Low-pass Filtering Operation Research Articles

On the mixed acoustic and vibration sensors for the cross-correlation analysis of pipe leakage signals

Pipe leak detection can be carried out by various acoustic/vibration sensors, such as acoustic pressure, velocity and acceleration sensors. The purpose of this study is to investigate the effectiveness of the mixed acoustic/vibration sensors for the determination of pipe leakage. Firstly, a propagation and attenuation model of leakage signal in the liquid-filled pipeline is established based on frequency response functions. Further, the expressions of the cross-correlation function of different sensor types are derived with respect to the differential properties of Fourier transform. Then, the influence of signal-to-noise ratio (SNR) on the cross-correlation function is analyzed. Numerical simulation is employed to generate pressure, velocity and acceleration signals by differentiating the leakage source signals. Simulation results indicate that the use of pressure sensor acts as low-pass filtering operation, whereas the velocity and acceleration sensors act as band-pass filtering. Finally, experimental work is carried out on a medium density polyethylene (MDPE) pipe. Leakage signals are collected synchronously by hydrophones and accelerometers. Experimental results show that the cross-correlation function of hydrophones fluctuates slowly with peak value being less obvious, while the cross-correlation function of accelerometers fluctuates rapidly and has a sharper peak. In comparison, there is a compromise in the fluctuation for the sake of peak sharpness in the cross-correlation function of the mixed sensors, indicating that the corresponding cross-correlation result are less affected by background noise.

Radio-over-fiber systems with delta-sigma-modulation for MMW-based indoor communication

In this paper, we propose and demonstrate a delta-sigma modulator (DSM) based radio-over-fiber (RoF) system for MMW indoor communication. In the experiment, 2-bit DSMs are used to quantize the high-order QAM orthogonal frequency division multiplexing (OFDM) signal into the 16QAM signal. At the receiver, the original high-order signal can be recovered by the low pass filter (LPF) and down-sampling operation. Based on the experimental results, up to 16,384 QAM signals can be transmitted over the DSM-based RoF system without noticeable performance degradation. When the received optical power is high enough, the corresponding error vector magnitude (EVM) and bit error rate (BER) are 0.53% and 7 × 10−3, respectively.

Magnetic Field Downward Continuation Iterative Method Based on Low-pass Filter

Magnetic field downward continuation is an important technique for expanding geomagnetic database, which can separate superimposed anomalies and enhance the resolution of local anomalies. However, the adaptability of the fixed filter operator of the existing downward continuation algorithm to the wavenumber domain frequency component of the observed data is not good enough, which leads to the poor applicability of the algorithm, resulting in poor applicability of the algorithm. The downward continuation iteration method based on a low-pass filter is proposed in this paper. First, the residual terms based on the remainder of the first and second vertical derivatives are constructed by using the stability of upward continuation; Secondly, the upward continuation low-pass filter operator, single parameter low-pass filter operator, and two-parameter low-pass filter operator are designed, and the effects of their filtering characteristics on the continuation stability are compared. At the same time, the cut-off frequency and order of the filter operator with parameters are optimized to update the iterative value. Simulation and experimental studies are carried out under different field source depths. Simulation results show that the convergence performance of this method is better when dealing with shallow source field, and the continuation accuracy is improved by 15%–25%. The stability and effectiveness of the magnetic field downward continuation method are verified by the measured data.

A new collaborative optimization method for a distributed energy system combining hybrid energy storage

• A distributed energy system with hybrid energy storage was proposed • For hybrid energy storage, two different system operation strategies were proposed • A new distributed energy system was optimized by a two-layer collaborative optimization method • A new distributed energy system was applied in the nearly zero-energy community • The energy balance, economy, and energy-saving, etc., of the new system, were studied The application of hybrid energy storage to distributed energy systems can significantly improve energy efficiency and reduce the investment operating cost of the system. However, inadequate efforts are found focusing on the investigation of the integration of the two systems and optimization configuration and operation strategy of systems. Therefore, a novel distributed energy system combining hybrid energy storage was proposed, and the system optimization configuration and operation strategy of the novel system were considered simultaneously. The hybrid system contained heat storage and two power storage forms, i.e., supercapacitors and lithium battery. The influence of hybrid energy storage on distributed energy systems was fully considered. Subsequently, a two-layer collaborative optimization method for the novel system considering energy efficiency, economy, and environmental protection was presented. The novel system was applied to a nearly zero-energy community. The results indicate that the primary energy-saving and pollutant equivalent emission reduction rates of the novel system are evaluated to be 54.8% and 63.6%, respectively, under the low-pass filter operation strategy combined with the secondary feedback adjustment of the supercapacitor charge state. The system configuration and operation strategy can be optimized simultaneously by the two-layer collaborative optimization method.

