On the motion of particles in turbulent duct flows

Existing knowledge on particle deposition rates on walls from turbulent pipe and channel flows is summarized and it is shown that discrepancies exist between experimental and theoretical findings. To contribute to the existing experimental information, laser Doppler measurements are reported of the flow field of a glass particle-air two-phase flow. The results reveal certain seemingly peculiar behaviors of the particles which obviously defy the predictions of the conventional analyses of turbulent two-phase suspension flows.In an accompanying approximate, yet pragmatic theoretical approach, an attempt is made to find a rational basis for the explanation of these experimentally observed particle behaviors. It is shown for the particles in the present study, there exists a limiting size above which their response to the agitation of the fluctuating motion of the surrounding fluid could be treated as if the flow were laminar. On this rational basis, these experimentally observed particle behaviors can then be qualitatively explained by the existing theory of particle excursion in a laminar shear flow field.Reported also is a suggestion to extend the present analysis to a dispersion of particles of multiple sizes.     

Coherent motion of turbulent thermal plume in stably stratified fluid

The purpose of this study is to clarify the existence of an ordered and large scale coherent motion in a turbulent plane thermal plume in a thermally-stable stratified fluid inside a comparatively large enclosure. First, the upper part of the thermal plume was carefully observed by a flow visualization. Secondly, a wave form of plume temperature variation was measured. Thirdly, a spectrum analysis was carried out on time series data of the thermal plume. Finally, physical characteristics were investigated on vortices in the thermal plume based on results of the wave form and the spectrum analysis of the plume temperature. As a result, the main conclusions are obtained as follows. (1) An existence of vortices near the upper part of the thermal plume was firstly found by careful flow visualization. (2) From the wave form of temperature variation and the spectrum analysis of the thermal plume, it was clarified that the vortices are generated in the transition state and are transported to the turbulent state. (3) The vortices are ordered and they behave as a large scale coherent motion in the turbulent thermal plume.     

Some restraints on the energy and spectrum of turbulent fluid motion

A thermodynamically nonequilibrium fluid tends to return to its stable equilibrium state owing to the dissipation and redistribution of part of its energy as a result of individual particle collisions. However, if the fluid deviates strongly from the state of thermodynamic equilibrium, it is more advantageous for it to go over into a stare of random motion, which permits the more rapid liquidation of the non-equilibrium condition in the active development of turbulent transfer processes exceeding in magnitude the classical processes of molecular transfer. Finding the spectrum and energy level of the resulting turbulent motion is a complex and, in some cases, mathematically impossible task. Therefore it is useful to have at least some restraints on their magnitudes, in order to be able to estimate the role of turbulent transfer processes in the pattern of evolution of the unstable state of the fluid. As these conditions we shall take the conditions of stability of the steady turbulent state of the fluid.     

On the origin of the organised motion in the turbulent far-wake of a cylinder

Spectra and spectral coherences of mainly the lateral velocity fluctuations, measured in the wake of a circular cylinder, strongly suggest that the origin of the far-wake organised motion can be traced back to the cylinder. A possible mechanism for the development of the organised motion is suggested.     

Eulerian prediction of the fluid/particle correlated motion in turbulent two-phase flows

An approximate equation governing the turbulent fluid velocity encountered along discrete particle path is used to derive the fluid/particle turbulent moments required for dispersed two-phase flows modelling. Then, closure model predictions are compared with results obtained from large-eddy simulation of particle fluctuating motion in forced isotropic fluid turbulence.     

On the non-persistence of irrotational motion in a viscous heat-conducting fluid

We consider the possibility of irrotational flow in a fluid exterior to a moving rigid obstacle, or interior to a moving rigid shell. Observations show that when a rigid body is impulsively set into motion an irrotational flow may exist initially but does not persist. The breakup of this irrotational flow and the associated phenomenon of generation of vorticity at the wall are generally attributed to the condition of adherence at the fluid-solid interface. Since this condition itself is derived from observation, one can ask whether there is another explanation for the phenomenon. The purpose of this paper is to show that a persistent irrotational flow is incompatible with the second law of thermodynamics.     

Some features of a turbulent wing–body junction vortical flow

Laser-Doppler velocimeter measurements of a wing/body junction flow field made within a plane to the side of the wing/wall junction and perpendicular both to a 3:2 elliptical nose—NACA 0020 tail wing, and a flat wall are presented. Reynolds number of the approach boundary layer was, Re_θ = 5940, and free-stream air velocity was, U_ref = 27.5 m/s. A large vortical structure residing in the outer region redirects the low-turbulence free-stream flow to the vicinity of the wing/wall junction, resulting in thin boundary layers with velocity magnitudes higher than free-stream flow. Lateral pressure gradients result in a three-dimensional separation on the uplifting side of the vortex. Additionally, a high vorticity vortical structure with opposite sense to the outer-layer vortex forms beneath the outer-layer vortex. Normal and shear stresses increase to attain values an order of magnitude larger compared to values measured in a three-dimensional boundary layer just outside the junction vortex. Bimodal histograms of the w fluctuating velocity occur under the outer-layer vortex near the wall due to the time-dependent nature of the horseshoe vortex. In such a flow the shear-stress angle (SSA) highly lags the flow-gradient angle (FGA), and the turbulence diffusion is highly altered due to presence of vortical structures.     

