Instantaneous Flow Research Articles

NUMERICAL SIMULATION OF PSEUDOPLASTIC FLUID FLOW OVER A SQUARE CYLINDER USING ANSYS FLUENT

Flow of any non-Newtonian fluid past a bluff body requires considerable attention due to its involvement in chemical, polymer, and process engineering. In the current study, numerical simulation of the flow past a square cylinder was conducted at Reynolds number Re = 100 in a pseudoplastic fluid flow environment (0.1 ≤ <i>n</i> < 1.0, where <i>n</i> represents the power index of non-Newtonian fluid) using ANSYS Fluent. Instantaneous and time-averaged flow fields along with the flow parameters were calculated to understand the behavior of pseudoplastic flow in the presence of an obstacle (i.e., the square cylinder). It was observed that the drag force value increased in the pseudoplastic fluid flow compared to that in Newtonian fluidflow (<i>n</i> = 1), whereas the vortex shedding frequency decreased as the n value decreased. Close to <i>n</i> = 0.4, clear evidence of flow instability was seen, which drastically changed the vortex shedding mechanism and affected the flow downstream from the cylinder. In addition, at <i>n</i> = 0.1 the fluid flow stood still downstream due to very high flow resistance-a common feature of flow fields with a very high pseudoplastic nature.

Particle transport in turbulent square duct flows with a free surface

Direct numerical simulation combined with a one-way coupled Lagrangian particle tracking technique is employed to investigate dilute particle-laden turbulent flows in open square ducts with a free surface. The focus is on examining the influence of the mean cross-stream secondary flow on particle transport near the wall, free surface, and across the duct cross section. Based on the duct half-width and mean friction velocity, a shear Reynolds number of Reτ = 300 is considered, with the corresponding particle Stokes numbers ranging from St+ = 0.31 to 260. The results reveal that particle concentration near the sidewalls is lower than that near the bottom wall, and the minimum particle concentration is observed at the free surface. Along the bottom wall centerline orientated upward, particle concentration gradually decreases. An exception to this is in the vicinity of the free surface where a slight increase is observed for the heavier particles (St+ ≥ 25), and the amplitude of this increase gradually declines as the Stokes number increases. In the streamwise direction near the free surface, heavier particles tend to preferentially concentrate in regions where the instantaneous transverse secondary flow velocity is negative. As the Stokes number increases, the position of the maximum streamwise velocity for heavier particles is closer to the free surface, and the rotation centers of inner and outer secondary particle motions gradually disappear. The streamwise root mean square velocity for the lightest St+ = 0.31 particles is higher than that for particles with higher inertia in the middle region of the free surface.

Numerical Study of the Boundary Layer Separation of a Low‐Pressure Turbine Cascade at Different Incidence Angles

Modern low‐pressure turbine (LPT) pursues high blade loading to improve its efficiency while the possibility of boundary layer separation on the blades is still a challenging problem. The incidence angle is one of the main factors affecting the flow separation and laminar‐turbulent transition, which is of key importance for the design of LPT. In this work, based on the initial simulation results obtained using the Reynolds‐averaged Navier–Stokes (RANS) method, the flow past a single LPT cascade at three representative incident angles (i = −10°, 0°, +10°) is investigated using large‐eddy simulation (LES) method with approximately 26 million structured grids. The simulations are carried out with Open Source Field Operation and Manipulation (OpenFOAM) for an off‐design low Reynolds number condition Re = 40,000 and Ma = 0.5. The time‐averaged aerodynamic performances and three‐dimensional instantaneous flow characteristics of the LPT cascade are discussed in detail, and the results show that the blade loading and the transverse pressure gradient (TPG) from the pressure side to the suction side of leading edge (LE) increase with increasing incidence angle. At the incidence angle of +10°, the LPT cascade undergoes separation and reattachment from the LE to around 20% Cax on the suction surface, which weakens the separation phenomenon at the rear half of the suction surface and the total pressure loss downstream of the trailing edge (TE). It is worth noting that the RANS method underpredicts the blade boundary layer separation region compared with LES under these extreme conditions. These results may offer off‐design condition data for the future improvement of RANS models or experimental studies.

