فهرست مطالب عبدالامیربک خوشنویس

Investigating the flow around a circular cylinder is one of the classical problems of fluid mechanics. Unsteady forces formed behind the cylinder are caused by vortices separated from its surface. Controlling the intensity of vortex shedding leads to controlling the intensity of forces caused by vortices. One of equipment that is used to reduce fluctuations caused by vortices is trip wires. Therefore, in the present study, the fluctuating velocities, Reynolds shear stress and drag coefficient around the circular cylinder with the trip wire have been investigated at different Reynolds numbers. For this purpose, the diameter of the circular cylinder is 20 mm and the diameter of the installed trip wires is 0.25, 0.5, 0.75, 1, 1.5 mm. Trip wires are installed at θ=±40 and ±140 positions. The results show that the use of trip wires with a diameter of 0.25mm causes a decrease of 68%, 50% and 55% of fluctuating velocities (u', v', w') and a decrease of 85% Reynolds shear stress compared to the smooth cylinder. Also, the presence of a peak in the fluctuating velocity indicates the presence of large vortices behind the cylinder. Reynolds shear stress has a direct relationship with the diameter of the trip wire.

Many arrangements are available for placing cylinders with respect to each other. Some arrangements of circular cylinders include side-by-side, tandem, and staggered at different angles. Thus, in the present study, the drag coefficient at three cylinders, A, B and C with diameters of 15.5, 21.3 and 31 mm at different angles (0°, 5°, 10°, 22.5°, 45°,67.5° and 90°) in L/D=2 and 4 at various Reynolds numbers (14700<Re<48000) is experimentally investigated. The results showed that the drag coefficient is completely dependent on the diameter of cylinders and the L/D ratio. The variation of drag coefficient in L/D = 2 is less dependent on the Reynolds number changes at different angles. Also, by increasing the diameter of the downstream cylinder, the effect of the Reynolds number on the drag coefficient was reduced. The drag coefficient in L/D = 4 is less sensitive to the angle changes except for the zero degree angle.

Today, measuring the flow parameters of the oscillating airfoil wake is of great importance due to its wide applications in industry and nature. Therefore, the aim of this research is to provide an innovative method for the experimental measurement of the NACA0012 oscillating airfoil. For this purpose, three one-dimensional hot-wire sensors were used, one of which is fixed, and the others are mobile. By using MATLAB code and the connection between the data of these three sensors, the spatial and time correlation of the flow velocity at all points of the wake was obtained. The results showed that measuring different parameters of the wake with less hardware equipment than other researches is possible. In this method, the flow parameters have been measured with higher accuracy by eliminating the errors caused by the large number of sensors and by increasing the number of data collection points in the wake. The results showed that the combination of the free stream flow momentum with the oscillating momentum of the airfoil creates a sinusoidal flow with the same frequency of the airfoil oscillation in the wake. Moving vortices area and the area of diffusing vortices momentum form the two parts of the oscillating airfoil wake.

Dynamic stall is a phenomenon which appears due to the vortex shedding on the oscillating wing section at high angles of attack. Occurrence of the dynamic stall causes a severe decrease in the lift force and huge increase in the drag force. The Co-Flow Jet (CFJ) is one of the active flow controls to prevent this phenomenon. In this paper, the effect of this active flow control on the NACA 0025 airfoil for different Reynolds numbers is investigated. For numerical solution of the fluid flow, the Reynolds-averaged Navier-Stokes equations in two-dimensional, incompressible, and unsteady form with the SST-k-ω turbulence model is solved using an in-house computer code. The developed code is validated with the previous experiment data, and a fairly good agreement is observed. In order to investigate the effects of the CFJ, three different momentum coefficients of 0.05, 0.07 and 0.08 and five Reynolds numbers of 5×104, 7.5×104, 105, 1.5×105, and 3×105 are studied. It is found in the examined cases that the baseline airfoil in the Reynolds numbers of 105 and lower has different behavior compared to the higher Reynolds numbers; while in order to eliminate the dynamic stall, it requires more jet momentum of 0.08, while for the higher investigated Reynolds numbers, by applying the jet momentum of 0.07, the dynamic stall is completely eliminated.

