Separation Position Research Articles

Aerodynamic Drag Reduction on Speed Skating Helmet by Surface Structures

The aerodynamic drag for speed skating helmets with surface structures was investigated in this work by using numerical and experimental methods. Computational fluid dynamic (CFD) research was performed to analyze the detail of the flow field around the helmets. The simplified helmet models, with riblet and bump surface structures, were analyzed using the CFD simulations. The pressure distribution and velocity field around the helmets were obtained through the CFD analysis. The CFD results showed that the boundary layer separation position was obviously delayed, and the pressure changed to a higher value at the back area for structured helmets. Therefore, the aerodynamic drag for the structured helmet was lower than that of the original model. According to the CFD results, three types of helmets, with the of riblet and bump surface structure printed on the helmets by using flexible film, were tested in a wind tunnel. A full-scaled skater mannequin of half a body was used in the experiment to simulate the actual skating process. Compared with the original helmet, a drag reduction rate of 7% was achieved for the helmet with the bump at the middle region in the wind tunnel experiment, at the average speed in competitions for skaters.

Wall-Modeled Large Eddy Simulation and Detached Eddy Simulation of Wall-Mounted Separated Flow via OpenFOAM

Considering grid requirements of high Reynolds flow, wall-modeled large eddy simulation (WMLES) and detached eddy simulation (DES) have become the main methods to deal with near-wall turbulence. However, the flow separation phenomenon is a challenge. Three typical separated flows, including flow over a cylinder at ReD = 3900 based on the cylinder diameter, flow over a wall-mounted hump at Rec = 9.36 × 105 based on the hump length, and transonic flow over an axisymmetric bump with shock-induced separation at Rec = 2.763 × 106 based on the bump length, are used to verify WMLES, shear stress transport k-ω DES (SST-DES), and Spalart–Allmaras DES (SA-DES) methods in OpenFOAM. The three flows are increasingly challenging, namely laminar boundary layer separation, turbulent boundary layer separation, and turbulent boundary layer separation under shock interference. The results show that WMLES, SST-DES, and SA-DES methods in OpenFOAM can easily predict the separation position and wake characteristics in the flow around the cylinder, but they rely on the grid scale and turbulent inflow to accurately simulate the latter two flows. The grid requirements of Larsson et al. (δ/Δx,δ/Δy,δ/Δz≈(12,50,20)) are the basis for simulating turbulent boundary layers upstream of flow separation. A finer mesh (δ/Δx,δ/Δy,δ/Δz≈(40,75,40)) is required to accurately predict the separation and reattachment. The WMLES method is more sensitive to grid scales than the SA-DES method and fails to obtain flow separation under a coarser grid, while SST-DES method can only describe the vortices generated by the separating shear layer, but not within the turbulent boundary layer, and overestimates the separation-reattachment zone based on the grid system in this paper.

Oblique shock train motion based on schlieren image processing

In this paper, shock train motion in a Mach number 2.7 duct is studied experimentally, and large numbers of schlieren images are obtained by a high-speed camera. An image processing method based on Maximum Correlation Detection (MCD) is proposed to detect shock train motion from the schlieren images, based on which the key structures, e.g., separation positions and separation shock angles on the top and bottom walls, can be analysed in detail. The oscillations of the shock train are generated by rhombus and ellipse shafts at various excitation frequencies. According to the analysis of MCD results, the distributions of the frequency components of shock train oscillation generated by the two shafts are distinctly different, in which the motion generated by the ellipse shaft is much smoother; shock train motion is mainly characterized by the oscillation of separation position while the separation shock strength is not so sensitive to downstream disturbance; there is a hysteresis loop relation between the downstream pressure and separation position.

