جستجوی مقالات مرتبط با کلیدواژه « عدد رینولدز » در نشریات گروه « مکانیک »

In this paper, the behavior of a non-Newtonian polymer through fluid hammer phenomenon in the pipe investigates. Special properties of this fluid such as nonlinear relation between stress and strain, having relaxation time make the pressure wave resulting from this phenomenon to behave differently from a Newtonian fluid. The system under investigation is reservoir-pipe-valve system and the equations representing the conservation of mass and momentum govern the transitional flow in the pipes. In the modeling of the equations finite difference numerical method is used. After defining non-dimensional numbers of governing equations, the effect of Deborah and Reynolds numbers on pressure historic at critical points such as at valve and midpoint investigate. The modeling results show that about investigating sensitivity to the Reynolds number, the pressure wave produced by non-Newtonian polymer shows a sensitivity similar to that of Newtonian fluids. It was also found that an increase in the Deborah number, indicating the elasticity of the polymer, affects the reduction of tensions and increases the oscillation height and consequently attenuation time of the created transient flow to be longer. It has been observed that in similar conditions, the phenomenon of line packing in viscoelastic fluid is slightly higher than in Newtonian fluid. The reason for this is definitely related to the constant characteristic of the relaxation time, which is strongly inclined to maintain the incoming potential energy and resists the damping of the transfer flow, which causes a long damping time compared to the Newtonian fluid have more.

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.

Friction plays an important role in velocity distribution, shear stress, boundary layer, energy loss, and erosion. In the pressure drop, the friction factor has a direct relation with the Reynolds Number in smooth, turbulent, and transitional flows from the transition to the turbulence. Today, among engineers and researchers in fluid science, due to its wide applications, accurate calculation of the relations governing the friction factor is of high significance. Many attempts were made to improve the most famous friction factor equation, named Colebrook Equation, in the last century and since the , the experimental data have not been fully enriched with experimental data in various Reynolds regions. The purpose of the present study was to improve the Colebrook implicit equation, provide a more accurate equation in the wider Reynolds Region, and adapt it to valid experimental and laboratory data. Therefore, the current researcher tried to perform the calculations with the least changes to the equation of Colebrook explicit and with the greatest accuracy using the experimental data. The method used in this research utilizes graph engineering software, and the first-generation solution method such as one of the three conventional methods matches the approximations and adjusts the curves to the obtained data. The number of errors in different equations in all Reynolds regions, specifically, the last equations presented, was investigated and researched as the last accurate and practical equations presented. Then, with the obtained information, the present research equation was corrected and matched with experimental data. Finally, in order to prove the accuracy of the equation of the present study, the accuracy was compared with other equations and diagrams were drawn in all common Reynolds regions. The results indicate the advantage of using this research and its equation accuracy in specific Reynolds regions compared to other equations. Accuracy and adaptation are much higher than previous values in the widely used areas.

Mixing different or identical fluids together plays an important role in many microstructures, so it is important to study the quality of mixing in microchannels. Numerical study of oxygen and nitrogen mixing in passive three-dimensional micromixer was performed by finite element method and the effect of Reynolds number, type, number and width of mixing units and obstruction in the flow path on mixing quality and pressure drop was investigated. While most previous studies have been on the mixing of liquids, in this study the mixing of gases in simple micromixers, as well as micromixers with mixing units, has been investigated. For this purpose, two new types of micromixers named A and B with new and multiple mixing units have been introduced. Mass performance evaluation of the new model's A and B with helical mixing units, compared to the base model without the mixing unit shows that the new models have a higher performance than the base model as micromixer A with five mixing units, in Reynolds 110, compared to the base model. Improves mixing quality 2 times and 1.25 times better than Model B. Also, the effect of the depth of cubic barriers in micromixer B with five mixing units indicates the improvement of mixing quality compared to the unobstructed model in higher Reynolds. As in Reynolds 110 model B with five mixing units, the mixing quality is about 95%.

