Experimental study on turbulent features in the negative transport region of asymmetric plane channel flow

Turbulent features of streamwise and vertical components of velocity in the negative transport region of asymmetric plane channel flow have been studied experimentally in details. Experiments show that turbulent fluctuations in negative transport region are suppressed, and their probability distributions are far from Gaussian. Besides, the skewness factors attain their negative maxima at the position of the maximum mean velocity, whereas the flatness factors attain their positive maxima at the same position. The project supported by the National Natural Science Foundation of China (19872043)     

Calculation of the transition region of a turbulent jet

The presently known methods for calculating plane and axisymmetric turbulent jets in a wake flow are based on dividing the flow region into two segments, initial and basic [1–3], Here the matching of the parameters of the initial and basic segments is of an artificial nature, since it permits the existence of a physically impossible discontinuity of the curves of the velocity distribution and the jet width along the axis.The aerodynamic characteristics of the transition segment, extending from the point of convergence of the boundary layers at the end of the initial segment to the section corresponding to the point of inflection of the curve u_m(x), differ significantly from the characteristics of the initial and basic segments. This difference is due not only to the sharp increase of the velocity pulsations, but also the marked deformation of the average longitudinal velocity component profile. Consequently, the calculation of the transition segment, in contrast to the initial and basic segments, cannot be based on the single-parameter method.Generally speaking, the flow development in the transition segment may be calculated with the aid of the method [4], which reduces the solution of the problem to an equation of the heat conduction type and assumes the use of an experimental curve of the velocity distribution along the jet axis. Abramovich has carried out the calculation of the transition segment of a plane submerged jet on the basis of certain assumptions which are based on the results of experimental studies [1].Below is presented an approximate method of calculating the transition segment of plane and axisymmetric turbulent jets in a wake flow in which the velocity profiles obtained for the extreme sections of this segment are used for calculating the flow parameters in the initial and basic segments.     

Computation of internal turbulent flow with a large separated flow region

An implicit two-equation turbulence solver, KEM. in generalized co-ordinates, is used in conjunction with the three-dimensional incompressible Navier–Stokes solver, INS3D, to calculate the internal flow in a channel and a channel with a sudden 2:3 expansion. A new and consistent boundary procedure for a low Reynolds number form of the κ-ε turbulence model is chosen to integrate the equations up to the wall. The high Reynolds number form of the equations is integrated using wall functions. The latter approach yields a faster convergence to the steady-state solution than the former. For the case of channel flow, both the wall-function and wall-boundary-condition approaches yield results in good agreement with the experimental data. The back-step (sudden expansion) flow is calculated using the wall-function approach. The predictions are in reasonable agreement with the experimental data.     

Sediment transport by turbulent flow above a bottom subject to erosion

Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, No. 4, pp. 61–69, July–August, 1992.     

Exact transport equation for eddy diffusivity in turbulent shear flow

Two-equation models that treat the transport equations for two variables are typical models for the Reynolds-averaged Navier–Stokes equation. Compared to the equation for the turbulent kinetic energy, the equation for the second variable such as the dissipation rate does not have a theoretical analogue. In this work, the exact transport equation for the eddy diffusivity was derived and examined for better understanding turbulence and improving two-equation models. A new length scale was first introduced, which involves the response function for the scalar fluctuation. It was shown that the eddy diffusivity can be expressed as the correlation between the velocity fluctuation and the new length scale. The transport equations for the eddy diffusivity and the length-scale variance were derived theoretically. Statistics such as terms in the transport equations were evaluated using the direct numerical simulation of turbulent channel flow. It was shown that the streamwise component of the eddy diffusivity is greater than the other two components in the whole region. In the transport equation for the eddy diffusivity, the production term due to the Reynolds stress is a main positive term, whereas the pressure–length-gradient correlation term plays a role of destruction. It is expected that the analysis of the transport equations is helpful in developing better turbulence models.     

Friction factor in micropipe gas flow under laminar,transition and turbulent flow regime

The present work deals with the compressible flow of nitrogen gas inside microtubes ranging from 30 to 500 μm and with different values of the surface roughness (<1%), for different flow regimes. The first part of the work is devoted to a benchmark of friction factor data obtained at DIENCA (University of Bologna) and the ENEA laboratories, using fused silica pipes of 50 and 100 μm. Data overlapping is excellent thus evidencing how the agreement of the experimental data with the classic theory is independent of the measurement system. The second part of the work demonstrates that classic correlations can predict friction factor in laminar flow without revealing any evident influence of the surface roughness. The laminar-to-turbulent transition starts for Reynolds number not lower than 2000 for smooth pipes, while tending to larger values (3200–4500) for rough pipes. Anyway, contrarily to other available results, no dependence of the critical Reynolds number on the L/D has been observed. Changes in the flow regime have been found of the sharp and smooth type, like for larger pipes; smooth transition looks typical of smooth pipes while the sharp transition in the flow pattern is associated with rough pipes. In the fully developed turbulent regime, obtained for both smooth and rough pipes, an agreement between experimental data and the Blasius correlation has been verified for smooth pipes, while for rough pipes the agreement with predictions given by the Colebrook equation is rather modest.     

Laminar,turbulent, and transition flow in porous sintered media

Summary The results of an experimental research on laminar, transition and turbulent flow through sintered bronze metallic filters are here reported. Assuming the square root of the permeability as characteristic dimension of the porous media, all the experimental results — obtained either with air or with water as working fluid — are presented on a plot of the friction factor versus the Reynolds number. A unique value of the characteristic constant allows a good fit for both fluids.

