جستجوی مقالات مرتبط با کلیدواژه « لایه مرزی » در نشریات گروه « فنی و مهندسی »

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.

This study investigates the effects of cascade blades that are passed through the flow. To this end, the characteristics of boundary layer flow is obtained on a fixed plate in downstream. A special moving grid has been introduced for the numerical simulation of flow. In this method, a zone of the grid moves inside the main grid. Regarding the defined plan, the regular changes of grid connections can be carried out by the program. Therefore, movement of different bodies in the flow can be simulated even in large sizes without reducing the quality of grid. In the written code, the unsteady form of the Navier-Stokes equations is solved using an averaging method. For turbulent modeling, a two-equation k −ε model is used. Velocity profile at the boundary layer on the flat plate is calculated. The results are validated with experimental data and good agreements among the results are observed. The results are validated with experimental data and good agreements among the results are observed.

In this paper, compressible flows inside converging-diverging nozzles with different shapes at off-design conditions are investigated. Shock waves and boundary layer separation are present in the flows. The area ratio (outlet to the throat) and the total combustion chamber pressure are constant while the nozzle shape and its length are variable. Compressible Reynolds averaged Navier Stokes equations were solved using the finite volume method. By comparing the results with available experimental data, the Spalaret-Almars model has been selected for simulations. Axisymmetric, unsteady and viscous flow is considered in all simulations. The characteristics method is used to design the nozzle shape and important parameters such as Mach number, wall shear stress, Thrust efficiency, wall pressure, axial pressure and boundary layer separation due to shock wave are studied in detail. By comparing the results for four shapes of the nozzles (i.e. perfect nozzles, 15° conical nozzle, 30° conical nozzle, and minimum length nozzle) it is concluded that at design condition, perfect nozzle gives higher thrust (7889 N). However, at off-design conditions (when the shock wave and boundary layer separation occurs), the conical nozzle gives higher thrust (9700 N).

In this paper, the flow field around a two-dimensional rectangular cylinder was numerically simulated and the effects of the flow suction on the separation bubble were investigated. The purpose was to estimate the effect of suction on the dimensions of the separation bubble. The flow field was assumed incompressible, turbulent, and two-dimensional. Gambit software was used for grid generation and FLUENT for numerical simulation.The problem was analyzed in two states: with and without suction, and the effect of suction on the separation bubble was investigated. According to the results, it can be said that the suction length of the separation bubble is smaller, so that in several different cases this length is reduced by about 50%.

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.

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.

In many industrial applications, fluids have non-Newtonian behavior and it is essential to access appropriate analytical models to study them. The Maxwell fluid model is one of the various models, proposed for the approximation of the non-Newtonian fluid behavior, which is used to study some of the industrial applications. In this research, the analytical investigation of a Maxwell flow on flat plate subjected to magnetic field in a porous media has been studied. For this purpose, the governing non-linear partial differential equations, are transformed to the ordinary non-linear equations by similarity variable and then analytically solved by the HPM. The results of the proposed method are compared to the numerical solution and validated. According to the results, increasing the magnetic field parameter, causes the velocity distribution decrement and increment of the boundary layer thickness. The most significant effect of the magnetic field parameter is observed for the stress on the plate. By increasing magnetic field parameter, the stress distribution on the plate substantially increases. Also increasing the magnetic field parameter, increases the skin friction coefficient on the surface and the velocity distribution.

This paper presents the implementation of the event-triggered technique on sliding mode control (SMC) method in the case of linear systems with bounded uncertainty. Event-triggered technique is a scheduling strategy to reduce the number of control updates in a feedback loop. The Main contribution of this paper is to implement event-triggered SMC on the linear multi-input linear systems with parameter uncertainties. Although the chattering is inevitable in SMC, to tackle this phenomenon, hysteresis bounds are offered to reduce the frequency of control updates; nevertheless it induces a limit cycle. By updating control law at hysteresis boundaries, as the event-triggering rule, system finally ends to limit cycle. Another contribution of this paper is to reduce the amplitude of this limit cycle according to the variable control input, while guaranteeing no request for successive control updates too. In this paper by properly designing dynamic control law and hysteresis bands based on state trajectory, as the system moves towards the stable point, its amplitude decreases and the limit cycle amplitude fades accordingly. This method is illustrated by an example of linear system with uncertainty in the system matrix. Simulation results depict the applicability of SMC combination with the event-triggered technique.

