Rectangular Obstacles Research Articles

Investigation of interaction between solitary wave and two submerged rectangular obstacles

The performance of coastal breakwaters in reducing wave height and energy is an important problem. This paper presents an experimental and numerical investigation of solitary wave interaction with two submerged rectangular obstacles. White light and particle image velocimetry (PIV) techniques were utilized to study the free surface profile of a solitary wave and flow field in an experimental procedure. The PIV test results revealed that two clockwise vortices are generated between and after the two obstacles, and the white light test results showed that three phenomena of wave breaking, crest-crest exchange, and air-water mixing occur in the solitary wave passage over the two obstacles. A transient two-dimensional numerical model was used to study solitary wave interaction with two rectangular obstacles. The numerical model was validated with experimental results in terms of free surface profile, velocity fields, and velocity profiles. Using this model, the effects of obstacles height and distance were investigated. The numerical results showed that when increasing the height of the obstacles, the drag force applied on the obstacles, the strength of vortices, the energy loss, and the height reduction of the solitary wave increased. The presence of the second obstacle and wave breaking occurrence constrain the movement of vortices and cause a negative drag coefficient on the obstacles in some cases. The energy loss, the height reduction of the solitary wave, and the strength of the vortex generated between the obstacles increased as the distance of the obstacles increased up to S/L = 1.5. In contrast, the strength of the vortex generated after the second obstacle was decreased by increasing the distance of two obstacles.

An experimental investigation of turbulent flow over a two-dimensional obstacle on a flat plate

Aeroacoustic and aerodynamic characteristics of the turbulent boundary layer encountering a large obstacle are experimentally investigated in this paper. Two-dimensional obstacles with a square and a semi-circular cross-section mounted on a flat plate are studied in wind tunnel tests, with particular interests in the shear layer characteristics, wall pressure fluctuations, and far-field noise induced by the obstacles. Synchronized measurements of the far-field noise and the wall pressure fluctuations were conducted using microphone arrays in the far-field and flush-mounted in the plate, respectively. Additionally, the streamwise and wall-normal velocity fluctuations behind the obstacle were measured using the X-wire probe. The measured velocity profiles, spectra, and wall pressure spectra are compared, showing that the rectangular obstacle has a significant impact on both the turbulent flow and far-field noise. The large-scale vortical structures shed from the obstacles can be identified in the wall pressure spectra, the streamwise velocity spectra, and the wall pressure coherence analysis. Within the shear layer, the pairing of vortices occurs and the frequency of the broadband peak in the velocity spectra decreases as the shear layer grows downstream. Further eddy convective velocities of large-scale vortical structures inside the shear layer were analyzed based on the wall pressure fluctuations.

A numerical simulation to effectively assess impacts of flow channels characteristics on solid oxide fuel cell performance

Solid oxide fuel cells (SOFCs) introduce a promising electrochemical conversion technology to generate electricity directly from fuel oxidization. A three-dimensional (3D) numerical model is proposed to evaluate the SOFC performance by employing computational fluid dynamics (CFD) approach based on the finite element method. This research includes simultaneously solving momentum, energy, and mass transport equations linked with the electrochemical reactions. First, the modeling results of a SOFC system with a rectangular channel in the absence of obstacles are compared with the experimental data, showing very good agreement. The effects of different shapes and numbers of obstacles on fuel cell behavior (voltage and power) are then evaluated to determine the optimal performance in the SOFC system. The impacts of operating conditions such as inlet gas velocity in anode and cathode channels and porosity and permeability of electrodes on the SOFC electric output as well as integrated transport phenomena and electrochemical reactions are also investigated. It is concluded that the average current density (SOFC performance) increases by 15% upon using seven triangular obstacles in comparison with the reference case. Fuel cell power is enhanced by almost 35%, compared to a cell with direct flow channels. It is found that the fuel cell performance is increased by using the trapezoidal and rectangular obstacles in the flow channels. The amount of hydrogen consumption at the anode channel is an important parameter affecting the SOFC performance; by increasing the efficiency of the fuel cell, hydrogen conversion is increased. According to the results, the average overpotential in SOFC decreases with increasing fuel cell efficiency. The new SOFC design proposed in this study can be used for optimal design and operation purposes.

