Straight Cavity Research Articles

Large Eddy Simulations of a Francis Turbine Operating at Ultra-Low Load to Overload Conditions

Due to the increasing global power demand, hydropower plants face the challenge of operating at flow rates far from their best efficiency point to comply with the electrical grid. Consequently, turbine units experience unstable high swirling flows under different load conditions, reducing the life of system components. Understanding the mechanisms behind the onset and sustenance of these instabilities in the swirling flow leaving the turbine runner is crucial for improving turbine performance. To achieve this, large-eddy simulations (LES) were conducted to characterize the unsteady flow inside an industrial-sized Francis turbine. The turbine was studied under a wide range of flow rates, 100, 80, 60, 40, and 120% of the design flow rate. A high swirl velocity was introduced as the flow rate deviated from 100% load, causing significant changes in the flow structure. An organized structure of vortex filaments in a helical pattern is seen inside the draft tube at 80 and 60% load, and the 40% load case showed an unorganized collection of small-scale vortices. A straight vortex cavity is seen in the draft tube center at 120% excess load. At the best design point, the magnitude of the swirl velocity was 0.1 times the tip speed. However, as the flow rate dropped to 40% of the design flow rate, the magnitude increased to 0.4 times the tip speed near the wall. In addition, the 40% load case had the highest magnitude of pressure fluctuations, over 25 times the peak design case near the runner. This showed a very unsteady flow field could damage the system or cause unstable operation at ultra-low partial loads. Generated power dropped dramatically as the flow rate decreased, and more noise was present in the power signal.

Wide-wavelength-tunable operation of Tm:GYAP disordered crystal laser with birefringence filtering

A disordered crystal of Tm:GYAP with a doping concentration of 5 at% was designed and grown. Test results show that the fluorescence spectral half-height width of this crystal is 203 nm, which implies that this crystal is capable of realizing a wide wavelength-tunable laser operation in the 2 µm band. In this study, the Tm:GYAP crystal based laser with a center wavelength of 1936.2 nm and a spectral width of 7.56 nm had been realized by using a compact straight cavity structure. Under a pumping of 8.55 W, a maximum output power of 0.841 W was obtained, and the corresponding optical-to-optical conversion efficiency and the slope efficiency were 9.8% and 12.7%, respectively. In addition, the wavelength-tunable laser operation of Tm:GYAP crystal was realized for the first time by inserting a birefringent filter. The tuning range of the output laser exceeded 120 nm (from 1911.8 nm to 2039.5 nm) even in the compact straight cavity.

A compact continuous-wave intracavity frequency doubling Pr:YLF ultraviolet laser at 360 nm

We demonstrate a continuous-wave (CW) ultraviolet (UV) laser operating at 360 nm, achieved through intracavity frequency doubling of a Pr:YLF laser. This UV laser is more compact and simpler compared to previously reported designs, due to the adoption of a straight cavity. The total length of the laser system measures 75 mm, with a resonant cavity length of only 25 mm. By utilizing an InGaN laser diode (LD) emitting at 444 nm with a maximum incident power of 3.1 W, we were able to achieve a CW UV laser with an output power of 252 mW. The optical to optical conversion efficiency reached up to 8.1%, and the output power stability remained within ±1.9% over a period of two hours. This innovative UV laser has potential applications in various fields such as spectral analysis, material analysis, bioengineering, optoelectronic detection, and medical treatment.

Narrow-linewidth stable-spectrum diode laser source basing on polarization-separated external cavity feedback technology

In this study, we propose a polarization-separated external cavity feedback structure based on a volume Bragg grating (VBG) that folds the optical path of a free-running diode laser into a non-straight direction for linewidth narrowing and central wavelength locking. By employing diode laser polarization splitting technology and external cavity feedback technology, a similar output power and narrow linewidth but a smaller current drift coefficient of central wavelength are demonstrated by comparing with the standard straight external cavity feedback structure. Experimental results show that the current drift coefficient of central wavelength reduce to 1/5 that of the traditional method. According to our research, this is the first report on folding the extended external cavity feedback optical path in a non-straight direction basing on polarization splitting technology, providing a new method for external cavity feedback based on the VBG.

