Laminar Part Research Articles

Natural laminar-turbulent transition delay by dielectric barrier discharge

The use dielectric barrier discharge for the delay of laminar turbulent transition excited by natural flow disturbances in a quiet wind-tunnel was investigated experimentally. Optimal electrodes location and the operational regime of high-voltage impulse generator provided maximal downstream shift of transition location were found. It was demonstrated that the 10% increase of the laminar part of boundary layer can be obtained using barrier discharge with the cross-flow electrodes. This gives up to 20% friction drag reduction.

Mean flow deformation in a laminar separation bubble: separation and stability characteristics

The mutual interaction of laminar–turbulent transition and mean flow evolution is studied in a pressure-induced laminar separation bubble on a flat plate. The flat-plate boundary layer is subjected to a sufficiently strong adverse pressure gradient that a separation bubble develops. Upstream of the bubble a small-amplitude disturbance is introduced which causes transition. Downstream of transition, the mean flow strongly changes and, due to viscous–inviscid interaction, the overall pressure distribution is changed as well. As a consequence, the mean flow also changes upstream of the transition location. The difference in the mean flow between the forced and the unforced flows is denoted the mean flow deformation. Two different effects are caused by the mean flow deformation in the upstream, laminar part: a reduction of the size of the separation region and a stabilization of the flow with respect to small, linear perturbations. By carrying out numerical simulations based on the original base flow and the time-averaged deformed base flow, we are able to distinguish between direct and indirect nonlinear effects. Direct effects are caused by the quadratic nonlinearity of the Navier–Stokes equations, are associated with the generation of higher harmonics and are predominantly local. In contrast, the stabilization of the flow is an indirect effect, because it is independent of the Reynolds stress terms in the laminar region and is solely governed by the non-local alteration of the mean flow via the pressure.

A multiscale subgrid model for both free vortex flows and wall-bounded flows

A new subgrid-scale (SGS) model which has an adequate behavior in both vortical flows and wall-bounded flows is proposed. In wall-bounded flows with “wall-resolved” large eddy simulation (LES), the theory predicts that the SGS dissipation should vanish as y+3 near the wall. In free vortex flows, one needs to have models which do not dissipate energy in the strongly vortical and essentially laminar part of the flow, e.g., in the vortex core regions. The wall adapting local eddy (WALE) viscosity model of Nicoud and Ducros [Flow, Turbul. Combust. 62, 183 (1999)] has the correct near-wall behavior. It is, however, demonstrated here that it produces values of effective eddy viscosity which are too high in vortical flows: this constitutes a major drawback for LES of vortex flows. On the other hand, the regularized variational multiscale models are suitable to simulate vortical flows as demonstrated by Cocle et al. [Complex Effects in LES (Springer, New York, 2007), p. 56], but they do not have a correct behavior in wall-bounded flows as shown by Jeanmart and Winckelmans [Phys. Fluids 19, 055110 (2007)]. The model presented here aims at combining the strengths of the two models: it is a multiscale model, thus acting on the high pass filtered LES field, and for which the SGS viscosity is evaluated using the WALE model, itself also computed using the high pass filtered field. Hence, this model is only active when there is locally a significant high wavenumber content in the flow and it has a natural near-wall damping behavior. The ability of this model to simulate vortex and wall-bounded flows is demonstrated on three test cases. The first case is the turbulent channel flow at Reτ=395 and Reτ=590. The second test concerns a counter-rotating four-vortex system at ReΓ=20 000. The third case concerns a two-vortex system in ground effect at ReΓ=20 000. It is shown that the model allows to perform successfully the LES of these flows with the proper dissipative behavior in both the near-wall and the vortical regions.

