Recently, a fully coupled formulation of thermo-fluid shape design problems has been developed, in which the unknown nodal coordinates appear explicitly in the formulation of the problem. This “direct design” approach is, in principle, generally applicable and has been successfully applied in the context of potential and Euler flow models. This paper focuses on the direct design of ducts using the ideal flow model and may be considered as an addendum to the paper entitled "Direct Design of Ducts [2]". However, a cell-vertex finite volume method is used and a different boundary condition implementation technique is applied as compared to the method presented in the above paper. The other new feature is that a non-linear algebraic method is used for grid generation. It is also shown that the method is capable of designing complex flow passages such as branched ducts.

Keywords:
Fully-coupled inverse method, Direct Design, Internal flow, Stagnation point.

In many applications, convection heat transfer is coupled with conduction and radiation heat transfer which generates temperature gradients along the walls and may greatly affect natural convection heat transfer. The main objective of this study is to calculate the heat-transfer characteristics for natural convection from a non-isothermal vertical flat plate into a supercritical fluid. The influence of the non-uniformity of wall temperature on the heat transfer by natural convection along a vertical plate having a linearly distributed temperature (characterized by the slope ) is also investigated. First, an equation for the thermal expansion coefficient ( ) was derived as a function of the temperature, the pressure, the van der Waals constants and the compressibility factor. The trend of the curves obtained with this equation and with values from tables of thermodynamic properties was similar and they diverge at critical point. These features confirmed the validity of equation obtained in this work. Then, the governing systems of partial differential equations are solved numerically using the finite difference method. The local Nusselt number was then calculated and plotted as a function of the local Rayleigh number. It was observed that positive slope of temperature distribution increases the heat transfer rate and negative slope decreases it.

Keywords:
Finite difference method, Natural convection, Nusselt number, Rayleigh number, Supercritical Fluids

In this work, rising of a single bubble in a quiescent liquid under microgravity condition was simulated. In addition to general studies of microgravity effects, the initiation of hydrodynamic convection solely due to the variations of interface curvature (surface tension force) and thus generation of shearing forces at the interfaces was also studied. Then, the variation of surface tension due to temperature gradient (Marangoni convection), that can initiate the onset of convection even in the absence of buoyancy was studied. The related unsteady incompressible full Navier-Stokes equations were solved using a finite difference method with a structured staggered grid. The interface was tracked explicitly by connected marker points via hybrid front capturing and tracking method. A one field approximation was used, where one set of governing equations is only solved in the entire domain and different phases are treated as one fluid with variable physical properties. While, the interfacial effects are accounted for by adding appropriate source terms to the governing equations. Also, Multi-grid technique in the context of the projection method improved convergences and computational stiffness. The results show that the bubble moves in a straight path under microgravity condition, compared to the zigzag motion of bubbles in the presence of gravity. Also, in the absence of gravity, variation of surface tension force due to interface curvature or temperature gradient can still cause the upward motion of the bubble. This phenomenon was explicitly shown in the results of this paper.

Keywords:
Marangoni Convection, Microgravity Condition, Hybrid Front Capturing and Tracking Method, Rising Bubble, Multi-grid Method.

In this work, a supersonic turbulent flow over a long axisymmetric body was investigated both experimentally and computationally. The experimental study consisted of a series of wind tunnel tests for the flow over an ogive-cylinder body at a Mach number of 1.6 and at a Reynolds number of 8×106, at angles of attack between -2 and 6 degrees. It included the surface static pressure and the boundary layer profile measurements. Further, the flow around the model was visualized using Schlieren technique. All tests were conducted in the trisonic wind tunnel of Qadr Research Center (QRC). Also, the same flow at zero angle of attack was computationally simulated using a multi-block grid (with patched method around the block interfaces) to solve the thin layer Navier-Stokes (TLNS) equations. The numerical scheme used was implicit Beam and Warming central differencing, while Baldwin-Lomax turbulence model was used to close the Reynolds averaged Navier-Stokes (RANS) equations. The static surface pressure results show that, the circumferential pressure at different nose sections vary significantly with angle of attack (in contrast to the circumferential pressure signatures along the cylindrical part of the body). While, the total pressure measurements in the boundary layer vary significantly both radially and longitudinally. Two belts with various leading edge angles were installed at different locations along the cylindrical portion of the model. The computational results obtained were compared with some of the experimental ones (found by these authors) showing considerably close agreements. Key Words: Supersonic Flow, Pressure Distribution, Boundary Layer, Long Axisymmetric Body, Multi-block, TLNS Equations.

