جستجوی مقالات مرتبط با کلیدواژه "جریان ناپایا" در نشریات گروه "مکانیک"

The purpose of this study is to investigate the behavior of a cylinder with three fins in the presence of free air flow. This study aims to investigate the unsteady aerodynamic flow with respect to its practical significance in aviation. Considering the degree of freedom of movement within the geometry and the complexity of the vortex masses that cause instability, it is essential to accurately predict the different dynamic behaviors. We have investigated the dynamic behavior of the model with different length ratios, free flow speeds, and angles of attack. According to the results of the study, rotational and oscillating behaviors, as well as a combination of both, are observed, and they are dependent upon the geometrical characteristics of the model, such as the ratio of the length of the plates to the radius of the cylinder, as well as the angle of attack When the aspect ratio and free flow speed are low, the motion pattern is damped around 60 degrees and the motion regime is oscillatory in nature, which changes to rotational motion as the aspect ratio and flow speed increase. The maximum oscillation or rotation is related to the initial angle of attack of zero degrees. It has been observed that rotational motion regimes occur with an increase in the longitudinal ratio and free flow speed at low initial angles of attack.

In this study, the effect of blowing jet parameters, which include location, blowing angle, momentum coefficient, and orifice area, on the average aerodynamic coefficients and aerodynamic performance parameter of the NACA0012 is investigated. The airfoil has a sinusoidal oscillating motion around a quarter of the chord. As a result of this movement, the angle of attack of the airfoil changes from -5 to 25 degrees. Five locations by 1, 4, 6, 10, and 20% of the chord length were examined and the results showed that the placement of the jet at 1% of the chord length is more appropriate than the other locations and significantly improves performance and have more impact on the flow control. Three angles of 30, 60, and 90 degrees were considered as the angles of the blowing jet and it was observed that the angle of 60 degrees is better in controlling the flow than the other two angles and this effective angle decreases when orifice length increases. The results show that there is no regular uptrend or downtrend for the angle effect. For the jet momentum coefficient, which actually aimed to investigate the effect of blowing jet velocity, the values of 0.14, 0.08, and 0.04 were considered while the orifice length is considered constants for these cases. The results showed that increasing the blowing jet velocity as well as increasing the jet orifice length improves aerodynamic performance, although increasing the orifice length at a constant momentum coefficient decreases the mean of lift coefficient.

The first step in turbine blade design is to select TSR. Therefore, an optimization procedure should be applied to find the best ratio since this directly affects the energy generated from the turbine. In this research the optimum TSR are calculated with regard to the dynamic stall phenomenon. The dynamic stall imposes large amplitude loading on airfoil sections and since it occurs in HAWT operating envelope in unsteady flow. The purpose of this study was to investigate the effect of unsteady flow with periodic oscillation on the performance of HAWT turbines. A DS model is implanted to analyze the static data. Then the Beddoes-Leishman DS model was tailored to the HAWT environment. Then, using this model and BEMT, the optimal TSR is calculated And the impact of the presence of dynamic stall is shown. Also, Cp and CT are plotted in several different TSR with different time steps to express the effects of Unsteady phenomenon. In addition to dynamic results, static and steady results are plotted in power and thrust graphs. This phenomenon affect the efficiency by -3% as compared to the static stall. Also the Optimum TSR increases in dynamic mode. Examination of the time average diagrams of the drag coefficient shows that the delay in separation starts approximately from the midpoints of the blade and reaches the maximum value at the root.

This paper presents a numerical algorithm for solving unsteady viscous incompressible two–dimensional (2D) Navier–Stokes equations. In the proposed method, for discretization of time derivatives and solving the Poisson equation of the pressure, Meshless Local Petrov-Galerkin (MLPG) and forward finite difference methods are employed, respectively. In the present analysis, the moving least-square (MLS) approximation is regarded for interpolation, and the Gaussian weight function is used as the test function. To satisfy the boundary conditions, the penalty approach is applied. In the numerical examples, the accuracy and efficiency of the method are compared with those of the exact solutions. The effects of the number of nodes, the size of time interval, as well as the nodes distribution (both regular and irregular) on the relative errors are investigated. Moreover, the Gaussian integral sub-domains with the circular and square shapes are considered, and the accuracy of the results is compared with each other. Analysis of these results for 2D benchmark geometries with different boundary conditions clearly displays that the accuracy of the suggested combined method for solution of the problem related to unsteady viscous incompressible 2D flows is high such that its differences with analytical solution is negligible. Since no limitations is considered on the design process of the regarded numerical algorithm; therefore, it is respected that this approach is successful and has sufficient efficiency to solve the governing equations.

