جستجوی مقالات مرتبط با کلیدواژه « drag coefficient » در نشریات گروه « مکانیک »

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.

This paper uses the dpd method to simulate the flow around a microairfoil. In this study, periodic boundary conditions were used, and the particles were balanced by applying these boundary conditions. The results include the analysis of velocity profiles in simple channel geometry and the comparison of flow simulations in channels by dpd and cfd. The error of comparing the velocity profiles with these two methods was 6%. Next, the flow around the rhombus-shaped airfoil was simulated, and with the expansion of the flow around the airfoil, the pattern of flow lines was obtained symmetrically. The results for naca0012 airfoil also expanded to 5 different angles. The diagram of aerodynamic coefficients and the diagram of aerodynamic force ratio in terms of attack angle has been compared with two simulation methods of dpd and cfd. The comparison error of these two methods was calculated to be less than 3% .

In this study, the growth of glaze and rime ice along the UAV’s wing span was studied, the cause of the formation of these ices on the surface and the effect of ice accretion on the aerodynamic performance of the wing was investigated. For this reason, a rectangular wing with NACA 0012 airfoil section at an angle of attack of 4 degree was studied at two different temperatures. A pressure-based solver and the Spalart-Allmaras turbulence model were used in commercial software. Calculations were performed at Re = 3×106. The results of the ice growth pattern indicate that there was no difference between the ice thickness on the wing span from root to middle, but from the middle to the tip, due to the increase in flow velocity, the rate of collision and the accumulation of droplets in the area increased. Also under glaze ice conditions, ice forms near the trailing edge due to the growth of the boundary layer. This calculation also proved the accuracy of this claim by performing similar calculation in the inviscid condition and the lack of ice growth near the trailing edge. On the other hand, the vortex phenomenon that occurs on the tip of the three-dimensional wings causes part of the droplets to hit the tip of the wing, resulting in the growth of a small amount of ice in this area. study of lift and drag coefficients showed that ice formation reduces the aerodynamic performance of the wing.

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.

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.