APPLICATION OF TRADITIONAL OIL-FLOW-VISUALIZATION TECHNIQUE IN DETERMINING SKIN-FRICTION FIELDS ON AXISYMMETRIC AFTERBODY MODEL

This study evaluated the application of the traditional oil-flow-visualization technique in determining skin-friction fields for surface flow of an axisymmetric afterbody model at lowspeed conditions. Oil liquid, which was mixed of kerosene, titanium dioxide and oleic acid, was painted on the surface of model before experimental process. The movement of the oil was recorded by camera for data processing. The algorithm for extracting skin-friction fields was developed by the author and was validated. The method applied for boattail model. The results indicated that the traditional oil-flow-visualization technique provided sufficiently good outcomes for extracting skin-friction fields. Lagrange multiplier in range from 5 to 500 and number of selected pair images between 10 and 20 have a small effect on the skin-friction topology. The flow streamlines of the proposed method were also discussed detail in this study. Although the separation position was not captured well by the method, the reattachment position showed close results to previous observations. Consequently, the technique can be applied to analyze skin-friction fields of moving vehicles.

Unsteady Motion of Shock Wave for a Supersonic Compression Ramp Flow Based on Large Eddy Simulation

A large eddy simulation (LES) is conducted to investigate shock wave/turbulent boundary layer interaction in a 24° compression ramp at a high inlet Mach number of Ma=2.9. The recycling/rescaling Method is used as the inflow turbulence generation technique. The shock wave structure and boundary layer flow in the interaction region are studied by flow visualization methods, such as vortex recognition and numerical schlieren. The distributions of turbulent kinetic energy and Reynolds normal stresses at different streamwise locations are analyzed. The results show that a strong anisotropy turbulent flow appears in the reattached boundary layer after the interaction of the shock wave. The large-scale unsteady motion of the separation shock wave is analyzed by using the intermittent factor and power spectrum. It is found that the shock moves around the averaged separation position, and the length scale is equal to 72% of the inlet boundary layer thickness. The power spectrum analysis reveals the existence of low-frequency instability in the separation region.

Theoretical analysis on the ignition of a combustible mixture by a hot particle

Mechanical spark is an important ignition source in various industrial processes involving combustible mixtures and it may cause serious safety issues. In this work, we analysed the ignition induced by hot particles in a combustible mixture. In Semenov's transient ignition criterion, we introduced a hypothetical heat loss coefficient accounting for the temperature inhomogeneity and obtained a revised ignition criterion which allows us to calculate the ignition delay time. Explicit expressions for the critical ignition temperature were derived and used to demonstrate the primary impacts of temperature inhomogeneity on the ignition process. Consistent with experimental and numerical results, the temperature inhomogeneity is intensified by either reducing the particle size or convective heat transfer at the particle surface, resulting in an increase of the critical ignition temperature. For flow separation on the particle surface, the boundary layer problem was solved based on a Blasius series. A temperature gradient for ignition was defined at the location of flow separation to reproduce the experimentally observed phenomenon that ignition prefers to occur first near the flow separation position. It is shown that the unsteadiness of particle cooling makes negligible contribution to the ignition process because of the exceedingly large density ratio between the particle and the ambient gas. In addition, the finite residence time of ignition for a fluid parcel due to its elevation from the particle surface leads to additional growth in the critical ignition temperature. However, such a correction appears to be inconsequential because the ignition of the fluid parcel restricted to the Frank–Kamenetskii region is close to the particle surface.

Pressure Fluctuation on the Blade of a Axial Fan Driven in a Cooling System Enclosure

W Suppressing the pressure fluctuations distributed on the blade surface of an axial fan is applied as one of the means to reduce the noise generated in the cooling system of construction machinery. For that purpose, it is effective to use computational fluid dynamics (CFD) to capture the 3D vortex structure around a fan blade. However, while there have been many reports on the flow field formed around the blades of a fan alone or an axial fan with a duct-shaped shroud, the influence of the vortex structure formed at the actual operating point of the axial fan on the pressure fluctuation on the blade surface has not been sufficiently discussed. In this study, the three-dimensional vortex structure around the fan blade installed in the cooling system is investigated, and in particular, the influence of the wing tip leakage vortex on the pressure fluctuation on the fan blade surface is clarified. Wing tip leakage vortex that flow from forward in the direction of rotation of the fan interfere with a blade while oscillating periodically in the y-axis direction. The oscillation of the forward wing tip leakage vortex causes the separation position of the newly formed wing tip leakage vortex to change, and as a result, the pressure fluctuation increases at the tip of the blade suction surface near the separation position.

