Wall Jet

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Israel J Wygnanski - One of the best experts on this subject based on the ideXlab platform.

  • on a turbulent Wall Jet flowing over a circular cylinder
    Journal of Fluid Mechanics, 1999
    Co-Authors: R Neuendorf, Israel J Wygnanski
    Abstract:

    The effect of surface curvature on the development of a two-dimensional Wall Jet was investigated experimentally. A comparison was made between a Wall Jet flowing around a circular cylinder and its plane equivalent. Velocity surveys and surface pressure measurements in the curved Wall Jet suggest the existence of two primary regions of interest. The first region, ranging from the end of the potential core to an approximate angular position of θ=120°, is characterized by a constant surface pressure and a self similarity of the mean flow. The second region is marked by an adverse pressure gradient leading to separation around θ=230°. The rate of spread of this flow, even in the initial region, is much higher than in the plane Wall Jet and so are the levels of turbulence and Reynolds stress. The dominant lengthscale in this flow is the radius of curvature R and the dominant velocity scale is the square root of the kinematic Jet momentum divided by the radius of curvature. Entrainment of ambient fluid which causes the Jet to adhere to the curved surface is also the main reason for its separation which is preceded by a rapid rate of spread of the flow leading to the failure of the boundary-layer approximation.

  • Film Cooling by a Pulsating Wall Jet.
    1997
    Co-Authors: Israel J Wygnanski, Alfonso Ortega, Hermann Fasel
    Abstract:

    Abstract : Turbulent Wall Jets have many important engineering applications. Much effort has been spent to investigate the plane turbulent Wall Jet without external stream (Launder and Rodi 1981,1983, Katz et al 1992, Wygnanski et al 1992) and with a relatively slow external stream (Zhou and Wygnanski 1993, Zhou et al 1996). However, many engineering applications seem to be described better by a Wall Jet embedded in a uniform stream of comparable velocity (the weak Wall Jet), for example, the cooling turbine blades and the flows over a wing equipped with a slotted flap (Fig. 1) represents such flows. The recently developed technique for separation control by periodic blowing/suction on the flap also belongs to category (Fig.2). Thus, it is important to provide a better understanding of the development of these flows. For example: the possibility of flow similarity, normalization of the mean velocity fields, scaling laws for the governing parameters, as well as the various responses to external excitations. This report represents but a single facet of the general effort endeavoring to use the Wall Jet for boundary layer control, film cooling and the exertion of force on a body through the use of what is commonly known as the Coanda Effect.

  • The Turbulent Wall Jet
    1993
    Co-Authors: Israel J Wygnanski, M D Zhou
    Abstract:

    Abstract : The flow of a Wall Jet embedded in an external stream or in a quiescent surrounding fluid was investigated experimentally. It was determined that the flow scales with the excess momentum injected into the stream and the viscosity of the fluid rather than the Jet efflux velocity and the dimension of the nozzle. In the presence of an external stream, a velocity ratio parameter R = (U i - U.)/(Uj + U ) had to be added in order to obtain a universal scaling of this flow. Low-amplitude external excitation, of the Wall Jet resulted in a reduction of skin friction and therefore a reduction of drag. The only possible cause for this behavior observed is the enhancement of the two-dimensionality of the large eddies as expressed by spanwise coherence and correlation measurements.... Turbulence, Control of Shear Flows, Wall Jet, Boundary Layer Control

  • on the applicability of various scaling laws to the turbulent Wall Jet
    Journal of Fluid Mechanics, 1992
    Co-Authors: Israel J Wygnanski, Y Katz, E Horev
    Abstract:

    The spatial distribution of the mean velocity in a two-dimensional turbulent Wall Jet was measured for a variety of nozzle Reynolds numbers. It was determined that the bulk of the flow is self-similar and it depends on the momentum flux at the nozzle and on the viscosity and density of the fluid. The width of the nozzle which was commonly used to reduce these data has no part in the similarity considerations as has already been suggested by Narasimha et al. (1973). This type of self-similarity can be easily applied to determine the skin friction, which can otherwise only be determined with considerable difficulty. It was also shown that the ‘law of the Wall’ applies only to the viscous sublayer. The Reynolds stress in the inviscid, inner portion of the flow is not constant thus the assumption of a ‘constant stress layer’ is not applicable. The applicability and universality of the ‘outer scaling law’ (i.e. Coles’ law of the wake) has been verified throughout the inviscid inner portion of the Wall Jet. The logarithmic velocity distribution cannot be derived by making the usual assumptions based on the constancy of the Reynolds stresses or on the thinness of the logarithmic region relative to the thickness of the inner layer.

R. G. Compton - One of the best experts on this subject based on the ideXlab platform.

  • Wall Jet electrodes: the importance of radial diffusion
    Journal of Applied Electrochemistry, 1993
    Co-Authors: R. G. Compton, A. C. Fisher, M. H. Latham, R. G. Wellington, C. M. A. Brett, A. M. C. F. Oliveira Brett
    Abstract:

    Existing models for mass transport to Wall Jet electrodes (WJE) are critically re-evaluated in the light of some new calculations and experiments which relate to steady state and transient currents observed at Wall Jet ring disc electrodes both in the presence and absence of homogeneous chemical kinetic complications. Specifically, it is concluded that in many cases quantitative descriptions are only realisable if transport to the electrode is described as having a significant contribution from radial diffusion in addition to the radial convection and normal diffusion that are usually only considered. This greatly increases the complexity of the mathematical solution of problems of interest and may prove an important limitation of WJEs vis-a-vis alternative hydrodynamic electrodes for other than analytical purposes.

