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Flow past a heated cylinder kept at constant surface temperature is computationally simulated and analyzed in the laminar regime at moderate buoyancy. In this study, we have restricted to moderate Reynolds numbers to completely eliminate the presence of mode-A and mode-B instabilities. The three dimensional transition due to the mode E instability is captured using a cell-centered finite volume method. The present study reveals the existence of two different kinds of coherent structures - the "surface plumes" and the "mushroom structures". The role of these mushroom structures in the heat transfer mechanism and the changes that the Prandtl number would bring into this coherent structure are discussed. The mushroom structures observed show high dependency on the changes in Prandtl number whereas the surface plumes are found almost unaffected. [ABSTRACT FROM AUTHOR]

Finite element solution for the classical problem of swirl flow in a cylinder with a rotating lid has been used to study the characteristic features of the stream-tube and identify the factors contributing to axial vortex breakdown. An increase of rotational Reynolds number has been found to result in (i) a decrease of total flow rate; (ii) an increase of flow rate through the boundary layer over the stationary walls; (iii) an increase of the throat area of the stream-tube, with the upstream axial vortex flow in some cases having a deficit in momentum flux needed to overcome the pressure and viscous forces; and (iv) an increase of distance for the axial flow to sustain deceleration in the diverging passage. Based on the analysis, it is hypothesized that "flow with particles in axial vortex motion having a deficit of momentum flux for axial flow when subjecting to a fluctuating radial force undergoes axial vortex breakdown." This explanation has been able to justify the disappearance of vortex breakdown at larger Re of laminar regime and the absence of vortex breakdown in small aspect ratio cylinders. We report novel results pertaining to total flow rate and its distribution within the vessel. The momentum flux of axial vortex, a main determinant of bubble breakdown, is found to be governed by the total flow rate, distribution of flow through the boundary layers, and the Reynolds number. The proposed hypothesis has been verified by analyzing two cases, one involving a passive and the other involving an active mechanism for regulating the axial momentum. [ABSTRACT FROM AUTHOR]

Methods are reported for less computationally expensive and more accurate implementations of the direct simulation Monte Carlo (DSMC) method for the simulation of high speed gas flows over arbitrarily shaped bodies. A new particle-tracking algorithm with a saving of computational time of up to 10% is reported in which tracking of particles is done with the help of big triangles having vertices lying on the boundary curves. An algorithm has been developed to generate DSMC cells for collision and sampling that contain a prescribed number of molecules. This algorithm is able to generate over 90% cells having the optimum number of seven or eight molecules for simulating collisions. Sampling for macroscopic properties is done on dynamic cells that contain a number of particles varying spatially as a function of the local number density. A criterion for finding the number of particles in sampling cells is presented. This criterion has been found to result in accurate and fast simulation of two-dimensional hypersonic flows of argon over a wedge, and argon and nitrogen over a circular cylinder. [ABSTRACT FROM AUTHOR]

Flow past a heated cylinder at constant surface temperature is computationally simulated and analyzed in the laminar regime at moderate buoyancy. The parameters governing the flow dynamics are the Reynolds number, Re, the Richardson number, Ri, and the Prandtl number, Pr. We perform our computations in the range 10 ≤ Re ≤ 35, for which the flow past an unheated cylinder results in a steady separation bubble, and vary the other two parameters in the range 0 ≤ Ri ≤ 2, 0.25 ≤ Pr ≤ 100. The heat transfer from the entire cylinder surface, quantified by the average Nusselt number Nuavg, is shown to obey Nuavg = 0.7435Re0.44Pr0.346 in the mixed convection regime we investigate. For a fixed Re and Pr, the flow downstream of the cylinder becomes asymmetric as Ri is increased from zero, followed by a complete disappearance of the vortices in the recirculation bubble beyond a threshold value of Ri. For a fixed Re and Ri, the vortices in the recirculation bubble are again observed to disappear beyond a threshold Pr, but with the reappearance of both the vortices above a larger threshold of Pr. In the limit of large Pr, the time-averaged flow outside the thermal boundary layer but within the near-wake region regains symmetry about the centerline and ultimately converges to a flow field similar to that of Ri = 0; in the far-wake region, however, we observe asymmetric vortex shedding for moderate Pr. The thermal plume structure in the cylinder wake is then discussed, and the plume generation is identified at points on the cylinder where the Nusselt number is a local minimum. The difference between the plume generation and the flow separation locations on the cylinder is shown to converge to zero in the limit of large Pr. We conclude by plotting the lift and drag coefficients as a function of Ri and Pr, observing that CD decreases with Ri for Pr < Prt (and vice versa for Pr > Prt), where Prt ≈ 7.5. [ABSTRACT FROM AUTHOR]

Flow past a heated two-dimensional cylinder in the laminar regime is investigated. A hybrid finite element–finite volume method is used to solve Navier–Stokes and energy equations. The vortex shedding frequency defined nondimensionally as Strouhal number is observed to be increasing with heating. Vortex shedding behind heated cylinder exhibits asymmetry in wake region due to stable and unstable density stratification below and above the cylinder, respectively. A negative lift is generated due to heating, in an otherwise symmetric zero time-averaged lift configuration, due to skewness in vortex shedding towards the direction of buoyancy. We also note that the heating brings down the drag for the cases considered here. The thermal energy transport due to convection, defined by nondimensional Nusselt number is found to increase only marginally with cylinder heating. [ABSTRACT FROM AUTHOR]

This paper reports a high accurate solution of the Blasius function f (ɳ) in the form of a converging Taylor's series for a higher range of ɳ ∊ [0;9]. The method used consists of conversion of the boundary value problem into an initial value problem and solution by differential transform method. The initial value of the second derivative of the Blasius function is determined from the final value of first derivative of another function. The final value of first derivative of the latter function is determined by the Taylor's series expansions with center at ɳ = 15. The series expansion for the Blasius function is obtained with center of expansion at ɳ = 4, is alternating and is accurately converging for higher values of h, with the number of used for summation equal to 2000. The present expansion is obtained without resorting to approximations and has a higher radius of convergence. The first 200 coefficients of the series, the second derivative of the function at ɳ = 0, the parameters of the asymptotic solution are reported with 21 decimal places accuracy. The level of accuracy of the results presented is higher than any other results reported so far. This note also reports the mathematical steps involved in the derivation of the similarity variable of Blasius problem. [ABSTRACT FROM AUTHOR]

A simple, robust, local divergence free, second-order accurate, stable solution procedure for incompressible flows using a finite element method for the evaluation of velocity components and a finite volume method for the evaluation of pressure is presented. In order to ensure pressure stability, the velocity components (at vertices) and pressure (at cell centres) are evaluated at different points of the elements. The finite element-finite volume combinations are taken from triangular and quadrilateral grids. These grids are used alone or together as mixed grids. A new element-cell configuration consisting of two adjacent triangular elements together as a finite volume quadrilateral cell for the evaluation of pressure is introduced. Predictions are found to be in good agreement with the available results for all types of grids used. The present method combines the pressure stability of the staggered grid finite volume method (FVM) and the flexibility of the finite element approach to analyse flow through complex geometries; it is easy to implement and locally divergence free. [ABSTRACT FROM AUTHOR]