Mathematical Principle Analysis of Separation Point Prediction

During the development of fluid mechanics, fluid separation is an important issue. So far, there is no mathematical formula to reveal and describe the essence of fluid separation. At the same time, due to the high cost and limitation of the experimental method, another method is urgently needed to predict the separation position of the fluid. After axiomatizing fluid mechanics and combining the principle of excited state of quantum mechanics, this paper reveals that fluid separation is a special form of fluid in an excited state, and deduces the state conditions of fluid separation. The research results of this paper provide new ideas for solving problems in fluid separation and engineering applications.

Flow Separation Control over an Airfoil Using Plasma Co-Flow Jet

A NACA0025 coflow jet airfoil model, including the inner and outer airfoils, was designed and manufactured through 3D printing technology. Using the duct connecting the leading- and trailing-edge slots formed by the inner and outer airfoils, eight plasma actuators are arranged in the duct and synchronously driven by two high-voltage alternating-current sources to realize the plasma coflow jet (PCFJ) flow control technique. The test angle of attack range is 8–30°, and the Reynolds number is 68,000. The induced airflow characteristics were studied, and the flow control mechanism of the PCFJ airfoil was explored using the detailed two-component particle image velocimetry technique. The results in the quiescent air show that in addition to the injection at the leading-edge slot and the suction at the trailing-edge slot, the actuators on the upper surface of the inner airfoil provide local flow acceleration and momentum injection. The results in the wind tunnel show that the PCFJ airfoil has a better performance in delaying flow separation, and its time-averaged flow separation position is delayed by 46% compared with the baseline airfoil. The mixing shear layer that becomes turbulent in advance and the subsequent coherent vortex structures, the injection at the leading edge and suction at the trailing edge, and local flow acceleration and momentum injection of the plasma actuators are the key factors for the mechanism of separation delaying of the PCFJ flow control technique.

Type-II GeC/ZnTe heterostructure with high-efficiency of photoelectrochemical water splitting

Efficient carrier separation and suitable band edge position are necessary conditions for photocatalytic water splitting. Based on density functional theory, the electronic properties and optical performance of the GeC/ZnTe vdW heterostructure are systematically explored. The heterostructure with inherent type-II band arrangement is conducive to constantly separate the hole–electron pairs, thus improving the utilization of solar energy. Meanwhile, an excellent optical absorption coefficient has been proved in the heterostructure with large carrier mobility. Within the strain range of –4% to +3%, the heterostructure possesses appropriate band edges for photocatalytic water splitting and the light absorption performance is obviously improved. High solar-to-hydrogen efficiency (26.81%) means that the heterostructure can effectively convert photon energy into water splitting. These findings predict the possibility of GeC/ZnTe heterostructures as photocatalysts for water splitting and other optoelectronic devices.

Workers`s Rights According to Islamic Law Persepective (The Principle of Mudharabah) That Can Be Applied In Trade and Investment Agreements

The practice of Foreign Direct Investment (FDI) in developing countries often cause the labor problems, such as the labor on strike demanding higher wages or the severance pay as the results of the “down-sizing” whose made changes etc. In the country who has inadequate labors regulation or even does not have a strong labors union, it is difficult to enforce the rights of the labors. With the extreme separation position system between the labors and the employer, the problem is always decreasing, so the concept of alternative working relationship to solve this problem. The mudharabah principle in working agreement may be the way to solve the problem. That principle puts the labor and the employer to the same position, so the worker can develop their self to be productive and will gain the the profit for the company and also increase the labors income. In that principle, the company does not need “downsizing” or facing the labors “strikes” or “layoffs”, because the labor welfares can directly obtain from the hard work for the company. Besides the welfare for the labors, this concept is also can attract the foreign investment in our country.