In the present study, a software has been developed using the unsteady Reynolds-averaged Navier–Stokes method to simulate time variation of turbulence coherent structure and reduce computational cost and it is used for numerical simulation of a jet in the cross nozzle flow, accurate determination of the flow structure and nozzle fluidic thrust vectoring. Since this software can capture time dependent physics, in this paper, its capability has been investigated in the simulation of pulse jet in nozzle cross flow and variation of the fluidic thrust vectoring for three frequencies, 50, 100, and 200 Hz. Firstly, software validation has been performed by comparison the results with some experimental data, next, variation of jet in cross flow and its effect on the exhaust flow field and nozzle surface pressure distribution have been studied. Governing equation on the unsteady Reynolds-averaged Navier–Stokes algorithm has been explained and time step and applied boundary conditions have been presented. The Finite volume approach has been used for numerical discretization and the advection upstream splitting method has been utilized for flux computing. Also, to improve the solution accuracy, the multi-block grid and 2nd order monotonic upwind scheme for conservation laws method have been applied and to reduce computational time, the open multi-processing parallel approach has been used.

In this study, the proper orthogonal decomposition has been used to develop a reduced order model of the unsteady incompressible flows. After projection of governing equations along high level energy modes, a low-dimensional dynamical system is constructed. Usually, this model is developed using velocity field modes therefore pressure term representation in momentum equation will have an important role for stability and accuracy of outcome reduced order model. Two approaches are available for pressure term deputation, the first is based on elimination of pressure term due to homogenous condition on control volume boundaries and the second used appropriately method for pressure term representation in momentum equation. The method, which is used in this research, is based on using pressure snapshots and velocity field modal coefficients for calculation pressure field modes. The obtained results from proposed model have been compared with related DNS results. Because the instability behavior of reduced order model is not only related to pressure term modeling therefore response of low-dimensional dynamical system is not exactly verified with DNS data. But the outcome results from proposed model have higher accuracy than classic reduced order models.

Unsteady flow structure, particularly in blade tip clearance region of turbomachines, is one of the main resources of blade vibrations, undesirable noises and losses which may eventuate to severe rotating stall and surge. So, analysis of flow behavior in tip clearance region is more significant. In this paper, the unsteadiness which caused by blade row tip leakage flow in a low speed axial compressor, is investigated. Analyses are based on results obtained through numerical simulation of unsteady three dimensional viscous flows. Analyses are based on flow simulation utilizing computational fluid dynamic technique. Two different circumstances at design point and near stall condition are considered for investigation and discussion. Tip leakage flow frequency spectrum was studied through surveying instantaneous static pressure signals imposed on blades surfaces. Frequency spectrum Results showed existence of some pressure peaks at near stall conditions. In this case, interaction between main inflow and tip leakage flow lead to unsteadiness. By occurrence of unsteadiness, tip leakage vortex flow starts to fluctuate at a frequency about the blade passing frequency. However, at design condition, flow is more affected by the main inflow instead of the tip leakage flow.

In this research, the experimental investigation of unsteady flow around a cylinder model with three plates perpendicular to it has been discussed. This study has been done at different speeds, primary model angle of attacks and length ratios. The results show that the model has steady and unsteady oscillation and rotational motions. These types of behaviors depend on the plate’s length ratio, primary object angle of attack, and free stream velocity. The model has damping motion around an angle of 60 degree, at low speed and length ratio. The greatest oscillational or rotational flow has appeared at initial angle of attack of 0 degree. In low length ratio, the regime of motion is oscillations that it changed to rotational motion when the length ratio and free stream velocity increase. The variation of angular velocity at length ratio of 1 and 4, with respect to time, are accompanied with reduction of vibrations range per motion. Also, over time, the vibrational velocity of the model is reduced. In addition, the results show that by increasing the Reynolds Number, Strouhal number becomes constant.

