فهرست مطالب غلامحسین پوریوسفی

This paper investigated a numerical simulation of the undesirable effects of swirl inlet distortion on a low-speed axial compressor rotor and the effect of plasma actuators as an active flow control method on reducing these flow instabilities. Initially, the effect of co and counter-rotating inlet swirl distortion at angles of ±5 and ±10° on the rotor's performance and efficiency was evaluated. Then, using simulations of plasma actuators' fluid dynamics effects, the effect of this method on mitigating the destructive effects of this type of bulk swirl distortion was examined. Co-rotation swirl distortion reduces the rotor's total pressure rise coefficient, whereas counter-rotation swirl distortion reduces the stall margin. Applying plasma actuators by modifying the rotor inlet flow angle, leading to stall margin improvement. Furthermore, in the 5 degree counter rotation swirl distortion case, plasma actuators, in addition to compensating the adverse effects of swirl distortion, increased the stall margin by 1.3%

In this research, the turbulent isotropic flow has been experimentally investigated. Hence, two different grids are made and a contraction channel is installed behind it inside the subsonic wind tunnel to generate an isotropic turbulence flow.  The grids with mesh sizes of 2/54 cm and 5/08 cm were cut on the wood with obstruction ratio of 0/34 and 0/17, respectively. One-dimensional hot wire was used to determine the perturbation velocities in the direction of flow, and an approximation was used to determine the components of other directions. At speeds of 5 m/s and 10 m/s, experiments were performed for each of the grids, which range from a Reynolds number of 8500 to about 33000. To determine the onset of the isotropic location, methods of velocity skewness, kurtosis, turbulence intensity, dissipation rate, and longitudinal scales such as Kolmogorov and Taylor lengths were used. For skewness and kurtosis, the numbers show 0 and 3, respectively, which indicate the isotropic flow. Results showed that with increasing the velocity, the isotropy of the flow was delayed. Also, in a grid with a lower obstruction ratio, the intensity of turbulence will be less near the grid, but as it moves away from the grid, the intensity of turbulence will increase.

Tip Leakage vortex in axial compressor is source of unsteadiness and loss in flow field, so it is very important to investigate its controlling method. The aim of this article is numerical simulation of plasma actuator effect on tip leakage vortex in low speed axial compressor rotor. By applying linear distribution of body force at upstream of rotor on casing, effects of plasma actuator on tip leakage vortex have been investigated. Numerical simulation confirm that plasma actuator at blade tip region boost the flow momentum and in addition to decreasing the size of tip leakage vortex and reduces blockage due to the flow separation from blade surface, leads to decreasing loss in tip region. In addition, Plasma actuator can eliminate leading edge spillage and Trailing edge backflow in tip clearance region. Furthermore, the results show that plasma actuator caused the stall margin of the rotor blade row to increase by 5.2%.

One of the conventional ways to increase velocity of a fluid flow is using a nozzle in stream wise. In this paper, in order to increase the flow velocity in a low speed wind tunnel, and to carry out experiments at higher velocities. An optimal nozzle profile, by considering constraints such as the nozzle inlet cross section, inlet velocity, and nozzle length has been designed using computational fluid dynamics. Design variables were the nozzle inlet cross section, outlet velocity, the turning point of the nozzle profile, the uniformity of the outlet flow and the effective length of the test section. The performance of the designed nozzle was experimentally investigated. According to the results, the velocity increased by 13 m/s and the uniformity of the flow decreased by ±1%.

The ability to control the flow, is one of the basic needs of Fluid Mechanics that constantly pursued by researchers. One of the new methods in this area, is using Dielectric barrier discharge (DBD) plasma actuators that by injecting momentum into the boundary layer, causing a delay in the phenomenon separation. The main object in this work was to help to optimize the electrical parameters to obtain stranger vortex and more effective ionic wind created by steady and unsteady plasma actuators on the air through the flat plate. For this reason, simulation is done for a flat plate with the compressible 5 m/s velocity airflow. The time averaged velocity profiles of the ionic wind show that averaged velocity come more and the position of the maximum velocity come near the surface by increasing the excitation voltage and frequency. The power, of the vortices that are shed form the unsteady actuator, increases by increasing duty cycle percentage. Our results on the ionic wind velocity on different position on the flat plate indicate that the maximum averaged velocity occurs in downstream of plasma actuator.

