Cavity Configuration Research Articles

An innovative small-sized double cavity PTC under investigation and comparison with a conventional PTC

In this study, a novel evacuated double cavity receiver parabolic trough collector is being examined in detail and compared with a conventional geometry. The collectors were investigated optically and thermally. Firstly, the optical efficiency of the cavity configuration was optimized and a tracking error analysis was conducted. Next, the collectors were optimized regarding the mass flow rate and the thermal analysis followed. Syltherm-800 was used as the working medium of this analysis. Several different inlet temperatures were examined (20°C–300°C) considering the optimum volumetric flow rate in each case. The comparison showed a clear prevalence of the suggested collector in the whole operating range with the mean enhancement reaching approximately 16%. These geometries were, also, compared with the same to the double cavity geometry, where the inner cavity surfaces are non-selective. The results indicated that the non-selective configuration exceeds the conventional one in thermal performance by 10.73% on average. The simulation results were, also, validated with several theoretical models and a sufficient agreement was achieved. In particular, the deviations were found to be less than 5% in both models for the Nusselt number and the Darcy friction factor. SolidWorks was used as the design and the simulation tool of this study.

Single-frequency lasing in a CW TEМ00 Nd:YVO4 disk laser with an M-type degenerate cavity and three-beam pumping

We demonstrate a continuous-wave TEM00 Nd:YVO4 disk laser with three-beam pumping and a two-mirror degenerate cavity configuration operating in a single-frequency regime without any intracavity wavelength selectors. The estimated lasing linewidth from the measurement was less than 200 MHz at the maximum output power of 1.35 W.

Physics-guided, data-refined modeling of granular material-filled particle dampers by deep transfer learning

This study presents a novel transfer learning (TL)-based multi-fidelity modeling approach for a set of granular material-filled particle dampers (PDs) with varying cavity height and particle filling ratio, targeting to realize vibration/noise mitigation across a broad frequency band. The dynamic characteristics of this kind of dampers are highly nonlinear and depend on a number of features such as particle material and size, cavity configuration, filling ratio, excitation frequency and amplitude, etc. While deep neural network (DNN) has demonstrated success in a variety of fields including nonlinear dynamics, DNN is a data-hungry modeling approach and tends to yield inaccurate or inadequate models for high-dimensional nonlinear problems when data are scarce or expensive to collect. In this paper, we propose a multi-fidelity approach for characterizing the dynamics of granular material-filled PDs by combining low-fidelity data from an approximate governing/constitutive equation and high-fidelity experimental data in the context of deep TL. Making use of the low-fidelity data, a DNN is first trained to represent a mapping between input parameters (cavity height, particle filling ratio, excitation frequency and amplitude) and output parameter (damper energy loss factor). Then, in compliance with the deep TL philosophy, the weights and biases in all layers of the pre-trained DNN except a few outermost layers will be frozen, while those in the outermost layers are re-trained using the experimental data to formulate a multi-fidelity DNN. The modeling capability of this multi-fidelity DNN model developed by the deep TL strategy is compared with a DNN model with the same architecture but trained using only the experimental data. Results show that the multi-fidelity DNN model offers much better performance than the DNN model trained using only the experimental data for characterizing the PD dynamics across a broad frequency band from 100 to 2000 Hz. Since the formulated model is versatile to varying cavity height and particle filling ratio and accommodates different excitation frequencies and amplitudes, it is amenable to use in the optimal design of PDs.

Acoustic instability prediction of the flow through a circular aperture in a thick plate via an impedance criterion

We investigate the mechanisms leading to acoustic whistling for a jet passing through a circular hole in a thick plate connecting two domains. Two generic situations are considered. In the first one, the upstream domain is a closed cavity while the downstream domain is open, leading to a class of conditionally unstable modes. In this case, the instability source lies in the recirculation region within the thickness of the plate, but coupling with a conveniently tuned resonator is needed to select the conditional instability range. In the second situation, the two regions, upstream and downstream of the hole, are considered as open, leading to a class of hydrodynamic modes where instability of the recirculation region is sufficient to generate self-oscillations without the need of any resonator. A matched asymptotic model, valid in the low Mach limit, is used to derive a global impedance of the system, combining the impedance of the hole and the modelled impedances of the upstream and downstream domains. It is shown that the knowledge of this global impedance along the real $\omega$ -axis provides an instability criterion and a prediction of the eigenvalues of the full system. Validations against the solutions of the eigenvalue problem obtained from the linearized fully compressible formulation confirm the accuracy of the approach. Then, it is subsequently used to characterise the range of existence of instabilities as a function of the Reynolds number, the Mach number, the aspect ratio of the hole and (for the cavity configuration) the dimensionless volume of the cavity.

