جستجوی مقالات مرتبط با کلیدواژه « ضریب پسآ » در نشریات گروه « مکانیک »

This study deals with the numerical investigation of the effects of the of multi row disks on the performance parameters of an axisymmetric supersonic air inlet of the mixed compression type at free stream Mach number of 2. Inlet performance parameters include drag coefficient, distortion coefficient, total pressure recovery and mass flow ratio. The multi row disk concept is one of the new methods to control the flow and the boundary layer. In this study, after validating the results of numerical simulation with the help of experimental results, the desired inlet has been investigated in terms of performance parameters in different back pressure ratios. Then, the effects of number of disks, length and depth of the cavities were also investigated. The results showed that using the multi row disks has a favorable effect on the drag coefficient and mass flow ratio. It was also observed that the use of multi row disks postpones the subcritical operating condition, which is operationally very beneficial for the aerial vehicle, because it makes the flight envelope of the vehicle wider.

The three low Reynolds number airfoils, NACA0015, NACA0018, and NACA0021, were examined. First, CFD and experimental testing were done in the wind tunnel using the numerical Vortex-Blade method, and it was used for the panel method of the KW-SST model. The findings demonstrated that the Reynolds numbers are more affected by airfoils when there are two airfoils present, as opposed to when there are more than two. The segmentation coefficient has been used to remove the tip radius and lift coefficient in the angle of attack less and less than other methods, according to comparison and results that showed that the method of vortex shedding and lift coefficient ratios in angle of attack between 10º and 18º in the laboratory were analyzed. In the attack angle of 10° and CFD model with a 1.8 and 2.3% error rate, increasing the attack angle of the CFD model, and Fortran at an attack angle of 18° with the 4 and 5.2% Fortran model and at the top of the CFD model for the analysis.

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.

In this study, the cylinder drag coefficient is reduced by using passive flow control. Installing a flat plate in two heights and different longitudinal distances in upstream flow increases the upstream flow momentum of the cylinder, leading to the higher boundary layer flow resistance against adverse pressure gradient which delays the flow separation. The flow separation delay enhances the pressure on the cylinder downstream. Then, the net pressure on the cylinder in the flow direction and, consequently, the cylinder drag coefficient are decreased. In case that the higher flat plate is utilized, the pressure on the upstream side is reduced more, leading to lower drag coefficient. However, for both heights of the flat plate at specific longitudinal distances from the cylinder due to the cavity flow formation between the cylinder and the flat plat, the vortex shedding is suppressed and the cylinder upstream is changed from the pressure side to suction side, leading to lower net pressure on the cylinder in the flow direction and as a result, less drag coefficient. At the optimal flat plate configuration at and , the minimum cylinder drag coefficient reached 90% reduction in comparison to the single cylinder case in the same flow condition. Results show that the drag coefficient reduction behavior is similar for different sub-critical Reynolds numbers due to the constant flow pattern and no considerable variation of the separation point. The entropy generation for the single cylinder and the case where the flat plate is located in its optimal configuration were investigated. The single cylinder has the highest entropy value, while the entropy of the optimal flat plate configuration with the cylinder reaches the lowest value, the same as the drag coefficient. Then, the drag coefficient is reduced by decreasing entropy generation, indicating the direct relation between drag coefficient and entropy generation.

Accurate aerodynamic coefficients and stability derivatives estimation is one of the feature of aeroballistic tests. This can be done by measuring the linear and angular coordinates, velocities and other relevant parameters from different sensors. The important question is the accuracy of the results, the method correctness and the coefficients estimation algorithm. Therefore, the effect of any measured parameters error on the estimated aerodynamic coefficients should be considered and analyzed.In addition, in order to investigate the effect of errors on the estimation of aerodynamic coefficients, a set of aeroballistic tests for measuring the sphere drag coefficient with error of less than 3% in this study. This shows the capability of high precision coefficient measurements in such tests. The errors involved in aeroballistic tests are mainly due to sensors, model manufacturing, data collection and the estimation algorithm. By modelling the error origins, the final error of the system can be predicted and analyzed. This investigation shows effect of each sensor error on the modelled collected data plus other flight simulation data that would help in designing the actual experiments.