Various studies on cars aerodynamics focusing on the Ahmed body model as a standard and simplified shape of a road vehicle have been carried out in recent years. In this paper plasma actuator as an active flow control method has been employed to control flow around the rear part of an Ahmed body with the rear slant angle of 25°. Experiments performed in a wind tunnel in free stream velocity of U=10m/s using steady and unsteady plasma actuator excitations. Pressure distribution on the rear part was measured by 52 sensors, and also total drag force was extracted by a load cell. More over smoke flow visualization was carried out to determine the flow pattern around the body. The results showed that employing plasma actuator not only has an effective influence on pressure distribution on the rear slant surface, but also reduces total drag force in steady and unsteady excitations 7.3% and 5%, respectively. As a result, based on flow visualization and pressure distribution tests, plasma actuator in steady state actuation, could distract D-shape vortices and suppress the separated flow over the rear slant.

Dielectric barrier discharge (DBD) plasma actuators are one of the new devices for active flow control, which has received substantial attention during the last decade. The performance of the actuator is optimum when it induces the highest velocity per unit of power consumption. Since the induced velocity and the power consumption of the actuator depend on many different variables, finding the optimal set, which results in the best performance, is of immense importance. In this paper, in order to optimize the performance of these actuators, at first, by using full factorial design of experiments the effect of electrical variables (including voltage and frequency) and geometrical variables (including the gap between electrodes, dielectric thickness, and covered electrode width) on induced flow velocity and power consumption in steady actuation is experimentally investigated. Then, by using the multi-layer perceptron neural network, a model is created for the ratio of induced velocity to power consumption. The model is validated both statistically and experimentally. The results indicate that the coefficient of determination for training and test data is higher than 95 percent. Finally, the surrogate model is optimized by genetic algorithm and the optimal value of electrical and geometrical variables is determined. In order to validate the result, an actuator is designed based on the optimal set of variables and it’s ratio of velocity to power is measured to be 29.71 (m/s)/(kW/m). The difference of 3 percent between the measured and the predicted value demonstrates high accuracy and correctness of the proposed model and method.

DBD plasma actuators are one of the effective devices for active flow control, which has received substantial attention during the last decade. In this paper, to find the optimum performance of plasma actuators, the effect of geometrical variables (including the gap between electrodes, dielectric thickness, and covered electrode width) and electrical variables (including voltage and frequency) on induced flow velocity and power consumption, is experimentally investigated. This is the first investigation in which design of experiment (DOE) approach is used to find the interaction among variables. The results show that each of the mentioned variables has a different effect on the start of uniform plasma discharge, induced flow velocity, power consumption, and the start of the filamentary plasma discharge regime. Furthermore, the results show that unlike the uniform discharge regime, in filamentary discharge regime increasing power consumption results in decreased induced flow velocity. The results of this investigation can be used to select the optimum actuator in which the ratio of induced flow velocity to power consumption is maximum, and to prevent from entering the filamentary discharge regime.

In this paper, an experimental research was carried out for investigating the characteristics of the flow pattern and separation region behind a backward-facing step (BFS) with low expansion ration (aerodynamic step), in the Reynolds number range of 18000

In this paper, Effects of runback ice accretion on NACA 23012 airfoil have been studied experimentally and numerically. For this purpose, experiments were applied on runback ice within Reynolds number of 0.6×〖10〗^6 over the angle of attack from 0 degree to 20 degree and then results were compared with the results of clean airfoil. Generally, Having examined behavior of the flow pattern and aerodynamic coefficients of the iced airfoil the results of which were compared to that of the clean airfoil, it can be concluded that icing phenomenon affects aerodynamic performance of the airfoil in two ways; in the first way that occurs at low angles of attack prior to stalling of the airfoil the effect is local. In this case ice accretion on the airfoil contributes to formation of a flow separation bubble behind the ice ridge on the upper surface of the airfoil. After numerical simulation of flow field, flow separation bubble behind the ice ridge was observed. The main effect of icing which is related to the second way occurs at angles of attack close to stall and post-stall. In this case flow pattern around the airfoil as well as aerodynamic coefficients undergo a fundamental change. In addition, it was made clear that runback ice causes stall angle decreases 2 degree and maximum lift reduces about 8 percent.