Flow Behaviors of Polymer Solution in a Lid-Driven Cavity.

In this work, a numerical study of polymer flow behaviors in a lid-driven cavity, which is inspired by the coating process, at a broad range of Oldroyd numbers (), is carried out. The Reynolds number is height-based and kept at . The fluid investigated is of Carbopol gel possessing yield stress and shear-thinning properties. To express rheological characteristics, the Herschel–Bulkley model cooperated with Papanastasiou’s regularization scheme is utilized. Results show that the polymer flow characteristics, i.e., velocity, viscosity, and vortex distributions, are considerably influenced by viscoplastic behaviors. Additionally, there exist solid-like regions which can be of either moving rigid or static dead types in the flow patterns; they become greater and tend to merge together to construct larger ones when increases. Furthermore, various polymer flow aspects in different cavity configurations are discussed and analyzed; the cavity width/aspect ratio and skewed angle are found to have significant impacts on the vortex structures and the formation of solid-like regions. Moreover, results for the critical aspect ratio at which the static dead zone is broken into two parts and the characteristic height of this zone are also reported in detail.

Conserved spacing multi-wavelength mode-locked erbium-doped fiber laser based on barium titanate saturable absorber

A compact multi-wavelength harmonic passive mode-locked (MWHML) erbium-doped fiber laser (EDFL) has been demonstrated by integrating a Barium Titanate (BaTiO3) in the ring cavity configuration. The BaTiO3 with 9.5% modulation depth serves as a saturable absorber (SA) and is optically deposited by the Top-Down method to form an end face fiber integrated saturable absorber. Both the saturable absorption property and the large third-order nonlinearity of BaTiO3 assisted in generating a second-order harmonic pulse beside the fundamental frequency. Moreover, in real-time, eight wavelengths around 1560 nm with 0.7 nm wavelength spacing were produced with a maximum output power of 2.7 mW pulse duration of 5.2 ps. These results further suggest that BaTiO3 could be a good candidate for the saturable absorber and high nonlinear photonic device for related applications.

Mixing and Evolution of Laterally Injected Liquid Fuel in Mach 2 Air Channel with Cavity

ABSTRACT The present study deals with the flow structure and mixing characteristics of liquid fuel injected laterally into a supersonic crossflow. The geometry, with a cavity downstream of the injector, mimics a supersonic combustor with a cavity flame holder. A variety of injection schemes and cavity configurations were investigated by means of high-speed schlieren photography. A reflected shock tunnel was employed to generate a Mach 2 air flow and to simulate the high-temperature environment in the combustor. JP-4 liquid fuel was injected at different jet-to-crossflow momentum flux ratios upstream of and from the front wall of the cavity. Results showed that in direct injection into the cavity adjusting the injection angle has significant effect on the local fuel-air ratio. Upstream injection far from the leading edge of the cavity yielded more effective fuel mixing and shorter dissipation length (Ld) than closer injection, due to the development of a vortex on the lee side of the jet core. These results suggest that a combination of upstream injection far from the leading edge of the cavity and inclined direct injection into the cavity may be beneficial for effective mixing in a supersonic combustor cavity flame holder.

Development of adaptive stereotactic radiotherapy method in treatment of primary malignant glial tumors in the brain

