Vortex dynamics in the wake of a mechanical fish

A novel Stereo PIV technique, with improvements over other techniques, is presented. The key feature of the new technique is the direct measurement of calibration data at each point in space on the measurement grid, so that no interpolation is necessary. This is achieved through the use of a contiguous target which can be analysed using standard PIV processing software. The technique results in three-dimensional measurements of high accuracy with a significantly simpler calibration phase. This has the benefit of improving ease of use and reducing the time taken to obtain data. Thorough error analysis shows that while previously-described error trends are correct, additional facets of the technique can be optimised to allow highly accurate results. The new technique is rigorously validated here using pure translation and rotation test cases. Finally, the technique is used to measure a complex swirling flow within a cylindrical vessel.     

多晶薄膜X-射线衍射

作者在普通X-射线衍射仪上实施了样品倾斜X-射线衍射(STD)技术,不需要专用薄膜附件(TFA)也能很好地进行薄膜结构的测定,并且给出其衍射能量方程和方位角方程,从而使理论、实验和结构解析结合为一体。目前,应用该技术已经成功地解决了一些其它简便方法所无法解决的诸如物相纵向分布变化测量等问题。本文还通过几种摩擦学用膜的测定结果阐明了该技术的应用及功能。     

Modified intelligent hybrid technique reducing experimental error over the entire target area

The intelligent hybrid technique proposed by Nishioka et al. can be used to analyze stress intensity factors from displacement data obtained by experiments with high accuracy and reliability. However, the hybrid technique suffers from inevitable error and noise involved in extracting displacement fields from the boundary experimentally the boundary obtained experimentally. In this paper, a modified intelligent hybrid technique is developed to improve the shortcomings of the current technique. The proposed hybrid technique reduces the influence of experimental errors not only on/along the boundary but also inside the target area. Taking the SaintVenant principle and the results of test simulations into account, the effectiveness of the proposed hybrid technique is discussed. Then, applying the proposed hybrid technique to the plate with a hole under tensile loading, the distribution of strain, stress, and displacement over the entire target area is calculated with high accuracy and reliability.     

Local stress measurement using the thermoelastic effect

A technique for measuring local stresses in metallic specimens is proposed and tested. The technique depends on the experimental measurement of temperature changes in stressed members due to adiabatic elastic deformation. At a free boundary in a body under plane stress, these temperature changes are directly related to the value of the tangential principal stress. The technique is suited for measurement of stress-concentration effects, since the temperature changes can be measured with thermocouples featuring extremely small junctions. A simple stress-concentration geometry, the finitewidth strip with a central circular hole, is chosen as a model system for this study. Heat transfer in this geometry due to the temperature gradients produced by elastic deformation is analyzed. It is shown that the ratio of the temperature change at a reference section to the change at the locale of the stress concentration can be used to determine the stress-concentration factor, allowing for heat-transfer effects. An experimental measurement system capable of obtaining reproducible results with the thermal-measurement technique is described, and experimental results are given for the model geometry which agree favorably with theoretical predictions. Application of the technique to other problems is discussed.     

A Procedure to Measure Biaxial Near Yield Residual Stresses Using the Deep Hole Drilling Technique

Deep Hole Drilling (DHD) is a mechanical strain relief technique used to measure residual stresses within engineering components. Such techniques measure strains or displacements when part of the component is machined away and typically assume elastic unloading. However, in components containing high levels of residual stress, elastic–plastic unloading can occur which may introduce substantial error. For the case of the DHD technique, a modification to the technique referred to here as the incremental or iDHD technique has been developed to allow such high levels of residual stress to be measured. Previous work has demonstrated the accuracy of the iDHD technique, although only for axisymmetric residual stress distributions. In the present investigation, the application of the iDHD technique has been extended to the general case of biaxial residual stress fields. Finite element simulations are first carried out to demonstrate the ability of the iDHD technique to measure biaxial residual stress. Experimental measurements were then made on shrink fit components and ring welded components containing biaxial residual stress to investigate the performance of the technique in practice. Good agreements between iDHD measurements, neutron diffraction measurements and FE predictions of the residual stresses were obtained, demonstrating the generally improved accuracy of the iDHD technique compared to the standard DHD approach.     

Three-component micro-PIV using the continuity equation and a comparison of the performance with that of stereoscopic measurements

Recently, a number of techniques have been presented for the determination of the third “out-of-plane” velocity component in micro particle image velocimetry (micro-PIV) data. In particular, the conventional macroscopic stereo-PIV technique has been converted to the micro scale by the use of stereo-microscopy. In this work a different technique is investigated, which uses conventional, two-component micro-PIV to generate velocity data on a number of planes. The in-plane velocity gradients are then calculated, which can be used in the continuity equation to produce the out-of-plane velocity gradients. These, together with the no-penetration boundary condition, can then be used to calculate the out-of-plane velocities. An algorithm is presented that is capable of handling up to one invalid vector per column of data by using a combination of first order and second order projections of the velocity. The advantage of the continuity based technique is that it uses the existing two-component micro-PIV technology, which at present is in a more advanced stage of development then stereo-microscopy based micro-PIV. The technique is investigated using a flow similar to one used previously to assess stereoscopic micro-PIV (Meas Sci Technol 17:2175–2185, 2006). This allows a comparison of the performance of the two techniques. The results show that the continuity based data agrees well with an independent computational fluid dynamics solution and has a smaller experimental uncertainty than the stereoscopic technique at a better spatial resolution. There are, however, potential limitations to the continuity based technique. These include the two-dimensionality of the data, which is limited to the planes on which the original images were taken, and the dependence of the technique on the data close to surfaces, where the experimental errors are often greatest. Stereoscopic micro-PIV does not have these limitations so, whilst at present it appears that continuity based techniques may be more accurate, there is sufficient potential for stereoscopic techniques to justify their further development.     

