arXiv subjects

J. C. Vassilicos

Publications and source records attributed to J. C. Vassilicos.

17 recordsLinked to original sources

arXiv2023

Spatio-temporal fluctuations of interscale and interspace energy transfer dynamics in homogeneous turbulence

We study fluctuations of all co-existing energy exchange/transfer/transport processes in stationary periodic turbulence including those which average to zero and are not present in average cascade theories. We use a Helmholtz decomposition of accelerations which leads to a decomposition of all terms in the Kármán-Howarth-Monin-Hill (KHMH) equation (scale-by-scale two-point energy balance) causing it to break into two energy balances, one resulting from the integrated two-point vorticity equation and the other from the integrated two-point pressure equation. The various two-point acceleration terms in the Navier-Stokes difference (NSD) equation for the dynamics of two-point velocity differences have similar alignment tendencies with the two-point velocity difference, implying similar characteristics for the NSD and KHMH equations. We introduce the two-point sweeping concept and show how it articulates with the fluctuating interscale energy transfer as the solenoidal part of the interscale transfer rate does not fluctuate with turbulence dissipation at any scale above the Taylor length but with the sum of the time-derivative and the solenoidal interspace transport rate terms. The pressure fluctuations play an important role in the interscale and interspace turbulence transfer/transport dynamics as the irrotational part of the interscale transfer rate is equal to the irrotational part of the interspace transfer rate and is balanced by two-point fluctuating pressure work. We also study the homogeneous/inhomogeneous decomposition of interscale transfer. The statistics of the latter are skewed towards forward cascade events whereas the statistics of the former are not. We also report statistics conditioned on intense forward/backward interscale transfer events.

physics.flu-dyn↗

arXiv2021

Interaction of two high Reynolds number axisymmetric turbulent wakes

The interaction between turbulent axisymmetric wakes plays an important role in many industrial applications, notably in the modelling of wind farms. While the non-equilibrium high Reynolds number scalings present in the wake of axisymmetric plates has been shown to modify the averaged streamwise scalings of individual wakes, little attention has been paid to their consequences in terms of wake interactions. We propose an experimental setup that tests the presence of non-equilibrium turbulence using the streamwise variation of velocity fluctuations between two bluff bodies facing a laminar flow. We have studied two different sets of plates (one with regular and another with irregular peripheries) with hot-wire anemometry in a wind tunnel. By acquiring streamwise profiles for different plate separations and identifying the wake interaction length for each separation it is possible to show that the interaction between them is consistent with non-equilibrium scalings. This work also generalises previous studies concerned with the interaction of plane wakes to include axisymmetric wakes. We find that a simple mathematical expression for the wake interaction length based on non-equilibrium turbulence scalings can be used to collapse the streamwise developments of the second, third and fourth moments of the streamwise fluctuating velocity.

physics.flu-dyn↗

arXiv2020

The role of Coherent Structures and Inhomogeneity in Near-Field Inter-Scale Turbulent Energy Transfers

We use DNS to study inter-scale and inter-space energy exchanges in the near-field of a turbulent wake of a square prism in terms of the KHMH equation written for a triple decomposition of the velocity field accounting for the quasi-periodic vortex shedding. Orientation-averaged terms of the KHMH are computed on the plane of the mean flow and on the geometric centreline. We consider locations between $2$ and $8$ times the width $d$ of the prism. The mean flow produces kinetic energy which feeds the vortex shedding coherent structures. In turn, these structures transfer energy to the stochastic fluctuations over all length-scales $r$ from the Taylor length $λ$ to $d$ and dominate spatial turbulent transport of two-point stochastic turbulent fluctuations. The orientation-averaged non-linear inter-scale transfer rate $Π^{a}$ which was found to be approximately independent of $r$ by Alves Portela et. al. (2017) in the range $λ\le r \le 0.3d$ at a distance $x_{1}=2d$ from the square prism requires an inter-scale transfer contribution of coherent structures for this approximate constancy. However, the near-constancy of $Π^a$ at $x_1=8d$ which was also found by Alves Portela et. al. (2017) is mostly due to stochastic fluctuations. Even so, the proximity of $-Π^a$ to the turbulence dissipation rate $\varepsilon$ in the range $λ\le r\le d$ at $x_1=8d$ requires contributions of the coherent structures. Spatial inhomogeneity also makes a direct and distinct contribution to $Π^a$, and the constancy of $-Π^a/\varepsilon$ close to 1 would not have been possible without it either in this near-field flow. Finally, the pressure-velocity term is also an important contributor to the KHMH, particularly at scales r larger than about $0.4d$, and appears to correlate with the purely stochastic non-linear inter-scale transfer rate when the orientation average is lifted.

