Time-periodic incompressible flow simulation using non-linear frequency domain approach

Istražuje se mogućnost predviđanja polja protoka vremenski periodičnog nestlačljivog toka pristupom područja nelinearne smanjene frekvencije - nonlinear reduced frequency domain (NLFD). Učinkovita NLFD metoda koristi predstavljanje Fourierovih serija u vremenu smatrajući da se pretpostavka periodičnosti rješenja i rezultirajuće nelinearne jednadžbe rješavaju pseudo-spektralnim pristupom. Na taj se način željeno periodičko rješenje može dobiti direktno bez potrebe rješenja inicijalnog tranzijentnog dijela. Tako se može postići značajno smanjenje troška bez gubitka generalizacije u usporedbi s uobičajenim vremenski preciznim metodama. U postojećem algoritmu (INLFD), NLFD pristup je primijenjen za rješavanje polja protoka u nestlačljivoj formulaciji a validacija se obavlja primjenom nekih periodičnih slučajeva ispitivanja s analitičkim rješenjima. Rezultati pokazuju da se obuhvatom samo ograničenog broja temporalnih načina može osigurati točna procjena polja protoka. Konačno, troškovi izračuna INFLED-a i vremenski točne metode također su uspoređeni i pokazuju učinkovitost INFLED metode u predstavljanju fizike nelinearnog polja protoka uštedom vremena izračuna do 50 posto.The capability of nonlinear reduced frequency domain (NLFD) approach in predicting the flow field of time-periodic incompressible flow is investigated. The efficient NLFD method uses Fourier series representation in time considering the assumption of solution periodicity and the resulting nonlinear equations are solved using pseudo-spectral approach. In this manner, the desired periodic solution can directly be obtained without the need to solve the initial transient part. Thus, without loss of generality a considerable computational cost reduction can be achieved in comparison with the common time-accurate methods. In the present algorithm (INLFD) the NLFD approach is used to solve the flow field in an incompressible formulation and the validation is performed using some periodic test cases with analytical solutions. The results show that capturing only a limited number of temporal modes can provide an accurate estimation of the flow field. Finally, the computational costs of the INLFD and a time-accurate method are also compared which demonstrate the efficiency of the INLFD method in representing nonlinear flow field physics by savings in computational time up to 50 percent

Predicting the tensile strength, impact toughness, and hardness of friction stir-welded AA6061-T6 using response surface methodology

In this research, an attempt has been made to develop mathematical models for predicting mechanical properties including ultimate tensile strength, impact toughness, and hardness of the friction stir-welded AA6061-T6 joints at 95 % confidence level. Response surface methodology with central composite design having four parameters and five levels has been used. The four parameters considered were tool pin profile, rotational speed, welding speed, and tool tilt angle. Three confirmation tests were performed to validate the empirical relations. In addition, the influence of the process parameters on ultimate tensile strength, impact toughness, and hardness were investigated. The results indicated that tool pin profile is the most significant parameter in terms of mechanical properties; tool with simple cylindrical pin profile produced weld with high ultimate tensile strength, impact toughness, and hardness. In addition to tool pin profile, rotational speed was more significant compared to welding speed for ultimate tensile strength and impact toughness, whereas welding speed showed dominancy over rotational speed in case of hardness. Optimum conditions of process parameters have been found at which tensile strength of 92 %, impact toughness of 87 %, and hardness of 95 % was achieved in comparison to the base metal. This research will contribute to expand the scientific foundation of friction stir welding of aluminum alloys with emphasis on AA6061-T6. The results will aid the practitioners to develop a clear understanding of the influence of process parameters on mechanical properties and will allow the selection of best combinations of parameters to achieve desired mechanical properties

COMPARISON BETWEEN NATIVE ARTERIOVENOUS FISTULA AND GRAFT IN PATIENTS REFERRED FOR HEMODIALYSIS ACCESS PLACEMENT

Arteriovenous fistula is considered as a prerequisite in patients with chronic renal failure undergoing hemodialysis. The purpose of this study was to compare the effectiveness and side effects if any of native vein and synthetic grafts in these patients. From April 2002 till July 2003, a total of 37 patients were referred to us for the utilization of these grafts. In 16 out of these patients we utilized native vein at the elbow joint, and in 21 patients we used polytetrafluoroethylene (PTFE) graft. In the native group, the results were better and side effects minimal and economically was in the interest of the patients. We advocate native vein as far as possible and give it a top priority

Optimum Design of 1st Gear Ratio for 4WD Vehicles Based on Vehicle Dynamic Behaviour

This paper presents an approach that allows optimizing gear ratio and vehicle dimension to achieve optimum gear transmission. Therefore, augmented Lagrangian multiplier method, defined as classical method, is utilized to find the optimum gear ratios and the corresponding number of gear teeth applied to all epicyclical gears. The new method is able to calculate and also to optimize the gear ratio based on dynamics of 4WD vehicles. Therefore, 4WD vehicles dynamic equations are employed assuming that the rear wheels or the front wheels are at the point of slip. In addition, a genetic algorithm is modified to preserve feasibility of the encountered solutions. The basic dimension of a sample commercial vehicle (2009 hummer H3 4dr AWD SUV) and its gearbox are optimized, and then the effects of changing slip angle, wheel base, and engine torque on optimum vehicle dimension are analyzed