Publications
168 results found
Melina G, Bruce PJK, Hewitt GF, et al., 2016, Heat transfer enhancement by grid-generated turbulence for a cylinder in crossflow, 5th International Conference on Jets, Wakes and Separated Flows (ICJWSF2015, Publisher: Springer, Pages: 125-132, ISSN: 0930-8989
The heat transfer coefficient is experimentally measured around a cylinder heated by ohmic effect. The heating technique approximates a uniform heat flux on the cylinder’s wall. The model is placed downstream of different perturbing grids in a wind tunnel. The Reynolds number based on the cylinder’s diameter varies between 10820 and 48830. At large distances from the grids, the heat transfer is significantly enhanced by using a fractal and a single square grid designed to increase turbulence intensity with a low blockage ratio. Significant differences exist between the heat transfer profiles measured in the production and in the decay region in positions where turbulence intensity is the same.
Alves Portela F, Papadakis, Vassilicos, 2016, Scale by scale energy budget in the near wake of a square cylinder, XXIV ICTAM
We study the energy cascade downstream of, but close to, a square cylinder Re = 3900 using two-point statistics, by means of DNS. Shortly after the mean recirculation region the turbulent energy spectrum exhibits a near −5/3 slope in the frequency domain. Our DNS analysis of the general Kármán-Howarth-Monin equation show that the production, transport, advection and fluctuating pressure terms cannot be neglected as they are in the Richardson-Kolmogorov equilibrium theory. Even so, the term which is interpreted as an interscale energy flux divergence in the case of homogeneous turbulence is close to −ε over a significant range of small scales, where ε is the turbulence dissipation rate.
Goto S, Vassilicos JC, 2016, Local equilibrium hypothesis and Taylor's dissipation law, FLUID DYNAMICS RESEARCH, Vol: 48, ISSN: 0169-5983
Vassilicos C, 2016, Streamlines in stationary homogeneous isotropic turbulenceand fractal-generated turbulence, Fluid Dynamics Research, Vol: 48, ISSN: 1873-7005
We compare streamline statistics in stationary homogeneous isotropic turbulence and in turbulence generated by a fractal square grid. We examine streamline segments characterised by the velocity difference ${\rm{\Delta }}u$ and the distance l between extremum points. We find close agreement between the stationary homogeneous isotropic turbulence and the decay region of the fractal-generated turbulence as well as the production region of the fractal flow for small segments. The statistics of larger segments are very similar for the isotropic turbulence and the decay region, but differ for the production region. Specifically, we examine the first, second and third conditional mean $\langle {[{\rm{\Delta }}u]}^{n}| l\rangle $. Noticeably, non-vanishing $\langle {[{\rm{\Delta }}u]}^{n}| l\rangle $ for $n=1,3$ are due to an asymmetry of positive and negative segments, i.e. those for which ${\rm{\Delta }}u\gt 0$ and ${\rm{\Delta }}u\lt 0$, respectively. This asymmetry is not only kinematic, but is also due to dissipative effects and therefore $\langle {[{\rm{\Delta }}u]}^{n}| l\rangle $ contains cascade information.
Melina G, Bruce PJK, Hewitt GF, et al., 2016, Heat transfer from a cylinder in the production and in the decay regions of grid-generated turbulence
Dairay T, Obligado M, Vassilicos JC, 2015, Non-equilibrium scaling laws in axisymmetric turbulent wakes, Journal of Fluid Mechanics, Vol: 781, Pages: 166-195, ISSN: 1469-7645
We present a combined direct numerical simulation and hot-wire anemometry study of an axisymmetric turbulent wake. The data lead to a revised theory of axisymmetric turbulent wakes which relies on the mean streamwise momentum and turbulent kinetic energy equations, self-similarity of the mean flow, turbulent kinetic energy, Reynolds shear stress and turbulent dissipation profiles, non-equilibrium dissipation scalings and an assumption of constant anisotropy. This theory is supported by the present data up to a distance of 100 times the wake generator’s size, which is as far as these data extend.
