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During the cruise status of an underwater high-speed vehicle, the flow around the vehicle's head typically transitions from a laminar to a turbulent state, triggering flow noise that can interfere with the normal operation of sonar. In order to accurately investigate this flow noise through numerical simulation, a high-fidelity turbulent flow field solution is essential. Common traditional turbulence numerical simulation methods, such as unsteady Reynolds-averaged simulation and large eddy simulation (LES), struggle to capture high-frequency turbulent fluctuations accurately due to their inability to directly resolve small-scale eddy structures, which results in compromising the simulation accuracy of high-frequency flow noise. To address this issue, this paper employs direct numerical simulation (DNS) to achieve high-fidelity resolution of the turbulent flow field, thereby enabling a more accurate assessment of flow noise distribution on the vehicle's surface. Meanwhile, considering significant computational resources required to solve the entire flow field in an underwater high Reynolds number environment, this study also incorporates the fixed transition modeling method and stability theory to confine the DNS computational domain to the vicinity of the transitional zone to improve simulation efficiency. Comparative analysis of flow noise monitoring results in the laminar, transitional, and turbulent zones revealed that the flow noise source in the laminar zone exhibits the lowest amplitude across all frequencies, while the flow noise source in the transitional zone features the highest amplitude, approximately 10 dB higher than that in the turbulent zone. Moreover, significant amplitudes in high-frequency components (above 30 kHz) are detected in both the transition and turbulent zones. Additionally, this study employs LES with the Smagorinsky model to simulate the flow field within the same computational domain as DNS, demonstrating the limitations of the Smagorinsky model-based LES in capturing high-frequency flow noise. [ABSTRACT FROM AUTHOR]

To date, research on defect-induced magnetism in SiC has been conducted to clarify the relationship between the ferromagnetic signal and the carrier concentration. It has been experimentally proven that there is an interaction between the d 0 magnetic moment and the hole carrier in p-type 4H-SiC. However, for n-type SiC, the existing theoretical predictions are insufficient to explain the variation in magnetization with the doping concentration. To solve this problem, we prepared 4H-SiC epitaxial layers with different nitrogen doping concentrations and introduced defects by ion implantation. By measuring and analyzing the magnetic properties, we found that the ferromagnetic composition depends on both the implantation dose and the doping concentration. By performing first-principles calculations, we studied the magnetic moments and interactions of defects with different charge states, which is related to defect-induced ferromagnetism. These defects include not only the paramagnetic centers reported in previous studies, such as silicon vacancies and divacancies, but also the N C V Si complex defect of recent interest, which are indicated by positron annihilation experiments. Combining experimental observations with theoretical calculations, we explained the relationship between magnetic properties and the nitrogen doping concentration in the epitaxial samples. Our research will help us to better understand the physical mechanism of defect-induced magnetism in doped semiconductors and provide a potential platform for the control of defect-induced magnetism by carrier density modulation and the fabrication of SiC spintronic devices without transition metals. [ABSTRACT FROM AUTHOR]

Significant amounts of work have been conducted in the area of plasma flow control, while the receptivity of plasma actuation in high-speed boundary layers has not had much attention over the last two decades. In the present study, the receptivity of a Mach 4.5 flat-plate boundary layer to plasma heating actuation produced by pulsed-DC surface dielectric barrier discharge (SDBD) has been studied by direct numerical simulation (DNS) and stability analysis. With the help of multimode decomposition technology, the amplitude of normal modes can be obtained. The results show that both fast and slow modes can be excited by plasma actuation, and the receptivity maximum is observed near the lower neutral branch. Because the pulsed-DC SDBD actuation is typical periodic pulse signals, when the total power remains constant, the Fourier components with multiples of actuation frequency have the same energy, regardless of the waveform, period, and width of the actuation signal. Such characteristics benefit the robustness of the pulsed-DC SDBD actuator. A theoretical prediction method by combining the receptivity model and linear parabolized stability equations is considered in the present study, and good agreement with the DNS results is achieved. [ABSTRACT FROM AUTHOR]

