Wall shear stress measurements in the plane of symmetry of a turbulent spot

Wall shear stress measurements were made in the plane of symmetry of turbulent spots. When represented in terms of the skin friction coefficient versus momentum thickness Reynold's number, the spot's wall shear distribution is composed of four linear segments each having a different slope. The spot's zone averaged wall shear stress is approximately equal to the zone averaged Reynolds's stress distribution in a spot while the ensemble mean wall shear is 10%–15% lower than that found in a turbulent boundary layer.     

Investigation of the elastoplastic deformation and the fracture criterion of a transversally isotropic material subject to uniaxial compression in a direction parallel to the isotropy plane

The stress-strain state and fracture of a transversally isotropic material subject to uniaxial compression in a direction parallel to the isotropy plane is studied. The deformation theory of the plasticity of a transversally isotropic body was used to analyze the stress-strain state. The fracture analysis of the material is carried out using a proposed variant of the strain-strength criterion. Theoretical and experimental data on the stress-strain state and the strength of a transversally isotropic material are compared. S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 36, No. 3, pp. 67–71, March, 2000.     

A hybrid technique for the solution of unsteady Maxwell fluid with fractional derivatives due to tangential shear stress

The article describes the unsteady motion of viscoelastic fluid for a Maxwell model with fractional derivatives. The flow is produced by cylinder, considering time dependent quadratic shear stress ft² on Maxwell fluid with fractional derivatives. The fractional calculus approach is used in the constitutive relationship of Maxwell model. By applying Laplace transform with respect to time t and modified Bessel functions, semianalytical solutions for velocity function and tangential shear stress are obtained. The obtained semianalytical results are presented in transform domain, satisfy both initial and boundary conditions. Our solutions particularized to Newtonian and Maxwell fluids having typical derivatives. The inverse Laplace transform has been calculated numerically. The numerical results for velocity function are shown in Table by using MATLAB program and compared them with two other algorithms in order to provide validation of obtained results. The influence of fractional parameters and material constants on the velocity field and tangential stress is analyzed by graphs.     

Distinguishing shear banding from shear thinning in flows with a shear stress gradient

Shear banding occurs in complex fluids that exhibit a non-monotonic constitutive instability, such as wormlike micelles, and potentially also in polymeric fluids with presumably monotonic constitutive behavior. However, velocity profiles for shear thinning fluids in geometries possessing a stress gradient, such as Taylor-Couette flow, could be misidentified as shear banding. To address this, we present a model-free experimental procedure to distinguish shear banding from strong shear thinning using high-resolution velocimetry. The approach is developed and validated using simulations using the d-Giesekus model and is based upon the behavior of the width of the apparent interface between the high and low shear rate regions. It is then tested using experimental data for model wormlike micellar solutions. The method allows shear banding to be distinguished from shear thinning in cases where this difference is otherwise indistinguishable. As a by-product, it also provides an estimate of the stress diffusivities for shear banding fluids.     

Instability of a plane axisymmetric flow with a shear discontinuity in the velocity profile

It is proposed to investigate the stability of a plane axisymmetric flow with an angular velocity profile (r) such that the angular velocity is constant when r < r_O – L and r > r_O + L but varies monotonically from ₁ to ₂ near the point r_O, the thickness of the transition zone being small L r_O, whereas the change in velocity is not small ¦₂ – ₁¦ ₂, ₁. Obviously, as L O short-wave disturbances with respect to the azimuthal coordinate (k=m/r_O 1/r_O) will be unstable with a growth rate-close to the Kelvin—Helmholtz growth rate. In the case L=O (i.e., for a profile with a shear-discontinuity) we find the instability growth rate _O and show that where the thickness of the discontinuity L is finite (but small) the growth rate does not differ from _O up to terms proportional to kL 1 and 1/m 1. Using this example it is possible to investigate the effect of rotation on the flow stability. It is important to note that stabilization (or destabilization) of the flow in question by rotation occurs only for three-dimensional or axisymmetric perturbations.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 111–114, January–February, 1985.     

Study of the nonlinear response of a polymer solution to various uniaxial shear flow histories

Summary A stiffened and automated Weissenberg Rheogoniometer has been used to obtain steady-state and transient stress data on a 12 wt.% polystyrene solution. While the results agree generally with those previously reported on the same solution, the present data-acquisition system provides a more accurate recording of short-time responses. Stress growth and relaxation data were compared with the responses predicted for a finite linear viscoelastic fluid with the same relaxation time spectrum as the test solution. Simple generalizations based on such a comparison have been sought.

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Zusammenfassung Ein versteiftes und automatisiertes Weissenberg-Rheogoniometer wurde dazu verwendet, stationäre und nicht-stationäre Spannungsmessungen an einer 12%igen Polystyrol-Lösung durchzuführen. Wenngleich die Ergebnisse durchweg mit den früher für die gleiche Lösung mitgeteilten übereinstimmen, so erlaubt doch das neu eingesetzte Daten-Verarbeitungs-System eine genauere Registrierung der Kurzzeit-Signale. Die Ergebnisse der Spann- und Relaxationsversuche werden mit den Voraussagen der Theorie einer finit linearviskoelastischen Flüssigkeit verglichen, welche dasselbe Relaxationsspektrum besitzt wie die Versuchslösung. Auf einem solchen Vergleich basierende einfache Verallgemeinerungen werden untersucht.

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With 17 figures     

Diffraction of a shear plane wave in a compound elastic space with a half-infinite crack parallel to the inhomogeneity line

Problems of stress wave propagation and diffraction in elastic inhomogeneous media are undoubtedly of interest to scientists from the viewpoint of investigation of fundamental laws of dynamic processes and of the use of the results in technical and technological applications. The paper deals with the dynamic contact problem of shear plane wave diffraction at the edge of a semi-infinite crack in a compound space consisting of two elastic half-spaces. The questions related to the onset of surface waves and the wave field behavior in far-field regions are also considered.     

