Compressive Channel Research Articles

Central mean temperature scaling in compressible turbulent channel flows with symmetric isothermal boundaries

Originating from the generalized Reynolds analogy theory, an empirical scaling for the central mean temperature in compressible turbulent channel flow with symmetric isothermal boundaries is proposed. The empirical scaling is quite accurate and most of the relative errors are below 1.5% as assessed by available direct numerical simulation data at various Reynolds and Mach numbers. The mean temperature profile can be quantitatively obtained through the mean velocity with empirical scaling.

Instabilities in a compressible hyperelastic cylindrical channel under internal pressure and external constraints

Pressurised cylindrical channels made of soft materials are ubiquitous in biological systems, soft robotics and metamaterial designs. In this paper, we study large deformation and subsequent instability of a thick-walled and compressible hyperelastic cylinder under internal pressure and external constraints. The applied pressure can lead to elastic bifurcations along the axial or circumferential direction. Perturbation theory is used to derive the partial differential equations that govern the bifurcation behaviour of the cylindrical channel. Two cases of boundary conditions on the outer surface of the cylinder, namely, free and constrained are studied to understand their influence on the instability behaviour. The derived equations are solved numerically using the compound matrix method to evaluate the critical pressure for instability. The effects of the wall-thickness of the cylinder and the compressibility of the material on the critical pressure is investigated for both the boundary conditions. The results reveal that for an isotropic material, the bifurcation occurs along the axial direction of the cylinder at lower critical pressure compared to circumferential direction for all cases considered herein. Finally, the tuneability of the bifurcation behaviour of transversely isotropic cylinder is demonstrated by considering reinforcements along the cylinder’s axis, triggering bifurcation in the circumferential direction in certain cases. The findings of the study indicate that the instability-induced pattern formation will be useful for designing shape changing material systems such as soft robotics and soft metamaterials.

Deep Compressed Sensing-Based Cascaded Channel Estimation for RIS-Aided Communication Systems

To reduce the pilot overhead of cascaded channel estimation for RIS-aided Massive MIMO communication system, we proposed a deep compressed sensing-based channel estimation scheme, where U-shaped network (U-Net), an encoder-decoder with skip connection, is used to recover the high-dimensional cascaded channel matrix from limited pilot overhead. The skip connections between encoder and decoder can fuse features of different scales and semantic by concatenating the feature map, which enhance the reconstruction performance of cascaded channel. To further improve the feature extraction ability of U-Net, we design a ResU-Net architecture with stacked residual units to increase the depth of network. Simulation results show the channel estimation of ResU-Net is more accurate than conventional algorithm and other network model. Meanwhile, ResU-Net has good generalization and robustness for different pilot lengths and phase quantization errors.

Hybrid Evolutionary-Based Sparse Channel Estimation for IRS-Assisted mmWave MIMO Systems

The intelligent reflecting surface (IRS)-assisted millimeter wave (mmWave) communication system has emerged as a promising technology for coverage extension and capacity enhancement. Prior works on IRS have mostly assumed perfect channel state information (CSI), which facilitates in deriving the upper-bound performance but is difficult to realize in practice due to passive elements of IRS without signal processing capabilities. In this paper, we propose a compressive channel estimation techniques for IRS-assisted mmWave multi-input and multi-output (MIMO) system. To reduce the training overhead, the inherent sparsity of mmWave channels is exploited. By utilizing the properties of Kronecker products, IRS-assisted mmWave channel is converted into a sparse signal recovery problem, which involves two competing cost function terms (measurement error and sparsity term). Existing sparse recovery algorithms solve the combined contradictory objectives function using a regularization parameter, which leads to a suboptimal solution. To address this concern, a hybrid multiobjective evolutionary paradigm is developed to solve the sparse recovery problem, which can overcome the difficulty in the choice of regularization parameter value. Simulation results show that under a wide range of simulation settings, the proposed method achieves competitive error performance compared to existing channel estimation methods.