physics.flu-dyn↗

arXiv2020

The energy cascade at the turbulent/non-turbulent interface

Interscale energy transfers at the vicinity of the turbulent/non-turbulent interface are from small to large scales in directions close to the interface's tangent plane where motions are predominantly stretching, but from large to small scales in the other directions where motions are predominantly compressive and significantly correlated with square angular momentum. An important role in this predominance is played by the extreme compressive motions which can be significantly more likely than extreme stretching motions even where motions are on average stretching. The most intense interscale transfer rates and dissipation occur when the interface is as far as possible from the wake centreline.

physics.flu-dyn↗

arXiv2019

The non-equilibrium part of the inertial range in decaying homogeneous turbulence

We use two related non-stationarity functions as measures of the degree of scale-by-scale non-equilibrium in homogeneous isotropic turbulence. The values of these functions indicate significant non-equilibrium at the upper end of the inertial range. Wind tunnel data confirm Lundgren's (2002, 2003) prediction that the two-point separation $r$ where the second and third order structure functions are closest to their Kolmogorov scalings is proportional to the Taylor length scale $λ$, and that both structure functions increasingly distance themselves from their Kolmogorov equilibrium form as $r$ increases away from $λ$ throughout the inertial range. With the upper end of the inertial range in non-equilibrium irrespective of Reynolds number, it is not possible to justify the Taylor-Kolmogorov turbulence dissipation scaling on the basis of Kolmogorov equilibrium.

physics.flu-dyn↗

arXiv2017

Vertical motions of heavy inertial particles smaller than the smallest scale of the turbulence in strongly stratified turbulence

We study the statistics of the vertical motion of inertial particles in strongly stratified turbulence. We use Kinematic Simulation (KS) and Rapid Distortion Theory (RDT) to study the mean position and the root mean square (rms) of the position fluctuation in the vertical direction. We vary the strength of the stratification and the particle inertial characteristic time. The stratification is modelled using the Boussinesq equation and solved in the limit of RDT. The validity of the approximations used here requires that$\sqrt{{L}/{g}}<{2π}/{\mathcal{N}}<τ\_η$,where $τ\_η$ is the Kolmogorov time scale, $g$ the gravitational acceleration, $L$ the turbulence integral length scale and $\mathcal{N}$ the Brunt-Väisälä frequency. We introduce a drift Froude number$Fr\_{d} = τ\_p g / \mathcal{N} L$. When $Fr\_{d} < 1$, the rms of the inertial particle displacement fluctuation is the same as for fluid elements, i.e.$\langle(ζ\_3 - \langle ζ\_3 \rangle)^2\rangle^{1/2} = 1.22\, u'/\mathcal{N} + \mbox{oscillations}$. However, when $Fr\_{d} > 1$,$\langle(ζ\_3 - \langle ζ\_3 \rangle)^2\rangle^{1/2} = 267 \, u' τ\_p$. That is the level of the fluctuation is controlled by the particle inertia $τ\_p$ andnot by the buoyancy frequency $\mathcal{N}$. In other words it seems possible for inertial particles to retain the vertical capping while loosing the memory of theBrunt-Väisälä frequency.

physics.flu-dyn↗

arXiv2017

Wavelet analysis of Wave motion

In this paper high resolution wave probe records are examined using wavelet techniques with a view to determining the sources and relative contributions of capillary wave energy along representative wind wave forms. Wavelets enable computations of conditional spectra and turn out to be powerful tools for the study of the development and propagation of capillary waves. They also enable the detailed analyses of the relative contributions to the spectrum of the wave peaks and troughs.

physics.flu-dyn↗

arXiv2014

The streamwise turbulence intensity in the intermediate layer of turbulent pipe flow

The spectral model of Perry, Henbest & Chong (1986) 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, Vallikivi, Bailey & Smits (2012, 2013). The finding of Dallas, Vassilicos & Hewitt (2009) that it is the eddy turnover time and not the mean flow gradient which scales with distance to the wall and skin friction velocity in the intermediate layer implies, when combined with Townsend's (1976) production-dissipation balance, that the mean flow gradient has an outer peak at the same location as the turbulent kinetic energy. This is seen in the data of Hultmark, Vallikivi, Bailey & Smits (2012, 2013). The same approach also predicts that the mean flow gradient has a logarithmic decay at distances to the wall larger than the position of the outer peak, a qualitative prediction which the aforementioned data also support.

physics.flu-dyn↗

arXiv2014

Influence of the spatial resolution on fine-scale features in DNS of turbulence generated by a single square grid

We focus in this paper on the effect of the resolution of Direct Numerical Simulations (DNS) on the spatio-temporal development of the turbulence downstream of a single square grid. The aims of this study are to validate our numerical approach by comparing experimental and numerical one-point statistics downstream of a single square grid and then investigate how the resolution is impacting the dynamics of the flow. In particular, using the Q-R diagram, we focus on the interaction between the strain-rate and rotation tensors, the symmetric and skew-symmetric parts of the velocity gradient tensor respectively. We first show good agreement between our simulations and hot-wire experiment for one-point statistics on the centreline of the single square grid. Then, by analysing the shape of the Q-R diagram for various streamwise locations, we evaluate the ability of under-resolved DNS to capture the main features of the turbulence downstream of the single square grid.