A theoretical and numerical evaluation of the steady-state burning rate of vertically oriented PMMA slabs

The steady state burning rate of vertically oriented slabs of poly-methyl-methacrylate (PMMA) is numerically investigated. Model predictions are compared with measurements and results of the laminar boundary layer (LBL) theory. The numerical model provides a solution of the Favre-averaged Navier–Stokes equations coupled with sub-models for turbulence, combustion, soot production and radiation. The modelling of condensed phase processes is based on the one-dimensional heat transfer equation and pyrolysis is treated as a phase change using the latent heat approach. Results show that the pyrolysing region can be divided into three regions. In the laminar part of the flow (Gr x < 4.3 × 107), the predicted normalised burning rate, ṁ″ p x/μ∞, is a power-law function of Gr x with an exponent close to that of the LBL theory, surface re-radiation being the primary source of discrepancies. From the LBL theory for free flow, it is demonstrated that the local burning rate is inversely proportional to the shear velocity gradient. This is globally confirmed by numerical model results. At Gr x = 4.3 × 107 the change in slope of the burning rate observed experimentally, which indicates the end of the laminar flow region, is reproduced numerically. From Gr x = 2.5 × 109 model results show that the surface mass flux of pyrolyzate increases with x, in agreement with experimental data in literature.

Anatomy of the Laminar Organs of Commelina erecta (Commelinaceae)

Abstract A study was undertaken to compare the anatomy of the laminar floral parts with that of the spathes and leaves of Commelina erecta L. Each flower has two types of petals and two types of sepals. In contrast to the other organs, the petals have a completely open venation system whose vein endings consist solely of modified bundle-sheath cells. Bundle sheaths of leaves and spathes, but not the floral organs, contain sclerified cells for support. The high density of hook-shaped trichomes on the outer surface of the spathe and of glandular microhairs on the inner surface might indicate protective and secretory functions, respectively. Anomalous stomatal apparatuses are more common on floral organs than on spathes or leaves. Leaves and spathes appear to have a more detailed developmental program than sepals and petals.

Direct numerical simulation of flow and heat transfer in a turbine cascade with incoming wakes

Direct numerical simulations (DNS) of flow in a turbine cascade with heat transfer have been performed. The set-up of the simulations was chosen in close accordance with previous experiments. Three of the experimental situations were simulated: one without free-stream turbulence and two with periodically incoming wakes of different frequency and with different levels of background fluctuation. Hence, the calculations allow us to study the influence of impinging wakes and background fluctuations on the development of the boundary layers and the local Nusselt number along the surfaces of the heated blade. Along the suction side, the pressure gradient is first favourable and then turns adverse near the trailing edge and the boundary layer remains laminar for the case without free-stream turbulence with the Nusselt number showing the typical decay from the leading to the trailing edge. With periodic wakes and background turbulence, transition occurs when the pressure gradient turns adverse, but intermittency persists so that the boundary layer is not fully turbulent when the trailing edge is reached. In this region, the heat transfer is increased significantly by an amount comparable to that found in the experiments. In the pre-transitional region with favourable pressure gradient, the flow acceleration stretches the free-stream vortices, aligning their axis with the flow direction, thereby forming streamwise vortical structures. These increase the laminar heat transfer in this region by 20–30%, which is, however, much less than observed in the experiments. On the pressure side, the pressure gradient is favourable along the entire blade so that the boundary layer remains laminar. Here, the wakes, through their impingement, also generate streamwise vortical structures which, because of the low convection speed on this side, have a very long lifetime compared to the structures along the suction side. Also these structures increase the laminar heat transfer by about 30%, which for the case with the highest wake frequency is again much less than in the experiments. The simulated average level of fluctuations in the laminar parts of the boundary layers is comparable or even higher than that in the experiments so that it seems likely that a difference in the spectral contents causes the discrepancies. The wake turbulence entering the calculation domain corresponds to that in far wakes with relatively small-scale structures, whereas in the experiments the wakes most probably still carried some large-scale fluctuations of the size of the wake width, which have been found to be more effective in increasing laminar heat transfer.

