Showing 12 results for Ahmadi
H. Ahmadi, Rahim Zolanwar,
Volume 8, Issue 1 (4-1990)
Abstract
K. Sadrnezhaad and H. Ahmadian,
Volume 17, Issue 2 (4-1998)
Abstract
In this research a Cu-Zn-Al alloy is produced by melting the raw materials in an electric resistance furnace and then pouring it into a steel mould. The optimum way to achieve the final analysis in the hypo-eutectoid range is determined and the influence of the alloying element, Ti on the grain size and the shape memory properties of the samples are investigated. Solution treatment (done at 850˚C) followed by quenching in ice-water mixture results in the formation of the martensitic structure and the shape memory effect. Aging at temperatures bellow 200˚C results in the reduction of transformation temperatures, while aging at temperatures between 200˚C and 350˚C results in the enhancement of these temperatures and at temperatures above 350˚C results in the destruction of the shape memory properties. Effect of super-elasticity at 40˚C (between Md and Ms) is observed and tensile tests are run at 25˚C and -55˚C to verify the influence of the prevailing phase.
M. T. Ahmadian and S.sadeghi,
Volume 19, Issue 1 (7-2000)
Abstract
A nonlinear model consisting “yaw, roll, longitudinal, lateral and pitch” has been developed in which, tire and suspension characteristics have been considered. Tire model is based on the elliptic concept and tire Calspan data. According to this tire model, cornering force and aligning moment are computed as a function of slip and camber (inclination) angles, normal load, tire adhesion characteristics and skid number. The effects of suspension systems and the component of lateral and longitudinal weight transfers, are considered. Finally the equations of motion are droven, vehicle handling behavior and effect of anti roll stiffness on handling characteristics are shown.
M. Izadi-Nia and M. T. Ahmadi,
Volume 19, Issue 2 (1-2001)
Abstract
Arch dams are generally built in cantilvever monoliths. Vertical contraction joints between monoliths control the stresses due to concrete shrinkage and temperature variations. Past studies in earthquake safety evaluation of the system often show the tensile fracture and local nonlinear deformation of joints. In this paper, some nonlinear joint element constitutive models are presented to examine the tensile and shear fractures of joints. Reservoir hydrodynamic interaction and water compressibility are considered in the finite element analysis of the system. Some primary examples were calculated. Regarding the numerical results, efficiency of the proposed models is promising for safety evaluation of arch dams under static and dynamic loads
E. Shirani, and H. Ahmadikia,
Volume 19, Issue 2 (1-2001)
Abstract
2-D and axisymmetric Navier-Stokes equations are solved using Reiman-Roe solver with different limiters for second-order accurate schemes. The results were obtained for supersonic viscous flows over semi-infinite axisymmetric and 2-D bodies. The free stream Mach numbers were 7.78 and 16.34. The stability of Roe method with different limiters and entropy conditions were considered. The results show that the limiters greatly affect the stability and accuracy of the numerical solution while the entropy conditions do not.
A. Nasirahmadi, L. Estehghari, and A. A. Soudi,
Volume 19, Issue 2 (1-2001)
Abstract
The compounds, lanthanium chromite, lanthanium magnetite and lanthanium ferric oxide were prepared through sol-gel methods in powdered form, then calcinated, peletized, and sintered at various temperatures. Electrical conductivity of these compounds was measured at various temperatures. The effectiveness of strontium as a dopant on electrical conductivity was also investigated. Electrical conductivity enhancement was observed in dopped compounds
A. R. Pishehvar and H. Ahmadi Kia,
Volume 20, Issue 2 (4-2001)
Abstract
In this study, the missile staging process by implementing a side-injected jet is simulated numerically. The problem is considered to be axisymmetric and the thin shear layer approximation of Navier-Stokes equations along with an algebraic turbulence model is used in a quasi-static form for the calculations. The free stream corresponds to a very high altitude flight condition with a Mach number of 10 and an injected jet pressure ratio of about 63000. An explicit Godunov-type scheme is used in the calculations, which is second-order in time and space. Computations are performed on the attached and separated geometry for a range of distances between the body and the warhead. The intense interactions between the jet flow and the main free-stream and its overall influences on the warhead aerodynamic loading are finally demonstrated.
