| 1. |
- 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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| 2. |
- Camarri, Simone, et al.
(författare)
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Stability analysis of experimental flow fields behind a porous cylinder for the investigation of the large-scale wake vortices
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 715, s. 499-536
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Tidskriftsartikel (refereegranskat)abstract
- When the linear stability analysis is applied to the time-averaged flow past a circular cylinder after the primary instability of the wake, a nearly marginally stable global mode is predicted with a frequency in time equal to that of the saturated vortex shedding. This behaviour has recently been shown to hold up to Reynolds number Re = 600 by direct numerical simulations. In the present work we verify that the global stability analysis provides reasonable estimation also when applied to experimental velocity fields measured in the wake past a porous circular cylinder at Re similar or equal to 3.5 x 10(3). Different intensities of continuous suction and blowing through the entire surface of the cylinder are considered. The global direct and adjoint stability modes, derived from the experimental data, are used to sort the random instantaneous snapshots of the velocity field in phase. The proposed method is remarkable, sorting the snapshots in phase with respect to the vortex shedding, allowing phase-averaged velocity fields to be extracted from the experimental database. The phase-averaged flow fields are analysed in order to study the effect of the transpiration on the kinematical characteristics of the large-scale wake vortices.
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| 3. |
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| 4. |
- 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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| 5. |
- 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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| 6. |
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| 7. |
- Fallenius, Bengt E. G., 1977-
(författare)
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Experimental design and vortex analyses in turbulent wake flows
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A new experimental setup for studies on wake flow instability and its control that successfully has been designed and manufactured, is introduced and de- scribed. The main body is a dual-sided flat plate with an elliptic leading edge and a blunt trailing edge. Permeable surfaces enable boundary layer suction and/or blowing that introduce the unique feature of adjusting the inlet condition of the wake created behind the plate. This, in combination with a trailing edge that is easily modified, makes it an ideal experiment for studies of different control methods for the wake flow instability as well as extensive parameter studies. Experimental validation of the setup has been performed by means of measurements of the wake symmetry and boundary layer velocity profiles at the trailing edge. Some preliminary results on the Strouhal number versus different inlet conditions are reported. Additionally, an in-house vortex detection (VD) program has been developed in order to detect, analyse and compare small-scale vortical structures in instantaneous velocity fields from flow measurements. This will be a powerful tool for comparison of wake characteristics for varying inlet conditions and control methods in the new experimental setup. Measurements from three completely separate experimental setups with different geometries and flow cases, have been analysed by the VD-program. i. In order to obtain improved ventilation we have studied the effect of pulsating inflow into a closed volume compared to having the inflow at a constant flow rate. We show that the number of small-scale eddies is significantly increased and that the stagnation zones are reduced in size, which enhances the mixing. ii. Instantaneous velocity fields in the wake behind a porous cylinder subjected to suction or blowing through the entire cylinder surface have also been analysed using the VD-program. The results show that the major change for different levels of blowing or suction is the location of vortices while the most common vortex size and strength are essentially unchanged. iii. Another study on how the geometry of a V-shaped mixer in a pipe flow affects the mixing have also been examined, where no general differences were found between different thicknesses, why a thickness that is favourable from an acoustic point of view can be chosen. We also propose a new method, using global mode analysis on experimental data, showing that randomly ordered snapshots of the velocity field behind the porous cylinder can be re-ordered and phase-averaged.
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| 8. |
- Fallenius, Bengt E. G., et al.
(författare)
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Experimental investigation of the influence of inlet conditions to a bluff body wake
- 2010
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Wind tunnel experiments have been performed in a bluff body wake with varying inlet conditions in order to enhance the physical understanding of the wake flow instability, which may lead to successful flow control and in turn reduced aerodynamic drag. The geometry consists of a rectangular-based forebody with permeable surfaces, an elliptic leading edge and a blunt trailing edge. Length, width and base height of the forebody is 2.3, 0.5 and 0.04 meters, respectively. Applying continuous suction or blowing, of different levels, through the permeable surfaces along the forebody, varies the wall-normal trailing edge velocity profile in a systematic way and hence the inlet condition to the wake. The streamwise velocity component has been measured both throughout the boundary layer and in the wake behind the body using hot-wire anemometry. High-speed stereo PIV has been used in the wake in order to collect statistics of vortical structures in the wake. The influence of boundary layer parameters on the wake flow characteristics, such as vortex shedding frequency and base pressure, will be presented.
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| 9. |
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| 10. |
- Fallenius, Bengt E. G., et al.
(författare)
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Experimental study on the effect of pulsating inflow to an enclosure for improved mixing
- 2013
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Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier. - 0142-727X .- 1879-2278. ; 44, s. 108-119
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Tidskriftsartikel (refereegranskat)abstract
- Optimal control of inlet jet flows is of broad interest for enhanced mixing in ventilated rooms. The general approach in mechanical ventilation is forced convection by means of a constant flow rate supply. However, this type of ventilation may cause several problems such as draught and appearance of stagnation zones, which reduces the ventilation efficiency. A potential way to improve the ventilation quality is to apply a pulsating inflow, which has been hypothesised to reduce the stagnation zones due to enhanced mixing. The present study aims at testing this hypothesis, experimentally, in a small-scale two-dimensional water model using Particle Image Velocimetry with an in-house vortex detection program. We are able to show that for an increase in pulsation frequency or alternatively in the flow rate the stagnation zones are reduced in size and the distribution of vortices becomes more homogeneous over the considered domain. The number of vortices (all scales) increases by a factor of four and the swirl-strength by about 50% simply by turning on the inflow pulsation. Furthermore, the vortices are well balanced in terms of their rotational direction, which is validated by the symmetric Probability Density Functions of vortex circulation (Γ) around Γ= 0. There are two dominating vortex length scales in the flow, namely 0.6 and 0.8 inlet diameters and the spectrum of vortex diameters become broader by turning on the inflow pulsation. We conclude that the positive effect for enhanced mixing by increasing the flow rate can equally be accomplished by applying a pulsating inflow.
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