Serpil Kocabiyik

The flow induced by a rotationally oscillating and translating circular cylinder

The temporal development of two-dimensional viscous incompressible flow induced by an impulsively started circular cylinder which performs time-dependent rotational oscillations about its axis and translates at right angles to this axis is investigated. The investigation is based on the solutions of the unsteady Navier-Stokes equations. A series expansion for small times is developed. The Navier-Stokes equations are also integrated by a spectral-finite difference method for moderate values of time for both moderate and high Reynolds numbers. The numerical method is checked with the results of the analytical solution. The effects of the Reynolds number and of the forcing Strouhal number S on the laminar asymmetric flow structure in the near-wake region are studied. The lift and drag coefficients are also extracted from numerical results. An interesting phenomenon has been observed both in the flow patterns and in the behaviour of drag coefficients for S = π/2 at Reynolds number R = 500 and is discussed. For comparison purposes the start-up flow is determined numerically at a low Reynolds number and is found to be in good agreement with previous experimental predictions.

Transient numerical simulation of buoyancy driven flow adjacent to an elliptic tube

Abstract In this paper the problem of laminar, transient, two-dimensional free convective heat transfer from the surface of a horizontal elliptic tube is considered. The tube, whose surface is suddenly subjected to uniform heat flux, is placed in a quiescent Boussinesq Newtonian fluid with its major axis horizontal. The details of both flow and thermal fields are obtained by solving the full governing Navier–Stokes and energy equations. These equations, expressed in terms of stream function, vorticity and temperature, are numerically solved using an implicit spectral finite difference procedure. The parameters involved are the modified Rayleigh number, Prandtl number and axis-ratio. The investigation covers a Rayleigh number range up to 10 7 . The minor–major axis ratio of elliptic cylinder ranges between 0.05 and 0.998 and Prandtl number ranges between 0.1 and 10. The effects of these parameters on the surface temperature distribution and heat transfer coefficients are determined and the different aspects of the results are discussed for some selected cases.

Rotational oscillations of a cylinder in cross-flow

The laminar unsteady periodic flow motion found in the wake of a rotationally oscillating circular cylinder is investigated numerically at a Reynolds number of 855 over a wide range of oscillation amplitude and frequency ratio. The governing Navier–Stokes equations are integrated to determine the flow field structure for large values of the time using an accurate spectral-finite difference method, but with the boundary vorticity calculated using global vorticity conditions rather than local finite-difference approximations. The lock-on phenomenon has been predicted and its effect on the flow hydrodynamics has been determined. The numerical scheme is verified by applying it to the special case of a steadily rotating cylinder (no forced oscillations) and good qualitative agreements are found.

A Numerical Study on Shear Stress and Heat Transfer of Segmented Flow Between Parallel Plates

The shear stress and heat transfer of moving liquid slugs between two parallel plates are studied numerically. The problem is solved initially as a steady state problem for the velocity field and shear stress, and then, the thermal problem is solved. The thermal boundary condition is constant wall temperature. The fluid properties are assumed to be constant. The finite volume method is applied using the ANSYS Fluent software package. The results show good agreement with the published literature. Effects of different interface shapes (contact angles) on wall shear stress and heat transfer is discussed. Dimensionless heat transfer plots are also presented.© 2011 ASME

Review of Advances in Thermal Spreading Resistance Problems

Thermal spreading resistance problems have been studied by many different researchers over the past six decades. In this paper, the literature on thermal spreading resistance from the past 50 years is chronologically presented, and the last decade of advances are specifically described. Focus is given to recent advances since much of the literature was reviewed in a handbook chapter published in 2003. For consistency throughout the paper, the rectangular slab and cylindrical disk heat spreader are referred to as flux channels and flux tubes, respectively. The thermal spreading resistance of compound rectangular flux channels and circular flux tubes with and without contact resistance are presented. The sink plane boundary condition is modeled using convective cooling with constant and/or variable heat transfer coefficient. Furthermore, the effects of discrete cooling in the heat-sink plane, orthotropic properties, and temperature-dependent thermal conductivity are also presented.

Thermal Behavior of Rectangular Flux Channels With Discretely Specified Contact Flux and Temperature

In this paper, an analytical solution for the thermal behavior of rectangular flux channels with discretely specified boundary conditions is presented. The boundary conditions along the source plane can be a combination of contact temperatures, heat fluxes, and/or adiabatic. Convective cooling is applied along the sink plane, and the edges of the channel are assumed adiabatic. The governing equation of the system is the Laplace equation which is solved using the method of separation of variables and the least squares method. The solution is presented in the form of Fourier series expansion. As a case study, a symmetrical flux channel with a combination of five discretely specified boundary conditions, including temperature, heat flux and adiabatic conditions is considered. Temperature profile along the channel is calculated and compared with the Finite Element Method (FEM) using COMSOL commercial software package [1]. A good agreement is observed between the analytical and FEM results.Copyright

Free surface wave interaction with an oscillating cylinder

Abstract The numerical solution of the special integral form of two-dimensional continuity and unsteady Navier–Stokes equations is used to investigate vortex states of a horizontal cylinder undergoing forced oscillations in free surface water wave. This study aims to examine the consequence of degree of submergence of the cylinder beneath free surface at Froude number 0.4. Calculations are carried out for a single set of oscillation parameters at a Reynolds number of R = 200 . Two new locked-on states of vortex formation are observed in the near wake region. The emphasis is on the transition between these states, which is characterized in terms of the lift force on the cylinder and the instantaneous patterns of vortex structures and pressure contours in the near wake.

On the Validity of Two-Dimensional Heat Transfer Simulation of Moving Droplets Between Parallel Plates

Use of moving droplets between two parallel plates has been investigated widely in recent years for cooling purposes. While the real shape of the droplets is a cylinder with curved side (convex or concave) i.e. a three dimensional shape, most of the researchers assumed a two-dimensional computational domain including vertical mid plane of the droplet, which is applicable for not realistic long droplets. In this paper, the differences between these two approaches are investigated numerically, using ANSYS Fluent package.Copyright

Temperature distribution in a circular flux tube with arbitrary specified contact temperatures

Temperature profile of electronic devices is one of the key factors that should be considered for designing an effective thermal management system. In this paper, an analytical solution for temperature distribution of a circular flux tube is presented. The boundary conditions along the source plane are specified as arbitrary temperatures and adiabatic. The boundary condition along the sink plane is convective cooling and the boundary condition along the walls is adiabatic. For solving the governing equation, the method of separation of variables and the least squares method are used. A case study is presented and the results are compared with the Finite Element Method (FEM). This analytical solution helps thermal engineers to have a better understanding of the thermal behavior of electronic devices.

Effects of Film Thickness on Heat Transfer in Taylor Flows Under Constant Wall Heat Flux Boundary Condition

Numerical simulations of heat transfer in two-phase Taylor flows in microchannels have been performed for different film thicknesses. Film thickness has been changed by adjusting surface tension and consequently Capillary number to investigate effects of film thickness on heat transfer processes under constant wall flux boundary condition. As stated in the early literature, film thickness is an important factor in Taylor flow hydrodynamics and governs the ratio between the portion of liquid which is in circulation and the portion which is bypassed through the thin liquid film around gas bubbles. It has been shown that film thickness has to be considered in the heat transfer correlations for this type of flow. Something which has not been considered in previous research.Copyright