Local Flow Field Research Articles

An axisymmetric flow analysis by means of tangent hyperbolic fluid with Cattaneo-Christov heat and mass flux model

In present analysis three dimensional steady, axisymmetric, incompressible, non-Newtonian tangent hyperbolic flow of nanofluid due to rotating disk is investigated. A rotating disc is a phenomena in which a disc rotates around an axis at a constant angular velocity and each point on the disc follows a circle's arc. The force necessary to move the disc to overcome frictional resistance is determined by the shear stress between the disc and the rotating fluid and the heat transfer is affected by the local flow field. The heat and mass transfer field is explored in the existence of chemical reaction and Cattaneo-Christov heat and mass flux model. By using similarities transformations, the supreme partial differential equations of the problem are turned into typical differential equations respectively. The determining differential equations are solved on MATLAB software by using bvp4c method that has been cracked through shooting technique. The deviation of velocity, temperature and mass profiles for different inputs of power-law index, Prandtl number, Brownian motion parameter, Schmidt number, thermophoretic parameter, heat and mass relaxation time parameters and chemical reaction parameter are displayed via graphs. The increment happens in radial velocity field whereas tangential and axial velocity fields reduces due to power index parameter n. The results show that for Cattaneo-Christov model, the temperature and concentration profiles exhibit reducing behaviour than that of Fourier's classical model. The concentration within fluid declined due to chemical reaction.

Numerical Simulation of the Flow around NACA0018 Airfoil at High Incidences by Using RANS and DES Methods

In this work, the flow around the NACA0018 airfoil with a wide range of attack angles was investigated based on the open-source computational fluid dynamics (CFD) platform OpenFOAM. Two numerical methods, Reynolds-averaged Navier–Stokes (RANS) and the detached eddy simulation (DES), were employed. Under the premise of a grid convergence analysis, the computed lift and drag coefficients were validated by the available experimental data. The pressure distribution, the complex flow mechanisms of the airfoil under the attached flow regime, the mild separation flow regime, and the post-stall flow regime, combined with the shedding vortex structures, streamlines, and vorticity distributions, are discussed. From the numerical results, it can be seen that the DES computation presents a better accuracy in the prediction of the lift and drag coefficients, with a deviation less than 10% at the largest angle of attack. Meanwhile, it also presents remarkable improvements in capturing the local flow field details, such as the unsteady separated flow and the shedding vortex structures.

Experimental investigations of hypersonic inlet unstart/restart process and hysteresis phenomenon caused by angle of attack

In this study, the unstart/restart process of a hypersonic inlet caused by the angle of attack (AOA) is experimentally performed at a Mach number of 5.0, and the unsteady flow characteristics during the test are investigated. The flow characteristics are simultaneously recorded using high-frequency pressure measurement and high-speed schlieren imaging. The results show that the inlet enters the unstarting state at AOA = 8.4° but restarts at AOA = 0.3°, exhibiting an obvious hysteresis phenomenon. During the unstart process, the forebody shock gradually approaches the body and then impinges on the inside of the cowl with the increasing angle of attack, indicating that the inlet works under an overrated mode. As the angle of attack increases further, the airflow chokes, and a high-pressure zone is first formed in the exit of the duct. Then, the high-pressure signal propagates upstream in the duct, leading to the emergence of an inlet unstart. When the inlet enters the unstarted pattern, a typical local unstarted flow field is formed. Low-amplitude oscillations occur in the internal contraction region, but further propagation to the isolator is weak. During the restart process, when the angle of attack decreases from 3.2° to 0.3°, two kinds of big buzz patterns, namely, intermittent high-amplitude oscillation and periodic high-amplitude oscillation, appear successively. The intermittent high-amplitude oscillation is characterized by the intermittent occurrence of a high-amplitude oscillation after the low-amplitude oscillations for a random length of time. In the periodic high-amplitude oscillation, the separation bubble is repeatedly disgorged and swallowed, resulting in a dominant fluctuation frequency in the entire duct. Furthermore, as the angle of attack decreases, the dominant frequency gradually rises from 280 Hz to 339 Hz. This work provides new insights into the flow evolutions and unsteady behaviors of hypersonic inlet unstart/restart owing to the AOA and provides potential applications on the detection and prediction of hypersonic inlet unstart/restart.