Nowadays, AC-current plasma actuators are more preferred to the DC-current plasma actuators in active flow control. So far, almost no research work has been done on DC-current plasma actuator. In this paper, using a direct current power supply and different geometries, plasma characteristics of the both corona and DBD, were studied. Using the results of this study, it was found that the induced flow of the DC current corona is weaker than that caused by AC-current ones, and it has perpendicular components to the flow direction. Also, the induced flow of the DBD actuators is stronger than the corona ones.

Aviation accidents due to the icing of aircraft wings in recent years raised the need for investigation of this phenomenon. In this paper, icing on NACA 23012 airfoil, due to the frequent usage in general aviation and also severe impact of the icing on this airfoil, has been studied. To study the effects of icing phenomenon, the experiments were carried out over a range of angles of attack from -8° to 22° at Reynolds number of 600000 on the clean airfoils, horn ice and spanwise ridge ice shape, and thereafter, the corresponding results were compared. According to the results, the icing phenomenon strongly affects the flow field around the iced airfoils, and it also has negative influence on the behavior of aerodynamic coefficients of the airfoil. Generally, it can be said that these two types of icing formation on the airfoil cause the formation of flow separation bubble on the upper surface at the downstream of the leading edge. In addition, it is found that the spanwise ridge ice has more negative effects on the aerodynamic parameters of the airfoil in comparison with the horn ice. In the case of spanwise ridge ice, ∝ stall and CL max decrease 8° and 50%, respectively, whereas, for the case of horn ice, these values reduce about 4° and 21%.

Boundary layer flow control, is one of the most important issues of aerodynamic.Generally, the purpose of flow control, is control of the boundary layer separation. Nowadays, researchers have considered the using of active electrohydrodynamic actuators due to the access and easy installation, no need for special repairs, very short response time and very low energy consumption. In this paper, using a similar geometry, the effects of dielectric barrier, on induced velocity in still air boundary layer, in the corona wind of direct and alternating fields was studied. The results showed that the using of dielectric barrier in the field of direct current, the flow velocity in the boundary layer is reduced, while with the dielectric barrier in the field of alternating current, greater speeds in boundary layer could be reached. with detection of smoke injection technique, it was found that the barrier dielectric has no effect on the flow pattern,but this pattern is in the direction of electric field in the direct current and is tangential to the boundary layer in the alternating current.finally, the using of the alternating current field, in addition to installing in the exact location due to small size of them, can cause to achieve higher speeds is still air in the boundary layer.

DBD plasma actuators are devices for active flow control, which recently have been widely of interest for aerodynamicists. Due to their advantages, there is a great focus on plasma actuators with the ability of unsteady actuation. In this paper, using a plasma power supply with the ability of unsteady actuation, the effective parameters on the performance of DBD plasma actuators in two conditions, i.e. steady and unsteady actuations, was experimentally measured and compared. In this regard, the effects of the applied voltage, carrier frequency, excitation frequency and duty cycle on the induced velocity, electric current and power consumption of the actuator was investigated. For the steady actuation, it was revealed that an increase in the applied voltage and carrier frequency leads to firstly an increase in the induced flow velocity, but afterwards, when streaky glow discharge regime starts (non-uniform plasma formation), induced velocity decreases. On the other hand, it wasfound that for the unsteady actuation, the variation of the excitation frequency does not significantly affect the electrical current and the induced velocity, but these values increase almost linearly with increasing the duty cycle. Generally, it was observed that using the unsteady actuation in comparison to the steady one, can cause considerable reduction in the electrical power consumption and also can have a better performance in the induced flow control.

This paper has investigated the flow interference between three circular cylinders of equal diameter in an equilateral-triangular arrangement and also between four circular cylinders in a square arrangement when subjected to a cross-flow. Wind tunnel experiments were conducted to measure force coefficients for six spacing ratios () varying from 1.5 to 4 at subcritical Reynolds number of. The pressure distributions on the surface of the cylinders were measured, using pressure transducers. It was found that for three cylinders at, the upstream cylinder experiences lower mean drag coefficient than that of the downstream ones. Also, the minimum drag coefficient values of the downstream cylinders occur at and. Moreover, It was revealed that for four cylinders at, due to severe flow interference between cylinders, there is a difference between lift coefficients for the upstream cylinders. Also, for, the mean drag coefficients for downstream cylinders are negative. In addition, it was concluded that the variations in strongly affect the aerodynamic coefficients. Also, by decreasing, the effects of the flow interference between the cylinders increase.