In this study, the wake behind a cylinder with helical strakes at Re = 20000 is experimentally investigated. Three helical strakes with a pitch of 3D and equal distances were installed on the cylinder. The ratios of strakes heights (thicknesses) to cylinder diameter (H/D) were 0.1, 0.09, 0.07 and 0.05. The effects of helical strakes on the mean velocity profile, turbulence intensity, velocity defect and Strouhal number of the cylinder on its center line were studied. The results showed that using helical strakes with varying heights results in different effects on the wake structure, which can be demonstrated in near (x≤D) and far (x>D) regions downstream, behind the model. In the near regions, planes with higher thickness resulted in improved wake and reduced velocity defect, while in the far regions, using the plane with an H/D of 0.07 led to up to 35% decrease in velocity deficit. Rotating planes helical strakes were also seen to reduce maximum turbulence by 14%.

In the present study, the effects of turbulent flow are experimentally investigated on the drag coefficient of tandem cylinders with non-equal diameters in different Reynolds numbers. Drag coefficient test for downstream cylinder in diameters A=15.5 mm and B=21.3 mm, was experimentally carried out at two longitudinal distance at five Reynolds numbers based on the upstream cylinder diameter for two turbulence intensity of 4.5 and 7% respectively. The position of the upstream cylinder to the downstream cylinder changes at five angels 0, 22.5, 45, 67.5 and 90 degrees. Results for downstream cylinder in the 4.5% turbulence intensity indicate that with increasing the diameter ratio of the cylinders and increasing the longitudinal distance between the two cylinders, the drag coefficient increases. However, with increasing aligning angels of the upstream cylinder with respect to the downstream cylinder, the drag coefficient varies depending on the nature of flow. Finally, by increasing the flow turbulence intensity, the drag coefficient decreases significantly at all stages of the experiment.

There are several ways to increase the aerodynamic efficiency ratio of the wing. One of the most important ones is the use of winglet at the tip of a finite wing. In the present study, four split blended winglets with different upper winglet height mounted on RQ170 Micro-Aerial Vehicle half model and its effects on the drag reduction in zero angles of attack compared with wing without winglet in low Reynolds number. Also, the effect of split blended winglet in different Reynolds number and angles of attack were investigated experimentally. The tests were carried out in a wind tunnel with a low speed and turbulence. The Reynolds number based on the Mean Aerodynamic Chord is changed from 22000 to 46500, and the angle of attack is adjusted from 0◦ to 20◦. The results are representing the drag reduction in all Reynolds number and angles of attack due to using split blended winglet. It was found that with increasing Reynolds number at zero angles of attack, the drag coefficient decreases. Also, the installation of the split blended winglet with the 3cm height of upper winglet has more reduction in total drag coefficient.

One of the methods for measuring the drag force is the momentum difference (or wake survey) method. In this method, using velocity and turbulence intensity distributions at the model downstream, drag coefficient is determined. In this experimental investigation, turbulence intensity downstream of a model (circular and elliptical cylinders) has been studied and minimum distance in which the flow turbulence can be considered with acceptable uniformity (homogeneity), has been determined. Turbulence intensities at the downstream of the elliptical and circular cylinders decrease with increase in x/B ratio. At x/B = 20, depending on Re number, the maximum turbulence intensity is about %14 and the error is about %2 and for elliptical cylinder is reduced to %10 and the error is about %2. At x/B = 25, the maximum turbulence intensity is reduced to about %12 and %8, respectively and the error is about %1. In this study, a hot wire anemometer has been used to investigate velocity distribution and air flow turbulence intensity at the downstream of the cylinder at Reynolds numbers of 15000 and 30000

In the present research, effects of flow acceleration magnitude on velocity profiles, near-wake and far-wake turbulences, and drag coefficients of a model vehicle are studied. Moreover, rate of changes in the mentioned parameters in an accelerated flow at various accelerations was considered. An open-circuit blowing wind tunnel was used for fluid simulation, whose maximum nominal turbulence and velocity were 0.1% and 30 m/s, respectively. Velocity was increased continuously at different accelerations using an inverter device. The results indicate that, geometry of the developed wake is consistent and independent of inlet flow acceleration. Furthermore, with the passage of time and increasing velocity, near-wake and far-wake velocity defects decreased largely and slightly, respectively. With increasing the inlet flow acceleration, momentum and turbulence components of drag coefficient increased. By enhancing the flow acceleration, drag coefficient increased. The lowest value of the drag coefficient often occurs in the current with minimal acceleration, which is approximately equal to the corresponding values in the flow at constant speed.

