Attributes of the large-scale coherent motions in a shear layer

Bade, Kyle M.; Foss, J. F.

doi:10.1007/s00348-010-0842-z

Cited by 3 publications

(2 citation statements)

References 26 publications

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“…Ill advised counsel led the present first author to incorrectly assign half saddles where obtuse angles are now understood to be appropriate. The affected publications are Foss (2007), Bade and Foss (2010) and Foss (2013). The examples cited in these two publications are kinematically simple enough that replacing the S′ values with obtuse angles accomplishes the required corrections.…”

Section: Required Correctionsmentioning

confidence: 99%

A topological evaluation procedure to assess the integrity of a PIV vector field

Foss

Hedden

Barros

et al. 2016

Meas. Sci. Technol.

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Particle image velocimetry (PIV) provides a field of discrete vectors to represent a continuum velocity field. Various methods have been adopted to evaluate the integrity of the discrete vectors. In contrast, the present communication provides a systematic technique whereby the integrity of the measured field can be assessed using basic topological principles. Starting with the recognition that PIV provides a vector field overlaid on a planar surface, the analyst can identify the holes (to be punched through the surface of a sphere) and the handles (to be added to the sphere's surface) that will represent the appropriate surface for the topological analysis. These operations define the a priori Euler characteristic (χ A ) for the subject PIV image. The experimental Euler characteristic (χ E ) will be known from the properties of the measured vector field: nodes, saddles, etc. A necessary condition for the integrity of the measured vector field is that χ E = χ A . The topological bases for the integrity evaluation, including the important constraint of ensuring a smooth collapsed sphere, are carefully explained and described with examples.

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Section: Required Correctionsmentioning

confidence: 99%

A topological evaluation procedure to assess the integrity of a PIV vector field

Foss

Hedden

Barros

et al. 2016

Meas. Sci. Technol.

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“…Conference presentations [7] Annual workshop on Measurement and Optical Techniques for Aerospace Research (MOTAR), Cologne, Germany, 2015 [8] 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 2014 [9] 16th International Symposium on Flow Visualization, Okinawa, Japan, 2014 [10] 16th STAB-workshop, Göttingen, Germany, 2013…”

Section: Appendix a Photographs Of Experimentsmentioning

confidence: 99%

Experimental investigation of coherent structures generated by active and passive separation control in turbulent backward-facing step flow

Ma¹

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This dissertation presents the experimental investigation of coherent structures which were generated by active and passive separation control devices in a turbulent backward-facing step (BFS) flow. The Reynolds number was Re h = 2.0×10 4 , based on the free-stream velocity and the backward-facing step height. Three types of flow control devices, which are referred to as acoustic tube (AT), oscillating flap (OF) and vortex generators (VGs), were implemented independently on the backward-facing step in order to control the turbulent flow separation downstream of the step. For each implementation of the devices, 2D-2C planar particle image velocimetry was used to measure the separated shear layer, recirculation region and reattachment area downstream. Moreover, 3D-3C(t) time-resolved tomographic particle image velocimetry was employed, within a three-dimensional volume at a high sampling frequency, to measure the three-dimensional fluid motions in the separated shear layer downstream of the vortex generators. The first flow control device under investigation in the present work was the acoustic tube. As an active flow control device, the acoustic tube generated periodic pressure perturbations influencing the flow through a spanwise thin slot close to the separation edge at an actuation frequency of f a = 100 Hz, which was close to the most-amplified frequency of the shedding instability of the turbulent shear layer. The flow control results show that the acoustic tube can suppress the recirculation region and reduce the reattachment length by 43.7%. The rolling-up and pairing processes of spanwise vortices lead to an increase of the Reynolds shear stress. The coherent structures are extracted by proper orthogonal decomposition (POD) and are represented by two pairs of the spatial modes. Both the primary and secondary series of vortices iv are reconstructed as travelling waves corresponding to the actuation frequency f a and the overtone frequency 2• f a , respectively. The second flow control device under investigation was the oscillating flap. The oscillating flap was driven by periodic Ampere force and thereby generated small perturbations over the separation edge at a perturbation frequency of f p = 55 Hz, which was close to the half of the most-amplified frequency of the shedding instability of the turbulent shear layer. The time-averaged velocity fields show that the small perturbations can reduce the recirculation region behind the step and reduce the reattachment length by 31.0%. The phase-averaged results indicate a small streamwise ejection with a velocity of 0.7• U 0 when the oscillating flap moves downwards. The rolling-up and breakdown processes of the spanwise vortices result in considerable increases of coherent and incoherent parts of the Reynolds shear stress. The coherent structures in the turbulent shear layer are further analyzed by snapshot proper orthogonal decomposition (POD) and spatial dynamic mode decomposition (DMD) in energy and dynamic approaches complementarily. It is shown that the per...

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Investigation of coherent structures generated by acoustic tube in turbulent flow separation control

Geisler

Agocs

et al. 2015

Exp Fluids

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Attributes of the large-scale coherent motions in a shear layer

Cited by 3 publications

References 26 publications

A topological evaluation procedure to assess the integrity of a PIV vector field

A topological evaluation procedure to assess the integrity of a PIV vector field

Experimental investigation of coherent structures generated by active and passive separation control in turbulent backward-facing step flow

Investigation of coherent structures generated by acoustic tube in turbulent flow separation control

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