Switching of several self-mode-locking states in an optically pumped semiconductor disk laser

Self-mode-locking (SML) of fundamental TEM00 mode and high-order transverse mode were demonstrated in an optically pumped semiconductor disk laser and these states can be switched by simply adjusting the elements of the laser. The repetition rate of the pulse train was 1 GHz, using a straight cavity. Furthermore, we measured the pulse durations and found that the pulse duration of SML pulse with higher-order transverse mode is shorter than that of fundamental mode at the same pump power. To the best of our knowledge, this is the first experiment to obtain two SML states in a single laser. By using the method of changing the cavity length and adjusting the angle of the output coupler, the repetition rate can be continuously tuned from 3 GHz to 3.2 GHz.

Curvature effect on the instabilities of lid-driven flow in a toroidal square cavity

The effect of curvature on the three-dimensional instabilities of lid-driven flow in a toroidal square cavity is numerically studied using the linear stability analysis based on the spectral element method. The critical stability curves are determined, and four types of instability modes are predicted. For small curvatures (δ ≤ 0.2), the most dangerous mode is a steady short-wavelength mode (λc ≈ 0.4), which is consistent with that in the straight square cavity. In a narrow range of 0.22 ≤ δ ≤ 0.28, the critical mode is oscillatory and of medium wavelength (0.84 < λc < 1.07). For δ ≥ 0.29, two types of steady long-wavelength modes (λc > 1.26) become the critical mode. The disturbance energy analyses show that all the instabilities are caused by the centrifugal instability mechanism.

Heat transfer intensification of NEPCM-water suspension filled heat sink cavity with notches cooling tubes by applying the electric field

The effects of an electric field on heat transfer, free convection, and entropy generation of nano-encapsulated phase change material (NEPCM) dissolved in water in a complex inclined chamber are studied. The enclosure consists of a rectangular cavity heated from below and the remaining walls are considered adiabatic. The fluid is cooled by eight cooling tubes, each with four notches. The effects of the electric field, inclination angle (ζ = 45° and ζ = 90°), and Rayleigh number (Ra = 103, 104, and 105) in equal concentration of NEPCM (ϕ = 2 %) are studied using ANSYS Fluent CFD code. The dimensionless form of fluid governing equations, electric potential equation, electric charge density equation, and entropy generation equation are used to set the primitive parameters. Grid verification and validation tests are performed to ensure the reliability of the solution. Calculations show that with increasing Ra, the average Nusselt number (Nuave) decreases from ζ = 45° to 90°, but when an electric field is applied, the results are reversed. As the Rayleigh number increases in conjunction with the electric field, the buoyancy effect becomes more pronounced. Furthermore, owing to its high heat transfer potential and lowest irreversibility, the straight cavity (ζ = 90°) is desirable at Ra = 105.

Sound absorption characteristics of thin parallel microperforated panel (MPP) for random incidence field

In this study, the absorption coefficient of a thin parallel MPP for random incidence field is investigated. A coiled backing air cavity realizes the thin structure, and the parallel configuration is applied to have a wider sound absorption bandwidth than a single MPP. The effects of the oblique sound incidence on the thin parallel MPP are investigated by numerical studies for both single oblique incidence and random incidence. These effects are significant from a practical standpoint, whereas the result of a coiled backing air cavity must be carefully evaluated. This paper focuses on developing, modeling, and evaluating parallel MPP with a coiled backing cavity. It is found that the oblique incidence can result in lower sound absorptions and shift in resonance absorptions compared to the normal incidence angle, especially for high incidence angles (>70°). In addition, the performance of the thin MPP at a normal incidence angle can be maintained up to the incident angle of 70°, where the sound absorption coefficients of 0.8 and above are present. Compared with a straight backing cavity, the use of coiled one does not introduce deviation in sound absorption characteristics except for the cavity modes slightly higher resonance frequency under the same axial length. Moreover, the proposed structure behaves as a locally reacting material as long as the cavity’s longest dimension is smaller than half an operating wavelength for the targeted frequency range. This allows the sound absorption characteristics of normal incidence and diffuse sound field excitation is insignificantly different. Hence, this characteristic can deal with two folded problems in terms of bulky absorber issue and sensitive performance on oblique incidence.