Design and numerical investigation of multi-channel cooling plate for proton exchange membrane fuel cell

Proton exchange membrane fuel cells need cooling plates with strong heat exchange capacity to maintain temperature balance. In order to obtain better cooling performance, four new types of flow field distribution of cooling plate are designed, including local serpentine channel, local parallel channel and the flow field in which the four channels are symmetrically distributed along the diagonal and horizontal centerline of the cooling plate. The maximum temperature, temperature difference, temperature uniformity index and pressure drop of the multi-channel cooling plate under different working conditions are analyzed by numerical simulation. The results show that the multi-channel design can effectively enhance the heat transfer effect of the cooling plate and reduce the pressure drop. The local serpentine channel design can increase the fluidity of the coolant, avoid local reflux, make the temperature distribution more uniform and avoid local overheating. The maximum temperature of local serpentine channel flow field is 1.2 K lower than that of local parallel channel flow field. The pressure drop of internal parallel flow field design is 2.58 kPa, which is 62% lower than that the design of internal serpentine flow field. Increasing the inlet coolant flow can strengthen the heat transfer capacity of the cooling plate, when the flow rate increases from 2e-6 m3/s to 4e-6 m3/s, the maximum decrease is 8.53% of Model6. But it will increase the pressure drop of the channel with long cooling channel, the maximum pressure drop increase is 924 kPa when the inlet flow increases from 6e-6 m3/s to 8e-6 m3/s.

Imaging of particle deposition and resulting flow field during flocculation filtration within a granulated activated carbon filter

Flocculation filtration onto granulated activated carbon (GAC) filters is an advanced treatment method to achieve lower phosphorous and micropollutant discharge in wastewater treatment plants. However, the suitability is put into question as the deposition of suspended solids (SS) onto a GAC filter is aligned with more frequent backwashing ongoing with abrasion of GAC and downtimes. To evaluate the filtration process of SS onto GAC filters on the pore scale, we resolved the particle deposition in a timely and spatially resolved image series by means of magnetic resonance imaging (MRI). Besides structural images, flow measurements were acquired to evaluate the impact of SS accumulation onto the local flow field. The column had an inner surface area of 1.54 cm2 and was filled with GAC up to 10 cm in height. In total, 6.4 L of wastewater originating from a secondary clarifier were added to the GAC filter in steps of 0.2 L. The wastewater contained approximately 8.4 mg/L TSS after flocculation and precipitation. Results of one experimental run are presented. Particle accumulations within the GAC filter as well as a formation of a deposit layer was found. During the filtration process preferential inflow points into the GAC filter were recognizable. The transition of a preferential inflow point to another was often aligned with the redistribution of upper parts of the GAC filter and a compaction of accumulated SS. Analysis of flow distributions revealed that the equivalent filter velocity of 28.8 m/h is 2.3 times higher compared to the average filter velocity (12.5 m/h including porosity). Further, the equivalent filter velocity increased up to 100.8 m/h at the end of the experiment. As particles do not distribute equally over the filter height, those velocities are not representative for the whole height of GAC filters. The current study provides insights into a real flocculation filtration process without using artificial materials for imaging purposes.

Numerical analysis of interaction between turbulent structures and transient sheet/cloud cavitation

This paper through the in-house code numerically examines the cavitation–vortex–turbulence interaction mechanism. The high grid resolution can obtain a more detailed flow field structure, which is helpful to reveal the relationship between cavitation occurrence and development and local turbulent flow field. Results are presented for a three-dimensional NACA66 hydrofoil fixed at an 8° angle of attack under a moderate Reynolds number of 1 × 106 and sheet/cloud cavitating conditions. Numerical simulations are performed via the boundary data immersion method coupled with the artificial compressibility method through a Fortran-based code. The results show that the numerical predictions are capable of capturing the unsteady cavitation characteristics, in accordance with the quantitative features observed in high-speed cavitation tunnel experiments. The evolution of the transient cavitating flow can be divided into three stages: growth of the attached sheet cavity, development of a re-entrant jet, and cloud shedding downstream. The Liutex method is applied to capture the vortex structure. Further analysis of the process of enstrophy transport reveals that cavitation promotes vortex production and increases the enstrophy as the cavity becomes more unstable. Moreover, the structure of the vortex gradually evolves from a vortex tube to a U-type vortex, Ω-type vortex, and streamwise vortex. Finally, the interaction between cavitation and turbulence is expounded using the turbulent energy transport equation, which demonstrates that cavitation promotes the production, diffusion, and dissipation of turbulent kinetic energy, while the viscous transport term only acts during the process of cloud cavity shedding.

