Vortex sound

Vortex motion is the only source of aerodynamic sound production in low Mach number flow: the unsteady part of the vorticity distribution contributes linearly to the sound field. The following fundamental model flows, which illustrate the vorticity as the predominant sound source in unsteady flows, are discussed: An initially planar elliptic vortex; two identical coaxial initially elliptic vortex rings, where a special case is the leap-frogging of two identical circular rings. For head-on collision of two identical circular vortex rings and for several cases of vortex-body interaction good agreement between theory and experiment exists. If the Mach number is not low, other mechanisms have also to be considered. Here the theory is not yet fully developed. Experimental results for a vortex-airfoil interaction in transonic flow show that local flow separation and boundary layer as well as compressibility effects play a basic role. However, if the motion of vorticity would be known in subsonic flow, essential parts of the sound field could be calculated by the theory. — In addition, it is shown that the general theory is well suited to provide a better understanding of the scattering of sound waves by vortex motion, at least for long wave lengths.     

Homogeneity of isostatic pressure-assisted sintering of agglomerated powder

This article is dedicated to the modeling of the pressure-assisted sintering of agglomerated powders by the grain-boundary and the surface diffusion transport. Agglomerates are treated as volumes with dense particle packing. Kinetics of sintering during consolidation is estimated by a direct numerical analysis of the matter redistribution by diffusion around a single neck between identical spherical particles. Type of packing is introduced into the model through the definition of the packing angle and a special symmetry boundary condition for diffusion fluxes. The numerical analysis of sintering parameters for a single neck allows the evaluation of macroscopic viscosities of the material for different types of the particle packing and enables the estimation of the densification rate of agglomerates and non-agglomerated elements of powder compacts. Calculations show that, despite low initial density and low initial viscosity of a loose powder around agglomerates, isostatic pressing cannot provide a complete equalization of local densities in an agglomerated powder. In all considered cases, agglomerates have reached final density faster than elements with looser packing.     

The blast resistance of sandwich composites with stepwise graded cores

Shock tube experiments were performed to study the dynamic response of sandwich panels with E-Glass Vinyl Ester (EVE) composite face sheets and stepwise graded styrene foam cores. Two types of core configurations, with identical areal density, were subjected to the shock wave loading. The core layers were arranged according to the density of the respective foam; configuration 1 consisted of low/middle/high density foams and configuration 2 consisted of middle/low/high density foams. The method to calculate the incident and reflected energies of the shock wave, as well as the deformation energy of the specimen, were proposed based on the shock wave pressure profiles and the high speed deflection images that were obtained. The experimental results showed that configuration 1 outperformed configuration 2 in regards to their blast resistance. Significant core material compression was observed in configuration 1, while in configuration 2 the core layers disintegrated and the front skin (blast side) fractured into two pieces along the midsection. The estimated energies were then calculated for both configurations. The total energy difference between the incident and reflected energies was almost identical, even though the deformation energy for configuration 2 was larger.     

Development of a high-speed, half-tracked truck with metal-coreless rubber track

A high speed half-tracked truck using a new type of metal-coreless rubber track for traveling and load-carrying both on- and off-road was developed. In this application a conventional embedded-metal rubber track was examined in a first step, and was found to be unsuitable because of its chain-type structure. Then a new type of positive drive metal-coreless rubber track with endless spiral cable reinforcement was developed to meet all of the desired performance, i.e. low driving resistance, low heat build-up due to flexing, low noise, low vibration, light weight, good flexibility, low wear, etc. The development of this track, along with the use of properly matched undercarriage and suspension, has renewed interest in the use of half-tracked trucks. This truck is based on a conventional four-wheel-drive dump truck of two tonnes payload with its rear wheels converted into rubber tracks. Both high mobility off-road and traveling speed faster than 60 km/h on-road are obtained without detracking of the rubber tracks from the undercarriage. Therefore, the truck is capable of transporting its load between on- and off-road without transshipment. An identical driver's license for the same category as the original truck has been granted by the Japanese Ministry of Transportation since 1992. In this paper, the structure of the truck, the metal-coreless rubber track developed especially for high-speed traveling, the test results, and some market applications are reported.     

