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Calibri 83ffff̙̙3f3fff3f3f33333f33333.9TU Delft Repositoryg n4uuidrepository linktitleauthorcontributorpublication yearabstract
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departmentresearch group programmeprojectcoordinates)uuid:9e6bbf793a5744ddb4e88e83a8d5d883Dhttp://resolver.tudelft.nl/uuid:9e6bbf793a5744ddb4e88e83a8d5d883#Gedaechtniseffekte in der TurbulenzHinze, J.O.qNOTE: REPORT IN GERMAN, ONLY THE ABSTRACT IS IN ENGLISH Memory effects in the flow of fluids are known long since. One has to reckon with such effects when studying the flow of nonNewtonian fluids, for instance when they exhibit a viscoelastic behaviour. Certain phenomena in turbulence also seem to point towards a viscoelastic behaviour. If, however, turbulence is considered as a hypothetical nonNewtonian fluid, there are essential differences with actual nonNewtonian fluids. First the requirement of objectivity concerning invariance of any constitutive equation against a timedependent bodily rotation, has to be dropped. Second, in rheology the principle of local action is used. This means that in the case of a simple fluid any deformationhistory can be described completely in terms of the local velocity gradient. In turbulence, action is not restricted to small regions, and higher derivatives of the mean velocity are often required to describe this action. In flows with a preferred mainflow direction, a distinction can be made between transport in axial and in transverse direction, when considering a "memory" behaviour that determines the degree of localness of action. If, for instance, turbulence shearstress is expressed in the transverse gradient of the mean velocity with an eddy viscosity (Boussinesq), this gradient may well vary in the transverse direction across "memory" distances, where the contribution to the total transport of momentum is still of importance. In some cases negative values may be obtained in a small region around a maximum of the mean velocity with an asymmetric distribution (wall jet) if the variation of this gradient is neglected. This would result in a "negative turbulence energy production ." These "memory" distances are of the order of the Lagrangian integral lengthscale or of the size of the bigger eddies. The action can then no longer be considered as being local in the rheological sense. In the axial, main flow direction, the situation may be different. Because of the relatively large convection velocity in this direction the memory distance is much larger than in transverse direction, i.e. many times the size of the bigger eddies. Though often the action may be described satisfactorily as if it were local, because of the size of the eddies involved the action is not strictly local. If we consider in those flows the effect of a nonconstant meanvelocity gradient on the turbulence shear stress by extending the simple Boussinesq relation with a term giving the change in axial direction of this velocity gradient. The obtained equations have been applied to a wake flow generated by a hemispherical cap on the wall of a constantpressure turbulent boundary layer, and the wakeflow of a circular cylinder in a uniform free stream. The result is that the extramemory effects are important in both the disturbed boundarylayer and the developing part of the wake at short distances from the cylinder. In the first case the eddy viscosity when corrected for the extramemory effect and rendered dimensionless with the local wallfriction velocity and boundarylayer thickness, still follows the same distribution as for the undisturbed boundarylayer. When applying the obtained equations to actual flows, an uncertainty is presented concerning the quantitative evaluation of the length Lambda_1 and of the function G, because of lack of knowledge of the Lagrangian autocorrelation and the relaxation time or memory function. In the present paper the function G has been approximated by an exponential function, and a relation for Lambda_1. As an estimate it has proven to be usef<ul, at least for the time being.PTurbulenz; Gedaechtniseffekte; memory effects; turbulence; Boussinesq; viscositydereportTU Delft, Department of Hydraulic Engineering!Civil Engineering and GeosciencesHydraulic Engineering)uuid:1d74d05023ea4d7aa69c5386f58ca92aDhttp://resolver.tudelft.nl/uuid:1d74d05023ea4d7aa69c5386f58ca92ayMemory Effect in a Turbulent BoundaryLayer Flow Due to a Relatively Strong Axial Variation of the MeanVelocity Gradient/Hinze, J.O.; Sonnenberg, R.E.; Builtjes, P.J.H.TU DelftMeasurements have been made of the distributions of mean velocity, turbulence intensities and turbulence shearstress in a turbulent boundarylayer downstream of a hemispherical cap attached onto the plane rigid wall. The eddyviscosity, when computed in the classical way according to Boussinesq's concept from the lateral gradient of the mean velocity and the turbulence shearstress, showed a very strong nonuniform lateral distribution, also across the outer region of the boundarylayer. More over , the nondimensional values of the eddy viscosity, using the wallfriction velocity and the boundarylayer thickness as the velocity scale and length scale respectively, were higher than those for the boundarylayer when not disturbed by the wake of the spherical cap. However, when account is taken of an axial memory effect of the streamwise variation of the lateral gradient of the meanvelocity, the values of the nondimensional eddy viscosity are close to those for the undisturbed boundarylayer.:turbulence; boundary layer; fluid mechanics; memory effectenDelft University of Technology)uuid:7ff6762fd28649e2b4a7df575464d8edDhttp://resolver.tudelft.nl/uuid:7ff6762fd28649e2b4a7df575464d8ed2Contribution la transition dans la couche limite:Hinze, J.O.; Leijdens, H.; Van den Brug, J.B.; Kleiweg, D.kA l'aide de la mthode du ruban vibrant de Schubauer et Klebanoff. on a tudi exprimentalement l'volution tridimensionnelle d'une perturbation sinusodale locale introduite dans une couche limite laminaire. Les distributions de la vitesse moyenne et de l'intensit de la composante axiale de la turbulence, de mme que le spectre d'nergie de celleci, ont t mesurs par anmomtre fil chaud. Les mesures mettent dairement en vidence le dveloppement tridimensionnel des tourbillons perturbateurs depuis leur naissance, en passant par la forme en boude jusqu' la forme en pingle cheveux. Les harmoniques superieure se dveloppent un stade ultrieur et ne prennent de l'importance que peu avant le passage au rgime turbulent, les premiers symptrnes tant l'apparition d'pines ou spikes (simple, doubles ou triples) et de bouffes de turbulence localises.3turbulence; boundary layer transition; measurementsfr Societe hydrotechnique de France
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