مجله مکانیک سازه ها و شاره ها
سال یازدهم شماره 3 (امرداد و شهریور 1400)

In this paper, using a preconditioning approach, a viscous all-speed flow solver has been developed based on JST and CUSP schemes and compared with Roe upwind scheme in unstructured database. The comparison between the preconditioning and non-preconditioning has been donned. The results show that the non-preconditioning compressible solvers hardly converged in low-speed regions while the preconditioned forms converged more rapidly. The results of all schemes were compared with experimental data that showed the all schemes have good agreement with experimental data. Although, Roe upwind and CUSP schemes have more agreement to the data. Also, the CUSP scheme is faster than the other schemes. In the next step, aerodynamic optimization based on discrete adjoint method has been developed over the schemes. The verification of the method done comparing the results with finite difference method in both preconditioning and non-preconditioning approaches. The agreement of results is good. In general, it can be concluded that the CUSP algorithm is somewhat faster than the other two algorithms and is a good option for optimization.

In the present study, the Charpy impact test on the 7075 aluminum sample performed in full size 55×10×10 mm with different notch depth and the fracture energy is measured. The force-displacement curves for Charpy samples with different notch depth were drawn by instrumented impact pendulum. Then the energy connection of the Charpy fracture is determined as a function of the changes in the initial crack depth. Using this relationship can determine the amount of error in measuring the energy fracture of the tested aluminum by measuring the available geometric dimensions of the initial crack including the allowable tolerances specified in the standard. Also, the microstructure of the fracture surface of several samples with different notch depth was investigated. Fractography observations showed that different fracture mechanisms occurred by changing the notch depth of the specimens, including ductile-brittle fracture, quasi-brittle fracture, and brittle fracture. As the depth of the notch depth increases, the fracture becomes more brittle, which is due to increase cleavage and decrease dimple in the fracture surface. Also, as the notch depth increases, the fracture energy decreases based on the E = 22.857e-0.355a exponential relationship, Using this equation, the exact amount of Charpy fracture energy can be calculated for each desired notch in the 7075 aluminum tested.

In the present study, a proper analytical model is developed for investigating nonlinear vibration behavior of asymmetric and symmetric rotors under unbalanced and misalignment induced forces in rotating coordinates. The rotor is connected to a motor through an asymmetric coupling. For a more accurate vibration analysis, Timoshenko's beam theory is applied and for a better modeling of misaligned coupling forces, Gibbons’ equations are used. The equations of motion are discretized by Rayleigh-Ritz method and derived from Hamilton's principle, thereafter. According to this investigation, for asymmetric rotors as opposed to symmetric rotors, a frequency range is detected in which resonance and instability occurs. Also, the dynamic behaviors of the symmetric and asymmetric rotors are analyzed at the same rotational speed to indicate specifically the effects of different parameters on both rotors. For investigating the vibration behaviors of the rotors more accurately, their time-domain vibration responses are plotted and then their Fast Fourier Transform (FFT) are presented to determine the vibration frequencies of the rotors, so the effects of different parameters can be well observed. As a whole, in this study, the effects of each of the defects such as shaft asymmetry, misalignment, mass unbalance and also nonlinear terms are investigated on the dynamic behavior of the rotor system.

Fluid contact structures as a branch of the Fluid–Structure Interaction (FSI) are among the physical models that have been able to present themselves as a new example of dynamic systems due to their very rich dynamics and Pay close attention to scientists. The dynamic behavior of two rigid straight articulated pipes conveying fluid is studied. The flow rate in the pipe is harmonic. The numerical results are compared with the present method and Runge–kutta 4th order for validation and an acceptable match between them is obtained. The method of multiple time scales is used to drive the time response and phase plane curves. The influence of the initial velocity , ratio fluid mass per total fluid mass and mass of pipes and flow frequency on the time response and phase plane curves are examined. The results show, by increasing "u" _"0" and and decreasing the system is closer to loss of stability and increasing dynamic chaos.