Dynamic tensorial eddy viscosity model: Effects of compressibility and of complex geometry

A previous paper by Cimarelli et al. [“General formalism for a reduced description and modelling of momentum and energy transfer in turbulence,” J. Fluid Mech. 866, 865–896 (2019)] has shown that every decomposition of turbulent stresses is naturally approximated by a general form of tensorial eddy viscosity based on velocity increments. The generality of the formalism is such that it can also be used to give a reduced description of subgrid scalar fluxes. In the same work, this peculiar property of turbulent stresses and fluxes has been dynamically exploited to produce tensorial eddy viscosity models based on the second-order inertial properties of the grid element. The basic idea is that the anisotropic structure of the computational element directly impacts, although implicitly, the large resolved and small unresolved scale decomposition. In the present work, this new class of turbulence models is extended to compressible turbulence. A posteriori analysis of flow solutions in a compressible turbulent channel shows very promising results. The quality of the modeling approach is further assessed by addressing complex flow geometries, where the use of unstructured grids is demanded as in real world problems. Also in this case, a posteriori analysis of flow solutions in a periodic hill turbulent flow shows very good behavior. Overall, the generality of the formalism is found to allow for an accurate description of subgrid quantities in compressible conditions and in complex flows, independent of the discretization technique. Hence, we believe that the present class of turbulence closures is very promising for the applications typical of industry and geophysics.

Statistical properties of pressure-Hessian tensor in a turbulent channel flow

A direct numerical simulation database of a weakly compressible turbulent channel flow with bulk Mach number 1.56 is studied in detail, including the geometrical relationships between the pressure-Hessian tensor and the vorticity/strain-rate tensor, as well as the mechanism of the pressure-Hessian tensor contributing to the evolution of invariants of the velocity gradient tensor. The results show that the geometrical relationships between the pressure-Hessian tensor and the vorticity/strain-rate tensor in the central region of the channel are consistent with that of isotropic turbulence. However, in the buffer layer with relatively stronger inhomogeneity and anisotropy, the vorticity tends to be aligned with the first or second eigenvector of the pressure-Hessian tensor in the unstable focus/compressing topological region, and tends to be aligned with the first eigenvector of the pressure-Hessian tensor in the stable focus/stretching topological region. In the unstable node/saddle/saddle and stable node/saddle/saddle topological regions, the vorticity prefers to lie in the plane of the first and second eigenvectors of the pressure-Hessian tensor. The strain-rate and the pressure-Hessian tensors tend to share their second principal direction. Moreover, for the coupling between the pressure-Hessian tensor and the principal strain rates, we clarify the influence on dissipation, the nonlinear generation of dissipation and the enstrophy generation. The decomposition of the pressure-Hessian tensor further shows that the slow pressure-related term dominates the pressure-Hessian tensor's contribution, and the influence of inhomogeneity and anisotropy mainly originates from the inhomogeneity and anisotropy of the fluctuating velocity. These statistical properties would be instructive in formulating dynamical models of the velocity gradient tensor for wall turbulence.

Active control of compressible channel flow up to Mab=3 using direct numerical simulations with spanwise velocity modulation at the walls

AbstractActive turbulence control has been pursued continuously for the last decades, striving for an altered, energetically more favorable flow. In this article, our focus is on a promising method inducing a spanwise wall movement in order to reduce turbulence intensity and hence friction drag, investigated by means of direct numerical simulation. This approach transforms a previously time dependent oscillatory wall motion into a static spatial modulation with prescribed wavelength in the streamwise direction [48]. Most procedures related to turbulence control including the present one have been overwhelmingly applied to incompressible flow. This work is different and novel to the effect, that this control method is applied to compressible, supersonic channel flow up to a bulk Mach number of . Due to substantial variations of viscosity, density, and temperature within the near‐wall region in supersonic flow, the impact of the control method is altered compared to solenoidal flow conditions. By creating a data set of different Mach‐/Reynolds numbers and control parameters, knowledge is gained in which way the effectiveness of oscillatory techniques and physical mechanisms change under the influence of compressibility. It is shown that the control method is able to effectively reduce turbulence levels and lead to large drag reduction levels in compressible supersonic flow. Variable property effects even enhance this behavior for the whole set of investigated parameters. Overall, the higher Mach number cases show a larger net power saving compared to the incompressible ones. Furthermore, we observe an increase of the optimum wavelength with increasing Mach number, which helps in guiding optimal implementations of such a control method.