Keywords: Missile Staging, Jet Interaction flow, TVD Scheme, Riemann problem
H. Ahmadikia, and G. A. Iravani,
Volume 26, Issue 1 (7-2007)
Abstract
Design of a natural dry cooling tower has been accomplished in two sections: the design of heat exchangers and the numerical solution of flow through the tower. Heat exchanger (Heller type) has been simulated thermodynamically and then coupled with a computer program, which calculated the turbulent natural convection flow through the tower. The computer program developed for this purpose can be used to obtain thermodynamic propertied of the cooling tower such as mass flow rate of air, temperature of outlet water, distribution of temperature, distribution of velocity, and distribution of pressure through the tower. Numerical results have been compared with experimental data of Shahid Montazery Thermal Power Plant under different environmental conditions. Comparison between numerical results and experimental data showed good agreement.
A. Eslami, H. Modaraei, and H. Ahmadi,
Volume 26, Issue 2 (1-2008)
Abstract
M. Ahmadi, N. K. A. Attari,
Volume 35, Issue 1 (9-2016)
Abstract
Using Vibro-Impact Nonlinear Energy Sinks (VI NESs) is one of the novel strategies to control structural vibrations and mitigate their seismic response. In this system, a mass is tuned on the structure floor, so that it has a specific distance from an inelastic constraint connected to the floor mass. In case of structure stimulation, the displaced VI NES mass collides with the inelastic constraint and upon impacts, energy is dissipated. In the present work, VI NES is studied when its parameters, including clearance and stiffness ratio, are simultaneously optimized. Harmony search as a recent meta-heuristic algorithm is efficiently specialized and utilized for the aforementioned continuous optimization problem. The optimized attached VI NES is thus shown to be capable of interacting with the primary structure over a wide range of frequencies. The resulting controlled response is then investigated, in a variety of low and medium rise steel moment frames, via nonlinear dynamic time history analyses. Capability of the VI NES to dissipate siesmic input energy of earthquakes and their capabilitiy in reducing response of srtructures effectively, through vibro-impacts between the energy sink’s mass and the floor mass, is discussed by extracting several performance indices and the corresponding Fourier spectra. Results of the numerical simulations done on some structural model examples reveal that the optimized VI NES has caused successive redistribution of energy from low-frequency high-amplitude vibration modes to high-frequency low-amplitude modes, bringing about the desired attenuation of the structural responses.
I. Ahmadi, D. Kouhbor, R. Taghiloo,
Volume 38, Issue 1 (8-2019)
Abstract
In this paper, a finite element model is presented for the transient analysis of low velocity impact, and the impact induced damage in the composite plate subjected to low velocity impact is studied. The failure criteria suggested by Choi and Chang and the Tsai-Hill failure criteria are used for the prediction of the damage in the composite plate; then the effect of various parameters on the impact induced damage is investigated. The first order shear deformation plate theory and the Ritz finite element method are employed for modeling the behavior of plate, and the modified Hertz contact low is used for the prediction of the contact force through the impact. In the numerical results, the time history of indentation, contact force and stress during the impact and the impact induced damage is investigated. The matrix cracking and delamination in the plies of the laminated composite plate subjected to low velocity impact are studied and the effects of various parameters are investigated.
S. A. Ahmadi, M. H. Pashaei, R. A. Jafari-Talookolaeilokoolaei,
Volume 39, Issue 1 (8-2020)
Abstract
In this paper, three-dimensional displacement response of a cylindrical sandwich panel with compressible core under the action of dynamic pulse loading is addressed using the extended high order sandwich panel theory. Also, local dynamic pulse buckling of facesheets is studied by considering the Budiansky-Roth buckling criterion. It is assumed that the sandwich panels consist of orthotropic face sheets and an isotropic viscoelastic foam core layer. The effects of various parameters including the panel span, core and facing thickness, pulse duration and maximum pressure on the non-linear dynamic response and buckling strength of the sandwich cylindrical panel are studied. The results obtained from the present method are compared with finite element solutions using the commercial software ANSYS and those reported in the literature, showing a good agreement. It is revealed that applied core non-linear theory could be satisfactory for the dynamic pulse response of sandwich viscoelastic panels. It is also shown that the pulse buckling strength of panel increases with a decrease of the panel radius or an increase of the panel thickness.
