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Experimental investigation of turbulence specifications of 3-D density currents
, Article 2007 5th Joint ASME/JSME Fluids Engineering Summer Conference, FEDSM 2007, San Diego, CA, 30 July 2007 through 2 August 2007 ; Volume 1 SYMPOSIA, Issue PART A , August , 2007 , Pages 789-796 ; 0791842886 (ISBN); 9780791842881 (ISBN) ; Afshin, H ; Baghaer Poor, A ; Sharif University of Technology
2007
Abstract
The present study investigates the turbulence characteristic of density current experimentally. The 3D Acoustic-Doppler Velocimeter (ADV) was used to measure the instantaneous velocity and characteristics of the turbulent flow. The courses of experiment were conducted in a three-dimensional channel for different discharge flows, concentrations, and bed slopes. Results are expressed at various distances from the inlet, for all flow rates, slopes and concentrations as the distribution of turbulence energy, Reynolds stress and the turbulent intensity. It was concluded that the maximum turbulence intensity happens in both the interface and near the wall. Also it was observed that turbulence...
The influence of vertical deflection of the supports in modeling squeeze film damping in torsional micromirrors
, Article Microelectronics Journal ; Volume 43, Issue 8 , 2012 , Pages 530-536 ; 00262692 (ISSN) ; Taghi Ahmadian, M ; Sharif University of Technology
Elsevier
2012
Abstract
The objective of this work is to create an analytical framework to study the problem of squeezed film damping in micromirrors considering the bending of the supporting torsion microbeams. Using mathematical and physical justifications, nonlinear Reynolds equation governing the behavior of the squeezed gas underneath the mirror is linearized. The resulting linearized equation is then nondimensionalized and analytically solved for two cases of the infinitesimal and finite tilting angle of the mirror. The obtained pressure distribution from the solution of the Reynolds equation is then utilized for finding the squeezed film damping force and torque applied to the mirror. The results show that...
Design, Analysis and Manufacturing of Gas Foil Bearing
, M.Sc. Thesis Sharif University of Technology ; Akbari, Javad (Supervisor) ; Movahhedy, Mohammad Reza (Supervisor)
Abstract
Foil gas bearings are compliant surface, self-acting hydrodynamic bearings that use ambient gas as their working fluid. They do not require external pressurization and are typically constructed from several layers of sheet metal foils. In high-speed, high-temperature applications the GFB’s have the advantage of developing higher load capacities than with fixed geometry gas bearings. Since GFB’s have a complex structure consisting of a spring-like substructure with an overlying flexible surface, there exists a great variety of permutations on any given design. At current work presents a 1D model for GFB with couple solving Reynolds and Euler-Bernoulli beam equation. This mean top foil assume to...
Coupled bending and torsion effects on the squeezed film air damping in torsional micromirrors
, Article Proceedings of the ASME Design Engineering Technical Conference, 12 August 2012 through 12 August 2012 ; Volume 5 , August , 2012 , Pages 49-55 ; 9780791845042 (ISBN) ; Kaji, F ; Ahmadi, M. T ; Sharif University of Technology
2012
Abstract
The current paper presents an analytical model for the problem of squeezed film damping in micromirrors considering the bending of the supporting torsion microbeams. At the first the nonlinear Reynolds equation governing the behavior of the squeezed gas underneath the mirror is linearized. The resulting linearized equation is then nondimensionalized and analytically solved for two cases of the infinitesimal and finite tiling angle of the mirror. The obtained pressure distribution from the solution of the Reynolds equation is then utilized for finding the squeezed film damping force and torque applied to the mirror. The results show that in the case of the infinitesimal tilting angle, the...
