zero value. First, the measurement principle has been analyzed, and subsequently, the relevant experimental platform has been established. To reduce random errors, we recalibrated the relationship between the frequency of the laser and the operating temperature. Furthermore, the incident angle has been accurately estimated using the method of non-linear least squares. Finally, the weighted average, considering correlated uncertainties for all measurements, is 1.873 ± 0.005 016 mm, which showcases clear advantages over conventional mechanical methods that risk potential damages. [ABSTRACT FROM AUTHOR]

An implicit large-eddy simulation is carried out to study turbulent boundary-layer separation from a backward-facing rounded ramp with active wall actuation control. This method, called spanwise alternating distributed strips control, is imposed onto the flat plate surface upstream of a rounded ramp by alternatively applying out-of-phase control and in-phase control to the wall-normal velocity component in the spanwise direction. As a result, the local turbulence intensity is alternatively suppressed and enhanced, leading to the creation of vertical shear-layers, which is responsible for the presence of large-scale streamwise vortices. These vortices exert a predominant influence on the suppression of the flow separation. The interaction between the large-scale vortices and the downstream recirculation zone and free shear-layer is studied by examining flow statistics. It is found that in comparison with the non-controlled case, the flow separation is delayed, the reattachment point is shifted upstream, and the length of the mean recirculation zone is reduced up to 8.49%. The optimal control case is achieved with narrow in-phase control strips. An in-depth analysis shows that the delay of the flow separation is attributed to the activation of the near-wall turbulence by the in-phase control strips and the improvement of the reattachment location is mainly due to the large-scale streamwise vortices, which enhance the momentum transport between the main flow and separated region. [ABSTRACT FROM AUTHOR]