Study on flow distribution of irrigation canal system based on image velocimetry

• The YOLOV5 algorithm combined with the monocular ranging principle is used for fast flow measurement of UAV images for UAV hovering type measurement of local river surface flow fields. • Based on the smooth circular arc channel distribution model, the velocity distribution law of Yongji section, Yonggang section and Yonglan section is studied and summarized. • Based on The Rule of Tyson polygon, combined with the representative point of average velocity, the measurement of flow through water section is calculated, and the contactless measurement is realized. With the development of computer technology, it has become possible to measure surface flow velocity using video technology. Ripples are produced when flowing, and flow velocity can be determined based on the ripples and floating objects in water. A UAV velocity measurement system based on optical flow method and YOLOV5 algorithm in deep learning has been established, to realize the monitoring of surface flow field of large rivers. The optical flow method and the deep learning algorithm are used to track the target, convert the pixel distance to actual distance by monocular ranging, and calculate the actual flow velocity. The method was successfully applied to the surface flow field measurement of the Yongji canal in the river-loop irrigation area of Inner Mongolia, and the results showed that the orthophoto graphic images obtained by the method were of high quality, the flow field calculation results were reasonable, and the calculation results agreed well with the measurements.

Preferential orientation of tracer spheroids in evolving Taylor–Green vortex flow

Rod- and disk-like particles preferentially align parallel and perpendicular, respectively, to the fluid vorticity, both at the early as well as later stages of the unsteady Taylor–Green vortex (TGV) flow. The early stage of the flow is laminar and comprises anisotropic large-scale Taylor–Green structures, while the later stages resemble homogeneous isotropic turbulence with Kolmogorov-type small-scale structures. The reason for the orientational behavior of inertialess spheroids in the early stage of the TGV-flow has been sought by examining the alignments of spheroidal particles, not only with vorticity but also with Lagrangian stretching and compression directions of the fluid elements in our earlier paper [Jayaram et al., “Alignment and rotation of spheroids in unsteady vortex flow,” Phys. Fluids. 33, 033310 (2021)]. This article is a sequel to the above paper in which the spheroids' alignments are studied locally, in contrast to the volume-averaged statistics studied previously, to observe the influence of the local flow field on the spheroidal alignment. It has been observed through our studies that the alignments vary periodically in space and these variations can be associated with the large-scale periodicity of the flow field originating from the initial conditions of the TGV flow. Additionally, the intense vortex stretching in the early stages of the flow evolution is seen to be largely influencing the orientation of the spheroids.

Simulation of scour and deposition in the flow field and adaptability of fishes around eco-friendly notched groins

Over the past few decades, eco-friendly channel construction has gained wide attention around the world including China, as it considers both navigational and ecological functions. The Groin system is a hydraulic structure that can effectively increase the area of fish habitats. Groin shape has an influence on the local flow field and the scour and deposition pattern near the structure, thereby affecting the adaptability of fishes to the groin fields. In this study, a new groin structure with a notch in its middle part was proposed. A lab experiment was conducted on a physical model to investigate the characteristics of the flow field including patterns of water level distribution, flow velocity distribution, and scour and deposition on different submergence levels and incoming flows. This experiment used four notch depths: 0 (unnotched), 1/3 of groin height, 2/3 of groin height, and full groin height. Then the distribution of habitat suitability index (HSI) was calculated based on the assessment index system of the indicator fish species (Cyprinidae). The results showed that the notches created in the middle of localized overflow groins had a significant effect on the surrounding flow pattern. Due to localized overflow, both the flow velocity and the velocity gradient behind each groin increased. The flow velocity behind each groin was positively correlated with notch depth, while the flow velocity in the main channel was negatively correlated with notch depth. Under the complex flow conditions, depositional features did not occur along the edges of the groin fields, ensuring connectivity between groin fields and the main channel. An analysis of water depth and flow velocity suitability indexes of Cyprinidae suggests when notch depth is 2/3 of groin height, the notched groins have good ecological effects, especially in the groin fields.