The Influence of an Impedance Obstacle on the Acoustic Field Inside a Rectangular Room

Abstract The problem of sound propagation inside a rigid-walled room containing a rectangular obstacle was solved by dividing an acoustic field into subregions and using the continuity conditions. Acoustic waves were generated by a point source. The formulas valid for an impedance obstacle extending from a room floor to its ceiling were obtained. The considered obstacle can model such elements as a ventilation shaft, furniture, or construction pillar. The solution was expressed in the form of convergent series. To obtain accurate results, the error resulting from the use of truncated series was controlled. Additionally, to check the correctness of the proposed solution and its computer implementation, the results obtained for a negligibly small obstacle were compared with those given by the empty room model. An excellent agreement was achieved which proves a high accuracy of the used methodology. The numerical analysis shows that the calculation time of acoustic pressure in a part of an empty room can be significantly reduced using the obtained solution. An appropriate source location for noise reduction was found. The distribution of acoustic field was illustrated and some conclusions were formulated. The changes in acoustic field due to the presence of an obstacle were predicted and discussed.

Modelling denitrification process in a static mixer–reactor using lattice-Boltzmann method

Nitrate removal in wastewater systems is crucially important to maintaining the quality of potable water as well as protecting vulnerable ecosystems. In this study, a denitrification model was incorporated into lattice-Boltzmann model (LBM) to investigate denitrification efficacy of wastewater treatment in a static mixer–reactor. The model was used for simulating the effects of six solid and porous obstacle configurations, different inlet velocities, and inlet methanol concentrations on denitrification within a static mixer–reactor with six different configurations. The results show that all the obstacles and porous blocks could improve the denitrification efficiency. However, the performances of these configurations could be affected differently by the inlet conditions. At a low velocity (u=0.5u0), the denitrification rates are almost twice as that at u=1.5u0. However, at a high inlet velocity, there were small differences of the denitrification efficiency among different configurations. Furthermore, the rectangular obstacle had the greatest effects on the denitrification rates at the low inlet velocity while its performance at a higher velocity was almost comparable to those of the porous blocks. It is also found that at a high inlet methanol concentration (C2-u = 2.0C2-u0), all the configurations increased the denitrification efficiency of up to 10%, compared to clear channel. However, the rectangular obstacle resulted in a decrease of about 3% in denitrification at a low inlet methanol concentration (C2-u = 0.5C2-u0) while all other configurations could improve the performance. Although at a low methanol concentration, the solid obstacles could dwindle the performance, the porous blocks did enhance denitrification rates in all the inlet methanol concentrations. This implies that the porous configurations have a wider range of operational conditions, compared to other obstacle configurations. A proper design of static mixer–reactors without any moving part is able to improve the denitrification efficiency in wastewater treatment. The present model is a useful tool to evaluate the performance of different reactor configurations under combinations of all significant parameters, such as inlet velocity and methanol concentrations.

Improvement of a Diagnostic Urban Wind Model for Flow Fields around a Single Rectangular Obstacle in Micrometeorology Simulation

In general, computational fluid dynamics (CFD) models incur high computational costs when dealing with realistic and complicated flows. In contrast, the mass-consistent flow (MASCON) field model provides a three-dimensional flow field at reasonable computational cost. Unfortunately, some weaknesses in simulating the flow of the wake zone exist because the momentum equations are not considered in the MASCON field model. In the present study, a new set of improved algebraic models to provide initial flow fields for the MASCON field model are proposed to overcome these weaknesses by considering the effect of momentum diffusion in the wake zone. Specifically, these models for the wake region are developed on the basis of the wake models used in well-recognized Gaussian plume models, ADMS-build and PRIME. The MASCON fields provided by the new set of wake zone models are evaluated against wind-tunnel experimental data on flow around a wall-mounted rectangular obstacle. Each MASCON field is compared with the experimental results, focusing on the positions of the vortex core and saddle points of the vortex formed in the near-wake zone and the vertical velocity distribution in the far-wake zone. The set of wake zone models developed in the present study better reproduce the experimental results in both the wake zones compared to the previously proposed models. In particular, the complicated recirculation flow which is formed by the union of the sidewall recirculation zone and the near-wake zone is reproduced by the present wake zone model using the PRIME model that includes the parameterization of the sidewall recirculation zones.

