An experimental investigation of combined turbulent free and forced evaporation

The wide variation in correlations available in the literature for predicting water evaporation rates in a moving air stream necessitated a new investigation to determine which correlations can be considered reliable. Water evaporation measurements were made from a heated pool (a class-A pan) into a low speed wind tunnel. The evaporation regime examined combined turbulent free and turbulent forced convection over the range 0.1 < Gr_m/Re² < 10.0. The data includes the range in which combined convection modes are important, as well as the limits where either free or forced convection effects may dominate. The data are compared to several evaporation correlations based on laboratory wind tunnel data. These historical correlations do not produce consistent estimates in predicting evaporation rates. It is believed that the apparent inconsistencies arise because many correlations do not adequately describe the appropriate evaporation regimes for which they are valid. A new correlation using the combined free/forced convection Sherwood number has been developed to predict evaporation rates for a moving air stream. This correlation allows the results of this study to be extended to other evaporating conditions (i.e. variation in surface geometry and air turbulence levels) than those described here. For a 95% confidence limit, the Sherwood number correlation matches the data within ±7.9%.     

On near-wall turbulence-generating events in a turbulent boundary layer on a riblet surface

The boundary layer over a drag reducing riblet surface is investigated using hot-wire anemometry and flow visualisation. The concept of a riblet sublayer is introduced, and a definition is proposed in terms of a region of reduced turbulence energy production formed near the wall by the addition of riblets. The hot wire records are examined using a modified form of quadrant analysis, and results obtained over plain and riblet surfaces are compared. Close to the wall, the addition of riblets produces a marked reduction in the occurrence of ejection (2nd quadrant) events. A corresponding increase in the incidence of sweep (4th quadrant) events is accompanied by the development of a strong tendency toward a preferred event duration, and a preferred interval between events. These changes diminish rapidly with distance from the surface, becoming almost undetectable beyondy ⁺=40. They are discussed in the light of flow visualisation results, and interpreted in terms of mechanisms associated with the interaction between the riblets and the inner boundary layer flow structures. A conceptual model of the flow mechanisms in the riblet sublayer is proposed.     

Heterogene Widerstandsverminderung bei turbulenten Rohrströmungen

Zusammenfassung Es wird über Experimente berichtet, bei denen eine viskoelastische Flüssigkeit in die Achse einer ausgebildeten turbulenten Rohrströmung eines newtonschen Fluids injiziert wurde. Bei der Injektion von höher konzentrierter Polymerlösung tritt eine deutliche Widerstandsverminderung auch dann auf, wenn die Lösung als elastischer Faden im Kernbereich mitgeführt wird, d. h. das Polymer noch nicht merklich in die wandnahe Schicht hineindiffundiert ist.Es werden Ergebnisse über die Veränderung des Druckabfalls längs des Rohres in Abhängigkeit von der Gesamtkonzentration des Polymers im Rohr, der Konzentration im injizierten Faden und der Reynoldszahl mitgeteilt und diskutiert. Weiterhin werden die Profile der Hauptströmung, der Turbulenzintensitäten in radialer und axialer Richtung sowie der Reynoldsschen Schubspannungen bei solchen Injektionsexperimenten bestimmt und mit denjenigen des reinen Lösungsmittels sowie einer homogenen Polymerlösung von gleicher, über den Rohrquerschnitt gemittelter Polymerkonzentration verglichen.Der Verlauf des dimensionslosen Geschwindigkeitsprofils und der Reynoldschen Schubspannungen zeigt, daß der Übergangsbereich zwischen viskoser Unterschicht und turbulentem Kern bei der Injektion wesentlich anders strukturiert ist als beim Vorliegen von homogener Widerstandsverminderung. Er ist sowohl im Vergleich zu einer newtonschen Flüssigkeit als auch zu einer homogenen Polymerlösung deutlich vergrößert, und es findet in ihm eine drastische Reduzierung der Reynoldsschen Schubspannungen statt. Die beobachtete Struktur der turbulenten Strömung deutet in Verbindung mit den gemessenen viskoelastischen Eigenschaften der injizierten Polymerlösung darauf hin, daß die großen Turbulenzelemente durch den Polymerfaden in ihrer Bewegung behindert werden. Infolgedessen übertragen sie weniger Energie auf die die Dissipation bewirkenden kleinen Wirbel, wodurch sich eine Widerstandsverminderung ergibt.

