Surface Flow Characteristics Research Articles

Desalination membranes with ultralow biofouling via synergistic chemical and topological strategies

Biofouling is a persistent problem for desalination and water treatment membranes because it critically degrades membrane performance. Here, we report desalination membranes with ultrahigh biofouling resistance fabricated using a synergistic anti-biofouling strategy that combines topological and chemical antifouling modifications. Specifically, both the surface topology and the chemistry of desalination membranes were tailored by creating a biomimetic Sharklet pattern and integrating hydrophilic tannic acid and biocidal silver nanoparticles (AgNPs). The resultant membrane significantly enhanced biofouling resistance compared to the membranes modified by either the topological or chemical modifications alone due to the combinatorial antifouling effect. Importantly, the topological anti-biofouling effect was much more significant than the chemical effect under dynamic flow conditions due to the unique surface flows generated by the Sharklet geometry. Interestingly, the surface flow characteristics of the Sharklet pattern also effectively suppressed the leaching of the incorporated AgNPs and thus prolonged their anti-biofouling effect, highlighting the synergistic effect of the proposed combination strategy. This approach opens a new avenue for the fabrication of functional membranes, coatings and surfaces with superior antifouling and self-cleaning functions by elucidating the underlying antifouling mechanisms.

Numerical analysis of wall surface adhesion and flow characteristics of microdroplets in oil mist separator

Numerical analysis of wall surface adhesion and flow characteristics of microdroplets in oil mist separator

Reynolds number and slant angle effects on the flow over a slanted cylinder afterbody

The cylinder with a slanted base is a simplified, canonical bluff body geometry that shares similarities to aircraft fuselages, which are known to produce a strong vortex pair due to their upswept afterbody. This work will examine in detail the surface flow and near-field characteristics of the flow over the slanted cylinder for slant angles of $20^{\circ }$ , $32^{\circ }$ and $45^{\circ }$ with spatially dense measurements. Principal flow features of the mean flow field are identified, showing the connection between the main counter-rotating vortex pair observed in the wake and the separation bubble observed at the leading edge of the slant. A full reconstruction of the three-dimensional mean flow field using stacked stereoscopic particle image velocimetry reveals intricate details of this flow and clearly shows the direct connection between the two features. To our knowledge, this is the first direct measurement of the full three-dimensional flow topology for this geometry. The separation bubble length is found to be directly proportional to the slant angle and inversely proportional to the Reynolds number. Furthermore, the circulation within the primary vortex pair increases with increasing slant angle. This strengthening of the vortices is correlated to the form drag of this body in the vortex-dominated regime. A bi-stable steady-state wake is also observed in this flow at a low Reynolds number for the slant angle of $45^{\circ }$ , where the formation of either a separated wake flow state or a vortex-dominated flow state is dependent upon initial conditions, i.e. the presence of overshoot of the free-stream velocity during wind tunnel start-up.

Visualization of asymmetric separation induced by lateral jet interaction on a slender body in supersonic flow

The lateral jet interaction on a slender body in supersonic flow was investigated by numerical simulation. The spatial and surface flow characteristics induced by jet interaction were shown. As a result, when the lateral jet is not in the longitudinal symmetry plane, the jet interaction causes asymmetric separation flow of surface and space, and destroys the pressure distributions of the slender body. With different angle of attack and circumferential positions of jet, the flow characteristic of the after body for jet in asymmetry plane changes greatly. The results with and without jet interaction also show that the far-field interaction played a major role in the lateral jet interaction.

A comparative study of fracture surface roughness and flow characteristics between CO2 and water fracturing

Fracture surfaces were measured using a 3-D optical topography scanner, and the fracture surfaces caused by CO2 fracturing are much rougher than those caused by water fracturing due to the stronger penetrability of CO2 through tiny pores and channels. The fracture roughness of siltstone with intensive macro-pores is greater than that of shale, because fractures preferably propagate along the orientation of macro-pores. Fracture flow conductivity is greatly enhanced after small shear displacement between fracture surfaces, because of the generation of flow pathways on rough fracture surfaces. Shear deformation perpendicular to flow direction creates relatively unobstructed and straight flow pathways along the flow direction, and flow conductivity is around one order of magnitude higher than that with shear deformation along the flow direction and 300–400 times that without any shear deformation. Our results suggest that CO2 fracturing has an advantage over water fracturing in improving flow capacity of created fractures.