Winglets are one of the tools used for reducing the effects of the tip vortex. In this research, the effects of installing split blended winglet on Micro-Aerial Vehicle half model in order to the reduction of wing tip vortex were investigated experimentally. The experiments were conducted in an open circuit wind tunnel at velocity 16 m/s and angle of attack 10ᵒ. The Reynolds number of the model was about 3.8×104 based on Mean-Aerodynamic chord. In order to measure the mean velocity and turbulence intensity, the Hot-Wire Anemometry (HWA) has been used. Experimental results in the absence of installing winglet indicated the existence of a vortex structure behind the tip of the wing which is due to the pressure difference created on the upper and lower surfaces of the wing. Turbulence intensity has it highest value in vortex core which is also observed to decrease gradually with downstream distance. The results show that in the presence of winglet, there are two clear vortex structures which one at the winglet/wing junction and the other at the tip of the winglet.

In this study, the characteristics of flow wake around an elliptic cylinder have been investigated experimentally. To this end, a cylinder with elliptic cross section with an axis ratio of AR=2 of Aluminum at Reynolds numbers of 15000 and 30000 has been tested in the wind tunnel. Using existing methods the characteristics of a flow wake and drag coefficients of the model in the different cases have been investigated. To measure the mean velocity and turbulence intensity, the hot-wire anemometry has been used. Free-stream turbulence intensity of wind tunnel is 0.15 percent which in this sense, it has high accuracy. The experiments indicates that the increase in Reynolds number causes significant changes in the mean velocity profile. The results show that by increasing the Reynolds number, the turbulence intensity in the wake increases. The results show that by increasing the Reynolds number at a constant axis ratio, the width of wake becomes narrow, and the Strouhal number of wake associated with the Karman vortex increases. It is found that by increasing the Reynolds number, the parameters of drag coefficient and velocity defect reduce.

In this study, the blade cross-section profile of horizontal wind turbine (NREL type) is investigated in the wind tunnel. In order to measure the velocity and turbulence intensity the hotwire anemometer is used. The tests are done in the wake of the S823 airfoil at velocities of 5, 7 and 12 m/s and at angles of attack of 0°, 7°, 10°, 13° and 15° and at stations of = , , ,1,2,3. The effects of variation of angles of attack and Reynolds numbers are investigated on velocity defect, additional flow, velocity distribution and turbulence intensity and the curves of effects of changing the downstream position on the velocity distribution are presented. The results show that the additional velocity in the near of the trailing edge of airfoil is great and increasing the angles of attack increase the additional velocity and increasing the Reynolds numbers decrease the additional velocity and also asymmetric wake of mean velocity profile and increasing region of the mean velocity profile at lager attack angle than 10° can as sign of vorticity developed at region of wake airfoil.

In this research, the behavior and characteristics of the wake of flow around an elliptic cylinder at zero angle of attack in the presence of a tripping wire were investigated experimentally. An aluminum elliptic cylinder with major axis, minor axis and height of 42.4 mm, 21.2 mm and 390 mm respectively, was used for this purpose. The cylinder model was examined in the test section of a blower type wind tunnel. The Reynolds numbers of the experiment based on major axis are 25700 and 51400 for 10 m/s and 20 m/s speeds, respectively. Tripping wires with the diameter of 0.5 mm, 1 mm, and 1.5 mm are placed symmetrically at both sides of the cylinder, and each are tested at angles of zero, 23.7, and 40.9 degrees with respect to the stagnation point. The drag coefficient of the smooth cylinder for both of the Reynolds numbers is about 0.6. The results indicate that in the best possible case, the drag coefficient for the 0.5 mm wire decreases by 75%. In the best cases, it is also reduced by 56.9% and 65.5% for the 1 mm and the 1.5 mm wires, respectively.

