Dust entrainment by means of a planar shock induced vortex over loose dust layers

Conclusions The recent study of Ben-Dor and Rayevsky (1994) regarding the interaction of planar shock waves with high density layers was reconsidered in order to demonstrate a possible dust entrainment mechanism which has not received appropriate attention so far. It was shown that, as a result of the interaction, a large vortex is generated. This vortex could in fact contribute to the entrainment of dust when planar shock waves interact with loose dusty layers.In addition, the effect of viscosity on the proposed dust entrainment mechanism was also considered.     

A new apparatus for studying mechanical behavior under pressure

Using a servohydraulic intensifier, a servohydraulic universal test machine and a novel test chamber, pressures of 750 MPa and axial stresses of almost any magnitude are simultaneously generated and applied to the gage section of a solid, cylindrical specimen. Under combined axial-stress/external-pressure loadings, a solid specimen experiences a truly three-dimensional, homogeneous stress state in which the axial stress equals σ₁ and the radial and circumferential stresses equal the negative of the pressure, —P. Through independent computer control of the pressure and axial stress, amterial behavior under monotonically and cyclically applied multiaxial stresses are investigated. This paper describes the experimental details of the test chamber design, high-pressure fluid production and sealing, load and strain measurement under high pressure, computer control and data acquisition. Experimental results obtained from triaxial monotonic tests on low alloy steel and triaxial cyclic tests on a Ni-Ti alloy using this unique apparatus are presented.     

A new photoelastic model for studying fatigue crack closure

The photoelastic analysis of crack tip stress intensity factors has been historically developed for use on sharp notches in brittle materials that idealize the cracked structure. This approach, while useful, is not applicable to cases where residual effects of fatigue crack development (e.g., plasticity, surface roughness) affect the applied stress intensity range. A photoelastic model of these fatigue processes has been developed using polycarbonate, which is sufficiently ductile to allow the growth of a fatigue crack. The resultant stress field has been modeled mathematically using the stress potential function approach of Muskhelishvili to predict the stresses near a loaded but closed crack in an elastic body. The model was fitted to full-field photoelastic data using a combination of a generic algorithm and the downhill simplex method. The technique offers a significant advance in the ability to characterize the behavior of fatigue cracks with plasticity-induced closure, and hence to gain new insights into the associated mechanisms.     

A new facility for time-resolved PIV measurements in rotating channels

A new facility to measure the time evolution of 2D velocity fields in a rotating channel is presented, and the accuracy is discussed in detail. Measurements are made by means of a time-resolved PIV system composed of a continuous laser diode, coupled by a fiber optics cable to a laser plane optical module, and a CMOS high-speed camera. Both the PIV system and divergent channel are fixed on a 2.5 m rotating disk. This allows a direct measurement of the relative velocity of flows with Reynolds numbers between 3 × 10³ and 3 × 10⁴ and Rotation numbers between 0.0 and 0.52. These values correspond to the flow conditions in small radial impellers and can be independently adjusted by a change of the relative flow velocity and RPM. It is shown that this new facility allows high signal-to-noise ratios, and that the direct acquisition of the data in a rotating frame drastically reduces the measurement error. The accuracy and high spatial and temporal resolution of the measurements allow a detailed analysis of boundary layer characteristics in stationary and rotating conditions.     

A note on the unsteady boundary layers over a flat plate

The unsteady boundary layer over a semi-infinite flat plate was investigated in this paper. The flow involves the unsteady flow over a flat plate with leading edge accretion or ablation. The momentum boundary layer was further analyzed and it was shown that the leading edge ablation had a similar effect to the wall mass injection or upstream wall movement making the fluid blown away from the wall. The thermal boundary layer of the same flow was also studied. Results show that the leading edge accretion or ablation can greatly change the fluid motion and the heat transfer characteristics.     