Novel plasma torch diagnostic method based on multiple exposition CCD and correlation analysis

Special diagnostic methods are needed for development of plasma sources as DC plasma torch, and for controlling their parameters in industry applications. Discussed methods are based on multiple exposition CCD camera equipped with fast rotating mirror wobble unit. In this configuration we gain several snapshots (typically eight) in single frame, with mutual time differences of the order of tens microseconds. Additional arrangement of three mirrors allows us to divide the frame into two perpendicular views, which enable a rough 3D reconstruction of optical self-emisivity of generated plasma. The acquired images can be analysed by mathematic apparatus with various methods including simple and very fast subtraction of adjacent snapshots, or by correlation analysis. Subtraction can be easily used for determination of structures with higher or lower luminosity, including their movements. This allows determination of velocities in the generated plasma jet without any disturbance. A more sophisticated correlation analysis is based on creating a vector field from the snapshots and computation of the correlations coefficients between these vectors. Construction of the vector fields can be done by various ways, and we can get a mutual correlation of the snapshots both in time and space-spatial correlations, or only in time-temporal correlations. Temporal correlations are more precise diagnostics than subtraction, but we can get almost the same sort of information about movements and changes in the plasma jet. On the other hand, spatial correlations show us the locations matching with laminar or turbulent parts in the jet.

Optic Nerve Invasion of Uveal Melanoma: Clinical Characteristics and Metastatic Pattern

To determine the frequency of optic nerve invasion in uveal melanoma, to identify clinical factors associated with optic nerve invasion, and to analyze the metastatic pattern and the association with survival. All iris, ciliary body, and choroidal melanomas (N = 2758) examined between 1942 and 2001 at the Eye Pathology Institute, University of Copenhagen, Denmark, and the Institute of Pathology, Aarhus University Hospital, Aarhus, Denmark, were reviewed. Cases with optic nerve invasion were identified and subdivided into prelaminar or laminar invasion and postlaminar invasion. Clinical characteristics were compared with those from 85 cases randomly drawn from all ciliary body and choroidal melanomas without optic nerve invasion from the same period. Survival data were obtained by the Kaplan-Meier method, and the Mantel-Cox log-rank test was used to test differences in survival among the three patient groups. Optic nerve invasion was found in 157 uveal melanomas (5.7%; 95% confidence interval [CI], 4.8%-6.6%). Frequency varied during the observation period between 5% and 7%. Only choroidal and ciliary body melanomas were found to invade the optic nerve. Eighty-five (54%) were confined to the prelaminar or laminar part, and 72 (46%) were confined to the postlaminar part. Increased intraocular pressure (IOP) and juxtapapillary location were associated with prelaminar or laminar invasion and postlaminar invasion. Age older than 70 years, reduced vision to light perception or worse, nonvisible fundus, and large (>15 mm) tumor size were associated with postlaminar spread. In univariate analysis, patients with postlaminar invasion had significantly higher all-cause and melanoma-related mortality than the other patients. Optic nerve invasion in uveal melanoma is found in 1 in 20 patients. Visible juxtapapillary melanoma or loss of light perception should make the clinician suspicious of melanoma with optic nerve invasion, and special awareness of postlaminar spread should be addressed when increased IOP is present independently of decreased visual acuity and tumor location.

Exposure to Diazinon Alters In Vitro Retinogenesis: Retinospheroid Morphology, Development of Chicken Retinal Cell Types, and Gene Expression

Developing embryos are more vulnerable than adults to acute cholinergic intoxication by anticholinesterases, including organophosphorus pesticides. These agents affect the process of neural development itself, leading to permanent deficits in the architecture of the nervous system. Recent evidence on direct roles of acetylcholinesterase (AChE) on neuronal differentiation provides additional grounds for investigating the developmental toxicity of anticholinesterases. Therefore, the effect of the organophosphate diazinon on the development of chick retinal differentiation was studied by an in vitro reaggregate approach. Reaggregated spheres from dissociated retinal cells of the E6 chick embryo were produced in rotation culture. During the whole culture period of 10 days, experimental cultures were supplemented with different concentrations of the pesticide, from 20 to 120 microM diazinon. The pesticide-treated spheres were reduced in size, and their outer surface was irregular. More importantly, inner structural distortions could be easily traced because the structure of control spheroids can be well characterized by a histotypical arrangement of laminar parts homologous to the normal retina. Acetylcholinesterase activity in diazinon-treated spheres was reduced when compared with controls. As a dramatic effect of exposure to the pesticide, inner plexiform layer (IPL)-like areas in spheroids were not distinguishable anymore. Similarly, photoreceptor rosettes and Müller radial glia were strongly decreased, whereas apoptosis was stimulated. The expression of transcripts for choline-acetyltransferase and muscarinic receptors was affected, revealing an effect of diazinon on the cholinergic system. This further proves the significance of cholinesterases and the cholinergic system for proper nervous system development and shows that further studies of debilitating diazinon actions on development are necessary.