Outdoor measurement of wall pressure on cubical scale model affected by atmospheric turbulent flow

Most studies on indoor ventilation have utilized wind-tunnel experiments (WTEs) or computational fluid dynamics based approaches under well-controlled flow conditions. However, the effects of urban boundary layer flow with variable wind directions and various turbulence scales on the ventilation driven by the pressure differences between the upwind and downwind sides of a building within a block array is still under discussion. Therefore, we conducted outdoor experiments at comprehensive outdoor scale model (COSMO) experiment sites in an urban climate to clarify the relationships between the building wall pressure differences and the approaching flow. The pressure coefficients for the outdoor site were comparable with those obtained during previous WTEs. Accordingly, temporal variations in the wind speed and pressure coefficient on the target block were investigated in detail using low-pass filtering operations. The relationships between the filtered wind speed and the pressure differences indicate that the slower temporal variations in the wall pressure showed good agreement with the filtered approaching flow. In addition, the correlation coefficient between the filtered wind speed and the wall pressure differences quantified the apparent coherence between the turbulent flow and the ventilation rate. Furthermore, the statistics of the ventilation rate were determined based on the conventional model to clarify the effects of the turbulent flow on the natural ventilation rate. The ratios between the mean and short-term ventilation rates imply that the short-term ventilation rate presented dramatic temporal fluctuations owing to the various scale turbulence generated by the atmospheric flow.

Blind Phase Search with Angular Quantization Noise Mitigation for Efficient Carrier Phase Recovery

The inherent discrete phase search nature of the conventional blind phase search (C-BPS) algorithm is found to introduce angular quantization noise in its phase noise estimator. The angular quantization noise found in the C-BPS is shown to limit its achievable performance and its potential low complexity implementation. A novel filtered BPS algorithm (F-BPS) is proposed and demonstrated to mitigate this quantization noise by performing a low pass filter operation on the C-BPS phase noise estimator. The improved performance of the proposed F-BPS algorithm makes it possible to significantly reduce the number of necessary test phases to achieve the C-BPS performance, thereby allowing for a drastic reduction of its practical implementation complexity. The proposed F-BPS scheme performance is evaluated on a 28-Gbaud 16QAM and 64QAM both in simulations and experimentally. Results confirm a substantial improvement of the performance along with a significant reduction of its potential implementation complexity compared to that of the C-BPS.

Fault feature extraction and enhancement of rolling element bearing in varying speed condition

In engineering applications, the variability of load usually varies the shaft speed, which further degrades the efficacy of the diagnostic method based on the hypothesis of constant speed analysis. Therefore, the investigation of the diagnostic method suitable for the varying speed condition is significant for the bearing fault diagnosis. In this instance, a novel fault feature extraction and enhancement procedure was proposed by the combination of the iterative envelope analysis and a low pass filtering operation in this paper. At first, based on the analytical model of the collected vibration signal, the envelope signal was theoretically calculated and the iterative envelope analysis was improved for the varying speed condition. Then, a feature enhancement procedure was performed by applying a low pass filter on the temporal envelope obtained by the iterative envelope analysis. Finally, the temporal envelope signal was transformed to the angular domain by the computed order tracking and the fault feature was extracted on the squared envelope spectrum. Simulations and experiments were used to validate the efficacy of the theoretical analysis and proposed procedure. It is shown that the computed order tracking method is recommended to be applied on the envelope of the signal in order to avoid the energy spreading and amplitude distortion. Compared with the feature enhancement method performed by the fast kurtogram and corresponding optimal band pass filtering, the proposed method can efficiently extract the fault character in the varying speed condition with less amplitude attenuation. Furthermore, do not involve the center frequency estimation, the proposed method is more concise for engineering applications.