In this research, the behavior and characteristics of the wake of the flow around an elliptic cylinder at zero angle of attack in the presence of a tripping wire of 1 mm diameter were investigated experimentally. An aluminum elliptic cylinder with major axis, minor axis, and height of 42.4 mm, 21.2mm, and 390 mm, respectively, was used for this purpose. The cylinder model was examined in the test section of a blower type wind tunnel. The Reynolds numbers of the experiment, based on major axis were 25700 and 51400 for 10 m/s and 20 m/s speeds, respectively. Tripping wire placed symmetrically at both sides of the cylinder, and each were tested at angles of zero, 23.7, and 40.9 degrees with respect to the stagnation point. The drag coefficient of the smooth cylinder for both of the Reynolds numbers was about 0.6. The results indicate that in the best possible case, the drag coefficient for the 1 mm wire reduces by 56.5% at angle of the 23.7 degree. In the best case, it also reduces by 8.6% for angle of the zero degree, while in the worst case, the drag coefficient for angle of 40. 9 degree increase by 20.7%.

This study empirically investigates the effect of turbulence promoter wire installation on the flow parameters of airfoil wake and wake flow characteristics such as average velocity and the turbulence intensity in the Reynolds number 3780 for airfoil and Reynolds number 30000 for the cylinder. The diameter of the wires was 1, 1.5, and .5 mm. The wire was installed with in the airfoil thickness of NACA0012 and in the circular cylinder α=140.40 angle. The diameter of the wire has changed to obtain different turbulence flow. The current study shows the width and height of the velocity profiles have increased with an increase in the diameter. The outflow of the wake area was the same for all of the parameters. The rate of change of velocity and the turbulence of cylinder and the airfoil has been compared. The results showed that the drag coefficient has increased with an increase in the diameter of the wire in airfoil, and the cylinder has changed depending on the wire diameter. For the cylinder in Re=30000, there is an optimal state in which the active drag coefficient on the cylinder reaches to the lower value of the cylinder coefficient drag. The experiments show that the values of the height profile of turbulence and the decrease of velocity in the cylinder wires have been decreased in comparison with that of a flat cylinder.

In this study, changes in velocity and turbulence energy in the wake of a Peugeot model and the drag coefficient changes in an Unsteady Flow, with increasing vehicle speed (inlet velocity), are Measured and evaluated. The blow open circuit wind tunnel to simulate fluid flow is used. The maximum disturbances and nominal maximum speed of the device, respectively are, 0. 1% and 30 m / s. The inlet wind velocity has been increased continuously by an inverter that causes the changes in rotational speed of the wind tunnels electro motor. The results showed three different regimes in the velocity profile of the near wake model. With increasing distance from the model and with increasing velocity, three regimes in the wake are close to each other. Turbulence intensity is measured in the wake and drag coefficient decreased with increasing the flow Speed and reached to a minimum value and then increased.

An experimental investigation on the cylinder having a length of 390 mm and a diameter of 20 mm at Reynolds number 18750 has been done. At the first, mean streamwise velocity profiles, then streamwise turbulence intensity profiles fixed and rotating circular cylinder (for spans 1000, 2000 and 3000 rpm) at three stations (x=30, 60,120 mm) behind the cylinder are discussed and compared. In order to measure the mean velocity and turbulence intensity, the hot-wire anemometry has been used. Also, the Strouhal number of fixed and rotating cylinders is compared. The results show that by increasing the rotational speed at a constant Reynolds number, if the direction of the cylinder is the agree of the fluid movement, the width of wake becomes smaller, and the Strouhal number of the wake associated with the Karman vortex increases. At the stations of 1 and 3 turbulences the rotating cylinder is more than the fixed cylinder, and the second station turbulences fixed cylinder is more than the rotating cylinder. It is found that by increasing the rotational speed, the parameters of drag coefficient and velocity defect reduce. By increasing the rotational speed, the drag coefficient reduced by about 10%.