Interaction between local corrosion and flow field of natural gas long-distance pipeline by artificial rectangular defect pit

Interaction between local corrosion and flow field of natural gas long-distance pipeline by artificial rectangular defect pit

Aerodynamic Behavior and Impact on Driving Safety of Spalling Blocks Comprising High-Speed-Railway Tunnel Lining

The lining of operating high-speed-railway tunnels suffers from cracks, the peeling of material, and other deteriorations and defects, all of which seriously affect driving safety. However, the trajectory of a falling block from the tunnel lining in a tunnel train wind environment and its impact on driving safety are unknown. To study the movement of falling blocks under a coupling effect of tunnel train wind and the local flow field of the falling blocks, a three-dimensional gas–solid-coupling numerical calculation model of spalling blocks–train–tunnel–air was established using FLUENT software The aerodynamic evolution mechanism governing the spalling blocks of the lining was analyzed, and we found that the falling process of a spalling block is affected by the coupling of its own characteristics and transient train wind. Train wind directly induced the horizontal motion of falling blocks, generated curves and eddy currents, and changed the motion state of spalled blocks. Furthermore, by comprehensively considering the motion trajectories of flaky and blocky blocks at different positions, the impact of spalling blocks on driving safety was obtained. The dangerous area of spalling blocks was approximately 5.5 m above the ground, which must be paid attention to. Our results provide guidance for the operation and maintenance of high-speed-railway tunnels.

On the scattering of focused wave by a finite surface-piercing circular cylinder: A numerical investigation

For nonlinear wave–structure interactions, the high-frequency scattered waves can be identified within the drag-inertia regime, especially in steep incident waves where viscous effects are not negligible. According to previous studies, this unexpected phenomenon is highly associated with the local flow field, posing challenges to the existing harmonic-based diffraction solutions (mostly up to second-order). To overcome these shortcomings in potential flows, we establish a high-fidelity numerical wave tank to solve this two-phase free surface flow in the open source computational fluid dynamics framework OpenFOAM. We implement the ghost fluid method to eliminate the spurious velocities, mostly reported in two-phase volume of fluid solvers, in the vicinity of the free surface and preserve a sharp air–water interface. A modified generating–absorbing boundary condition is employed to achieve high computational efficiency without passive relaxation zones. Good agreement with experimental data demonstrates the reliability and accuracy of the present numerical wave tank in extreme wave conditions. On this basis, this paper numerically investigates the wave scattering of the focused wave by a finite surface-piercing circular cylinder, with emphasis on the flow mechanism. Three types of high-frequency scattered waves are identified in the near field, namely, Type-1, Type-2, and Type-1* waves. The typical mechanisms of each type are analyzed in depth with detailed flow field data, which confirms and complements the observations from previous experiments. More importantly, the primary vortical structures involved in scattering are extracted by the Liutex vortex identification method. The behaviors of these vortical structures could characterize the evolution of the high-frequency scattered waves and provide new insights into this strongly nonlinear phenomenon. An overall schematic of the wave scattering evolution in this complex condition is summarized for a straightforward understanding.

Numerical investigation on underwater towed system dynamics using a novel hydrodynamic model

A new hydrodynamic model is proposed to investigate the dynamic characteristics of an underwater towed system consisting of an unmanned surface vehicle (USV), a towing cable, and a towed vehicle under different operation modes. A depth-tracking control model with a PID algorithm is proposed. The towing cable is divided into a series of discrete elastic catenary segments connected by nodes. The hydrodynamic force acting on the cable is determined by the Morrison equation. The flow field velocity around the towing cable is determined by the local flow field velocity at the nodes. The flow field around the underwater towed system is simulated using computational fluid dynamics method. Experiments were performed to verify the proposed method. The results by numerical simulation show that the draft of the USV increased by 12% under the influence of the towing cable and the vehicle, and sailing resistance of the USV significantly in non-controlled operation. Moreover, the control model is robust in depth tracking operation, the deviation between the prescribed vertical trajectory and the controlled one is acceptable, and the dynamic factors of the underwater towed vehicle are closely related to the longitudinal motion of the shifting weight.