Fog Formation Related to Gravity Currents Interacting with Coastal Topography

An interesting mixing-fog event was identified during the C-FOG field campaign, where a cold-frontal airmass arriving from the north-east collided with The Downs peninsula in Ferryland, Newfoundland, Canada, to produce misty/foggy conditions. A comprehensive set of field observations suggests that this collision caused turbulent mixing of nearly saturated ambient air with an almost saturated cold-frontal airmass, creating conditions for mixing fog. To delve into the physical processes underlying this phenomenon, laboratory experiments were performed on the interaction of lock-exchange-induced gravity currents with a rectangular obstacle. Instantaneous velocity and density fields were obtained using particle image velocimetry and planar laser-induced fluorescence. The observations suggest that the obstacle starts affecting the approaching gravity-current propagation at an upstream distance of 2H and, upon collision, the mixing occurs over a length of 0.83H, where H is the depth of the ambient fluid layer. The time for larger-scale turbulent stirring to permeate to the smallest scales of turbulence and activate the condensation nuclei is estimated as \(3t^*\), where \(t^*=\sqrt{H/g'}\) is the intrinsic time scale of the gravity current, and \(g'\) is the reduced gravity. Extrapolation of laboratory results to field conditions shows a good agreement with observations.

Mixing Behavior and Pressure Drop Analysis of Micromixer with Different Geometric Conditions

A 3-D design of and analysis of fluid flow in the micromixer with different configurations is carried out in this dissertation. The main purpose of this research is to obtain minimum mixing length as rapid mixing is essential in many of the micro-fluidic systems used in biochemistry analysis, drug delivery, sequencing, or synthesis of nucleic acids. Also effect on various parameters such as mixing behavior, volume arrow, mixing length, maximum velocity, maximum pressure, pressure drop, and velocity distribution were analyzed by changing the mixing angle between inlets. Micromixers with square cross-section rectangular mixing chamber with various types of obstacle place in fluid flow paths such as rectangular obstacles, elliptical obstacle, and circular obstacle in split and recombination manner were designed for the analysis. The micromixer has 3 inlets and 1 outlet. Water and ethanol were used as working fluids. For computational fluid dynamics analysis, COMSOL Multiphysics 5.0 is used. From various results, we have found that size, the geometry of mixing chambers and obstacles, and mixing angle effect mixing length, pressure, and velocity. With a decrease in mixing angle mixing length, pressure drop, and maximum velocity decrease i.e it gives better mixing performance. Also with an increase in the number of obstacles mixing length and maximum velocity decreases and pressure drop increases. Micromixer with mixing angle 60 degree and circular obstacles gives minimum mixing length than any other models consisting rectangular or elliptical obstacle and mixing angle greater than 60 degrees.

Nano particles distribution characteristics in multi-phase heat transfer between 3D cubical enclosures mounted obstacles

The innovation of this paper is hybrid heat transfer simulation of two different multi-phase nanofluid simultaneously between two 3D enclosures under heat flux boundary condition. This present work with arrangement of different shape of mounted obstacles effects has practical applications such as energy storage systems. Velocity and temperature fields of fluid flow are modeled based on the D3Q19, Lattice Boltzmann Method. Nanoparticle volume fraction, temperature fields, mean and local velocity in 24 cases of enclosures with 5, 7 and 9 obstacles in hot zone and 10, 14 and 18 obstacles in hot and cold zones have been investigated for arrangement rectangular, circle, vertical and horizontal ellipse obstacles. rates of mean temperature in each enclosure have been analyzed. The results presented that minimum of fluid velocity in left enclosure was in 0.4<y/L<0.5, and maximum was in 0.15<y/L<0.25 and 0.7<y/L<0.85. Also, maximum and minimum of mean temperature in left enclosure on the wall (y=0) were in enclosures with 9 rectangular obstacles in hot zones that was equal to 48.37 °C (%8.3) and 10 horizontal ellipse obstacles in hot and cold zone that was equal to 45.03 °C (%0.8), respectively.

Numerical and Experimental Study of Abrupt Wave Interaction with Vertical and Inclined Rectangular Obstacles

Numerical and Experimental Study of Abrupt Wave Interaction with Vertical and Inclined Rectangular Obstacles

Modifications of Heat Transfer and Induced Flow Rate of a Solar Chimney by an Obstacle in the Air Channel

Using renewable energy resources in buildings has been increasing for saving energy. Among widely used methods, solar chimneys absorb solar radiation and induce an airflow for ventilation, heating, or cooling buildings. The performance of a solar chimney depends on the heat transfer in its air channel, which can be modified by changing the shape of the heat exchange surface. In this study, we examine the effects of a rectangular obstacle placed on the heated surface of the air channel of a solar chimney. A numerical model was built with the Computational Fluid Dynamics (CFD) technique. By changing the geometries of the chimney and of the obstacle, and location of the obstacle, it was found that the obstacle had strong effects on the local flow and heat transfer properties close to the object, particularly the Nusselt number and the temperature field. However, average quantities of the whole air channel flow, such as the averaged Nusselt number and the induced flow rate, were modified insignificantly, except for the temperature rise through the chimney and the thermal efficiency. The change of the flow rate and the average Nusselt number was 5.0% while the temperature rises and the thermal efficiency was up to 13.0%. The height of the obstacle had more influence than the length. This study suggests more investigations to achieve the objective of enhancing the induced flow rate for ventilation of buildings with an obstacle in the air channel of a solar chimney.