Intermittency of space charge waves

Intermittency of the nonlinear waves in one-dimensional flow of charged particles is studied. A two-mode solution is obtained in the frame moving with the space charge wave. The second mode corresponds to modulation of the fundamental (first) mode. A resonance interaction of the two modes can lead to a sharp change in the phase trajectory parameters, which is manifested by random variation of the wave amplitude. Calculations are simplified by using a method based on the formal quantization of the classical solution. The corresponding Schrodinger equation is obtained, the solution of which formally coincides with that of the well-known Coulomb problem. The probability of transitions between levels with different azimuthal quantum numbers corresponding to different amplitudes of the classical solution is calculated. The probability distribution function W(ΔΦ), where ΔΦ is the phase length of a regular (laminar) part of the trajectory, is determined using the “azimuthal number-angle” uncertainty relation.

Morphology and ultrastructure of megaspores and microspores of Isoetes savatieri Franchet (Lycophyta)

The general morphology, organization, structure and ornamentation of the sporoderm of megaspores and microspores of Isoetes savatieri Franchet have been studied with a stereoscopic microscope, light microscopy, transmission and scanning electron microscopy. The megaspores are trilete, 420–580 μm in equatorial diameter, subtriangular to globose in polar view with an evident equatorial flange (zone). The ornamentation is rugulate. A siliceous cover with a three-dimensional network overlies an exospore sporopolleninous apparently laminar with an equatorial–distal separation of laminae. Each exospore lamina is fused to those of other planes delimiting lacunae. The lacunae located in the outer part of the exospore are filled with silica. The endospore is thick, its structure is fibrillar. In section it is apparently composed of two zones. The microspores are monolete, 35–40 μm long and 20–25 μm wide and elliptic in polar view. A supra-laesural expansion is present. The sculpture is microechinate. A contrasted perispore composed of thin joined threads that form a lacunose structure can be distinguished in the microspore sporoderm. Two parts of the underlying exospore structurally different are evident: a laminar outer part and an inner part, mostly compact. An equatorial–distal separation between both parts of the exospore is evident. The endospore is fibrillar. The ultrastructural similarity of the spores of I. savatieri with those previously studied in Isoetes can be regarded as indicative of the homogeneity of this group of living lycopsids.

Interpretation of Repulsive Bifurcation of the Kuroshio and Gulf Stream with Classification of Their Branches

Distinctive features of the repulsive bifurcation of Kuroshio and Gulf Stream jets, induced from various observations, have been examined by equations of motion of a 1 1/2 -layer model in natural coordinates, although causes of the bifurcation are discussed little. From the conservation of volume flux through paths above a main thermocline, we have derived the equation MnKIn + MsKIs = 0 for northern (n) and southern (s), steady-state curved jets, just downstream of the bifurcation from a straight jet, where M is momentum flux through the path and KI, positive for cyclonic deflection of the streamline, is the curvature of a certain intermediate streamline in the laminar path. Assuming concentric streamlines in the curved path, which is narrower than 1–0.5 times the radius of curvature of the center streamline of the path, and symmetric momentum flux through a path element of unit width with respect to the center streamline, we found that KI approximates to the curvature averaged across the path, relative errors being <0.25−(0.25)2. Bifurcation starts with a ‘Big Bang’ and the leading turbulent front of a split jet may be unsteady, but the succeeding laminar part can be regarded as stationary or as translating at a constant velocity, as if frozen. Thus, such features are described by the equation. A provisional classification is given of various warm extrusions along branches of both Streams, all of which exemplify the repulsive bifurcation, except for the mainstreams with very narrow (< 30–40 km) branches, possibly due to misapplication of the model.