Effect of Chordwise Struts and Misaligned Flow on the Aerodynamic Performance of a Leading-Edge Inflatable Wing

Leading-edge inflatable (LEI) kites use a pressurized tubular frame to structurally support a single skin membrane canopy. The presence of the tubes on the pressure side of the wing leads to characteristic flow phenomena for this type of kite. In this paper, we present steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations for a LEI wing for airborne wind energy applications. Expanding on previous work where only the leading-edge tube was considered, eight additional strut tubes that support the wing canopy are now included. The shape of the wing is considered to be constant. The influence of the strut tubes on the aerodynamic performance of the wing and the local flow field is assessed, considering flow configurations with and without side-slip. The simulations show that the aerodynamic performance of the wing decreases with increasing side-slip component of the inflow. On the other hand, the chordwise struts have little influence on the integral lift and drag of the wing, irrespective of the side-slip component. The overall flow characteristics are in good agreement with previous studies. In particular, it is confirmed that at a low Reynolds number of Re=105, a laminar separation bubble exists on the suction side of this hypothetical rigid wing shape with perfectly smooth surface. The destruction of this bubble at low angles of attack impacts negatively on the aerodynamic performance.

A local approximation model for macroscale transport of biased active Brownian particles in a flowing suspension

A dilute suspension of motile microorganisms subjected to a strong ambient flow, such as algae in the ocean, can be modelled as a population of non-interacting, orientable active Brownian particles (ABPs). Using the Smoluchowski equation (i.e. Fokker–Planck equation in space and orientation), one can describe the non-trivial transport phenomena of ABPs such as taxis and shear-induced migration. This work transforms the Smoluchowski equation into a transport equation, in which the drifts and dispersions can be further approximated as a function of the local flow field. The new model can be applied to any global flow field due to its local nature, unlike previous methods such as those utilising the generalised Taylor dispersion theory. The transformation shows that the overall drift includes both the biased motility of individual particles in the presence of taxis and the shear-induced migration in the absence of taxis. In addition, it uncovers other new drifts and dispersions caused by the interactions between the orientational dynamics and the passive advection–diffusion of ABPs. Finally, the performance of this model is assessed using examples of gyrotactic suspensions, where the proposed model is demonstrated to be most accurate when the biased motility of particles (i.e. taxis) is weak.

A novel interpolation-free sharp-interface immersed boundary method

This paper describes a novel 2nd order direct forcing immersed boundary method designed for simulation of 2D and 3D incompressible flow problems with complex immersed boundaries. In this formulation, each cell cut by the immersed boundary (IB) is reshaped to conform to the shape of the IB. IBs are modeled as a series of 2D planes in 3D space that connect seamlessly at the edges of the cut cells, in a way that mimics conformal grid. IBs are represented in a continuous and consistent fashion from one cell to another, thus eliminating spatial pressure oscillations originating from inconsistent description of the IB as well as the traditional stair-step problem, leading to a more accurate resolution of the boundary layer. Boundary conditions are enforced at the exact location of the IB devoid of interpolation, which guarantees sound simulations even on grids with high aspect ratio, and enables simulations of flow packed with multiple IBs in close proximity. Boundary conditions for each phase across the IB are enforced independently, yielding a unique capability to solve flows with zero-thickness IBs. Simulations of a large number of 2D and 3D test cases confirm the prowess of the devised immersed boundary method in solving flows over multiple loosely/closely-packed IBs; stationary, moving and highly morphing IBs; as well as IBs with zero-thickness. Results show that predictions from the proposed scheme agree remarkably well with theoretical models and experimental data for both integral variables and local flow fields and they are often with less than 1% deviation from solutions obtained by conformal grid of similar resolution.

Effect of Bottom Blowing Mode on Fluid Flow and Mixing Behavior in Converter

Bottom blowing agitation plays a crucial role in improving the reaction kinetics condition of molten bath during the steelmaking process. Herein, the influence of bottom blowing mode on the flow and mixing characteristics of molten bath and the abrasion characteristics of refractory lining in a 6:1 scaled-down model of a 100 t converter were investigated using physical and numerical simulations together. Eight bottom blowing modes were designed (uniform, three-point linear co-direction, three-point linear unco-direction, two-point linear, circumferential linear, A-type, V-type, and triangle alternating). The results indicated that bottom blowing mode has a significant effect on the local flow field at the inner ring of bottom tuyeres, the velocity interval distribution, and the turbulent kinetic energy, which in turn determines the tracer diffusion path and rate as well the mixing time of molten bath. Reasonable non-uniform bottom blowing modes promote the interaction between the various stirring sub-zones of the molten bath. Among them, the three-point linear co-direction mode and A-type mode have the highest mixing efficiency under the conditions of bottom blowing and combined blowing, respectively, which is superior to the uniform mode. In addition, the bottom blowing mode changed the location and degree of abrasion of the refractory lining, and the total abrasion of the non-uniform mode was reduced. The average value and fluctuation degree of integral wall shear stress for the A-type mode were minimal.

