Stagnation Velocity Research Articles

The investigation on the ceiling of inlet velocity regarding to fine particle separation in a cyclone

The maximum efficiency inlet velocity (MEIV) serves as the upper limit for the inlet velocity that defines the separation efficiency in cyclone design and operation. In this paper, a combination of numerical and experimental methods is used to study MEIV. Experimental findings indicate that the MEIV is 22 m/s for a median particle size of 12.39 μm (coarse powder) and 35 m/s for a median particle size of 2.93 μm (fine powder). Meanwhile, the amount of escaped fine powder is reduced by 25% compared to that at an inlet velocity of 22 m/s. Computational fluid dynamics (CFD) simulations have shown that the inconsistency between tangential and axial velocity growth of inlet velocity with respect to various powder diameters can explain this phenomenon. As the inlet velocity increases, the peak axial velocity exhibits a stepwise increase. When the peak value remains constant, the peak width increases. This phenomenon is called stagnation of the axial velocity. During the axial velocity stagnation step, the residence time of back-mixed particles vary. In contrast, the tangential velocity increases linearly with the inlet velocity, resulting in an enhanced secondary separation of the inner vortex. Both factors hinder the escape of fine particles due to entrainment by a rapid upward airflow. The inlet velocity range corresponding to the stagnation step of the fine powder is larger than that of the coarse powder. Therefore, the MEIV of the fine powder is higher.

Thermal design of composite cold plates by topology optimization

Topology optimization has been extensively utilized to generate cold plates with efficient cooling performance. However, the design of composite cold plates remains challenging. This study presents a two-solid topology optimization method for conjugate heat transfer based on ordered SIMP (solid isotropic material with penalization). The proposed method aims to develop high-performance composite cold plates by leveraging the differences in thermal conductivity of various materials. The optimization results were investigated for four inlet and outlet combinations at various Reynolds numbers. The results indicate that using a staggered distribution of multiple inlets and outlets yields higher heat transfer than a single distribution. Furthermore, composite cold plates derived from the two-dimensional optimized models were numerically compared with rectangular flow channel and non-composite cold plates. The optimized channels exhibited superior hydrothermal performance over the rectangular flow channel, which was primarily attributed to the weakening of the velocity stagnation region. The average temperature (T¯) and root mean square temperature (RMST) of the heat source were reduced by a maximum of 2.53% and 57.67%, respectively. The friction factor decreased by up to 50.42%. Compared to non-composite cold plates, the decrease in RMST reached 16.24% without any additional increase in the pumping power. Composite cold plates further promote temperature uniformity by accelerating the heat transfer rate through their high thermal conductivity, thereby mitigating the occurrence of hotspots. Finally, the performance of the three test samples was experimentally compared to validate the numerical results. The optimized design enables the strategic allocation of highly thermally conductive material on a cold plate while employing more cost-effective alternatives in other areas. The proposed method is anticipated to expand the design possibilities for cold plates and provide insights into addressing concerns related to hotspots in equipment.

An improved method for calculating the wave run-up on a vertical cylinder based on the velocity stagnation head theory

The estimation of wave run-up is a concern in the design of certain marine engineering structures. Accurate estimation of the wave run-up is important for the safety and cost of the structure. To better predict the wave run-up on structures, a series of physical experiments for wave run-up on vertical cylinder under regular waves and focusing waves were conducted in this study. The influences of the wave steepness kA, relative structural scale ka, and A/D on the wave run-up were analyzed. By fitting the experimental data, a semi-empirical formula that calculates the wave run-up, based on the velocity stagnation head theory, is proposed. In the improved formula, a second order theory is employed to calculate the wave velocity in the crest, and the two important factors, viz., the relative structural scale ka and A/D, are also considered. The formula was evaluated by utilizing certain experimental data in the literature, demonstrating that it can provide a satisfactory wave run-up prediction. Furthermore, the wave run-up on the cylinder under the focused waves was predicted by the new formula. It can predict it well as local wave steepness is small.