Study on the Dependence of Reverse Simulation for Identifying a Pollutant Source on Grid Resolution and Filter Width in Cavity Flow

When a hazardous substance is diffused, it is necessary to identify the pollutant source and respond immediately. However, there are many cases in which damage is caused without a clear understanding of where the pollutant source is located. There are three groups of identifying pollutant source information (Liu and Zhai, 2007): the probability method, forward method, and backward method. In our previous study, we proposed reverse simulation, which is categorized as a backward method (Abe and Kato, 2011). Numerical instability by negative diffusion is a principal problem in the backward method. In order to improve the problem, we applied a low‐pass filter operation to the concentration flux in the RANS analysis. The simulation secured the numerical stability. However, reverse simulation accuracy is expected to depend on the grid resolution and filter width. In this paper, we introduce reverse simulation results in cavity flow. In particular, we survey the dependence of reverse simulation accuracy on the grid resolution and filter width. Moreover, we discuss the dependence of reverse simulation on the grid resolution and filter width with a one‐dimensional diffusion equation. As a result, we found that the simulated negative diffusion varies greatly among the grid resolution and filter width.

THE DEPENDENCE ON GRID RESOLUTION AND FILTER WIDTH OF REVERSE SIMULATION FOR IDENTIFYING POLLUTANT SOURCE

Reverse simulation, which is the solution of the transport equation in negative time, is a method for identifying pollutant source with CFD. Then the negative diffusion amplifies numerical errors and reverse simulation cannot be carried out. We proposed a method for improving numerical stability (9). This method applies a low-pass filter operation to concentration flux in RANS (Reynolds averaged numerical simulation) In this paper, for application of actual problems, we introduce reverse simulation result in cavity flow, which has greatly-bent streamline and forms complex flow. In addition, we survey the dependence of reverse simulation accuracy on grid resolution and filter width.

Direct Numerical Simulation of Discrete Roughness on a Swept-Wing Leading Edge

Direct numerical simulation is employed in order to describe the subsonic flow past an array of micron-sized discrete roughness elements, which were mounted near the leading edge of a 30-degree swept wing at a chord Reynolds number of 7.4 x 10 6 . The flow conditions correspond to flight receptivity experiments that were conducted to investigate the effects of roughness on crossflow instabilities. To make the computations tractable, the geometry is scaled by the radius of the wing leading edge, which magnifies the region of interest and enhances resolution. The leading-edge region is then approximated by the flow past an infinite parabolic cylinder. The numerical method is based upon a sixth-order-accurate time-implicit scheme to attain high fidelity and was used in conjunction with an eighth-order low-pass Pade-type nondispersive filter operator to maintain stability. A high-order overset-grid approach preserved spatial accuracy on a local mesh system representing the roughness elements, using domain decomposition to perform calculations on a parallel computing platform. The direct simulation for the flow about the roughness elements was used to capture crossflow vortices and served as input to the nonlinear parabolized stability equations, which were then solved in order to determine receptivity of the flow to the geometric perturbations. Three different geometric roughness elemental shapes were investigated in the study. For one shape, the effect of element height was examined. Features of the roughness-element flowfields are elucidated, and findings of the stability calculations are compared. Results are presented for receptivity of the crossflow instability to the size and shape of elements, as obtained by the direct numerical simulation and by two different stability approaches.

Pre- and Post-Shift Filtering for Blocking Removing in Downsizing Transcoding

When high-quality video is delivered via a bandwidth-limited network, one solution is to reduce the spatial resolution of the bitstream before transmission. Downsizing transcoding is adopted as an efficient tool to reduce the spatial resolution of the precoded bitstream. Since the downsizing process and the coding process both apply a low-pass filtering operation on the picture, the reconstructed video may result in serious blocking artifact. In this paper, pre- and post-shift filters are considered in the transcoding process. The block boundary is shifted between the downsizing process and the coding process. This reduces the blocking effect in downsizing transcoding. Experimental results show that the proposed scheme can efficiently remove the blocking effect without introducing high computation.

Wavefront reconstruction and three-dimensional shape measurement by two-step dc-term-suppressed phase-shifted intensities

A wavefront reconstruction and three-dimensional (3-D) shape measurement method by a two-step phase-shifting algorithm with arbitrary phase shift in (0,pi) is proposed. In this method, only two phase-shifted intensities, with the removal of the dc term by an averaging technique in spatial domain or low-pass filter operation in the frequency domain, are needed, and the other additional measurements are no longer required. The simulation for an irregular wavefront has shown the feasibility, and the optical experiment for a 3-D face mask in the case of a sinusoidal fringe projection system has illustrated the validity of the proposed method.