Experimental study of cavitating flows of protuberance on the surface of underwater vehicle

The high-speed movement of the underwater vehicle causes cavitation in the flow field. For the underwater vehicle with protuberances on the surface, the geometric shape of the protuberance is bound to affect the local flow field of the underwater vehicle. Furthermore, it affects the generation, development and shedding of cavitation around the vehicle. In order to explore the influence of the geometric shape of the protuberance on the generation and development of the cavitation phenomenon, the experimental and high speed imaging technology are carried out to study the cavitation characteristics of the protuberance on the surface of the underwater vehicle. The results show that with the continuous decrease of vertical motion pressure, the cavitation number decreases gradually, the cavitation scale increases continuously and the phenomenon of periodic shedding occurs. Different shapes of protuberances produce different cavitation effects. The increase of protuberance height can restrain the shedding of cavitation bubbles and form larger cavitation bubbles. The shape of the arc-shaped protuberance can inhibit the occurrence of cavitation and produce fewer cavitation bubbles. The size of the cavitation bubbles produced by the bevel-shaped protuberance is larger, but the cavitation bubbles shedding frequency is also higher.

Data-driven model for improving wall-modeled large-eddy simulation of supersonic turbulent flows with separation

A machine learning algorithm is presented, serving as a data-driven modeling tool for wall-modeled large eddy simulations (WMLESs). The proposed model is formulated to address the problems of log layer mismatch and inaccurate prediction of skin friction, particularly for supersonic separated and reattached flows. This machine learning algorithm uses random forest regression to map the local mean flow fields to the discrepancies in the skin friction (heat flux) while complying with Galilean invariance as the flow features input is provided using relative velocities. The model is tested on two different supersonic flows, namely, flow over a flat plate and flow around an expansion-compression corner. The performance is evaluated by comparing the skin friction (heat flux) and flow properties with exact values. The ultimate goal is to build a robust and generalizable machine learning model to improve the prediction of WMLES of supersonic flows. To this end, the model is trained by a set of flows containing some essential flow physics to devise a generalizable model. Although the general machine learning model shows some advantages over the baseline WMLES model, it is concluded the data set is far from being representative of the rich flow physics model; therefore, the machine learning model should be trained and tested by a broader set of flows.

Experimental and computational study of hull–propeller–rudder interaction for steady turning circles

The hull–propeller–rudder interaction of the Korea Research Institute of Ships & Ocean Engineering Container Ship is studied using a combined experimental fluid dynamic (EFD) and computational fluid dynamics (CFD) method with an innovative approach employed for the analysis of steady state circular motions. The force and moment balances are analyzed by decomposing into contributions from the bare hull, rudder, and propulsor. Detailed investigation of the computed local flow fields is performed including the hull vortices, surface pressure and streamlines, and propeller and rudder inflows. The force and moment balances mostly have a similar trend for both the EFD and CFD. The propeller inflow in port and starboard turning shows different trends, and the propeller is more heavily loaded with reduced efficiency as compared to the straight-ahead condition. The port side shows larger magnitudes of the hull vortices, more propeller load, lower propeller efficiency, larger drift angle, and smaller circle radius than the starboard side turning. These differences are explained via the hull–propeller–rudder force and moment balances with the aid of transformed circular motion equations of centrifugal force. The surge (X) force is hardly changed, but the lateral (Y) force is reduced (largely due to the rudder force) for the port side turning, which induces a larger drift angle and more speed loss. The overall conclusion is that the circular motion induces the centrifugal force and drift angle, which induce the hull vortices, off-design propeller inflow, reduced propeller efficiency, increased propeller thrust, and speed loss in addition to the propeller rudder interactions.