Sharif Digital Repository

Nowadays, hybrid LES/RANS approach is being widely used by many researchers. This approach uses the advantages of both LES and RANS approaches simultaneously. Hybrid approach is as accurate as LES, but its cost is much lower. The biggest problem of hybrid approach is transformation of data between RANS and LES regions. Before, precursor simulation and synthetic methods, such as Fourier series and synthetic eddy methods (SEM) have been used extensively to solve such problems. Precursor simulation is expensive, because of high cost of generation of data. On the other hand, Fourier series method is confined to simple geometries, while SEM has problem of programming. Interface condition in this... 

Partially Averaged Navier-Stokes (PANS) turbulence approach provides a closure model for any degree of velocity field filtering, ranging from completely resolved direct numerical simulation (DNS) to completely Reynolds averaged Navier-Stokes (RANS) approach. Preliminary investigations of PANS show promising results, but there is still computational and physical issues that must be addressed. This study investigates the performance of the PANS method for turbulent flow around a cylinder in ground effect with Reynolds number of 13,200. The cylinder flow is a benchmark flow problem which has significant experimental results available for validation of PANS approach. Three different filters... 

The phenomena of formation and detachment of droplets are of fundamental importance in studying two-phase flows, such as spraying processes, ink jet printing, emulosin, etc. Droplets are formed under the effects of surface tension forces. After formation, forces like gravity detach the droplet from the rest of the fluid. Recent advancements in computational fluid dynamics and computers have made it feasible to have advancement in simulation of complex flows, including two-phase phenomenon. On the other hand, the lattice Boltzmann method (LBM) has been developed into an alternative and promising numerical scheme for simulating multi-component fluid flows.
In this project, formation and... 

Nowadays, microfluidic flow appears in many applications, such as medical, biological, and chemical industries. Where as, micromixing, which deals with mixing of microfluidic flow in micro channels, appears to be an important issue to many researchers. In such systems, molecular diffusion plays an important role. On the other hand, lattice Boltzmann method is a relatively new simulation technique for complex fluid systems and has become interesting to many researchers in computational physics. In this study, computational simulation of such mixing process, using LBM is the main objective. Different obstacle layouts inside a microchannel have been investigated. Chaotic advection and jet... 

In recent years, LBM has shown to be a suitable computational method for most flow simulations. Its simplicity, adaptability for complex geometries, and capability in parallel processing are among many reasons for broad implementation of LBM these days. In this research, we have tried to take advantage of these features in the simulation of a turbulent flow over a moving circular cylinder at low CPU cost. In order to use LBM in a high Reynolds number incompressible flow over a moving solid body, one needs to pay special attention to grid quality, curved solid wall boundary condition, turbulence model, moving boundary, etc. In order to obtain a suitable practical LBM computer code, the... 

In recent years, lattice Boltzmann method (LBM) has been developed to be used as an alternative and promising computational technique to simulate various flows. It originates from classical statistical physics. The ability to simply solve complex flows, simulating of multiphase and multi-component without need to follow the boundaries of different phases, and the inherent ability of parallel processing are notable features of this approach. On the other hand, finite element method (FEM) is widely used in many practical engineering fields, especially in solid mechanics. In this study, in addition to simulating flow over a rigid body, flow over an elastic body is also simulated with a... 

Today energy sources, in any kind, have their special situation in human’s life and we must use our knowledge to manage them for effective usage by decreasing losses and finding modern renewable sources. One of the best way to achieve this purpose is increasing efficiency and improve energy generator performances under different environmental conditions. In spite of the fact that wind turbines stall dynamics behavior is under investigation for 6 years, study of dynamic stall phenomenon especially over series blades still needed. By addition to this phenomenon, improper wind turbine arrange decreases efficiency too. As a matter of fact, scrutiny of dynamics stall of wind turbines serial... 

In this study, an arbitrary Lagrangian-Eulerian (ALE) approach is incorporated with a mixed finite- volume-element (FVE) method to establish a novel moving boundary algorithm to simulate unsteady incompressible flow on non-stationary meshes. The method collects the advantages of both finite-volume and finite-element methods as well as the ALE approach in a unified algorithm capable of solving laminar, transient, and turbulent flows in fluid flow problems with moving boundaries. To enhance the robustness of the extended algorithm, we treat the convection terms at the cell faces using a physical influence upwinding scheme, while the diffusion terms are treated using bilinear finite-element... 

In this work, a direct design approach for designing a surface shape (inverse design problem) has been developed in which both the target surface pressure and the unknown nodal coordinates appear explicitly in the formulations. The final discretized form of the governing equations (unified formulation) can be used for both analysis and shape design problems. Shape design problems in the context of the steady inviscid and compressible flow, based on the three-dimensional Euler equations, were directly solved to achieve a prescribed pressure along the solid boundaries. The AUSM+ scheme was used to discretize the flux terms in the Euler equations, in which the inviscid flux is splitted into... 

In this research, surface reaction phenomenon in porous media has been numerically simulated in pore scale. The main interesting part in this kind of physics is surface reaction exposed to a flow field. In this regard, an in-house code, based on LBM was developed. To speed-up our simulation, parallel computing was used on a GPU platform leading to a significant speed-up. Also, to model a real complex geometry, X-CT and MRI medical imaging technique were used to accurately model several complex 3D geometries. While, image processing technique was used to model our 2D geometries. The basic equilibrium equation of isopropanol dehydrogenation was considered to simulate and to analize chemical... 

