Statement of Problem: A recent technique (MBT) has been introduced for easy and efficient orthodontic treatment by means of straight wire fixed appliances. Although this technique has many advantages, in space closure stage causes up righting problem in upper incisors, which should be adjusted by applying additional torque in anterior region. The purpose of this clinical trial was to determine whether there is a significant difference in final incisor inclination (torque) of two groups one treated with MBT technique and the other by changing the point of application of the closing force (in anterior region). Twelve patients (8 girls, 4 boys) with Cl I malocclusion and crowding, whom were candidate for first premolars extraction, were selected. For space closure, 6 anterior teeth (canine to canine) in upper and lower jaws ligated and the applied force was from the hooks on canines to hooks on the first molars. After space closure the results compared with the results of a previous study which treatment plan was according to original MBT technique (anterior hooks soldered to arch wire). Forces applied randomly among quadrants by NiTi coil spring (American Orthodontics Co.) or active tie backs with elastic modules (Dentaurum Co.) The results showed a statistically significant difference between final torques of two techniques (P=0.0001). Also, the rate of space closure with NiTi coils was significantly greater than active tie back (P=0.0001). Anchorage loss in two groups compared and the results indicated significantly lower anchorage loss in NiTi coil spring group (P=0.035).

The wake flow and the onset of Karman-vortex shedding corresponding to periodic forces of a heated circular cylinder are investigated numerically with a SIMPLEC finite volume code. This study is performed for Reynolds numbers ranging from 20 to 200 and a Prandtl number of 0.7.The first bifurcation of flow over bluff bodies, which is strictly a Hopf type, is modelled using the Stuart-Landau equation. In this study, it is shown that the flow behind a circular cylinder is steady up to Re=49 and beyond this critical value, the flow develops to a periodic state, corresponding to the vortex shedding and force oscillation on the body. The wake structure is studied close to the oscillation threshold and also at the higher Reynolds numbers using streamlines, pressure, temperature, Nusselt numbers and wall vorticity profiles and iso-lines. Some useful physical quantities, such as time mean separation angle, Nusselt number, the dominating wake frequency (Strouhal and Roshko numbers), mean drag and the RMS value of lift are computed and compared with numerical and experimental results. Based on the results of this study, the empirical relations for the Reynolds dependence of the mean Nusselt number, mean separation angle, Strouhal and Roshko numbers are obtained.

When check valve works in long distance or high lift liquid pipeline system, it is often subjected to water hammer. In this study, the UDF program was used to simulate the closing process of an axial flow check valve at the moment of pump shutdown, and the porous media model was applied to simulate the complete closing of the valve disc. It was found that there was local vacuum at the end of the valve disc at the moment when the valve was completely closed. The water hammer and characteristics of the force acting on the valve disc in the whole closing process were also obtained. In order to reduce the pressure surges on the valve disc and seat, a built-in critical damping was designed and added to the to the valve disc drive system. Since the spring force is directly proportional to the movement displacement of the valve disc, the elastic force and the speed of the valve disc reach the peak value when the valve is fully closed, while the damping force is directly proportional to the speed of the valve disc, therefore, the damping force increases gradually with the speed of the valve disc, which only produces the maximum damping force at the moment of fully closing, so as to reduce the slam shut, but has little effect on the closing time, thus adding damping is more effective than reducing the elastic force of spring. The current study provides a possible approach to protect the valve disc and seat of check valves in liquid supply and drainage systems.

Alignment of the teeth as the first stage of comprehensive orthodontic treatment has widely been addressed. Several methods such as using super elastic arch wires, multi-strand wires and incorporation of various loops into the round stainless steel wires have been suggested in this stage. The aim of this study was to investigate and compare the horizontal forces produced by six various loop designs in different amounts of activation using the finite element method. The Finite Element Model (FEM) was used to design and compare the horizontal forces of round 0.014 inches stainless steel vertical open loop, vertical closed loop, vertical helical open loop, vertical helical closed loop, plain vertical loop and the squash loop. The forces were measured at 0.1, 0.4, 0.8 and 1mm activations. For all activations, vertical open loop had the highest horizontal force; while vertical helical closed loop showed the lowest force. The design of vertical loops influences the horizontal force in all activations, immensely.

