Search

The transient response of porous media is an important aspect of dynamic research. However, existing studies seldom provide solutions to the transient response problem of layered unsaturated porous media. Based on the Biot-type unsaturated wave equations, dimensionless one-dimensional wave equations are established. An appropriate displacement function is introduced to homogenize the boundary conditions. Subsequently, the transfer matrix method is used to obtain the eigenvalues and eigenfunctions of the homogeneous governing equations. Leveraging the orthogonality of the eigenfunctions, the original problem is transformed into solving a series of initial value problems of ordinary differential equations. The temporal solution within the time domain is then obtained through an improved precise time integration method. The validity of the solution presented in this paper is verified by comparing it with existing solutions in the literature. Analysis of numerical examples shows that reflection waves of opposite phases will be generated at the hard-soft and hard-harder interface, which helps in the accurate identification of weak interlayers in practical engineering applications. With increasing saturation, there is a noticeable increase in the velocities of the P 1 and P 3 waves, whereas the velocity of the P 2 waves tends to decrease, which can be used to assess the mechanical property of medium. The peak value of pore pressure in unsaturated can be 1.64 times higher than those in saturated condition. [ABSTRACT FROM AUTHOR]

The multi-parameter and nonlinear characteristics of the Smith Watson Topper (SWT) equation present considerable challenges for predicting the fatigue life of 2024-T3 clad Al alloy. To overcome these challenges, a novel model integrating traditional fatigue analysis methods with machine learning algorithms is introduced. An improved SWT fatigue life prediction equation is developed by incorporating key factors such as the mean stress effect, stress concentration factor, and surface roughness coefficient. Extreme gradient boosting, Random Forest, and their derived models are used to construct the fatigue life prediction model. The L-BFGS algorithm was then integrated with the established machine learning model to solve for the multi-parameter of the improved SWT equation. Thus, an accurate modified SWT prediction equation for 2024-T3 clad Al alloy was obtained. To further optimize the solution, the deep deterministic policy gradient and deep reinforcement learning algorithms are introduced to dynamically optimize the nonlinear equation, achieving a more efficient and accurate solution. The improved SWT fatigue life prediction equation and its solution method proposed in this study provide new insights for fatigue life prediction of clad metallic materials. [ABSTRACT FROM AUTHOR]

High-altitude platforms (HAPs) are considered to be the most important equipment for next-generation wireless communication technologies. In this paper, we investigate the channel characteristics under the configurations of massive multiple-input multiple-output (MIMO) space and large bandwidth at millimeter-wave (mmWave) bands, along with the moving essence of the HAP and ground terminals. A non-stationary three-dimensional (3D) geometry-based stochastic model (GBSM) is proposed for a HAP communication system. We use a cylinder-based geometric modeling method to construct the channel and derive the channel impulse response (CIR). Additionally, the birth–death process of the scatterers is enclosed using the Markov process. Large-scale parameters such as free space loss and rainfall attenuation are also taken into consideration. Due to the relative motion between HAP and ground terminals, the massive MIMO space, and the wide bandwidth in the mmWave band, the channel characteristics of HAP exhibit non-stationarities in time, space, and frequency domains. By deriving the temporal auto-correlation function (ACF), we explore the non-stationarity in the time domain and the impact of various parameters on the correlations across the HAP-MIMO channels. The spatial cross-correlation function (CCF) for massive MIMO scenarios, and the frequency correlation function (FCF) in the mmWave bands are also considered. Moreover, we conduct simulation research using MATLAB. Simulation results show that the theoretical results align well with the simulation results, and this highlights the fact that the constructed 3D GBSM can characterize the non-stationary characteristics of HAP-MIMO channels across the time, space, and frequency domains. [ABSTRACT FROM AUTHOR]