Well placement optimization for large-scale geothermal energy exploitation considering nature hydro-thermal processes in the Gonghe Basin, China

Doublet well system composed by extraction and injection wells is widely used for heat production from deep geothermal reservoirs. The large-scale implementation of doublet well systems lead to hydro-thermal interaction with the natural hydro-thermal flow system; this affects heat production efficiency and sustainable management of geothermal energy and groundwater resources. However, the interaction between artificial operations and nature hydro-thermal status was seldom considered in the well placement optimization. In this study, the natural status of hydro-thermal flow was determined by fitting numerical modeling output to the groundwater level and temperature observations, in a realistic site of the Gonghe Basin, China. The well separation and relative position were then optimized against background water flow direction. It was found that a well separation of 1200 m with the injection well located upstream (south) to the extraction well can maximize the volume of injected water entering extraction well, without influence on the outflow temperature. Based on this finding, we further compared the effects of “extensive well placement” where the wells were located sparsely in entire geothermal field, and “intensive well placement” where the wells were located centrally in a local region. The calculation showed that the performance of intensive well placement was superior to extensive well placement with higher outflow temperature and lower interruption in groundwater levels. Further optimization of intensive well placement resulted in the outflow temperature higher than 92 °C under the total outflow rate of 30,000 m3/d for 30 years, and the groundwater level changes can be controlled below 6.0 m. The multiple-step well placement optimization approach and novel intensive heat production mode regarding nature status of hydro-thermal flow in geothermal reservoirs, provides important reference to the other geothermal field for large-scale geothermal exploitation.

Towards the ultimate strength of iron: spalling through laser shock

The ultimate strength of materials is reached at strain rates approaching the Debye frequency, when the deformation time at the atomic scale approaches the time for atoms to move away from the equilibrium to their extreme separation position (~5.5 × 1013 s−1 for iron). We conducted high-power pulsed laser experiments on single, poly-, and nanocrystalline iron, generating tensile pulses with strain rates approaching the Debye frequency, 106 s−1 – 107 s−1, and nanosecond time durations. We find iron strengths varying between 5 and 10 GPa, a factor of ten higher than the static tensile strength. Ultrafine-grained iron samples exhibit a lower tensile strength, ~4-6 GPa, than single crystal iron, ~10 GPa. MD simulations show that this is due to differences in the initiation sites for voids, primarily at grain boundaries for the nano- and polycrystalline conditions. Sparse runaway voids (~5 μm diameter) and evidence of surface melting are observed for the single crystal iron and are likely due to strain-induced melting when sufficient deformation occurs. The process of separation leading to spalling is modeled by molecular dynamics, and the mechanisms observed in the experimentally recovered specimens are determined: in single crystals voids nucleate at the intersection of twins, while in nanocrystalline specimens grain boundaries are the principal sources of void nucleation. Analytical calculations are applied to the dislocations generated by the emission of shear loops from the void surfaces and the geometrically necessary dislocation densities are found to be consistent with predictions from molecular dynamics calculations.

Inspection of structures interaction in laminar separation bubbles with extended proper orthogonal decomposition applied to multi-plane particle image velocimetry data

This work reports the application of an extended proper orthogonal decomposition (E-POD) procedure to multi-plane particle image velocimetry (PIV) measurements describing the evolution of laminar separation bubbles (LSBs). Measurements were performed over a flat plate installed between adjustable end-walls providing a prescribed adverse pressure gradient for two Reynolds numbers (Re = 70 000, 150 000) and free-stream turbulence intensity levels (Tu = 1.5%, 2.5%). A wall-normal and two wall-parallel measuring planes located at different distance from the wall were considered. POD was applied to the entire PIV planes as well as on their sub-domains, showing the main flow features occurring in the different regions of the LSB. Then, the application of E-POD on different plane partitions revealed the existing correlation between the main dynamics observed in the forward part of the bubble and the breakup events occurring in the reattachment region. The E-POD modes computed in the breakup region resemble streaky structures when PIV snapshots are projected onto the POD eigenvectors of the near wall plane. Otherwise, Kelvin–Helmholtz rolls dominate the E-POD modes obtained by projection of the snapshot matrices on the basis computed in the plane located far from the wall. The main scales of the coherent structures highlighted by the E-POD modes were also characterized by means of the streamwise and spanwise autocorrelation functions of E-POD filtered fields. Data in this work clearly highlight the similarity properties of the main flow features observed in LSBs once scaled with the momentum thickness of the boundary layer at the separation position.

