Using numerical simulations and experimental tests, the temperature and velocity fields of the molten pool during selective laser melting (SLM) were investigated, where the laser scanning speed ...ranging from 2.5 m/s to 0.3 m/s was employed. Experiments for single tracks and part samples were conducted for verification. Three kinds of molten pool states were identified and investigated: unstable state, transition state and stable state. The unstable state is characterized by numerous balling defects, where the bulk density is severely deteriorated. The transition state is featured by the transition region where the melt velocity is relatively lower, and the molten pool is vulnerable to the necking defect. The molten pool with a depression region is identified as the stable state. A small depression is favorable for improving the surface quality of single track and the bulk density. However, exorbitant energy input will convert the depression into a keyhole. Additionally, a threshold of the scanning speed was found, where the bulk density peaked. Over the threshold, the density decreased continuously with the speed increasing. However, the density slightly decreased by 1.5% when the speed was below the threshold; this anomaly was ascribed to the residual pores induced by the recoil pressure.
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•A molten pool has three different states due to different laser scanning speeds.•Recoil pressure confines the molten pool temperature around the boiling point.•There existing a threshold in the scanning speed where the bulk density peaks.•Residual pores can be caused by the evolution of the molten pool with a keyhole.
•Numerical simulation study of the multiple Faraday wave's evolutions.•Pressure difference in the Faraday wave is the internal driving force of fluid flow.•The Faraday wave unstable evolution is ...essentially an energy conversion process.•The effect of liquid density on atomization characters is predicted by simulation.
Ultrasonic atomization has attracted a great deal of attention from the industrial and research communities in recent years. However, current research lacks attention to the internal pressure and velocity fields in the unsteady evolution of multiple Faraday waves, leading to an insufficient understanding of the ultrasonic atomization mechanism. Here, we use computational fluid dynamics to conduct a detailed study of multiple Faraday waves under ultrasonic frequency vibrations, including phase, pressure, and velocity field evolution, and atomization characteristics. In the middle of the ligament, a pressure peak point induced by surface tension induces the capillary pinch-off phenomenon. Essentially, Faraday wave instability evolution is the process of storing, consuming kinetic energy, then converting it into pressure potential energy, and finally into droplet surface energy and kinetic energy. The effects of different parameters (vibration amplitude, vibration frequency, liquid surface tension, viscosity, density, and liquid film thickness) on the atomization characteristics (amplitude threshold, atomization time, and droplet size) are related to this energy evolution. This study provides a new physical explanation of the unstable evolution mechanism of multi-Faraday waves from the perspective of energy conversion, improving the scientific validity of ultrasonic atomization techniques in applications.
Vertical movements of the solid Earth surface reflect crustal deformation and deep mantle related phenomena. For Holocene times, coastlines displaced from the present mean sea-level are often used ...together with past relative sea-level (RSL) prediction models to decipher the vertical deformational field.
Along the coastline from southwest Turkey eastward to Israel and Cyprus, field data that constrain Holocene vertical movements are already published, leaving a gap only along the Mediterranean coast of the Central Anatolian Plateau (CAP). Based on new field observations between Alanya and Adana (Mersin, southern Turkey), together with AMS 14C dating, we fill that gap, allowing for the construction of a continuous overview of Holocene vertical differential movements along the Eastern Mediterranean coast. We apply the most recent Glacial Isostatic Adjustment (GIA) models to correct for the glacio-hydro isostatic component of the RSL. Different solutions from the ICE-6G(VM5a) and ICE-7G(VM7) models (developed by W.R. Peltier and co-workers at the Toronto University), and a GIA model developed by K. Lambeck and collaborators at the Australian National University, have been applied to 200 middle-to-late Holocene RSL markers.