The photo-induced superconducting phase transition is widely used in probing the physical properties of correlated electronic systems and to realize broadband photodetection with extremely high responsivity. However, such photoresponse is usually insensitive to electrostatic doping due to the high carrier density of the superconductor, restricting its applications in tunable optoelectronic devices. In this work, we demonstrate the gate voltage modulation to the photoresponsivity in a two-dimensional NbSe2-graphene heterojunction. The superconducting critical current of the NbSe2 relies on the gate-dependent hot carrier generation in graphene via the Joule heating effect, leading to the observed shift of both the magnitude and peak position of the photoresponsivity spectra as the gate voltage changes. This heating effect is further confirmed by the temperature and laser-power-dependent characterization of the photoresponse. In addition, we investigate the spatially-resolved photocurrent, finding that the superconductivity is inhomogeneous across the junction area. Our results provide a new platform for designing tunable superconducting photodetector and indicate that the photoresponse could be a powerful tool in studying the local electronic properties and phase transitions in low-dimensional superconducting systems. [ABSTRACT FROM AUTHOR]

In this paper, we propose a physical layer secret key (PLSK) generation scheme for secure encrypted transmissions in orbital angular momentum (OAM) communication systems which improves the secret key capacity via taking advantage of OAM orthogonal modes. Based on the line-of-sight (LoS) quasi-static channel conditions in OAM systems, we propose a mode-based active defense phase in the PLSK scheme to invalidate keys generated by an eavesdropper when it is extremely close to the legitimate user. Our simulation results highlight that the key information generated by the eavesdropper is almost zero in the case of highly correlated legitimate and eavesdropping channels under LoS conditions. In addition, our proposed OAM PLSK scheme achieves a higher key generation rate (KGR) than existing works since mode-based active defense can disrupt the process of PLSK by the eavesdropper. [ABSTRACT FROM AUTHOR]

Lane detection is essential for autonomous vehicles, and vision-based lane detection is widely used in the field of intelligent driving cars because of its low cost. Aiming to solve the problem of false detection caused by surrounding vehicles during lane detection, a new multi-lane detection method with strong anti-interference ability is proposed. The main contribution of this work is the utilization of object detection neural network which is applied to remove vehicles from road images as much as possible. Compared with the traditional feature-based detection algorithms, this method can preserve lane line features more accurately and effectively. Firstly, binarization of global optimal threshold method is utilized to extract the basic features of lanes. In order to further remove non-lane line noises and extract lane features as clear as possible, YOLOv4-tiny object detection network is introduced to detect and filter vehicles on the road. Then the DBSCAN (Density-Based Spatial Clustering of Applications with Noise) algorithm with denoising function is applied to cluster the different lane features. Finally, an improved RANSAC (Random Sample Consensus) method is used to perform quadratic curve fitting on the clustered features. The experimental results on dataset show that consumed time of a single frame image is only 0.119 second and the precision of the proposed method can reach 94.4%. Compared with other detection algorithms, the proposed algorithm has good real-time performance, stability and robustness in a variety of complex scenarios. [ABSTRACT FROM AUTHOR]

Niobium diselenide (NbSe2) is a layered transition metal dichalcogenide with novel quantum phases at low temperatures (T) such as superconductivity and charge density wave order. While its electronic correlations and the interaction between electrons and other collective modes have been explored extensively, a detailed study of the transport behavior of photo-excited charge carriers still remains elusive. Here, we report a systematic investigation of the photoresponse generated in homogenous NbSe2 nano-flakes near the superconducting critical temperature (Tc). By combining scanning photocurrent microscopy and classic photoconductivity measurements, three distinctive mechanisms of the photoresponse are established, including the band bending at the NbSe2–metal junction, the perturbation of the superconducting state, and the photo-bolometric effect. Among them, the photo-induced phase transition from the superconducting to normal state results in an extremely large photocurrent, which is tunable by the bias voltage and is consistent with the observation via the electrical transport characterization. The photoresponsivity of our device reaches 42.3 A/W, and the response time is less than 2 μs at T = 3.8 K for an excitation in the visible wavelength, whose performance could be further improved by optimizing the device design and the experimental condition. Our result sheds light on ultrasensitive broadband photodetection with atomically thin NbSe2 and points to a potential means of probing the correlated electronic phases by exploring light–matter interactions. [ABSTRACT FROM AUTHOR]