预折纹管在低速冲击载荷作用下的能量吸收

为了降低结构的初始载荷、增加有效塑性变形面积,进而提高其吸能效率,研究一种以新型的预折纹管,在普通管的管壁上引入特别设计的折角。基于有限元软件ABAQUS/EXPLICIT的数值分析验证了预折纹在低速冲击载荷作用下可以引导预期的大变形模式,预折纹管的这种大变形模式相较于普通方管的对称变形模式有更低峰值载荷和更高的平均载荷。通过低速落锤实验获得了与有限元模拟结果相似的载荷-位移曲线和变形模式,验证了数值结果的可信性和预折纹方管的高效吸能特点。

     

A study of the causes of cracks in bed plates of a ship's diesel engines

In cargo vessels, large machines such as engines in which part of the assembly consists of cast-iron structures are often exposed to the risk of crack formation. The present study is concerned with two diesel engines of identical type. A number of fractures were observed in the bed plates. Analysis of the type of loading and crack propagation shows that the fractures could have been predicted. The study shows a picture of the fracture causes which indicates fatigue due to pulsating loads. The fractures and cracks emanated from sharp notches. The material of the bed plates was cast iron of low strength. Fatigue with stable crack propagation has taken place.     

An equation set for non-equilibrium two phase flow,and an analysis of some aspects of choking,acoustic propagation,and losses in low pressure wet steam

An equation set for multidimensional, time variant, inviscid flow of a condensing vapour is presented. The equations include the effects of relative motion between the primary gas phase and the suspended liquid droplets. They have been formulated with steam turbine applications in mind but are also relevant to problems of gas-particle and liquid bubble flow.It is shown that the critical velocity in one dimensional choking of low pressure wet steam is identical with the “frozen” speed of acoustic propagation, and the variation of choking mass flow with respect to equilibrium based calculations is described. Results obtained with two different models of droplet growth are compared, and simple formulae for calculating limiting values of choking flow are given. A generalised loss coefficient including the effects of thermodynamic and kinematic non-equilibrium is introduced.     

相对弯度对低雷诺数流动中翼型动态气动力特性的影响

以固定翼微型飞行器为研究背景,研究了相对弯度对低雷诺数流动中翼型动态气动力特性的影响规律。采用Roe迎风差分格式和双时间步迭代方法,数值求解拟压缩性修正不可压Navier-Stokes方程组,给出了数值算法与实验数据的对比验证。以翼型弦长为特征长度,在Re=500~50000情况下,选取不同最大相对弯度和不同最大相对弯度位置的翼型,计算了其等速上仰时的动态气动力,结果表明后者对气动力的影响比较显著,把最大弯度位置布置在翼型弦向40%的地方要比布置在30%和50%两处所获得的动态升阻比大。     

Kinematics and normal stress differences of a viscoelastic fluid undergoing wiggle flow

An experimental program was carried out to determine the laminar regime kinematics and normal stress differences of a viscoelastic fluid in wiggle flow employing non-contact measurement techniques. The viscoelastic fluid was a 5% by weight solution of polyisobutylene dissolved in Primol 355, a high purity mineral oil.The kinematics were determined by Laser-Doppler Anemometry and compared with the data obtained for a Newtonian fluid, Primol 355, under identical flow conditions. It was found that the normalized axial velocity versus axial position curves along the centerline for both fluids superimposed at very low flow rates, an experimental verification that a viscoelastic fluid behaves like a Newtonian fluid under very low shear rates. However, at higher flow rates the behaviour of the viscoelastic fluid curves changed appreciably whereas the Newtonian fluid curves did not change at all. Thus, the effect of flow rate on viscoelastic fluid behaviour was also experimentally established.The normal stress differences were determined using a stress-birefringence apparatus. Data obtained along the centerline clearly exhibited a delayed growth of stress which should be attributed to the expected memory effects in viscoelastic fluid flow.     