Starting from southwest Turkey, we find subsidence between −0.9 mm/yr and − 2.3 mm/yr, corroborating estimates from previous studies. Velocities from the new markers along the CAP Mediterranean coast are positive, ranging between 0.9 and 1.5 mm/yr. These two first blocks are separated by a sharp velocity jump, occurring along the Isparta Angle Fault System one. Such high vertical velocities for the CAP southern margin were predicted by recently published papers that report a rapid uplift phase that peaked during themiddle to late Pleistocene. Moving to the east, velocities are also positive, from 0.2 to 0.6 mm/yr along the coast between the Hatay Gulf and southern Lebanon. The highly variable velocity along the Lebanese sector is likely due to co-seismic deformation along the Lebanese Restraining Bend (LRB) faults. To the south, in contrast, the Israeli coast shows stability, according to some unique archaeological RSL markers named piscinae, whereas other markers indicate slow subsidence (−0.2 mm/yr on average). Hence, another velocity jump of at least 0.5 mm/yr is recognizable between Israel and Lebanon. This jump is probably associated with mapped, active tectonic structures. In Northern Cyprus, the only Holocene sea-level marker confirms the near zero vertical velocity values already obtained for the MIS 5e marine terrace. Therefore, a vertical velocity jump occurs between stable Northern Cyprus and the uplifting CAP southern margin, although they occur on the same overriding plate of the Eastern Mediterranean subduction system. High-angle normal faults at the northern margin of the Adana-Cilicia Basin could explain these strongly distinct late Holocene vertical velocity fields.
These results depict a complex framework of independently moving crustal blocks, with kinematic separation along well-known regional fault zones. The drivers of the block movements could be related either to regional tectonics, as it is probably the case for the LRB coast, or to mantle dynamics, such as for the uplifting Turkish sector, where deeper processes should be considered.
Nine modifications of Stairmand high-efficiency type cyclone with various cylindrical and conical heights were used to investigate their effects on pressure drop and flow field within cyclones. An ...experimental setup was built and experiments were conducted on various cyclone geometries at inlet velocities ranging from 10 to 18.5m/s. The body heights ranged from D to 2D (D being the diameter of the cyclone body), while the conical heights were between 2D and 3D. The experimental results were used to calibrate CFD (Computational Fluid Dynamics) model. Experimental results showed that the pressure drop ranged from 191 to 235Pa and 690 to 920Pa at the lowest and highest inlet velocities, respectively, and that pressure drop is a function of both cylindrical and conical heights with reduced pressure drops as cylindrical and/or conical heights increase. Maps showing the change in pressure drop with respect to cylindrical and conical heights were prepared and interpreted to determine optimum ratio of conical-to-cylindrical height for reduced pressure drop. Optimum ratio was found experimentally as between 1.67 and 2.5. CFD results agreed well with the experimental data and the results helped gain insights into complicated phenomena taking place in cyclones. CFD results showed that increased axial velocities at the lip of vortex finder distrub radial velocity profiles and that tangential velocities in the stabilized vortex can be as high 1.4 times the mean inlet velocity. Both findings indicate that collection efficiency of the cyclone may be influenced by the height of the cyclone.
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•Tangential velocities can reach 1.4 times the inlet velocity depending on body and cone heights.•Increased axial velocity at vortex finder lip disturbes radial velocity reducing efficiency.•The lowest pressure drop is obtained by setting cone-to-body height ratio greater than 1.67.•CFD models could be confidently used for designing high-performance cyclones.
Ensuring optimal conditions for fish, especially in terms of uniform velocity fields, is crucial for aquaculture systems to attain self-cleaning efficiency in rearing tanks and better fish behavior, ...which ultimately impacts their survival and growth. The current study presented a full-scale computational fluid dynamics (CFD) model of three types of circular, square and square arc angle tanks used in aquaculture farms to choose the most optimal tank based on the flow uniformity and velocity field. At the next step, the optimal arrangement of the inlet number, inlet location, inlet angle, as well as the number of nozzles on each inlet was reconfigured to create the most optimal tank. The simulations were done using the CFD software FLUENT distributed by the ANSYS Corporation. The software calculates velocities in the tanks by solving the Reynolds-averaged Navier-Stokes equations of continuity and momentum that govern the mean turbulent flow of water. The results illustrated circular and square arc angle tanks had significantly more uniform fluid velocity compared to the square tanks, which the square arc angle tank due to better flow velocity near the wall and space utilization was selected for further rearrangements. Additionally, the implementation of two entrance pipes near the corner of the two parallel walls with an entry angle of 0° in a square arc angle tank resulted in a more consistent water velocity. As the number of inlet nozzles increased to six, the water streamlines became more uniform, suggesting a more consistent water velocity throughout the tank. However, when the number of input nozzles increased to nine, the tension on the nozzle apertures were increased probably due to the decrease of distance between them. Findings of the present study concluded that two inlets in the corner with an entry angle of 0° and six nozzles on each inlet pipe could provide the optimum water velocity in the square arc angle tanks.