Theoretical and Experimental Studies on Microwave Pulse Compressor Storage Efficiency

The energy storage efficiency of a microwave pulse compressor (MPC) formed by an inductive iris, a straight rectangular waveguide, and an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> -plane T-junction is theoretically and experimentally studied. The MPC storage efficiency is examined considering a cavity drive source with limited pulse duration. In this context, a theoretical model is developed to conclude to an optimal MPC cavity design taking into account the drive source pulse duration. The proposed theoretical model addresses both steady and transient state calculations at noncritical cavity-coupling conditions. The numerical results show that the MPC cavity storage efficiency optimization is feasible by selecting properly the iris’s opening width as well as the lengths of the straight waveguide cavity and the T-junction’s side arm. The validity of the theoretical design guidelines is verified by independent simulation as well as by experiments at 1.3 GHz.

Theoretical and Experimental Research on the Mode Modulation Regulation for the Mode-Tunable Vortex Laser Based on Mode Conversion and Intra-Cavity Modulation

The vortex laser beam has been widely applied in many fields for its unique properties. However, researchers have to conduct extensive and recurring experiments to find the modulation abilities of the vortex beam modes for a given resonant cavity. In this paper, a mode modulation regulation acquisition (MORA) method, investigating the relationship between the modes of the vortex beam and modulation parameters, is proposed and verified. A typical mode-tunable vortex laser, consisting of a classic plano-concave straight cavity, a vortex beam generation beamline, and a reference beam acquisition beamline, is used as the analysis and experiment object. The principle and working process of the MORA method is analyzed in the simulation, and its validity is verified in the experiment. Based on the obtained theoretical relationship between the modes of the vortex beam and modulation parameters, the MORA method could be used to help researchers in designing the practical vortex laser with target vortex beams output by optimizing the structure of the vortex laser, selecting the suitable intra-cavity modulation elements (IMEs), and pre-positioning the location of the IMEs.

Drag reduction mechanisms on a generic square-back vehicle using an optimised yaw-insensitive base cavity

Regulations on global greenhouse gas emission are driving the development of more energy-efficient passenger vehicles. One of the key factors influencing energy consumption is the aerodynamic drag where a large portion of the drag is associated with the base wake. Environmental conditions such as wind can increase the drag associated with the separated base flow. This paper investigates an optimised yaw-insensitive base cavity on a square-back vehicle in steady crosswind. The test object is a simplified model scale bluff body, the Windsor geometry, with wheels. The model is tested experimentally with a straight cavity and a tapered cavity. The taper angles have been optimised numerically to improve the robustness to side wind in relation to drag. Base pressures and tomographic Particle Image Velocimetry of the full wake were measured in the wind tunnel. The results indicate that a cavity decreases the crossflow within the wake, increasing base pressure, therefore lowering drag. The additional optimised cavity tapering further reduces crossflow and results in a smaller wake with less losses. The overall wake unsteadiness is reduced by the cavity by minimising mixing in the shear layers as well as dampening wake motion. However, the coherent wake motions, indicative of a balanced wake, are increased by the investigated cavities.Graphical abstract

Influence of circular hole defect on dynamic crack propagation behavior under impact loads

