Transition to turbulence on the initial section of a circular pipe

All the available data indicate that transition to turbulence in a circular pipe takes place within the initial section. This is confirmed by the conclusions of the linear theory of hydrodynamic stability, according to which the velocity profiles on the initial section of the pipe are unstable [1]. So far, however, there have been few investigations of initial-section flow at different values of the initial perturbation level ₀ at the pipe inlet and different values of the length to diameter ratio of the pipe 2/d. We have now investigated the transition to turbulence in the boundary layer on the initial section of a circular pipe for various ratios of the thickness of the layer to the radius of the pipe and various levels of initial turbulence. The transition point in the boundary layer was found experimentally, since at present there are no reliable methods of calculating it. In particular, the susceptibility problem has not been solved, i.e., the problem of the initial amplitude of the Tollmien—Schlichting wave, the development of which results in transition to turbulence. It may be assumed that the initial amplitude of this wave is determined by the interaction of higher-frequency waves on the section preceding its growth zone [2]. Moreover, different views are held concerning the mechanism of transition to turbulence at ₀ > 0.5%. At the same time, the results of the transition calculations for ₀ > 0.5% based on the three-parameter turbulence model [3] require experimental verification.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 52–56, July–August, 1985.     

A solution birefringence study of polymer elongation in spatially periodic stretching flows

The flow birefringence of dilute polymer solutions in periodically converging/diverging channels has been employed to study the dynamics of flexible chain molecules under transient stretching stresses. The onset of periodic birefringence for chains of high molecular weight is only observed after the chains have experienced several cycles of stress, at a point deep into the channel. This slow onset indicates that the solutions possess a memory on time scales much greater than that normally associated with a relaxed flexible coil. This behavior is recorded for both aqueous and nonaqueous solutions, at concentrations both above and below the entanglement concentration. Centerline birefringence, however, which is associated with purely elongational flow, is only observed for aqueous polymer solutions. An explanation for these birefringence results is suggested, based on a configuration-dependent dumbbell model for polymer chains in dilute solution.     

Stability of ribbed cylindrical shell under local edge loads

We examine a hinged cylindrical shell (l, r, h are the length, radius, wall thickness) that is regularly stiffened by stringers (k is the number of stringers; F, I, I^t are the cross-section area and the moments of inertia in bending and torsion) and frames (K₁ is the number of frames; I₁, I ₁ ^t are the moments of inertia in bending in the plane of the ring and in torsion). Compatibility of the deformation of the skin and the ribs is satisfied with respect to deflections and angles of rotation, the eccentricity of the placement of the ribs relative to the middle surface of the skin and the discreteness of arrangement of the stiffeners are not considered. To the edge of the shells there are applied cyclically K₂ local axial loads, which are represented in the form of uniformly distributed normal stresses P₂ on elementary regions (areas) of arc length b. The shell is additionally loaded by an axial force that is applied in the form of uniformly distributed over the end normal stresses P, and also by internal pressure of intensity q, applied to the side surface. The stability problem is solved in the linear formulation for the moment-free precritical state; the energy method in the Timoshenko form is used.S. P. Timoshenko Institute of Mechanics of the Ukraine Academy of Sciences, Kiev. Translated from Prikladnaya Mekhanika, Vol. 30, No. 1, pp. 45–50, 1994.     

Optimum conditions of turbulence reduction with screens

The results of an experimental investigation of the conditions of flow turbulence suppression with a set of screens, each of which operates in the optimum regime, are presented. Recommendations are made concerning the choice of the number of screens in the set, their geometric parameters and their location in the flow which guarantee the most efficient reduction of free-stream turbulence.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.1, pp. 181–191, January–February, 1993.     

Maximization of the lift/drag ratio of airfoils with a turbulent boundary layer

The problem of seeking the maximum life/drag ratio for airfoils in an incompressible separationless high-Re flow is considered in the turbulent boundary layer approximation. Within the framework of this model, the maximization of the L/D ratio is redwed to the determination of a conditional extremum for a convex functional. An explicit analytical expression for the extremal solution is obtained. The angle-of-attack dependence of the L/D ratio is demonstrated. The global maximum of the L/D ratio is determined. The results are compared with the known data of numerical optimization. Kazan’. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 177–184, May–June, 1998.     

Numerical study of boundary layer transition in flowing film evaporation on horizontal elliptical cylinder

A numerical study of the onset of longitudinal transition between turbulent and laminar regimes during the evaporation of a water film is presented. These water film streams along a horizontal elliptical tube under the simultaneous effects of gravity, pressure gradients, caused by the vapor flow and curvature, and viscous forces. At the interface of water vapor, the shear stress is supposed to be negligible. Outside the boundary layer, the vapor phase velocity is obtained from potential flow. In the analysis Von Karmans turbulence model is used and the inertia and convection terms are retained. Transfers equations are discretised by using the implicit Keller method. The effects of an initial liquid flow rate per unit of length, Froude number, temperature difference between the wall and the liquid–vapor interface and ellipticity on the transition position have been evaluated. The transition criterion has been given in term of the critical film Reynolds number (Re)_C.     

Direct Statistical Simulation of a Supersonic Flow of a Binary Mixture of Rarefied Gases around a Transversely Positioned Cylinder

A supersonic flow of a binary mixture of gases in a wide range of rarefaction (from a flow with a Knudsen number K _n = 0.1 to a free-molecular flow) around a cylinder is studied by means of direct statistical Monte Carlo simulations (DSMC method). The influence of a small fraction of heavy particles in a light gas flow on the region of significant nonequilibrium near the cylinder and on the heat flux is considered.__________Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 5, pp. 53–59, September–October, 2005.     

Extended glare point velocimetry and sizing for bubbly flows

A novel measuring technique for bubbly flows, named glare point velocimetry and sizing (GPVS), was developed in order to measure both bubble size and velocity with high accuracy in a 2D plane. This is accomplished by observing glare points in-focus under an observation angle of 96°. When a second laser-sheet is added, even higher accuracies are obtained and the relative refractive index of the bubble can be measured. It also allows non-spherical bubbles to be rejected and arbitrary angles to be used (e.g. 90°). The accuracy of the size and refractive index measurements was found to be within 1.3%.     

Random Lattices with Fractal Bond Permeabilities

The macroscopic permeability of random lattices has been studied when the permeability of each link is a power law of its length with an exponent . When they are sufficiently long, the link lengths are shown to follow exponential laws which depend on the density. The macroscopic permeability is studied as a function of ; it is compared to a modified effective medium theory (EMT).*Author for correspondence: e-mail: adler@ipgp.jussieu.fr**e-mail: le_chic@mail.ru     

Turbulent flow past a transverse cavity with inclined side walls. 1. Flow structure

The process of vortex formation in a cavity with inclined walls, which has a moderate aspect ratio, is experimentally studied, and the distribution of pressure coefficients is measured. The angle of inclination of the side walls ϕ is varied from 30 to 90°. It is found that the flow in the cavity becomes unstable in the range of inclination angles ϕ = 60–70°. Flow reconstruction occurs, which substantially alters the surface-temperature and static-pressure distributions. Large changes in these characteristics and their nonuniform distributions for these angles are observed across the cavity on its frontal wall and on the bottom. For small angles (ϕ = 30 and 45°), the pressure on the rear wall drastically increases, which leads to a small increase in pressure averaged over the entire cavity surface. __________ Translated from PrikladnayaMekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 5, pp. 68–76, September–October, 2006.