Experimental investigation of the Aero-Acoustics of a rectangular jet impinging a slotted plate for different flow regimes

Impinging jets are found in many industrial applications and specifically in ventilation systems. Mostly, these jets are turbulent and under certain conditions can be a source of discomfort in closed areas due to the high level of noise it can generates. Therefore, it is very important to understand the flow dynamics that is responsible of generating the acoustic field in order to control and reduce such phenomenon. In this paper, an experimental study of a rectangular impinging air jet on a slotted plate is considered for three different Reynolds numbers producing self-sustaining tones (Re = 4100, 5100, and 5900). The sound pressure and spatial velocity field are obtained simultaneously using four microphones located at different locations and SPIV technique. Results show that Re = 5100 correspond to a critical regime where there is a significant drop in the sound pressure level (SPL) that reached 5 dB when compared to a lower Reynolds number Re = 4100. The flow dynamic analysis suggests that this drop in SPL could be contributed to the path followed by the large coherent structure at Re = 5100, where they are deviated in the transverse direction along the wall leading to low energy transfer from the dynamic field to the acoustic one. However, at Re = 4100 and 5900 the peak in SPL could be contributed to the two paths followed by the large coherent structures and particularly to the path where vortices hit the slotted plate before escaping through it. This path is responsible for the optimization of energy transfer from the dynamic field to the acoustic field. Moreover, at Re = 5900 the spectrogram of the instantaneous frequency and instantaneous flow dynamics results show that the acoustic frequencies (160 Hz and 320 Hz) are generated by symmetric and anti-symmetric modes respectively. This unusual aspect is related to the sudden changes in vortex mode.

CHARACTERIZATION OF SEPARATED SHEAR LAYER OF ELEVATED JET IN CROSSFLOW AT LOW-VELOCITY RATIO

The present study reports an experimental investigation of the flow dynamics of separated shear layers of an elevated jet-in-crossflow (EJICF). The current work aims to enhance the understanding of the downwash of jet fluid into the stack-wake region, akin to plume downwash in chimney flows, particularly at a low-velocity ratio. One end of the square stack with the aspect ratio of 9.0 is kept on a flat plate while the jet emerges from the free-end open to crossflow. Flow visualization and particle image velocimetry (PIV) experiments are conducted in a recirculating, low-speed water tunnel for three Reynolds numbers, namely Re = 1000, 2000, and 4000, based on the side of the square stack (<i>D</i>) and the freestream velocity (<i>U</i><sub>∞</sub>). The study mainly focuses on the jet shear layer characteristics and the downstream wake near the free-end formed by the interaction of crossflow and the jet shear layer. The instantaneous flow behavior includes the analysis of the shedding of the jet shear layer in a symmetric plane (<i>z</i> = 0) at a constant velocity ratio of <i>R</i> = 0.5. On the windward side, the jet shear layer reveals the formation of small-scale vortices due to the Kelvin-Helmholtz (KH) instability for Re ≥ 2000. Also, it is found that at a high Reynolds number (Re = 4000), an unstable vortex appears above the jet exit on the upstream side, mentioned as a hovering vortex. The mean and fluctuating flow behavior are investigated using streamline and contour plots of mean velocity and fluctuating components.

Flood Risk Mapping of Kankai Basin: A Case Study of Shivasatakshi Municipality and Kankai Municipality

