Turbulent Flow Parameters Research Articles

Drag reduction by polymer solutions in a riblet-lined pipe

The flow of 3 to 100 wppm aqueous solutions of a polyethyleneoxide polymer,Mw=6.2×;106, was studied in a 10.2 mm i.d. pipe lined with 0.15 mm V-groove riblets, at diametral Reynolds numbers from 300 to 150000. Measurements in the riblet pipe were accompanied by simultaneous measurements in a smooth pipe of the same diameter placed in tandem. The chosen conditions provided turbulent drag reductions from zero to the asymptotic maximum possible. The onset of polymer-induced drag reduction in the riblet pipe occurred at the same wall shear stress, τ* w =0.65 N/m2, as that in the smooth pipe. After onset, the polymer solutions in the riblet pipe initially exhibited linear segments on Prandtl-Karman coordinates, akin to those seen in the smooth pipe, with specific slope increment\(\delta /\sqrt c = 6\). The maximum drag reduction observed in the riblet pipe was independent of polymer concentration and well below the asymptotic maximum drag reduction observed in the smooth pipe. Polymer solution flows in the riblet pipe exhibited three regimes: (i) Hydraulically smooth, in which riblets induced no drag reduction, amid varying, and considerable, polymer-induced drag reduction; this regime extended to non-dimensional riblet heightsh+ 0; this regime extended from 5 70, and was observed in all polymer solutions at highh+, the more so as polymer-induced drag reduction increased, withR′ 8. The greatest drag enhancement in polymer solutions,R′=−7±1 ath+=55 whereS′=20, considerably exceeded that in solvent. Three-dimensional representations of riblet- and polymer-induced drag reductions versus turbulent flow parameters revealed a hitherto unknown “dome” region, 8<h+<31, 0<S′<10, 0<R′<1.5, containing a broad maximum at (h+,S′,R′) = (18, 5, 1.5). The existence of a dome was physically interpreted to suggest that riblets and polymers reduce drag by separate mechanisms.

95/01770 The production of premixed flame surface area in turbulent shear flow

In the present work, we use three-dimensional Direct Numerical Simulation (DNS) of premixed flames in turbulent shear flow to characterize the effect of a mean shear motion on flame surface production. The shear is uniform in the unburnt gas, and simulations are performed for different values of the mean shear rate, S. The data base is then used to estimate and compare the different terms appearing in the Sigma-equation as a function of S. The analysis gives in particular the relative weights f the turbulent flow and mean flow components, a(sub T) and A(sub T), of the flame surface production term. This comparison indicates whether the dominant effects of a mean flow velocity gradient on flame surface area are implicit and scale with the modified turbulent flow parameters, kappa and epsilon, or explicit and scale directly with the rate of deformation.

Numerical modelling of the spiral plate heat exchanger

The spiral plate heat exchanger (SHE) is widely used in plenty of industrial services in full counter current flow liquid-liquid heat exchange. We have produced a thermal modelling of the heat exchanges in both steady-state and time dependent cases with 2D spiral geometry, allowing computation with different materials, forced convective heat transfer models in turbulent flow and geometrical parameters options. We will display here some results in steady-state conditions in order to improve the exchanger performances.

Dependence of the water depth at the mouth of cavity-free drains on the parameters of turbulent flow

1. A method was developed and the similarity criteria were obtained for modeling turbulent seepage flows in cavityfree drainage. 2. A graph was obtained for determining the depth of water in the mouth of cavity-free drains with free flow from them as a function of the specific inflow to the drain, coefficient of turbulent seepage flow of the filling, and width and slope of the drain.

Integral method of computing the diffusion of a polymer impurity in the turbulent flow at a permeable surface

An integral method of computing turbulent flow parameters at a permeable wall through which fluid with a drag-reducing polymer impurity is delivered is proposed on the basis of an asymptotic analysis of velocity and concentration profiles. Formulas are obtained to compute the effective impurity concentration.

Laser-Doppler measurements of turbulent-flow parameters in a stirred mixer

It is recognized that a detailed knowledge of turbulence parameters, as well as velocities, can aid in understanding and modelling mixing rate dominated phenomena in stirred vessels. Turbulent-flow parameters were measured in a baffled, Rushton turbine agitated vessel with a laser-Doppler velocimeter. The necessary corrections for the periodic, nondissipative velocity fluctuations in the near impeller region were made by an autocorrelation method. Two components of periodic fluctuation, one corresponding to impeller blade frequency, the other corresponding to twice that frequency, were found to be significant. With the periodicity removed, meaningful turbulence intensities, autocorrelation functions, turbulence scales, energy spectra, and turbulence energy dissipation reates were obtained. Integral scales and turbulence energy dissipation rates were a particular objective in this work because of their usefulness in modelling local mixing rates in turbulence flows. From an energy balance around the impeller and impeller stream, it was found that 60% of the energy transmitted into the tank via impeller was dissipated in that region, and 40% was dissipated in the bulk of the tank. An equation for calculating local energy dissipation rates from resultant fluctuation velocities and resultant turbulence macroscales, ▪, appeared adequate. Constant A was found to be 0.85.

