Journal of Aerospace Mechanics
Year:2008 | Volume:4 | Issue:2 (12)|Papers Count:7

In this work, Heat transfer and pressure drop for a three-dimensional flow in the enterance region of a pipe, which is equipped with one or more inserts, is numerically investigated. The inserts were inter-connected by a thin rod and were kept in the center of the pipe. The Reynolds number for the flow ranged between 450 to 1800 and the Prandtl number was 0.72. The number of inserts, the distance between them, and their axial length were the variables whose effects on the heat transfer and on the pressure drop were investigated. Finite volume method was employed to transfer the governing equations into algebraic ones. To evaluate simultaneous effects of the inserts on heat transfer and on pressure drop, an overall enhancement ratio (OER) was introduced and applied. The results show that in many cases, thinner inserts lead to a better OER’s. Also, in lamiar flows, the shape of the inserts affects ORE. On the other hand, inserts with a square shape lead to logitudinal vortices, which increase ORE.

In this work, potential flow with supercavitation and partial cavitation has been studied, using boundary element method. For this purpose, vortex distribution is used in the boundary of the flow. The impermeable condition is used on the wetted surface of the hydrofoil-cavity (zero vertical velocity), while constant tangential velocity condition is used on the wetted surface of the cavity; the Kutta condition is also used at the trailing edge of the hydrofoil for partial cavitation and the continuity of tangential velocity condition is used for supercavitation. In this method, the length of the cavity is assumed to be known and an initial guess for the cavity shape is considered. The results of the numerical solution of partial cavitation flow around NACA0009 hydrofoil and supercavitation from over a flat surface and a wedge are compared with the experimental results. The results in partial cavitation show the accuracy of this numerical method for cavity length of less than 75% of the cord. Thus, for better accordance with the experimental results, a new method is used in which this method, the effect of a counter clockwise independent vortex, which is formed downstream of the flow in partial cavitation, has been studied. Furthermore, the effect of changing the cavity starting point has been studied, which shows that it does not have great influence on the ratio of thickness to length of the cavity.

In this work, numerical modelling of porous radiant burners (PRB) with sidewall heat loss has been studied. A cylindrical (axisymmetric) burner is used in methane - air premixed combustion. The stoichiometric mixture of reactant has been selected to maximize the effects of sidewall heat transfer. Also, a lean mixture of reactant has been added. It has been observed that the one-dimensional modelling of PRB is not valid due to the sidewall heat transfer. The present study indicates that the lateral heat loss, which is found to be 6% of the supplied fuel energy, results in highly two-dimensional behavior. The temperature of the two - dimensional flame is generally lower than the one-dimensional one. This temperature reduction is observed to be more significant at the sidewall than at the centre line, where the maximum flame temperature is experienced. Due to a lower temperature, less NOx is formed with respect to the one - dimensional modelling. The new results are much closer to the experimental data than that in previous works.

In this work, unsteady laminar and turbulent supersonic flows of a discharged projectile from a tube have been numerically simulated. The equations of axisymmetric viscous compressible flow has been computed by Van Leer flux vector splitting method, using time and space second order accuracy and moving boundary considerations.When a projectile is discharged from a tube, a complicated flow in its front and back is created which this flow includes expansion and shock waves. Complicated intractions between these waves make the analysis of this flow very difficult. The numerical simulation predicts expansion wave, bow shock wave in front of projectile, barrel shock wave, Mach disk, vortex ring, and shear layer. Results show that, there are not many differences between the laminar and turbulent flow arrangements, but the aerodynamic forces are different. The variation of acceleration and aerodynamic forces, when projectile is discharging from a tube, due to more intractions between the expansion and shock waves, is large and causes vibration of the projectile and change in its path. When projectile starts to discharge from a tube, the acceleration will be maximum, but when discharges entirely from tube, the resistant force are increased and the acceleration is decreased.

In this work, numerical simulation of flow with spray and combustion has been performed in a direct injection diesel engine. The grid was structured and all phenomena that affect the operation of a diesel engine, such as injection, evaporation, droplet collision, droplet impinging on walls, heat transfer to walls, combustion, ete were taken into account. The kinetic of combustion and the effects of turbulence on fuel-air mixture and on rate of reaction were considered. The KIVA-3V code (with some changes) was used for the computations. The prediction of performance factors, pollution and energy release rate, as well as the investigation of input load effects on performance and pollution was the main purpose of this work. The geometry and operation data of OM355 engine were introduced as input data and the results of simulation and experiment were compared, which were relatively close.

This paper presents cutting force prediction algorithm for low immersion end-milling, using an efficient dynamic force modeler. The geometric simulation of milling process was performed, using the B-Rep solid modeling techniques. For predicting stability lobes, two methods were used for prediction of stability lobes: the frequency domain technique and the time finite element analysis (TFEA). The frequency domain technique is faster and except for low immersion milling, is accurate. The TFEA method forms an approximate solution by dividing the cutting time into a finite number of elements. This approximated solution is then matched with the exact one for free vibration to obtain a discrete linear map. The comprehensive time domain simulation is used, in order to verify stability lobes diagram obtained by frequency domain technique and TFEA method.

In the work, modification of wall shear stress due to air bubble injection on a rotary apparatus was experimentally investigated. In this device, water flow field causes a rotary motion of hub by shear forces. Injected air bubbles cross the near wall region of the hub surface and affect its rotary behavior (in constant water flow rates). Void fraction increase leads to decrease of rotational velocity provided by the flow shear stresses. In high-void fractions, the hub motion stops completely. The amount of skin friction reduction was estimated by measurement of hub rotational velocity. Rotational velocity decrease, due to bubble injection leads to the reduction of wall shear stress in all range of water flow rates. More than 90 percent of rotational velocity reduction was achieved in maximum void fraction.This result expresses the significant reduction of shear stress on the rotary hub. The considerable amount of skin friction reduction obtained in this special test case indicates the effectiveness of gas bubbles injection on skin friction reduction in some rotary parts for special applications.