Wave run-up on composite bucket foundation due to random waves: Model tests and prediction formulae

Wave run-up around foundations is a hot research topic in offshore wind engineering, particularly for novel composite bucket foundations (CBFs). This study investigates wave run-up on a CBF due to random waves via physical model experiments. The variation in wave run-up around a CBF and the influence of relative wave height on the maximum relative wave run-up is analysed. The experimental results show that the maximum relative wave run-up is found at the 0° marking, and the lowest wave run-up heights occur between the 108° and 144° markings. Moreover, the velocity stagnation head theory (VSH theory) and M5ʹ model tree are employed to predict the wave run-up and obtain an optimum prediction model. Statistical indicators, including the coefficient of determination, agreement index, scatter index, and bias, were used to evaluate the performance of the generated formulae. The prediction formula developed by the VSH theory combined with the second-order Stokes wave theory based on the particle velocity at the mean free surface has good performance in the prediction of wave run-up on the CBF due to random waves. The performance of the formula for relative wave run-up was developed using the M5ʹ model tree based on the scatter parameter, relative wave height, and wave steepness and was optimal according to statistical indicators. The relative wave height plays a crucial role in wave run-up prediction. The relative wave run-up height increases with an increase in the scatter parameter, relative wave height, and decrease in wave steepness. These two methods are recommended for predicting wave run-up on CBF owing to random waves.

Clustering in laboratory and numerical turbulent swirling flows

We study the three-dimensional clustering of velocity stagnation points, of nulls of the vorticity and of the Lagrangian acceleration, and of inertial particles in turbulent flows at fixed Reynolds numbers, but under different large-scale flow geometries. To this end, we combine direct numerical simulations of homogeneous and isotropic turbulence and of the Taylor–Green flow, with particle tracking velocimetry in a von Kármán experiment. While flows have different topologies (as nulls cluster differently), particles behave similarly in all cases, indicating that Taylor-scale neutrally buoyant particles cluster as inertial particles.

Numerical research on mechanism of the effect of propeller shaft brackets on wake field and propulsion performance

Ship drag and self-propulsion simulations of a twin-screw high-speed displacement vessel were performed using the unsteady Reynolds-averaged Navier–Stokes (RANS) method. The sliding mesh and overset mesh methods were applied concurrently to simulate the coupling of the propeller rotation and the ship motion. The effects of the propeller shaft brackets on the drag, propulsion performance, wake field, and propeller excitation force were studied. The influence of the shaft brackets on the wake field was visualized. The high numerical accuracy of the proposed simulation method was proven through verification and validation analyses. The low-speed zones of nominal wake within the propeller disc were mainly caused by vortex shedding from the vicinity of the shaft brackets and the speed reduction of the frictional wake resulting from the velocity stagnation in front of the rudder. The tangential wake velocity, which was affected by the twist angle and vortex shedding, had a considerable effect on the propulsion performance. The wake non-uniformity coefficient was highly correlated with the vortex shedding. The spatial structure of the shaft bracket and the inward curved streamline around the stern aggravated the non-uniformity further, but the shaft bracket had a limited impact on the unsteady propeller bearing force in the absence of cavitation.

Experimental Investigation of Wave Load and Run-up on the Composite Bucket Foundation Influenced by Regular Waves

In the design of wind turbine foundations for offshore wind farms, the wave load and run-up slamming on the supporting structure are the quantities that need to be considered. Because of a special arc transition, the interaction between the wave field and the composite bucket foundation (CBF) becomes complicated. In this study, the hydrodynamic characteristics, including wave pressure, load, upwelling, and run-up, around the arc transition of a CBF influenced by regular waves are investigated through physical tests at Shandong Provincial Key Laboratory of Ocean Engineering, Ocean University of China. The distributions of the wave pressures and upwelling ratios around the CBF are described, and the relationship between the wave load and the wave parameters is discussed. New formulae based on the velocity stagnation head theory with linear wave theory and the second-order Stokes wave theory for wave kinematics are proposed to estimate the wave run-up. Moreover, the multiple regression method with nonlinear technology is employed to deduce an empirical formula for predicting run-up heights. Results show that the non-dimensional wave load increases with the increase in the values of the wave scattering parameter and relative wave height. The wave upwelling height is high in front of the CBF and has the lowest value at an angle of 135° with the incoming wave direction. The performance of the new formulae proposed in this study is compared using statistical indices to demonstrate that a good fit is obtained by the multiple regression method and the analytical model based on the velocity stagnation head theory is underdeveloped.