Analytical model of motion of turbulent vortex rings in an incompressible fluid

An analytical model describing the motion of vortex rings in an incompressible fluid is constructed. The model is valid both for homogeneous and inhomogeneous vortices buoyant in the gravity field, as well as for combined vortices. The expansion angle of a buoyant vortex is found from the characteristic parameters that define the flow rather than specified on the basis of experiments. Significant differences in the expansion angles of homogeneous and buoyant vortex rings are explained. The calculation results for the proposed model are compared with the results of laboratory experiments and data on the rise of the cloud produced by an atomic explosion.     

Elastic waves in a turbulent fluid

It is shown that a turbulent flow can support elastic waves for which the fluctuating stress is essentially proportional to the strain field. These fluctuations are described by a hyperbolic system and hence they do not decay away from a solid boundary. Their behaviour is also consistent with Taylor's frozen field hypothesis that turbulent fluctuations are advected with the local mean flow speed.     

On a model of fluid motion near a solid surface

The possibility is examined of using the model of micropolar media [1] to describe the anomalous change in the mechanical properties and the associated appearance of the scale effect in the boundary layers of certain fluids.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 70–75, May–June, 1981.     

Development of turbulent mixing in a fluid

The development of turbulent mixing produced by a linear source in a flat cell is studied with dyes and a laser thermal marker. The velocity field outside the mixing region is determined. The agreement of region size determined by dye diffusion and thermal marker deformation is shown.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 2, pp. 91–95, March–April, 1973.     

An orthogonal-plane PIV technique for the investigations of three-dimensional vortical structures in a turbulent boundary layer flow

An experimental investigation was carried out regarding a three-dimensional topology of a zero-pressure gradient turbulent boundary layer. In this study, the polarization separation technique has been applied to the PIV measurements. Two mutually perpendicular measurement planes have been employed in x–y and x–z planes, respectively. Synchronization between a stereoscopic PIV with another plane PIV system was made toward the detection of such salient features of the coherent structure as the legs and the head of the hairpin vortices. Polarization rotation via a half-waveplate and subsequent particle image separation using polarizer minimized the spurious particle images. The PIV results clearly demonstrate the presence of hairpin-like coherent vortical structures and coincidence between the near-wall quasi-streamwise vortex pair and the legs of the hairpin vortex.     

Development of vortical motions of an incompressible fluid in the cavity of a rotating body

Vortical nonstationary viscous incompressible flows in the space between coaxial cylinders or hemispherical segments rotating with a constant angular acceleration about a stationary axis of symmetry are analyzed numerically for Reynolds numbers Re — 1–10. It is shown that laminar circulating motions are realized. Two vortices form in the flow. The positions of these vortices depend substantially on the geometry of the rotating cavity.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 47–52, March–April, 1995.     

Large-scale vortical structure of turbulent separation bubble affected by unsteady wake

The large-scale vortical structure of a turbulent separation bubble under the influence of an unsteady wake was investigated. The unsteady wake was generated by a spoked-wheel type wake generator installed in front of the separation bubble. This wake generator was rotated either clockwise or counter-clockwise at Re _H=5.600. The mechanism of vortex shedding from the separation bubble was analyzed in detail by taking a conditional average as well as a phase average. Spatial box filtering (SBF) was used to extract the large-scale vortical structure from the turbulent separation bubble affected by the unsteady wake. To elucidate the influence of the unsteady wake on the large-scale vortical structure, conditional averages of the velocity, vorticity and turbulent kinetic energy were calculated. The nature of the convection of the vortical structure under the influence of an unsteady wake was analyzed. The dipole acoustic pressure level was predicted using Curle's integral of wall-pressure fluctuations.     

On the motion of particles in a fluid under the influence of a large velocity gradient

The motion of seeding particles as used in laser Doppler anemometry is investigated in the presence of a large velocity gradient across aerodynamic shocks under different flow conditions. Experimentally obtained results are presented and compared with theoretical predictions based upon the size distribution of the seeding particles used. It is found that the agreement of experimental and theoretical results depends on the flow conditions as well as on the particle material.     

Modeling of the turbulent motion of particles in a vertical channel

The results of modeling of the statistical parameters of a turbulent particle motion in a vertical plane channel are presented. The model is based on a kinetic equation for the particle velocity probability density function. The results are compared with direct numerical simulation.     

Motion of discrete particles in a turbulent fluid

Summary Various approximations to Basset's equation for the motion of a particle in a viscous fluid have been applied to the complex phenomenon of dispersion in a turbulent fluid. The deviations of the particle motion from the fluid motion, as predicted by the various approximations, is explored, and the frequencies for which this deviation is large are described. The approximations are found to be invalid for such cases as sediment transport and motion of gas bubbles in liquids. For small, 7 micron, liquid or solid particles in air, however, all approximations are shown to be valid for turbulent frequencies below 812 cps.Nomenclature a parameter in equation (2.3) - b parameter in equation (2.3) - c parameter in equation (2.3) - d diameter of sphere - E _f energy spectrum of the fluid - E _p energy spectrum of the particle - F frequency of oscillation - f ₁ parameter defined by equation (2.10) - f ₂ parameter defined by equation (2.10) - g acceleration of gravity - N _S , Stokes number - s density ratio - t time - t ₀ initial time - u _f fluid velocity - u _p particle velocity - V velocity of sphere - phase angle - parameter in equation (2.8) - amplitude ratio - parameter in equation (2.8) - dynamic viscosity - kinematic viscosity - f density of the fluid - p density of the particle - parameter in equation (2.8) - parameter in equation (2.8) - circular frequency of the motion