مجله مکانیک سیالات و آیرودینامیک
سال سوم شماره 3 (پاییز 1393)

An experimental study has been performed to investigate the flow physics of an under-expanded axisymmetric supersonic jet exhausting transversely from a flat plate into a subsonic compressible cross flow. The goal was to study the effects of boundary layer thickness ratio (δ/D) and the jet to free stream dynamic pressure ratio (J) on surface pressure. The resulting measurements distinctly show that both the magnitude and the width of the positive and the negative pressure coefficient regions in front and behind the jet increase with J. In addition, the side region of the jet affects the jet plume and as J increases, the pressure coefficient decreases at this region. The data acquired upstream of the jet reveal that, with increasing δ/D, the magnitude of the positive pressure coefficient region decreases, While the recovery of the back pressure downstream of the jet moves in the vicinity of the nozzle exit.

In this paper, the effect of bioethanol- gasoline fuel blends on exhaust emissions of a four-cylinder spark ignition (SI) engine was investigated experimentally. Pure gasoline fuel was blended with various percentages of bioethanol (0, 20, 40, 60 and 85%), and the HC, CO, CO2 and NOX exhaust emissions were measured at different engine speeds and two throttle valve positions (full throttle and 50% throttle valve).The experimental results showed that as the ratio of bioethanol fuel is increased in the blend, the CO and HC emissions are decreased but CO2 and NOX are increased at two throttle valve positions. Also when the speed of engine was increased, the HC and CO2 emissions were decreased but CO and NOX emissions were increased. The statistical analysis indicated that the effect of bioethanol percentage in fuel blends on emissions at 5% level was significant.

In this paper a double-shaft turbofan engine is considered to be in the design phase. Therefore its environmental condition and critical source of failures has been studied and the subsystems and components has been identified. Since it is in design phase, the project encounters to the lack of specified failure data. To overcome this problem, the generic data or expert judgments on the basis of environmental condition is used. Then Simulation process utilizing Reliability Block Diagrams and implementation of Monte Carlo Method has been done. The results involve reliability evaluation of system and Reliability Importance. The engine has less than 97 percent reliability in 150 flight hours. Finally a Reliability Allocation method has been utilized to improve the reliability measure to 97 percent in 300 flight hours. Results reveal that oil and fuel filters and pumps and the disks and blades of rotating subsystems are the most sensitive and critical parts of engine.

In this paper the numerical simulation and operation comparison of two sizes of air gun pellets with 4.5 and 5.5 mm calibers is performed. These pellets affected by drag force and are encountered to continuous velocity variations and experience an unsteady flow. In order to solve this problem numerically, the Navier- Stokes equations in an unsteady compressible turbulent flow with the dynamical equations of pellet motion solved simultaneously in a moving computational grid by a computer code. The geometrical modeling of pellets is obtained by accurate photography of actual sample and digitization. According to pellets geometry, the axisymmetric cylindrical coordinate system is used. The numerical simulation is based on discretization of convective flaxes by second-order central difference scheme and an implicit finite volume method. Because of the time importance in solution, the second-order time accurate is applied by dual time stepping approach. Then, in order to validate the operation of computer code the results are compared with experimental data. The results obtained from this investigations show that the shoot velocity, mass and pellets size are the effective factors on trajectory time, height loss and hitting momentum at a certain distance.

Determining characteristics and behavior of fluid flow and obtaining the aerodynamic coefficients are important and considerable tasks in investigation of flow over the bodies. In this research, leading edge radius effects on the flow behavior over a delta wing with a sweep angle of 50 degree at different Reynolds numbers between 0.5×106 and 2.5×106 are investigated numerically. Three different leading edge models including one sharp and two round (having different radius) are studied. Validation of the results is performed using available experimental results which showed good agreements. Leading edge radius effects on the aerodynamic coefficients and size and location of the vortices are investigated. Based on the obtained results, on a fixed angle of attack, increasing the wing leading edge radius reduces the moment coefficient. Also, it decreases size and strength of the preliminary vortex and magnitude of the lift and drag forces. When the Reynolds number varies Between 0.5×106 and 0.97×106, the lift coefficient tends to experience a 6 to 8 percent raise, while at higher Reynolds numbers, no considerable raisings observes. Also, at angle of attacks of 20 and 25 degrees, by increasing the Reynolds number, the lift coefficient graph experiences a noticeable reduction. At the sharp leading edge delta wing, stronger vortices generate which results higher lift coefficient. By rounding the leading edge, due to weakening the aforementioned vortices, the lift coefficient decreases.

In this research, the effect of surface roughness and bubble injection on skin friction and hydrodynamic drag force has been numerically considered. This research is based on Taylor-Couette flow. Considered flow includes two concentric cylinders which internal cylinder is movable and external is immovable. The effective equations on the flow includes: equations of momentum, continuous, turbulent and bubble distribution. These equations are based on finite volume numerical method and have been solved by second order upwind discrete method. For modeling of two-phase flow used Euler-Lagrange method. In all of observed views, turbulent flow and Taylor vortex has been revealed between spaces of two cylinders. Air flow has been injected into the flow as the second phase. Selected Reynolds range is 104 to 2 × 105. For validation of the results an experimental study has been used. Results showed that friction coefficient will be decreased by Reynolds increasing and also, drag force will be decreased by roughness decreasing.