Numerical investigation of steady density currents flowing down an incline using v2̄ - F turbulence model
, Article Journal of Fluids Engineering, Transactions of the ASME ; Volume 129, Issue 9 , 2007 , Pages 1172-1178 ; 00982202 (ISSN) ; Firoozabadi, B ; Farhanieh, B ; Sharif University of Technology
2007
Abstract
The governing equations of two-dimensional steady density currents are solved numerically using a finite volume method. The v2̄-f turbulence model, based on standard k - s model, is used for the turbulence closure. In this method, all Reynolds stress equations are replaced with both a transport equation for v2̄ and an elliptic relaxation equation for f, a parameter closely related to the pressure strain redistribution term. The Simple-C procedure is used for pressure-velocity coupling. In addition, Boussinesq's approximation is used to obtain the momentum equation. The computed height of the progressive density current is compared to the measured data in the literature, resulting in good...
3-D modeling of particle laden density current
, Article 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006, Chicago, IL, 5 November 2006 through 10 November 2006 ; 2006 ; 08888116 (ISSN); 0791837904 (ISBN); 9780791837900 (ISBN) ; Firoozabadi, B ; Ghasvari Jahromi, H ; Moosavi Hekmati, S. M. H ; Sharif University of Technology
American Society of Mechanical Engineers (ASME)
2006
Abstract
The gravity currents on the inclined boundaries are formed when the inflow fluid has a density difference with the ambient fluid and a tangential component of gravity becomes the driving force. If the density difference arises from the suspension of particles, the currents are known as particle-driven density currents, in which the local density of the gravity current depends on the concentration of particles. A low Reynolds k-ε turbulence model is used to simulate three dimensional turbidity currents. Also a laboratory apparatus was built to study the 3D flow resulting from the release of particle laden density currents on a sloping surface in a channel of freshwater via a sluice gate and...
Dynamics of multi layer microplates considering nonlinear squeeze film damping
, Article 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006, Chicago, IL, 5 November 2006 through 10 November 2006 ; 2006 ; 1096665X (ISSN); 0791837904 (ISBN); 9780791837900 (ISBN) ; Moghimi Zand, M ; Borhan, H ; Sharif University of Technology
American Society of Mechanical Engineers (ASME)
2006
Abstract
This paper presents a model to analyze pull-in phenomenon and dynamics of multi layer microplates using coupled finite element and finite difference methods. Firstorder shear deformation theory is used to model dynamical system using finite element method, while Finite difference method is applied to solve the nonlinear Reynolds equation of squeeze film damping. Using this model, Pull-in analysis of single layer and multi layer microplates are studied. The results of pull-in analysis are in good agreement with literature. Validating our model by pull-in results, an algorithm is presented to study dynamics of microplates. These simulations have many applications in designing multi layer...
Dynamic Behavior and Stability Analysis of Rotor-Seal System
, M.Sc. Thesis Sharif University of Technology ; Mohammmad Navazi, Hossein (Supervisor)
Abstract
The present research aims to achieve two main objectives in the Rotordynamic. The first one is, study and calculate the linear dynamic coefficients of a tapered gas seal, and the second one is, analysis the dynamic behavior and stability of the Timoshenko rotor supported with tapered gas seal. Classical Reynolds equation is used to calculate the dynamic coefficients of tapered gas seal. At first, using the perturbation method, the perturbed equations for gas film thickness and pressure are obtained. The finite difference method (FDM) is utilized to transfer the obtained PDEs to the algebraic system of equations. The algebraic system of equations then solved, and the zeroth and first-order...
Modeling squeezed film air damping in torsional micromirrors using extended Kantorovich method
, Article Meccanica ; Volume 48, Issue 4 , 2013 , Pages 791-805 ; 00256455 (ISSN) ; Ahmadian, M. T ; Farshidianfar, A ; Sharif University of Technology
2013
Abstract
The current paper uses the Extended Kantorovich Method (EKM) to analytically solve the problem of squeezed film damping in micromirrors. First a one term Galerkin approximation is used and following the extended Kantorovich procedure, the solution of the Reynolds equation which governs the squeezed film damping in micromirrors is reduced to solution of two uncoupled ordinary differential equation which can be solved iteratively with a rapid convergence for finding the pressure distribution underneath the micromirror. It is shown that the EKM results are independent of the initial guess function. It is also shown that EKM is highly convergent and practically one iterate is sufficient for...