Visualization of separation and reattachment in an incident shock-induced interaction

An experimental study is conducted on the qualitative visualization of the flow field in separation and reattachment flows induced by an incident shock interaction by several techniques including shear-sensitive liquid crystal coating (SSLCC), oil flow, schlieren, and numerical simulation. The incident shock wave is generated by a wedge in a Mach 2.7 duct flow, where the strength of the interaction is varied from weak to moderate by changing the angle of attack α of the wedge from 8° and 10° to 12°. The stagnation pressure upstream was set to approximately 607.9 kPa. The SSLCC technique was used to visualize the surface flow characteristics and analyze the surface shear stress fields induced by the initial incident shock wave over the bottom wall and sidewall experimentally which resolution is 3500 × 200 pixels, and the numerical simulation was also performed as the supplement for a clearer understanding to the flow field. As a result, surface shear stress over the bottom wall was visualized qualitatively by SSLCC images, and flow features such as separation/reattachment and the variations of position/size of separation bubble with wedge angle were successfully distinguished. Furthermore, analysis of shear stress trend over the bottom wall by a hue value curve indicated that the relative magnitude of shear stress increased significantly downstream of the separation bubble compared with that upstream. The variation trend of shear stress was consistent with the numerical simulation results, and the error of separation position was less than 2 mm. Finally, the three-dimensional schematic of incident shock-induced interaction has been achieved by qualitative summary by multiple techniques, including SSLCC, oil flow, schlieren, and numerical simulation.

Particles motion simulation and application exploration of three-stage cone water-only cyclone

ABSTRACT In this study, the particle motion simulation and application exploration of three-stage cone water-only cyclone (TWOC) were performed. The discrete phase model was applied to simulate the particles motion during the separation process of TWOC. The particle distribution curve of light products of TWOC obtained by the simulation is well consistent with that of the validation experiment, demonstrating the credibility of the particles motion simulation. The separation characteristic of this cyclone was clarified. The cone part is the main separation region, and the axial separation position of particles is deepened with the growth of the particle density. The “T-F” flow, consisting of the TWOC separation and flotation, was designed to separate the coal-slime efficiently. The separation experiments indicate that the pyrite sulfur rejection rate and the ash rejection rate of the NO.1 experiment are all greater than 79.00%. The “C-T-F” flow, consisting of the crushing process and the “T-F” flow, was designed to improve the desulfuration performance of the high-sulfur lump-coal. The crushing size was determined as 3 mm, and the pyrite sulfur rejection rate of the “C-T-F” flow is greater than 90.00%. The superiorities of the two separation flows promote the practical application of TWOC.

Simulation study on the performance of a vapor-bypassed evaporator for heat pump applications

• A mathematical model for the vapor-bypassed evaporator is developed. • Corresponding experiments were conducted to validate the model accuracy. • Entropy generation is applied to evaluate the evaporator overall performance. • Phase separation position and channel number of the evaporator are optimized. This paper presents a simulation study on the vapor-bypassed evaporator for heat pump applications. The vapor-bypassed evaporator, in which a phase separator is set in the middle of the refrigerant circuit to bypass vapor refrigerant, can reduce the refrigerant pressure drop for enhancing the evaporator overall performance. A mathematical model for the vapor-bypassed evaporator is developed and the experiments were conducted to validate its accuracy. Based on the entropy generation minimization theory, the optimum phase separation positions and refrigerant channel numbers of the evaporator for various operation conditions are selected out. According to the simulation results, as the required heat transfer capacity ranges 4~10 kW, the vapor-bypassed (VB) mode obtains 25.6~84.6 kPa lower refrigerant pressure drop and 3.1~25.3% lower entropy generation than the conventional operation (CO) mode. Moreover, when the ambient temperature ranges 5~15°C, the entropy generation of VB mode is decrease by 22.3~11.5% than CO mode. In general, applying the vapor-bypassed technique is a promising way for improving the evaporator performance especially under high heat transfer capacity and low ambient temperature conditions.

