Kerbala Journal for Engineering Sciences

This paper proposes how to balance a non-linear triple inverted pendulum, which is represented
by a gymnastic robot in the vertical plane. This robot simulates a human acrobat. This type of robot
is installed on ball bearings and connected to a high bar that rotates freely. It is made of three
joints; the first joint is idle with no actuator (not powered). The second joint is active. As for the
third joint, it is active. The first joint presents a major challenge in how to control the balance of
the three-link gymnastic robot. To achieve that goal, Discrete Linear Quadratic Regular (DLQR)
and Linear-quadratic-Gaussian (LQG) control theory is used and implemented on the time discrete
controller by using the mathematical model of the robot. The simulation results showed the
difference between LQR and LQG control systems to balance and control the gymnastics robot
system. Finally, the LQR is better where superior results for the three links (first, second,
third) were obtained in terms of overshot (23.37%, 176.8%, 73.125%), rise time (0.05013s,
0.07519s, 0.02506s), and setting time (3.208s, 3.233s, 4.16s) compared to the than the LQG.

This study investigates the impact of specimen size on the flexural strength of concrete reinforced
with optimized polyethylene terephthalate (PET) fibers. A series of experimental tests were conducted
using various PET fiber ratios (0.5 to 1.5% by the total volume of the mix) and lengths (15, 30, and 45
mm) to highlight the optimized PET fiber in concrete. Flexural strength tests were carried out on
prismatic specimens of varying sizes (100x100x400 mm, 100x150x500 mm, and 100x200x700 mm) and
different concrete grades (20, 35, and 45 MPa). The findings indicate a significant improvement in the
splitting tensile strength of the concrete, reaching up to 18.43% with the addition of PET fibers. These
fibers effectively act as bridges across cracks, reducing their width and halting their propagation. The
optimal PET fiber length was identified as 30 mm, added to the concrete at a rate of 1% by the total
volume of the mix. Moreover, the results revealed a noticeable effect of the specimen size on the flexural
strength of PET fiber-reinforced concrete, with an inverse relationship between specimen size and
measured flexural strength. As the size of the specimen increased, the flexural strength decreased. Based
on the experimental data, an empirical relationship was developed to quantify the flexural strength of
optimized PET fiber-reinforced concrete considering the size effect. These relationships provide
practical tools for engineers to accurately account for the size effect in the design and analysis of fiber
reinforced concrete structures.

Computational Fluid Dynamic modeling of a heat pipe can be utilized for investigating the
complex physical phenomena of phase change process during the evaporation and condensation in
thermosyphon heat pipes. In this study, FLUENT (ANSYS 22) was used to perform a numerical
simulation of two-phase flow inside a thermosyphon heat pipe to investigate the effect of both the filling
ratio and the inclination angle of the heat pipe on the thermal performance in terms of thermal resistance
and temperature distribution. The results were compared with a published research, displaying a good
agreement with a highest deviation in the temperature distribution of about 6.3%. According to the
results, at a filling ratio of 65%, the average temperature of the evaporator walls was at its lowest, while
at a filling ratio of 25%, it was at its highest. At all values of heat input, the lowest total thermal resistance
was attained at a tilt angle of 90° and a filling ratio of 65%. Thermal resistance decreases as the heat
input increases, and the effects of the filling ratio and tilt angle also become more significant as the heat
input rises. Additionally, at a heat input of 400 W, a filling ratio of 65%, and a tilt angle of 90°, the
bubbles begin to form at time 1 second, and the vapor rises to the condenser section starting at time 20
second, carrying with it the thermal energy. While condensation begins at time 27 second.

Using nanofluids as a working fluid in heat pipes can increase the ability of heat pipes to transfer
thermal energy and improve thermal performance. This paper reviews a group of previous experimental
and numerical research that dealt with the impact of nanofluids on the thermal characteristics of heat pipes
to understand more about these fluids and their thermal conductivity and to know the difference between
using them and using traditional fluids, which are characterized by limited thermal conductivity. Previous
researchers have studied the effect of thermal energy input, the type of nanofluid, the size and
concentration of the nanoparticles, and the materials from which these nanoparticles are made. In general,
nanofluids are characterized by high thermal conductivity when compared to conventional fluids.
Therefore, their use in heat pipes leads to a very significant improvement in thermal properties. All of this
depends on the operating conditions, the type of nanofluid, the size, and the concentration of the
nanoparticles. It was pointed out that the heat pipes attain the highest thermal performance at a certain
concentration of nanofluids. In addition, a conflict between the enhancement in the effective thermal
conductivity and an increase in viscosity due to the dispersion of nanoparticles should be carefully
observed.

In this work, functionally graded Materials (FGMs) were created by centrifugal force mixing
polyester resin with silicon dioxide (SiO2) to create five layers, each having a thickness of 1.2 mm.
Tensile composite specimens with varying volume fractions (0, 10, 15, 20, 25, and 30)% are taken before
the fabrication of the FGMs specimens, and their modulus of elasticity is subsequently computed. The
fourth specimen was made using the exponential equation to describe the property distribution
throughout the thickness of the three-points specimen, whereas the other three specimens were made
using the power law equation when the power law index (β) was (0.5, 1, and 5). Flexural modulus of the
functionally graded composite (FGMs) loaded from the neat polyester, raised by 37.1% for (β=0.5).
When compared to pristine polyester, the flexural modulus improved by 33.3% for (β=1). The increase
in (β=5)'s flexural modulus was (27.78%). The flexural modulus improved by 24.5% with exponential
FGMs.Good correlations are observed, with the greatest absolute discrepancy between the numerical
and experimental outcomes being 14.6% when the power law index (β) is at (0.5).The results of the
experimental flexural test were verified using Design Modeler (ANSYS Workbench R2 2021).