In this paper, the proper orthogonal decomposition method has been used for gappy data reconstruction of the unsteady incompressible viscous flow and calculation of related forces (such as aerodynamics coefficients). The present approach is based on combination of POD method and solution of an optimization problem to get a reduced order model. This model reconstructs missed data or incomplete snapshots with relevant accuracy and fast speed. Two kinds of model have used in this manner. In the first approach, a simple iterative approach is used to reconstruct of the relative missed or incomplete basis functions of snapshots ensemble. The Second method, which is proposed recently by the author, used a time advancing and domain decomposition method along time direction, for reconstruction of incomplete ensemble members with high missing percentage. Essential trait of this method is about application of step by step reconstruction of primary ensemble to reduce number of unknown variables and obtaining an accurate model to recover the missed data. The outcome results of two approaches have compared with direct simulation CFD data which are shown appropriate ability and good agreement

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.

In this study, experimental and analytical unsteady flow around a cylinder model with rotational degrees of freedom is discussed. Experimental studies at different speeds and angles of attack for two cylinder models with different length ratios have been done. Meanwhile the analyses of numerical technique known as vortex panel method have been used. Analytical and experimental results show that the rotational and vibrational motion and a combination of these behaviors occur. These types of behaviors depend on ratio of length plates to cylinder radius, primary object angle of attack and free stream velocity. At different speeds and at all angles of attack for a length of less than 1, the model has vibrational Motion around a specific angle. This angle for cylinder with two plates is 90 degrees. Generally, the model tends to vibrational motion at low angles of attack with increasing length ratio and free stream velocity occurs and by increasing the Primary angle of attack is the desire to vibration motion around a specific angle. Also, in free stream velocity 10(□(m/sec)) and Higher, for length ratio 4, the model had a steady rotational motion. In addition, angular velocity models and Strouhal number on rotational motion is calculated. The results show that Strouhal Number is a fixed amount, by increasing the Reynolds Number.

Aerodynamic study of flows at low Reynolds for special applications such as micro unmanned underwater vehicles, underwater robots and explorers are interested. In this paper, an improved progressive preconditioning method named power-law preconditioning method, for analyzing unsteady laminar flows around hydrofoils is presented. In this method, the 2D Navier-Stokes equations modifies by altering the time derivative terms of the governing equations. The preconditioning matrix is adapted from the velocity flow-field by a power-law relation. The governing equation is integrated with a numerical resolution derived from the cell-centered Jameson’s finite volume algorithm and a dual-time implicit procedure is applied for solution of unsteady flows. The stabilization is achieved via the second- and fourth-order artificial dissipation scheme. Explicit four-step Runge–Kutta time integration is applied to achieve the steady-state condition. The computations are presented for unsteady laminar flows around NACA0012 hydrofoil at various angles of attack and Reynolds number. Results presented in the paper focus on the velocity profiles, lift and drag coefficient and effect of the power-law preconditioning method on convergence speed. The results show satisfactory agreement with numerical works of others and also indicate that using the power-law preconditioner improves the convergence rate and decreases the computational cost, significantly.

In this paper the POD method has been used for development of a reduced order model of the unsteady incompressible flow fields. After projection of the governing equations along POD modes, a low-dimensional dynamical system is achieved. Due to variations in energy level of POD modes and large differencing in modes’ energy, the outcome dynamical system can be converted to a reduced order model with a few amount of error. The standard low-dimensional models, due to some reasons, do not predict time variations of flow field accurately. One of these reasons is due to non-divergence free modes. Accuracy of dynamical system has been improved by using combined form of continuity and linear momentum equations which has been proposed by the author. The obtained reduced order model can predict time variations of flow field with fast computational speed and relatively good accuracy. The results from this low-dimensional model have been compared with direct numerical simulation data and shown good accuracy and salient simplicity in computations.

In this study, accelerating motion of a sphere in incompressible fluid has been nvestigated by the Navier-Stokes equations. The motion is set in a small time interval, having the constant velocities out of it.The laminar and unsteady flow has been solved in various Reynolds numbers. In a short moment the Reynolds number is rised which cause large variations in the drag force. This method is applicable to other accelerating motions such as oscillation and also spherical submarines.

In this paper, reduced order proper orthogonal decomposition method has been used for simulation of unsteady incompressible flow fields. After projection of the governing equations along POD modes, a low-dimensional dynamical system is achieved. Normally, standard low-dimensional models, due to some reasons, do not predict time variations of flow field accurately. Accuracy of dynamical system has been improved by using a linear calibration term. The presented method is based on combination of POD and an optimization problem, which is solved using least square approach. The obtained reduced order model can predict time variations of flow field with relatively fast computational speed and good accuracy. This method is based on a local minimization technique proposed by these authors. The results have been compared with direct numerical simulation (DNS) data and shows good accuracy and simplicity of the computations.