Exact mathematical formulas for wall-heat flux in compressible turbulent channel flows

In this paper, several exact expressions for the mean heat flux at the wall ($q_w$) for the compressible turbulent channel flows are derived by using the internal energy equation or the total energy equation. Two different routes, including the FIK method and the RD method, can be applied. The direct numerical simulations data of compressible channel flows at different Reynolds and Mach numbers verify the correctness of the derived formulas. Discussions related to the different energy equations, and different routes are carried out, and we may arrive at the conclusion that most of the formulas derived in the present work are just mathematically ones which generally are lack of clear physical interpretation. They can be used to estimate $q_w$, but might not be suitable for explore the underlying physics.

Stress dependence of the gas permeability of montmorillonite sediments in the presence of methane hydrate

The evolution of porosity and permeability of silty clay sediments in the South China Sea has significant effect on gas hydrate production due to the stress effect of submarine overburden rocks. The gas permeability of silty clay specimens under different effective stresses was measured in a cylindrical stainless steel reaction vessel. The experimental results in this paper show that application of effective stress destroys gas permeability of methane hydrate specimens. The power function relation between permeability and effective stress and exponential function relation between porosity and effective stress are given. The experimental data show that the formation of methane hydrate improves the compressible channel of montmorillonite deposits. The permeability damage rate increases with increase of effective stress, and stress sensitivity coefficient increases first and then decreases with increase of effective stress. Finally, an effective permeability equation of sediment containing hydrate saturation and effective stress parameters is established. The results are of guiding significance for the study of the stress dependence of porosity and permeability of gas hydrate silty clay reservoirs in the South China Sea. • Power law is superior to exponential law for showing permeability stress dependency. • Hydrate formation improves the compressible space of montmorillonite sediments. • Permeability damage rate increase with increasing effective stress in sediments. • Stress sensitivity coefficient show a trend of decreasing first and then increasing. • Relational expression between effective stress and permeability is proposed.

Joint Synchronization and Compressive Channel Estimation for Frequency-Selective Hybrid mmWave MIMO Systems

Synchronization is a fundamental procedure in cellular systems whereby user equipment (UE) acquires the time and frequency information required to decode the data transmitted by a base station (BS). Due to the small antenna aperture at millimeter wave (mmWave), large antenna arrays are used to compensate for the low signal-to-noise ratio (SNR) at the receiver. For this reason, synchronization is often performed jointly with beam training as in 5G New Radio (NR) to boost the receive SNR. However, if the mmWave multiple-input multiple-output (MIMO) channel is to be estimated, then synchronization must be performed in the low SNR regime owing to the lack of directional beamforming during compressive channel estimation. Current mmWave channel estimation algorithms implicitly take synchronization for granted, which is a not practical assumption in the low SNR regime. To solve this problem, this work proposes the first synchronization framework for hybrid mmWave MIMO that is robust to carrier frequency offset (CFO) and phase noise (PHN) synchronization errors. We propose two novel multi-stage algorithms to jointly estimate the various unknown parameters, based on the expectation-maximization (EM) algorithm. By using the QuaDRiGa 5G NR channel simulator, we show that compressive channel estimation without prior synchronization is possible, and the proposed approaches outperform current solutions for joint beam training and synchronization currently considered in 5G NR.

Kinetic-energy-flux-constrained model using an artificial neural network for large-eddy simulation of compressible wall-bounded turbulence

Kinetic energy flux (KEF) is an important physical quantity that characterizes cascades of kinetic energy in turbulent flows. In large-eddy simulation (LES), it is crucial for the subgrid-scale (SGS) model to accurately predict the KEF in turbulence. In this paper, we propose a new eddy-viscosity SGS model constrained by the properly modelled KEF for LES of compressible wall-bounded turbulence. The new methodology has the advantages of both accurate prediction of the KEF and strong numerical stability in LES. We can obtain an approximate KEF by the tensor-diffusivity model, which has a high correlation with the real value. Then, using the artificial neural network method, the local ratios between the real KEF and the approximate KEF are accurately modelled. Consequently, the SGS model can be improved by the product of that ratio and the approximate KEF. In LES of compressible turbulent channel flow, the new model can accurately predict mean velocity profile, turbulence intensities, Reynolds stress, temperature–velocity correlation, etc. Additionally, for the case of a compressible flat-plate boundary layer, the new model can accurately predict some key quantities, including the onset of transitions and transition peaks, the skin-friction coefficient, the mean velocity in the turbulence region, etc., and it can also predict the energy backscatters in turbulence. Furthermore, the proposed model also shows more advantages for coarser grids.