Structural tailoring of sharkskin-mimetic patterned reverse osmosis membranes for optimizing biofouling resistance

Controlling biofouling is critical for membrane materials exposed to aquatic environments. Specifically, the sharkskin-mimetic, so-called Sharklet, surface pattern has proven effective for suppressing biofilm formation on desalination membranes. In this study, a series of Sharklet patterns with different unit and pattern spacings were designed on reverse osmosis (RO) membrane surfaces to identify the effect of the Sharklet pattern dimension on membrane biofouling. A high fidelity of Sharklet-patterned RO membranes with different spacing dimensions were successfully fabricated by micromolding combined with layered interfacial polymerization. The biofouling behavior of the fabricated Sharklet-patterned RO membranes was systematically characterized under both static and dynamic conditions. Importantly, dynamic biofouling results showed that the anti-biofouling effect of the Sharklet pattern was optimized when the unit and pattern spacings were both 2 μm. Computational fluid dynamics simulation elucidated the surface flow characteristics of the Sharklet patterns depending on the spacing dimensions. The maximized anti-biofouling performance of the Sharklet pattern with 2 μm spacings was hypothesized to be determined by the balance between the intrinsic biofouling propensity (under static conditions) and surface flow characteristics such as vortex and primary/secondary flows (under dynamic conditions).

A numerical study on flow field and maneuvering derivatives of KVLCC2 model at drift condition

ABSTRACT This paper investigates the flow field around the KRISO Very Large Crude Carrier (KVLCC2) model at different drift angle using the RANS method in combination with the Re-Normalization Group (RNG) and Shear Stress Transport (SST) turbulence models. The hydrodynamic forces and yaw moment acting on the KVLCC2 model are calculated under the different turbulence models and drift angles, and the numerical results are compared with previous experimental results. The accuracy of the numerical results is found to be acceptable, especially under the SST turbulence model. Three different mesh conditions are employed to perform the verification and validation based on the methodology and procedures suggested by ITTC. The surface pressure distribution, vorticity distribution, and surface flow features around the hull are observed and analysed. The maneuvering derivatives are calculated based on numerical results to create the mathematic model of ship maneuvering motions in the marine simulator and predict the ship maneuvering motions.

Flow characteristics and dynamic responses of a parked straight‐bladed vertical axis wind turbine

AbstractWith the development of urbanization and the application of renewable energy, wind turbine is becoming an important approach for wind energy reservation and utilization. This study provides a numerical investigation on understanding the surface pressure distribution, flow characteristics and dynamic responses of a parked straight‐bladed vertical axis wind turbine (VAWT), which is helpful for its design. Together with the two‐way coupling method between simulation platforms such as STAR‐CCM+ and ABAQUS, the SST k‐ω turbulence model is used to obtain the surface pressure and surrounding flow of the VAWT, and the finite element method is used to obtain the dynamic responses of its structural components. The results show that the contours of the pressure distribution on the windward surface of the VAWT are similar even under a few different conditions, and the deformation of the VAWT can lead to changes in surface pressure; the turbulent flow characteristics and the wake effect become more obvious as the wind velocity increases; the blades and support arms of the VAWT need to be reinforced during the design, and the effect of the parked condition on the dynamic responses of the VAWT can be neglected. The two‐way coupling method as well as the numerical simulation results is expected to provide references for the design of VAWTs subjected to coming wind action.

Numerical simulation and optimization study of surface pressure and flow field around a triangular prism behind a porous fence

The flow structure around upright porous fences and a triangular prism was numerically investigated by using the RNG k-e turbulence model. The numerical method was validated by comparing the simulations with the experimental data, and then the effects of porosity and height of the wind fence on the flow field around the prism model and surface pressures were studied. The aim of this paper is to achieve the best shelter effect of a porous fence on the triangular prism located behind the fence with the height of 0.5h and 1.0h respectively, where h is the height of the prism. The gradient algorithm was adopted as the optimization algorithm to determine the optimum porosity of the wind fence. Three objective functions were considered to minimize (1) the sum of the absolute values of the mean pressure coefficients on the prism surface; (2) the total turbulence kinetic energy around the prism model, and (3) the total values of the peak velocity ratios involving velocity and turbulence. The numerical results showed that a porous fence with the height of about 0.5h–1.0h seems to be most effective in reducing the absolute value of the mean pressure coefficient on the model surface. Additionally, all three objective functions were capable of finding an optimal shelter performance of porous fences on the prism with surface pressure or flow characteristics considerations.

