Axial flow over a blunt circular cylinder with and without shear layer reattachment

Flow over a circular cylinder with its axis aligned with the free stream was investigated experimentally. Both upstream and downstream faces of the cylinder are sharply truncated. The fineness ratio (length to diameter ratio) was varied and the behavior of the leading-edge separating shear layer and its effect on the wake were studied in water using both flow visualization and PIV techniques. For the moderately large fineness ratio, the shear layer reattaches with subsequent boundary layer growth, whereas over a shorter cylinder the shear layer remains detached. This causes differences in the wake recirculation region and the immediate wake patterns. The shear layer structure was analyzed using the proper orthogonal decomposition (POD). The model in the water channel was sting-mounted and in some cases the effect of model support was detected in the wake measurements. To avoid such disturbance from the model support, an experiment was initiated in air using a magnetic model support and balance system. The drag variation with fineness ratio is presented and discussed in light of the flowfield measurements.     

Pore-Scale Study of Rarefied Gas Flows Using Low-Variance Deviational Simulation Monte Carlo Method

Transport in Porous Media - Gaseous flow through ultra-tight porous media, e.g. shale and some high-performance insulation materials, is often rarefied, invalidating an analysis by the continuum...     

A machine learning method for computing quasi-potential of stochastic dynamical systems

Nonlinear Dynamics - The concept of quasi-potential plays a central role in understanding the mechanisms of rare events and characterizing the statistics of transition behaviors in stochastic...     

The effect of end conditions on the vortex-induced vibration of cylinders

In the present investigation we study the effect of end conditions on the vortex-induced vibration of an elastically mounted rigid cylinder. This work was triggered by some initial controlled vibration experiments which showed that spanwise end conditions can have a large effect on measured fluid forces on a cylinder, and this suggested that some of the disparity amongst previous free vibration studies may possibly be attributed to differences in end conditions. In the principal experiments here, we are concerned with a vertical cylinder piercing the clean free surface of a water channel, and attached to a carriage system mounted atop the channel. The upper end of the submerged cylinder is thus the free surface, while the lower end is manipulated to yield three different conditions, namely: an attached endplate; an unattached endplate fixed to the channel floor (with a variable gap between cylinder and plate); and a condition of no endplate at all. Interestingly, we find that the free vibration response for the attached and unattached endplate cases were nearly identical. One expectation was that the case without an endplate would lead to a flow around the end of the body, modifying the vortex dynamics, and thereby reducing the correlation of the induced fluid forces on the body. Surprisingly, over the entire response plot, the vibration amplitude is markedly higher in the absence of an endplate, with the exception of the peak amplitude, which remains nearly unchanged. Unexpectedly, the vibrations become much more steady at flow velocities in the vicinity of the peak response, if the endplate is removed. In a further set of experiments, we undertake controlled vibration, where we vary the gap between cylinder and endplate. We discover a large discontinuous jump in the magnitude of fluid excitation, when the gap exceeds 15% of a diameter. For larger gaps, the fluid excitation becomes independent of the gap size, effectively equivalent to having no plate at all. This study is consistent with some of the disparity between the character of vibration response plots in previous studies, if one takes into account the particular end conditions chosen in those studies.     

AN IMPROVEMENT OF THE TIME DELAYED QUASI-STEADY MODEL FOR THE OSCILLATIONS OF CIRCULAR CYLINDERS IN CROSS-FLOW

This paper is devoted to an improvement of the time-delayed quasi-steady model for the prediction of the movement-induced vibrations of tubes mounted in tandem or in an in-line bundle. The improvement lies in the physical interpretation and calculation of the time lag which is based on measured steady force coefficients. In order to calculate the time lag, a mean convection velocity between cylinders is estimated at each time step. A linear analysis of the present model gives a critical velocity which is in good agreement with an unsteady experiment found in the literature. Moreover, limit-cycle amplitudes are relatively well predicted and show that instability is triggered by the time lag, whereas amplitude is governed by nonlinear aerodynamic damping as for a Van der Pol oscillator.     

A diffusion model of the pervaporation separation of ethylene glycol-water mixtures through crosslinked poly(vinyl alcohol) membrane

The pervaporation separation of ethylene glycol-water mixtures was carried out over the full range of compositions at temperatures varying from 60 to 80°C, using chemically crosslinked PVA dense membrane which had been developed in our laboratory. A new thermodynamic diffusion coefficient equation is derived based on the modified Vigne equation. Combining Lee-Thodos equations, Wilke-Chang equations, Vrentas-Duda's free volume theory, diffusion equations and swelling equilibrium equations, the permeation fluxes of individual components in ethylene glycol-water mixture through crosslinked poly(vinyl alcohol) (PVA) dense membrane have been calculated and showed to be in agreement with the experimental values.     

Employing controlled vibrations to predict fluid forces on a cylinder undergoing vortex-induced vibration

In the present study, we measure the fluid forces on a vertical cylinder that is forced to vibrate transversely to a water channel flow, and compare directly to the forces encountered by freely vibrating cylinders, under conditions where we carefully match the amplitude, frequency, and Reynolds number (Re) of the two cases. A key point is that we use precisely the same cylinder and submerged flow configuration for both the free and controlled cases. Where the free vibration exhibits closely sinusoidal motion, the controlled sinusoidal motion yields forces in close agreement with the free vibration case. Although this result might be expected, previous comparisons have not been uniformly close, which highlights the importance of matching the experimental conditions precisely, and of accurately measuring the phase between the force and body motion. For a lightly damped system, which is perhaps the most significant case to analyze, one typically finds that the maximum response amplitude is quite unsteady. One might conventionally expect prediction of forces to be difficult in such cases. However, it is of practical significance that, even in this case, a quasi-steady approximation is effective. This is a significant point because it suggests that controlled vibration measurements for constant amplitude motion might remain applicable to free vibration systems undergoing even transient or intermittent motions.     

Numerical simulation of water entry of twin wedges

The hydrodynamic problem of twin wedges entering water vertically at constant speed is analysed based on the velocity potential theory. The gravity effect on the flow is ignored based on the assumption that the ratio of the entry speed to the acceleration due to gravity is much larger than the time scale of interest. The problem is solved using the complex velocity potential together with the boundary element method through three stages. When the body touches water, the similarity solution is obtained for each wedge in isolation. This is used as the initial solution at the second stage for the time stepping technique for each wedge in a stretched system defined through the ratio of the Cartesian system to the distance the wedge travelled into water. When the disturbed zone of each wedge begins to affect the flow generated by the other wedge, the stretched system is abandoned and the original system is used. At the third stage the full interactions between the two wedges are included. Various results are provided for the wave elevation, pressure distribution and force at different deadrise angles. They are compared with those obtained from a single wedge and the interaction effect is investigated.     

Closed form solutions of Euler–Bernoulli beams with singularities

The problem of the integration of the static governing equations of the uniform Euler–Bernoulli beam with discontinuities is studied. In particular, two types of discontinuities have been considered: flexural stiffness and slope discontinuities. Both the above mentioned discontinuities have been modeled as singularities of the flexural stiffness by means of superimposition of suitable distributions (generalized functions) to a uniform one dimensional field. Closed form solutions of governing differential equation, requiring the knowledge of the boundary conditions only, are proposed, and no continuity conditions are enforced at intermediate cross-sections where discontinuities are shown. The continuity conditions are in fact embedded in the flexural stiffness model and are automatically accounted for by the proposed integration procedure. Finally, the proposed closed form solution for the cases of slope discontinuity is compared with the solution of a beam having an internal hinge with rotational spring reproducing the slope discontinuity.