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M E Goldstein - One of the best experts on this subject based on the ideXlab platform.

  • effect on a laminar boundary layer of small amplitude Streamwise Vorticity in the upstream flow
    Journal of Fluid Mechanics, 2001
    Co-Authors: David W Wundrow, M E Goldstein
    Abstract:

    This paper is a generalization of a previous analysis of the effects of a small-amplitude, steady, Streamwise Vorticity field on the flow over an infinitely thin flat plate in an otherwise uniform stream. That analysis, which is given in Goldstein & Leib, required that the disturbance Reynolds number (i.e. the Reynolds number based on the disturbance velocity and length scale) be infinite while we consider the more general case where this quantity can be finite. The results show how an initially linear perturbation of the upstream flow ultimately leads to a small-amplitude but nonlinear cross-flow far downstream from the leading edge. This flow can, under certain conditions, cause the Streamwise velocity profiles to develop distinct shear layers in certain localized spanwise regions. These shear layers, which are remarkably similar to the ones that develop in Tollmien-Schlichting-wave transition, are highly inflectional and can therefore support the rapidly growing inviscid instabilities that are believed to break down into turbulent spots. Numerical computations are carried out for input parameters which approximate the flow conditions of some recent experimental studies of the so-called Klebanoff-mode phenomenon

  • effect on a laminar boundary layer of small amplitude Streamwise Vorticity in the upstream flow
    Journal of Fluid Mechanics, 2001
    Co-Authors: David W Wundrow, M E Goldstein
    Abstract:

    This paper is a generalization of a previous analysis of the effects of a small-amplitude, steady, Streamwise Vorticity field on the flow over an infinitely thin flat plate in an otherwise uniform stream. That analysis, which is given in Goldstein & Leib (1993), required that the disturbance Reynolds number (i.e. the Reynolds number based on the disturbance velocity and length scale) be infinite while the present paper considers the more general case where this quantity can be finite. The results show how an initially linear perturbation of the upstream flow ultimately leads to a small-amplitude but nonlinear cross-flow far downstream from the leading edge. This flow can, under certain conditions, cause the Streamwise velocity profiles to develop distinct shear layers in certain localized spanwise regions. These shear layers, which are remarkably similar to the ones that develop in Tollmien–Schlichting-wave transition (Kovasznay, Komoda & Vasudeva 1962), are highly inflectional and can therefore support the rapidly growing inviscid instabilities that are believed to break down into turbulent spots (Greenspan & Benney 1963, and, subsequently, many others). Numerical computations are carried out for input parameters which approximate the flow conditions of some recent experimental studies of the so-called Klebanoff-mode phenomenon. The results are used to explain some of the experimental observations, and, more importantly, to explain why the averaged quantities usually reported in these experiments do not correlate well with the turbulent-spot formation and therefore with the overall transition process.

  • three dimensional boundary layer instability and separation induced by small amplitude Streamwise Vorticity in the upstream flow
    Journal of Fluid Mechanics, 1993
    Co-Authors: M E Goldstein, S J Leib
    Abstract:

    We consider the effects of a small-amplitude, steady, Streamwise Vorticity field on the flow over an infinitely thin flat plate in an otherwise uniform stream. We show how the initially linear perturbation, ultimately leads to a small-amplitude but nonlinear cross flow far downstream from the leading edge. This motion is imposed on the boundary-layer flow and eventually causes the boundary layer to separate. The Streamwise velocity profiles within the boundary layer become inflexional in localized spanwise regions just upstream of the separation point. The flow in these regions is therefore susceptible to rapidly growing inviscid instabilities.

David W Wundrow - One of the best experts on this subject based on the ideXlab platform.

  • effect on a laminar boundary layer of small amplitude Streamwise Vorticity in the upstream flow
    Journal of Fluid Mechanics, 2001
    Co-Authors: David W Wundrow, M E Goldstein
    Abstract:

    This paper is a generalization of a previous analysis of the effects of a small-amplitude, steady, Streamwise Vorticity field on the flow over an infinitely thin flat plate in an otherwise uniform stream. That analysis, which is given in Goldstein & Leib, required that the disturbance Reynolds number (i.e. the Reynolds number based on the disturbance velocity and length scale) be infinite while we consider the more general case where this quantity can be finite. The results show how an initially linear perturbation of the upstream flow ultimately leads to a small-amplitude but nonlinear cross-flow far downstream from the leading edge. This flow can, under certain conditions, cause the Streamwise velocity profiles to develop distinct shear layers in certain localized spanwise regions. These shear layers, which are remarkably similar to the ones that develop in Tollmien-Schlichting-wave transition, are highly inflectional and can therefore support the rapidly growing inviscid instabilities that are believed to break down into turbulent spots. Numerical computations are carried out for input parameters which approximate the flow conditions of some recent experimental studies of the so-called Klebanoff-mode phenomenon