Breaking Wave Interaction With a Group of Four Vertical Slender Cylinders in Two Square Arrangements

The main purpose of the study is to investigate the breaking wave interaction with a group of four circular cylinders. The physical process of wave breaking involves many parameters, and an accurate numerical modeling of breaking waves and the interaction with a structure remain a challenge. In the present study, the open-source computational fluid dynamics (CFD) model REEF3D is used to simulate the breaking wave interaction with multiple cylinders. The numerical model is based on the incompressible Reynolds-averaged Navier–Stokes (RANS) equations, the level set method for the free surface, and the k–ω model for turbulence. The numerical model is validated with experimental data of large-scale experiments for the free surface elevation and the breaking wave force on a single cylinder. A good agreement is obtained between the numerical results and experimental data. Two different configurations with four cylinders are examined: in-line square configuration and diamond square configuration. For both configurations, three different tank widths and four different spacings between the cylinders are investigated. The breaking wave forces on each cylinder in the group are computed for each case for the two configurations, and the results are compared with the breaking wave force on a single isolated cylinder. Furthermore, the study investigates the water surface elevations and the free surface flow features around the cylinders. For the closely spaced cylinders in a relatively narrower tank, the cylinders in both configurations experience the maximum forces lower than the maximum force on a single cylinder. But for the widely spaced cylinder in a relatively wider tank, the forces are higher and lower for the upstream cylinders and downstream cylinders, respectively, than the maximum force on a single isolated cylinder. The results of the present study show that the interference effects from the neighboring cylinders in a group strongly influence the kinematics around and the breaking wave forces on them.

On the mean structure of sharp-fin-induced shock wave/turbulent boundary layer interactions over a cylindrical surface

Interactions between an oblique shock wave generated by a sharp fin placed on a cylindrical surface and the incoming boundary layer are investigated to unravel the mean features of the resulting shock/boundary layer interaction (SBLI) unit. This fin-on-cylinder SBLI unit has several unique features caused by the three-dimensional (3-D) relief offered by the cylindrical surface that noticeably alter the shock structure. Complementary experimental and computational studies are made to delineate both the surface and off-body flow features of the fin-on-cylinder SBLI unit and to obtain a detailed understanding of the mechanisms that dictate the mean flow and wall pressure features of the SBLI unit. Results show that the fin-on-cylinder SBLI exhibits substantial deviation from quasi-conical symmetry that is observed in planar fin SBLI. Furthermore, the separated flow growth rate appears to decrease with downstream distance and the separation size is consistently smaller than the planar fin SBLI with the same inflow and fin configurations. The causes for the observed diminution of the separated flow and its downstream growth rate were investigated in the light of changes caused by the cylinder curvature on the inviscid as well as separation shock. It was found that the inviscid shock gets progressively weakened in the region close to the triple point with downstream distance due to the 3-D relief effect from cylinder curvature. This weakening of the inviscid shock feeds into the separation shock, which is also independently impacted by the 3-D relief, to result in the observed modifications in the fin-on-cylinder SBLI unit.

On the Near-Wake of a Ground-Effect Diffuser with Passive Flow Control

A ground-effect diffuser is an upwardly-inclined section of an automobile’s underbody which increases aerodynamic performance by generating downforce. To understand the diffuser flow physics (force behaviour, surface and offsurface flow features), we established the near-wake (within one vehicle width of the base) velocity profiles and flow structures of an automotive ground-effect diffuser using a bluff body with a 17 degree slanted section forming the plane diffuser ramp surface (baseline geometry), and endplates extending along both sides of the ramp. Wind tunnel experiments were conducted at a Reynolds number of 1.8 million based on the bluff body length, and laser Doppler velocimetry was used to measure two-dimensional velocity components on three planes of the diffuser near-wake. We also measured the velocity field in the near-wake of diffusers with modified geometry (with an inverted wing or a convex bump) as passive flow control devices. The near-wake velocity profiles indicated that the passive flow control methods increased the diffuser flow velocity and that the longitudinal vortices along the diffuser determined the shape of the flow structures in the near-wake of the diffuser bluff body.

Optimal Hyperelliptic Cambered Span Configurations for Minimum Drag

The current study considers an analysis of hyperelliptic cambered span wing configurations, optimized for minimum drag. This study encompasses a multidimensional design optimization approach wherein the wing geometries are optimized with respect to both span load and spanwise camber to achieve minimum aerodynamic drag. A discrete vortex element method was coupled with a constrained gradient-based optimization routine to achieve these drag-optimal design configurations. Optimum drag configurations were obtained at a fixed design lift coefficient as well as a fixed bending moment constraint. Navier–Stokes simulations were also performed, which revealed an gain of 29.01 and 25.35% at design conditions, relative to an elliptically loaded planar wing for the upward and downward cant configurations, respectively. Different wake trace characteristics for the two hyperelliptic cambered span configurations were observed along with surface flow features indicating different wake vortex and boundary-layer transition behaviors for the two hyperelliptic cambered span configurations.