The Wake of Self-Propelled and Over-Thrusted Slender Bodies Near a Simulated Free Surface

Measurements were performed in the turbulent wake of a propeller-driven axisymmetric body with a plane of symmetry. A flat plate strut was attached to the upper surface of the axisymmetric body, giving a configuration like that of a SWATH-type ship, with the free surface replaced by the plane of symmetry. All mean flow and turbulent flow parameters were measured at three streamwise stations. The measurements were performed for the self-propelled condition and 100 percent over-thrust condition. In the far wake, the center of the wake was found to migrate towards the plane of symmetry. Some interactions were noted between the wakes of the propeller-driven axisymmetric body and that of the flat plate strut—yielding lower axial velocities, higher turbulence intensities and larger static pressure changes compared to regions free of such interference. Comparisons of these effects in the self-propelled case, 100 percent over-thrust case and a previous unpropelled case are given. Spectral measurements were also performed in both near-wake and far-wake regions.

Variation of turbulent flow parameters in the presence of flow acceleration and deceleration

The variation of the turbulent flow parameters along the main section of a cylindrical channel in the presence of flow acceleration and deceleration has been experimentally investigated. The results of the experiments are presented.

Pulsating flow in a pipe

Turbulent and laminar pulsating flows in a straight smooth pipe are compared at identical frequencies and Reynolds numbers. Most measurements were made at a mean Reynolds number of 4000, but the influence of Re was checked for 2900 < Re < 7500. The period of forcing ranged from 0.5 to 5 s, with corresponding change in the non-dimensional frequency parameter α = R √(ω/ν) from 4.5 to 15. The amplitude of the imposed oscillations did not exceed 35% of the mean in order to avoid flow reversal or relaminarization. Velocities at the exit plane of the pipe and pressure drop along the pipe were measured simultaneously; velocity measurements were made with arrays of normal hot wires. The introduction of the periodic surging had no significant effect on the time-averaged quantities, regardless of the flow regime (i.e. in both laminar and turbulent flows). The time-dependent components at the forcing frequency, represented by a radial distribution of amplitudes and phases, are qualitatively different in laminar and turbulent flows. The ensemble-averaged turbulent quantities may also be represented by an amplitude and a phase; however, the non-harmonic content of these intensities increases with increasing amplitude of the imposed oscillations. A normalization procedure is proposed which relates phase-locked turbulent flow parameters in unsteady flow to similar time-averaged quantities. An integral momentum equation in a time-dependent flow requires that a triad of forces (pressure, inertia and shear) will be in equilibrium at any instant of time. All the terms in the force-balance equation were measured independently, providing a good check of data. The analysis of the experimental results suggests that turbulence adjusts rather slowly to the local mean-flow conditions. A simple eddy-viscosity model described by a complex function can account for ‘memory’ of turbulence and explain the different phase distribution in laminar and turbulent flows.

Unsteady convective heat transfer for turbulent flows of gases and liquids in tubes

The paper presents an analysis and generalization of experimental data on the unsteady heat transfer of gases and liquids flowing in tubes under the conditions of heating/cooling and variation in the flow rate, heat release in the tube walls, and entrance flow temperature. The comparison between the results of theoretical analysis and experiments has made it possible to find the distribution of turbulent flow parameters over the tube radius and to determine the effect of variable flow structure on heat transfer. Different findings concerning the non-stationarity effects on turbulent flow characteristics are compared with the data of experiments. As a result, the correlations have been derived to calculate the unsteady heat transfer coefficient. Engineering calculation methods for real unsteady thermal processes are suggested.

A Study of Some Turbulence Models for Flow and Heat Transfer in Ducts of Annular Cross-Section

A variable property finite-difference calculation procedure is used to predict turbulent flow and heat transfer parameters in annular passages. Predictions from several turbulence models are compared with measurements over a range of flow and thermal conditions. Of the models considered, one utilizing transport equations for turbulence kinetic energy and characteristic mixing length scale gave the best overall performance. The inclusion of turbulence kinetic energy in the turbulence modeling was found not to be crucial for predicting isothermal flows or for predicting all parameters except the temperature distribution for flows with heat transfer at Reynold numbers greater than 110,000.

Optical Doppler method for investigating turbulent flow by spectral analysis of a signal

A method for determining the parameters of turbulent flow of a gas or liquid from the energy spectrum of a signal obtained in an optical Doppler velocity meter is described. An analysis is made of the limits of validity of a method suggested for measuring the distribution of the local velocity density. Apparatus is described and the results are given of measurements of the average velocity and intensity of turbulence in an axisymmetic submerged water stream of initially small diameter.

A radioactive tracer technique for particle velocity measurement in solids‐gas systems

AbstractTo record the velocity history of a particle in complex problems of solid‐gas flow with greater accuracy than is possible with existing methods, a technique was developed which in essence consists of following the flight of the particle under test—which has been previously tagged with a suitable gamma‐emitting nuclide—by means of Geiger probes placed at predetermined points along the outside surface of the duct or reactor and parallel to its axis. Sharp timing pulses are thus obtained on the screen of a cathode‐ray oscilloscope, which permit the estimation of the time elapsed between probe stations to less than 1/1000 second. Complete description of the electronic aspects of the method is given, and details of the particle preparation are outlined.A brief description is given of the application of the method to the determination of the rate of mass and heat transfer from particles in accelerated motion in a turbulent co‐current gas stream and to the study of the effects of the turbulent flow parameters on the drag coefficients of particles in highly turbulent fields.