Amirmahdi Ghasemi

Computational Simulation of the Tethered Undersea Kites for Power Generation

The dynamic motion of tethered undersea kites (TUSK) is studied using numerical simulations. TUSK systems consist of a rigid-winged kite moving in an ocean current. The kite is connected by tethers to a platform on the ocean surface or anchored to the seabed. Hydrodynamic forces generated by the kite are transmitted through the tethers to a generator on the platform to produce electricity. TUSK systems are being considered as an alternative to marine turbines since the kite can move in high speed motions to increase power production compared to conventional marine turbines. The two-dimensional Navier-Stokes equations are solved on a regular structured grid that comprises the ocean current flow, and an immersed boundary method is used for the rigid kite. A two-step projection method along with Open Multi-Processing (OpenMP) is employed to solve the flow equations. The reel-out and reel-in velocities of the two tethers are adjusted to control the kite angle of attack and the resultant hydrodynamic forces. A baseline simulation was studied where a high net power output was achieved during successive kite power and retraction phases. System power output, vorticity flow fields, tether tensions, and hydrodynamic coefficients for the kite are determined. The power output results are in good agreement with established theoretical results for a kite moving in two dimensions.Copyright

Modeling and parallel computation of the non-linear interaction of rigid bodies with incompressible multi-phase flow

A computational tool is developed to capture the interaction of solid object with two-phase flow. The full two-dimensional Navier-Stokes equations are solved on a regular structured grid to resolve the flow field. The level set and the immersed boundary methods are used to capture the free surface of a fluid and a solid object, respectively. A two-step projection method along with Multi-Processing (OpenMP) is employed to solve the flow equations. The computational tool is verified based on numerical and experimental data with three scenarios: a cylinder falling into a rectangular domain due to gravity, transient vertical oscillation of a cylinder by releasing above its equilibrium position, and a dam breaking in the presence of a fixed obstacle. In the first two validation simulations, the accuracy of the immersed boundary method is verified. However the accuracy of the level set method while the computational tool can model the high density ratio is confirmed in the dam breaking simulation. The results obtained from the current method are in good agreement with experimental data and other numerical studies. The applicability of the current computational tool for the interaction of a buoy in a water wave tank with two types of waves; symmetrical and asymmetrical waves; has also been studied.

Application of Multi-Objective Optimization for Pollutants Emission Control in an Oil-Fired Furnace

This paper is aimed at the reduction of soot and NOx emissions, while maintaining reasonable temperature. For this goal, a computational model, Sprint CFD code, is incorporated with genetic algorithm (GA) to solve multi-objective optimization problem. Sprint CFD code analyzes the pollutants emissions, temperature and chemical species of the axisymmetric cylindrical furnace. An extended Genetic Algorithm called the ”Non-dominating Sorting Genetic Algorithm” (NSGA-П) is used as an optimizer thanks to its ability to derive high accurate solutions. The target purpose functions are exit temperature, NOx, and soot emissions. The design variables are air inlet axial velocity, air inlet tangential velocity, diameter of droplets and air inlet preheating. The Pareto optimum solutions obtained from Sprint-NSGA-П are very useful to obtain optimal operational conditions. The solution shows the amount of NOx and soot emissions being kept under regulated values while the exit temperature is in the range of 1890 to 1990k.

Nanoparticle migration effects at film boiling of nanofluids over a vertical plate