Cavities and crack defects usually coexist in deep earth rock mass structures, which together affect the structural safety and stability of rock masses. In order to study the effect of circular cavity on crack propagation behavior in rock mass under dynamic loads, circular opening specimens with straight crack cavity (COSSCC) specimen were proposed in this study, and a large-scale drop hammer impact device was applied to conduct impact tests. Crack propagation gauges were implemented to measure fracture mechanics parameters, such as dynamic crack initiation time and crack propagation velocity. Then a modified finite difference code Autodyn was applied to carry out the numerical simulation analysis of crack propagation path and stress field around the circular hole. The traditional finite element code Abaqus was also used to calculate the dynamic initiation toughness and dynamic propagation toughness. The results indicate that: (1) when the inclination θ of the circle hole is less than 10°, the crack propagation path deflects and passes through the surface of the circle hole; when the inclination θ of circle hole is 20° and 30°, the crack propagation paths deflects in the direction of the hole but does not pass through the circular hole, indicating that the circular hole has obvious guiding effect on crack propagation; when the inclination θ of circle hole is 40° and 50°, crack propagation path does not deflect, and the guiding effect of the circular hole is obvious weaken. (2) When the crack propagation path reaches the vicinity of the circular hole, the tensile stress zone at the crack tip coincides with the tensile stress zone at the edge of the circular hole. At this time, the crack propagation speed increases significantly, and the dynamic fracture toughness of the crack decreases significantly. (3) The deflection direction of the crack is basically the same as the direction of the maximum circumferential stress at the crack tip. (4) The dynamic fracture toughness of the crack is always smaller than the crack initiation toughness, and the dynamic fracture toughness of the crack has a linear relationship with the dynamic crack growth rate. The larger the dynamic crack growth rate, the smaller the dynamic fracture toughness of the crack.

Study the effect of circular hole on dynamic fracture properties of cracked PMMA specimen under impact loads

Hole defects and crack defects usually coexist in a real engineering rock masses, they together affect the structural security of rock masses. With the purpose of investigating the effect of circular hole on crack initiation and propagation process under dynamic loads, a circular opening specimen with straight crack cavity (COSSCC) model was proposed in this study and polymethyl methacrylate was chosen to make the specimens. A series of dynamic experiments were carried out utilizing a drop hammer impact test system (DHITS), crack speeds and crack initiation time were determined utilizing crack propagation gauge (CPG). The maximum principal stress field evolution in the crack evolution process was simulated using a modified finite element difference method software, and dynamic fracture toughness was calculated by a traditional finite element method software. Some significantly conclusions can be obtained: As the inclination angle θ of the circular hole is less than 10o, crack propagation trajectories will propagate to the surface of the circular hole; As the inclination angle θ of circular hole is 20o and 30o, crack propagation trajectories will turn in the direction of circular hole but not reach the circular hole; As circular hole inclination angle θ is 40o and 50o, the crack shows a pure mode I crack propagation. As crack propagates near the circular hole, crack propagation speeds increase significantly. The dynamic propagation toughness during crack propagation is not constant and is less than dynamic initiation toughness (DIT) and the dynamic propagate toughness (DPT) is inversely proportional to the crack speeds.

Compact dual-wavelength vertical-external-cavity surface-emitting laser with simple elements.

Dual-wavelength lasers with separation from several nanometers to tens of nanometers at 1 µm waveband are attractive light sources for terahertz generation. This work reports a compact dual-wavelength vertical-external-cavity surface-emitting laser with simple elements. The gain chip is regularly designed and epitaxially grown, and the laser cavity is the most common straight line cavity. By the use of a blade as the tuning element in cavity, the laser wavelength can be tuned continuously, and the stable dual-wavelength oscillation can be obtained when the blade is situated at a certain position. The total output power of 85 mW, which is limited by our pump source, is produced when the laser wavelengths are at 961 nm and 970 nm. We have also analyzed the evolution mechanism and the stability of this dual-wavelength laser.