Dissipation estimates in turbulent flows using the zero-wire-length technique

Azad and Kassab (1989) presented a new technique for estimating dissipation in turbulent flows and they referred to the method as the zero-wire-length technique. The validity of the approach has been here checked experimentally for the flow in the far wake of a circular cylinder for which Browne et al. (1987) had obtained reasonable estimates of the dissipation. It has been found that the zero-wire-length technique provides no more than an estimate of the isotropic dissipation: the actual dissipation values cannot be estimated by this technique.     

Application of the laser beam deflection to the study of temperature fields in Rayleigh–Benard convection

An optical integral and unobtrusive method is developed to determine the temperature gradient field by measurement of laser beam deflection induced by optical index changes. The technique is inexpensive and easy to perform. It allows the study of the temperature gradient at a large number of points simultaneously (over 400/cm²). This technique is particularly adapted to two-dimensional convection flow in both unsteady and steady conditions. For illustration purposes, such a technique is applied to the roll pattern of Rayleigh-Bénard convection. The agreement between theory and experiments is fair.     

Sweep technique to measure the relative performance of dampers

The sweep technique described in this paper represents a new method for measuring the relative performance of different dampers. Based on laboratory measurements, the technique is easy to use and keeps test execution time to a minimum. It also allows measurement of the relative performance of nonlinear dampers for which it is difficult to derive the analytical expression for their mechanical impedance. In addition to describing the test setup, the paper also presents results obtained from the application of this technique to four types of commercial dampers.     

An estimate for the small parameter in the asymptotic analysis of non-linear systems by the method of averaging

The Method of Averaging is an asymptotic method that can be used to obtain approximate solutions for many parameter dependent non-linear systems. The resulting approximate solutions are as accurate as desired provided the system parameter is sufficiently small. In this paper, a general technique is developed to obtain a relationship between the magnitude of the small parameter and the permissible error in the approximate solution. The technique is then demonstrated by its application to two examples.     

Vision-based measurement of spatio-temporal deformation of excavated soil for the estimation of bucket resistive force

This paper reports a vision-based technique of measuring the spatio-temporal deformation of excavated soil for estimating the bucket resistive force. The proposed measurement technique uses two depth cameras to determine three-dimensional soil-surface displacement. The technique consists of the following two processes: the first is related to image correlation between the two cameras, and the second involves data filtering and smoothing for generating soil deformation as a continuously curved surface. The proposed technique delivers measurement accuracy to the nearest centimeter. Typical experimental results of the three-dimensional measurement of soil deformation using the proposed technique are presented in the paper. Further, this study updates an interaction model for the resistive-force estimation while a bucket excavates soil. The model introduces a correction variable that changes with the bucket wrist angle by exploiting the experimental measurement of soil deformation. The model estimates the resistive force with an error of less than one quarter of the maximum force. These updates also exhibit the effectiveness of the proposed technique.     

Calibration of the X-Ray Diffraction Technique for Stress Quantification in Metallic (Steel) Structures

The X-Ray Diffraction technique has been widely applied for decades in many industrial sectors for the quantification of residual stresses in metallic parts. The present paper describes the laboratory calibration of this technique with the aim of adapting it to the quantification of global stresses (non residual) in metallic structures, in service for civil engineering and building. A small structure specifically built for this research has been repeatedly loaded at laboratory. In each load level the global stresses in a bar of the structure have been quantified by means of X-Ray Diffraction technique. The experimental procedure allows one to discern the residual stresses and the structural (mechanical) stresses in service. The correlation between the stresses deduced experimentally and the applied stresses is excellent. As conclusion, it can be stated that the X-Ray Diffraction technique as a non-destructive technique, has been calibrated to be used for stress deduction in metallic elements in service.     

Solving free surface fluid flow problems by the minimal kinetic energy functional

In this paper a more accurate minimization technique, namely the minimal kinetic energy method, is developed and used to investigate the free surface fluid flow caused by an obstacle on the bottom of a channel whose exact shape and location are unknown a priori. The fluid flow is assumed to be two‐dimensional, steady, inviscid, incompressible, irrotational and under the effect of the gravitational force. The minimization technique is based on the combination of the boundary integral method and the variational principle technique. This technique is extensively used in identifying unknown bottom surfaces. To illustrate this technique the free surface profile to be applied in the inverse analysis has been generated following a direct formulation when the solid bottom boundary possesses a double hump/double depression, a hump in front of a step, and a depression and a hump in front of a step. For all problems considered, the numerical results are in excellent agreement with the known analytical solution. In fact the computed profiles for both the bottom and free surfaces are graphically indistinguishable from the analytical results. Copyright © 2001 John Wiley & Sons, Ltd.     

