احسان خواصی

One of the coastal protection techniques is the use of breakwaters. This study aims to investigate the effect of pile breakwaters on coastal waves. These breakwaters are similar to a porous structure. They are preferred over other impenetrable coastal structures due to their increased roughness and resistance to the current, their relatively low cost, economic savings, greater environmental compatibility, and preservation of natural landscapes. Considering rigid cylindrical obstacles on the coast with constant slopes, the effect of their roughness on flow patterns and waves by numerical modeling in OpenFOAM software was investigated. The method used in flow modeling is the RANS method and k-ω, SST model. Modeling was performed in two modes with and without barriers for three different heights of wave. The results were compared with laboratory data. The absorbed force for wave height of 6, 9, and 12 cm in the numerical model was 17.14, 4.23, and 7.86 percent, respectively, with the laboratory model, Also, the mean square root of normal error was 0.07, and the correlation coefficient was 0.99, which indicates the conformity of two numerical and laboratory models and the appropriate performance of Open FOAM software in modeling.

More than 90% of people’s lives are spent indoors and indoor air pollution is the cause of more than 2.2 million deaths annually worldwide. Contrary to popular belief, the dangers of pollutants in enclosed spaces are greater than in open environments. In this study, the ability of the computer models to simulate flow behavior in the interior space of an industrial workshop was investigated to may check the satisfaction of the environmental standards

The flow pattern and flow mixing and dilution were investigated by developing a specific solver in OpenFOAM and comparing the results with the ex[perimental data.

Due to the initial buoyancy, the plume enters from the lower opening with high concentration and after diluting in indoor space, goes out from the roof opening. It is then observed that while there are no openings, the concentration of Carbon Monoxide in the breathing height of workers will reach the dangerous limit of (100 ppm <) in different models.

In this study, the concentration of Carbon Monoxide in the building’s interior space was estimated far higher than the maximum allowable centration mentioned in the standards. Therefore, the high level of pollution concentration in indoor spaces indicates the inadequacy of natural ventilation and the need for mechanical ventilation to could decrease pollution down to the breathing standard of ocupants.

In the present study, the two-dimensional lock-exchange turbidity current under the influence of wind flow is modeled using open-source software. To solve this, the large eddy simulation method has been used in order to observe turbulent phenomena more accurately. By developing the two-phase solver of the software so that the equations of the volume of fluid method are coupled with the scalar equation of concentration, the three-phase problem is simulated as a phase of a mixture of dense fluid and pure water next to the air phase. The results show that an increase in wind speed reduces the buoyancy force driving the turbidity current and increases the entrainment, which means faster pollution of water areas. This increase in wind speed also increases the wall shear stress, with the difference that the amount of wall shear stress at low wind speeds is not significant. So this prevents a significant change in the deposition behavior of the current. Studying the current's sedimentation behavior, showed that at high wind speeds, the co-current wind flow corresponding to the turbidity current has more harmful effects than the reverse wind flow and its sediment accumulation is getting higher.

Gravitational currents are important currents in atmospheric and oceanic studies. Gravity current is caused when a fluid with different density moves into another fluid. If the fluid of a given density enters the stratified ambient, such that its density is lower than the underneath layers and higher than the upper layers, the gravity current is of the intrusive type. The Kelvin-Helmholtz and the Holmboe instabilities are seen in the interface. The decisive parameters in the type of instability are the Richardson number local (J) and the ratio of shear layer thickness to the density layer (R). In this study, two-dimensional numerical simulation of the shear instabilities (Holmboe waves) with the Eulerian-Eulerian approach on intrusive gravitational flow is investigated. OpenFOAM code was used to perform this simulation, and due to the turbulence of the flow, the LES method was used to model the turbulence. The obtained results show that with increasing the intrusive current density, the value of Richardson number decreases and the R parameter increases. Also, as the density increases, the frequency of the Holmboe waves first increases, then decreases. An increase in the wavelength of Holmboe waves is observed with increasing the intrusive current density but in the wave number of the waves, the opposite behavior is seen. The phase velocity of Holmboe waves also does not have a specific trend with density changes.

The placement of a pier causes a three-dimensional complex flow pattern around the pier, which lead to scouring hole around the pier. In this research, modeling of scouring around piers was performed both experimentally and numerically using pimpleLPTdensFoam solver in OpenFOAM. In the first section, two different scenarios were used to simulate the scouring process. In the two numerical models, the effect of phase coupling and drag models on the scoured bed was investigated. One-way coupling with sphere drag model was implemented in the first scenario and four-way coupling with nonsphere drag model in the second one. According to the results, the first model was not satisfying, but the result of the second model was in good agreement with experimental results. The maximum depth of scour at the cross-section in numerical and experimental results was equal (6 percent error). All the simulations were done using the Euler-Lagrange approach and k-w turbulence model to simulate turbulent flow. In the second section of this research, to investigate effective parameters on the reduction of scour, three numerical models including a simple cylindrical pier and two more models using pier with collar were simulated. In order to study the effect of a collar on the reduction of scouring, the simple cylinder without a collar and then with the presence of collar at two different levels were performed. The results show that using collar in level 3.5 and level 4 may respectively lead to 49.2 and 29.7 percent reduction in maximum depth of scour.

