Journal of Aerospace Mechanics
Year:2008 | Volume:3 | Issue:4|Papers Count:7

In this work, turbulent supersonic flows over axisymmetric bodies, including the base, are investigated, using multi-block grid to solve the Navier-Stokes equations. Patched method has been used near the interfaces. Our numerical scheme was implicit Beam-Warming centeral differencing, while Baldwin-Lomax turbulence modeling was used to close the Reynolds averaged Navier-Stokes equations. In order to take advantage of structured grids, multi-block grid has been used widely in the past for complex geometries. On the other hand, different parts of such flow may require different forms of the governing equations. For supersonic flows over missile geometries, the thin layer Naveir-Stokes equations (TLNS) are usually used. But, the easiest and the fastest approach would be the use of parabolized Naveir-Stokes equations (PNS) everywhere. Note, for regions such as the missile’s blunt nose, near fins, and the base flow, we do not have to use the PNS equations. Therefore, this leads to the use of both TLNS and PNS equations with multi-block solution approaches. The resultes of this new version of our code (MBPTLNS), which is an extension of our original code (MBTLNS) [1], were compared with both computational and experimental benchmark data and showed close agreements.

In this work, the characteristics of the near wake of a rectangular cylinder was investigated in a two-dimensional analysis, using hot-wire anemometer. The time-averaged and the fluctuating components of velocity were measured for the cylinder having various width-to-height ratios. The measurements were carried out in a low-speed wind tunnel. The results show that the wake turbulence intensity decreases, as the width-to-height ratio of the cylinder increases. Also, the characteristics of the flow field in different stream wise locations were evaluated. The spectral analysis of the signals was also carried out for two different Reynolds numbers. This showed that the strouhal number remains nearly constant in high Reynolds numbers. The results of our experiments were in close agreements with the most widely accepted results in the literature.

In the present study, experimental investigations on flow boiling heat transfer enhancement (using twisted tape inserts inside an evaporator) and on its simultaneous effect on pressure drop were carried out. The main goal of this research was to obtain the maximum heat transfer enhancement with the least increase in pressure drop. The experimental set-up used was a well instrumented vapor compression refrigeration cycle. This set-up consisted of a test-evaporator as well. In addition, there were a pre-evaporator and an after-evaporator to achieve the required vapor quality. Refrigerant R-134a was electrically heated by the coil around it. The experimental investigation was carried out for a plain tube and for four twisted tapes inserted tubes with different twist ratios of 6, 9, 12, and 15. The results showed that the twisted tapes, in the best condition (twist ratio of 6), enhanced the heat transfer coefficient up to 68%, and in the worst condition (again twist ratio of 6), increased the pressure drop up to 180% above the plain tube values on a nominal area basis. Also, it was observed that among the tapes with different twist ratios, the tape with twist ratio of 12 in low mass velocities and the tape with twist ratio of 9 in high mass velocities, had the best performances.

In the present study the generation and the propagation of sound wave in low Mach number flows are numerically studied. Since one of the major problems in aeroengine design is the reduction of noise pollution, the exact prediction of sonic behavior is of special importance. The target of this study is to simulate a leakage, as a free jet. The basis of this study is the hybrid method for which benchmark Reynolds stress turbulence model is applied in solving the flow field and the Lighthill analogy is used for acoustic field analysis. Meanwhile, the continuous filter white noise is applied for solving the instantaneous velocity. Accordingly, the sound pressure level profiles for axial and transverse directions are plotted. Comparison of our results with existing benchmark data is satisfactory.

Pre-swirl is often used in the internal cooling-air systems of gas turbines to reduce the temperature of the cooling air relative to the rotating turbine blades. In a "direct-transfer" system, the air passes axially across the wheel-space from stationary pre-swirl nozzles to receiver holes in the rotating turbine disc. This paper investigates the effects of inlet flow conditions and rotational speed on the flow and the heat transfer in such system, using a simplified computational model for three-dimensional steady, incompressible turbulent flow based on an in-house solver developed by the author. The computed results are compared with measurements of tangential velocity and Nusselt number and show that there are significant mixing losses near the inlet nozzles, resulting in a reduced “effective” pre-swirl ratio at inlet. The computed tangential velocity distributions suggest free-vortex-type behavior for the flow between the pre-swirl nozzle radius and that of the receiver holes. There is mainly good agreements between the computed and the measured velocities, although the measured values are generally lower than the computed ones and follow free-vortex behavior less closely. There is less agreements between the computed and the measured values of Nusselt number, although the correct trends are obtained for the effects of the main flow parameters.

Environmental cross winds usually distord the uniform distributions of both air flow and air resistance at inlet to cooling towers and subsequently cause hot areas due to insufficient local cooling of water by air. Curative devices, such as windbreaks, are developed to enhance the performance of the power station cooling towers. At present experimental investigation, the effects of curative devices on performance of a 1/1000 scaled isothermal model of a 660MW wet cooling tower were quantified, using intake dynamic and total pressure losses. Dimensional similitude’s were used in the isothermal modeling of the wet cooling tower. A dimensionless pressure loss coefficient in the tower was defined, as the performance indicator, which is the ratio of total pressure drop in the heat exchanger region (i.e. combined packing and rain zone) to the dynamic pressure. Over twenty three curative devices were designed, manufactured, and their effects on the pressure loss coefficient were quantified. Pressure drop coefficient with no curative device was selected as the base condition and was compared with the presence of curative devices. Up to 33% improvement was noticed in the pressure loss coefficient. The top three most efficient curative devices were selected and presented here.

Ricochet of a tungsten long-rod projectile from oblique steel plates was investigated numerically, using two explicit finite element methods. These two methods are lagrange and SPH (smooth particle hydrodynamic). Critical ricochet angles were calculated for various impact velocities and strengths of the target plates in Lagrange and SPH methods. It was predicted that in both methods, the critical ricochet angle increases with decreasing the impact velocities and that higher ricochet angles were expected, if higher strength target materials were employed. The experimental results were compared with those predicted by the simulations and with the existing two-dimensional analytical model. Through investigation of the angles in which projectile only ricochets, both SPH and lagrange methods represent approximately, the same results. However, in the cases that projectile begins to crack in head region out of high impact angles, the SPH method yields better results. One other advantage of the SPH method is that no erosion occurs when using the SPH method. This means better satisfaction of the conservation of mass principle. Therefore, the correlation between the numerical and the available experimental data demonstrates that the SPH approach is a very accurate and effective analysis technique for long rod ricochet phenomena in ricochet of Tungsten rod with RHA target.