Aerodynamic Numerical Analysis of the Low Reynolds Number Diamond Joined-Wing Configuration Unmanned Aerial Vehicle

Aerodynamic numerical simulation of low Reynolds number long-endurance Diamond Joined-Wing configuration unmanned aerial vehicle was performed. First, low Reynolds number aerodynamic characteristics of a typical wing are numerically simulated by solving the Reynolds averaged Navier–Stokes (RANS) equations based on the structural grid technology and the k–kL–ω transition model. To prove the accuracy of this method, experimental data and numerical simulation results were compared. Second, the performance of low Reynolds number Diamond Joined-Wing configuration UAV (unmanned aerial vehicle) aerodynamic characteristics and control surfaces was studied by numerical simulation and theoretical analysis. The analysis of the interference between the Frt-wing (front wing) and the Aft-wing (after wing) of the Diamond Joined-Wing configuration UAV and the reason for the change in the pitching moment in the lateral-directional motion is carried out. The results show that the Diamond Joined-Wing configuration UAV has a typical low Reynolds number surface flow characteristics, Frt-wing and Aft-wing flow interference are relatively minor at a small angle of attack, which has a high basic aerodynamic performance in addition to good stall characteristics, with longitudinal and lateral-directional static stability. The performance of control surfaces and flaps is higher to meet the design requirements. Finally, the accuracy of the numerical simulation results was verified by vehicle aerodynamic testing.

Flowfield Measurements in a Mach 2 Fin-Generated Shock/Boundary-Layer Interaction

An experimental study is conducted on the interaction of a swept shock wave with a turbulent boundary layer. The shock wave is generated by a sharp unswept fin in a Mach 2 flow, where the strength of the interaction is varied from weak to moderate by changing the angle of attack of the fin from 10 to 15 deg, which corresponds to a normal Mach number of 1.3 and 1.4, respectively. Surface oil-flow visualization is used to study the topography of the interaction where surface flow features such as the upstream influence line and the separation lines are identified. By taking advantage of the quasi-conical symmetry of the flowfield, velocity field measurements are acquired in the conical reference frame for the interaction at the moderate interaction strength of at two locations from the fin apex. Flowfield features such as the -shock structure, slip line, and the separation bubble with the reverse flow are clearly visible in the velocity field. These results are also examined in the spherical coordinate frame, and good agreement in the spatial location of the critical features is found, providing direct quantitative, experimental evidence of quasi-conical symmetry of the flowfield above the surface. An examination of the velocity field downstream of the rear-foot of the -shock shows a region (a “streamtube,” bounded on one side by the slip line emanating from the triple point) where the flow accelerates to transonic and supersonic speeds. This flow eventually turns toward and impinges upon the flat plate, a phenomenon referred to as an “impinging jet” in the literature. It is believed to be the principal cause of the high mean, unsteady pressures, very high heating, and skin friction coefficients that the present measurements confirm. An examination of the turbulence kinetic energy reveals a significant increase in amplitude in the separated flow region that is bounded by the flattened vortex under the -shock.

Experimental study of the mean structure and quasi-conical scaling of a swept-compression-ramp interaction at Mach 2

This study investigates the mean flow structure of two shock-wave boundary-layer interactions generated by moderately swept compression ramps in a Mach 2 flow. The ramps have a compression angle of either $19^{\circ }$ or $22.5^{\circ }$ and a sweep angle of $30^{\circ }$. The primary diagnostic methods used for this study are surface-streakline flow visualization and particle image velocimetry. The shock-wave boundary-layer interactions are shown to be quasi-conical, with the intermittent region, separation line and reattachment line all scaling in a self-similar manner outside of the inception region. This is one of the first studies to investigate the flow field of a swept ramp using particle image velocimetry, allowing more sensitive measurements of the velocity flow field than previously possible. It is observed that the streamwise velocity component outside of the separated flow reaches the quasi-conical state at the same time as the bulk surface flow features. However, the streamwise and cross-stream components within the separated flow take longer to recover to the quasi-conical state, which indicates that the inception region for these low-magnitude velocity components is actually larger than was previously assumed. Specific scaling laws reported previously in the literature are also investigated and the results of this study are shown to scale similarly to these related interactions. Certain limiting cases of the scaling laws are explored that have potential implications for the interpretation of cylindrical and quasi-conical scaling.