Energy conversion efficiency of water to the water flow depends on the blade of the turbine wheel depends on the blade surface. Heat water in a collision with turbine blades, friction on the surface of the water during passage through the turbine blades, nozzle head loss and air friction resistance pelton turbine energy resources are wasted. When the water jet passes around the turbine blades, ideal against the current because of non-slip surface and thus the boundary layer is created. The formation of the boundary layer and its thickness depends on the blade surface roughness. In this research the mathematical relationships to help the border layer, a model for the calculation of the flow passing through the wasted power of pelton turbine blade is presented in which the effect of the friction and pressure changes on the flow along the path of the blade come into account.

In this paper, a quantum intelligent robust controller via a combination of sliding mode control with boundary layer and quantum neural networks, for uncertain nonlinear systems in presence of external disturbances, is presented. Based on the adjustable time variant rejection regulator and rejection parameter, a time variant sliding surface as an adaptive chain of the first ordered low pass filters is defined. A three layers quantum neural network is designed to identify the uncertain nonlinear functions in system dynamics. In this method, the control gain and the break-frequency bandwidth are tuned adaptively. Also, the effects of uncertainties and the un-modeled frequencies are eliminated and chattering phenomenon doesn’t occur. Also, for facilitating analytical theory of the presented method and derivation of the adaptive laws a theorem is proved. Finally, the simulated examples show that the proposed method presents an intelligent adaptive robust tracking control such that the control amplitudes and the integral absolute error index of the tracking trajectory are much less than the other methods. Therefore, effective identification, eliminating the effects of system uncertainties, adjustable control gain and break-frequency bandwidth and more accurate tracking are some of the advantages of this method.

In this paper, the forced convective heat transfer of cu-water nanofluid over a wedge is investigated numerically. It is assumed that the flow of nanofluid is two-dimensional, laminar and incompressible. The boundary layer approximations are used to simplify the equations of momentum and energy. For solving the differential equations of momentum and energy, the similarity solution and the numerical Keller-Box method have been used. The effects of volume fraction of nanoparticles and wedge angle on the flow field and heat transfer rate are investigated. Numerical results for the dimensionless velocity and temperature profiles, the local friction coefficient and local Nusselt number are obtained. With the addition of copper nanoparticles , the hydrodynamic boundary layer thickness decreases and the thermal boundary layer thickness increased. The results indicate that the addition of nanoparticles increases the friction factor and Nusselt number. Also, increasing the wedge angle has the same effect on the friction factor and Nusselt number

Flow around a circular cylinder placed in an incompressible uniform stream is investigated via two-dimensional numerical simulation in the present study. Some parts of the cylinder are replaced with moving surfaces, which can control the boundary layer growth. Then, the effects of the moving surfaces locations on the power and drag coefficients are studied at various surface speeds. The flow Reynolds number is varied from 60 to 180. To simulate the fluid flow, the unsteady Navier-Stokes equations are solved by a finite volume pressure-velocity coupling method with second-order accuracy in time and space which is called RK-SIMPLER. In order to validate the present written computer code, some results are compared with previous numerical data, and very good agreement is obtained. The results from this study show that some of these surfaces reduce the drag coefficients and the coefficient of the total power requirements of the system motion. The optimum location and the speed of the surfaces which cause the minimizing the power coefficient are also obtained; By observing the results it is found that in all Reynolds numbers, the minimum power coefficient or in other word, the optimum drag coefficient is occurred at surface angle of 70 deg.

The study of the boundary layer flow is considered as one of the fundamental issues in fluid mechanics that attracted many of the researchers’ attention in this field. The most of previous researches on boundary layer problem are limited to Newtonian fluids and a few numbers of researches are considered the non-Newtonian fluids. The main objective of this research is to better recognize of the viscoelastic properties effect on characteristics of the boundary layer. In this study, the hydrodynamic and thermal boundary layer of viscoelastic Falkner-Skan problem is investigated numerically. Here, the second order model has been used as the viscoelastic constitutive equation and analyzed with MATLAB software. Both constant temperature and constant heat flux at walls are used as thermal boundary conditions. The effect of Reynolds number, the coefficient of the first normal stress difference, Prandtl number and wedge shape factor on the thickness of dynamic and heat transfer boundary layer, momentum thickness, displacement thickness, drag coefficient and Nusselt number are studied. The effects of both adverse pressure gradient and favorable pressure gradient in heat and hydrodynamics boundary layer characteristics are investigated. The magnitude of the non-Newtonian hydrodynamic and heat boundary layer are found to increase with increasing the coefficient of the first normal stress and finally at constant pressure gradient, average Nusselt number decreases along plate by increasing first normal stress coefficient.