Estimation of Compressible Channel Impulse Response for OFDM Modulated Transmissions

Channel estimation scheme for OFDM modulated transmissions usually combines an initial block-pilot-assisted stage with a tracking one based on comb or scattered pilots distributed among user data in the signal frame. The channel reconstruction accuracy in the former stage has a significant impact on tracking efficiency of the channel variations and the overall transmission quality. The paper presents a new block-pilot-assisted channel reconstruction procedure based on the DFT-based approach and the Least Square impulse response estimation. The proposed method takes into account a compressibility feature of the channel impulse response and restores its coefficients in groups of automatically controlled size. The proposition is analytically explained and tested in a OFDM simulation environment. The popular DFT-based methods including compressed sensing oriented one were used as references for comparison purposes. The obtained results show a quality improvement in terms of Bit Error Rate and Mean Square Error measures in low and mid ranges of signal-to-noise ratio without significant computational complexity growth in comparison to the classical DFT-based solutions. Moreover, additional multiplication operations can be eliminated, compared to the competitive, in terms of estimation quality, compressed sensing reconstruction method based on greedy approach.

Drag reduction via opposition control in a compressible turbulent channel

The compressibility effect on opposition drag control is investigated using direct numerical simulation of turbulent channel flows at a bulk Reynolds number Re${}_{b}$ = 3000 for three different bulk Mach numbers: Mb = 0.3, 0.8, and 1.5. With increasing Mb, drag reduction slightly decreases at small sensing plane location ${y}_{d}^{+}$ but increases at large ${y}_{d}^{+}$. Interestingly, for large ${y}_{d}^{+}$ cases, a resonance buffer layer characterized by a streamwise periodic array of spanwise-coherent rollers is established, one of the primary reasons for the degradation of drag reduction performance.

Preliminary analysis of the sensitivity of the FEM model of the process of dry ice extrusion in the die with a circularly converging channel on the changing its geometrical parameters

The article presents the results of a preliminary analysis of the numerical model susceptibility for simulating the process of dry ice compaction utilizing single-channel and multiple channel dies. The work focuses on a preliminary comparison of the influence of changes in the geometrical parameters of the 4 types of compression channels. Based on the results of the performed analyses, conclusions were formulated for a basis and direction of further study regarding improving the energy efficiency of the indicated manufacturing process.

THE MINIMUM WAGE AND OCCUPATIONAL MOBILITY

AbstractThis article quantifies the effect of minimum wages on workers' occupational mobility. I show that minimum wages decrease younger, less‐educated workers' occupational mobility and are associated with more mismatch. A search‐and‐matching model highlights two channels by which the minimum wage decreases occupational mobility. First, it compresses wages and reduces the gain from switching, leading to lower occupational mobility and more mismatch. Second, it decreases vacancy posting. Calibrating the model to the U.S. economy, the results suggest that a 15 dollar minimum wage can damp aggregate output by 0.4%, of which the wage compression channel accounts for 80%.

Influence of hearing aid compression speed and channels on measures of psychophysical tuning curves

While much work has been completed on spectral resolution in listeners with SNHL, little attention has been paid to the impact of hearing-aid signal processing on access to spectral information. In this study, we hypothesized that increasing the hearing aid compression speed and number of compression channels would improve access to spectral information by providing better audibility across frequency. However, limited outer hair cell function or inhibition could curtail the benefit of improved audibility on spectral resolution. In this experiment, psychophysical tuning curves were measured for 25 participants with hearing loss. Both an excitation and phenomenological model of the auditory system were used to examine potential mechanisms that contributed to the observed results. The provision of amplification improved the measured sharpness of tuning for the low-frequency side. While decreasing the compression speed did not have a systematic effect on the measured psychophysical tuning curves, the low frequency side improved as the number of compression channels was increased. Threshold estimates from the excitation and phenomenological models were able to account for some of these observed trends in the psychophysical tuning curves, but the threshold estimates from the phenomenological model were more consistent with the measured thresholds.