To study the effect of magnetohydrodynamic actuation on wedge shock waves under hypersonic flow, experiments were conducted in a Mach 7 hypersonic shock tunnel. A schlieren method was used to visualize the shock waves and the flow, which showed that the shock waves raised and moved forward under 0.37 T. The experimental results were analyzed in detail by numerical simulation. A theoretical model was designed to investigate the effect of Joule heat and the Lorentz force. The load factor (K) was defined to show the dominant factor, and the results of the calculation showed that Joule heat was the dominant factor in this experiment. From further analysis, we conclude that increasing the magnetic field intensity, improving the gas conductivity, arranging the actuation near the wedge head, and increasing the width of the actuation region properly may be helpful in achieving a better control effect. [ABSTRACT FROM AUTHOR]

GaSb and InSb epilayers grown on GaAs (001) vicinal substrates misoriented toward (111) plane were studied using high resolution x-ray diffraction. The results show that GaSb and InSb epilayers take on positive crystallographic tilt, and the asymmetric distribution of 60° misfit dislocations in {111} glide planes have an effect on the tilt. In addition, the vicinal substrate influences the distribution of the threading dislocations in {111} glide planes, and the density of dislocation in the (111) plane is higher than in the [formula] plane. A model was proposed to interpret the distribution of full width at half maximum, which can help us understand the formation and glide process of the dislocations. [ABSTRACT FROM AUTHOR]

We investigated the magnetic structure in epitaxial Fe films on GaAs(001) by taking advantage of planar Hall effect combining with static and dynamic magnetization measurements. The depth dependence of the magnetic structure was evidenced as a result of competition between in-plane interfacial uniaxial and bulk cubic magnetic anisotropies. The competing results exposed by these techniques allow us to image a spiral magnetic structure nearby the interface of the Fe/GaAs(001) system. This work provides an insight for electrical and magnetic properties in an ultrathin hybrid ferromagnet/non-magnet system. [ABSTRACT FROM AUTHOR]

Presents an X-ray diffraction study of a GaAs/InAs/GaAs ultrathin single quantum well. Simulation method; Results; Discussion.

Presents a study that investigated gallium arsenide epilayer films on silicon substrates grown by molecular-beam epitaxy by the x-ray double-crystal diffraction method. Growth process of the epilayers; Function of SLS in improving the quality of gallium arsenide epilayers; Relative perfection of epilayers at different depths.

Nanostructured hexagonal InN overlayers were heteroepitaxially deposited on vertically oriented c-axis GaN nanorods by metal-organic chemical vapor deposition. InN overlayers grown in radial directions are featured by a nonpolar heteroepitaxial growth mode on GaN nanorods, showing a great difference from the conventional InN growth on (0001) c-plane GaN template. The surface of InN overlayers is mainly composed of several specific facets with lower crystallographic indices. The orientation relationship between InN and GaN lattices is found to be [0001]InN∥[0001]GaN and [11_00]InN∥[11_00]GaN. A strong photoluminescence of InN nanostructures is observed. [ABSTRACT FROM AUTHOR]

The uniaxial strain-induced electronic structure variations in intrinsic and Al doped (3C-, 4H-) SiC are studied by using first-principle calculations. The main findings are summarized as follows: (I) the tensile strain leads to a structural transformation in Al doped 3C-SiC, which is signed by the total energy and lattice characteristics; (II) the band gap Eg modulation with large reductions is achieved by applying strains up to 2% in all compounds; (III) The high-symmetry points of valence band maximum and conduction band minimum in intrinsic and Al doped 4H-SiC show the abnormal characteristic under strain, corresponding to the maximum Eg as the strain is ∼-1%. These results suggest that the strain approach could be used to tune the electronic structures of SiC compounds. [ABSTRACT FROM AUTHOR]

The influences of microdefects and dislocations on the lattice parameters of undoped semi-insulating GaAs single crystals were analyzed, and a novel nondestructive method for measuring stoichiometry in undoped semi-insulating GaAs was established in this letter. The comparison of this method with coulometric titration indicates that the method of nondestructive measurements is indeed convenient and reliable. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]