Experimental study of flow structures of a solitary wave over two rectangular tandem obstacles

In this research, interactions between a solitary wave and two submerged rectangular obstacles with different distances were studied experimentally. The particle image velocimetry (PIV) technique was used to measure the velocity field on the weather and lee sides of the two obstacles. Two submerged rectangular obstacles with 12-cm height and 8-cm length were installed in an open water channel with 7-m length. The velocity fields were obtained with 5 fields of view (FOVs) and then synchronized together. The vortices that formed on the weather and lee sides of the rear and front obstacles were compared to each other. It was found that the vortex on the weather side of the rear obstacle was flatter and smaller than that on the weather side of the front obstacle. Moreover, the weather-side vortex of the front obstacle grew more. After the wave passes over the two obstacles, the vortices on the weather and lee sides of the front obstacle are merged together and form a larger vortex. A higher vorticity at the center of the weather side vortex of the front obstacle was observed. By increasing the distance between the obstacles, the vortex between the obstacles expands more, and the interaction of the flow with the obstacles is more intensive so that more and larger vortices are formed. These vortices depreciate the wave energy. The measurement of the wave height before and after the obstacles showed that the larger the distance between the obstacles, the higher wave energy loss will be.

Numerical Investigation of Newtonian Fluid Flow around Sharp &amp; Rounded Corner Rectangular Obstruction

In the present study, very stable and converging Least Square finite element method (LSFEM) is employed to calculate the approximate solution of steady state Navier – Stokes equation, consisting continuity and momentum equations in two dimensions. The 2D rectangular domain is considered to study the behavior of linear fluid passing two parallel rectangular obstructions in an open channel. The current numerical analysis is based on two instances, in the first one two rectangular obstructions possesses sharp corners set to face the entering flow while in the other case, obstructions having round corners are analyzed concerning to examine the enhancement in the size of vortex formed due to the flow blockage, intensity of vortex and the recirculation flow rate in the dead zone for Reynolds number from 250 to 2000. The stream line patterns are also presented to monitor the changes appears in the shape of vortex for different Re. The information gather through the analysis suggests the cutoff corner obstruction is congenial in restricting the vortex length and favorable for slow down the flow circulation rate in the lower stream of the channel. The acquired results are compared with established data in past literature which turn out to be in good agreement.

Turbulent flow characteristics around a non-submerged rectangular obstacle on the side of an open channel

The three-dimensional flow structure and turbulence characteristics around a non-submerged rectangular obstacle in an open channel are explored using numerical simulation. In particular, a low length-to-depth ratio condition, shown to be associated with three-dimensional flow features in our previous study, is considered. To sufficiently resolve all the important details of the three-dimensional turbulent flow around and in the entire wake of an obstacle, high-resolution large-eddy simulation (LES) employing is carried out on a parallel supercomputer. The LES results were compared with the measurements and analyzed to examine the horseshoe vortex structure, free-surface vortex structure, recirculation zones, vortex shedding process, turbulence characteristics, and wall shear stress distribution around an obstacle. The results provide important insights into the complete three-dimensional flow structure and wall shear stress patterns around the obstacle.

Assessment of Numerical Methods for Plunging Breaking Wave Predictions

This study evaluates the capability of Navier–Stokes solvers in predicting forward and backward plunging breaking, including assessment of the effect of grid resolution, turbulence model, and VoF, CLSVoF interface models on predictions. For this purpose, 2D simulations are performed for four test cases: dam break, solitary wave run up on a slope, flow over a submerged bump, and solitary wave over a submerged rectangular obstacle. Plunging wave breaking involves high wave crest, plunger formation, and splash up, followed by second plunger, and chaotic water motions. Coarser grids reasonably predict the wave breaking features, but finer grids are required for accurate prediction of the splash up events. However, instabilities are triggered at the air–water interface (primarily for the air flow) on very fine grids, which induces surface peel-off or kinks and roll-up of the plunger tips. Reynolds averaged Navier–Stokes (RANS) turbulence models result in high eddy-viscosity in the air–water region which decays the fluid momentum and adversely affects the predictions. Both VoF and CLSVoF methods predict the large-scale plunging breaking characteristics well; however, they vary in the prediction of the finer details. The CLSVoF solver predicts the splash-up event and secondary plunger better than the VoF solver; however, the latter predicts the plunger shape better than the former for the solitary wave run-up on a slope case.

