مجله مکانیک سیالات و آیرودینامیک
سال دوم شماره 2 (تابستان 1392)

Fan wing is a unique design which its main difference with common wings is that a cross flow fan is embedded along its span at the leading edge. The most important property of the fan wing is the ability to produce high lift force in low flight speeds and also thrust generation due to the fan rotation. Since the rear wall, which was not considered in the original model, is one of the most effective parts on the cross-flow fan performance, in this article the effect of this fan geometrical parameter has been investigated. The results show that by considering the rear wall, lift coefficient increases and decreases in small and large angles of attack, respectively. Besides, it causes the drag coefficient to increase and the magnitude of moment coefficient to decrease in all attack angles.

In this research، turbulence parameters such as turbulence intensity، Kolmogorov length scale، dissipation rate and velocity profiles at a cross section of flow over a flat plate are analyzed by using hot wire anemometer. Furthermore، the behaviors of parameters at unknown heights of plate are predicted by utility of advanced neural networks. One can find that there is well agreement between results of neural network at unidentified points and experimental data that guaranties the use of neural network to determine turbulent parameters at each point that there are not any experimental data for them. In addition to، frequency analysis and the effect of number of samples on the turbulence parameters are considered. The results show that the turbulence intensity، Kolmogorov length scale and Kurtosis are decreased with increase the height where dissipation rate and mean velocity are increased. Furthermore، the amplitude of FFT is increased near the plate.

In this paper، a fully coupled formulation of the surface shape design problem، called the direct design approach، has been proposed. In this method both the target surface pressure and the unknown nodal coordinates appear explicitly in the formulations. The proposed method is based on the compressible Navier-Stokes equations، but in the past work it has only been applied in the context of inviscid fluid flows. A cell-centered finite volume method and AUSM+ scheme are used to discretize the Navier-Stokes equations. The details of linearization and imposition of the target pressure are discussed there. The validation test case is presented which show the robustness of the method in handling complex geometries and complex physical phenomena (such as shock waves). The importance role of viscosity dictates aerospace designers to consider this phenomenon especially in thick boundary layer regions (hypersonic nozzles)، diffusers with separation possibility and many other cases. Stability of this method in viscous flow decreased extensively in comparison with the last version the method based on the Euler equations. The significant results of this study، no single answer that is where the separation occurs.

In this paper، due to the importance of incoming flow turbulence intensity into combustion chamber، tripping wire effect on the flow wake has been experimentally investigated within a linear compressor cascade. To do this، two wires were implemented along each blade and their effects on average velocity، turbulence intensity and vorticity frequencies at Reynolds number 45500 were accurately considered. To measure wake parameters، single channel hot wire anemometer was used. Turbulence creation in response to the turbulence promoters made the separation to take place within the boundary layer in a distance farther from the edge of attack and also a decrease in wake width was observed. It is found that turbulence promoters increased the maximum turbulence intensity in blades wake and also reduced corresponding frequency in maximum amplitude and the Strouhal number، consequently.

In this paper، the characteristics of both the natural and ventilated supercavities occurred behind a 30 degree conical cavitator were investigated experimentally. For all cases، the drag force of the cavitator and the streamwise distribution of pressures downstream of the cavitator were measured using the corresponding electronic devices. Both of the maximum diameter and the length for the supercavities، formed behind the cavitator، were measured from the captured pictures using a high speed camera. Experimental results of France and Michel reported for natural supercavitation flow over a flat disk was utilized for validating the present test setup. For the natural supercavitating cases، the results show that the lengths of supercavities decreased as the inlet velocity decreased، while their lengths were many times shorter than those related to the ventilated cases. Also، the sensitivity of the length of the supercavities to the inlet velocity of flows was reduced for the ventilated cases. For the ventilated cases، the lengths of supercavities were influenced most by the mass flow rate of the injected air.

Study of presence of bubble cloud in liquids has been of interest to researchers because of its positive or negative effects in many industrial processes. This paper aims to understand the behavior of a homogeneous mixture of bubbles and liquid. Due to usual simplifications، the equations governing bubble cloud behavior do not offer a complete analysis of this phenomenon. In this research، the works of the other scientists are reviewed especially the studies carried out by Wang and Brennen. In this study، first، the code devised according to numerical algorithm of this research is validated by results of Wang and Brennen. Then، by taking into account the compressibility property at the bubbles boundary for homogeneous mixture of bubbles in liquid، the governing equations are derived. Next، these equations under different applied sound pressure levels to the cloud، and a variety of gas percentage in liquid are numerically solved by the devised code. The severe reduction in maximum growth of bubbles with compressibility considerations، and the occurrence of collapses under some conditions are of the obtained results. Also، when bubble presence ratio is low the behavior of bubble cloud does not relate to compressibility. Some other outcomes of this research are manners of bubbles collapse in outer and inner layers of bubble cloud، serious compressibility effects on intensity and time of collapses، and maximum bubbles growth.

In this research، effects of superhydrophobic nano-coating on frictional drag coefficient have been investigated. A rotating disc apparatus is used as the experimental set-up to compare the frictional drag force on an aluminum disc with TiO2 superhydrophobic nano-coating and a smooth coatless aluminum disc. The superhydrophobic nano-coating is prepared، using sol-gel method and is able to produce a contact angle of 160 degrees. Experiments were performed in the range of Reynolds number from 105 to 4×106. The results show about 30% drag reduction in laminar flows as well as about 15% drag reduction in turbulent flows.