Land vehicles are among the blunt body objects. When a vehicle moves forward, the movement of air around it produces a pressure gradient that varies along the body. This can lead to separation and appearance of a turbulent wake region in the rear of the vehicle. The present study numerically investigates the aerodynamic effects of vortex generators and their arrangement in different positions of 6 and 15 numbers, each with linear, rectangular and triangular arrangements on the back of a car model. Reynolds-Averaged Navier-Stokes (RANS) equations and turbulent models have been used to analyze the changes in drag and lift coefficients obtained from different arrangements of the vortex generators. The results show that the best case for reducing the drag force is related to 6 numbers of vortex generators with linear and triangular arrangement, which reduces the drag coefficient by 2% compared to the car model without vortex generators. In addition, the best case to improve the downforce; in order to increase the stability of the car, is the arrangement of 15 vortex generators with a rectangular alignment, which reduces the lift coefficient by 23.1% compared to the car model without the vortex generator. Also, with increasing the number of vortex generators from 6 to 15, the drag coefficients generally increase.

The falling and sedimentation of solid particles in liquids occur in many natural and industrial processes such as water and waste water industries, biotechnologies, environmental engineering, marine engineering, etc. This study represents the results of the experimental study of the falling velocity of steel balls in the water channel for different ball diameters (in the range of 8 to 25mm). The tests are done far from the channel walls. Moreover, as a case study, the wall effect on falling velocity of steel ball (i.e. diameter=12mm) is examined. A high-speed camera is used to determine the coordinate of a falling sphere and estimate the ball velocity and drag coefficients. In addition, a numerical method is used to solve the governing equations in comparison with experimental data. Comparing experimental and numerical results for transient and terminal velocities shows the maximum difference of 12 and 4.5% respectively. Experimental drag coefficients have good agreement with other published data. In addition, falling near the wall leads to a negligible effect on velocity but path diversion is observed.

In the present study, the instantaneous velocity profile behind an airfoil at two different Reynolds numbers has been measured experimentally. Data are used to study the wake profile and the corresponding drag coefficient force of the airfoil in different conditions. In the conventional and common methods for calculation of the drag force coefficient through the velocity measurement behind an airfoil, turbulence velocity terms of the momentum equation are ignored. However at moderate to high angles of attack where the flow becomes turbulent and separation occurs, the nature of the flow becomes three dimensional and disregarding the components of the fluctuation of velocity (in three dimensions) in calculation of the drag coefficient of airfoil may result in erroneous information. In the present study, in order to increase the accuracy of the experimental drag coefficient of the airfoil for moderate to high angles of attack, turbulence velocity terms in experimental drag coefficient calculation are considered and this causes an acceptable compatibility between experimental and numerical results whereas for low angles of attack, disregarding the effects of turbulence velocity terms in experimental drag coefficient calculation will improve the accuracy of the experimental drag coefficient and a desired compatibility between experimental and numerical data will be established.

Investigation of the intake performance is one of the most important topics in the aerodynamic design of the aerial vehicles. In this study, the performance of a supersonic axisymmetric mixed compression intake that was designed for free stream Mach number of 2, has been investigated at zero degrees angle of attack. Then by applying a heat source and changing its position, effects of this source on the intake performance has been studied. The flow has been simulated by a computational fluid dynamic code which in RANS equations have been discretized using the explicit method. The practical results were achieved by testing in the wind tunnel at Imam Hossein University have been used to validate the numerical simulation. Results show that a heat source located at the proper location could have desirable effects on the total pressure recovery, mass flow ratio and drag coefficient of the intake. The critical back pressure ratio also increases which widens the intake operating region. However, the flow distortion will increase a bit. At design Mach number and critical condition, 9.68% increase in total pressure recovery, 26.6% decrease in drag coefficient, 16.16% increase in mass flow ratio, 8.7% increase in critical back pressure ratio and 17.13% increase in flow distortion have been observed.

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.

The flow separation at the rear of a vehicle generates more pressure drag. A vortex generator can cause delay in developing of separation by chang-ing the distribution of momentum in boundary layer. The comparison be-tween the results of with and without vortex generator reveals the effects of vortex generator on drag reduction considerably. In this study, an effi-cient vortex generator is designed for the Peugeot 405 sedan. The numeri-cal simulations are performed using ANSYS FLUENT and also the model and mesh are generated by ICEM.

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.