This study focuses on transition of laminar to turbulent flow around a symmetrical airfoil at a low Reynolds number in free flow and flow near the ground at different angles of attack. Finite volume method is adopted to solve the unsteady Reynolds-averaged Navier–Stokes (RANS) equation. Flow around the symmetrical airfoil SD8020 at a low Reynolds number (4000) at 5 and 8 degree angle attack has been simulated in free stream and near the groundnumerically. Current numerical result is compared with other’s experiment and numerical result in free flow at low Reynolds number and flow in ground effect that good agreement has been obtained in aerodynamic coefficient prediction. SIMPLEC method is used for pressure and velocity coupling and flow equations discrete with Quick method. Transition-SST model is used for modeling turbulence of flow. Result shows that the current numerical method can detect adverse pressure gradient، laminar separation bubble and transition of laminar flow to turbulent. According to the result، in ground effect location of laminar separation bubble، length of bubble and location of transition is moved to leading edge and pressure distribution is effected by location of laminar separation bubble.

In this research, an aerodynamic design of axisymmetric diffuser is performed via linking a solver of boundary layer equations, Genetic Algorithm and Ball Spine inverse design algorithm (BSA). A numerical boundary layer code is incorporated to the genetic algorithm to reach an optimum pressure distribution on the diffuser wall in such a way that maximum pressure recovery is obtained without separation. To validate the developed boundary layer code, the calculated quantities are compared with Blasius and Howart’s analytical results. Then, the optimized pressure distribution is considered as the "target pressure distribution" for the inverse design algorithm to find out the relevant optimum geometry. For inverse design, Ball-Spine algorithm as the geometry modification algorithm is compined by the Fluent software as the flow solver. Implementation of this combination is completed through User Defined Function (UDF) feature of Fluent. Having examined the performance of the proposed inverse design method, quantitative effect of this method on the performance improvement of an axisymmetric diffuser is studied. The numerical results of the optimized diffuser shows that its pressure recovery coefficient has been increased considerably.

The most important factors for the sudden increase in drag coefficient in transonic flight regime are the occurrence of shock waves and its interaction with boundary layer. Using shock control bump to weaken the interaction of shock and boundary layer makes it possible to experience lower drag coefficient in a range of off-design Mach numbers. In this study, a differential evolution algorithm is used to find the optimum shape and location of shock control bump respect to free stream condition. This study clearly shows that the effectiveness of this method in improving the aerodynamic performance and reducing drag is totally a function of flight condition; The method is effective in fairly wide range of off-design Mach numbers; However, boundary layer separation and oscillatory shock wave in downstream of bump region is the main limitation of this method for the case of strong shock waves.

Heat transfer and fluid flow in a porous medium over a stretching flat surface with internal heat generation and suction/injection have been investigated using a two-temperature model of heat transfer. On assuming non Darcy flow، similarity solutions are obtained for the governing steady laminar boundary layer equations. The surface is maintained at a non uniform temperature. The coupled momentum and energy equations for both fluid and solid phases are obtained and then transformed to the similarity format. The system of governing equations along with the related boundary conditions have been solved using the classical fourth-order Runge–Kutta method allied with the shooting technique. The resulting velocity and temperature distributions are shown for different values of parameters entering into the problem. Also، the values of the local surface heat flux for both solid and fluid phases and critical values of the Prandtl number are reported as a function of the permeability parameter.

In this research, numerical simulation of the interaction between a large bubble and a boundary layer, considering various parameters in a low Reynolds number flow, have been studied. This work can be a start for more complicated tasks with higher Reynolds numbers. The considered flow has been solved using the well-known front tracking method. The results indicate that the wall shear stresses considerably depend on the bubble location. Also,when bubbles are inside the boundary layer, the overall shear stresses and the skin friction drag increase.

Different schemes have been implemented for Solution of boundary layer equations by many of researchers in the past. These methods are commonly based on numerical or analytical techniques. In this study, boundary layer equations are solved by two different methods. At first, the integral forms of boundary layer equations are expressed and considered by using Thwaites-Walz method. Then, explicit form of finite difference equation is implemented for discretization of boundary layer equation. Finally, laminar two dimensional flow on a flat plate is analyzed by employing both Thwaites-Walz and Finite difference method. Obtained results from the suggested methods are in good agreement with each other.