The effect of rear cavity modifications on the drag and flow field topology of a Square Back Ahmed Body

This work presents a numerical investigation of turbulent flow over a simplified vehicle model called the Square Back Ahmed Body (SBAB). The simulations are performed at Reynolds number 1 × 105 using the k − [Formula: see text] SSTSAS turbulence model. The numerical results for the standard reference model, that is, SBAB, have been validated against available experimental data and numerical simulations. The performance of a passive flow controller with four variants on the mean wake topology and the resultant drag reduction is evaluated. The four variants are; (i) straight cavity, (ii) straight cavity with rounded edges, (iii) C-shaped cavity, and (iv) tapered cavity. For a straight cavity with a depth equal to 33.33% of the body height, drag is lowered by 5.63%. With the same cavity depth, rounding the straight cavity edges reduces the drag by 10.67% owing to the streamlining and shortening of the recirculation region. For a C-shaped cavity, the amount of drag reduction increased slightly by 1% more than that off straight cavity with rounded edges, due to improvement in the base pressure distribution compared to that of the latter case. Tapering the cavity edges at an angle of 6° gave a significant drag reduction of 22.55% primarily due to a tremendous decrease in wake size. The drag reduction achieved in all the cases results from the modification in the mean wake topology induced by afterbody shape remodeling. The power spectra of the evolution of the lift coefficient over time reveal a noticeable decrement in the flow randomness with the inclusion of a cavity and its modifications, which interprets the mitigation of the chaotic nature in the wake. The cavity presence has also increased the vorticity spreading rate in the mixing layer and produced significant attenuation of Turbulent Kinetic Energy (TKE).

Drag reduction on a three-dimensional blunt body with different rear cavities under cross-wind conditions

The use of rear cavities at the base of a square-back Ahmed body has been experimentally evaluated as a passive control device under cross-wind conditions with yaw angles β≤10°, by means of pressure, force and velocity measurements. A comparative study has been performed at a Reynolds number Re=105, considering the reference square-back body (i.e. the body without any passive control device), and the same body implementing both straight and curved cavities as add-on devices. It is shown that the performance of a straight cavity, which is widely acknowledged as a robust drag reduction device for car models, is hindered under moderate cross-wind conditions, and does not constitute an efficient control strategy, especially when compared with a curved cavity.In particular, when the freestream is aligned with the body, the curved cavity provides a stronger attenuation of the fluctuating nature and the bi-stable dynamics of the wake (characteristic of the wake behind a square-back Ahmed body) than the straight one. Besides, the reduced size of the near wake, which is provoked by flow re-orientation and the reduced span between the rear edges of the curved cavity, leads to an important base pressure recovery, that translates into relative reductions of the drag of 9.1% in comparison with the reference case (i.e. 2.6%, with respect to the straight cavity). The results are considerably improved under cross-wind conditions, since the increase with the yaw angle of the force is particularly intense for the body with the straight cavity and attenuated for the model with the curved cavity. Thus, the relative reduction of the drag coefficient with respect to the reference body becomes negligible for the straight device at a yaw angle of 10°, while it still represents approximately a 10% for the curved cavity. Furthermore, flow visualizations show that the wake is deflected as the incident flow is increasingly yawed, leading to the formation of a single leeward vortex core that approaches progressively the body, decreasing the base pressure. This phenomenon is minored when a curved cavity is implemented, increasing the low pressure induced at the body base.

Peroperative epicardial ultrasonography of distal coronary artery bypass graft anastomoses using a stabilizing device. A feasibility study

BackgroundWidespread use of intraoperative epicardial ultrasonography (ECUS) for quality assessment of coronary artery bypass graft anastomoses during coronary artery bypass grafting (CABG) has not occurred - presumably due to technological and practical challenges including the need to maintain stable and optimal acoustic contact between the ultrasound probe and the target without the risk of distorting the anastomosis. We investigated the feasibility of using a stabilizing device during ultrasound imaging of distal coronary bypass graft anastomoses in patients undergoing on-pump CABG. Imaging was performed in both the longitudinal and transverse planes.MethodsSingle-centre, observational prospective feasibility study among 51 patients undergoing elective, isolated on-pump CABG. Ultrasonography of peripheral coronary bypass anastomoses was performed using a stabilizing device upon which the ultrasound transducer was connected. Transit-time flow measurement (TTFM) was also performed. Descriptive statistical tests were used.ResultsLongitudinal and transverse images from the heel, middle and toe were obtained from 134 of 155 coronary anastomoses (86.5%). After the learning curve (15 patients), all six projections were obtained from 100 of 108 anastomoses scanned (93%). Failure to obtain images were typical due to a sequential curved graft with anastomoses that could not be contained in the straight cavity of the stabilizing device, echo artefacts from a Titanium clip located in the roof of the anastomoses, and challenges in interpreting the images during the learning curve. No complications were associated with the ECUS procedure. The combined ECUS and TTFM resulted in immediate revision of five peripheral anastomoses.ConclusionsPeroperative use of a stabilizing device during ultrasonography of coronary artery bypass anastomoses in on-pump surgery facilitates imaging and provides surgeons with non-deformed longitudinal and transverse images of all parts of the anastomoses in all coronary territories. Peroperative ECUS in addition to flow measurements has the potential to increase the likelihood of detecting technical errors in constructed anastomoses.Trial registrationThe study was registered on September 29, 2016, ClinicalTrials.gov ID: NCT02919124.