Direct Numerical Simulations of Flow Past an Array of Distributed Roughness Elements

Direct numerical simulation was used to describe the subsonic flow past an array of distributed cylindrical roughness elements mounted on a flat plate. Solutions were obtained for element heights corresponding to a roughness-based Reynolds number (Re k ) of both 202 and 334. The numerical method used a sixth-order-accurate centered compact finite difference scheme to represent spatial derivatives, which was used in conjunction with a tenth-order low-pass Pade-type nondispersive filter operator to maintain stability. An implicit approximately factored time-marching algorithm was employed, and Newton-like subiterations were applied to achieve second-order temporal accuracy. Calculations were carried out on a massively parallel computing platform, using domain decomposition to distribute subzones on individual processors. A high-order overset grid approach preserved spatial accuracy on the mesh system used to represent the roughness elements. Features of the flowfields are described, and results of the computations are compared with experimentally measured velocity components of the time-mean flowfield, which are available only for Re k = 202. Flow about the elements is characterized by a system of two weak corotating horseshoe vortices. For Re k = 334, an unstable shear layer emanating from the top of the cylindrical element generated nonlinear unsteady disturbances of sufficient amplitude to produce explosive bypass transition downstream of the array. The Re k = 202 case displayed exponential growth of turbulence energy in the streamwise direction, which may eventually result in transition.

Numerical Study of Active Flow Control for a Transitional Highly Loaded Low-Pressure Turbine

Active control was simulated numerically for the subsonic flow through a highly loaded low-pressure turbine. The configuration approximated cascade experiments that were conducted to investigate a reduction in turbine stage blade count, which can decrease both weight and mechanical complexity. At a nominal Reynolds number of 25,000 based upon axial chord and inlet conditions, massive separation occurred on the suction surface of each blade due to uncovered turning. Vortex generating jets were then used to help mitigate separation, thereby reducing wake losses. Computations were performed using both steady blowing and pulsed mass injection to study the effects of active flow control on the transitional flow occurring in the aft-blade and near-wake regions. The numerical method utilized a centered compact finite-difference scheme to represent spatial derivatives, that was used in conjunction with a low-pass Pade-type nondispersive filter operator to maintain stability. An implicit approximately factored time-marching algorithm was employed, and Newton-like subiterations were applied to achieve second-order temporal accuracy. Calculations were carried out on a massively parallel computing platform, using domain decomposition to distribute subzones on individual processors. A high-order overset grid approach preserved spatial accuracy in locally refined embedded regions. Features of the flowfields are described, and simulations are compared with each other, with available experimental data, and with a previously obtained baseline case for the noncontrolled flow. It was found that active flow control was able to maintain attached flow over an additional distance of 19–21% of the blade chord, relative to the baseline case, which resulted in a reduction of the wake total pressure loss coefficient of 53–56%.

Numerical Simulation of Separation Control for Transitional Highly Loaded Low-Pressure Turbines

The subsonic flow through highly loaded low-pressure turbines is simulated numerically using a high-order method. The configuration approximates cascade experiments that were conducted to investigate a reduction in turbine stage blade count, which can decrease both weight and mechanical complexity. At a nominal Reynolds number of 25 × 10 3 based upon axial chord and inlet conditions, massive separation occurs on the suction surface of each blade as a result of uncovered turning. Pulsed injection vortex generator jets were then used to help mitigate separation, thereby reducing wake losses. Computations were performed for both uncontrolled and controlled cases and reproduced the transitional flow occurring in the aft-blade and near-wake regions. The numerical method utilizes a centered compact finite difference scheme to represent spatial derivatives, which is used in conjunction with a low-pass Pade-type nondispersive filter operator to maintain stability. An implicit approximately factored time-marching algorithm is employed, and Newton-like subiterations are applied to achieve second-order temporal accuracy. Calculations were carried out on a massively parallel computing platform, using domain decomposition to distribute subzones on individual processors. A high-order overset grid approach preserved spatial accuracy in locally refined embedded regions. Features of the flowfields are elucidated, and simulations are compared with each other and with available experimental data. Relative to the uncontrolled case, it was found that pulsed injection maintained attached flow over an additional 15% of the blade chord, resulting in a 22% decrease of the wake total pressure loss coefficient.