A new formulation for the analysis of elastic layers bonded to rigid surfaces

Elastic layers bonded to rigid surfaces have widely been used in many engineering applications. It is commonly accepted that while the bonded surfaces slightly influence the shear behavior of the layer, they can cause drastic changes on its compressive and bending behavior. Most of the earlier studies on this subject have been based on assumed displacement fields with assumed stress distributions, which usually lead to “average” solutions. These assumptions have somehow hindered the comprehensive study of stress/displacement distributions over the entire layer. In addition, the effects of geometric and material properties on the layer behavior could not be investigated thoroughly. In this study, a new formulation based on a modified Galerkin method developed by Mengi [Mengi, Y., 1980. A new approach for developing dynamic theories for structural elements. Part 1: Application to thermoelastic plates. International Journal of Solids and Structures 16, 1155–1168] is presented for the analysis of bonded elastic layers under their three basic deformation modes; namely, uniform compression, pure bending and apparent shear. For each mode, reduced governing equations are derived for a layer of arbitrary shape. The applications of the formulation are then exemplified by solving the governing equations for an infinite-strip-shaped layer. Closed form expressions are obtained for displacement/stress distributions and effective compression, bending and apparent shear moduli. The effects of shape factor and Poisson’s ratio on the layer behavior are also investigated.     

A new framework for studying the stability of genus-1 and genus-2 KP patterns

The Kadomtsev-Petviashvili equation - or KP equation - is a model equation for waves that are weakly two-dimensional in a horizontal plane, and models water waves in shallow water with weak three-dimensionality. It has a vast array of interesting genus—k pattern solutions which can be obtained explicitly in terms of Riemann theta functions. However the linear or nonlinear stability of these patterns has not been studied. In this paper, we present a new formulation of the KP model as a Hamiltonian system on a multi-symplectic structure. While it is well-known that the KP model is Hamiltonian - as an evolution equation in time - multi-symplecticity assigns a distinct symplectic operator for each spatial direction as well, and is independent of the integrability of the equation. The multi-symplectic framework is then used to formulate the linear stability problem for genus-1 and genus-2 patterns of the KP equation; generalizations to genus—k with k > 2 are also discussed.     

A new approach to studying the dynamics of a thin curved vortex

The paper studies the dynamics of a thin curved vortex in a potential flow of an ideal incompressible fluid. The flow is specified by a number of geometrical restrictions and does not satisfy the Biot–Savart law. The form of the derived equation of the vortex dynamics coincides with the form of the wellknown equation of local induction for selfinduced vortex motion. The parameters of the new equation are simultaneously flow parameters, and in this sense, they do not show uncertainty typical of classical equations. The coefficient of the new equation can take any specified values (not necessarily much greater than unity, as required according to the concept of local induction) and generally is a function of a natural filament parameter.     

A flow environment for studying vortex interactions

The vortex formed at the tip of a propeller interacting with the vortex formed at the tip of a stator vane provides a unique environment for the study of vortex interactions. Changes in the relative vortex strengths and vortex rotational directions were determined to impact the resulting vortex structures and are easily implemented with the experimental apparatus described herein. Study of the development of the vortex interaction was determined to be possible by increasing the initial separation between the two vortices. Vortex interaction phenomenon has been observed using smoke flow visualization.The authors would like to thank the NASA Lewis Research Center for their funding of propeller related research from which this experiment evolved and the National Sciences and Engineering Research Council of Canada for R. Johnston's Post Graduate Scholarships.     

A new sliding joint to accommodate recoil of a free-piston-driven expansion tube facility

This paper describes a new device to decouple free-piston driver recoil and its associated mechanical vibration from the acceleration tube and test section of The University of Queensland’s X3 expansion tube. A sliding joint is introduced to the acceleration tube which axially decouples the facility at this station. When the facility is fired, the upstream section of the facility, which includes the free-piston driver, can recoil upstream freely. The downstream acceleration tube remains stationary. This arrangement provides two important benefits. Firstly, it eliminates nozzle movement relative to the test section before and during the experiment. This has benefits in terms of experimental setup and alignment. Secondly, it prevents transmission of mechanical disturbances from the free-piston driver to the acceleration tube, thereby eliminating mechanically-induced transducer noise in the sensitive pressure transducers installed in this low-pressure tube. This paper details the new design, and presents experimental confirmation of its performance.     

