10:30
Jets and Wakes 1
Chair: Carlo Massimo Casciola
10:30
15 mins
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Three-component velocity measurements in a momentum-conserving, axisymmetric, turbulent jet
F. Gökhan Ergin, Clara Marika Velte
Abstract: Experiments have been performed on a momentum conserving axisymmetric turbulent jet, the turbulence characteristics of which are well known [1]. Simultaneous three-component velocity measurements are acquired with high spatial and temporal resolution, using a new triple-sensor hotwire probe. Velocity and directional calibrations are performed using a dedicated automatic calibration system. Two experiments are performed; one for capturing the average velocity field in a 3D volume, and one for investigating the turbulence spectra in specific points in space. In the first experiment, measurements are performed in 9 equidistant cross-planes, from 10- to 50-diameters downstream of the nozzle using a computer-controlled traversing system. The spatial resolution is as low as 1 mm and the sampling rate was 10 kHz. In the second experiment, long velocity time histories are acquired with 50 kHz sampling rate to perform power spectral density computations for each velocity component. Preliminary results of velocity capture confirm the general characteristics of the turbulent jet. The power spectra at different positions indicate that the turbulent fluctuations are not isotropic at lower frequencies.
[1] H.J. Hussein, S.P. Capp, and W.K. George. Velocity measurements in a high-Reynolds-number, momentum-conserving, axisymmetric, turbulent jet. J. Fluid Mech. 258: 31–75, 1994.
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10:45
15 mins
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Turbulent wakes of plates with non-equilibrium similarity scalings
Martin Obligado, John Christos Vassilicos
Abstract: We have conducted hot wire anemometry measurements of six different axisymmetric turbulent wakes which demonstrate the existence in all these wakes of non-equilibrium mean flow profile scalings and of the underlying self-preservation and non-equilibrium dissipation scalings. These mean flow profile scalings are different from those of all documented canonical boundary-free turbulent shear flows to date, all of which have been established for very far downstream regions.
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11:00
15 mins
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MEAN FLOW GENERATION DUE TO LONGITUDINAL LIBRATIONS OF SIDE-WALLS OF A ROTATING ANNULUS
Torsten Seelig, Abouzar Ghasemi V., Michael Kurgansky, Marten Klein, Andreas Will, Uwe Harlander
Abstract: Laboratory experiments with rotating annuli are reported that reveal a prograde jet, which is adjacent either to a (longitudinally) librating inner straight cylinder or to a librating inner truncated cone (frustum), whereas the outer cylindrical wall and bottom and top lids rotate with constant angular velocity. In the frustum case, the jet is located on a straight cylindrical surface which is circumscribed about the frustum and joins the bottom lid. These findings are supported by direct numerical simulations which show good agreement between experimental data and numerical results and, when the centrifugal instability of the Stokes boundary layer near the oscillating sidewall does not set in, highlight the important role of local dynamical processes in the corners, between the inner cylinder and the lids, in producing the prograde jet.
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11:15
15 mins
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Triple decomposition of a fluctuating velocity field in a multiscale flow
Pawel Baj, Paul J.K. Bruce, Oliver R.H. Buxton
Abstract: A method for the triple decomposition of fluctuating velocity in a multiscale flow, suitable for a spatiotemporal data set, is presented. It is applied to experimental data gathered by means of particle image velocimetry (PIV). The basic properties of the decomposed parts are shown. The presented method is then used to perform a conditional study on the residual transverse velocity fluctuations. It was found that phase locking occurs between the stochastic fluctuations embedded in the wakes of different bars, appearing after the wakes have merged.
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11:30
15 mins
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Modulation of fine-scale velocity gradient phenomena by concurrent large-scale velocity fluctuations in a developed shear flow
Oliver Buxton
Abstract: The interaction between the large-scale velocity fluctuations (uL) and the small-scale velocity gradient phenomena, such as dissipation and the vortex stretching term, are examined in a turbulent free-shear flow. The difference between the probability density functions of these small-scale quantities conditioned on the sign of uL is quantified by means of the Kullback-Leibler divergence. It is observed that the interaction between uL and the velocity gradient phenomena is maximised at a filter length of 4λ, where λ is the Taylor length-scale. It is postulated that this is consistent with a mean shear mechanism.
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11:45
15 mins
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Evolution of Coherent Structures in Under-expanded Supersonic Impinging Jets
Paul Stegeman, Julio Soria, Andrew Ooi
Abstract: This study looks at the spatio-temporal dynamics of the coherent structures found in under-expanded supersonic impinging jets from a circular nozzle at a pressure ratio of 3.4 and standoff distances of 2d and 5d. In these jets the development of coherent structures within the shear layer and their interaction with a standoff-shock are the principle components of a fundamental non-linear acoustic feedback mechanism. Temporally resolved and phase averaged data for each case was generated from a three dimensional hybrid large-eddy simulation on a non-uniform structured cylindrical grid with computational domains consisting of approximately 30 million nodes. From these datasets we investigate the development of the energy, topology and turbulence interactions of the coherent structures as a function of their distance travelled along the shear-layer.
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12:00
15 mins
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Direct numerical simulation of heat transfer of round subsonic impinging jets at high Reynolds number
Robert Wilke, Joern Sesterhenn
Abstract: Impinging jets provide an effective cooling method for various applications such as the cooling of aircraft turbine blades. The latest generation of high performance computers allows us to investigate those at practically relevant Reynolds numbers Re by means of direct numerical simulations. In order to analyse the heat transfer of a confined round impinging jet, two direct numerical simulations are performed at Re=3300 and Re=8000 using a grid of 512 x 512 x 512 respectively 1024 x 1024 x 1024 points. Each configuration is fully turbulent. The first one features two annular regions with local maxima of heat transfer at the impinging plate. These effects are related to high wall-normal turbulent heat fluxes caused by vortical structures of the turbulent flow field. The second simultion is ongoing. Its results will also be presented on the conference.
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12:15
15 mins
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Relation between enstrophy production and geometry near the turbulent/non-turbulent interface in free shear flows
Tomoaki Watanabe, Carlos da Silva, Yasuhiko Sakai, Kouji Nagata
Abstract: In many free shear flows, such as mixing layers, wakes and jets exhibit a sharp turbulent/non-turbulent interface (TNTI) separating regions of turbulent and non-turbulent or potential flow. In the present work the dependence of enstrophy production on the interface geometry near the TNTI is investigated by using direct numerical simulations (DNS) of a shear free turbulence (SFT) and a temporally developing planar jet (PJET). It is shown that the geometry of the TNTI has impacts on the mechanism governing enstrophy dynamics within the interface layer itself. In particular it is shown that enstrophy production within the turbulent sublayer is primarily associated with a convex shape of the interface both the SFT and PJET.
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