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Precise ascertainment of the diffusion coefficient as the index of penetration of one species through another is essential for accurate physical modeling. The most precise and careful method for computation of these properties is Molecular Dynamics (MD). Unfortunately these methods have huge computational cost. The aim of this article is to introduce a suitable mesoscopic method and an efficient algorithm for calculating diffusion coefficient using this method. In this study we use Dissipative Particle Dynamics (DPD) for calculating diffusion coefficient of the water and lipid through the biological bilayer membranes. Eventually we validate our results with MD simulation and also experimental results. Then we can conclude about this method that besides being efficient from point of computational cost, the proposed algorithm is an accurate method for calculating diffusion coefficient. Therefore using this method, we are able to study the equilibrium behavior of bilayer membranes like calculating diffusion coefficient which with Molecular Dynamics simulations, it maybe takes more than hundreds of times.

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In this paper the effect of horizontal control surfaces (stern fins) angle on the drag force of the Subsea R&D Autonomous Underwater Vehicle (AUV) is investigated using both experimental fluids dynamic and numerical fluids dynamic methods. The experiments were conducted in the Subsea R&D towing tank using a 1:1 scale model of the AUV, at various stern angles and in a speed range of 1 to 3 m/s. A pair of Naca shaped struts was used to connect the AUV to the carriage dynamometer. The stern drag force was experimentally calculated at various stern angles and towing speeds. The results obtained by experimental method compared with those obtained numerically by commercial computational fluid dynamics CFX code. Both experimental and numerical results showed that as the stern angle increases, the total AUV drag force increases, and the drag force coefficient can be estimated by a second order polynomial. The results showed that, at a speed of 1.5m/s, as the stern angle increases to 45 degree, the drag coefficient increases up to 174 percent It was also observed that at a specific stern angle, the drag force due to stern fin increases with the AUV speed. Variation of axial force as a function of stern angle was determined by using both experimental and numerical methods. The results obtained by both methods showed that the expensive experiments conducted in towing tanks can be replaced by numerical simulations.

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The radios of particles in Dissipative Particle Dynamics (DPD) method is investigated numerically taking into account size and type of conservative force. In the most of previous studies, the DPD particles have been considered as a point center of repulsion with zero radios and hence sphere size is prescribed by the creation of a structure of frozen DPD particles. Although only in ideal gas state or zero conservative force the DPD point particle is meaningful and with conservative force the DPD particles have an intrinsic size which is assigned by the spherical impenetrable domain occupied by each particle when immersed in a sea of other particles. At first the appropriate method should be define to calculate the size of DPD particle. Different methods including Stokes-Einstein relation, Stokes law and radial distribution function (RDF) are studied and it is concluded that according to limitation of Stokes-Einstein and Stokes relations the RDF is the best method for evaluation of DPD particle size. In the following, the trend of DPD particle size changing and their distribution in the system with linear and exponential conservative force examined. At the end we demonstrate that the employing of exponential conservative forces for the colloid-colloid and colloid-solvent interactions but keep the conventional linear force for the solvent-solvent interactions achieve a well-dispersed suspension with different particle sizes without extra computation