Floods are probably the most frequent, widespread, and disastrous hazards in the world. It causes loss of life, casualties, financial loss, and displacement. Heavy monsoons, fragile geography, and constructions along the embankments cause the river to flood. The increase of population and squatter settlement of landless people living at the bank of the river has tremendous pressure on encroachment of flood plain making them vulnerable to flood damage. This study aims to develop HEC-RAS models for the Kankai River basin for preparation of 1D flood plain maps, vulnerability, risk assessment of RCC building in Shivasatakshi Municipality ward 10, and Kankai Municipality ward 4. Flood frequency analysis for 2, 10, 25, 50, 100, 200 and 1000 years return was carried out by Log Pearson III, Gumbel and Log Normal method based on maximum instantaneous flow recorded at Mainachuli station. The result of flood frequency analyzed by Log Normal method for 2, 10, 25, 50, 100, 200 and 1000 years return period floods shows higher among the results obtained using different methods. The flooding impact of the Kankai River being a large catchment with braided river form inundates vast downstream flood plain regions during high flood levels. With numerous people living along the river and downstream plains, flood risk is predominately high in the Kankai River catchment. With increasing flood intensity, higher flood depth increases and lower flood depth decreases. The inundation of a large area of Shivasatakshi Municipality ward 10 and Kanaki Municipality ward 4 at the bank of the Kankai river indicates that human lives and the physical infrastructure will be more vulnerable to flood disasters in the future. According to a study, the largest areas affected by floods with return periods of 2, 10, 25, 50, 100, 200, 1000 years, respectively, are 1843.24 ha, 1967.96 ha, 2016.32 ha, 2049.48 ha, 2079.64 ha, 2109.4 ha and 2176.08 ha of the whole basin. As a result, the study may help in future disaster planning and management.

Resolution assessment of Reynolds stress model-based DES methods for high-lift device flow over 30P30N configuration

Flows over high-lift devices (HLD) contain abundant complex physical characteristics such as flow separation, free shear layer transition, and turbulent vortex evolution, which cause challenges to the numerical prediction of unsteady flow characteristics over the HLD. The recently developed detached-eddy simulation (DES) based on the Reynolds-stress model (RSM) is expected to resolve the complex flow characteristics with high fidelity due to the RSM model holding fewer modeling assumptions than classical eddy-viscosity models (EVM). To assess the flow resolution performance of the RSM-based DES method and its dynamic version on the HLD, they are adopted in the numerical investigation of flow over a 30P30N multielement airfoil, a canonical HLD configuration with sufficient experiment measurements. The widely used EVM-based DES method, the shear-stress transport (SST)-based DES method, is also applied for the numerical investigation as the benchmark DES method for the flow resolution performance assessment of RSM-based DES methods. The results are compared with the available experimental data and the benchmark DES method, including average quantities, fluctuation quantities, and instantaneous flow field. It is found that RSM-based DES methods can overcome the prediction delay matter of free-shear layer instability appearing in the SST-based DES method. The dynamic RSM-based DES method can predict the fluctuation flow characteristics in the slat cove of 30P30N configuration more exactly than the other two methods and resolve finer vortex structures in the flow field.

Fast estimation of airflow distribution in an urban model using generative adversarial networks with limited sensing data☆

Fast estimation of instantaneous urban airflow distribution can protect pedestrians and objects from high-speed winds and promote high-efficiency natural ventilation. Statistical values based on instantaneous airflow distribution can also provide valuable data for wind environment assessments and urban construction. In this study, we evaluated the potential of generative adversarial networks (GANs), a deep-learning-based generative model that learns the probability distribution of training data and generates new data for estimating instantaneous flow fields in an urban model. It utilizes the velocity obtained from sensors as input, and we investigated the influence of constraints on the estimated results. We used two non-linear models based on GANs: Wasserstein GAN with gradient penalty (WGAN), which has no constraints, and conditional WGAN (CWGAN), which has constraints. We employed a flow field dataset obtained using a large-eddy simulation (LES) for model training and testing. The feasibility and accuracy of the flow field generated by the GANs were verified using the results of the LES and proper orthogonal decomposition–linear stochastic estimation (POD–LSE). For the estimated results, although the WGAN learned the probability distribution of the training data, it was unable to determine the temporal relationship among the estimated data. Conversely, the CWGAN successfully estimated the airflow distribution data for reconstruction and prediction, including the temporal relationship. Furthermore, the statistical quantities of the results from the CWGAN were closer to those of the LES than those of the POD–LSE.