In this work, a plate heat exchanger with an inconsistent flow is investigated in different working conditions. Four different models in laminar and turbulent flows with rigid and elastic plates at different Reynolds numbers have been studied, and the effect of the separator plate elasticity on the heat transfer rate and the efficiency of the heat exchanger has been investigated. A sinusoidal profile is used at the entrance to the cold passage. The results of this study showed that the elasticity of the intermediate plate in both laminar and turbulent flow has raised the heat transfer rate. Of course, in cases where the middle plane was elastic, the friction coefficient increased relative to the rigid plate. The efficiency of the heat exchanger is defined and is obtained for the heat exchanger to consider both heat transfer rate and pressure drop. Results showed that in most cases, the efficiency of the heat exchanger with elastic separator is more than one. The results showed that the maximum efficiency is 5.2 which was observed in Re=3000 and the minimum efficiency is 0.9 which occurs in Re=200.

The twist is one of the most important parameters in the design of the flying wing and tailless aircraft that causes eliminate some aerodynamic challenge at these categories of aircrafts. The present study was performed for an aerodynamic investigation of the geometrical twist at a subsonic flying wing and evaluate this parameter at different flight phases. The study geometry is a lambda-shaped flying wing that has a wing with a 56-degree sweepback. The twist angle applied on wingtips is washout, which is linearly distributed along the wingspan. The study is conducted in the framework of numerical simulation and based on solving Reynolds-Averaged Navier-Stokes (RANS) equations by finite volume method. The simulation process was performed after validation with experimental data, for twist angles of 0 and 6 degrees and range of attack angles of 5 to 20 degrees; also, to investigate the twist performance in the range of landing and take-off phase and cruise phase, studies have been performed in two different Reynolds numbers. The results show that by applying twist, the aerodynamic efficiency is improved at high angles of attack, but this characteristic will drop significantly at the zero-degree angle of attack. Also, by applying the twist, the conditions required for longitudinal stability are satisfied, and the pitch up phenomenon will be delayed.As speed increases, aerodynamic efficiency improves over a wide range of attack angles; also, aerodynamic efficiency changes due to twist increased, and twist will be more effective. Pitch moment analysis shows that as speed increment, the degree of stability will increase, and the pitch-up behavior will improve.

Dynamic stall is a phenomenon which appears due to the vortex shedding on the oscillating wing section at high angles of attack. Occurrence of the dynamic stall causes a severe decrease in the lift force and huge increase in the drag force. The Co-Flow Jet (CFJ) is one of the active flow controls to prevent this phenomenon. In this paper, the effect of this active flow control on the NACA 0025 airfoil for different Reynolds numbers is investigated. For numerical solution of the fluid flow, the Reynolds-averaged Navier-Stokes equations in two-dimensional, incompressible, and unsteady form with the SST-k-ω turbulence model is solved using an in-house computer code. The developed code is validated with the previous experiment data, and a fairly good agreement is observed. In order to investigate the effects of the CFJ, three different momentum coefficients of 0.05, 0.07 and 0.08 and five Reynolds numbers of 5×104, 7.5×104, 105, 1.5×105, and 3×105 are studied. It is found in the examined cases that the baseline airfoil in the Reynolds numbers of 105 and lower has different behavior compared to the higher Reynolds numbers; while in order to eliminate the dynamic stall, it requires more jet momentum of 0.08, while for the higher investigated Reynolds numbers, by applying the jet momentum of 0.07, the dynamic stall is completely eliminated.

Two phases flows particularly motion droplets into a second fluid have variety of application in different industries including oil and gas industry, medicine and pharmaceutical, extraction of metals, power plants as well as heat exchangers. In this paper, dynamic of a Newtonian drop falling in laminar regime is studied. A fluid which its viscosity is 340 cP is used in order to produce an inertia flow. In experimental section, de-ionized water and glycerin solutions with volume concentrations of 21:79 and 17:83 are used for drop phase. By increasing the volume of drop that leads to rising the inertia force, the drop shape is changed from spherically and a dimple at the rear of drop is appeared. Inertial forces, surface tension and hydrodynamic tension play a significant role on drop shape. Increasing the drop volume causes expanding the dimple consequently drag force is enhanced and terminal velocity of drop is decreased as well. According to the experimental observations, variations of viscosity ratio does not have a profound effect on drop deformation. Moreover, increasing the Reynolds number leads to reduction of pressure coefficient. It is shown that the experimental observations have an appropriate agreement with analytical results.

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.