A study on the average wind load characteristics and wind-induced responses of a super-large straight-cone steel cooling tower

As a novel typical wind-sensitive structure, the wind load and wind-induced structural behaviors of super-large straight-cone cooling towers are in an urgent need to be addressed and studied. A super large straight-cone steel cooling tower (189 m high, the highest in Asia) that is under construction in Shanxi Power Plant in China was taken as an example, for which four finite element models corresponding to four structural types: the main drum; main drum + stiffening rings; main drum + stiffening rings + auxiliary rings (auxiliary rings are hinged with the main drum and the ground respectively); and main drum + stiffening rings + auxiliary rings (auxiliary rings are fixed onto the main drum and the ground respectively), were established to compare and analyze the dynamic properties and force transferring paths of different models. After that, CFD method was used to conduct numerical simulation of flow field and mean wind load around the cooling tower. Through field measurements and wind tunnel tests at home and abroad, the reliability of using CFD method for numerical simulation was confirmed. On the basis of this, the surface flow and trail characteristics of the tower at different heights were derived and the wind pressure distribution curves for the internal and external surfaces at different heights of the tower were studied. Finally, based on the calculation results of wind-induced responses of the four models, the effects of stiffening rings, auxiliary rings, and different connecting modes on the dynamic properties and wind-induced responses of the tower structure were derived and analyzed; meanwhile, the effect mechanism of internal suction on such kind of cooling tower was discussed. The study results could provide references to the structure selection and wind resistance design of such type of steel cooling towers.

Numerical Investigation of Focused Waves and Their Interaction With a Vertical Cylinder Using REEF3D

For the stability of offshore structures, such as offshore wind foundations, extreme wave conditions need to be taken into account. Waves from extreme events are critical from the design perspective. In a numerical wave tank, extreme waves can be modeled using focused waves. Here, linear waves are generated from a wave spectrum. The wave crests of the generated waves coincide at a preselected location and time. Focused wave generation is implemented in the numerical wave tank module of REEF3D, which has been extensively and successfully tested for various wave hydrodynamics and wave–structure interaction problems in particular and for free surface flows in general. The open-source computational fluid dynamics (CFD) code REEF3D solves the three-dimensional Navier–Stokes equations on a staggered Cartesian grid. Higher order numerical schemes are used for time and spatial discretization. For the interface capturing, the level set method is selected. In order to test the generated waves, the time series of the free surface elevation are compared with experimental benchmark cases. The numerically simulated free surface elevation shows good agreement with experimental data. In further computations, the impact of the focused waves on a vertical circular cylinder is investigated. A breaking focused wave is simulated and the associated kinematics is investigated. Free surface flow features during the interaction of nonbreaking focused waves with a cylinder and during the breaking process of a focused wave are also investigated along with the numerically captured free surface.

Numerical simulation and experimental validation of free surface flows during low pressure casting process

The present work analyzes the free surface flow characteristics during mold filling in a low pressure casting process (LPC) through water model experiments and numerical simulations. The flow patterns visualized through the water analog experiments using different input pressure sequences have been verified with numerical simulation results for the entire LPC process. The benchmarked numerical model has been further used to study the impact of different in-gate shapes on fluid free surface behavior inside the mold cavity. The mold filling and solidification simulation is then carried out using magnesium alloy as liquid for the same geometry and the same input pressure profile to highlight the effect of thermo physical properties of actual melts on the free surface flow behavior.

Cross-stream stereoscopic particle image velocimetry of a modified turbulent boundary layer over directional surface pattern

A turbulent boundary layer developed over a herringbone patterned riblet surface is investigated using stereoscopic particle image velocimetry in the cross-stream plane at$Re_{\unicode[STIX]{x1D70F}}\approx 3900$. The three velocity components resulting from this experiment reveal a pronounced spanwise periodicity in all single-point velocity statistics. Consistent with previous hot-wire studies over similar-type riblets, we observe a weak time-average secondary flow in the form of$\unicode[STIX]{x1D6FF}$-filling streamwise vortices. The observed differences in the surface and secondary flow characteristics, compared to other heterogeneous-roughness studies, may suggest that different mechanisms are responsible for the flow modifications in this case. Observations of instantaneous velocity fields reveal modified and rearranged turbulence structures. The instantaneous snapshots also suggest that the time-average secondary flow may be an artefact arising from superpositions of much stronger instantaneous turbulent events enhanced by the surface texture. In addition, the observed instantaneous secondary motions seem to have promoted a free-stream-engulfing behaviour in the outer layer, which would indicate an increase turbulent/non-turbulent flow mixing. It is overall demonstrated that the presence of large-scale directionality in transitional surface roughness can cause a modification throughout the entire boundary layer, even when the roughness height is 0.5 % of the layer thickness.