| 1. |
- Bailey, S. C. C., et al.
(författare)
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Obtaining accurate mean velocity measurements in high Reynolds number turbulent boundary layers using Pitot tubes
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 715, s. 642-670
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Tidskriftsartikel (refereegranskat)abstract
- This article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from Reθ = 11 100 to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here.
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| 2. |
- Downs, Robert S. , I I I, et al.
(författare)
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Evaluation of miniature vortex generators for flow control in Falkner-Skan boundary layers
- 2016
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Ingår i: 54th AIAA Aerospace Sciences Meeting. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624103933
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Konferensbidrag (refereegranskat)abstract
- Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress growth of unsteady disturbances which would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works have demonstrated the versatility of these miniature vortex generators (MVGs) in zero pressure gradient boundary layers. In this work, mean-flow disturbances developing from MVGs in adverse and favorable pressure gradient boundary layers are measured systemically to assess the possibility of extending MVG-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values and an empirical scaling is found in congruence with existing results. The effect of streaks on transition in an adverse pressure gradient boundary layer is also tested and moderate increases to laminar flow extents are observed.
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| 3. |
- Downs, R. S. , I I I, et al.
(författare)
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Miniature vortex generators for flow control in falkner-skan boundary layers
- 2017
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Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics Inc.. - 0001-1452 .- 1533-385X. ; 55:2, s. 352-364
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Tidskriftsartikel (refereegranskat)abstract
- Vortex generators with heights comparable to displacement thickness are an effective means of producing persistent mean-flow streaks in laminar boundary layers. Inducing streaky base flows can suppress the growth of unsteady disturbances that would otherwise incite laminar-to-turbulent transition. Previous experimental and numerical works demonstrated the versatility of these miniature vortex generators in zero-pressure-gradient boundary layers. In this work, mean-flow disturbances developing from miniature vortex generators in adverse and favorable pressure-gradient boundary layers are measured systemically to assess the possibility of extending miniature vortex generator-based flow control to these scenarios. Boundary-layer streak amplitudes are measured across a range of Falkner-Skan m values, and an empirical scaling is found based on existing results. The effect of streaks on transition in an adverse pressure-gradient boundary layer is also tested, and moderate increases to laminar flow extents are observed.
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| 4. |
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| 5. |
- Fallenius, Bengt E. G., et al.
(författare)
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Assessment of wall vibrations in the long pipe facility at CICLoPE
- 2019
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Ingår i: Springer Proceedings in Physics. - Cham : Springer Nature. ; , s. 203-208
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Konferensbidrag (refereegranskat)abstract
- The present investigation aims at finding out whether there are pipe vibrations in the higher Reynolds number range at the Long Pipe Facility at the CICLoPE facility and to quantify their amplitude and frequency. Since vibrations are natural to any wind-tunnel facility, similar vibration measurements have also been performed in an established high-quality wind-tunnel facility, viz. the Minimum Turbulence Level (MTL) wind tunnel at KTH Royal Institute of Technology, in order to provide reference data. Results affirm that the amplitudes observed in Willert et al. (J Fluid Mech 826, 2017) are most likely the result of an amplification due to the optical set-up that is attached to the window plug, rather than vibrations of the pipe structure.
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| 6. |
- Fransson, Jens H. M., 1973-, et al.
(författare)
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An experimental setup for idealised studies on transition to turbulence on a generic compressor outlet guide vane
- 2018
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Ingår i: Proceedings of the ASME Turbo Expo. - : ASME Press. - 9780791850992
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Konferensbidrag (refereegranskat)abstract
- The understanding of flow phenomena in turbomachinery has come far with respect to three-dimensional flow patterns and pressure distributions. Much is due to improved measurements and a continuously evolving fidelity in computational fluid dynamics (CFD). Turbulence and transition in boundary layers are two classical areas where improvements in modeling are desired and where experimental validation is required. Apart from this, fundamental improvements in efficiency can be obtained by developing experimental resources where technologies affecting transition can be studied. The reduction in friction drag can be considerable if the transition to turbulence can be delayed. An experimental setup in an idealized configuration has been designed and built with the objective to study transition on a very large-scale guide vane profile at low speed. The purpose of the rig is to enable high quality fundamental studies of technologies to delay transition, but also to see how effects of manufacturing or other constraints may affect the boundary layer. In the present paper we report the first validation of the experimental setup, by comparing the first test results to CFD calculations performed during the rig design, i.e. no post-calculations with experimental data as input to the simulations have been done yet. The pressure distribution is in line with the design intent, which is a good indicator that the tunnel design is suitable for the intended purpose. At last we report some velocity measurements performed in the wake and we calculate the total drag based on the wake velocity deficit for various Reynolds numbers and with and without turbulence tripping tape. We illustrate that a two dimensional tripping around 7% of the chord from the leading edge can increase the total drag by 50% with respect to the reference case without tripping tape.
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| 7. |
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| 8. |
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| 9. |
- Fransson, Jens H.M., et al.
(författare)
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On the disturbance growth in an asymptotic suction boundary layer
- 2003
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Ingår i: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645. ; 482, s. 51-90
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Tidskriftsartikel (refereegranskat)abstract
- An experimental and theoretical study on the effect of boundary layer suction on the laminar-turbulent transition process has been carried out. In the study an asymptotic suction boundary layer was established in a wind tunnel with a free-stream velocity of 5.0 m s(-1). Wall-normal suction (suction velocity 1.44 cm s(-1)) was applied over a large area and the boundary layer was nearly constant over a length of 1800 mm. Measurements were made both with and without suction so comparisons between the two cases could easily be made. Measurements of the development of the mean velocity distribution showed good agreement with theory. The Reynolds number based on the displacement thickness for the suction boundary layer was 347. Experiments on both the development of forced Tollmien-Schlichting (TS) waves and boundary layer disturbances introduced by free-stream turbulence were carried out. Spatial linear stability calculations for TS-waves, where the wall-normal velocity component is accounted for, were carried out for comparison with the experiments. This comparison shows satisfactory agreement even though the stability of the asymptotic suction profile is somewhat overpredicted by the theory. Free-stream turbulence (FST) was generated by three different grids, giving turbulence intensities at the leading edge of the plate between 1.4% and 4.0%. The FST induces disturbances in the boundary layer and it was shown that for the present suction rate the disturbance level inside the boundary layer is constant and becomes proportional to the FST intensity. In all cases transition was prevented when suction was applied whereas without suction the two highest levels of grid turbulence gave rise to transition. Despite a twofold reduction in the boundary layer thickness in the suction case compared to the no suction case the spanwise scale of the streaky structures was almost constant.
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| 10. |
- Fransson, Jens H.M., et al.
(författare)
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On the hydrodynamic stability of channel flow with cross flow
- 2003
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 15:2, s. 436-441
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Tidskriftsartikel (refereegranskat)abstract
- We study plane channel flow, with a homogeneous cross flow through porous walls, mainly with respect to the stability to two-dimensional wave disturbances. Since the stability of a shear flow depends both on the velocity distribution and the Reynolds number we partly investigated this flow under the conditions that the flow Reynolds number was constant. The flow exhibits some interesting and unexpected stability characteristics. The effect of the cross flow was for certain parameter regions stabilizing and for others destabilizing. The latter result is in contrast to previous studies.
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