A general theory for two- and three-dimensional wall-mode instabilities in boundary layers over isotropic and anisotropic compliant walls

An asymptotic theory is developed for two- and three-dimensional disturbances growing in a two-dimensional boundary layer over a compliant wall. The theory exploits the multideck structure of the boundary layer to derive asymptotic approximations at a high Reynolds number for the perturbation wall pressure and viscous stresses. These quantities can be regarded as driving the wall and, accordingly, the equation(s) of motion for the wall is (are) used as the characteristic equation(s) for finding the eigenvalue(s). The main assumptions are that the amplitude of the disturbance is sufficiently small for linear theory to hold, the Reynolds number is large, the disturbance wavelength is long compared with the boundary-layer thickness, and the critical and viscous wall layers are well separated. The theory was developed to study the travelling-wave flutter instability discussed by Carpenter and Garrad, i.e., the Class B instability of Benjamin and Landahl. Under certain limiting processes both the upper-branch and conventional triple-deck scalings for the Tollmien-Schlichting instability can be obtained with the present approach. Accordingly, the theory also gives a reliable qualitative guide to the effect of anisotropic wall compliance on the Tollmien-Schlichting instability.The theory is applied to various cases including two- and three-dimensional disturbances, developing in boundary layers over isotropic and anisotropic compliant walls. The disturbances can be treated as either temporally or spatially growing. Eigenvalues are very accurately predicted by means of the theory, especially near points of neutral stability. The computational requirements are trivial compared with those required for full numerical solution of the Orr-Sommerfeld equation. For isotropic compliant walls the theory confirms the earlier result of Miles and Benjamin that the phase shift in the disturbance velocity across the critical layer plays a dominant role in destabilization of the Class B travelling-wave flutter through making irreversible energy transfer possible due to the work done by the fluctuating pressure at the wall. The theory elucidates the secondary role played by the phase shift occurring across the wall layer. Viscous effects are much more important for anisotropic compliant walls which admit substantial horizontal, as well as vertical, displacement. For these walls an important mechanism for irreversible energy transfer is the work done by fluctuating shear stress. This almost invariably has a stabilizing effect on the travelling-wave flutter. In addition there is a weaker effect arising from the effect of anisotropic wall compliance on the phase shift across the wall layer. This may be stabilizing or destabilizing.This work was carried out with the support of the Ministry of Defence (Procurement Executive) and the Office of Naval Research and was completed while P.W.C. and J.S.B.G. were on study leave at the Department of Aerospace Engineering, The Pennsylvania State University, and the Department of Mathematics, Iowa State University, Ames, respectively. They would like to express their gratitude to those institutions and the Office of Naval Research for financial support during their study leaves.     

弹性半平面中边界裂纹研究的新方法

讨论了载荷作用在裂纹面上的弹性半平面边界裂纹问题.研究以线弹性断裂力学为基础,采用复变函数方法以及Riemann-Hilbert(R-H)边值问题的一般理论,将问题分拆为含有限裂纹的全平面问题与无裂纹的半平面问题的叠加,计算得到裂纹尖端的应力强度因子.与文献结果比较,该方法具有精度高的优点.     

A model for adiabatic supersonic turbulent boundary layers

An asymptotic analysis of the equations describing supersonic turbulent flow over an adiabatic wall is carried out for high Reynolds numbers, Re, and mainstream Mach numbers, M _e=O(1). A general expression for the adiabatic-wall temperature is derived. The asymptotic theory constrains the types of turbulence models that are suitable to represent the effects of viscous dissipation. A simple algebraic turbulence model is proposed and comparisons with measured total enthalpy profile data show good agreement, capturing the overshoot observed in total enthalpy near the boundarylayer edge.This work was supported by NASA Langley Research Center under Grant NAG-1-832 and the Air Force Office of Scientific Research under Grants AFOSR-91-0069 and F49620-93-0130; Dr. Ruban was supported by a grant from United Technologies Corporation.     