Influence of number of hearing aid compression channels on spatial release from masking

Listeners with NH take advantage of differences in the spectrum of speech and noise, dips in the noise level, and spatial separation between sound sources to improve their understanding of speech. Listeners with SNHL are less able to take advantage of these differences. Possibly by not considering the influence of access to spectral information under conditions of amplification, hearing-aids only provide marginal improvement in complex listening environments. This experiment examined whether modifying the number of compression channels can improve spatial release from masking (SRM). We obtained sentence recognition for AzBio sentences presented from in front of each participant. Noise that varied temporally and spectrally over time was presented in a collocated condition and in a spatially separated condition. Test conditions included unaided and binaural amplification with an open-source hearing aid. Four and 16 channels of compression were tested. These experimental conditions were based on prior data that documented improved measures of spectral resolution with 16 relative to 4 channels of compression. We hypothesized that SRM would be greater for the condition with 16 channels of compression, relative to the condition with 4 channels of compression. SRM was hypothesized to be poorest for the unaided condition.

Theoretical analysis of quasi-one-dimensional compressible magnetohydrodynamic channel flow

A theoretical analysis of quasi-one-dimensional, steady, inviscid, compressible channel flow at a low magnetic Reynolds number with variable cross-section is performed in this paper. A second-order nonlinear dynamical system describing the variation of physical parameters is investigated in the phase plane. The characteristics of all possible channel flow with a constant electromagnetic field are obtained by the phase plane and the isomorphism of labeled graphs. It is revealed that the magnetic interaction number novelly derived from the variation rate of the cross-sectional area could significantly affect the phase trajectory in the phase plane of dimensionless velocity and Mach number. Meanwhile, the phase trajectory is only dependent on this magnetic interaction number. Further analysis of the second-order dynamical system discovers five critical values in the range of the magnetic interaction number. These five critical values can divide the whole range into eight subsets under the isomorphism of labeled graphs, forming eight equivalence classes of the phase plane. The study of such eight equivalence classes reveals that the variable cross-section flow in the phase plane can be viewed as a linear superposition of constant cross-section magnetohydrodynamic flow and isentropic flow, the weight ratio being exactly the magnetic interaction number. Consequently, for the divergent channel, the acceleration region in the supersonic zone is larger than that of the constant cross-section flow, while for the convergent channel, only the acceleration region in the subsonic zone is larger than that of the constant cross-section flow.

Site-specific millimeter-wave compressive channel estimation algorithms with hybrid MIMO architectures

In this paper, we present and compare three novel model-cum-data-driven channel estimation procedures in a millimeter-wave Multi-Input Multi-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system. The transceivers employ a hybrid analog-digital architecture. We adapt techniques from a wide range of signal processing methods, such as detection and estimation theories, compressed sensing, and Bayesian inference, to learn the unknown virtual beamspace domain dictionary, as well as the delay-and-beamspace sparse channel. We train the model-based algorithms with a site-specific training dataset generated using a realistic ray tracing-based wireless channel simulation tool. We assess the performance of the proposed channel estimation algorithms with the same site's test data. We benchmark the performance of our novel procedures in terms of normalized mean squared error against an existing fast greedy method and empirically show that model-based approaches combined with data-driven customization unanimously outperform the state-of-the-art techniques by a large margin. The proposed algorithms were selected as the top three solutions in the "ML5G-PHY Channel Estimation Global Challenge 2020" organized by the International Telecommunication Union.

Joint Channel and AoA Estimation in OFDM Systems: One Channel Tap With Multiple AoAs Problem

Recently, a joint channel and angle-of-arrival (AoA) estimation scheme for pilot-assisted multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems with digital arrays has been proposed; in that scheme, compressive sensing-based time-domain channel estimation is applied. In the scheme, it is assumed that each channel tap has only one AoA. However, in reality, one channel tap may contain responses from multiple paths with different AoAs. In this letter, we propose an improved approach to solve the aforementioned problem. The underlying concept is to transmit multiple OFDM symbols with different beamforming vectors, rendering the time-invariant channel equivalently time-variant. Subsequently, the correlation matrix, imbedded with AoA information, can be estimated. Finally, subspace-based algorithms, such as multiple signal classification (MUSIC), can be used for AoA estimation. The advantage of the proposed method is that the number of detectable AoAs is much larger than that in conventional MUSIC. The performance of the proposed method was also theoretically analyzed. Simulation results demonstrated that with a small number of OFDM symbols, both channel gains and AoAs could be satisfactorily estimated and the theoretical analysis results were accurate.