Analytical modeling of squeeze film damping in micromirrors
, Article Proceedings of the ASME Design Engineering Technical Conference, 28 August 2011 through 31 August 2011, Washington, DC ; Volume 7 , 2011 , Pages 79-85 ; 9780791854846 (ISBN) ; Ahmadian, M. T ; Farshidianfar, A ; Sharif University of Technology
2011
Abstract
In the current paper, Extended Kantorovich Method (EKM) has been utilized to analytically solve the problem of squeezed film damping in micromirrors. A one term Galerkin approximation is used and following the extended Kantorovich procedure, the solution of the Reynolds equation which governs the squeezed film damping in micromirrors is reduced to solution of two uncoupled ordinary differential equation which can be solved iteratively with a rapid convergence for finding the pressure distribution underneath the micromirror. It is shown that the EKM results are independent of the initial guess function. It is also shown that since EKM is highly convergent, practically one iterate is...
Experimental investigation of drag and lift forces on microparticles in low reynolds number poiseuille flow in microchannel
, Article Journal of Dispersion Science and Technology ; Volume 37, Issue 12 , 2016 , Pages 1767-1777 ; 01932691 (ISSN) ; Saidi, M. H ; Sharif University of Technology
Taylor and Francis Inc
Abstract
In this paper, the hydrodynamic drag and lift forces on microparticles in a dilute suspension in low Reynolds number Poiseuille flow in a microchannel is experimentally investigated using microscopic image analysis. An effective technique is applied to manipulate single-particle tracking in order to determine the velocity field and hydrodynamic forces on microparticles with diameter-to-channel height ratio of 0.1 (Formula presented.) in microchannels. The results show that there is a distance near the walls of the channel in which the hydrodynamic coupling between the particles and channel walls is considerable. Also, the magnitude of drag force that is imposed on the particles with...
Transportation and Settling Distribution of Microparticles in Low-Reynolds-Number Poiseuille Flow in Microchannel
, Article Journal of Dispersion Science and Technology ; Volume 37, Issue 4 , 2016 , Pages 582-594 ; 01932691 (ISSN) ; Saidi, M. H ; Sharif University of Technology
Taylor and Francis Inc
Abstract
In this article, velocity field and settling distribution of microparticles in a dilute suspension in low-Reynolds-number Poiseuille flow in a microchannel is experimentally investigated using microscopic image analysis. An effective technique is applied to manipulate single-particle tracking in order to determine the controlling parameters on transportation and settling of microparticles in microchannels. The results show that the velocities of dispersed phase are affected by the hydrodynamic properties, and this velocity deviation can be significant when the hydrodynamic coupling between particles and channel walls is considerable. Increasing the Reynolds number would result in decrease in...
A numerical investigation on the dynamic stall of a wind turbine section using different turbulent models
, Article World Academy of Science, Engineering and Technology ; Volume 58 , 2009 , Pages 290-296 ; 2010376X (ISSN) ; Sharif, S ; Jamshidi, R ; Sharif University of Technology
2009
Abstract
In this article, the flow behavior around a NACA 0012 airfoil which is oscillating with different Reynolds numbers and in various amplitudes has been investigated numerically. Numerical simulations have been performed with ANSYS software. First, the 2-D geometry has been studied in different Reynolds numbers and angles of attack with various numerical methods in its static condition. This analysis was to choose the best turbulent model and comparing the grids to have the optimum one for dynamic simulations. Because the analysis was to study the blades of wind turbines, the Reynolds numbers were not arbitrary. They were in the range of 9.71e5 to 22.65e5. The angle of attack was in the range...
Sensitivity analysis of a heat exchanger tube fitted with cross-cut twisted tape with alternate axis
, Article Journal of Heat Transfer ; Volume 141, Issue 4 , 2019 ; 00221481 (ISSN) ; Esfahani, J. A ; Sharif University of Technology
American Society of Mechanical Engineers (ASME)
2019
Abstract
Numerical simulations are used to analyze the thermal performance of turbulent flow inside heat exchanger tube fitted with cross-cut twisted tape with alternate axis (CCTA). The design parameters include the Reynolds number (5000 < Re < 15; 000), cross-cut width ratio (0:7 < b=D < 0:9), cross-cut length ratio (2 < s=D < 2:5), and twist ratio (2 < y=D < 4). The objective functions are the Nusselt number ratio (Nu=Nus), the friction factor ratio (f =fs), and the thermal performance (g). Response surface method (RSM) is used to construct second-order polynomial correlations as functions of design parameters. The regression analysis shows that heat transfer ratio decreased with increasing both...