Deformation and breakup of drops are the basisof many interfacial flow studies and appear in a number of industrial applications, e.g., spray painting, spray combustion, emulsion, foam, sedimentation, and rain. Following their formation, drops may enter a region where hydrodynamicforces are large enough to cause their significant deformation and breakup. When a drop breaks apart into a multitude of small fragments due to disruptive hydrodynamicforces, the process is termed secondary atomization or breakup. Due to many engineering and scientific applications of multiphase and multi-component flows, they have been the main topic of many researchers for many years. Particularly, interfacial... 

This work deals with the computational investigation of film cooling technique, which is one the best practical way to protect gas turbine components form high thermal loads. In this regards, previous works are extensively reviewed and most important effective parameters are classified into three general categories, as geometrical parameters, flow characteristics, and physical surface factors. Each of these categories is then divided into subcategories and more details studies of each are performed. Then, a novel near-wall flow control technique of using staggered arrangement of small injection ports near a film cooling hole (combined-triple-jet; CTJ) is introduced. The fluid injected from... 

Smoothed particle hydrodynamics (SPH) is a meshfree method, based on Lagrangian formulation of Navier-Stokes equations, in which fluid is discretized to particles. This method is suitable and gives much better results for problems in which other methods have severe difficulties, (e.g., tracking of particles, moving boundaries, and problems with large deformations and, physical discontinuity). No need to follow a regular geometric structure in solution process and simple adaptation and coupling with methods such as finite element, are the positive characteristics of this numerical method. In this work, a systematic study of the processes, which are part of a membrane failure in a... 

In this research, the feasibility of the usage of the Wells turbine for direct conversion of the hydro-mechanical energy of the surface waves is studied via analytical and numerical methodologies. The results produced, showed for the first time that the Wells turbine can be employed for such waves directly and the performance variables are quite reasonable and even efficient considering low cost of HKWT system and abundant availability of those waves. For the low-frequency waves, the turbine is tested for different wave velocities to discover the best angular velocity of the rotor in terms of performance and the rotational velocity of 300 RPM has been found to be the optimum amount. Further,... 

Formation and detachment of drops are of fundamental importance in studying two-phase flows, such as ink jet printing, emulsion, and spray. Drops are formed under the effects of surface tension forces. After formation, forces like gravity detach the drop from the rest of the fluid. The flow rate is varied from dripping to jetting regime. In this project, formation and detachment of drops were simulated, using LBM with phase field model, which can simulate flows with high-density ratios and is a robust method for applying wetting condition on the walls. Results were validated using single-phase and two-phase flows. After code validation, the oscillations caused by formation and detachment of... 

The limitations of metals in tolerating thermal stresses is one of the main obstacles in increasing temperature of combustion products. In addition, strong desires to use higher temperatures than are allowed for metals, have led to use of different cooling methods for protecting surfaces adjacent to hot gases. Previous valid studies show that more than 25% of research in the field of gas turbine is related to their blades cooling. On the other hand, one of the very important methods of cooling such surfaces is film cooling. The results of this and previous researches conducted by the team of this thesis’ supervisors can for instance help designers to predict more suitable positions for... 

Boundary layer control has long been and still is considered to reduce drag. In this research, the effect of quasi-spherical structures on the control of boundary layer is investigated. First, flow over a NACA0015 airfoil was investigated. Then, flow over the same airfoil with quasi-spherical bumps on its upper surface was numerically simulated and the result of these two situations were compared. The dimensions and numbers of the bumps in longitudinal and lateral directions (different case studies) were chosen using trial and error. The three-dimensional, incompressible, and turbulent flow governing equations were numerically solved, using FVM and the OpenFOAM software. To get preliminary... 

In the present project, a stringed cavity, specifically the body of the Persian Setar, was studied using computational simulation. This study was carried out via acoustic approaches, based on structural and sound wave interactions. At first, the effects of different structural components were considered, separately. Based on vibrational modal analysis, dynamic characteristics of the elastic structure was investigated in terms of calculated vibrational normal modes. Then, the details of the whole structure was analyzed and the effects of these details on the quality of the produced sounds were investigated by applying some specific changes. The eigenmodes of Setar’s sound board and body, and... 

The interactions between jets and cross flows appear in many engineering applications (such as film cooling) and thus their study is of great importance. In film cooling, the coolant fluid is transferred from the compressor to the surface of the turbine blades via small holes. An increase of about 170⁰C of the engine working temperature increases the engine efficiency about 5%. To reach higher temperatures of the inlet air flow, one tries to have the least amount of interactions between the film cooling jets and the the cross flow. In this work, we computationally studied turbulent film cooling of a flat plate, in which we used the combined triple jet and investigated the effects of density... 

Numerical Simulation of two-dimensional steady compressible fluid flow on unstructured grids was accomplished using a fast implicit algorithm. To solve the copmlete two-dimensional Navier-Stokes equations, implicit time stepping was used which results in a large sparse linear system in each iteration. To solve the linear system, the biconjugate gradient method which belongs to Krylov subspace methods family, with an ILU(0) preconditioner was used. For accelerating the solution in large problems, parallel processing was used for linear system to be solved faster. Two upwind methods, namely Roe’s and AUSM+ methods were used for spatial descritizaion of inviscid fluxes with a MUSCL algorithm...