Purpose The purpose of this paper is to theoretically investigate the effects of nanoparticle migration on the heat transfer enhancement at film boiling of nanofluids. The modified Buongiorno model is used for modeling the nanofluids to observe the effects of nanoparticle migration. Design/methodology/approach The governing partial differential equations including continuity, momentum, energy and nanoparticle continuity are transformed to ordinary ones and solved numerically. For nanoparticle distribution, an analytical expression has been found. The results have been obtained for different parameters, including the Brownian motion to thermophoretic diffusion NBT, saturation nanoparticle volume fraction ϕsat and normal temperature difference. Findings A closed-form expression for nanoparticle distribution is obtained, and it is indicated that nanoparticle migration significantly affects the flow fields and thermophysical properties of nanofluids. It was shown that temperature gradient at heated wall grows as the migration of nanoparticles increases, which has positive effects on the heat transfer rate. However, decrement of thermal conductivity at heated wall because of nanoparticle depletion plays a negative role in heat transfer enhancement. In fact, there is a tradeoff between thermal conductivity reduction and an increment in temperature gradient at the wall, which determines the net enhancement/deterioration of the heat transfer rate. Research limitations/implications Flow has been assumed to be laminar, and the vapor temperature is constant such that boiling is the only heat transfer mechanism between the liquid-vapor interface. Also, the shear stress at the liquid-vapor interface is assumed to be negligible. The film thickness is small relative to the plate length to justify the boundary layer assumptions. Inertia forces are neglected relative to shear stress forces. Practical implications Outcomes of the present study are suitable for several heat exchange purposes such as evaporation and condensation in heat pipes, immersion, microchannel cooling of microelectronics and crystal growth. Originality/value The novelty of this paper has three aspects: modeling the film boiling of nanofluids considering the effects of nanoparticle migration; how it influences the cooling performance; and an analytical expression for the nanoparticle distribution at film boiling of nanofluids.

Parallelized numerical modeling of the interaction of a solid object with immiscible incompressible two-phase fluid flow

Purpose A numerical method is developed to capture the interaction of solid object with two-phase flow with high density ratios. The current computational tool would be the first step of accurate modeling of wave energy converters in which the immense energy of the ocean can be extracted at low cost. Design/methodology/approach The full two-dimensional Navier–Stokes equations are discretized on a regular structured grid, and the two-step projection method along with multi-processing (OpenMP) is used to efficiently solve the flow equations. The level set and the immersed boundary methods are used to capture the free surface of a fluid and a solid object, respectively. The full two-dimensional Navier–Stokes equations are solved on a regular structured grid to resolve the flow field. Level set and immersed boundary methods are used to capture the free surface of liquid and solid object, respectively. A proper contact angle between the solid object and the fluid is used to enhance the accuracy of the advection of the mass and momentum of the fluids in three-phase cells. Findings The computational tool is verified based on numerical and experimental data with two scenarios: a cylinder falling into a rectangular domain due to gravity and a dam breaking in the presence of a fixed obstacle. In the former validation simulation, the accuracy of the immersed boundary method is verified. However, the accuracy of the level set method while the computational tool can model the high-density ratio is confirmed in the dam-breaking simulation. The results obtained from the current method are in good agreement with experimental data and other numerical studies. Practical/implications The computational tool is capable of being parallelized to reduce the computational cost; therefore, an OpenMP is used to solve the flow equations. Its application is seen in the following: wind energy conversion, interaction of solid object such as wind turbine with water waves, etc. Originality/value A high efficient CFD approach method is introduced to capture the interaction of solid object with a two-phase flow where they have high-density ratio. The current method has the ability to efficiently be parallelized.

Effects of Droplet Size and Air Preheating on Soot Formation in Turbulent Combustion of Liquid Fuel

The present study is concerned with the effect of fuel droplet size, air inlet preheating and air swirl number on complex soot process in a turbulent liquid-fuelled combustor. A hybrid Eulerian-Lagrangian method is employed to model the reactive flow-field inside the combustor. Equations governing the gas phase are solved by a control volume based semi-implicit iterative procedure while the time-dependent differential equations for each sizes of the fuel droplets are integrated by a semi-analytic method. The processes leading to soot consist of both formation and combustion. Soot formation is simulated using a two-step model while a finite rate combustion model with eddy dissipation concept is implemented for soot combustion. Also, mathematical models for turbulence, combustion, and radiation are used to take account the effects of these processes. Results reveal the significant influence of liquid fuel droplet size, air inlet temperatures and swirl numbers on soot emission from turbulent spray flames. The predictions show that reduction of spray droplet size and increases of air inlet temperature and swirl numbers considerably, increases soot emission from spray flames.© 2010 ASME