At present, there are some scattered evidence data pertaining to the use of an adaptive technique of radiotherapy in treatment of malignant glial tumors of the brain. Our evidence data obtained in MRI in the course of treatment show that the initial treatment plan may become irrelevant due to some changes observed in the tumor configuration. In its turn, it bears witness to the topicality of developing and introducing adaptive methods and techniques in the brain tumor treatment, which are capable to increase efficacy and tolerability in patients with primary malignant tumors of the brain. Aim. Our aim has been to develop an adaptive stereotactic method of radiotherapy in treatment of primary malignant brain tumors, which shall be capable to increase efficacy and tolerability of radiation therapy as well as reduce radiation dose to normal structures in the brain. Materials and methods. Our method has been elaborated with recruiting a group of 10 patients diagnosed with primary glioblastoma G IV, which have received specialized treatment at the National Medical Research Centre for Oncology at the Ministry of Health, the Russian Federation, in the period 2021-2022. The average age of the above patients is 43,4 years. All patients have undergone microsurgery covering the total resection of the tumor (3 patients) and subtotal removal of the malignant tumor (7 patients). The average time interval between the surgery and radiotherapy is 32,5 days. Before treatment, an intravenous contrast enhancement MRI of the brain with an axial pitch of 1 mm has been conducted employing the contrast T1, contrast-free T1 and T2 FLAIR modes. Planning of radiotherapy for this sort of patients has been carried out employing BrainLab Elements и Varian Medical Systems Eclipse. The CTV was defined as a 2,0 cm margin around GTV with an anatomical correction. The CTV-to-PTV margin was 0,1 cm. Doses have been administered as follows: a single dose of 2 Gy up to a total dose of 60 Gy in 30 fractions. The brain has been MRI-scanned in all patients with the use of intravenously introduced contrast agents with an axial pitch of 1 mm employing enhanced contrast/contrast-free T1 sequences and T2 FLAIR to redefine the radiotherapy targets (GTV, CTV, PTV). At fraction 30 we have completed MRI in order to estimate the treatment outcome. In the average, the radiotherapy course has taken 42-45 days, holidays included. For the purpose of the radiation therapy, used have been the Novalis Tx Varian Tx linear accelerator of kinetic energy of the beam of 6 MeV. Results We have developed our own adaptive stereotactic method of radiotherapy to treat the primary malignant glial tumors in the brain, which is capable of tracing the configuration of the post-surgery cavity, the residual tumor and the brain structures in the course of radiotherapy and adapting the therapy plan thereto that makes possible to reduce tissue volumes exposed to radiation due to a decrease in the tumoral and peritumoral volumes of the tumor and post-operative cavity. Conclusion. Our analysis has shown that in the course of radiotherapy some anatomical changes in the tumor configuration are found. An adaptive approach applied to radiation therapy allows monitoring the above changing volumes and correcting the treatment plan.

Tunable multiwavelength Erbium-doped fiber laser based on in-fiber Fabry-Perot interferometer fiber Bragg gratings in linear and ring cavity configurations

Multiwavelength Erbium-doped fiber laser (MWEDFL) in the ring and linear cavities operating at 1.5-μm wavelength region was demonstrated. The multiwavelength fiber laser comprises a 3-m Erbium-doped fiber as an active gain medium, an in-fiber Fabry-Perot interferometer Fiber Bragg Gratings (FPI FBG) as a filter, and a 10-km dispersion compensating fiber (DCF) as a nonlinear gain medium. A comparison investigation was conducted. The MWEDFL in the linear cavity generates 12 stable lasing lines, with an optical signal-to-noise ratio (OSNR) of 45 dB. In the case of MWEDFL operated in a ring cavity, 8 stable lasing lines were generated, with an OSNR of 47 dB. The free spectral range (FSR) of 0.08 nm was measured for the FPI FBG in both linear and ring cavities MWEDFL spectrum. Besides, the stability measurement was conducted for 150 min for both MWEDFL cavities. Both lasers had a peak power fluctuation of less than 1.0 dB and negligible wavelength drifts below 0.1 nm. Furthermore, the wavelength tunability of MWEDFL in both configurations was performed by heating the FPI FBG from 28 °C to 100 °C. At the temperature of 100 °C, the MWEDFL shows a wavelength shift of 1.0 nm, and this can be further increased by heating the FPI FBG to its highest temperature limit. This MWEDFL employs in-fiber FPI FBG offers a compact, highly tunable, and simple design applicable for optical communications and optical sensing applications.

Probing the mode-locking pattern in the parameter space of a Figure-9 laser.

Due to the many available cavity configurations, a generalized approach for identifying the optimal operating state of a Figure-9 mode-locked laser has proved a challenge. In this Letter, we probe the output pulsation states of an exemplar Figure-9 laser by meticulously scanning its parameter space. Regions corresponding to mode-locked operations are identified periodically in the map of the output states. We correlate these regions to a set of band-like cavity transmission functions that fundamentally allow ultra-short pulse formation. Interestingly, a clear correlation between the mode-locking pattern and the cavity configuration is observed. For example, with the decrease of the fiber loop symmetry in the cavity, half of the solutions in the mode-locking pattern are found to transit to forbidden states. Numerical calculations based on the Jones matrix are used to explain the experimental observations. In addition, the dynamic change of the map of output states is illustrated by using a setup with an automatic algorithm. Our results provide a visually-rich yet simple way for evaluating and optimizing a Figure-9 laser.

Dense-pattern multi-pass cavity based on spherical mirrors in a Z-shaped configuration for Raman gas sensing.