Numerical investigation of bubble dynamics at a corner

This paper is concerned with bubble dynamics at a corner formed by two flat rigid boundaries associated with applications in ultrasonic cleaning and cavitation damage. This phenomenon is modeled using the potential flow theory and the boundary integral method. The Green’s function is obtained to satisfy the impenetrable conditions at the rigid boundaries using the method of images with the corner angle α = π/k, where k is a natural number. To evaluate the numerical model, experiments were carried out with a spark-generated bubble in water and recorded using a high-speed camera. The predicted bubble shapes are in excellent agreement with those from the experiments. A jet forms toward the end of the collapse, pointing to the corner when initiated at the bisector of the two walls but pointing to the near wall and inclined to the corner when initiated near one of the two walls. The Kelvin impulse theory predicts the jet direction well. As compared to a bubble near a flat wall, the oscillation period and the jet width increase but the jet velocity decreases. The bubble migrates away from the near wall and the corner during its expansion and moves back toward them during its collapse, but at a much larger speed and amplitude. A velocity stagnation point forms at the start of the collapse, and a high-pressure zone is generated at the base of the jet during the late stages of the collapse, which drives the jet and the bubble toward the near wall and the corner.

Recirculating structures and combustion characteristics in a reverse-jet swirl pulverized coal burner

Industrial pulverized coal boilers play an important role in the use of distributed energy, but still suffer from severe pollutant emissions, combustion instabilities and poor fuel adaptabilities. In this paper, numerical simulation and full-scale experiments were carried out to study the recirculating and combustion characteristics of a novel reverse-jet swirl pulverized coal burner with an adding bluff body and flexibly adjusted secondary air. The axial velocity stagnation contour and the out-of-plane vorticity contour were used to characterize the recirculation structure of reverse swirl flows. The original design achieves a moderate char burnout with the carbon content in fly ash to be 18.63% under a residence time of 0.158 s, which demands for the further improvements. However, if only the injection velocity of the secondary air is increased by halving the axial range of the swirl vane, the recirculating flow is strengthened to such an undesired degree that coal ignition appears in the primary air pipe, the burner-wall overheating and near-wall particle enrichment can occur. To overcome it, a bluff body was incorporated with the enhanced secondary air flow. The recirculation zone was restricted downstream the bluff body, thus eliminating possible early coal ignition. Besides, the width of the recirculating flow is broadened, resulting in a more stable combustion with a less fluctuated pressure signal at the furnace exit. The near-wall coal particle concentration is reduced by approximately 90%. As a result, the use of bluff body decreased the carbon content in fly ash to 8.55% by increasing the coal particle residence time to 0.171 s. In addition, the NO concentration at the burner outlet is decreased by 8% owing to a longer coal particle residence time in the reducing environment.

The effect of uniform magnetic field on spatial-temporal evolution of thermocapillary convection with the silicon oil based ferrofluid

A uniform axial or transverse magnetic field is applied on the silicon oil based ferrofluid of high Prandtl number fluid (Pr ? 111.67), and the effect of magnetic field on the thermocapillary convection is investigated. It is shown that the location of vortex core of thermocapillary convection is mainly near the free surface of liquid bridge due to the inhibition of the axial magnetic field. A velocity stagnation region is formed inside the liquid bridge under the axial magnetic field (B = 0.3-0.5 T). The disturbance of bulk reflux and surface flow is suppressed by the increasing axial magnetic field. There is a dynamic response of free surface deformation to the axial magnetic field, and then the contact angle variation of the free surface at the hot corner is as following, ?hot, B = 0.5 T = 83.34? > ?hot, B = 0.3 T = 72.16? > > ?hot,B = 0.1 T = 54.21? > ?hot, B = 0 T = 43.33?. The results show that temperature distribution near the free surface is less and less affected by thermocapillary convection with the increasing magnetic field, and it presents a characteristic of heat-conduction. In addition, the transverse magnetic field does not realize the fundamental inhibition for thermocapillary convection, but it transfers the influence of thermocapillary convection to the free surface.

Asymmetric magnetic reconnection driven by ultraintense femtosecond lasers

Three-dimensional asymmetric magnetic reconnection (AMR) driven by ultraintense femtosecond (fs) lasers is investigated by relativistic particle-in-cell (PIC) simulation. The reconnection rate is found to be only one-third of that in the previous symmetric reconnection PIC simulations. Similar to the case of dayside reconnection at geomagnetopause, magnetic X- and velocity stagnation points are not colocated, with the X-point at the lower field side and the stagnation point at the higher field side. Moreover, the moving direction of the X-point as reconnection evolving with the laser irradiation is determined by δBH/δBL, and the moving of stagnation point is dominated by neHBL/neLBH, where δB and ne are the magnetic field disturbance and the electron density with the subscripts “H” for the higher field side and “L” for the lower field side, respectively. Then, the hosing instability triggered by AMR and the merging of two parallel currents resulting in the tilt of the electron beam generated by the weak laser are also investigated.