Flow Field Analysis of a Turbulent Channel Controlled by Scalloped Riblets

Riblets are small protruding surfaces along the direction of the flow, and are one of the most well-known passive turbulent drag reduction methods. We investigated a scalloped riblet, the shape of which was constructed by smoothly connecting two third-order polynomials and was not as sharp in the tip as corresponding triangular riblets with the same height-width ratio. Numerical simulations were performed for turbulent channel flow with and without riblet control at an estimated optimum width of W+=20 and a height-width ratio of 0.5. A drag reduction rate of 5.77% was obtained, which is generally larger than the reported drag reduction rates of corresponding triangular riblets from the literature. Mean flow fields and second-order statistics of velocity, vorticity, and Liutex, a quantity introduced to represent vortices, were reported. It was found that streamwise vortices just above the riblet tips, which have a length scale of 200−300 in wall units, have an important influence on those statistics and thus the turbulence generation cycle and the drag reduction mechanism. Pre-multiplied energy spectra of streamwise velocity and the Liutex component were reported to reveal the length scales in the flow field. Instantaneous vortical flow fields visualized by the Liutex method were provided with emphases on the streamwise vortices just above riblet tips. It should be noted that the class of scalloped riblets is suitable for investigations on the influences of curvatures at the tip and the valley of the riblet in future.

Insights into the flame transitions and flame stabilization mechanisms in a freely falling burning droplet encountering a co-flow

The present study investigates the flame dynamics of a contactless burning fuel droplet under free fall subjected to a co-flow. The dynamic external relative flow established due to co-flow and droplet acceleration results in a series of droplet flame transitions. Different flame structures were observed, including a wake flame, reversed wake flame and enveloped flame. Following ignition, the droplet is allowed to fall through the central tube of a co-flow arrangement, and, at its exit, the droplet flame encounters the co-flow. The wake flame, which was established based on the droplet's instantaneous velocity of descent, encounters the abrupt relative velocity jump due to the co-flow. This causes the droplet flame to go through various transitions as it approaches equilibrium with the surrounding flow. Once it equilibrates, the droplet flame evolves in response to the instantaneous relative flow velocity. The droplet flame evolves by altering both its shape and the stabilization mechanism. Two stabilization mechanisms were identified for the droplet wake flame: edge-flame stabilization and bluff-body stabilization. The stabilization mechanism for different flame structures and the transition events have been theoretically analysed, and the relation between flame shape evolution and flow velocity has been determined based on the flow-field characteristics at the corresponding Re (Reynolds number) range. Furthermore, these correlations are employed in a mathematical formulation based on the spring–mass analogy, which predicts the droplet flame evolution after encountering the co-flow, including all the transition events.

Analysis of Single-Blade Passage and Full Circumference Large-Eddy Simulations of Turbine Rim Seal Flows

Abstract The flow in a 1.5-stage axial flow turbine with 16 vanes and 32 blades is investigated by large-eddy simulations with special focus on the analysis of the hot gas ingress through the rim seal into the upstream wheel space. Two setups with different solution domains are used. In the first large-eddy simulation, the full circumference of the turbine is resolved on a mesh with approximately 1 billion mesh cells. The second setup includes only a single-blade passage and is solved on a mesh with approximately 75 million cells. The analysis shows that the flow fields inside the upstream wheel space, i.e., between the disk of the upstream stator and the rotor disk strongly differ. In the 360 deg domain, two large-scale rotating vortex structures are observed, which have a strong impact on the ingress of main annulus gas into the wheel space. These structures cannot be captured in the single-blade passage domain. The instantaneous flow fields are further analyzed by dynamic mode decomposition to determine similarities and differences in the two flow fields. The comparison of the modes from the two setups reveals that the hot gas ingress from modes at the blade passing frequency generates a reduced sealing efficiency only at the outer radius of the wheel space. The additional presence of large-scale modes, however, reduce the sealing efficiency also in the inner wheel space. These results suggest that a single-blade passage simulation cannot be used for a reliable prediction of the hot gas ingress for the investigated turbine setup and operating condition.

Hydrodynamics and shape reconstruction of single rising air bubbles in water using high-speed tomographic particle tracking velocimetry and 3D geometric reconstruction