In this paper, a complete four axles rail vehicle model with 70 degrees of freedom (DOFs) is addressed, which includes

a carbody, two bogies, and four axles. In order to include track irregularities’ effects on the vehicle behavior, a simplified track model for straight line is proposed. As the performance of the suspension components, especially for air springs, has significant effects on rail–vehicle dynamics and ride comfort of passengers, a complete nonlinear thermo–dynamical air spring model, which is a combination of two different models, is introduced and then implemented in the complete rail–vehicle dynamics. By implementing Presthus formulation [1], the thermo–dynamical parameters of air spring are estimated and then tuned based on the experimental data. Effects of air reservoir volume and connecting pipes’ length and diameter on the system performances are investigated. For improving passengers’ comfort during their trip, air suspension parameters of the modeled rail vehicle are tuned to minimize Sperling ride comfort index. Results showed that by modification of air suspension parameters, passengers comfort is improved and ride comfort index is reduced about 10%. Genetic Algorithm (GA) optimization method is also used to optimize air suspension parameters. Result showed that improved air suspension configuration is more practical comparing to the optimized one.

Keywords:
Rail vehicle dynamics
Air spring model
Air suspension parameters
GA
Optimization

Cavitating flow is investigated around marine propellers experimentally and numerically. Two different types of conventional model propellers are used for the study. The first one is a four bladed model propeller, so called model A, and the second one is a three bladed propeller, model B. Model A is tested in different cavitation regimes in K23 cavitation tunnel. The results are presented in characteristic curves and related pictures. Finally, the results are discussed. Model B is investigated based on existing experimental results. In addition, model B is used for validation of numerical solution prior to testing of the model A. Cavitation phenomenon is predicted numerically on a two dimensional hydrofoil, NACA0015, as well as propellers models A and B. The cavitation prediction on hydrofoil is carried out in both the steady and unsteady states. The results show good agreement in comparison with available experimental data. Propeller models are simulated according to the cavitation tunnel conditions and comparisons are made with the experimental results quantitatively and qualitatively. The results show good agreement with experimental data in both cavitating and noncavitating conditions. Furthermore, Propeller cavitation breakdown is well reproduced in proceeding. The overall results suggest that the present approach is a practicable tool for predicting probable cavitation on propeller during design processes. Key Words: cavitation, marine propeller, cavitation tunnel, experimental, CFD

Nonlinearity characteristics such as backlash in various mechanisms limit the performance of feedback systems with causing delays, undesired oscillations and inaccuracy. Backlash influence analysis and modeling is necessary to design precision controller for this nonlinearity. Backlash between meshing gears in an electromechanical system is modeled by use of differential equations and nonlinear spring-damper. According to this model, the paper shows, oscillations and delays can’t be compensated by a state feedback controller. Therefore an adaptive algorithm is designed based on different regions of system angular position error. Since this controller needs an estimation of backlash value, it is estimated by a learning unit in the adaptive controller. Simulations show that the presented adaptive controller can eliminate backlash oscillations properly in accordance with previous works. Keyword: Backlash nonlinearity, Backlash estimation and compensation, Adaptive controller

ABSTRACT Subsonic wind tunnel experiments were conducted to study the turbulence level in the test section. Measurements were performed by introducing trip strip and/or damping screens on the flow field. The results indicate that the introduction of the trip strips not only reduced the turbulence intensity compared to cases without it but also it flattened the variations. Further, the experiments which investigated the impact of the damping screens indicated similar reduction of the turbulence intensity

however, the pattern remained the same. Furthermore, the results for cases in that both the trip strip as well as the damping screens were placed on the contraction and in the settling chamber, respectively, showed that the turbulence intensity was even more reduced than the previous cases. It is believed that the combination of several damping screens with the trip strip could be a sound method for turbulence reduction in subsonic wind tunnels.

Keywords:
Wind tunnel- Turbulence Reduction-Trip Strip-S

A series of supersonic visualization tests were performed on an airplane model in both static and dynamic pitching cases. After image processing, the wave angles originating from different parts of the model have been carefully measured and averaged over several oscillation cycles. These findings were then compared with the corresponding normal force at similar conditions. The results reveal a hysteresis loop in the variations of the model shock angles with instantaneous angle of attack during upstroke and down stroke motions. Having compared with the normal force hysteresis loop, it has been found that there is an interesting relationship between the shape of the hysteresis loops of the shock angle and the corresponding loop observed in the normal force data. Further, the oscillation frequency has been shown to have similar effects on both the shock angle and the aerodynamic force variations with the instantaneous angle of attack. Key Words: Hysteresis, Pitching Motion, Vortex Bursting, Reduced Frequency, Schlieren, Upstroke