Sharif Digital Repository

The electrohydrodynamic (EHD) vortices produced by an electric current in freely suspended liquid crystal (LC) films of N-(4-methoxybenzylidene)-4- butylaniline (MBBA), convert to a pure rotation in the presence of external electric field (E ext) perpendicular to the current direction. Here, the direction and strength of the rotation are precisely under control by our self-made device called "liquid-film motor". In this paper, we present experimental observations of the EHD fluid flow when external electric field varies from zero to a value in which pure rotation on the liquid crystal (LC) film is observed. We also show experimentally that the presence of external electric field causes a... 

Extensive low-speed wind-tunnel tests are performed to study the flow field structure over a dual-surface canard, known as split canard, in comparison to the conventional configurations. Surface pressure for both steady deflections and unsteady oscillations of the canard are measured and compared for both canard-alone and split-canard cases. The split canard shows a superior advantage at high angles of attack, where a delay in the vortex breakdown phenomena on the canard surface is observed. For the split-canard configuration, the downwash of the front canard reduces the effective angle of attack at the inboard section of the rear element which postpones formation of the leading-edge vortex.... 

Particle dispersion in the vortex flow has been one of the most interesting subjects in recent years. Bidirectional vortex flow field is an industrial sample of the rotating flow which is used to obtain advantages of better mixing and combustion. In this work penetration and dispersion quality of the particles which are entering from various positions on the vortex engine walls have been numerically predicted. Head side, end side, and sidewall are considered as the entering positions. The particle has been assumed to be a rigid sphere. Initial velocity, diameter, and density of entering the particles are assumed to be known. If the particle length scale is considered not to be comparable... 

We study the rotational motion of nano-flake ferromagnetic disks suspended in a Newtonian fluid, as a potential material owing the vortex-like magnetic configuration. Using analytical expressions for hydrodynamic, magnetic and Brownian torques, the stochastic angular momentum equation is determined in the dilute limit conditions under applied magnetic field. Results are compared against experimental ones and excellent agreement is observed. We also estimate the uncertainty in the orientation of the disks due to the Brownian torque when an external magnetic field aligns them. Interestingly, this uncertainty is roughly proportional to the ratio of thermal energy of fluid to the magnetic energy... 

This research has investigated the effect of certain geometric parameters on cooling performance of three vortex tubes. The influencing parameters include three length/diameter ratios L/D = 10, 25, 40, three nozzle cases and each case with number n = 2, 4, 6 nozzles, three cold orifice/diameter ratios β = 0.389, 0.5, 0.611 and three inlet pressures Pi = 2, 2.5 and 3 bar. The experiments are conducted based on three factors, two-level and central composite face-centred design with full factorial. The results are analysed according to the principle of response surface methodology. The goodness of fit of the regression model is inspected using the analysis of variance and F-ratio test. The... 

Underwater surface of man-made structures is quickly covered by unwanted aquatic organisms. They change the flow regime around the structure. Despite the important contributions of previous studies on the vortex induced vibration (VIV) of bluff bodies, the flow induced vibration of non-stationary cylinders covered by marine fouling appears not to have received due attentions in the literature. The current paper reports on an attempt for better understanding the marine fouling effects on the VIV of circular cylinders. A structured review of the literature related to the fluid structure interactions in marine fouled cylinders is first presented. Results of an experimental study on the VIV of... 

In this paper, flow separation control over a NACA 4415 wing section using arrays of discrete quasi-radial wall jets is numerically investigated. This novel flow control method is studied under conditions which the wing section angle of attack is α=18 degrees and Reynolds number based on chord length is Rec=550000. Contours of mean streamwise velocity indicated that quasi-radial wall jets cover wing surface rapidly. Therefore, arranging them in a spanwise row can be an effective way in adding momentum to the near wall fluid particles. According to the results, if blowing parameters of the jets be chosen correctly, arrays of quasi-radial wall jets would lead to considerable decrease in wake... 

In the present experimental study, the effect of operating parameters such as inlet and outlet pressure as well as inlet temperature on the mass separation capability of vortex tubes (VTs) for a gas mixture is investigated. Specifically, concentrating heavier components of a gas mixture in the hot outlet stream is considered. Proposing a semi-empirical index from the experimental study, the simulation of different arrangements of VTs, and the effect of recycling a portion of the hot outlet stream on its mass separation performance are investigated. Moreover, the proposed separation method by VTs is economically compared with commonly used methods by distillation columns and two-phase... 

Previous studies of analysis and prediction of marine risers responses usually focus on vortex-induced vibration (VIV) of cross-flow (CF) direction rather than in-line (IL). Recent studies show that responses of IL direction tend to dominate in some cases. Responses of long riser due to biaxial bending of IL and CF VIV are investigated. Closed-form formulas are derived for estimating maximum normal stress due to the biaxial moment of CF/IL VIV and relations for estimating biaxial stress using CF values are presented. Analytical results are compared with numerical results of the time domain model and a good correlation is observed. It is shown that for tension and bending-controlled modes of...