Reological behaviour of concentrated monodisperse suspensions as a function of preshear conditions and temperature: an experimental study

 The influence of preshearing on the rheological behaviour of model suspensions was investigated with a stress-controlled cone-and-plate rheometer. The used matrix fluids showed Newtonian behaviour over the whole range of applied shear stresses. Highly monodisperse spherical glass spheres with various particle diameters were used as fillers. By applying steady preshearing at a low preshear stress, where a diffusion of particles can be expected, it was found for all model suspensions investigated at volume fractions ranging from 0.20 to 0.35 that the time-temperature superposition in the steady shear and in the dynamic mode holds within the chosen temperature range. Furthermore, all presheared model suspensions displayed a high and a low frequency range which are either separated by a shoulder or by a plateau value of G′ at intermediate frequencies. It could clearly be demonstrated that the low frequency range strongly depends on the preshear conditions. Hence, the features observed in the low frequency range can be attributed to a structure formation of a particulate network. In the high frequency range a frequency-dependent behaviour was observed which obeys the classical behaviour of Newtonian fluids (G′∝ω², G′′∝ω). The resulting temperature shift factors from the dynamic and the steady shear mode are identical and independent of the volume fraction and the particle size of the filler. Received: 29 November 2000 Accepted: 12 July 2001     

Suppression of the unsteady vortex wakes of a circular cylinder pair by a doublet‐like counter‐rotation

The present investigation examines the suppression of unsteady, two‐dimensional wake instabilities of a pair of identical circular cylinders, placed side‐by‐side normal to freestream at a low Reynolds number of 150. It is found that when the cylinders are counter‐rotated, unsteady vortex wakes can be completely suppressed. At fast enough rotational speeds, a virtual elliptic body is produced by a closed streamline, strongly resembling a doublet potential flow. Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental analysis of dynamic mechanical properties for artificially frozen clay by the split Hopkinson pressure bar

A device for impact compression experiments is the split Hopkinson pressure bar with a refrigerating attemperator. Data for incident and reflected waves are obtained by the measuring technique with strain gauges, and data for transmitted waves are obtained by the measuring technique with semiconductor gauges. Static compression tests of frozen clay are conducted at an identical temperature and different strain rates of 0.001 and 0.01 sec ⁻¹ . Dynamic stress-strain curves are obtained at strain rates of 360–1470 sec ⁻¹ . The low and high temperatures correspond to high and low strain rates, respectively. It is shown that both the temperature and strain rate affect the frozen soil deformation process. Different dynamic stress-strain curves obtained at the same temperature but different strain rates are found to converge. The test results indicate that frozen soil has both temperature-brittleness and impact-brittleness.     

Macroscopic theory of orientation transitions in the extensional flow of side-chain nematic polymers

A macroscopic continuum mechanical model for incompressible side-chain nematic polymers, under isothermal conditions is given. The model is a synthesis of a transient network model and the standard nematorheological model. Simplifications in the model yield constitutive equations that are identical to well known Theological models for polymer melts and for low molar mass nematics. A detailed analysis of four possible composite orientation modes of polymer backbone and mesogenic side groups in uniaxial extensional flow is given. It is shown that the thermal sensitivity of the viscoelastic parameters leads to thermally-induced orientation transitions. The extension rate sensitivity of the competition between elastic and flow orienting effects leads to flow-induced orientation transition. The role of smectic A fluctuations in thermally-induced transitions during uniaxial extensional nematic flow is elucidated. The model is able to predict and explain the experimentally observed orientation modes and thermally-induced orientation transitions of a side-chain nematic polymer subjected to uniaxial extensional flow.     

Dual cylinder inviscid vortex sound of low Mach number

The unsteady motions of an inviscid vortex under the influence of a cylinder pair in the presence of a low Mach number mean flow and the corresponding sound generation are examined in the present study. The two cylinders are in close proximity. A semi-analytical approach using the conformal mapping together with the potential theory is adopted. The results show that the vortex will interact intensively with the cylinders under the right combinations of mean flow direction and initial vortex position. Such interactions result in a high rate of change of vortex propagation velocity, strong fluctuating forces on cylinder and strong sound radiations. However, it is found that much stronger acoustic energy radiation will result when the vortex approaches the cylinder pair from the bottom than from the top, unless the mean flow is nearly perpendicular to the horizontal cylinder pair axis. Stronger sound radiation is also observed for the identical cylinder cases in general, except the flow direction is close to some critical values.     

