نشریه مهندسی مکانیک مدرس
سال بیست و دوم شماره 9 (شهریور 1401)

4D printing is an emerging phenomenon of additive manufacturing which is achieved by the 3D printing of shape memory materials. The printed parts can change their shapes when exposed to an external stimulus by controlling the 4D printing process. In this study, a relationship between applied pre-stress in the printing process and the amount of bending shape recovery was obtained using numerical simulations. Then, the effect of printing parameters of fused deposition modeling including layer thickness, printing speed, and nozzle diameter on the applied pre-stress in the printed polylactic acid parts was investigated. To reduce the cost and time as well as to increase the validity and accuracy, the central composite design method was used. Seventeen experimental tests were performed and a model was obtained between the mentioned parameters and applied pre-stress. In the following, R2 and Adj R2 of the model were obtained above 0.99 and 0.98, respectively, which indicates the high accuracy of the model. The model showed that the layer height, nozzle diameter, and printing speed have a significant effect on the applied pre-stress. Also, the applied pre-stress is inversely related to the layer height, nozzle diameter and is directly related to the printing speed. Simulations were performed under the conditions of maximum bending shape recovery and compared with the experimental model. The results showed that the error of the relationship between applied pre-stress and amount of shape recovery is 1.5% and the experimental model of printing parameters effect on the shape recovery is completely matched.

In this study, the purpose is to evaluate the effective factors in the increase of projectile velocity in the gas-gun device. At first, the effect of various factors such as the shape and dimensions of the sabot, groove depth and the shape of the Rupture Disk holder, the path of the projectile from the moment of movement to the exit from the tube, and the type of gas used for launching are investigated. In each step of the experiments, a factor is evaluated and its effect on improving the performance of the gas-gun device is investigated. Based on the results of the study of effective factors the velocity of the projectile the shape of the sabot, and the projectile movement path with a 24.12 and 20.81% increase in projectile velocity respectively, the maximum and the type of gas used with 2.99% increase in projectile velocity had the minimum effect on the performance of the gas-gun device. In the second part of the present study, using the finite element method by LS-Dyna, the required pressure and rupture shape of the Rupture Disk in different grooves were investigated and compared with the experimental results. The shape of the rupture and the pressure required to rupture the Rupture Disk from the numerical method are in good agreement with the experimental results.

In the current paper, downstream flow field of a propeller at low Reynolds numbers and at static conditions (zero flight speed) is investigated experimentally. This propeller can be utilized in UAVs. Propeller diameter is 56 centimeter and it is investigated at 2550 to 5670 rpm experimentally. Experiment results show that propeller rpm increasing, increases induction velocity. Flow swirl ratio and axial flow coefficient decrease along propeller radius at different propeller rpm. Experimental results of absolute velocity of swirl flow at the propeller airfoil trailing edge downstream is fairly similar to the free vortex flow theory at static condition along the blade radius.  At static condition for r/R<0.8, semi-empirical equations are suggested for variation of flow swirl ratio and axial flow coefficient at downstream of propeller. The propeller is also simulated with numerical simulations. Relative standard deviation of numerical and experimental results for propeller thrust and power are 0.4 and 4.1, respectively. The exponential coefficient (n) which predicts numerical axial flow downstream of propeller for r/R<0.8 has a 7.7 relative standard deviation with experimental results at static condition.

Flap-type WECs are used On-Shore to generate electricity and pump. The draft depth and incident wave frequency are parameters affecting the performance of this type of converters. In this paper, the effect of water draft depth and incident wave frequency on the performance of a converter at a scale of 1: 8 investigated experimentally. The power take-off system is hydraulic. The Caspian Sea was also selected as the target sea. After calibration and uncertainty analysis, experimental tests performed in the wave-flume of BNUT by regular waves. Considering the period of the Caspian Sea ([4-8] s), Froud scaling, the tests were performed in the period interval of [1.6-2.5] s, which is equivalent to the frequency interval [0.4-0.63 ] Hz. Also, due to the importance of the converter's draft, the converter's performance was evaluated from the draft of -0.1 (submerged flap) to 0.6 m. According to the results, the best converter performance was at the lowest frequency; the converter performance decreased with increasing frequency. The best converter performance was obtained at the dimensionless draft of 0.43 (equivalent to 0.4 m draft), and the converter power was reduced at larger and smaller draft. It is worth noting that at a negative draft (submerged flap), the converter has the lowest performance. The maximum values ​​of flow, power and pressure on a laboratory scale were 0.14 liters per second, 21.3 watts and 156.8 kPa, respectively, which were measured at 18 liters per second, 22.66 kW, respectively, using Froud scaling method. And will be 1249/61 kPa

In recent years, linear electromagnetic actuators have gained special attention in small robot actuation and calibration of milli newton thruster stands. In this paper, a linear electromagnetic actuator with a force range of milli newtons is designed and manufactured. In this regard, first the analytical relationships of the magnetic field and Lorentz force were derived and then, based on the desired design criteria meaning high force sensitivity, low heat loss and minimum geometric dimensions and weight, the appropriate design parameters of the electromagnetic actuator is obtained. According to the results, the  obtained force constant is approximately 1 mN/A while the maximum power loss is 1 mW at available stroke of 10 mm. Finally, a prototype of the linear electromagnetic actuator is manufactured and experiments are performed to validate the electromagnetic actuator. For this purpose, a precision scale with an accuracy of 0.01 gr and a power supply with a resolution of 1 mA is utilized. The results showed that the maximum difference between the calculated and measured force was 2.5%. Therefore, there is a good correlation between the experimental data and the corresponding analytical values.

Low-cost and highly effective noise reduction has recently become one of the substantial challenges for industrial manufacturers. This paper presents the design and construction of a cost-effective system for attenuating single-frequency annoying noise generated from industrial products and machines. To achieve this goal, narrowband active noise control using Filtered-x Least Mean Square (FxLMS) method has been used with the help of a two-factor digital adaptive filter, called the adaptive notch filter. Therefore, a duct structure has been designed and experimental tests have been performed on it. To reduce implementation costs, the Arduino Uno board, which has an AVR microcontroller (ATmega328P), has been used as the controller. About 15dB noise attenuation at 400Hz and 750Hz frequencies and about 30dB noise attenuation at 650Hz and 950Hz frequencies have been achieved. Then, active noise control for two separate and simultaneous frequencies has been performed, which had somewhat effective results, and in one of these frequencies, noise attenuation of about 18dB has been observed.