Influence of the obstacles on dielectrophoresis-assisted separation in microfluidic devices for cancerous cells

The development of a separation lab-on-a-chip device based on the particle size is critical in the early detection of circulating tumor cells. In this work, combining alternating current dielectrophoresis and drag force are used to precisely separate MDA-MB231 and red blood cells in the microfluidic platform. Four microfluidic devices with sidewall electrodes and different obstacle shapes were employed to both generate non-uniform fields inside the channels and condense the particles near the electrodes. Besides that, dividing the particles inlet into two main channels enable fast and accurate separation inside the device, and then particles deflected to the different outlets. Choosing the frequency is critical in the magnitude and sign of the dielectrophoresis force. Negative dielectrophoresis force was acting on both cancerous and red blood cells when the frequency was 10 kHz. Finite element simulation was conducted to predict the particle trajectories and revealed that the device with rectangular obstacles had the higher electric field non-uniformity when the voltage applied was set to 6 V and − 6 with the frequency of 10 kHz, Simulation represented 100% separation efficiency and purity for cancerous cells in the specific outlets. Decreasing the voltage from 6 V and − 6 V to 5 V and − 5 V could reduce the separation efficiency to 46%. The appropriate value of voltage (6 V, − 6 V) was applied to the electrodes resulting in the lower Joule heating and sufficient dielectrophoresis forces.

Ambient Entrainment Mechanisms of Partially Unconfined Turbidity Currents Interacting with a Continuous Rigid Obstacle

Abstract In this study, the entrainment mechanisms of unconfined turbidity currents that interact with a linear rectangular obstacle will be investigated and compared with confined studies. Laborat...

Lamb Wave Scattering Analysis for Interface Damage Detection between a Surface-Mounted Block and Elastic Plate.

Since stringers are often applied in engineering constructions to improve thin-walled structures’ strength, methods for damage detection at the joints between the stringer and the thin-walled structure are necessary. A 2D mathematical model was employed to simulate Lamb wave excitation and sensing via rectangular piezoelectric-wafer active transducers mounted on the surface of an elastic plate with rectangular surface-bonded obstacles (stiffeners) with interface defects. The results of a 2D simulation using the finite element method and the semi-analytical hybrid approach were validated experimentally using laser Doppler vibrometry for fully bonded and semi-debonded rectangular obstacles. A numerical analysis of fundamental Lamb wave scattering via rectangular stiffeners in different bonding states is presented. Two kinds of interfacial defects between the stiffener and the plate are considered: the partial degradation of the adhesive at the interface and an open crack. Damage indices calculated using the data obtained from a sensor are analyzed numerically. The choice of an input impulse function applied at the piezoelectric actuator is discussed from the perspective of the development of guided-wave-based structural health monitoring techniques for damage detection.

Equilibrium configuration of a rectangular obstacle immersed in a channel flow

Fluid flows around an obstacle generate vortices which, in turn, generate lift forces on the obstacle. Therefore, even in a perfectly symmetric framework equilibrium positions may be asymmetric. We show that this is not the case for a Poiseuille flow in an unbounded 2D channel, at least for small Reynolds number and flow rate. We consider both the cases of vertically moving obstacles and obstacles rotating around a fixed pin.

Numerical Study Of Natural Convection In Square Cavity Using Lattice Boltzmann Method: Obstacles Effect

In this work, the natural convection is studied in a closed square cavity differentially heated, using the Lattice Boltzmann numerical Method. Completely filled with a newtonian fluid, the cavity is managed by two rigid walls in right and left sides, and is provided with two rectangular adiabatic obstacles interposed on the mid-horizontal walls. The height of the two obstacles is varied from 10% to 40% of the side (L). The calculation code was performed for a laminar flow and an incompressible fluid with a Prandtl number Pr = 0.71 (the case of air). The computation code thus developed was used to analyse the effect of the length of obstacles in the mid-square cavity. The obtained results were validated by comparing with those of a previously published works. It has been deduced that the addition of the obstacles influences the thermal and hydrodynamic profiles of the flow inside the cavity and its thermal performances.