Time-resolved tomographic particle tracking velocimetry (TR-3D-PTV), also called 4D-PTV, is used here to obtain the instantaneous 3D liquid flow field information in the wake of a single rising bubble in water. Simultaneously, the bubble shape, size and velocity are determined by tomographic reconstruction of the 3D bubble shape. Both, tracer particles for PTV and bubbles, are imaged in a shadow mode with background illumination. The Lagrangian method used in this paper, especially combined with the shake the box algorithm, has big advantages compared to particle image velocimetry, in situations, where only low particle per pixel values can be obtained. In this research, single air bubbles of different sizes, with diameters of around 2.4 mm, 4.0 mm, 6.0 mm and 9.6 mm, were injected into stagnant de-ionized water. Their shape was reconstructed in 3D, and an equivalent bubble diameter was determined from this reconstruction. Compared to conventionally used 2D shadow imaging, this diameter is about 13% smaller. The 3D bubble trajectory can be analysed and decomposed into a sinusoidal function curve lateral projection and an ellipsoidal shape vertical projection. As the bubble diameter increases, the radius of the spiral trajectory is decreasing as well as the amplitude of vertical sinusoidal oscillation. The wake structure in the liquid behind the bubbles is also changing with bubble size: from simple vortex pairs for smaller bubbles to an intertwined structure of several twisted vortices for the bigger ones.Graphical abstractThree-dimensional bubble reconstruction (grey surface) and liquid stream lines coloured with velocity magnitude around an ascending air bubble in de-ionized water.

Effects of Riblet Dimensions on the Transitional Boundary Layers Over High-Lift Turbine Blades

Abstract Substantial research exists in the literature on reducing the profile loss of transitional boundary layers over low-pressure turbine (LPT) blades via different mechanisms such as freestream turbulence, upstream wakes, and surface roughness. These mechanisms have proven to be beneficial in mitigating the separation bubble-related losses in ultra-high-lift blade designs, despite an increase in the loss due to increased turbulent wetted area (TWA). In this work, we adopt a strategy of employing surface roughness in the transitional regime to minimize the separation bubble-related losses and flush-mounted riblets downstream to further mitigate the skin-friction drag and boundary layer losses due to an increase in the TWA. Several high-fidelity scale-resolving simulations are performed on this “rough-ribbed blade surface” to discern the effect of varying the riblet spacing (s+) and height (h+). The streamwise evolution of skin-friction coefficient, boundary layer integral parameters, and shape factor are compared and contrasted among riblets of different dimensions. The instantaneous flow features and second-order statistics such as the Reynolds stress, turbulent kinetic energy, and its production are analyzed for different test cases to determine the impact of riblets on these quantities. When compared to the roughness alone configuration, the scalloped shape riblets with s+ = 17 and h+ = 22 reduced the net skin-friction drag by 7.3% and the trailing edge momentum thickness by 14.5%, thereby demonstrating the efficacy of riblets in reducing the mixing losses under adverse pressure gradients. Through an analysis of flow blockage introduced by the application of riblets, the deleterious effects of increasing the riblet height along with the necessity of optimizing the riblet ramp are highlighted.

Mode identification and decomposition analysis of self-excited thermodynamic oscillations in hypersonic inlet/isolator of a scramjet

The self-excited pressure and velocity oscillations caused by the downstream back pressure in a hypersonic inlet/isolator directly affect the operational stability of a scramjet. In this work, we conduct the two-dimensional unsteady RANS (Reynolds averaged Navier–Stokes) simulation to predict the flow field in the Mach 7 inlet/isolator with 147 times the freestream static pressure. With the model validated, the time evolution process of the instantaneous flow fields in the isolator is analyzed first. It is shown that the structure of the shock train exhibits an up-down asymmetry. The flow oscillations as observed near the upper wall are more complex than those near the lower wall. Furthermore, the dominant frequency of the wall pressure oscillations is approximately 520 Hz. However, the self-excited oscillations in the isolator flow are dominated with approximately 510 Hz mode but coupled with multiple harmonic higher frequencies. To identify and decompose the modes of these oscillations in the unsteady flow of the isolator, different mode decomposition technologies are applied to further examine the mechanism of such oscillations. By conducting Proper Orthogonal Decomposition (POD) analysis on the x-axis velocity and Mach number fields, it is shown that the first two modes consist of more than 77% of the total oscillations’ kinetic energy. Additionally, the oscillations of the quadrilateral separation zone and low-speed zone near the upper wall largely determine the dominant frequency of the overall unsteady flow field. As Dynamic Mode Decomposition (DMD) investigation is conducted on the x-axis velocity and Mach number fields, it is found that the identified first mode is almost the same as that identified by using POD. It is also found that both DMD and POD can well predict the viscous flows such as the separation zone, shear layer, and low-speed zone. Finally, the dominant frequencies predicted by the POD and DMD are revealed to be very close to the dominant frequency of the wall static pressure oscillations. The movements of multiple shock waves near the upper wall are the major incentive for the complicated change in the entropy generation rate. In general, the self-excited thermodynamic oscillations phenomenon as observed in the hypersonic inlet/isolator in a scramjet could be well analyzed by decomposing and identifying the dominant modes, which contribute mostly to the total energy of such oscillations.