Sum of positive Lyapunov exponents for Kuramoto–Sivashinsky equation

The sum of positive Lyapunov exponents expresses the entropy loss of the systems under study. The sum of positive Lyapunov exponents for Kuramoto–Sivashinsky equation is studied. Its time series behaves like the on–off intermittency. They have laminar and burst parts alternately. It is remarkable that the system in turbulent state loses its entropy intermittency-like.

Morphological variation and phylogenetic relationships in the genus Anginon (Apiaceae)

The variation in habit and morphology of the twelve species of Anginon, a poorly known southern African genus of woody Apiaceae, is discussed and illustrated. The genus Glia is shown to be the obvious outgroup, sharing with Anginon two convincing synapomorphies: 1, the reduction or partial reduction of the laminar part of the leaves, and 2, the heavily cutinized outer walls of the fruit epidermis. Several characters of the internal structure of the fruit, together with other morphological characters, have been analysed by the cladistic method. Our interpretation of character evolution within the genus and a first hypothesis of phylogenetic relationships, showing three distinct infrageneric groups, is presented.

Implantation of the nail plate in plastic surgery

A series of cases are presented to illustrate the possible uses of the nail plate as an alternative support in reconstruction of some free edges (nasal ala, eyelid, auricle of the ear), and laminar bone parts, such as the floor of the orbit. This technique was used experimentally by the authors many years ago and has subsequently been used in clinical practice.

Large-eddy simulation of transition to turbulence in a boundary layer developing spatially over a flat plate

It is well known that subgrid models such as Smagorinsky's cannot be used for the spatially growing simulation of the transition to turbulence of flat-plate boundary layers, unless large-amplitude perturbations are introduced at the upstream boundary: they are over-dissipative, and the flow simulated remains laminar. This is also the case for the structure-function model (SF) of Métais &amp; Lesieur (1992). In the present paper we present a sequel to this model, the filtered-structure-function (FSF) model. It consists of removing the large-scale fluctuations of the field before computing its second-order structure function. Analytical arguments confirm the superiority of the FSF model over the SF model for large-eddy simulations of weakly unstable transitional flows. The FSF model is therefore used for the simulation of a quasi-incompressible (M∞ = 0.5) boundary layer developing spatially over an adiabatic flat plate, with a low level of upstream forcing. With the minimal resolution 650 × 32 × 20 grid points covering a range of streamwise Reynolds numbers Rex1 ε [3.4 × 105, 1.1 × 106], transition is obtained for 80 hours of time-processing on a CRAY 2 (whereas DNS of the whole transition takes about ten times longer). Statistics of the LES are found to be in acceptable agreement with experiments and empirical laws, in the laminar, transitional and turbulent parts of the domain. The dynamics of low-pressure and high-vorticity distributions is examined during transition, with particular emphasis on the neighbourhood of the critical layer (defined here as the height of the fluid travelling at a speed equal to the phase speed of the incoming Tollmien–Schlichting waves). Evidence is given that a subharmonic-type secondary instability grows, followed by a purely spanwise (i.e. time-independent) mode which yields peak-and-valley splitting and transition to turbulence. In the turbulent region, flow visualizations and local instantaneous profiles are provided. They confirm the presence of low- and high-speed streaks at the wall, weak hairpins stretched by the flow and bursting events. It is found that most of the vorticity is produced in the spanwise direction, at the wall, below the high-speed streaks. Isosurfaces of eddy viscosity confirm that the FSF model does not perturb transition much, and acts mostly in the vicinity of the hairpins.

A Proposal for a New Turbulent Premixed Combustion Model and Its Evaluation. (Development of the Model).

The authors propose a new turbulent premixed combustion model based on a hyperbolic tangent approximation of the reaction progress variable distribution in a one-dimensional laminar flame. Several different conventional models including FL (flamelet), EBU (eddy break up), ED(eddy dissipation) and FS (flame sheet) models are compared with the proposed model. Main features of the proposed model are as follows: (1) The model in constructed from a laminar flamelet part and a distributed flame part. (2) The model is applicable to laminar and turbulent flame analyses, and it does not depend upon the types of turbulent models. (3) Its distributed flame part includes EBU, ED and FS models as approximations.