Numerical study of heat transfer between shell-side fluid and shell wall in the spiral-wound heat exchangers
, Article International Journal of Refrigeration ; Volume 120 , December , 2020 , Pages 285-295 ; Afshin, H ; Sharif University of Technology
Elsevier Ltd
2020
Abstract
Heat transfer between heat exchangers and the surrounding environment, referred to as heat-in-leak, is a crucial phenomenon in the cryogenic applications which can substantially degrade the heat exchanger performance. Present research is organized to investigate the mechanism of heat transfer between the shell-side fluid and the shell wall of spiral wound heat exchangers (SWHEs) to determine the heat transfer coefficient used in the heat-in-leak calculations. The heat transfer characteristics are studied using computational fluid dynamics (CFD) tools. First, 20 dissimilar SWHE models with respect to the geometrical parameters are built and then numerically simulated at different Reynolds...
Jet into cross flow boundary layer control an innovation in gas turbine blade cooling
, Article 35th AIAA Fluid Dynamics Conference and Exhibit, Toronto, ON, 6 June 2005 through 9 June 2005 ; 2005 ; 9781624100598 (ISBN) ; Taeibi Rahni, M ; Darbandi, M ; Sharif University of Technology
2005
Abstract
New standpoint of turbulent coolant jets into crossflow, which have numerous applications in traditional and modern technology, especially in gas turbine blades, is presented in this work. It is more than half a century that, many researchers have been studying jet into cross flow to understand its behavior and to predict and control it better. Previous studies indicate that, the main attentions had been on: a- geometrical parameters such as: inclined and compound jet angles, hole's shape, jet's array arrangements, jet's spacing, and jet's channel depth, b- flow characteristics like: blowing ratio, density ratio, jet and cross flow Reynolds numbers, and turbulence intensity. Here, we have...
Application of piezoelectric and functionally graded materials in designing electrostatically actuated micro switches
, Article Proceedings of the ASME Design Engineering Technical Conference, 15 August 2010 through 18 August 2010 ; Volume 4 , August , 2010 , Pages 613-620 ; 9780791844120 (ISBN) ; Ahmadian, M. T ; Design Engineering Division and Computers in Engineering Division ; Sharif University of Technology
2010
Abstract
In this research, a functionally graded microbeam bonded with piezoelectric layers is analyzed under electric force. Static and dynamic instability due to the electric actuation is studied because of its importance in micro electro mechanical systems, especially in micro switches. In order to prevent pull-in instability, two piezoelectric layers are used as sensor and actuator. A current amplifier is used to supply input voltage of the actuator from the output of the sensor layer. Using Hamilton's principle and Euler-Bernoulli theory, equation of motion of the system is obtained. It is shown that the load type (distributed or concentrated) applied to the microbeam from the piezoelectric...
Numerical simulation of vortex engine flow field: One phase and two phases
, Article Journal of Thermal Science ; Volume 18, Issue 3 , 2009 , Pages 226-234 ; 10032169 (ISSN) ; Saemi, S. D ; Saidi, M. H ; Sharif University of Technology
2009
Abstract
Aiming at improving efficiency in combustion systems, the study on droplet behavior and its trajectory is of crucial importance. Vortex engine is a kind of internal combustion engine which uses swirl flow to achieve higher combustion efficiency. One of the important advantages of designing vortex engine is to reduce the temperature of walls by confining the combustion products in the inner vortex. The scopes of this investigation are to study vortex engine flow field as well as effective parameters on fuel droplet behavior such as droplet diameter, droplet initial velocity and inlet velocity of the flow field. The flow field is simulated using Reynolds Stress Transport Model (RSM). The...