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Experiments in which a viscoelastic fluid is injected along the axis of a fully developed pipe flow of a Newtonian fluid are described. For relatively high polymer concentrations the injected polymer solution forms a thread, which remains intact down the entire length of the pipe. Under these conditions remarkable drag reduction is also observed, although the polymer has not diffused noticeably into the layers near the wall.The dependence of the local drag reduction along the pipe on the total polymer concentration in the pipe, the concentration of the injected polymer solution and the Reynolds number was investigated. Furthermore, velocity profiles, turbulence intensities and Reynolds shear stresses were measured and compared with those for the solvent and a premixed homogeneous polymer solution with the same effective concentration.The shape of the velocity profiles and the Reynolds shear stresses demonstrate that the structure of turbulence in the transition or buffer layer between the viscous sublayer and the turbulent core is markedly different in the injection experiments from that in a premixed homogeneous polymer solution. The size of this region is significantly larger than in a Newtonian fluid or a homogeneous polymer solution and the Reynolds shear stresses are drastically reduced. From details of the structure of the turbulence and the rheological properties of the injected polymer solution it may be concluded that the large-scale structures in the pipe are influenced by the polymer thread. It seems that their motion is restricted leading to a reduced transport of energy to the dissipating small eddies and thus to the occurrence of drag reduction.

Von der Abteilung Chemietechnik der Universität Dortmund genehmigten Dissertation; auszugsweise vorgetragen auf der Jahrestagung der Deutschen Rheologischen Gesellschaft in Berlin vom 11. – 13. Mai 1981 und der Ersten Europäischen Rheologen-Konferenz in Graz vom 14.–16. April 1982.     

Space-time correlations in turbulent Rayleigh–Bnard convection

The recent development of the elliptic model(He,et al. Phy. Rev. E, 2006), which predicts that the space-time correlation function Cu(r, τ) in a turbulent flow has a scaling form Cu(rE, 0) with rEbeing a combined space-time separation involving spatial separation r and time delay τ, has stimulated considerable experimental efforts aimed at testing the model in various turbulent flows. In this paper, we review some recent experimental investigations of the space-time correlation function in turbulent Rayleigh–B′enard convection. The experiments conducted at different representative locations in the convection cell confirmed the predictions of the elliptic model for the velocity field and passive scalar field, such as local temperature and shadowgraph images.The understanding of the functional form of Cu(r, τ) has a wide variety of applications in the analysis of experimental and numerical data and in the study of the statistical properties of small-scale turbulence. A few examples are discussed in the review.     

Numerical prediction of turbulent thermocapillary convection in superposed fluid layers with a free interface

The present paper introduces a new numerical method for predicting the characteristics of thermocapillary turbulent convection in a differentially-heated rectangular cavity with two superposed and immiscible fluid layers. The unsteady Reynolds form of the Navier–Stokes equations and energy equation are solved by using the control volume approach on a staggered grid system using SIMPLE algorithm. The turbulence quantities are predicted by applying the standard k–ε turbulence model. The level set formulation is applied for predicting the topological changes of the interface separating the two fluid layers and to provide an accurate and robust modeling of the interfacial normal and tangential stresses. The computational results obtained showed good agreement when compared with the previous experimental, numerical and analytical benchmark data for different validation cases in both laminar and turbulent regimes. The present numerical method is then applied to predict the velocity and temperature distribution in two immiscible liquid layers with undeformable interface for a wide range of Marangoni numbers. The laminar-turbulent transition is demonstrated by obtaining the turbulence features at high interfacial temperature gradient which is characterized by high Marangoni number. The effect of increasing Marangoni number on the interface dynamics in turbulent regime is also investigated.     