Today, one of the useful methods of flow control, especially external aerodynamics, is plasma DBD actuators. In this study, the effect of plasma DBD actuators on cylinders in tandem arrangement is investigated. The actuators are considered on upstream cylinder. The cylinders are placed in distance (L/D) relative to each other. Investigation is done at two Reynolds number (100 and 200) with two different conditions of applying actuators. Cases with Vp-p=55kv and Vp-p=1kv are selected from references. The results of the present study are validated against the previous available experimental and numerical data and close agreement is found. Finite volume method is applied to solve equation of motion. Plasma actuators caused downstream cylinder experience upper values of drag coefficient and Nusselt number in all cases of study. Also, the growth of drag coefficient and Nusselt number are decreased by rising the Reynolds number, so that increasing the Nusselt number is 2% more at cases with Re=100 compared to cases with Re=200.

According to the development of technology of automated highways, the platooning can be benefited for improvement in fuel consumption. Platooning has already been exercised in the nature and industry as an effective tool to reduce the drag coefficient. The goal of this article is to investigate the effect of longitudinal distance between two passenger cars on drag coefficient and hence the fuel consumption numerically and experimentally. It has already been shown by various investigators that a passenger car travelling in the wake of another car will experience a reduction in drag coefficient. The optimized distance and speed to attain the least drag coefficient and fuel consumption are reported. The car’s speeds of 70, 90 and 110 kilometers per hour have been examined in this study. To evaluate the numerical results, some field tests are done. A device which was particularly designed and fabricated to measure the fuel consumption was installed on a sedan midsize car. The results show that the car platooning with an optimized longitudinal distance between two cars can reduce the drag coefficient by 7 percent.

Present study is a numerical solution for incompressible fluid-elastic structure interaction by use of immersed boundary method. The solid phase is an elastic filament hinged on rear of a rigid cylinder in the presence of fluid flow. The purpose of this study is investigation of interaction of flexible filament and rigid cylinder. In the immersed boundary method, solid and fluid domains are solved in separated regions. For solving the flow field and momentum equations, the lattice Boltzmann equations are employed. In present study in contrast to previous studies, the elastic filament is also modeled by a mass-spring network that provides elastic properties like young’s and bending modulus. The mass of filament is discretized on nodes of spring network. These two solution regions are connected by applying suitable boundary conditions at each time step. Results show that the mass, geometry and location of filament behind the rigid cylinder have significant effect on the filament’s stability and drag reduction. Indicating and analyzing a special point behind the cylinder, where the drag is minimum, is the another purpose of this study.

The high-speed submersion projectiles are designed to move in the supercavitation regime. In these conditions, the frictional drag decreases and the pressure drag becomes the dominant force against the projectile movement. A number of cavitation models have been suggested by researchers in order to simulate various types of cavitation flows. In the present work, by choosing a proper cavitation model, the supercavitation flow is modeled around a disk cavity. The supercavitation parameters are measured and compared with the experimental and analytical results. In this research, the supercavitating flow is simulated in different angles of attack and the drag coefficient is obtained in different cavitation numbers. The results of numerical analysis are compared with the results of quasi-empirical relations. Finally, using the results of numerical analysis, a correlation coefficient is proposed for calculating the drag coefficient at different attack angles for a range of cavitation numbers.

In this research the effect of roughness on airfoil sections with one dimensions hot-wire anemometer has been studied. The factors of oscillating and the average time of flow velocity was measured for two positions. Soft and rough surfaces in 5 and 10 m/s speed, also changes in the drag coefficient was obtained and examined in two modes. For measuring the parameters the sequence method (experimental) for calculating the speed component measurements have been performed without dimension at x/c=0.01, 0.5, 1, 2 and 3 (c means chord length and x means distance from the back of airfoil) was used. According to the graphs and results of experiments it is observed that with increasing distance from the longitudinal direction the width of the trail increased more surface will affect. Also moving away from blade in the longitudinal direction makes minimum speed peaks spread eventually it will go away but in case of a rough surface width of the trail increase and to achieve free-stream velocity we need more distance from blade.

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.

Nowadays, with regard to the new methods of simulation, the designof airfoils have greatly changed and this allows the evaluation of the effect of various factors on the efficacy of airfoils. Thus, choosing the type of wing would be so important; in another term, designing a proper wing is what an airplane designer is mainly concerned with. In this project, the effect of airfoil wave surface on the changes of coefficients of lift and drag are studied. Graphs of lift and drag coefficients, Cantor Distribution Pressure and Cantor Distribution speed are alsoinspected in different attack angles. The achieved results indicate that wavy airfoils atspecific attack angle and also near the angle of extinction have a better performance than airfoils without wavy surface. The results of airfoil performance in different attack angles shown throughgraphs indicate the proper performance of wavy wing.