The most favorable feature of nickel-titanium (Ni-Ti) alloys in orthodontics is producing constant forces in a wide deformation range. Ni-Ti closed coils produce compressive strength and can be used for several purposes such as space closure. The aim of this study was to evaluate the effects of the aging process (temperature changes and prolonged strain) on Ni-Ti closed coils and to assess the effects of preactivation on forces generated by these coils. A total of 60 Ni-Ti closed coils (G&H Wire Co., Indiana, USA) were divided into four groups (n = 15). Two groups were kept in room temperature and two were incubated in 37°C in normal saline for 45 days. All the samples were extended to 30% of their original length. One group of the incubated coils and one group of room temperature coils were preactivated the same amount of 30%. The incubated groups also received 1000 thermocycles on days 22 and 45. The unloading forces were measured by a universal testing machine on days 0, 22 and 45. The data were analyzed using SPSS. Signifi cance was set at 0.05. The unloading forces of the Ni-Ti closed coils was not affected by prolonged heat and moisture either in preactivated groups (P = 0.8) or the nonpreactivated groups (P = 0.6). Furthermore, preactivation had no signifi cant effect on the unloading forces of Ni-Ti closed coils (P = 0.7). Within the limitations of this study, both aging process and preactivation do not affect on mechanical properties of nickel-titanium closed coil springs. Preactivation had no effect on the forces generated by Ni-Ti closed coils.

Statement of Problem: Clinicians use the NiTi coil springs frequently for its appropriate mechanical properties. The aim of this study was to determine the effect of recycling and autoclave sterilization on the unloading forces of NiTi closed coil springs. Fourteen NiTi closed coil spring with the length of 9mm were selected. Each coil was stretched to a peak extension of 12 mm. A universal testing machine was used to acquire load/deflection curve of the coil springs at 25±2°C. The influence of thermocycling (1000 cycles,5-55°C), autoclaving (134°C, 32PSI, 3min) and mechanical strain (9mm extension) which simulated the oral condition, were considered. Data were statistically analyzed by adopting Repeated Measures MANOVA Paired t-Test. Autoclaving in the 1, 4, 6 steps increased the force levels of coil springs about 2-5gf (p< 0.01). Thermocycling reduced their force levels about 4-6gf. Prolonged strain at 3, 5 steps decreased the magnitude of forces levels about 3-4gf. Concerning all the limitations; according to the results of this study; it is possible to recycle Ni-Ti closed-coil springs without significant reduction in their force levels.

The dynamic features of flow field in three models of the unidirectional and bidirectional grooves of the dry gas seal of a centrifugal compressor are simulated using computational fluid dynamics. The computational domain is the gas film between two rotating and stationary rings and inside the grooves. The seal performance is investigated under different thicknesses. The continuum and Navier-Stokes equations are solved by assuming the ideal gas fluid by using structural and hexagonal cells. The flow is considered laminar. the results include pressure distribution, opening force, leakage, and stiffness of the gas film. First, the unidirectional seal is modeled and the results are compared with the experimental results. The results show that the effects of turbulence can be ignored. In the following, two different geometrical models of the bidirectional grooves are used and the results are compared. The results show that the opening force and stiffness of the gas film in two bidirectional groove models are higher than the unidirectional groove. Also, the leakage values in these two models are reduced compared to the unidirectional groove.

In this review article, the potential functions of the reactive force field ReaxFF used in reactive molecular dynamics (RMD) simulations are introduced and described. This force field with its special potential functions is used to simulate chemical reaction systems with accuracies close to those of quantum chemical calculations within reasonable computation times, even with about one million atoms on high-performance computing facilities. Variations of the partial charges of atoms during chemical reactions can also be calculated and probed within RMD simulations with accuracies very close to those calculated by density functional methods. In addition to the general potential functions incorporated in non-reactive force fields, in ReaxFF force field, there are a number of special (usually complex) potential functions describing details of the bonding structures and charge states of atoms in different coordination states and chemical and solvent environments. All of these potential functions are described in this paper, and examples are calculated and plotted to demonstrate clearly their behavior and their corresponding chemical and physical backgrounds. Finally, important evolutions of the ReaxFF force field and a few of its applications have been presented.

Introduction
The aim of this study was to compare the characteristics of deflection load of three kinds of nickel titanium closed coil springs.
Methods and Materials
This research was an experimental study. Research sample contains three kinds of NiTi closed coiled springs (heavy, medium, light) from GAC, 3M, and RMO factories. 10 springs with 9 mm length for each group (totally 90 springs) were subjected to tensile test. They were pulled to 12 mm extension by Dartec machine and then were released. Our variables were maximum force at 12 mm extension, force at the start of the plateau, deflection at the start of plateau, mean force of plateau, plateau slope, force at the end of plateau, deflection at the end of plateau. For tensile test, we used DARTEC universal machine. The data were analyzed by ANOVA and Duncan tests.
Results
In comparision among identical coils from different factories, the deflection at the end of plateau hadn’t significant difference (P=0.107) but the other parameters had significant differences (P

In most literature on lubrication theory, the assumption of is considered to solve the Navier-Stokes equations in which inertial forces are neglected in comparison with the viscous ones. In problems of bearing with high speed motion, neglecting the inertial forces can not be utilized. In this study, the Navier-Stokes equations for a slider bearing are solved, considering both the inertial and the viscous forces. Then, the effective forces were computed and analyzed. Finally, the results were compared with the available approximate ones, which showed close agreements.