The turbulence vorticity as a window to the physics of friction-drag reduction by oscillatory wall motion

DNS data for channel flow, subjected to spanwise (in-plane) wall oscillations at a friction Reynolds number of 1025, are used to examine the turbulence interactions that cause the observed substantial reduction in drag provoked by the forcing. Following a review of pertinent interactions between the forcing-induced unsteady Stokes strain and the Reynolds stresses, identified in previous work by the present authors, attention is focused on the equations governing the components of the enstrophy, with particular emphasis placed on the wall-normal and the spanwise components. The specific objective is to study the mechanisms by which the Stokes strain modifies the enstrophy field, and thus the turbulent stresses. As such, the present analysis sheds fresh light on the drag-reduction processes, illuminating the interactions from a different perspective than that analysed in previous work. The investigation focuses on the periodic rise and fall in the drag and phase-averaged properties during the actuation cycle at sub-optimal actuation conditions, in which case the drag oscillates by around ±2% around the time-averaged 20% drag-reduction margin. The results bring out the important role played by specific strain-related production terms in the enstrophy-component equations, and also identifies vortex tilting/stretching in regions of high skewness as being responsible for the observed strong increase in the spanwise enstrophy components during the drag-reduction phase. Simultaneously, the wall-normal enstrophy component, closely associated with near-wall streak intensity, diminishes, mainly as a result of a reduction in a production term that involves the correlation between wall-normal vorticity fluctuations and the spanwise derivative of wall-normal-velocity fluctuations, which pre-multiplies the streamwise shear strain.     

Evaluation of two RANS turbulence models in predicting developing mixed convection within a heated horizontal pipe

The developing weakly turbulent regime of mixed convection in a uniformly heated horizontal pipe was first studied experimentally, by means of heat transfer measurements in the following ranges of dimensionless numbers: 3.19 < Re × 10^? 3 < 6.39, 1.80 < Gr _h  × 10^? 8 < 4.20. The working fluid was FC-72?, with Pr = 12.4.

In order to gain a better insight into the thermo-fluid dynamics involved in the phenomenon and obtain the velocity and temperature fields at every point of the fluid domain, numerical simulations were performed by means of commercial software. Turbulence was modelled by using the Reynolds averaged Navier–Stokes equations (RANS) approach. Two closures of the governing equations were evaluated: realizable κ–? (RKE) model and renormalization-group κ–? (RNG) model.

Both models were capable of reproducing the observed physical trends. However, deviations from the experimental data lower than 20% were obtained only in the entry-zone with the RKE model, while the RNG model gave fair predictions only in developed or quasi-developed flow.     

Simulations of dispersed turbulent multiphase flow

Direct numerical simulations of homogeneous isotropic turbulence are used to investigate the effects of turbulence on the transport of particles in gas flows or bubbles in liquid flows. The inertia associated with the bubbles or the particles leads to locally strong concentrations of these in regions of instantaneously strong vorticity for bubbles or strain-rate for particles. This alters the average settling rates and other processes. If the mass-loading of the dispersed phase is significant a random “turbulent” flow is generated by the particle settling. A simple demonstration of this is given, showing the statistically axisymmetric character of this flow and how it can modify an ambient turbulent flow.     

Heat transfer enhancement in a turbulent natural convection boundary layer along a vertical flat plate