Alves Portela F, Papadakis G, Vassilicos, 2015, Scale-by-scale energy fluxes in anisotropic non-homogeneous turbulence behind a square cylinder, 68th Annual Meeting of the American Physical Society Division of Fluid Dynamics
Sponfeldner T, Soulopoulos N, Beyrau F, et al., 2015, The structure of turbulent flames in fractal- and regular-grid-generated turbulence, Combustion and Flame, Vol: 162, Pages: 3379-3393, ISSN: 0010-2180
This study reports on the use of fractal grids as a new type of turbulence generators in premixed combustion applications. Fractal grids produce turbulence fields which differ from those formed by regular turbulence generators such as perforated plates or meshes. Fractal grids generate high turbulence intensities over an extended region some distance downstream of the grid with a comparatively small pressure drop. Additionally, the integral scale of the flow does not change downstream of the grid. The extended region of high turbulence can also be optimized for the specific application at hand by changing certain parameters of the grid which makes it possible to design the downstream development of the turbulence field. Four space-filling fractal square grids were designed to independently vary the resulting turbulent field and a regular square mesh grid with similar turbulent intensity acted as a reference case. The structure of the resulting premixed V-shaped flames was investigated using Conditioned Particle Image Velocimetry (CPIV). At the same downstream position, flames in the turbulence field of fractal grids showed larger turbulent burning velocity compared to flames in regular grid generated turbulence. However, when compared for the same turbulence intensity, flames in fractal grid generated turbulence produced similar turbulent burning velocities compared to flames in regular grid generated turbulence. In particular, it could be shown that theories such as Taylor's theory of turbulent diffusivity and Damköhler's theory of premixed flame propagation, which were deduced from regular turbulence fields, adequately described the increase of effective flame surface area due to the increase in turbulence intensity. Using fractal grids allows the independent variation of the turbulent fluctuations, the integral length scale and the turbulent Reynolds number. An unexpected finding was that the burning velocity ratio, s t/s l was negligible influenced by the i
Laizet S, Nedic J, Vassilicos JC, 2015, The spatial origin of-5/3 spectra in grid-generated turbulence, Physics of Fluids, Vol: 27, ISSN: 1089-7666
A combined wind tunnel and computational study of grid-generated turbulencealong the centreline shows that the close to −5/3 power law signature of energyspectra in the frequency domain originates relatively close to the grid not only wherethe velocity derivative statistics become quite suddenly isotropic but also wherethe turbulent fluctuating velocities are very intermittent and non-Gaussian. As theinlet flow velocity increases, these power laws are increasingly well defined andincreasingly close to −5/3 over an increasing range of frequencies. However, thisrange continuously decreases with streamwise distance from the grid even though thelocal Reynolds number first increases and then decreases along the same streamwiseextent. The intermittency at the point of origin of the close to −5/3 power spectraconsists of alternations between intense vortex tube clusters with shallow broad-bandspectra and quiescent regions where the velocity fluctuations are smooth with steepenergy spectra.
Vassilicos JC, Laval J-P, Foucaut J-M, et al., 2015, The streamwise turbulence intensity in the intermediate layer of turbulent pipe flow, Journal of Fluid Mechanics, Vol: 774, Pages: 324-341, ISSN: 1469-7645
The spectral model of Perry et al. (J. Fluid Mech., vol. 165, 1986, pp. 163–199) predicts that the integral length scale varies very slowly with distance to the wall in the intermediate layer. The only way for the integral length scale’s variation to be more realistic while keeping with the Townsend–Perry attached eddy spectrum is to add a new wavenumber range to the model at wavenumbers smaller than that spectrum. This necessary addition can also account for the high-Reynolds-number outer peak of the turbulent kinetic energy in the intermediate layer. An analytic expression is obtained for this outer peak in agreement with extremely high-Reynolds-number data by Hultmark et al. (Phys. Rev. Lett., vol. 108, 2012, 094501; J. Fluid Mech., vol. 728, 2013, pp. 376–395). Townsend’s (The Structure of Turbulent Shear Flows, 1976, Cambridge University Press) production–dissipation balance and the finding of Dallas et al. (Phys. Rev. E, vol. 80, 2009, 046306) that, in the intermediate layer, the eddy turnover time scales with skin friction velocity and distance to the wall implies that the logarithmic derivative of the mean flow has an outer peak at the same location as the turbulent kinetic energy. This is seen in the data of Hultmark et al. (Phys. Rev. Lett., vol. 108, 2012, 094501; J. Fluid Mech., vol. 728, 2013, pp. 376–395). The same approach also predicts that the logarithmic derivative of the mean flow has a logarithmic decay at distances to the wall larger than the position of the outer peak. This qualitative prediction is also supported by the aforementioned data.