A Proposal for a New Turbulent Combustion Model and Its Evaluation. 1st Report. Development of the Model.

The authors propose a new turbulent combustion model based on a hyperbolic tangent approximation of the reaction progress variable distribution in a one-dimensional laminar flame. Several different models including FL (flamelet), EBU (eddy break up), ED (eddy dissipation) and FS (flame sheet) models, are compared with the present model. Main features of the present model are as follows (1) The model is constructed from a laminar flamelet part and a distributed flame part. (2) The model is applicable to laminar and turbulent flame analysis, and it is independent of type of turbulence model. (3) Its distributed flame part includes EBU, ED and FS model as approximations.

Entrainment and evaporation of drops in the laminar part of a two-dimensional developing mixing layer

A formulation has been developed that combines the simplicity of an experimentally derived well-establishedcorrelation for describing the development of a mixing layer, and a rigorous approach for the description of the dynamics and evaporation of dense or dilute clusters of drops in large coherent vortices. An extensive parametric study has been performed by varying the radius of the drops in the drop-ladenstream both for high and low air/fuel mass ratio, as well as for constant initial drop number density, but at varying air/fuel mass ratio. The air/fuel mass ratio has also been varied at fixed drop radius in the drop-laden stream. Additional parameters independently varied were the temperature of the hot air stream, its velocity, and the velocity ratio between the two streams. The results show that it is possible to optimize the relative number of drops (with respect to the initial value) entrained into the coherent vortices of the mixing layer by using the velocity ratio as control parameter. The eventual liquid mass entrained in the cluster is an increasing function of the drop radius in the drop-carrying stream for typical drop number densities in sprays. The mass fraction of the evaporated fuel in the clusters can be optimized by using the velocity of the hot air stream as a control parameter. It is also shown that, in agreement with existing observations, the average drop radius may increase withaxial distance from the mixing layer inception point, and the reasons for this are explained.

Stratigraphy and genesis of Durorthids and Haplargids on dissected alluvial fans, western Mojave Desert, California

Presumed slow weathering rates in arid-region soils have given rise to the hypotheses that (1) the major carbonate source in most Aridisols is from eolian deposition, and (2) soil horizons cemented by carbonates and opaline silica are indicators of geomorphic surfaces of mid-Pleistocene age. To test these hypotheses we studied the stratigraphy and genesis of four soils, a Typic Durorthid (Alko series), two Typic Haplargids (Neuralia and Garlock series) and a Typic Torripsamment (Cajon series), found in close association on a dissected alluvial fan in the western Mojave Desert. The study area has a rolling topography with the calcareous Alko soil occurring on narrow summits and on the edges of broad summits, Neuralia (calcareous) on broad summits and on sideslopes, Garlock (noncalcareous) on sideslopes, and Cajon in the drainage ways. Soil stratigraphy in an excavated trench indicates that these soils are forming on deposits of differing ages. Paleomagnetic data indicate that at least four aggradational events have occurred in the past 783 ka (thousand years). The sequence of alluvial units in the area is similar to other sequences observed in the Mojave Desert and eastern San Joaquin Valley. The Garlock soil, with 5YR hues and a distinct carbonate-free argillic horizon, occurs on an older geomorphic surface, but younger deposit, than the Alko and Neuralia soils. Rodents are active in the profiles of Alko and Neuralia soils. Bioturbation of the upper soil mantle brings calcareous soil and duripan fragments to the surface, thus creating a pedogenically rejuvenated geomorphic surface on which profile development has been retarded. The occurrence of calcareous Haplargids adjacent to Haplargids that are noncalcareous to a depth of 65 cm, indicates that the atmospheric input of CaCO 3 in the study area has been minimal during the Holocene. The rate of CaCO 3 accumulation in soils of the study area is estimated to be 0.03 to 0.05 g cm −2 1000 yr −1 over the past 200 ka. A rate of accumulation of 0.27 g cm −2 1000 yr −1 is required to form the 2-cm-thick laminar cap of the upper Alko duripan during the Holocene. This amount could easily be added to the surface by bioturbation from calcareous duripan fragments in the profile. Thus, much of the laminar part of the duripan is probably a Holocene phenomenon.