A New Procedure to Measure Near Yield Residual Stresses Using the Deep Hole Drilling Technique

Mechanical strain relief techniques for estimating the magnitude of residual stress work by measuring strains or displacements when part of the component is machined away. The underlying assumption is that such strain or displacement changes result from elastic unloading. Unfortunately, in components containing high levels of residual stress, elastic-plastic unloading may well occur, particularly when the residual stresses are highly triaxial. This paper examines the performance of one mechanical strain relief technique particularly suitable for large section components, the deep hole drilling (DHD) technique. The magnitude of error is calculated for different magnitudes of residual stress and can be substantial for residual stress states close to yield. A modification to the technique is described to allow large magnitudes of residual stress to be measured correctly. The new technique is validated using the case of a quenched cylinder where use of the standard DHD technique leads to unacceptable error. The measured residual stresses using the new technique are compared with the results obtained using the neutron diffraction technique and are shown to be in excellent agreement.     

Application of a POD-Galerkin based method to time resolved and time unresolved data for the determination of the Convective Velocity of Large-Scale Coherent Structures in High Speed Flows

Motivated by the aero-acoustic feedback loop phenomenon in high speed free jets and impinging jets, a thorough examination of a POD (Proper Orthogonal Decomposition)-Galerkin method to determine the average convection velocity of coherent structures in the shear layer is presented in this paper. The technique is shown to be applicable to both time resolved as well as time unresolved data, if the data set meets certain requirements. Using a detailed sensitivity analysis on a synthetic data set, a quantitative estimate on the required time resolution for the technique has been found, which can be useful for both experimental, as well as numerical studies investigating the aero-acoustic feedback loop in high speed flows. Moreover, some innovative ways to apply the technique are also demonstrated using a simulated data set, showing the effectiveness of the technique to any general problem in supersonic jets, heat transfer, combustion or other areas in fluid mechanics, where an advection process can be identified.     

Semi-analytical technique for isolating the pseudo-Rayleigh component of the field induced by a transiently responding submerged cylindrical shell

A semi-analytical technique is proposed for isolating the pseudo-Rayleigh (A₀) component of the field radiated into the surrounding fluid by a submerged elastic cylindrical shell. The technique is based on the simultaneous use of two fluid–shell interaction models, one based on the Reissner–Mindlin shell theory, and the other on the Kirchhoff–Love shell theory, and is aimed at offering the possibility of analyzing the pseudo-Rayleigh component in its pure form, an approach that has certain rather important advantages over analyzing the component as a part of the overall radiated hydrodynamic pattern. The technique is applied to the analysis of the radiation by two elastic shells with parameters that are common in industry, and the high computational efficiency of the technique is demonstrated.     

Surface flow visualization using the thermal wakes of small heated spots

 This note describes a surface flow visualization technique which uses the thermal wakes of an array of small heated spots to infer the local flow direction. The thermal wake is made visible using wide band thermochromic liquid crystals. The technique is illustrated using the endwall flow under a horseshoe vortex at the base of a right circular cylinder in a turbulent boundary layer. Comparisons to results generated using the oil of wintergreen technique were in good agreement. In addition to surface flow direction, the technique has the potential to be used to measure the heat transfer coefficient at each spot. Data are presented in terms of photographs of the actual visualization surface. The techniques is suitable for low speed flows. Received: 11 November 1996 / Accepted: 12 December 1997     

Evaluation of the performance of a soft X-ray charger for the bipolar charging of nanoparticles

The use of soft X-rays in a neutralizer represents an alternative technique that could replace conventional radioactive sources. In this study, we evaluated the charging characteristics of a soft X-ray aerosol neutralizer. In addition, the results from the evaluation of the soft X-ray charger were compared with results obtained using a neutralizer incorporating an ²⁴¹Am radioactive source. The tandem differential mobility analyzer technique was used previously to determine the size-dependent positive, negative, and neutral charge fractions of a soft X-ray neutralizer. This technique was used to show that the neutral fractions obtained using the soft X-ray charger agreed well with the predictions of bipolar diffusion charging theory, and that the soft X-ray charger could be used as a neutralizer for a scanning mobility particle sizer system.     

On the methods of non-linear analysis of dynamical systems

This paper is concerned with the methods of non-linear analysis of dynamical systems and the associated bifurcation and stability problems. Attention is focused on the intrinsic harmonic balancing (IHB) technique, and the interrelationship between this technique and the methods of normal forms and averaging. Recent improvements and a complex formulation of the technique, which facilitates comparisons with other methods, are described. Thus, it is demonstrated that the simplified equations of an autonomous system, obtained by both the IHB and averaging techniques are identical, and these equations are, in fact, normal forms. Hilbert's 16th problem is analyzed as an illustrative example. It is observed that the IHB technique lends itself to a symbolic computer language (MAPLE) more efficiently compared to other methods; furthermore, its efficiency increases with the complexity of the system analyzed.