Gomes-Fernandes R, Ganapathisubramani B, Vassilicos JC, 2015, The energy cascade in near-field non-homogeneous non-isotropic turbulence, Journal of Fluid Mechanics, Vol: 771, Pages: 676-705, ISSN: 0022-1120
We perform particle image velocimetry (PIV) measurements of various terms of the non-homogeneous Kármán–Howarth–Monin equation in the most inhomogeneous and anisotropic region of grid-generated turbulence, the production region which lies between the grid and the peak of turbulence intensity. We use a well-documented fractal grid which is known to magnify the streamwise extent of the production region and abate its turbulence activity. On the centreline around the centre of that region the two-point advection and transport terms are dominant and the production is significant too. It is therefore impossible to apply usual Kolmogorov arguments based on the Kármán–Howarth–Monin equation and resulting dimensional considerations to deduce interscale flux and spectral properties. The interscale energy transfers at this location turn out to be highly anisotropic and consist of a combined forward and inverse cascade in different directions which, when averaged over directions, gives an interscale energy flux that is negative (hence forward cascade on average) and not too far from linear in r , the modulus of the separation vector r between two points. The energy spectrum of the streamwise fluctuating component exhibits a well-defined −5/3 power law over one decade, even though the streamwise direction is at a small angle to the inverse cascading direction.
Valente PC, Vassilicos JC, 2015, The energy cascade in grid-generated non-equilibrium decaying turbulence, PHYSICS OF FLUIDS, Vol: 27, ISSN: 1070-6631
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- Citations: 40
Laizet S, Nedic J, Vassilicos C, 2015, Influence of the spatial resolution on fine-scale features in DNS of turbulence generated by a single square grid, INTERNATIONAL JOURNAL OF COMPUTATIONAL FLUID DYNAMICS, Vol: 29, Pages: 286-302, ISSN: 1061-8562
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- Citations: 33
Nedic J, Supponen O, Ganapathisubramani B, et al., 2015, Geometrical influence on vortex shedding in turbulent axisymmetric wakes, PHYSICS OF FLUIDS, Vol: 27, ISSN: 1070-6631
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- Citations: 24
Goto S, Vassilicos JC, 2015, Energy dissipation and flux laws for unsteady turbulence, Physics Letters A, Vol: 379, Pages: 1144-1148, ISSN: 0375-9601
Direct Numerical Simulations of unsteady spatially periodic turbulence with time-dependent rms velocity u′(t) and integral length-scale L(t) show that not only the instantaneous energy dissipation rate but also the instantaneous energy flux at intermediate wavenumbers scales as View the MathML source where U0 and L0 are velocity and length scales characterizing initial or overall unsteady turbulence conditions. These high Reynolds number scalings are qualitatively different from the well-known u′(t)3/L(t) cornerstone scalings of equilibrium turbulence where the energy flux and dissipation are exactly balanced at all times.
Laizet S, Vassilicos JC, 2015, Stirring and scalar transfer by grid-generated turbulence in the presence of a mean scalar gradient, Journal of Fluid Mechanics, Vol: 764, Pages: 52-75, ISSN: 0022-1120
The stirring of a passive scalar by grid-generated turbulence in the presence of a mean scalar gradient is studied by direct numerical simulations (DNS) for six different grids: one fractal square grid with three fractal iterations, one fractal square grid with four fractal iterations, one fractal I grid and three different regular grids. Our results can be summarised as follows. (i) For all these grids, the turbulence intensity averaged over time and over a plane parallel to the grid takes its peak value when the streamwise position of this plane is between 0.75Meff and 1.5Meff where Meff is the effective mesh size introduced by Hurst & Vassilicos (Phys. Fluids, vol. 19, 2007, 035103). (ii) Downstream of the location of this peak, the turbulence intensity averaged in this way is greatly enhanced by the fractal grids relative to the regular grids even though the fractal grids have comparable or even lower blockage ratios. The novelty of this result lies in the fact that it concerns turbulence intensities averaged over lateral planes (as well as time). (iii) The pressure drop is about the same across grids of the same blockage ratio whether fractal or not, but the pressure recovery is longer for the fractal grids. (iv) Even so, the fractal grids enhance turbulent scalar fluxes by up to an order of magnitude in the region downstream of the aforementioned peak and they also greatly enhance the streamwise growth of the fluctuating scalar variance in that region. (v) We demonstrate on a simple planar model problem that the cause of this phenomenon lies in the fractality of the grids. (vi) The turbulence scalar flux coefficient is constant far enough downstream of all the present grids and is significantly dependent on the nature and details of the turbulence-generating grid.