Numerical Study on Flow Field Characteristics of Interval control valve in Intelligent Well

Interval control valve (ICV) is an important part of intelligent well. An ICV was designed and its flow field characteristic was analyzed by the software CFX. With the increase of the ICV opening, the thickness of laminar bottom layer became thinner and the average velocity of the main flow zone increased. The opening had greater effect on the average velocity than the pressure differential. When the ICV was opened or closed, there might be a great shear resistance due to the large inlet flow rate. There was a velocity stagnation point at the intersection of ICV entrance and its axis. The turbulent kinetic energy decreased gradually with the increase of the ICV opening. It was proved that the ICV had a good flow capacity, and the downhole fluid did not produce large pressure drop when it flew through the ICV. The ICV opening - flow coefficient curve could guide the operation of the ICV to achieve the purpose of accurately regulating the production of a reservoir.

Modelling mist flow for investigating liquid loading in gas wells

This paper presents a dynamic simulation approach to investigating liquid loading in gas wells for a fluid flowing in the mist flow regime. Two-component gas-liquid two-phase flow is considered, using coupled thermodynamic and hydrodynamic models as well as constitutive equations that incorporate the Peng-Robinson equation of state and the convex hull algorithm. The behaviour of the flow properties is investigated as phase change occur during flow. The accumulation of liquids is explored by investigating the distribution of the liquid density in the tubing, which is explicitly determined from the flow variables. The calculated phase densities are validated using data obtained from NIST RefProp and the results show good agreement. This procedure can provide substantial benefits in investigating the phenomena of liquid loading in gas wells compared to critical velocity predicting models that determine the stagnation velocity under isothermal conditions.

A CFD model for simulating wave run-ups and wave loads in case of different wind turbine foundations influenced by nonlinear waves

A hydrodynamic simulation of wave run-up heights and wave loads on three types of wind turbine foundations, i.e. monopile, gravity-based and tripod support structures, was conducted using a RANS solver and employing k-ε turbulent closure. Due to the contribution of the present CFD model, a semi-empirical formula is calibrated based on velocity stagnation head theory for crest kinematics. Eventually, the results indicate that the difference among the maximum normalized run-up heights of these support structures is smaller for lower wave steepness than those for higher wave steepness. In contrast, it is shown that the difference among the wave loads of these foundations is larger for lower wave steepness than those for higher wave steepness. A calibrated run-up parameter is also obtained by means of numerical simulation and found that the value of calibrated run-up parameter becomes smaller accompanied with larger values of wave steepness and the maximum normalized run-up height. It is relevant that the tendency of run-up heights is positively correlated with higher nonlinearity, whereas an opposite trend is observed in the relationship between larger calibrated run-up parameter and lower nonlinearity.

Improving predictions of heat transfer in indoor environments with eddy viscosity turbulence models

Heat transfer modelling in indoor environments requires an accurate prediction of the convective heat transfer phenomenon. Because of the lower computational cost and numerical stability, eddy viscosity turbulence models are often used. These models allow modification to turbulent Prandtl number, and near wall correction which influences stagnation points, entrainment, and velocity and time scales. A modified v2–f model was made to correct the entrainment behaviour in the near wall and at the stagnation point. This new model was evaluated on six cases involving free and forced convection and room airflow scenarios and compared with the standard k–e, and k–ω–SST models. The results showed that the modification to the v2–f model provided better predictions of the buoyant heat transfer flows while the standard k–e failed to reproduce and underestimate the convective heat transfer. The k–ω–SST model was able to predict the flow field well only for a 2D square cavity room, and 3D partitioned room case, while it was poor for the other four cases.

Hyperbolicity in temperature and flow fields during the formation of a Loop Current ring

Abstract. Loop Current rings (LCRs) are among the largest mesoscale eddies in the world ocean. They arise when bulges formed by the Loop Current in the Gulf of Mexico close off. The LCR formation process may take several weeks, and there may be several separations and reattachments before final separation occurs. It is well established that this period is characterized by a persistent saddle point in the sea surface height field, as seen in both model and satellite data. We present here a detailed study of this saddle region during the formation of Eddy Franklin in 2010, over multiple days and at several depths. Using a data-assimilating Gulf of Mexico implementation of the HYbrid Coordinate Ocean Model (HYCOM), we compare the vertical structure of the currents and temperature fields on 5 and 10 June 2010. Finite-time Lyapunov exponents (FTLE) are computed from the surface down to 200 m to estimate the location of relevant transport barriers. Several new features of the saddle region associated with LCR formation are revealed: the ridges in the FTLE fields are shown to be excellent surrogates for the manifolds delineating the material flow structures with only slight degradation at depth. The intersection of the ridges representing stable and unstable manifolds drops nearly vertically through the water column at both times; remarkably, the material boundary shapes are maintained even as they are advected. Moreover, velocity stagnation points and saddle points in the temperature field are consistently found near the intersections at all depths, and their geographic positions are also nearly constant with depth.