Using proper dimensionless coefficients that are insensitive to various operating conditions is a crucial issue during the utilization of a yawmeter probe. These dimensionless coefficients produce the deviation angle of flow, stagnation and static pressures. In the current study, these coefficients are analyzed using SPM analytical and experimental methods. A comparison of experimental and analytical results shows that SPM analytical method predicts the flow deviation coefficients satisfactorily at the operational angle range of three-hole probe. This method also calculates the stagnation pressure coefficient precisely at the deviation angle range of ±10 degrees. The experimental results show that due to the assumption of constant speed on the probe, the analytical method cannot calculate the static pressure accurately. Experimental observations also demonstrate that velocity is increased and pressure is decreased over the probe. This is due to the suction region at the downstream of probe. Unlike analytical results, experimental observations depict that at zero degrees, the flow static pressure is equal to the average of pressure at the left and the right side of probe. Due to sensitivity of dimensionless coefficients of flow static pressure to variation of Reynolds number, various values are reported at different kinds of literature for these coefficients. These coefficients change with Reynolds number variations and their accuracies are decreased. In the current study, a new proper dimensionless coefficient is introduced which represents minimum sensitivity to Reynolds number.

In this study, the effect of heat transfers on lift and drag coefficients of two symmetric and camber airfoils with plunging motion in laminar flow is numerically investigated. The compressible Navier–Stokes equations are discretized using a finite volume method and they are solved by PIMPLE algorithm. In this simulation, fluid dynamic viscosity and thermal conductivity are considered to be variable. Also dynamic mesh method is used for oscillating motion. The results show that, by reducing the airfoil surface temperature less than the free flow temperature, lift to drag ratio increases. Moreover, influence of heat transfer on vortexes pattern at airfoils trailing edge and therefore its impact on the aerodynamic coefficients are investigated.

In the present study, three different fluids with different volumes of nano materials(Al2O3) in the Reynolds range of 3 to 48 thousand were investigated for the purpose of evaluating the effect of using nanofluid in heat exchangers. The converter in the submarine engine come to use of as a motor oil cooler. The results show that the addition of nanoparticles to the base fluid in the lower reynolds results in a higher performance factor in the exchanger. By increasing the volumetric percentage of nanoparticles, the converter's performance factor decreases. Optimum mode and maximum heat exchanger performance, in all volumetric percentages, occurs in Reynolds 20,000 for water and 12,000 for ethylene glycol. In other words, the choice of base fluid with a higher performance factor depends on the Reynolds number range. Also, this study examined sea water as a cooling fluid available to the marine diesel engine.

This work is devoted to find an appropriate criterion for thermal judging of nanofluids. The importance of this issue arises from the possible misleading which asserts an existence of extra heat transfer for nanofluid compared to the base fluid, neglecting the hydraulic effects such as increasing the pressure drop. To clarify the issue, an experimental apparatus with ability of making fixed Reynolds number and constant pumping power was constructed and thermal behavior of silicon oxide/ water nanofluid and distilled water are investigated in the laminar and developing regions. In this regard, heat transfer coefficient inside the finned air cooled heat exchanger is evaluated. Results are provided for different inlet temperatures and different nanofluid concentrations for two criteria of fixed Reynolds number and constant pumping .According to the results, the concentration of the nanoparticles in the base fluid will have a huge impact on the amount of deflection of these two measures, so that by increasing the concentration of nanofluids, the difference between these two measures becomes greater, hence in this work due to the low concentration of nanofluids, the use of any of the criteria did not make a significant difference in the results.

In this study, the blade cross-section profile of horizontal wind turbine (NREL type) is investigated in the wind tunnel. In order to measure the velocity and turbulence intensity the hotwire anemometer is used. The tests are done in the wake of the S823 airfoil at velocities of 5, 7 and 12 m/s and at angles of attack of 0°, 7°, 10°, 13° and 15° and at stations of = , , ,1,2,3. The effects of variation of angles of attack and Reynolds numbers are investigated on velocity defect, additional flow, velocity distribution and turbulence intensity and the curves of effects of changing the downstream position on the velocity distribution are presented. The results show that the additional velocity in the near of the trailing edge of airfoil is great and increasing the angles of attack increase the additional velocity and increasing the Reynolds numbers decrease the additional velocity and also asymmetric wake of mean velocity profile and increasing region of the mean velocity profile at lager attack angle than 10° can as sign of vorticity developed at region of wake airfoil.

