Numerical visualization of vortex flow behavior in square jets in cross-flow

Direct numerical simulations have been performed in this study to visualize the flow behavior of single and multiple square jets issuing normally into a cross-flow. Three configurations are considered, a single jet located in the centre of the domain, twin jets in side-by-side (SBS) arrangement in the spanwise direction and triple jets in tandem arrangement with twin jets at the front and a third jet in downstream along the centre line. Simulation uses a jet to cross-flow velocity ratio of 2.5 and the Reynolds number 225, based on the free-stream quantities and the jet width. While the vortical structures predicted from single jet case were in good qualitatively agreement with the findings of other researchers, our results show that the process of merging between two counter-rotating vortex pairs (CRVP) in twin jets configurations is strongly dependent on the jet-to-jet edge distance. Further downstream in the far-field, results from the SBS twin jets show a most dominating larger CRVP accompanied with a smaller inner vortex pair. The observations are in good qualitative agreement with the experimental findings in the literature. The resulting flow structures of triple jets in tandem configuration have revealed, for the first time, more complicated flow interactions between individual jets and cross-flow, providing further insights of complex flow physics and its potential engineering applications.     

On the number of tube rows required to study cross-flow induced vibrations in tube banks

Careful experiments in which a triangular array of tubes with a pitch ratio of 1·375 was used have been undertaken to determine the minimum number of tube rows required to study flow induced vibration phenomena typical of a tube bank. Individual tube responses were monitored to examine the effects of different numbers of upstream and downstream tube rows. It was found that the critical tubes for fluid-elastic instability are in the third and fourth rows. From the point of view of studying vibrations induced by turbulence, vorticity phenomena and fluid-elastic instability, it is recommended that six tube rows be used.     

Experimental simulation of turbulent boundary layer induced vibrations by using a synthetic array

Of particular interest is the experimental feasibility of replacing wind tunnel measurements with a substitute experiment to simulate Turbulent Boundary Layer (TBL) induced vibrations. This paper describes the development of an experimental procedure to address this issue. The proposed approach is based on the concept of uncorrelated wall pressure plane waves, which is introduced as a prelude to this study. Concretely, an array of acoustic monopoles can be used to generate these wall pressure plane waves. To this end, the design parameters of the array are studied numerically. However, when dealing with experimental applications, the number of monopoles required being prohibitive, the principle of synthetic array is applied instead. This technique allows simulating the effect of an array of acoustic monopoles from sequential measurements. To assess the validity of the proposed approach, the whole procedure is applied to simulate TBL induced vibrations of a thin elastic plate. The results obtained are in good agreement with those obtained from random vibration theory.     

Effect of shroud eccentricity on suppression of flow induced vibrations

The object of the research described was to examine the vibrational characteristics (amplitude and frequency) and the resulting drag for an eccentrically shrouded cylinder. The results revealed that an eccentricity in any direction and of any magnitude always reduces the amplitude of vibration in the subcritical flow region in comparison with that of a concentrically shrouded cylinder. This trend was terminated within the transitional region where there was a converging of the concentric and eccentric vibrational responses. The drag of the cylinder enclosed in the axial-rod shroud was found to be less than the drag of the plain cylinder in the subcritical region but the opposite effect was obtained in the transitional region. The drag was measured on the eccentrically shrouded cylinder for various upstream, lateral and downstream eccentricities and the results were compared with those for the concentrically shrouded cylinder.     

Measurement of wall pressure fluctuations in the presence of vibrations induced by a turbulent flow

A systematic study of the methods of measuring wall pressure fluctuations against a background of intense vibrations is carried out. The method of separating the turbulent signal from noise on the basis of monitoring the level of vibration interference is considered. Active methods of vibration control are developed for a miniature receiver of turbulent pressure fluctuations. It is shown that the methods based on the spatial filtering of the noise field offer the greatest promise. The filtering properties of vibration-proof receivers and measuring systems are investigated.     

Sensitivity investigation of the different direction vibrations of a SNOM probe

In this article, we analyze the sensitivity of lateral, axial, and torsional vibration of a scanning near-field optical microscope (SNOM) probe. An exact solution for the title problem is obtained using the theory of beam and torsional vibrations. Sensitivities are obtained for a wide range of the intervening physical parameters. This paper discusses the relationship between the sensitivity of a SNOM probe and its geometry. The analysis is presented for a probe in three different cases lateral vibration, axial vibration, and torsional vibration, for a fixed-free configuration. To derive the mathematical relation between the probe and sample (by utilizing springs and damping to simulate different environments) through the process of theory analysis for different environments. Thus a universal touchless SNOM probe sensitivity controller is developed for appropriate control and adjustment of the probe sensitivity suitable under different environments. Such controller helps the SNOM probe achieve the nanoscale resolution through proper control and adjustment probe sensitivity in accordance to changes to different environments and reduce the chance of being damaged during the scan, which in the end further advances the sample inspection technique.     