  • effect on a laminar boundary layer of small amplitude Streamwise Vorticity in the upstream flow
    Journal of Fluid Mechanics, 2001
    Co-Authors: David W Wundrow, M E Goldstein
    Abstract:

    This paper is a generalization of a previous analysis of the effects of a small-amplitude, steady, Streamwise Vorticity field on the flow over an infinitely thin flat plate in an otherwise uniform stream. That analysis, which is given in Goldstein & Leib (1993), required that the disturbance Reynolds number (i.e. the Reynolds number based on the disturbance velocity and length scale) be infinite while the present paper considers the more general case where this quantity can be finite. The results show how an initially linear perturbation of the upstream flow ultimately leads to a small-amplitude but nonlinear cross-flow far downstream from the leading edge. This flow can, under certain conditions, cause the Streamwise velocity profiles to develop distinct shear layers in certain localized spanwise regions. These shear layers, which are remarkably similar to the ones that develop in Tollmien–Schlichting-wave transition (Kovasznay, Komoda & Vasudeva 1962), are highly inflectional and can therefore support the rapidly growing inviscid instabilities that are believed to break down into turbulent spots (Greenspan & Benney 1963, and, subsequently, many others). Numerical computations are carried out for input parameters which approximate the flow conditions of some recent experimental studies of the so-called Klebanoff-mode phenomenon. The results are used to explain some of the experimental observations, and, more importantly, to explain why the averaged quantities usually reported in these experiments do not correlate well with the turbulent-spot formation and therefore with the overall transition process.

Dorian Liepmann - One of the best experts on this subject based on the ideXlab platform.

  • the role of Streamwise Vorticity in the near field entrainment of round jets
    Journal of Fluid Mechanics, 1992
    Co-Authors: Dorian Liepmann, Morteza Gharib
    Abstract:

    The role of Streamwise vortex structures in the near-field (x/d < 10) evolution of a round jet is examined. In free shear layers the Streamwise Vorticity develops into Bernal-Roshko structures which are Streamwise vortex pairs. Similar structures are shown to exist in round jets. These structures, which evolve and amplify in the braid region between primary vortical structures, are shown to drastically alter the entrainment process in the near field and to increase the rate at which fluid is entrained into the jet. As the flow evolves downstream, the efficiency of the Streamwise Vorticity in entraining fluid increases relative to that of the azimuthal Vorticity. Beyond the end of the potential core regime, the entrainment process is mainly controlled by Streamwise Vorticity. These processes are identified via flow visualization and confirmed by detailed global entrainment measurements.

  • Streamwise Vorticity and entrainment in the near field of a round jet
    Physics of Fluids, 1991
    Co-Authors: Dorian Liepmann
    Abstract:

    The transition in round jets is accompanied by the transport of azimuthal to Streamwise Vorticity in the near field of the flow. In free shear layers, this forms Bernal–Roshko structures which are Streamwise vortex pairs. Similar structures are shown to exist in round jets. The vortical ring structures form, grow, and amalgamate as they move downstream; they develop azimuthal instabilities. The resulting radial component of Vorticity in the flow is tilted backward and stretched by the axial shear in the braid region of the flow. As the vortex filaments stretch backward, the Vorticity increases and the induced velocity of the resulting vortex pair moves it farther outward, away from the shear layer into the stagnant fluid. The end of the pair is then left behind the braid and forms a streamerlike structure. This process is identified via flow visualization and confirmed by detailed entrainment measurements. The structures are shown to grow in the braid region. The Streamwise structures have a major impact on the instantaneous entrainment field and, therefore, on the mixing process of the jet.

Surya Pratap Vanka - One of the best experts on this subject based on the ideXlab platform.

  • large eddy simulation of turbulence driven secondary flow in a square duct
    Physics of Fluids, 1991
    Co-Authors: Ravi K Madabhushi, Surya Pratap Vanka
    Abstract:

    The fully developed turbulent flow in a straight duct of square cross section has been simulated using the large eddy simulation (LES) technique. A mixed spectral‐finite difference method has been used in conjunction with the Smagorinsky eddy‐viscosity model for the subgrid scales. The simulation was performed for a Reynolds number of 360 based on friction velocity (5810 based on bulk velocity) and duct width. The simulation correctly predicted the existence of secondary flows and their effects on the mean flow and turbulence statistics. The results are in good qualitative agreement with the experimental data available at much higher Reynolds numbers. It is observed that both the Reynolds normal and shear stresses equally contribute to the production of mean Streamwise Vorticity.

Hassan Peerhossaini - One of the best experts on this subject based on the ideXlab platform.