Postprocessing for vector-quantized images based on projection onto hypercubes

In this paper, in order to reduce blocking artifacts in vector-quantized images, we propose a new postprocessing algorithm, based on the projection onto a subset of quantization constraint set (QCS). The notion behind the projection onto QCS is to prevent the postprocessed data from diverging from QCS, i.e., blurring, which is usually caused by a low-pass filtering operation. First, we theoretically analyze the projection onto QCS, and show that the projection onto a subset of QCS could yield a better performance than the projection onto QCS case. Since the quantizer regions in the vector quantizer (VQ) are arbitrarily shaped unless the VQ has a structural codebook, it is not easy to implement a projector for QCS. In order to simplify the projection, we introduce hypercubes for a subset of QCS, where the hypercubes are the elements of the subset. Hence, the proposed postprocessing algorithm has two steps: linear space-invariant low-pass filtering (or projecting onto smoothness constraint sets) and then projecting onto hypercubes. Simulation results show that the proposed algorithm can reduce blocking artifacts without blurring the edge components, and achieve a 0.5-2.0-dB gain. Furthermore, the contouring effect can also be removed by iteratively applying the proposed postprocessing algorithm, based on the constrained minimization problem.

Detection of low-contrast fiducial marks for electron beam direct write

Precise registration to substrate fiducial marks is crucial to high density, fine-line device fabrication. Mark detectability is a function of both the mark contrast with respect to its surrounding substrate and also its interior topography. Since the accuracy and speed of detection of any scheme is related to the signal-to-noise ratio (SNR) of the mark image presented to the detection filter, one is compelled to develop higher contrast marks in order to enhance the effective SNR and thereby keep pace with the more stringent registration requirements of today’s processes. Unfortunately, some higher contrast marks, being developed by more advanced processes, are characterized by a rather severe interior topography. Standard edge detectors are hampered by the apparent interior ‘‘edges’’ of such marks. Another complication arises from the pulsed type data acquisition scheme used on the AT&T in-house EBES4 system that demands high bandwidth detection electronics. This prohibits the reduction of the detection noise power spectral density by low-pass filtering operations. While this acquisition architecture has the potential for accurate mark location with little electron beam induced mark degradation or errors due to charging, the detection filter must cope with a relatively strong noise energy content. To register to both low contrast and moderate contrast ‘‘rough terrain’’ marks under these circumstances, a two-stage matched filter has been developed. This variant of the standard matched filter was designed to improve its edge localization characteristics. The approach incorporates computationally intensive signal processing algorithms executed on special purpose hardware. Our detection system consists of a custom digitizer that feeds a VMEbus based array processor. The digitizer is equipped with a large, fast memory buffer so that scan data may be processed as a new scan is being acquired. This parallelism results in detection times commensurate with high throughput. Detection accuracy from chrome marks has been investigated using three detection techniques. Optimum ‘‘‖mean‖+3σ’’ overlay errors of ≤0.06 μm were achieved by utilizing the two-stage filter.

Detection of objects in quantum-noise-limited images

A method is presented that allows the automatic detection of single particles against an extremely noisy background, e.g., biological macromolecules in a low dose electron micrograph. In this method each image element is replaced by the image variance in its environment. This convolution-like operation leads to the “variance image” which is an efficient detector of objects with the same average density as the background (such as phase objects). The variance image is essentially a simple measure of texture. The human visual system appears to be equipped with an equally powerful detector. A separate class of objects is comprised of objects that have an average density different from that of the background. These objects are often best detected by a low-pass filtering operation. The quantum-noise limits of both methods of detection are calculated and compared to the noise limits of “matched filtering” methods. Fast algorithms are provided to calculate the variance image. Model calculations confirm the theoretical noise limits. The Rose equation is discussed in the context of the two operators treated in this paper.

Bandpass filtering with lowpass filters

Lowpass filters may achieve the effects of bandpass filtering by heterodyning the bandpass wave to obtain the “in-phase” and “quadrature” components and filtering these with suitable lowpass filters. The impulse responses for the lowpass filters are determined in a simple way from the desired bandpass impulse response. When the filtering is to be arbitrary and the signal to be filtered has arbitrary phase modulation, four lowpass filtering operations are necessary. In the special cases where (1) the signal has only amplitude and/or binary phase modulation, or (2) the transfer function of the filter has “conjugate symmetry,”, only two lowpass filtering operations are necessary. The outputs of the lowpass filters may be combined simply to give: 1. 1. The instantaneous envelope of the filtered bandpass wave. 2. 2. The instantaneous phase or frequency of the filtered bandpass wave. 3. 3. The filtered bandpass wave itself at almost any carrier frequency. The techniques described above can be extended straightforwardly to linear time-variable filters.