Derivation of a universal relation between tangential stress and shear strain intensities in describing reversible martensitic deformation within the framework of a synthetic model

Conclusion The concept of slipping can be used in designing modern phenomenological models for the nonlinear deformation of polycrystals of various nature. Among the approaches based on the concept of slipping, the synthetic approach is one of the most effective and mathematically justified. The proposed synthetic model of phase deformation was used to describe a reversible isothermal martensitic reaction. The process of accumulation and recovery of strain under loading and unloading was described. Allowance for the microstructural peculiarities of martensitic transformations leads to understanding of macroscopic regularities in the deformation behavior of polycrystals. Use of the above averaging method enables one to describe analytically reversible changes in material properties for various types of stressed states. A universal relationship between the tangential stress and shear strain intensities is derived. The constitutive relations of the model are brought to a form analogous to the relations of the deformation theory of plasticity. Good qualitative agreement with the experimental data was obtained. In addition to the transition considered, phase reactions of the first kind under different strength and thermal conditions can be described within the framework of this model. L'vov Polytechnic University, L'vov 290013, Ukraine. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 37, No. 3, pp. 178–185, May–June, 1996.     

Disturbance spectrum of a plane fluid-fluid interface in the field of tangential high-frequency vibrations underweightlessness

The spectra of time-dependent disturbances of the equilibrium state and the stability of a plane interface between two fluid layers having different but comparable densities and being in the field of tangential high-frequency vibrations under weightlessness are investigated. Plane, spiral, and three-dimensional disturbances are considered. The cases of the same and different layer thicknesses are analyzed (in the latter case one of the layers is thicker than the other by a factor of ten). It is established that it is monotonic plane disturbances that are most hazardous. It is found that at high values of a vibration parameter growing spiral oscillatory disturbances (traveling waves) appear. With intensification of the vibrations the oscillatory disturbances vanish from the spectrum. In the case of the layers of different thicknesses it is established that the wavelength of the most hazardous disturbances is of the order of the thinner layer thickness. The experimentally observed generation of alternating strata in two-layer systems in the high-frequency vibration filed under weightlessness is attributed to the growth of disturbances having the greatest growth rate. The results obtained are quantitatively compared with the experimental data.     

Structures and regimes of shear flow in a plane cavity with translating boundaries

Stability of shear flow in a plane cavity whose boundaries translate in opposite directions is analyzed by solving the nonstationary Navier-Stokes equations numerically. It is shown that, depending on the Reynolds number and the cavity aspect ratioH/R, there may exist either a single-vortex, or a multi-eddy, or an intermediate flow regime with a bridge, all of which are stable. No oscillatory regime was found forH/L=0.1–10 and Re=1–3000.Nizhnii Novgorod, Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 53–56, March–April, 1995.     

Direct numerical simulation of a turbulent plane Couette-Poiseuille flow with zero-mean shear

Direct numerical simulations of a turbulent Couette-Poiseuille flow with zero-mean-shear at the moving wall (SL-flow) is performed to examine flow features compared to those for a turbulent pure Poiseuille flow (P-flow). Profiles of the streamwise mean velocity, indicator function and ratio of production to dissipation show that the logarithmic region is significantly elongated for the SL-flow compared to that for the P-flow at a similar Reynolds number. In addition, the magnitudes of the Reynolds stresses are found to be larger in both inner and outer layers for the SL-flow than those for the P-flow. The spanwise spectra of the production term in the turbulent kinetic energy equation are examined to provide a structural basis for explaining the statistical behaviors. In addition, because the growth of the energy-containing motions extends to the outer layer further for the SL-flow due to the presence of a positive mean shear throughout the entire wall layer, the self-similar behavior of the energy balance between the production and transport terms with respect to the self-similar wavenumber is found far from the wall. We also find the increase in the number of uniform momentum zones in the SL-flow, revealing the hierarchical distribution of the energy-containing eddies which are composed of multiple uniform momentum zones. These coherent motions lead to the elongation of the logarithmic region for the SL-flow. Finally, investigation of the turbulent energy transfer process in a spectral domain for the SL-flow demonstrates importance of outer layer very-long structures, and these structures attribute to the energy transport process in an entire flow field.     

Transient contact shear stress in a layered elastic quarter space subjected to anti-plane shear loads

In this paper the transient behaviour of a contact shear stress in a layered elastic quarter space subjected to anti-plane shear loads is investigated. The loads are suddenly applied to upper and side edges of the layer. The effects of the reflected waves, the loaded position and the material properties to the contact shear stress are shown graphically.     

Characteristics of normal and tangential forces acting on a single lug during translational motion in sandy soil

Lugs (i.e., grousers) are routinely attached to the surfaces of wheels/tracks of mobile robots to enhance their ability to traverse loose sandy terrain. Much previous work has focused on how lug shape, e.g., height, affects performance; however, the goal of this study is to experimentally confirm the effects of lug motion on lug–soil forces. We measured normal and tangential forces acting on a single lug as functions of inclination angle, moving direction angle, sinkage length, horizontal displacement, and traveling speed. The experimental results were mathematically fitted by using least square method to facilitate quantitative analyses on effects of changes in these motion parameters. Moreover, we compared the measured tangential forces to values calculated from a conventional tangential force model to evaluate the effects of the lug-tip surface, which is generally ignored in existing terramechanics models. The conclusions from this study would be useful for estimating the traveling performance of locomotive mechanisms equipped with lugs, modeling interaction mechanics between lugged wheels and soil, etc.