Solar active regions (ARs) that produce major flares typically exhibit strong plasma shear flows around photospheric magnetic polarity inversion lines (MPILs). It is therefore important to ...quantitatively measure such photospheric shear flows in ARs for a better understanding of their relation to flare occurrence. Photospheric flow fields were determined by applying the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) method to a large data set of 2548 coaligned pairs of AR vector magnetograms with 12-min separation over the period 2012 – 2016. From each AR flow-field map, three shear-flow parameters were derived corresponding to the mean (
〈
S
〉
), maximum (
S
max
) and integral (
S
sum
) shear-flow speeds along strong-gradient, strong-field MPIL segments. We calculated flaring rates within 24 h as a function of each shear-flow parameter and we investigated the relation between the parameters and the waiting time (
τ
) until the next major flare (class M1.0 or above) after the parameter observation. In general, it is found that the larger
S
sum
an AR has, the more likely it is for the AR to produce flares within 24 h. It is also found that among ARs which produce major flares, if one has a larger value of
S
sum
then
τ
generally gets shorter. These results suggest that large ARs with widespread and/or strong shear flows along MPILs tend to not only be more flare productive, but also produce major flares within 24 h or less.
A methodology is presented for efficient and accurate modeling of correlated wind velocities along long span structures at a virtually infinite number of points. Currently, the standard approach is ...to model wind velocities as discrete components of a multivariate stochastic vector process, characterized by a Cross-Spectral Density Matrix. To simulate sample realizations of the vector process, the Spectral Representation Method is one of the most commonly used, which involves a Cholesky decomposition of the Cross-Spectral Density Matrix. However, it is a well-known issue that as the length of the structure, and consequently the size of the vector process, increases, this Cholesky decomposition breaks down numerically. Alternatively, this paper introduces the use of the frequency–wavenumber spectrum to model the wind velocities as a stochastic “wave”, continuous in both space and time. This allows the wind velocities to be modeled at a virtually infinite number of points along the length of the structure. In this paper, the relationship between the Cross Spectral Density Matrix and the frequency–wavenumber spectrum is first examined. The frequency–wavenumber spectrum is then derived for wind velocities. Numerical examples are carried out demonstrating that the simulated wave samples exhibit the desired spectral and coherence characteristics. The efficiency of this method, specifically through the use of the Fast Fourier Transform, is also demonstrated.
•We present a frequency–wavenumber based representation of wind velocity fields.•Closed form frequency-wavenumber spectra for typical wind field models are derived•Efficient simulation of continuous wind fields, not limited by Cholesky decompositions.•Verification against the prescribed auto- and cross-spectra through numerical examples.
NASA’s
Solar Dynamics Observatory
(SDO) spacecraft was launched 11 February 2010 with three instruments onboard, including the
Helioseismic and Magnetic Imager
(HMI). After commissioning, HMI began ...normal operations on 1 May 2010 and has subsequently observed the Sun’s entire visible disk almost continuously. HMI collects sequences of polarized filtergrams taken at a fixed cadence with two
4096
×
4096
cameras, from which are computed arcsecond-resolution maps of photospheric observables that include line-of-sight velocity and magnetic field, continuum intensity, line width, line depth, and the Stokes polarization parameters
I
,
Q
,
U
,
V
. Two processing pipelines have been implemented at the SDO Joint Science Operations Center (JSOC) at Stanford University to compute these observables from calibrated Level-1 filtergrams, one that computes line-of-sight quantities every 45 seconds and the other, primarily for the vector magnetic field, that computes averages on a 720-second cadence. Corrections are made for static and temporally changing CCD characteristics, bad pixels, image alignment and distortion, polarization irregularities, filter-element uncertainty and nonuniformity, as well as Sun–spacecraft velocity. We detail the functioning of these two pipelines, explain known issues affecting the measurements of the resulting physical quantities, and describe how regular updates to the instrument calibration impact them. We also describe how the scheme for computing the observables is optimized for actual HMI observations. Initial calibration of HMI was performed on the ground using a variety of light sources and calibration sequences. During the five years of the SDO prime mission, regular calibration sequences have been taken on orbit to improve and regularly update the instrument calibration, and to monitor changes in the HMI instrument. This has resulted in several changes in the observables processing that are detailed here. The instrument more than satisfies all of the original specifications for data quality and continuity. The procedures described here still have significant room for improvement. The most significant remaining systematic errors are associated with the spacecraft orbital velocity.
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