An experimental study on heat transfer enhancement for a turbulent natural convection boundary layer in air along a vertical flat plate has been performed by inserting a long flat plate in the spanwise direction (simple heat transfer promoter) and short flat plates aligned in the spanwise direction (split heat transfer promoter) with clearances into the near-wall region of the boundary layer. For a simple heat transfer promoter, the heat transfer coefficients increase by a peak value of approximately 37% in the downstream region of the promoter compared with those in the usual turbulent natural convection boundary layer. It is found from flow visualization and simultaneous measurements of the flow and thermal fields with hot- and cold-wires that such increase of heat transfer coefficients is mainly caused by the deflection of flows toward the outer region of the boundary layer and the invasion of low-temperature fluids from the outer region to the near-wall region with large-scale vortex motions riding out the promoter. However, heat transfer coefficients for a split heat transfer promoter exhibit an increase in peak value of approximately 60% in the downstream region of the promoter. Flow visualization and PIV measurements show that such remarkable heat transfer enhancement is attributed to longitudinal vortices generated by flows passing through the clearances of the promoter in addition to large-scale vortex motions riding out the promoter. Consequently, it is concluded that heat transfer enhancement of the turbulent natural convection boundary layer can be substantially achieved in a wide area of the turbulent natural convection boundary layer by employing multiple column split heat transfer promoters. It may be expected that the heat transfer enhancement in excess of approximately 40% can be accomplished by inserting such promoters.     

Experimental studies on drag reduction and rheology of mixed cationic surfactants with different alkyl chain lengths

Experimental studies of the effects of mixtures of cationic surfactants on their drag reduction and rheological behaviors are reported. Cationic alkyl trimethyl quaternary ammonium surfactants with alkyl chain lengths of C₁₂ and C₂₂ were mixed at different molar ratios (total surfactant concentrations were kept at 5 mM with 12.5 mM sodium salicylate (NaSal) as counterion). Drag reduction tests showed that by adding 10% (mol) of C₁₂, the effective drag reduction range expanded to 4–120 °C, compared with 80–130 °C with only the C₂₂ surfactant. Thus mixing cationic surfactants with different alkyl chain lengths is an effective way of tuning the drag reduction temperature range. Cryo-TEM micrographs revealed thread-like micellar networks for surfactant solutions in the drag reducing temperature range, while vesicles were the dominant microstructures at non-drag reducing temperatures. High extensional viscosity was the main rheological feature for all solutions except 50% C₁₂ (mol) solution, which also does not show strong viscoelasticity. It is not clear why this low extensional viscosity solution with relatively weak viscoelasticity is a good drag reducer. Received: 3 November 1999/Accepted: 5 January 2000     

The Onset of Darcy–Forchheimer Convection in Inclined Porous Layers Heated from Below

It is well known that the onset of convection in an inclined porous layer heated from below takes the form of longitudinal vortices when Darcy’s law is valid. In this paper we consider briefly how the onset criterion alters when form drag, as modelled by the Forchheimer terms, is significant. In general, the critical Rayleigh number increases substantially as form drag effects strengthen, but the wavenumber rises by only a small amount. This numerical study is supplemented by a brief asymptotic analysis of the case when the Forchheimer terms dominate and it is shown that the critical Rayleigh number increases in direct proportion with the form drag parameter.     

Turbulent Flow and Dispersion of Inertial Particles in a Confined Jet Issued by a Long Cylindrical Pipe

In this work we examine first the flow field of a confined jet produced by a turbulent flow in a long cylindrical pipe issuing in an abrupt angle diffuser. Second, we examine the dispersion of inertial micro-particles entrained by the turbulent flow. Specifically, we examine how the particle dispersion field evolves in the multiscale flow generated by the interactions between the large-scale structures, which are geometry dependent, with the smaller turbulent scales issued by the pipe which are advected downstream. We use Large-Eddy-Simulation (LES) for the flow field and Lagrangian tracking for particle dispersion. The complex shape of the domain is modelled using the immersed-boundaries method. Fully developed turbulence inlet conditions are derived from an independent LES of a spatially periodic cylindrical pipe flow. The flow field is analyzed in terms of local velocity signals to determine spatial coherence and decay rate of the coherent K–H vortices and to make quantitative comparisons with experimental data on free jets. Particle dispersion is analyzed in terms of statistical quantities and also with reference to the dynamics of the coherent structures. Results show that the particle dynamics is initially dominated by the Kelvin–Helmholtz (K–H) rolls which form at the expansion and only eventually by the advected smaller turbulence scales.     