Large eddy simulation of dynamic stall flow control for wind turbine airfoil using plasma actuator

To solve aerodynamic performance deterioration caused by dynamic stall, a large eddy simulation numerical calculation based on dynamic grid and sliding grid technology was conducted and the dynamic flow control mechanism of unsteady pulsed plasma was explored. The results showed that plasma aerodynamic actuators can effectively control the airfoil’s dynamic stall, improve the mean and transient aerodynamic forces, and reduce the negative peak value of the pitch moment and hysteresis loop area. A negative pressure “bulge” appears in the plasma application areas, and the peak suction of the airfoil’s upper surface obviously increases. Two unsteady control parameters, the pulsed frequency and duty cycle, significantly influence the flow control. When the dimensionless pulsed frequency is 1.5, plasma control improves, and when the duty cycle is 0.8, it is close to the aerodynamic benefits under the continuous working mode. In the deep stall state, plasma impels the flow separation position to obviously move backward, resisting large-scale dynamic stall vortices. The structure of the separation vortices is broken, dissipated, and reattached to the airfoil by the plasma, and the influence area of the vortices is reduced. In the light stall state, the plasma actuator easily controls the shear layer, inducing the transition of the airfoil’s boundary layer and promoting the momentum mixing with the main flow. “Vortex clusters” near the airfoil’s leading edge induced by plasma actuation play a role in the virtual aerodynamic shape. The harmonic oscillation of aerodynamic force/moment is caused by the nonlinear and strong coupling effect between dynamic vortex structures with different scales and frequencies and plasma aerodynamic actuation. The amplitude of low-order mode energy concentration is relatively large, which is mainly caused by the pitching motion. The high-order fluctuation energy concentration is caused by the evolution process of starting vortices and derived secondary vortices with different frequencies induced by plasma.

A rock mechanics calculation model for identifying bed separation position and analyzing overburden breakage in mining

During coal seam mining, the overlying rock can deform, break, and produce a large number of fractures, including horizontal and vertical fractures; the horizontal fractures are also known as bed separation. A bed separation water disaster (BSWD) is a destructive release of roof water, and preventing BSWDs requires accurate location of bed separation positions. A calculation model for overburden deformation and fracture analysis during mining is therefore needed. First, the deformation and breakage area of overlying rock above an active mining face were reduced to a triangular overburden deformation area (TODA), based on the results of similar material simulation experiments. Second, the hierarchy comparison merging method (HCMM) was proposed to analyze the combination and separation of rock layers in the overlying strata. The triangular calculation model (TCM), which can be used to identify bed separation locations and analyze overlying strata breakage, was then obtained by combining TODA with HCMM, and the discriminant principle and related calculation formulas of the model were given. Finally, the model was applied and validated using a mining case, showing that the TCM is a practical tool for analyzing bed separation position and overburden breakage during mining.

CFD-based numerical study on the ventilated supercavitating flow of the surface vehicle

When supercavitation technique was used in surface vehicle, supercavitating flow was formed on the surface of the submerged body via artificial ventilation, which greatly reduced the navigation resistance and enhanced the navigation speed. By combining the VOF model and k−ω SST turbulence model, this study performed numerical simulation on the formed supercavitating flow field on the submerged body of the surface vehicle during artificial ventilation and examined the existence of the strut on the cavity form when the navigation parameters varied and the change regulation of the resistance coefficient of navigation body. As the navigation inclination angle changed, the pressure coefficient exhibited an asymmetrical distribution on two sides of the strut. At a large navigation inclination angle, the supercavities were more significantly affected by the strut and the drag resistance performance was poor. With increasing navigation depth, high-pressure and low pressure regions around the strut expanded, and the sunken separation position of the cavities moved forward, resulting in the coating area of the vehicle by the formed cavities dropped and the resistance increased. As the navigation speed increased, the ratio of drag reduction increased gradually. After the navigation speed exceeded to a certain value, further increasing navigation speed did not increase, but gradually decreased the ratio of drag reduction.