جستجوی مقالات مرتبط با کلیدواژه « vortex » در نشریات گروه « مکانیک »

Deficiency of potable water has created many problems for human and human society, Therefore, The production of fresh water from saline water is an important issue. One of the method of production fresh water from saline water is the use of solar stills. This paper is the numerical simulation of the conventional solar stills with setting Rectangular, Triangular, Wavy barriers on the left and right walls inside the solar still. Setting barriers causes a change in the pattern of humid airflow in the solar still that it affect water productivity and convective heat transfer rate. Also, changes in the size and number of barriers cause changes it will be in the result. The continuity, momentum, energy and concentration equations are discretized by finite volume method and the results are presented as flow function and concentration and temperature contours. The simulation results show that setting wavy barriers with A=0.01(m) and N=2 at left wall and A=0.075(m) and N=5 at right wall water productivity and convective heat transfer rate can be increased by about 31% and 31.34%.

In this article, the effects of the vortex rope in the Francis turbine draft tube in one of Iran hydropower plant are studied experimentally and numerically. First, the experimental results of the unit at four operational loads of 22.52, 45.74, 68.73 and 85.60 MW was extracted by a pressure gauge which is located at a distance of 3.199 meters below the runner. According to the fast fourier transform, the load with a power of 45.74 MW had the most pressure fluctuations. Then numerical simulation of the whole turbine at different condition was performed using transient solution and SAS-SST turbulence model. At five injection condition namely 1.5% and 3% only water, 1% and 2% only air, and the combined injection of water and air (3% water and 2% air) order of vortex fluctuations were investigated. Also, an algorithm to optimize the parameter of efficiency multiplication in the pressure recovery coefficient over the peak-to-peak amplitude of fluctuations at different air and water injection was presented. The results showed that injection with 1.58% of water and 0.02% of air, cause the mentioned ratio to be maximum, which is the most optimal condition of operation.

In this research, the effects of LEX angle on flow pattern over the diamond wing by using hot-wire and five-hole probe are investigated. The pressure coefficient tests over the wing at three different angles of attack respectively 5, 10 and 15 degrees and on two section was conducted by five hole probe. Also turbulence intensity measurements were conducted at three sections for 10 degree angle of attack by using hot wire anemometer. Experimental tests were conducted in a closed circuit wind tunnel with acceptable flow quality at the velocity of 12.5 (m/s) equal to 192500 Reynolds number. At a constant angle of attack with a downward movement the flow; the vortex diameter formed due to the Leading edge of the wing. Therfore, the LEX vortex increased which has led to growing pressure suction on the wing. Amplified suction pressure at the center of the LEX vortex core and the leading edge vortex have carried the two vortices closer together. Thus, the larger the vortex core lead to higher turbulence intensity by moving to downstream. Frequency analysis near the center of the leading edge vortex and Lex vortex showed that at constant section over the wing, the turbulence intensity near vortex core exposed, increasing turbulence intensity that caused to rise the amplitude spectra of fluctuation. Furthermore, frequency analysis indicate that the maximum domain of power spectra increases for LEX angle of 16 degree with rising of angles of attack.

.To achieve clean and sustainable energy for power generation and displacement, the engine designers demand a high performance and powerful propulsion. The compressor blades have the task of increasing the loss coefficient and should be considered in the design to prevent the destructive phenomena such as the flow separation. If the reverse pressure on the vane could be engineered in such a way as to prevent the flow separation and control the vortices, a higher loss coefficient would be achieved. A reliable way to achieve this goal is to use a tandem, which is obtained by placing a small secondary blade behind the main blade. In the present numerical analysis, a tandem rotor and stage designed and tested at NASA's Lewis Research Center are studied. The desired geometry is extracted from the mentioned source and a high-quality network with about 896 thousand nodes is applied to it, and then considering the  SST turbulence model, it is analyzed by the CFX commercial software. The rotor and its stage are studied in 5 rounds and therefore 5 different pressure ratios, and the resulting vortices are also subject to investigation and interpretation. Finally, it can be seen that at 2105 revolutions (half of the nominal revolution), both in the rotor and in the tandem compressor stage, the vortices are not fully restrained and occupy a very small area of the pressure diagram in terms of chords, thus demonstrating inappropriate performance. At a pressure ratio of 0.9, we also see a lot of turbulence after the vane, which is not suitable for the operation of the compressor. On the other hand, at 4210 rpm (nominal rpm), the vortices are well restrained and a good reduction in the pressure is observed. It also occupies a lot of space in the graph of pressure by chord. Also, at the pressure ratio of 1.1038, we see a proper formation and control of vortices to reduce the turbulence.

Plasma actuator is one of the newest devices in flow control techniques which can delay sepration by inducing external momentum to the boundary layer of the flow. The purpose of this paper, Dynamic stal behavour of a NACA0012 airfoil undergoing pitching motion has been studied by a numerical approach in the present and without plasma actuator. The oscillation frequency and amplitude and the Reynolds number were found to be the major contributors in dynamic stall. The flowfield structure and the associated vortices for this airfoil as well as the impact of the oscillation frequency on aerodynamic efficiency were also studied. The simulations were two dimensinal and the k-ω SST turbulence model were utilized for the present analysis. The results show that in without plasma actuator increasing the oscillation frequency and amplitude, postpones the dynamic stall to higher angles of attack. Furthermore, as increasing the Reynolds number, both the lift coefficient and the width of the associated hysteresis loop decrease. But when plasma actuator is on, dynamic stall not happen and aerodynamic coefficients improved. The flow field structure revealed that the main cause of the dynamic stall is a series of low pressure vortices formed at the leading edge which shed into downstream and separate from the surface. A secondary vortex will then appear and increases the lift coefficient dramatically. But when plasma actuator is on, sepration is delay and power and size vortex much reduced.

In this study, the effectiveness of three types of winglets such as Blended , Multi-tip, and Raked on a specific wing,is investigated by using a numerical method based on finite volume and pressure-based algorithms. In thisnumerical method, the Spalart-Allmaras turbulence model is used. In this simulation Reynolds number is 1.5×105and SD7032 airfoil is used for wing section by 4.6 aspect ratio. The stresses on the wing surface are calculated bythe wall functions, and the convective fluxes are computed by the second-order upwind accuracy. In this research,the specific airfoil is used for wing and winglet and the effectiveness on the aerodynamic performance of arectangular wing has been investigated. Evaluation of aerodynamic coefficients and flow physics have shown thatusing the same airfoil for wing and winglet with different angles of attack, It has increased the performance of theMulti-tip winglet by 3.1 percent compared to the case of using two airfoil and also for Blended and Multi-tipwinglets compared to the wing without winglet will increase an average aerodynamic performance by 10.5 and 10percent. But for Raked winglet, it has only a small effect on reducing the power of the vortex core.

Flow in a centrifugal electrical submersible pump is simulated. The purpose of this study is to find the effect of changing the axial distance between impeller and scroll on pump’s operation. Flow in the impeller and scroll is simulated with the Fluent software. Finite volume method with SST k-ω turbulance model is used for numerical solution. In order to evaluate the simulation results, values of head, power and efficiency are calculated at the design conditions and are compared to the experimental results. Simulations are then performed with different impeller to scroll axial distances. The results show that by increasing the distance between impeller and scroll, each of the three properties, head, power and efficiency will decrease. Increasing the distance between impeller and scroll from 0.5 to 5.0 mm, head, power and efficiency will drop by 33%, 18% and 20% respectively. Also, effect of changing the axial distance between impeller and scroll on vortices created in the fluid passing through the impeller and the axial force imposed on the impeller are calculated.