In this study, forced convection heat transfer in channels with various cross sections area was investigated. Also, the effects of addition of nanoparticles, type of nanoparticles, and cross section area on heat transfer coefficient, Nusselt number, and pressure drop were studied. Due to the investigation of nanoparticles addition effects on convection heat transfer, three nanoparticles of Al2O3, Cuo, AgO with water as base fluid were utilized. The results showed that the heat transfer and pressure drop enhancing by addition of nanoparticles. The nanofluids of Al2O3, and AgO had the maximum and minimum heat transfer, respectively. In order to investigate the effects of cross section area on heat transfer, four square, circular, triangle, and rectangular channels were used. The circular and triangle cross sections had the minimum and maximum heat transfer coefficient and pressure drop, respectively.

Department of Aerospace Engineering, Sharif University of Technhlogy, Tehran, IranReceived: 2016.2.25, Received in revised form: 2016.6.21, Accepted: 2016.7.2In this paper, the operating parameters of a swirl injector were studied as a function of Reynolds number (Re) and Weber number (We). Spray cone angle, liquid sheet breakup length and injector discharge coefficient are the parameters which their dimensionless variations were studied to characterize the injector behavior in terms of Re and We. At the end, these experimental results were compared with those of some famous models, such as LISA. All the experiments were prformed using a simplex swirl injector. Moreover, water was used as the operating fluid sprayed in atmospheric conditions. The Shadowgraphy imaging method was used to visualize the injection spray. Experimental results show that by increasing Re, the injection regime varies from Dripping Mode to Atomization Mode at Re=3×104, and by further increasing Re, the fully developed mode (that creates a clear conical sheet) is achieved at Re=3.7×104. Also, results show that the breakup lengthvaries with We0.5Re0.6 linearly. Although increasing Re initially increases the spray cone angle, but beyond a specific Re, this trend is stopped.

In this work steady flow of fluid in shell and coiled tube heat exchangers has been simulated then analyzed. Effect of pitch, coil’s diameter, tube’s diameter, shell’s diameter, coil’s height, shell’s height and Reynolds number on the friction factor of coil side has been investigated using numerical method. Forty cases have been analyzed in numerical work. The working fluid of both sides is water which its viscosity and thermal conductivity were assumed to be dependent on temperature. The standard K-έ model was used for turbulence. Results indicate the diameter of the coil is the most effective geometrical parameter on the friction factor of the coil side so that by remaining other parameters constant, if the coil’s diameter increases 60%, the friction factor will decrease 30.6%. Also by remaining other parameters constant if the tube’s diameter is doubled the friction factor of the coil side will increase 16.5%, if the shell’s diameter is doubled the friction factor of the coil side will increase 11.7% while the effect of other geometrical parameters on the friction factor of the coil side is much less than the effect of coil’s diameter, tube’s diameter and shell’s height. Also a correlation has been proposed for prediction of the friction factor of the coil side that contains the effect of all defined geometrical parameters in addition to Reynolds number. This correlation is applicable for wide range of Reynolds number (2700

The effect of Reynolds number and inclination angle on the behavior of drops suspended on an inclined surface at high viscosity ratio is studied by numerical simulations at non-zero Reynolds number. The flow is driven by the acceleration from gravity without any pressure gradient in the flow direction. It is found that by increasing Reynolds number, drops close to the floor or close to the free surface move to the center of channel and the equilibrium position moves away from the channel floor, increasing the fluctuation energy as well. The same trend is observed when the inclination angle with respect to horizontal direction increases. That is, when the inclination angle of the channel with respect to horizontal direction increases, drops close to the floor or close to the free surface move to center of the channel. The equilibrium position moves away from the channelfloor and fluctuation energy increases.

This paper presents the results of numerical simulations of the laminar and turbulent flows inside diverging channels using random vortex methods (RVM). Random Vortex Method is a mesh-free method which solves unsteady vorticity equation instead of solving Navier-stokes equations directly to determine the velocity field. In this paper، the velocity potential، which is used as the initial condition، is derived by the Schwarz-Christoffel mapping. A FORTRAN code is developed to solve the equations and good agreement is achieved by comparing the results with the CFD code of the FLUENT. Furthermore، the effects of the angle of divergence of the channel and the Reynolds number on the flow separation are investigated.