A simple equilibrium model for shallow-cumulus-topped mixed layers

A new equilibrium model for shallow-cumulus-topped mixed layers is presented. A variant on the w _* closure for the shallow-cumulus mass flux is applied that retains the convective area-fraction in its formulation. As opposed to being constant, the fraction is explicitly modeled using a statistical closure as a function of the saturation deficit and humidity variance at cloud base. As a consequence, important new interactions are introduced between the convective transport, humidity, and depth of the mixed layer. This mechanism, which we call the mass-flux humidity feedback, helps determine the character of the equilibrium state such that the mixed-layer top is maintained close to the cloud-base height. Due to the strong sensitivity of the mass flux to the area fraction, the latter thus acts as a regulator or valve mechanism on moist convective transport. As a consequence, the mixed-layer model is able to explain the robustness of many aspects of the shallow-cumulus boundary layer that is typically found in observations and large-eddy simulations (LESs). The model is evaluated for a single-LES case as well as for global climatology obtained from a 40-year reanalysis of meteorological data by the European Centre for Medium-range Weather Forecasts (ECMWF). LES characteristics of convective mass flux, cloud fraction, humidity variance, cloud-base height, and surface fluxes of heat and humidity are reproduced. The solution on reanalysis fields reproduces the spatial structure of mixed-layer temperature and humidity and their associated surface fluxes in the subtropical Atlantic and Pacific trade wind regions. Furthermore, the spatial structure of the convective area-fraction matches that of synoptic surface observations of frequency of occurrence of shallow cumulus. Particularly striking is the smooth onset of the convective area-fraction and mass flux along the trade-wind trajectory that is reproduced, from zero to typical trade-wind values. The cumulus onset represents the necessity for shallow-cumulus mass flux to occur in order to close the mixed-layer budgets of heat, moisture, and mass, as a response to the changing magnitude of large-scale subsidence and free tropospheric humidity along the trajectory. Finally, the mass flux model is implemented in an intermediate-complexity tropical climate model to study its behavior when fully interactive with the larger-scale flow. A climate run then shows that the model is stable, due to the mass-flux humidity feedback acting to keep the shallow-cumulus boundary layer close to its equilibrium state for long, climatological timescales.     

A numerical technique for three-dimensional compressible boundary layers

The non-linear two-point boundary value problem for three-dimensional compressible boundary layers is solved through the application of a boundary value technique for a range of parameters characterizing the nature of stagnation point flows. The analytical boundary conditions, at infinity, are applied at the edge of the computational mesh with iterations on the size of the domain. The solutions obtained show excellent agreement with the established similarity solutions for three-dimensional flows. The present method has the potential advantage of yielding the wall values of f″_w, g″_w and θ′_w as a part of the solution, contrary to the previously used ‘shooting’ methods. The algorithm is computationally simple and numerically stable and extremely suitable for engineering design applications.     

Development of a new mixing rheometer for studying rheological behaviour of concentrated energetic suspensions

The overall objective is to present a procedure based on a Couette analogy to quantitatively analyse torque/rotor speed data and extract viscosity/shear-rate curves using a non-conventional geometry. Diphasic flows of energetic concentrated suspensions of melt-cast insensitive explosives exhibit particular rheological properties. The characterization of these complex fluids may be a challenging task when conventional rheometers are used. Placing these dense suspensions in a classic cylindrical geometry may lead to a partial destruction of the internal fluid structure. To prevent that, a “RheoXF” a mixer-type rheometer has been developed: it consists of a mixing device with quite a complex geometry rotating in a cylindrical tank. To evaluate the rheological constants (virtual radius, virtual shear rate and stress constants) of thus mixing rheometer, we used five Newtonian fluids. After this calibration, the rheological characterizations were carried out on five formulations. The unique parameter which changes in these formulations is the batch's origin of a secondary explosive: the 3-nitro-1,2,4-triazole-5-one. These energetic particles differ by their morphology, maximum packing density and may be by their process synthesis. After having determined pseudoplastic parameters, a correlation has been made with the evolution of maximum packing density values calculated with De Larrard model.