In the present study, the propagation of a continuous three-dimensional, in the collision with obstacle and bi-disperse particle-laden turbidity current with a large eddy simulation method was modeled. Due to the presence of large number of suspended particles, the Eulerian-Eulerian method has been used and for each particle a concentration equation, which the particle settling velocity has been added to, is solved. The density current is simulated for particles with three different diameters 12, 20 and 30 microns. The inlet concentration effect of the particles has been studied by changing the inlet concentration to 0.6 and 0.3. The results show that in the same entrance conditions, before the obstacle, there is no significant change in the current speed profile in with and without obstacle state, but presence of an obstacle decreases the maximum velocity by 10%, also the amount of suspended particles on the obstacle decreases in channel width. In the final semi-stable state, the maximum concentration (after obstacle) 15.3% is reduced in compared to without obstacle state. By increasing the particle diameter to 20 and 30 microns, the maximum concentration is increased by 12.5% and 22.3%, the amount of suspended particles also decreases by 68% and 21%, respectively. As a result, particles with larger diameter precipitate more and rapidly and fine particles are transported over longer intervals by current. Changing the inlet concentration in the case of smaller diameter particle increases the amount of suspended particles by 11.2% and current will have more capability for carrying suspended particles.

One of the important subject in the analysis of compressor performance is estimating the surge limit. In this paper, the numerical simulation of surge in the first stage blades of axial compressor of GE-Frame 6 gas turbine has been done. Simulations done with 34 degrees angle of attack and k-ω (sst) turbulence model has been used. Obtained velocity vectors indicate that during the surge, the reverse flow which is the most important characteristic of surge occurs and the flow returns backward. Also, in this paper the coupled ordinary differential equations of pressure and flow changes for different values of the stability parameter B has been solved. It was found that for B = 0.6, which is less than critical value, rotating stall occurs and the pressure fluctuations damp after several fluctuations. While for B = 1.6, which is more than its critical value, a deep surge occurs and the pressure and flow disturbances fluctuate with constant amplitudes. These fluctuations of pressure and flow can cause fatigue or failure of the compressor blades.

An important types of density currents is called turbidity currents, where the density difference is due to the presence of suspended solid particles. In this paper, 3D numerical simulation of lock-exchange particle laden density current is presented in order to investigate the hydrodynamic of the current in density stratified environment. Simulations are carried out using Large Eddy Simulation method, three-dimensional Box filter, dynamic Smagorinsky method and Van driest damping function. The obtained results in stratified case are in good agreement with experimental data. Also, the presence of stratified environment significantly reduces the current velocity, but does not have any significant effect on the sedimentation pattern. In addition, the results show that increasing the slope leads to increase in sedimentation. It was also observed that increasing the particle diameter reduces the current momentum and increased sedimentation. For more accurate representation of the particle interaction, the particle settling velocity also varies with concentration. The analysis show that assuming the variable settling velocity in the early stages of the current leads to insignificant change in the front velocity, but when the flow advances more, the faster front velocity will be predicted.

Density currents flow due to the density difference between the current and surrounding environment. An important category of density currents is called turbidity currents, which density difference created as a result of suspended solid particle presence in fluid. In the present study, it is tried to use both Eulerian-Eulerian and Eulerian-Lagrangian methods, to take advantage of each one. In this way, the larger particle that have a more effective role in sedimentation mechanism due to the more falling velocity are calculated as Lagrangian and smaller particles by the Eulerian method. In order to obtain a criterion for particle assortment, seven currents with different particle sizes in the Eulerian-Eulerian model have been numerically simulated in a simple channel and it is compared with no particle case, and also the Eulerian-Eulerian method has been verified with experimental results and identified when the particle sizes is less than 12 micron, the sedimentation process is not appreciable, and the presence effect of these kind of particle can be ignored. Therefore, the Eulerian-Eulerian method is a suitable method for this case. The Eulerian-Lagrangian method validation has been performed with experimental results. Finally, the current inside the channel with a spectrum of particle dimensions is simulated and described the results by the proposed method (the combination of two methods). To perform numerical simulations, the development of open-source OpenFOAM codes has been used to take into account the effect of particle. Due to the current’s turbulence, a Large Eddy Simulation method has been used for turbulent modeling.