مجله مکانیک سیالات و آیرودینامیک
سال پنجم شماره 1 (بهار و تابستان 1395)

The objective of this paper is to obtain acoustical behavior of flow around 50° tailless, 8 /65° double and 53° diamond delta wing configurations, using large eddy simulation. Validation of the aerodynamic coefficients is performed using available experimental results, showing good agreements. CFD simulations were performed for 15 degree angles of attack at a Mach number of 0.147 and a Reynolds number of 1.2 million, based on the root chord root chord of the wings (is 360 mm). Acoustic measurements, such as power spectral density, acoustic pressure, sound pressure level, and sound amplitude were taken, using 3 microphones in the wake region of the mentioned wings. The amount of sound pressure level of microphone, which is placed at 1.835 meter from apex of above wings in DK Strouhal number 0 to 1 is, 24 to 69, 10 to 54 and 9 to 44. Hence, for purposes of being silence, aeroacoustics values indicate that diamond delta wing is more preferable than tailless and double delta wing.

An experimental study has been performed to study the nature of breakup and penetration of liquid jets injected transversely into cross gas streams. The Schlieren technique was used to study the liquid jet breakup and penetration. This technique include concave mirrors and convex lens which will be useful when used with a high speed camera and a suitable light source. The resulting images are then processed and analyzed, using a processing software for studying the effects of various parameters on spray penetration. Experiment was carried out at ambient conditions and subsonicgas flow. Liquid-to-air momentum flux ratio (q- varied from 32 to 57) and Weber number (from 0.5 to 1) were the principal liquid jet operating parameters. Two modes of breakup have been noticed: column breakup and surface breakup. A correlation has been developed to predict the jet penetration with respect to q. Decrease of cross-flow velocity and increase of gas to liquid ratio are found to increase the penetration. Moreover, surface breakup produces droplets more than column breakup.

The Darrieus lift-based VAWT has been studied by many scientists due to its simplicity of design and the wind direction independency. Due to increasing angle of attack and dynamic stall at low tip speed ratios, these wind turbines have the inherent problem of self-starting inability and produce less power's compared to that of horizontal axis wind turbines. In this study, the effects of variable blade pitch angle mechanism on decreasing dynamic stall have been investigated. Finally, a pitching system (variable pitch Darrieus-type wind turbine) has been proposed, which can reduce both the blades, oscillating motion and the magnitude of the angle of attack in one revolution, compared to that of conventional VAWT. In this study, unsteady two-dimensional simulation is conducted, using CFD with moving mesh the rotating rotor. Simulation of different preset pitch angles of ±3° and ±6°¬, ±9° and ±12°, it is concluded that adjustment of pitch angle about -3°, causes a delay in flow separation and control dynamic stall. It was also observed that a variable-pitch blade turbine can suppress flow separation on its blades at low tip speed ratios. This leads to delay or elimination of dynamic stall and results in a higher efficiency during the range of turbine operation compared to that of fixed-pitch blade Darrieus turbines.

In this paper, a two-dimensional laminar flow over two elliptical cylinders with different arrangements, confined in a channel is numerically investigated, using lattice Boltzmann method (LBM) for different Reynolds numbers. A second order curved boundary condition used for the no-slip boundary condition at the solid curved boundaries. The results were validated for flow past a single circular cylinder. According to the results, the first cylinder has maximum drag force in tandem arrangement, but it has minimum value of drag in the side-by-side configuration. Of course, the second cylinder has more complicated behavior, being in the wake of the first one. In steady flows, the lift force is maximal in the side-by-side arrangement, while in unsteady flow, it is maximal in staggered arrangement, due to vortex shedding and the dominance of wake interference effects. The Strouhal number in tandem arrangement is greater than that of staggered arrangement and no vortices is shed in side-by-side arrangement.

In this paper, design and production of a high power pulsating illuminator for particles imaging in shadowgraph technique is presented. Based on high contrast of shadowgraph images and no need for high power illuminator, commercial light emitted diode (LED) is used as an alternative for laser light. The designed pulsating high power light source works with minimum pulse width of 125 ns, frequency of 500 Hz, and maximum electrical current of about 15 A. A trigger compatible with camera and light source is designed for synchronizing light pulses and camera shutter. In addition, commercial DSLR camera was used for implementation of single frame-multi pulse technique with no need for high speed camera. The images recorded by a DSLR camera shows drops with velocity of about 20-30 m/s and size of about 100-200 um being captured.