Coupled dynamics of the moving Antarctic krill trawl structure and its hydrodynamics behavior using various catch sizes and door spreads based on wavelet-based and Fourier analysis

Antarctic krill is a key element of the complex ecology of the Antarctic Ocean and a target species for commercial fisheries for aquaculture supply, krill oil production, and as a major food source for various natural predators. Therefore, understanding the interaction between the fluid structures of the Antarctic krill trawl structure is the key factor in solving problems such as selectivity, energy efficiency, catchability, and ecological sustainability in this fishery. Thus, this study experimentally investigated the influence of catch sizes, door spread, and inflow velocities on the bridle tension, trawl motions, trawl deformation, and flow field inside and around a scaled trawl model in flume tank based on the electromagnetic current velocity meter measurements. Fourier analysis using power spectrum density and the continuous wavelet transform is used to analyze the time-frequency contents of the instantaneous flow velocities, bridle tension, and trawl motions; and in the interaction between the unstable turbulent flows and the fluttering trawl motions, wavelet coherency analysis was employed to detect coherent structures. Results indicated that the bridle tension and trawl motions increased as the catch size, door spread, and inflow velocity increased. Owing to the significant trawl deformation, a complex fluid–structure interaction occurred and a strong positive correlation between the bridle tension and trawl motions was obtained. Furthermore, a significant decrease in the flow field occurred inside and outside different parts of the trawl. Fourier and wavelet analysis results showed that the trawl motions and bridle tension are mainly of a low-frequency activity and of another component related to the unsteady turbulent flow street, and decreased with the increasing catch size and door spread. A complex fluid–structure interaction is then demonstrated where the hydrodynamics of the moving Antarctic krill trawl structure are an intricate interplay between trawl fluttering motions and unsteady turbulent flow influenced by catch size and door spread. The knowledge of such trawl hydrodynamic behavior is of great importance to understand and design the optimal structure of trawl nets to maintain the sustainability of the Antarctic krill fishery.

A Mathematically Exact and Well-Determined System of Equations to Close Reynolds-Averaged Navier–Stokes Equations

Since Sir Osborne Reynolds presented the Reynolds-averaged Navier–Stokes (RANS) equations in 1895, the construction of complete closure for RANS equations has been regarded as extremely challenging. Taking into account that the Navier–Stokes equations are not coherent for instantaneous and mean flows, a body of knowledge outside the scope of classical mechanics may be amenable to the closure problem. In this regard, the methodology of physics-to-geometry transformation, which is coherent for both flows, is applied to RANS equations to construct six additional equations. The proposed equations stand out from existing RANS closure models and turbulence quantity transport equations in two respects:they are mathematically exact and well-determined.

A Thermal Anemometry Method for Studying the Unsteady Gas Dynamics of Pipe Flows: Development, Modernisation, and Application.

A detailed study of the gas-dynamic behaviour of both liquid and gas flows is urgently required for a variety of technical and process design applications. This article provides an overview of the application and an improvement to thermal anemometry methods and tools. The principle and advantages of a hot-wire anemometer operating according to the constant-temperature method are described. An original electronic circuit for a constant-temperature hot-wire anemometer with a filament protection unit is proposed for measuring the instantaneous velocity values of both stationary and pulsating gas flows in pipelines. The filament protection unit increases the measuring system's reliability. The designs of the hot-wire anemometer and filament sensor are described. Based on development tests, the correct functioning of the measuring system was confirmed, and the main technical specifications (the time constant and calibration curve) were determined. A measuring system for determining instantaneous gas flow velocity values with a time constant from 0.5 to 3.0 ms and a relative uncertainty of 5.1% is proposed. Based on pilot studies of stationary and pulsating gas flows in different gas-dynamic systems (a straight pipeline, a curved channel, a system with a poppet valve or a damper, and the external influence on the flow), the applications of the hot-wire anemometer and sensor are identified.