Ultrasonic levitation has a high potential to be used in different applications due to its independence from the levitated material. Among effective parameters of this process, the distance between the reflector and the transducer and applied voltage plays an important role in the process. Presenting a precise numerical model would be helpful in the study of the process. The levitated particle conditions as well as the levitated capacity, such as the maximum levitated particle density, can be predicted using the model. In this paper, a precise numerical model was presented to study of the process. In the model, by simultaneous solving the equations of the piezoelectric and the wave equation in the solid and fluid medium as well as considering the three-dimensional levitation and particle tracing in Comsol software, the simulation conditions were similar to the experiments. Hence results agreed well with the experimental results. The effect of voltage and distance between the transducer and the reflector on the levitated particle was investigated using the model. Particle motion during levitation was explained simply and effectively using the proposed certain parameters. Defining the parameters makes it possible to compare the particle conditions at different voltages and distances between the transducer and the reflector. The results showed that in order to properly levitated object condition, the adjustment of the parameters should be made based on each other's values. Moreover, the maximum density in different working conditions with an accuracy of 0.45% was presented using the proposed numerical model and halving algorithm.

Repairing cracked parts using patches is a common method to restore mechanical properties and tensile strength. The efficiency of such patches can be achieved by obtaining the maximum amount of force borne by the repaired part and comparing it with the non-repaired part. The purpose of this study is to investigate the diffusion method used in the repair of cracked aluminum parts and to evaluate the patches and fibers used. In this study, the diffusion method is used to connect the aluminum patch to repair the central crack in the aluminum thin sheet. Examination of microstructures showed that using the diffusion method, parts can be welded together and a suitable part can be obtained without the use of glue. The process of repairing cracked parts made of aluminum or its alloys in this study involves the use of three types of patches and different fibers and different conditions, and finally pressing the patch and the piece together under heat for a certain period of time. The repaired parts are subjected to quasi-static tensile loading and the maximum amount of force borne by the part is obtained in different states.

In this paper, frequency analysis of Bi-directional Porous functionally graded beams with variable cross section which are resting on elastic foundation based on Reddy third order shear deformation theory is studied. Mechanical property gradients defined in accordance with two models of exponential and volume fraction power law. Governing equation which is obtained with the aid of third order shear deformation theory and by considering elastic foundation effect in conjunction with Hamilton’s principle. Due to intrinsic closed form solution of equations, differential equations solved with using Generalized Differential Quadrature Method by considering various end conditions. In order to validate the results comparisons are made with solutions which are available for other papers. This study reveals that the difference between the results of this paper and the results of others is negligible. Eventually the effects of geometrical parameters, power and exponential law indexes and elastic foundation coefficients on natural frequencies of Bi-directional FGM beams is studied. The results reveal that non dimensional frequencies increase with the rise of elastic foundation coefficients and the soar of porosities and material gradients in two directions causes a sharp decrease in non dimensional frequencies.The results of this study can be used in optimal design, vibration control and detection of failure structure of functional graded structures

In this paper, the effects of input parameters on the roughness and cylindricality of a hole were investigated using fused deposition modeling (FDM). The roughness and dimensional accuracy are very important for a 3D printed part. The investigated input parameters are injection speed, solid percentage, and layer thickness. The experiments were designed using the response surface method and 51 PLA samples were printed. To measure the roughness and dimensions of the manufactured products, a roughness-testing machine and a coordinate measuring machine (CMM) were used. The basis of this study is the comparison of roughness and deviation from the center to find the optimal set of the investigated parameters to achieve higher quality.The optimal levels of parameters for the minimum simultaneous roughness (0.016 mm) and cylindricality (5.98 microns) are a layer thickness of 0.1 mm, injection speed of 40 mm / s, and solidity percentage of 15% when the pattern is triangular. The results also suggest that the layer thickness has the strongest effect on the surface quality in comparison with the injection speed and percentage of solidity.

Thin-walled tubes are widely used to absorb energy in road accidents. In this paper, a novel circumferentially corrugated tubes with a tangential circular profile is studied. The LS-Dyna finite element code is used to evaluate the crashworthiness. In order to verify the models and the solution method, the results are compared with the experimental results. These studies show that the simulation correctly predicts the behavior of the structure. These profiles are investigated under quasi-static, axial, lateral and oblique loads. Then, the results are compared with the results of the conventional square, circular and cellular square tubes. The results indicate that the present profiles under simultaneous axial and lateral loads, compared with the conventional cellular square tubes, increase the specific energy absorption and the crush force efficiency parameter by 26% and 10%, respectively. In addition, the results show that use of cross blades inside the corrugated profiles prevents the creation of a general deformation mode. Under lateral loading, compared with the conventional cellular square tubes, the amount of specific energy absorption is about 20% less, but the crushing force efficiency is significantly higher.