Transition to escape in a system of coupled oscillators

We study a Hamiltonian system of coupled oscillators derived from two forced pendulums, connected with a torsional spring. The uncoupled limit is described by two identical oscillators, each possessing a homoclinic orbit separating bounded from unbounded motion. We focus on intermediate energy levels which lead to detained motions, defined as trajectories that, though unbounded as t → ∞, oscillate within the region defined by the homoclinic orbit of the unperturbed system for a long but finite time. We analyze the existence and behavior of these motions in terms of equipotential surfaces. These curves provide bounds on the motion of the system and are shown to be closed for low energies. However, above some critical energy level the equipotential curves become open. The detained trajectories are shown to arise from the region of phase space that was, for appropriate energies, stochastic. These motions remain within this region for long times before finally “leaking out” of the opening in the equipotential curves and proceeding to infinity.     

A comparative study of constant-voltage and constant-temperature hot-wire anemometers Part I: The static response

The static response of the constant-voltage anemometer (CVA) was investigated analytically for both subsonic and supersonic flow, and a corroborative experiment was performed at Mach 3.5 using both CVA and CTA systems. This experiment allowed a direct comparison of the static sensitivities of the two systems by utilizing the identical flow conditions and the same wire sensors. The subsonic analysis of the CVA indicates that the anemometer has primary sensitivity to velocity fluctuations at high overheat ratios and to temperature fluctuations at low overheat ratios. The theoretical and empirical relative static sensitivity of the CVA system to mass-flux and total-temperature variations appears very similar to that of the CTA and CCA systems over a wide range of overheat ratio. Received: 19 July 1998/Accepted: 26 April 1999     

Normal stress distribution in highly concentrated suspensions undergoing squeeze flow

Using pressure-sensitive films, the normal stress distribution is measured in suspensions of glass spheres in a Newtonian liquid undergoing constant-force squeeze flow. At volume fractions of solids up to 0.55, the normal stress distribution is independent of volume fraction and almost identical to the parabolic pressure distribution predicted for Newtonian fluids. However, at higher volume fractions, the normal stresses become an order of magnitude larger near the center and very low beyond that region. At these high volume fractions, the normal stresses decrease in the outer regions and increase in the inner regions as the squeezing proceeds. The normal stress distribution that results when the glass spheres without any fluid are subjected to squeeze flow is very similar to that for suspensions with volume fractions above 0.55, suggesting that the cause for the drastic changes in the normal stress distribution is the jamming of the particles in the suspension.     

Controlling the Cut in Contour Residual Stress Measurements of Electron Beam Welded Ti-6Al-4V Alloy Plates

The multiple cut contour method is applied to map longitudinal and transverse components of residual stress in two nominally identical 50 mm thick electron beam welded Ti-6Al-4V alloy plates, one in the as-welded condition and a second welded plate in a post weld heat treated (PWHT) condition. The accuracy and resolution of the contour method results are directly linked to the quality of the electro-discharge machining cut made. Two symmetric surface contour artefacts associated with cutting titanium, surface bowing and a flared edge, are identified and their influence on residual stresses calculated by the contour method is quantified. The former artefact is controlled by undertaking a series of cutting trials with reduced power settings to find optimal cutting conditions. The latter is mitigated by attaching 5 mm thick sacrificial plates to the wire exit side of the test specimen. The low level of noise in the measured stress profiles for both the as-welded and PWHT plates demonstrates the importance of controlling the quality of a contour cut and the added value of undertaking cutting trials.     

The alternating between complete synchronization and hybrid synchronization of hyperchaotic Lorenz system with time delay

The various cases of synchronization in two identical hyperchaotic Lorenz systems with time delay are studied. Based on Lyapunov stability theory, the sufficient conditions for achieving synchronization of two identical hyperchaotic Lorenz systems with time delay are derived, and a simple scheme only with a single linear controller is proposed. When the parameters in the response system are known, the alternating between complete synchronization and hybrid synchronization (namely, coexistence of antiphase and complete synchronization) is observed with the control feedback gain varying. Furthermore, when the parameters in the response system are unknown, for the same feedback controller, the complete synchronization and the hybrid synchronization can be obtained, respectively, as the associated parameters updated laws of the unknown parameters are chosen. Numerical simulation results are presented to demonstrate the proposed chaos synchronization scheme.