We report a dense-pattern multi-pass cavity (MPC) based on four spherical mirrors placed in a Z-shaped cavity configuration for improving the Raman signals from gases. The folding structure of the cavity causes dense patterns of spots, and at least 420 beams are reflected in the cavity. Raman spectra of ambient air, methane, and ethylene are recorded to demonstrate the performance of our apparatus. At atmospheric pressure, ppm-level detection limits of the gases are achieved with 10 s of exposure time. The Raman signal intensities of the gases show excellent linearity with the gases' partial pressures, which means that high-accuracy detection is also feasible.

Synchronously Intracavity-Pumped Picosecond Optical Parametric Oscillators for Sensors.

The research and development of laser systems for intracavity phase interferometry is described. These systems are based on an intracavity synchronously pumped optical parametric oscillator (OPO), enabling the generation of two trains of picosecond pulses inside a single cavity. In such a configuration, it is possible to measure the beat note frequency between two pulses and to very precisely determine the phase difference between them. The pump source is a diode-pumped passively mode-locked Nd:YVO laser. A periodically poled magnesium-doped lithium niobate crystal is used as the optical parametric oscillator crystal coupling the pump and the signal cavities. We designed a synchronously pumped OPO in a linear and ring cavity configuration allowing generation in a dual-pulse regime. By a mutual detuning of both cavity lengths, the quasi-synchronous regime of pumping was achieved and high harmonics of repetition rate frequencies were generated. Such a system can be useful for applications such as pump-probe spectroscopy or for testing telecommunication systems. We also realized the subharmonic OPO cavity as a source of two independent trains of picosecond pulses suitable for intracavity phase interferometry; we also measured the beat note signal.

Mode III analysis of a piezoelectric rectangular plane weakened by multiple cracks and cavities

This study presents an analytical model for a piezoelectric rectangular plane weakened by multiple cracks and cavities, under in-plane electric displacement and anti-plane shear stress. The defects are considered either electrically permeable or impermeable. First, the piezoelectric rectangular plane is weakened by a Volterra-type screw dislocation. The electric and stress displacement fields of the domain under consideration are determined using finite Fourier transform. Then, the problem is reduced into a set of Cauchy integral equations in the piezoelectric rectangular plane using the distributed dislocation approach. The solution to these integral equations determines electric displacement and stress intensity factors and hoop stress around cavities in the piezoelectric rectangular plane. Also, the boundary of the plastic zone is found using the stress field in the vicinity of crack tips and the Von Mises yield criterion. Finally, multiple numerical results are provided to illustrate the capability of the dislocation method in handling different cases of crack and cavity configurations and arrangements.

Measurement Uncertainty, Purity, and Entanglement Dynamics of Maximally Entangled Two Qubits Interacting Spatially with Isolated Cavities: Intrinsic Decoherence Effect.

In a system of two charge-qubits that are initially prepared in a maximally entangled Bell’s state, the dynamics of quantum memory-assisted entropic uncertainty, purity, and negative entanglement are investigated. Isolated external cavity fields are considered in two different configurations: coherent-even coherent and even coherent cavity fields. For different initial cavity configurations, the temporal evolution of the final state of qubits and cavities is solved analytically. The effects of intrinsic decoherence and detuning strength on the dynamics of bipartite entropic uncertainty, purity and entanglement are explored. Depending on the field parameters, nonclassical correlations can be preserved. Nonclassical correlations and revival aspects appear to be significantly inhibited when intrinsic decoherence increases. Nonclassical correlations stay longer and have greater revivals due to the high detuning of the two qubits and the coherence strength of the initial cavity fields. Quantum memory-assisted entropic uncertainty and entropy have similar dynamics while the negativity presents fewer revivals in contrast.

Enhanced energy trapping in a nonreciprocal phononic crystal cavity

Acoustic energy trapping using defect modes in the band gap frequencies of acoustic metamaterials has been widely explored. Unlike this extensively used mechanism, the present work demonstrates the use of non-reciprocity in the transmission band to trap energy inside a phononic crystal cavity. A phononic crystal with broken parity and time reversal symmetry can be used to generate linear nonreciprocal transmission of the ultrasound waves. A gradient induced differential dissipation (GIDD) based passive non-reciprocal phononic crystal with asymmetric scatterers was employed to create three configurations of cavities. The parity of the present system is broken by the asymmetric shape of the scatterers, and the time reversal symmetry is naturally broken by viscous dissipation. The cavity configurations were based on the orientations of the asymmetric scatterers in part of the crystal that allowed utilization of non-reciprocity involved in only one of the cavities' configurations. Enhancement of energy trapping at a frequency of 622.5 kHz was observed experimentally for the cavity utilizing nonreciprocity compared to other cavity configurations. Experimental results were further confirmed and comprehended using finite element method based computational outcome. This energy trapping device is linear, robust, and allows sound energy trapping without an external energy source.