The flow characteristics around an elliptic cylinder with an axis ratio of AR=2 located near a flat plate were investigated experimentally. The elliptic cylinder was located on the inside and outside a turbulent boundary layer region whose thickness (δ) is 0.38B. The Reynolds numbers based on the height of the cylinder cross-section were 15000 and 30000. Measurements of mean velocity and turbulence intensities have been made using the hot-wire anemometry when the wake of elliptic cylinder interact with the boundary layer on a flat plate. In the near-wake region, streamwise mean velocity profiles and turbulence intensities were strongly dependent on gap ratio (G/B) and Reynolds number (Re). It is found that, wake and boundary layer interaction except in G/B=0.1, at Reynolds number of 30000 is faster than the 15000. The wake-boundary layer interaction at fluctuating velocity quantities begins earlier than the mean velocity. As the gap ratio increases, the drag coefficient of the cylinder itself decreases, which the decrease value of the drag coefficient at Reynolds number of 30000 is more than the 15000. The results show that the Strouhal number by increasing the gap ratio increases and nearly independent of δ/B. The wake region behind the elliptic cylinder is relatively small and the velocity profiles tend to approach rapidly to those of a flat plate boundary layer.

In this paper, the existence of statistical relationship between high- order velocity moments as flatness and skewness in the plain wake behind a circular cylinder were studied. Hot- wire anemometer was used to measure turbulent parameters in various situations downstream the flow behind the cylinder. Turbulence intensity of wind tunnel’s free line is 0.1 precent which in this sense, it has high accuracy. In these experiments, the Reynolds number is 28000. Non- dimensional values of mean velocity, skewness S and flatness F in various x/d’s has been plotted. By comparing existent relationship for predicting flatness values (which are demonstrated for turbulent boundary layer and free jet) with actual values, it has been found that new relationship can be found which can have much more agreement with their actual cases. Considering this point, and by using fitting method, in this paper some new relationship from which was can predict high- order moments with much more accuracy was provided. Therefore for further accuracy, two polynomial of orders 2 and 3 were used to fit the data which has taken in a wind tunnel. Though the accuracy of these estimations in larger x/d’s reduses significantly.

In this study, the characteristics of the wake around a rotating circular cylinder have been investigated experimentally. To this end, a cylinder with a circular cross-section of Plexiglas with the length of 400 mm and a diameter of 20 mm have been tested in the wind tunnel. Using existing methods, the characteristics of the wake and the drag coefficients of the model in different cases have been investigated. In order to measure mean velocity profile and turbulence intensity, hot-wire anemometry has been used. The experiments carried out on the rotating cylinder indicates that the rotation of the cylinder causes significant changes in mean velocity profile. The rotation of the cylinder causes increase in the velocity in the vortex and decrease in the velocity of the wake. The variations become more and more by increase in rotational speed. Also, the results show that by increasing the rotation speed, the turbulence intensity in the wake increases. It is found that by increasing the rotational speed, the parameters of drag coefficient and velocity defect reduce.

In this study optimization of objective function of axial compressor by using Genetic Algorithm with weighted method has been investigated. The effects of variations in combination of different weighted coefficients of objective function on optimization have been carried out. The objective function contains linear isentropic efficiency and static pressure rise coefficient by considering mean stage diameter, angle of inlet air at rotor and flow coefficient. In each steps of optimization the fitness function is created by weighted coefficients and the new objective function is produced and the new optimization problem has been established. The effect of variations in weighted coefficients on objective function was studied. Results showed that the objective function has been reduced in each step which explains the optimization has been achieved.

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، inflow turbulence effects on the flow wake parameters around a NACA001 airfoil at zero incidence angle and Reynolds number of 38،700 were investigated. Various turbulence promoter networks were used to achieve turbulence intensity of 5% and 6%. Experiments have been conducted for three different turbulence intensities in which mean velocity parameters، turbulence intensity، frequency response، and Strouhal number were measured. Half of the wake width (b1/2) in all three cases of turbulence، was increased with increasing x/d and turbulence intensity. It was also found that vortex formation length is decreased with turbulence intensity. Besides، it became clear that Strouhal number is reduced with any increase in turbulence intensity or in the distance from the trailing edge of the airfoil.