Numerical simulation of the dynamic behaviors of a gas tungsten welding arc for joining magnesium alloy AZ61A

Based on the Magneto-Hydro-Dynamic (MHD) theory, a united three-dimensional (3D) transient numerical model is developed to investigate the dynamic behaviors of arc plasma for a magnesium alloy AZ61A gas tungsten arc welding (GTAW) arc. The arc, electrode and workpiece are integrated into one calculation domain to avoid both presumed distribution of the current density at the electrode tip and the assumption of constant conditions of interface between welding arc and workpiece. The distributions of electric potential, current density, magnetic flux density, electromagnetic force, velocity, temperature, and pressure of the arc plasma in the 3D space are analyzed by using the numerical model. Results indicate that the maximum gradient of the electric potential in the whole arc space exists around the electrode tip, where the electric current density, electromagnetic force, and temperature are also the maximum. However, maximum pressure is found at the velocity stagnation, which is above the workpiece. Comparison between predicted temperature and measured one in arc region shows a good agreement.

Extinguishment of counterflow methane/air diffusion flame by polydisperse fine water droplets

In the present study, we investigated the effect of fine water droplets on extinguishment of a methane–air counterflow diffusion flame. Fine water droplets were generated by piezoelectric atomizer. Water droplet mass loading was varied by changing the number of activated piezoelectric units. Droplet characteristics were measured by a phase Doppler particle analyzer and temperature measurements were performed using a silica-coated fine thermocouple. With an increase in the non-dimensional fuel ejection parameter, the non-dimensional flame stand-off distance increases, but it is not affected by water droplets. When the stagnation velocity gradient is small enough, all the droplets are decelerated simultaneously along the stagnation stream line. However, non-equilibrium in droplet velocities appears when the stagnation velocity gradient is increased. Large droplets move faster than small ones when the Stokes number is larger than unity. On the other hand, thermophoretic velocities were negligibly small over all sizes of droplets as compared to convection velocity. When the surface area parameter is small, the critical velocity gradient at extinguishment decreases rapidly with increase in the surface area parameter and water droplets are significantly effective in fire suppression. However, the critical velocity gradient at extinguishment becomes less sensitive to the surface area parameter when it increases. The effect of humidity variation in the ambient air is not discernible. This fact suggests that the water vapor included in the ambient air is not so effective to reduce the maximum flame temperature and the critical velocity gradient at extinguishment. With increase of the water droplet mass loading, the maximum flame temperature decreases as well as the critical velocity gradient at extinguishment. However, the maximum flame temperature at the limit of extinguishment changes only slightly with increase of the water droplet mass loading and is of the order of 1570K.

A stochastic method for wave run-up on slender circular cylinders due to long-crested and short-crested non-linear random waves

A practical stochastic method for estimating the wave run-up height on a slender vertical circular cylinder for long-crested (two-dimensional (2D)) and short-crested (three-dimensional (3D)) non-linear random waves is provided. This is achieved by using the velocity stagnation head theory in conjunction with a stochastic approach. Here the waves are assumed to be a stationary narrow-band random process. The effects of non-linear waves are included by adopting the Forristall wave crest height distribution representing both 2D and 3D non-linear random waves. A parameter study is provided and, for both 2D and 3D non-linear random waves, the wave run-up height is larger than for linear random waves. In shallow water, when the water is sufficiently shallow, the maximum wave run-up height is larger for 3D waves than for 2D waves. Comparisons are also made with measurements of the maximum wave run-up heights for 2D random waves in a finite water depth by De Vos et al., and it appears that the data are well predicted if the predictions are calibrated with the data. Therefore the present approach represents a model which can be validated against and adjusted to field data.

Wave run-up on slender circular cylindrical foundations for offshore wind turbines in nonlinear random waves

A practical method for estimating the wave run-up height on a slender circular cylindrical foundation for wind turbines in nonlinear random waves is provided. The approach is based on the velocity stagnation head theory and Stokes second order wave theory by assuming the basic harmonic wave motion to be a stationary Gaussian narrow-band random process. Comparisons are made with measurements by De Vos et al. (2007), and some of the highest wave run-up events that were predicted agree with those measured.