  • heat transfer enhancement by artificially generated Streamwise Vorticity
    Eurotherm 2012 6th European Thermal Sciences Conference, 2012
    Co-Authors: Akram Ghanem, Thierry Lemenand, Dominique Della Valle, Charbel Habchi, Hassan Peerhossaini
    Abstract:

    Vortex-induced heat transfer enhancement exploits longitudinal and transverse pressure-driven vortices through the deliberate artificial generation of large-scale vortical flow structures. Thermal-hydraulic performance, Nusselt number and friction factor are experimentally investigated in a HEV (high-efficiency vortex) mixer, which is a tubular heat exchanger and static mixer equipped with trapezoidal vortex generators. Pressure gradients are generated on the trapezoidal tab initiating a Streamwise swirling motion in the form of two longitudinal counter-rotating vortex pairs (CVP). Due to the Kelvin-Helmholtz instability, the shear layer generated at the tab edges, which is a production site of turbulence kinetic energy (TKE), becomes unstable further downstream from the tabs and gives rise to periodic hairpin vortices. The aim of the study is to quantify the effects of hydrodynamics on the heat- and masstransfer phenomena accompanying such flows for comparison with the results of numerical studies and validate the high efficiency of the intensification process implementing such vortex generators. The experimental results reflect the enhancement expected from the numerical studies and confirm the high status of the HEV heat exchanger and static mixer.

  • effects of embedded Streamwise Vorticity on turbulent mixing
    Chemical Engineering and Processing, 2009
    Co-Authors: Hakim Mohand Kaci, Thierry Lemenand, Dominique Della Valle, Hassan Peerhossaini
    Abstract:

    Abstract This work concerns the characterization of turbulent flow underlying mixing in the presence of Streamwise Vorticity. An experimental test section made of a cylindrical tube equipped with seven rows of Streamwise vortex generators was designed and constructed for this study. Each row is composed of four vortex generators fixed symmetrically on the tube wall. This new type of mixer, called a high-efficiency vortex (HEV) mixer, generates coherent structures in the form of longitudinal counter-rotating vortices. The resulting flow enhances radial mass transfer and thus facilitates particle dispersion and mixing. The energy cost of this mixer used as an emulsifier has been evaluated as up to a thousand times less than that of other static mixers for a given interface area generation (Lemenand et al. [1] , [2] ). The aim of this work is to study experimentally and numerically the turbulence structure and mixing properties of the flow composed of Streamwise vortices superimposed on a turbulent flow, in particular the more energetic structures present in the base flow. Experiments were carried out in the test section in a flow loop by measuring instantaneous velocities by laser Doppler anemometry. Numerical simulations of the velocity distribution and turbulence field inside the flow were conducted for various turbulence models using a computational fluid dynamic CFD package. Attention is focused on the evolution and distribution of turbulent kinetic energy dissipation as the underlying mechanism for turbulent mixing. Mean and turbulent quantities are compared with experimental results. Both laboratory experiments and numerical simulations show a vortex zone behind each tab that could explain the efficiency of the HEV mixer. This study provides a basis for understanding the physical mechanisms in the mixing and homogenizing of the flow and therefore the efficiency of the mixer.

  • intensification of heat transfer and mixing in multifunctional heat exchangers by artificially generated Streamwise Vorticity
    Applied Thermal Engineering, 2006
    Co-Authors: Sebastien Ferrouillat, Patrice Tochon, C Garnier, Hassan Peerhossaini
    Abstract:

    Abstract Compact heat exchangers are well known for their ability to transfer a large amount of heat while retaining low volume and weight. The purpose of this paper is to study the potential of using this device as a mixer as well as a chemical reactor, generally called a multifunctional heat exchanger (MHE). Indeed, the question arises: can these geometries combine heat transfer and mixing in the same device? Such a technology would offer many potential advantages, such as better reaction control (through the thermal aspect [S. Ferrouillat, P. Tochon, H. Peerhossaini, D. Della Valle, Open-loop thermal control of exothermal chemical reactions in multifunctional heat exchangers, Int. J. Heat Mass Transfer, in press]), improved selectivity (through intensified mixing, more isothermal operation and shorter residence time, and sharper residence time distribution (RTD)), byproduct reduction, and enhanced safety. Several geometries of compact heat exchanger based on turbulence generation are available. This paper focuses on one type: vortex generators. The main objective is to contribute to the determination of turbulent flow inside various geometries by computational fluid dynamics methods. These enhanced industrial geometries are studied in terms of their thermal-hydraulic performance and macro-/micro-mixing ability [S. Ferrouillat, P. Tochon, H. Peerhossaini, Micromixing enhancement by turbulence: application to multifunctional heat exchangers, Chem. Eng. Process., in press]. The longitudinal vortices they generate in a channel flow turn the flow perpendicular to the main flow direction and enhance mixing between the fluid close to the fin and that in the middle of the channel. Two kinds of vortex generators are considered: a delta winglet pair and a rectangular winglet pair. For both, good agreement is obtained between numerical results and data in the literature. The vortex generator concept is found to be very efficient in terms of heat-transfer enhancement and macro-mixing. Nevertheless, the micro-mixing level is poor due to strong inhomogeneities: the vortex generator must be used as a heat-transfer enhancement device or as a static mixer for macro- and meso-mixing.