Temporal large-eddy simulation of unstratified and stably stratified turbulent channel flows

Recently, Pruett et al. [Pruett, C.D., Gatski, T.B., Grosch, C.E., Thacker, W.D., 2003. The temporally filtered Navier–Stokes equations: properties of the residual stress. Phys. Fluids 15, 2127–2140] proposed an approach to large-eddy simulation (LES) based on time-domain filtering; their approach was termed temporal large-eddy simulation or TLES. In a continuation of their work, Pruett and collaborators tested their methodology by successfully performing TLES of unstratified turbulent channel flow up to Reynolds number of 590 (based on channel half-height and friction velocity) [Pruett, C.D., Thomas, B.C., Grosch, C.E., Gatski, T.B., 2006. A temporal approximate deconvolution model for LES. Phys. Fluids 18, 028104, 4p]. Here, we carefully analyze the TLES methodology in order to understand the role of its key components and in the process compare TLES to more traditional approaches of spatial LES. Furthermore, we extend the methodology to stably stratified turbulent channel flow.     

Drag reduction effectiveness and shear stability of polymer-polymer and polymer-fibre mixtures in recirculatory turbulent flow of water

The turbulent drag reduction caused by polymer-polymer and polymerfibre mixtures has been measured in recirculatory flow of water. Shear stability studies have also been made on a number of drag reducing polymers, asbestos fibres and their mixtures in recirculatory turbulent flow of water. Reynolds numbers ranged from 20,000 to 57,000. Both positive and negative deviations from linear additive behaviour have been observed in drag reduction caused by the polymer-polymer mixtures depending upon their compositions, flow rate and polymer species in the mixture. The drag reduction by the mixtures has been predicted by using simple mixture rule equations including an interaction parameter. This interaction parameter is believed to depend upon the polymer interaction in the polymer mixture. The random coil size and rigidity of the polymer molecules appear to be responsible for the synergism observed in the drag reduction caused by the mixture. In general, mixtures having larger solvation number seem to give positive synergism.Synergism in drag reduction by the polymer-fibre mixtures has also been observed. The simple mixture law equation with interaction parameter is also applicable in predicting the drag reduction by the mixtures as above. The random coil size of the polymer molecules and the rigidity of the polymer-fibre system appear to be responsible for the synergism observed in drag reduction. In the shearstability studies it has been observed that the decrement in drag reduction (DR) is higher than the the decrement in absolute viscosity in most cases. Carboxymethyl cellulose is found to be the most shear stable polymer followed by guar gum, xanthan gum and polyacrylamide. The mixtures exhibiting synergism in causing drag reduction are found to be more shear stable.     

Heat transfer characteristics and Nusselt number correlation of turbulent pulsating pipe air flows

Heat transfer characteristics to turbulent pulsating pipe flows under a wide range of Reynolds number and pulsation frequency were experimentally investigated under uniform heat flux condition. Reynolds number was varied from 8462 to 48540 while the frequency of pulsation ranged from 1 to 29.5 Hz. The results showed that the relative mean Nusselt number is strongly affected by both pulsation frequency and Reynolds number. Enhancements in mean Nusselt number of up to 50% were obtained at medium pulsation frequency between 4.1 and 13.9 Hz for Reynolds number range of 8462 to 14581. An enhancement of up to 50% in mean Nusselt number was obtained at high pulsation frequency range between 13.9 and 29.5 Hz, specially as Reynolds number is close to 15000, while a reduction was observed at higher Reynolds number more than 21200. This reduction, at high Reynolds number, increased as pulsation frequency increased. Also, there was a reduction in mean Nusselt number of up to 20% that obtained at low pulsation frequency range between 1 and 4.1 Hz for Reynolds number range of 8462 to 48543. A significant reduction in mean Nusselt number of up to 40% was obtained at medium pulsation frequency between 4.1 and 13.9 Hz for Reynolds number range of 21208 to 48543. Empirical equations have been developed for the relative mean Nusselt number that related to Reynolds number and dimensionless frequency with about uncertainty of 10% rms.The support of both King Fahd University of Petroleum and Minerals and Cairo University for this research is acknowledged.