High sensitivity fiber displacement sensor based compound ring laser cavity with linear variation of beat frequency signal

A novel fiber sensor based compound ring laser cavity with linear variation of frequency is proposed and demonstrated experimentally. The compound ring laser cavity is comprised of a ring cavity and a straight cavity. This system can generate the beat frequency spectrum by employing an erbium doped fiber amplifier, a fiber Bragg grating is used as a sensor head and the straight cavity reflector, a π phase shifted fiber Bragg grating serves as a microwave photonic passband filter. The principle of the proposed sensor is theoretically analyzed, showing that as the displacement increases the beat frequency decreases, and there exists a linear relationship between displacement change and beat frequency shift. In experiment, it is shown that the sensor has a high sensitivity of about 86.19 kHz/mm and can achieve a good linear response (&lt;i&gt;R&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; = 0.9973), and that the minimum monitored displacement is about 10 μm. The measurement results demonstrate that the sensor is accurate, sensitive, and the proposed sensor system has a compact and simple structure, which makes it convenient for more applications in future.

Calibration of high-accuracy spectrometers using stabilized 11-GHz femtosecond semiconductor laser.

We demonstrate for the first time the calibration of the wavelength scale of high-performance spectrometers using a fully stabilized optical frequency comb from an ultrafast optically pumped semiconductor disk laser (SDL) as a traceable reference. The SDL is a modelocked integrated external-cavity surface-emitting laser (MIXSEL) with the gain and saturable absorber layers fully integrated into one wafer chip, which forms one end mirror of the simple straight cavity with a pulse repetition rate of 11 GHz. This MIXSEL comb is actively stabilized and opens new possibilities for easier and more accurate frequency calibrations of standard laboratory instruments.

Effects of rear cavities on the wake behind an accelerating D-shaped bluff body

We investigate experimentally and numerically the transient development of the wake induced by a constant acceleration of a D-shaped bluff body, starting from rest and reaching a permanent regime of Reynolds number Re = 2000, under different values of acceleration and implementing three distinct rear geometrical configurations. Thus, alongside the classical blunt base, two control passive devices, namely, a straight cavity and an optimized, curved cavity, recently designed using adjoint optimization techniques, have also been used to assess their performance in transient flow conditions. Particle image velocimetry measurements were performed in a towing tank to characterize the near wake development in the early transient stages. It has been observed that the flow first develops symmetric shear layers with primary eddies attracted toward the base of the body due to the flow suction generated by the accelerated motion. Eventually, the interaction between the upper and lower shear layers provokes the destabilization of the flow and the symmetry breaking of the wake, finally giving rise to an alternate transitional vortex shedding regime. The transition between these phases is sped-up when the optimized cavity is used, reaching earlier the permanent flow conditions. In particular, the use of the optimized geometry has been shown to limit the growth of the primary eddies, decreasing both the recirculation and vortex formation length and providing with a more regularized, more organized vortex shedding. In addition, numerical simulations have been performed to evaluate the distribution of forces induced by the addition of rear cavities. In general, the aforementioned smoother and faster transition related to the use of optimized cavity translates into a lower averaged value of the drag coefficient, together with less energetic force fluctuations, regardless of the acceleration value.