  • Non-uniform accessibility and the use of hydrodynamic electrodes for mechanistic studies: A comparison of Wall-Jet and rotating disc electrodes
    Journal of Applied Electrochemistry, 1991
    Co-Authors: R. G. Compton, A. C. Fisher, G. P. Tyley
    Abstract:

    The merits of non-uniformly accessible electrodes for discriminating between electrode reaction mechanisms are established. In particular a comparison of the theoretical behaviour of the uniformly accessible rotating disc electrode and the highly non-uniformly accessible Wall-Jet electrode towards a wide range of different types of electrode process shows that mechanistic resolution is better achieved with the latter electrode geometry.

  • The Wall-Jet electrode and the study of electrode reaction mechanisms: The EC′ (catalytic) mechanism
    Journal of Applied Electrochemistry, 1991
    Co-Authors: R. G. Compton, A. C. Fisher, G. P. Tyley
    Abstract:

    The theory of EC′ (catalytic) reactions at the Wall-Jet electrode is developed using a computational procedure based on the Expanding Grid Backwards Implicit method. In particular, it is shown that the variation of the transport limited current with solution flow rate provides a means of characterising the EC′ mechanism. A working curve is presented which shows how the effective number of electrons transferred depends on a normalized rate constant and this permits the analysis of experimental data and the deduction of the homogeneous rate constant of the catalytic chemical reaction, for arbitrary electrode geometry, without recourse to further computation.

G. P. Tyley - One of the best experts on this subject based on the ideXlab platform.

A. C. Fisher - One of the best experts on this subject based on the ideXlab platform.

  • Wall Jet electrodes: the importance of radial diffusion
    Journal of Applied Electrochemistry, 1993
    Co-Authors: R. G. Compton, A. C. Fisher, M. H. Latham, R. G. Wellington, C. M. A. Brett, A. M. C. F. Oliveira Brett
    Abstract:

    Existing models for mass transport to Wall Jet electrodes (WJE) are critically re-evaluated in the light of some new calculations and experiments which relate to steady state and transient currents observed at Wall Jet ring disc electrodes both in the presence and absence of homogeneous chemical kinetic complications. Specifically, it is concluded that in many cases quantitative descriptions are only realisable if transport to the electrode is described as having a significant contribution from radial diffusion in addition to the radial convection and normal diffusion that are usually only considered. This greatly increases the complexity of the mathematical solution of problems of interest and may prove an important limitation of WJEs vis-a-vis alternative hydrodynamic electrodes for other than analytical purposes.

  • Non-uniform accessibility and the use of hydrodynamic electrodes for mechanistic studies: A comparison of Wall-Jet and rotating disc electrodes
    Journal of Applied Electrochemistry, 1991
    Co-Authors: R. G. Compton, A. C. Fisher, G. P. Tyley
    Abstract:

    The merits of non-uniformly accessible electrodes for discriminating between electrode reaction mechanisms are established. In particular a comparison of the theoretical behaviour of the uniformly accessible rotating disc electrode and the highly non-uniformly accessible Wall-Jet electrode towards a wide range of different types of electrode process shows that mechanistic resolution is better achieved with the latter electrode geometry.

  • The Wall-Jet electrode and the study of electrode reaction mechanisms: The EC′ (catalytic) mechanism
    Journal of Applied Electrochemistry, 1991
    Co-Authors: R. G. Compton, A. C. Fisher, G. P. Tyley
    Abstract:

    The theory of EC′ (catalytic) reactions at the Wall-Jet electrode is developed using a computational procedure based on the Expanding Grid Backwards Implicit method. In particular, it is shown that the variation of the transport limited current with solution flow rate provides a means of characterising the EC′ mechanism. A working curve is presented which shows how the effective number of electrons transferred depends on a normalized rate constant and this permits the analysis of experimental data and the deduction of the homogeneous rate constant of the catalytic chemical reaction, for arbitrary electrode geometry, without recourse to further computation.

Brian Launder - One of the best experts on this subject based on the ideXlab platform.

  • on the spreading mechanism of the three dimensional turbulent Wall Jet
    Journal of Fluid Mechanics, 2001
    Co-Authors: Tim Craft, Brian Launder
    Abstract:

    The paper explores, using different levels of turbulence closure, the computed behaviour of the three-dimensional turbulent Wall Jet in order to determine the cause of the remarkably high lateral rates of spread observed in experiments. Initially, to ensure accurate numerical solution, the equations are cast into the form appropriate to a self-similar shear flow thereby reducing the problem to one of two independent variables. Our computations confirm that the strong lateral spreading arises from the creation of streamwise vorticity, rather than from anisotropic diffusion. The predicted ratio of the normal to lateral spreading rates is, however, very sensitive to the approximation made for the pressure–strain correlation. The version that, in other flows, has led to the best agreement with experiments again comes closest in calculating the Wall Jet, although the computed rate of spread is still some 50% greater than in most of the measurements. Our subsequent calculations, using a forward-marching scheme show that, because of the strong coupling between axial and secondary flow, the flow takes much longer to reach its self-preserving state than in a two-dimensional Wall Jet. Thus, it appears very probable that none of the experimental data are fully developed.