Axisymmetric Separated Flow Research Articles

Numerical Analysis of Cavitation Inception and Desinence Behind Orifices

Abstract Experimental results and trends for cavitation inception and desinence behind orifices in microchannels are quite different from the data obtained during previous experiments in much larger facilities. The objective of this paper is to explain these differences via a numerical analysis. The employed computational procedure is divided into two parts. The first part is computation of an axisymmetric separated flow around the orifice. The second part is determination of characteristics of cavities appearing within separation zones. The provided analysis of the experimental data of other researchers pointed out two sources of the above-mentioned differences. First, for larger orifices, the cavities appear in the cores of drifting vortices. For such a situation, cavitation inception and desinence number increases with the inflow speed due to an impact of turbulence, but there is no such an increase for microbubbles with laminar flows. Second, because of the difficulty to measure the cavity pressure in microbubbles, cavitation number is usually defined with employment of the vapor pressure, and this leads to misinterpretation of the measurements and their trends.

Investigation of rib’s effect in cavitation on an axisymmetric separated flow over a longitudinal blunt circular cylinder

The current investigation, determines the effects of rib shape and its parameters on an inner flow in which a circular cylinder exist in the middle. The problem is solved in 2 dimensional methods and the distance from the channel’s entrance to cylinder is 125 mm. Ansys 14.5 is used for modeling the geometry and meshing of the problem and for modeling the turbulence the k − e (realizable) is employed. In this study rib and its four parameters of pitch (p), length (h), rib’s height (e) and the distance from the cylinders’ head to start of rib (a) in a cavitation flow is investigated. The results show that none of rib’s parameters has a direct relation with void fraction. The values for these parameters are different for the maximum or the minimum cavitation; namely if a rib’s pitch increases and surpasses a specific value, it will reduce the effect of the previous rib in the flow. In fact, the turbulence eradicates before it become more turbulent by the next rib. On the other hand, if the rib’s pitch is too small the flow traps between the two ribs and stagnation happens which is the key element in plummeting the pressure and forming cavity. For other parameters either no correlation with void fraction is observed.

Response of axisymmetric separated flow to its spatially localized perturbation

The flow past an axisymmetric body with laminar boundary-layer separation in a low-velocity air stream has been studied. The hot-wire technique was employed to identify the variation of velocity field induced by a local stationary perturbation of separation region at the stern of the experimental model. A large-scale influence upon the near-wall flow due to a cylinder roughness element provided on the model surface was observed. The obtained data substantiate the possibility of controlling the laminar boundary-layer separation on an axisymmetric body using a local external forcing.

Heat transfer performance and the transition to chaos of mixed convection around an isothermally heated sphere placed in a uniform, downwardly directed flow

This study aims to reveal the temporal and spatial heat transfer performance and the transition process from a steady axisymmetric separated flow to a chaotic flow of an isothermally heated sphere placed in a uniform, downwardly directed flow by experimentation and transient 3D numerical simulation. Experiments were carried out using air with a Grashof number of 3.3 × 10 5 and with several Reynolds numbers up to 1900. Four types of flow patterns appear showing that the chaotic flow around the sphere is a spatiotemporal chaos by the calculation of a set of Lyapunov exponents from the calculated time series of temperatures.

압력비에 따른 박리 노즐 유동의 수치적 해석

Axisymmetric separated flows in a converging-diverging conical nozzle are investigated through numerical simulations for various pressure ratios. We employ AUSM scheme for spatial derivatives and Pulliam`s 2nd order subiteration time stepping scheme for implicit time integration. Numerical results indicate that the separated flow structures are very complex when compared to the simple quasi-one dimensional flow. Depending on the pressure ratio, the flow within the nozzle is either separated or non-separated. Various separated flow patterns with distinctive features are illustrated and discussed in detail.

Separation control by tangential blowing inside the bubble

Experiments have been carried out investigating the effectiveness of steady tangential blowing (inside the separation bubble) to control an axisymmetric separated flow at low speeds. Turbulent boundary separation was induced on a contoured afterbody and the separated shear layer reattached on a narrow cylindrical sting. Measurements made consisted of model surface pressures, mean velocity, turbulent shear stress and kinetic energy profiles using a 2-component LDV system. The results explicitly demonstrate that blowing downstream of the separation location, but within the bubble, can be an effective means of separation control, considering both wall and wake flow reversals.

Asymptotics of steady axisymmetric flow of incompressible fluid past a bluff body at high Reynolds number

Steady axisymmetric separated flow at high Reynolds number is considered. It is shown that the body scale flow does not correspond to the Kirchhoff model because near the body there is a fairly strong reversed flow causing the secondary separation which is observed in numerical computations. Quantitative theoretical results are compared with the numerical results.

Development of an iterative boundary-layer-type solver for axisymmetric separated flows

On presente un schema de couplage totalement implicite. Comparaison de la vitesse de convergence de ce schema avec d'autres methodes de couplage. Resultats numeriques pour des ecoulements laminaires incompressibles decolles sur differentes geometries

Nonexistence of axisymmetric separated flow.

Axisymmetric separated flow in Chapman model shown nonexistent by flow visualization technique