Vassilicos JC, 2015, Dissipation in turbulent flows, Annual Review of Fluid Mechanics, Vol: 47, Pages: 95-114, ISSN: 0066-4189
This article reviews evidence concerning the cornerstone dissipation scaling of turbulence theory: ε = CεU3/L, with Cε = const., ε the dissipation rate of turbulent kinetic energy U2, and L an integral length scale characterizing the energy-containing turbulent eddies. This scaling is intimately linked to the Richardson-Kolmogorov equilibrium cascade. Accumulating evidence shows that a significant nonequilibrium region exists in various turbulent flows in which the energy spectrum has Kolmogorov’s −5/3 wave-number scalingoverawidewave-numberrange,yetCε ∼ RemI /RenL,withm ≈ 1 ≈ n, ReI a global/inlet Reynolds number, and ReL a local turbulence Reynolds number.
de la Cruz JMG, Rossi L, Vassilicos JC, 2015, Experimental evidence of the scalar spiral range in vortical flows, EXPERIMENTS IN FLUIDS, Vol: 56, ISSN: 0723-4864
de la Cruz JMG, Rossi L, Vassilicos JC, 2014, Three-dimensional effects in quasi two-dimensional free surface scalar experiments, EXPERIMENTS IN FLUIDS, Vol: 55, ISSN: 0723-4864
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- Citations: 2
Garcia de la Cruz JM, Vassilicos JC, Rossi L, 2014, Topologies of velocity-field stagnation points generated by a single pair of magnets in free-surface electromagnetic experiments, PHYSICAL REVIEW E, Vol: 90, ISSN: 1539-3755
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- Citations: 2
Gomes-Fernandes R, Ganapathisubramani B, Vassilicos JC, 2014, Evolution of the velocity-gradient tensor in a spatially developing turbulent flow, JOURNAL OF FLUID MECHANICS, Vol: 756, Pages: 252-292, ISSN: 0022-1120
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- Citations: 37
de la Cruz JMG, Vassilicos JC, Rossi L, 2014, Topologies of velocity-field stagnation points generated by a single pair of magnets in free-surface electromagnetic experiments., Phys Rev E Stat Nonlin Soft Matter Phys, Vol: 90
The velocity fields generated by a static pair of magnets in free-surface electromagnetically forced flows are analyzed for different magnet attitudes, ionic currents, and brine depths. A wide range of laminar velocity fields is obtained despite the forcing simplicity. The velocity fields are classified according to their temporal mean flow topology, which strongly depends on the forcing geometry but barely on its strength, even through the bifurcation to unsteady regimes. The mean flow topology possesses a major influence on the critical Reynolds number Rec under which the steady velocity fields remain stable. The qualitative comparison of the dependence of Rec on the topology is in agreement with previous works. The unsteady configurations evidence the advection of smaller flow structures by the largest scales, commonly known as "sweeping."
Nicolleau FCGA, Vassilicos JC, 2014, WAVELET ANALYSIS OF WAVE MOTION, INTERNATIONAL JOURNAL OF APPLIED MECHANICS, Vol: 6, ISSN: 1758-8251
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- Citations: 7
Valente PC, Vassilicos JC, 2014, The non-equilibrium region of grid-generated decaying turbulence, JOURNAL OF FLUID MECHANICS, Vol: 744, Pages: 5-37, ISSN: 0022-1120
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- Citations: 49
Onishi R, Vassilicos JC, 2014, Collision statistics of inertial particles in two-dimensional homogeneous isotropic turbulence with an inverse cascade, JOURNAL OF FLUID MECHANICS, Vol: 745, Pages: 279-299, ISSN: 0022-1120
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- Citations: 13
Soulopoulos N, Kerl J, Sponfeldner T, et al., 2013, Turbulent premixed flames on fractal-grid-generated turbulence, FLUID DYNAMICS RESEARCH, Vol: 45, ISSN: 0169-5983
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- Citations: 20
Nedic J, Ganapathisubramani B, Vassilicos JC, 2013, Drag and near wake characteristics of flat plates normal to the flow with fractal edge geometries, FLUID DYNAMICS RESEARCH, Vol: 45, ISSN: 0169-5983
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- Citations: 35
Laizet S, Sakai Y, Vassilicos JC, 2013, Special issue of selected papers from the second UK-Japan bilateral Workshop and First ERCOFTAC Workshop on Turbulent Flows Generated/Designed in Multiscale/Fractal Ways, London, March 2012, FLUID DYNAMICS RESEARCH, Vol: 45, ISSN: 0169-5983
Laizet S, Vassilicos JC, Cambon C, 2013, Interscale energy transfer in decaying turbulence and vorticity-strain-rate dynamics in grid-generated turbulence, FLUID DYNAMICS RESEARCH, Vol: 45, ISSN: 0169-5983
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- Citations: 30
Nedic J, Vassilicos JC, Ganapathisubramani B, 2013, Axisymmetric Turbulent Wakes with New Nonequilibrium Similarity Scalings, PHYSICAL REVIEW LETTERS, Vol: 111, ISSN: 0031-9007
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- Citations: 74
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