In this paper, the slider-on-flat-surface (SOFS) test was used to study the galling in automobile body drawing dies (made of GGG60 alloy). The SOFS tests were conducted on the DC06 (EN 10130) and DC04 (MSC standard) alloy sheet metals; according to the following two approaches: in the first approach, the slider was forced to move in a single linear path, repeatedly. While, in the second approach, the slider was moved on separate parallel paths. The second approach was performed using both flat and curved surfaces of the slider to simulate galling conditions on flat and curved surfaces of the dies, respectively. Moreover, the effect of hardness, surface roughness, and mechanical properties of the DC04 and DC06 sheet materials on die galling was investigated. The galling tests’ results were in agreement with the galling wear that occurred in automobile body drawing dies. The results showed that forming DC04 sheet metal will lead to more galling wear. Furthermore, a qualitative coincidence was observed between the results of the aforementioned approaches. Nevertheless, in the first approach, wear emerged in a shorter period; while, in the second approach, the slider was moved on a much longer path for creating observable wear.

Backdraft is a special phenomenon of fire that occurs in a closed environment with limited ventilation and it can increase temperature, pressure and intensify fire. In this paper, with the method of Large Eddy Simulation and using Fire Dynamics Simulator, the behavior of the backdraft phenomenon in a closed enclosure is investigated. Changing the aspect ratio of opening, fuel injection from the combustion source and displacement of the combustion source are the three basic parameters whose effect on the dynamic and thermodynamic behavior of the backdraft to extinguish or delay it was investigated. The results indicate that if the fuel participates in reaction with a lower concentration, backdraft can occur at low pressures. By reducing the aspect ratio of opening, the time of occurrence of the backdraft phenomenon was delayed and was able to reduce the peak dynamic pressure resulting from this phenomenon by nearly 4 Pascals; However, due to the presence of semi-combustible materials, the possibility of a second backdraft occurred. Also, by displacement the combustion source and bringing it closer to the opening, it was observed that the backdraft phenomenon occurred on a smaller scale and it is possible to extinguish it completely.

Cyclone separators are used to separate the solid particles from the gas streams in industrial applications. A strong swirling turbulent flow inside the aero-cyclone causes the particle separation due to their inertia. Two important parameters for cyclone design are the pressure drop and separation efficiency. The cyclone geometry strongly affects the efficiency and pressure drop. Therefore, in this article, different new geometries for the vortex finder of the cyclone are considered and tested experimentally. The efficiency and pressure drop have been measured and compared for different fabricated vortex finders. A typical high-efficiency Stairmand cyclone was considered and the guide vanes with an angle of 10 and 20 degrees to the horizontal plane were added to the original vortex finder. Experimental results show that for the guide vanes with 20 degrees angle, and at the gas velocities between 10 and 20 m/s, the pressure drop is reduced by 58% and the efficiency is increased by 2.3%.

In this study, the flow pattern, temperature distribution and thermal comfort index have been experimentally investigated in a room with personalized confluent jets ventilation (CJV) system for different diffusers’ heights of 30 and 160cm from the floor and for two inlet temperatures of 16 and 24°C and three different inlet angles of 0, 22.5 and 45 degrees. The results showed that the above cases can be significantly affected by changing in location of inlet diffuser. So that by placing the inlet diffuser at a height of 160 cm, the occupants’ upper body is strongly affected by the flow and the maximum air velocity in this region, depending on the change of inlet flow angle from zero to 45 degrees, can respectively reach about 1.5 to 1 m/s and this causes the draught dissatisfaction in the occupants’ upper body. However, by placing the diffuser under the desk and at a height of 30cm, the most dissatisfaction caused by draught occurs in the legs and feet. Finally, it can be concluded that the draught dissatisfaction is the most important factor that can limit the use of personalized confluent jets ventilation system.

