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| 2. |
- Cimarelli, Andrea, et al.
(författare)
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Scalings of the outer energy source of wall-turbulence
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- By means of the multidimensional description given by the Kolmogorov equation we study the energy transfer physics and the production mechanisms of wall-turbulent flows at moderately high Reynolds numbers. Two driving mechanisms are identified for the energy fluxes. The first stronger one, here called driving scale-range (DSR), belongs to the near-wall cycle. As expected, its topology remains unaltered with Reynolds number while its intensity is found to slightly increase with Re. The second mechanism, here called outer scale-range (OSR), takes place in the overlap layer and highlights different features in agreement with the attached eddies hypothesis usually considered to describe the overlap dynamics.
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| 3. |
- Discetti, S., et al.
(författare)
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Characterization of very-large-scale motions in high-Re pipe flows
- 2019
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Ingår i: Experimental Thermal and Fluid Science. - : Elsevier. - 0894-1777 .- 1879-2286. ; , s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- Very-large-scale structures in pipe flows are characterized using an extended Proper Orthogonal Decomposition (POD)-based estimation. Synchronized non-time-resolved Particle Image Velocimetry (PIV) and time-resolved, multi-point hot-wire measurements are integrated for the estimation of turbulent structures in a pipe flow at friction Reynolds numbers of 9500 and 20000. This technique enhances the temporal resolution of PIV, thus providing a time-resolved description of the dynamics of the large-scale motions. The experiments are carried out in the CICLoPE facility. A novel criterion for the statistical characterization of the large-scale motions is introduced, based on the time-resolved dynamically-estimated POD time coefficients. It is shown that high-momentum events are less persistent than low-momentum events, and tend to occur closer to the wall. These differences are further enhanced with increasing Reynolds number.
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| 4. |
- Facciolo, Luca, et al.
(författare)
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A study of swirling turbulent pipe and jet flows
- 2007
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 19:3
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Tidskriftsartikel (refereegranskat)abstract
- Axially rotating turbulent pipe flow is an example in which the rotation strongly affects the turbulence, which then also influences the mean flow properties. For instance, in the fully developed flow as well, the fluid is not in solid body rotation due to the influence of the cross-stream Reynolds stress. The present paper reports new measurements from a rotating pipe flow and the streamwise mean velocity distribution is compared with recent scaling ideas of Oberlack [J. Fluid Mech. 379, 1 (1999)] and good agreement is found. A second part of the paper deals with the initial stages when the flow leaves the pipe and forms a swirling jet. The measurements in the jet show that at some distance downstream (approximately five jet diameters) the central part of the jet actually rotates in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross-stream Reynolds shear stress.
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| 5. |
- Fransson, Jens H. M., 1973-, et al.
(författare)
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Advanced Fluid Research On Drag reduction In Turbulence Experiments
- 2011
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6596.
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Konferensbidrag (refereegranskat)abstract
- A hot topic in today's debate on global warming is drag reduction in aeronautics. The most bene cial concept for drag reduction is to maintain the major portion of the airfoil laminar. Estimations show that the potential drag reduction can be as much as 15%, which would give a signi cant reduction of NOx and CO emissions in the atmosphere considering that the number of aircraft take os, only in the EU, is over 19 million per year. An important element for successful ow control, which can lead to a reduced aerodynamic drag, is enhanced physical understanding of the transition to turbulence process.
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| 6. |
- Fransson, Jens H. M., 1973-, et al.
(författare)
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Base flow modulations for skin-friction drag reduction
- 2011
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Ingår i: Journal of Physics. - : IOP Publishing.
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Konferensbidrag (refereegranskat)abstract
- Recent experimental investigations have shown that spanwise modulations of thebase flow may delay transition to turbulence. In this paper we explore the possibility togenerate streaks of much larger amplitude than previously reported by using a row of miniaturevortex generators (MVGs). Here, we present the first boundary layer experiment where streakamplitudes exceeding 30% have been produced without having any secondary instability actingon them. Furthermore, the induced drag due to the streaky base flow is quantified and it isdemonstrated that the streaks can be reinforced by placing a second array of MVGs downstreamof the first one. In this way it is possible to make the control more persistent in the downstreamdirection. We conclude that the specially designed set of MVGs, as a boundary layer modulator,is a promising candidate for successfully setting up robust and persistent streamwise streaks,which is a prerequisite for a successful flow control.
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| 7. |
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| 8. |
- Fransson, Jens H. M., et al.
(författare)
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Experimental and theoretical investigation of the nonmodal growth of steady streaks in a flat plate boundary layer
- 2004
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 16:10, s. 3627-3638
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Tidskriftsartikel (refereegranskat)abstract
- An experimental and theoretical investigation aimed at describing the nonmodal growth of steady and spanwise periodic streamwise streaks in a flat plate boundary layer is presented. Stable laminar streaks are experimentally generated by means of a spanwise periodic array of small cylindrical roughness elements fixed on the plate. The streamwise evolution of the streaks is measured and it is proved that, except in a small region near the roughness elements, they obey the boundary layer scalings. The maximum achievable amplitude is mainly determined by the relative height of the roughness elements. Results are compared with numerical simulations of optimal and suboptimal boundary layer streaks. The theory is able to elucidate some of the discrepancies recently noticed between experimentally realizable nonmodal growth and optimal perturbation theory. The key factor is found to be the wall normal location and the extension of the laminar standing streamwise vortices inducing the streaks. The differences among previous experimental works can be explained by different dominating streak generation mechanisms which can be linked to the geometry and to the ratio between the roughness height and the boundary layer scale.
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| 9. |
- Fransson, Jens H M, et al.
(författare)
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Experimental study of the stabilization of Tollmien-Schlichting waves by finite amplitude streaks
- 2006
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Ingår i: Sixth IUTAM Symposium on Laminar-Turbulent Transition. - DORDRECHT : SPRINGER. - 1402034598 ; , s. 299-304
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Konferensbidrag (refereegranskat)abstract
- The stabilization of Tollmien-Schlichting (TS) waves in a spanwise modulated Blasius boundary layer has been experimentally verified in the MTL wind tunnel at KTH. The alternating high and low speed streaks were created by regularly spaced cylindrical roughness elements mounted on the flat plate. It is shown that the larger the streak amplitude the larger is the damping effect of the TS-waves, which is in agreement with recent theoretical work.
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| 10. |
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