Comparison of multimode GRIN-fiber Raman lasers with FBG and random DFB cavity

Abstract Raman lasing in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to wavelength-agile fiber lasers of new type. Here we compare power scaling and brightness enhancement capabilities of Raman laser based on multimode GRIN-fiber of 62.5/125 um core/cladding diameters pumped by ∼700 W multimode source with beam quality M2∼10, performed in two different cavity configurations: 1) linear cavity based on two fiber Bragg gratings and 2) half-open cavity with one FBG and random distributed feedback via Rayleigh backscattering along the GRIN fiber.

Machine Learning to Predict Quasi TE₀₁₁ Mode Resonances in Double-Stacked Dielectric Cavities

We have applied machine learning in a neural network to calculate the quasi TE₀₁₁ mode of a cylindrical microwave cavity with two symmetrically stacked dielectric resonators (DRs) inside, with aspect ratios of the overall cavity being limited to the range of 0.25&#x2013;4. The neural network was trained with 99 970 samples and evaluated using 9564 samples from a holdout dataset. The samples were created using a supercomputer to solve random cavity configurations via finite-element method (FEM) programming. The trained neural network predicts the resonant frequency of the quasi TE₀₁₁ mode and expresses the mode in terms of expansion coefficients of empty cavity TE<inline-formula> <tex-math notation="LaTeX">$_{\mathrm {0\,np}}$ </tex-math></inline-formula> modes, from which plots of the electric and magnetic fields can be made. The predictions are extremely quick, taking &#x007E;0.05&#x2013;0.2 s running on a typical personal computer, and are very accurate when judged against the FEM results: the overall median error in the frequency neural network is 0.2&#x0025;, and the overall median error of the expansion coefficients neural network is 0.003&#x0025;. This should allow designers to much more rapidly determine optimal cavity and DR dimensions and other parameters in order to achieve the frequency and mode they desire, with a speedup of approximately <inline-formula> <tex-math notation="LaTeX">$10\,\,000\times $ </tex-math></inline-formula> compared with FEM calculations alone. A link to the Python implementation of our FEM code and our trained neural network code is provided.

Simulation of near ideal-Bessel beam generation by a thin-disk laser configuration

The annular Bessel beam output from an axicon-based thin-disk laser resonator has been investigated under the particular assumption of the cavity configuration. The numerical results show the purity of the output beam, which is one of the essential properties of the thin-disk laser. Cavity diffraction loss, stability, and full width at half minimum of the annular beam are surveyed in detail.

The investigation of inclined AFT wall cavities in a circular scramjet combustor

Achieving good mixing with high combustion efficiency, stable flame holding, and low stagnation pressure losses are still open issues in supersonic reacting flows. Mixing and vorticity in supersonic flows are mainly driven by the baroclinic term that is correlated to the density and pressure gradients. An increase of the baroclinic term has been observed to strongly depend on shock waves arising within the combustion chamber. This work numerically investigated the interaction between the fuel jet and the air stream as a function of the aft wall angle inclination. In fact, this parameter has a primary impact on the shock wave inclination arising from the cavity leading edge and therefore on the baroclinic term. Four different combustor geometries have been investigated: without cavity and with three different aft wall inclinations (90°–15°, 60°–15°, 30°–15°). 3D RANS numerical simulations showed that the primary ramp angle has a critical impact on the fuel-air mixing and consequently on the combustion efficiency. A correlation between the combustor geometry and the shock angle inclination has been proposed. Greater combustion efficiency, homogeneous H2O, and temperature distribution across the combustor are found with the 30–15 aft wall cavity configuration. Lowering the primary aft wall of the cavity from 90° to 30°, resulted in the rising of a stronger bow shock, extended recirculation volume within the cavity, and increased fuel-air mixing.

Tunable 2.3–3 μm optical vortex parametric laser

We generate a tunable (2.3–3 μm) optical vortex output with an order of 1 or 2 from a 1 μm optical-vortex-pumped singly resonant parametric oscillator based on a Mg doped periodically poled lithium niobate crystal. The orbital angular momentum (OAM) from the pump vortex beam was transferred to the mid-infrared idler output in this signal singly resonant high-Q cavity configuration. A maximum vortex output energy of 1.52 mJ was achieved within a wavelength tuning range of 2.27–2.96 µm. The signal beam with the Gaussian spatial profile was also generated within a 1.66–2 μm tuning range. The wide spectral bandwidth was experimentally measured near the wavelength degeneracy and explained by theoretically.