In this study, the possibility of indoor cooking in an indirect solar cooker integrated with phase change material is experimentally investigated. Moreover, the effect of using acetanilide and magnesium nitrate hexahydrate (as the phase change materials) on the performance of the cooking unit of the solar cooker is compared. For this purpose, the phase change materials are charged by the indirect solar cooker initially. The stored energy in these materials is then utilized to heat 0.5 L and 1 L of water in the afternoon. The investigated parameters include oil temperature at the inlet of the cooking unit, water temperature, phase change materials temeprture and the rate of the thermal energy absorbed by water. Based on the results, acetanilide cannot reach its melting point during the charging process; however, thermal energy is stored as sensible and latent heat in magnesium nitrate hexahydrate. Using the stored energy in the phase change materials, the time required to boil 0.5 L and 1 L of water in the afternoon is 8 min and 12 min respectively. In the cooking unit with magnesium nitrate hexahydrate, the temperature of 0.5 L of boiling water reduces to 80 ºC after passing 4 h and 42 min, which is 1 h and 50 min higher than that of the cooking unit with acetanilide. Moreover, in the cooking unit with acetanilide, the average rate of the thermal energy absorbed by 1 L of water is about 373.68 W in the afternoon cooking.

In this article, three configurations of using compressor in absorption heat transformers are introduced and analyzed thermodynamically and compared with simple configuration of them. Water-lithium bromide, ammonia-lithium nitrate and ammonia sodium thiocyanate solutions are used as working fluid in these systems. The results show that systems using water-lithium bromide as working fluid have lower delivered heat temperature, higher minimum generator and evaporator temperatures and higher possibility of crystallization occurrence than systems using ammonia-lithium nitrate and ammonia sodium thiocyanate fluids as working fluid. In addition, the results show that using compressor in absorption heat transformers increase the temperature of delivered heat, decrease the minimum temperatures of absorber and generator and increase PER, ECOP especially at high absorber temperature. Compared to the systems using ammonia-lithium nitrate, the systems using ammonia sodium thiocyanate starts to perform at lower generator and evaporator temperatures and delivers heat at higher temperatures, but they have lower primary energy ratio and second low efficiency except at high absorber temperatures.

Geometry of primary nozzle is one of the parameters that affect the performance of a supersonic ejector. In this paper, the effect of using parallel-flow primary nozzle on the performance of a supersonic ejector is experimentally investigated. For this purpose, two primary nozzles, conical and parallel-flow, with the same converging portion and the same ratio of exit surface to throat surface, have been used. The parallel-flow nozzle diverging curve is calculated using the characteristic method. At various positions of the primary nozzle, characteristic curves of the ejectors with conical and parallel-flow nozzles have been compared.The results show that by changing the primary nozzle diverging curve from conical to parallel-flow, at the same position of primary nozzle, the entrainment ratio increases in both critical and subcritical regions. In the critical region, entrainment ratios obtained by the primary nozzle positions 5 mm, 10 mm, and 15 mm are increased by 9.5%, 4.5% and 4.7%, respectively.The optimum position of the primary nozzle changes with changing the primary nozzle diverging curve. At the suitable position of the primary nozzles and without any reduction in critical pressure, the maximum relative increase of entrainment ratio in the critical region is 8%.

In this study, the performance of viscoelastic constrained layer damping (CLD) in suppressing the flutter and vibration of panels is investigated with the special focus on the effects of three practically important features including the in-plane edge constraint, location of the CLD patch on the panel, and cutting the CLD patch. The panel is assumed to have infinite width and the higher-order shear deformation theory accounting for the through-the-thickness deformation is used for modeling the core. The finite element model is constructed using one-dimensional three-node elements with Lagrangian and Hermitian shape functions, and the piston theory is used for the aerodynamic pressure. Parametric studies are performed to determine the effects of in-plane constraints, partial CLD location, and segmeting the CLD. Results show that in-plain constraints can considerably affect flutter boundaries and also by adjusting the CLD location, the performance can be improved. However, the optimal location for maximum vibration damping would differ from the best location obtained for highest flutter suppression. Moreover it is found that the patch segmentation would not be beneficial for achieving better flutter suppression, while it would increase the damping of the structure.