Quantifying instantaneous flow reversal of tracer particles in subsonic, transonic and supersonic flows past a circular cylinder

Tracer pathline statistics have rarely, if at all, been used in the investigation of compressible subsonic, transonic and supersonic flows past a circular cylinder. Here, we report pathline histogram, conditional histogram and zonal histogram data, focusing on the characteristics of particle instantaneous flow reversal and their dependencies on Mach number and tracer release position. The far-upstream Mach number and the diameter-based Reynolds number used for these three flow regimes are ( 0.2 , 10000 ) , ( 0.9 , 3900 ) and ( 1.2 , 10000 ) , respectively. The subsonic surface-release histogram profiles display unexpected persistent oscillations at a frequency 50 times of the periodic vortex shedding frequency, reflecting the effect of a very small magnitude zig-zagging type of particle motion in a region very close to the wall and slightly behind the mean flow separation location. No similar high-frequency oscillation is observed in the corresponding transonic and supersonic histograms likely because, at Ma ≥ 0.9 , periodic vortex shedding ceases to exist and the near-wake becomes a quasi-laminar recirculation zone bounded by a pair of converging slip-layers with a neck opening to the far-wake. At the instant of 5 subsonic vortex-shedding periods after being released from the cylinder surface, there are 1.67 % , 35.2 % and 39.7 % particles that experience flow reversal in the subsonic, transonic and supersonic flows, respectively. Surprisingly, in the present transonic flow, growth of the turbulent far-wake behind the pair of lambda-shocks is constrained over a considerable streamwise distance.

Comparison and validation of various drag models for fluidization characteristics of bubble fluidized beds with a high-speed particle image velocimetry experiment

Bubbling liquefaction of dense particles is one of the most common forms of industrial fluidization in gas–solid flow systems. Computational fluid dynamics and the discrete element method are important tools for studying dense gas–solid flows. In these methods, the momentum transfer between phases relies on a drag model, so a reasonable choice of drag model is crucial for accurately predicting the hydrodynamic behavior of dense gas–solid flows. This paper investigates the effect of different drag models on the flow behavior prediction of dense gas–solid flow for the “Small-Scale Challenge Problem-I” published by the National Energy Technology Laboratory in 2013. The gas–solid fluidization characteristics, such as instantaneous particle flow processes, particle velocity vector distributions, changes in the fluidized bed height, and average gas phase pressure drops, were compared for different drag models. A detailed validation analysis of each dominant drag model was carried out in conjunction with the experimental data. The results show that the drag model significantly affects the numerically predicted results of particles’ hydrodynamic behavior, especially in terms of the bed height variation and the remixing behavior of particles. These research results are expected to improve the predictive accuracy of gas–solid flow hydrodynamic behavior and provide guidance for designing and optimizing fluidized beds, which has theoretical and practical significance.

Flow-Induced Vibration of Cantilever Type Elastic Material in Straight Tricylinder

The hydrodynamic performance and wake evaluation results of a cantilever flexible cylindrical triangular array at Re = 9000 and with L/D spacing ratios ranging from 3 to 7 are presented in this paper. The improved delayed separation eddy current simulation (IDDES) SST k-ω turbulence model and tetrahedral mesh are employed to solve the three-dimensional instantaneous flow field. The objective is to find the L/D ratio that the upstream cylinder has the most pronounced effect on the downstream cylinders, to quantify whether a flexible upstream cylinder is more favorable than the rigid upstream cylinder, and to know the flexible cylinder wake distribution. The findings indicate that, at Re = 9000, when the spacing ratio is 3~7 and the spacing ratio L/D is 6, the amplitude of the downstream cylinder is most affected by the wake of the upstream cylinder; the amplitude of downstream cylinders of the rigid upstream cylinder exerted a 1% or 2% greater effect than the flexible cylinder, with vortices predominantly concentrated in the middle and upper parts of the flexible cylinder rather than at its top.