---

Sommario Si presentano i risultati di una ricerca sperimentale sul moto di filtrazione in regime laminare, di transizione e turbolento attraverso filtri metallici di bronzo sinterizzato. Assumendo la radice quadrata della permeabilità quale dimensione caratteristica degli elementi porosi, tutti i risultati ottenuti — sperimentando sia con aria che con acqua — vengono correlati in funzione del fattore di attrito e del numero di Reynolds, con un unico valore della costante caratteristica dei mezzi porosi.

---

     

Three-component,time-resolved velocity statistics in the wall region of a turbulent pipe flow

Three-component, coincident, time-resolved velocity measurements were obtained in the near wall region, y ⁺ < 100, of a fully developed turbulent pipe flow. The measurements were conducted in the ARL/PSU glycerin tunnel at a Reynolds number (Re _h), based on pipe radius and centerline velocity, of 6436 and an Re of approximately 730. The reported data include velocity statistics up to fourth order, Reynolds stresses and three component, coincident turbulent velocity spectral estimates. The current data are generally in quite good agreement with the fully developed channel flow direct numerical simulation (DNS) results of Antonia et al. (1992) at Re 700 - 700. The accuracy of the current experimental data and the very good agreement with the DNS results provides evidence for the accuracy of the DNS solutions and thus Antonia's conclusions of very near wall, y ⁺ < 20, Re dependence on turbulent velocity statistics. The very good agreement between the low Re rectangular channel flow DNS results and the low Re flat plate turbulent boundary layer statistics of Karlsson and Johansson (1988) suggests that for y ⁺ < 30 statistics of similar flows of differing geometry may be compared on the basis of equal Re . The current data are available on disk or by anonymous ftp by the first author.     

Effect of velocity and pipeline configuration on dispersion in turbulent hydrocarbon-water flow using laser image processing

Drop size distribution and concentration profile data for hydrocarbon-water mixtures are obtained in a 8.2 cm dia pipe at a range of velocities for a straight horizontal pipe, horizontal and vertical flow after one bend and vertical flow after three bends. The laser image processing technique employed in this project is proven reliable.

The maximum drop size (d₉₉), is more dependent on the number of upstream interactive bends than on the velocity. The drop size distributions follow a Rosin-Rammler power law. The values of Rosin-Rammler exponents, based on this work, are on average 2.1 for all the configurations studied.

The concentration profiles as a function of velocity for straight horizontal flow are obtained and show the transition from stratified to adequately dispersed flow at about 2.3 m/s velocity. The concentration profiles for horizontal or vertical flow after one bend show dispersed flow in some cases; however, in other cases swirling makes representative sampling more difficult.

Vertical downflow after three interactive bends breaks the droplets to a finer size, and concentration profiles obtained in this location are more uniform than the other configurations studied. Representative sampling can be accomplished in this location even at 0.7–1.0 m/s velocity, in a 8.2 cm pipe.     

Local measurements of turbulent angular momentum transport in circular Couette flow

We report on velocity fluctuations and the fluctuation-driven radial transport of angular momentum in turbulent circular Couette flow. Our apparatus is short (cylinder height to gap width ratio Γ ~ 2) and of relatively high wall curvature (ratio of cylinder radii η ~ 0.35). Fluctuation levels and the mean specific angular momentum are found to be roughly constant over radius, in accordance with previous studies featuring narrower gaps. Synchronized dual beam Laser Doppler Velocimetry (2D LDV) is used to directly measure the r − θ Reynolds stress component as a function of Reynolds number (Re), revealing approximate scalings in the non-dimensional angular momentum transport that confirm previous measurements of torque in similar flows. 2D LDV further allows for a decomposition of the turbulent transport to assess the relative roles of fluctuation intensity and r − θ cross-correlation. We find that the increasing angular momentum transport with Re is due to intensifying absolute fluctuation levels accompanied by a slightly weakening cross-correlation.     

Rollup region of a turbulent trailing vortex issued from a blade with flow separation

This paper presents the results of an experimental investigation on the near field of a tip vortex generated by a blade at moderate incidence. The experiments were conducted at Re=15 000 and the boundary layer over the blade separated around midchord on the upper surface. Laser-Doppler measurements of the turbulent flow (Tu=1.5%) were performed at various stations downstream of the blade. The three components of the mean velocity field and turbulent attributes were quantified at cross-planes, characterizing both the blade wake and the tip vortex structure. This allowed the analysis of the rollup and initial stages of decay of the tip vortex in the light of known theories and models. The axial velocity defect at the center of the vortex core evolved as x⁻¹ log x, without displaying any significant outgrowth imposed by the separated flow upstream. Momentum balances were also carried out at a station downstream to the conclusion of vortex rollup. The approximate axisymmetry of the flow field in the trailing vortex was used to formulate the balances in a cylindrical coordinate system. Among other observations, it was seen that an adverse axial pressure gradient developed in the vortex core, which reinforced the tenacity of the axial velocity defect. In contrast, an area influenced by a favorable pressure gradient was found outside the core.     

Study of the turbulent flow of a gas over long blunt bodies in a viscous shock layer

Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, No. 1, pp. 69–75, January–February, 1993.