Interaction of induced sound with flow past a square leading edged plate in a duct

The interaction of resonant sounds with the flow past a thick, blunt, flat plate in a rigid walled square duct has been examined. Sound pressure levels of up to 146 dB (re 20 μPa) have been recorded. It has been established that the resonant sound can initially be excited at a harmonic of the normal vortex shedding frequency. In some cases, the sound “feeds back” on the vortex shedding process causing a step change in the shedding frequency, increasing the Strouhal number for the plate by up to twice the normal value. This excited vortex shedding and associated resonances can be suppressed by locating the plate at incidence to the air flow direction. Complex duct modes can be generated by the vortex shedding resulting in different regions of the plate shedding at different Strouhal numbers.     

Large amplitude free vibrations of a square plate of variable thickness

The large amplitude free vibrations of a square plate of exponentially varying thickness are investigated by using von Kármán's equations expressed in terms of displacement components. The numerical results are given in tabular form.     

Dipolar bosons in a planar array of one-dimensional tubes

We investigate bosonic atoms or molecules interacting via dipolar interactions in a planar array of one-dimensional tubes. We consider the situation in which the dipoles are oriented perpendicular to the tubes by an external field. We find various quantum phases reaching from a "sliding Luttinger liquid" phase to a two-dimensional charge density wave ordered phase. Two different kinds of charge density wave order occur: a stripe phase in which the bosons in different tubes are aligned and a checkerboard phase. We further point out how to distinguish the occurring phases experimentally.     

Thermally induced vibrations of a viscoelastic plate

A method for the study of thermally induced vibrations of a viscoelastic plate of arbitrary shape is proposed. The method is based upon the concept of iso-amplitude contour lines on the surface of the plate. It is shown that the time behaviour can be found by assuming a normal mode expansion in terms of the eigenfunctions for the associated elastic plate problem, and the deflection is obtained by using the elastic-viscoelastic analogy. As an illustration of the technique, the vibration of a viscoelastic circular plate under a thermal shock at its centre is discussed, all details of which are illustrated by graphs.     

Forced flexural vibrations of a pipe with a liquid flow

The effect of a liquid flow on the forced vibrations of a pipe under an external driving force is studied both theoretically and experimentally. The analytic dependences of the displacement field on the frequency, the liquid flow velocity, the force application point, and the parameters of the pipe and the liquid are determined by means of the small perturbation method. Experimental results agree well with theoretical dependences. It is shown that the effect of the liquid flow on the pipe vibrations is maximal in the vicinity of the resonance of the second vibration mode.     

Stability boundaries for flow induced motions of tubes with an inclined terminal nozzle

A theoretical and experimental study is made of self-induced non-planar vibrations of a flexible tube conveying a fluid. The tube is fixed at one end and the fluid issues from a nozzle inclined to the axis of the tube at the free end. The effect of the inclined nozzle is to cause a pronounced sinusoidal static deformation of the tube. When a critical flow rate is exceeded the tube begins to flutter. The flutter oscillations occur either in the plane of the static deformation or perpendicular to this plane, depending upon the parameters of the system. An analysis of this phenomenon is carried out for small nozzle angles by a perturbation method. The results of the analysis are presented in terms of stability boundaries as a function of the system parameters. These results are confirmed qualitatively by means of simple experiments.     

On variable length induced vibrations of a vertical string

The purpose of this paper is to study the free lateral responses of vertically translating media with variable length, velocity and tension, subject to general initial conditions. The translating media are modeled as taut strings with fixed boundaries. The problem can be used as a simple model to describe the lateral vibrations of an elevator cable, for which the length changes linearly in time, or for which the length changes harmonically about a constant mean length. In this paper an initial-boundary value problem for a linear, axially moving string equation is formulated. In the given model a rigid body is attached to the lower end of the string, and the suspension of this rigid body against the guide rails is assumed to be rigid. For linearly length variations it is assumed that the axial velocity of the string is small compared to nominal wave velocity and the string mass is small compared to car mass, and for the harmonically length variations small oscillation amplitudes are assumed and it is also assumed that the string mass is small compared to the total mass of the string and the car. A multiple-timescales perturbation method is used to construct formal asymptotic approximations of the solutions to show the complicated dynamical behavior of the string. For the linearly varying length analytic approximations of the exact solution are compared with numerical solution. For the harmonically varying length it will be shown that Galerkin?s truncation method cannot be applied in all cases to obtain approximations valid on long timescales.     

The 3-D electrostatic potential surrounding a square array of surface charges

Simple scaling laws have been derived which relate the 3-D potential to the inter-surfacestate spacing, the field-plate spacing, the dielectric constants of both insulators bordering the interface, and the dimensions x,y,z. This 3-D potential, obtained by using a pair of images and Neumann boundary conditions, contains the usual 1-D potential as the zeroth harmonic in a Fourier series. The 3-D and 1-D potentials give equivalent densities of conduction electrons in an accumulation layer only for a surface state density σ < 3 × 10¹¹/cm². For higher σ, the 1-D potential seriously underestimates the amount of space charge possible. There is a low saddle point in the 3-D potential configuration which allows surface conduction electrons to transport laterally in the presence of a transverse field.