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arXiv2014

The energy cascade in grid-generated non-equilibrium decaying turbulence

We investigate non-equilibrium turbulence where the non-dimensionalised dissipation coefficient $C_{\varepsilon}$ scales as $C_{\varepsilon} \sim Re_{M}^{m}/Re_{\ell}^{n}$ with $m\approx 1 \approx n$ ($Re_M$ and $Re_{\ell}$ are global/inlet and local Reynolds numbers respectively) by measuring the downstream evolution of the scale-by-scale energy transfer, dissipation, advection, production and transport in the lee of a square-mesh grid and compare with a region of equilibrium turbulence (i.e. where $C_{\varepsilon}\approx \mathrm{constant}$). These are the main terms of the inhomogeneous, anisotropic version of the von Kármán-Howarth-Monin equation. It is shown in the grid-generated turbulence studied here that, even in the presence of non-negligible turbulence production and transport, production and transport are large-scale phenomena that do not contribute to the scale-by-scale balance for scales smaller than about a third of the integral-length scale, $\ell$, and therefore do not affect the energy transfer to the small-scales. In both the non-equilibrium and the equilibrium decay regions, the peak of the scale-by-scale energy transfer scales as $(\overline{u^2})^{3/2}/\ell$ ($\overline{u^2}$ is the variance of the longitudinal fluctuating velocity). In the non-equilibrium case this scaling implies an imbalance between the energy transfer to the small scales and the dissipation. This imbalance is reflected on the small-scale advection which becomes larger in proportion to the maximum energy transfer as the turbulence decays whereas it stays proportionally constant in the further downstream equilibrium region where $C_{\varepsilon} \approx \mathrm{constant}$ even though $Re_{\ell}$ is lower.

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arXiv2014

The non-equilibrium region of grid-generated decaying turbulence

The previously reported non-equilibrium dissipation law is investigated in turbulent flows generated by various regular and fractal square grids. The flows are documented in terms of various turbulent profiles which reveal their differences. In spite of significant inhomogeneity and anisotropy differences, the new non-equilibrium dissipation law is observed in all these flows. Various transverse and longitudinal integral scales are measured and used to define the dissipation coefficient $C_{\varepsilon}$. It is found that the new non-equilibrium dissipation law is not an artefact of a particular choice of the integral scale and that the usual equilibrium dissipation law can actually coexist with the non-equilibrium law in different regions of the same flow.

physics.flu-dyn↗

arXiv2011

Dependence of decaying homogeneous isotropic turbulence on initial conditions

We conduct a careful analysis of the data provided by Krogstad & Davidson (2011) and show that their data do not support their conclusions. According to their published data, their decaying approximately homogeneous isotropic turbulent flows are, invariably, clearly different from Saffman turbulence; and very clearly marked differences exist between the far downstream turbulence behaviors generated by their conventional grid and by their multiscale cross grids.

physics.flu-dyn↗

arXiv2009

Turbulent pair separation due to multi scale stagnation point structure and its time asymmetry in two-dimensional turbulence

The pair separation model of Goto and Vassilicos (S Goto and J C Vassilicos, 2004, New J.Phys., 6, p.65) is revisited and placed on a sound mathematical foundation. A DNS of two dimensional homogeneous isotropic turbulence with an inverse energy cascade and a k^{-5/3} power law is used to investigate properties of pair separation in two dimensional turbulence. A special focus lies on the time asymmetry observed between forward and backward separation. Application of the present model to this data suffers from finite inertial range effects and thus, conditional averaging on scales rather than on time has been employed to obtain values for the Richardson constants and their ratio. The Richardson constants for the forward and backward case are found to be (1.066 +/- 0.020) and (0.999 +/- 0.007) respectively. The ratio of Richardson constants for the backwards and forwards case is therefore g_b/g_f = (0.92 +/- 0.03), and hence exhibits a qualitatively different behavior from pair separation in three dimensional turbulence, where g_b > g_f (J Berg et al., 2006, Phys.Rev.E, 74(1), p.016304). This indicates that previously proposed explanations for this time asymmetry based on the strain tensor eigenvalues are not sufficient to describe this phenomenon in two dimensional turbulence. We suggest an alternative qualitative explanation based on the time asymmetry related to the inverse versus forward energy cascade. In two dimensional turbulence, this asymmetry manifests itself in merging eddies due to the inverse cascade, leading to the observed ratio of Richardson constants.

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arXiv2004

Acceleration Statistics as Measures of Statistical Persistence of Streamlines in Isotropic Turbulence

We introduce the velocity (Vs) of stagnation points as a means to characterise and measure statistical persistence of streamlines. Using theoretical arguments, Direct Numerical Simulations (DNS) and Kinematic Simulations (KS) of three-dimensional isotropic turbulence for different ratios of inner to outer length scales L/eta of the self-similar range, we show that a frame exists where the average =0, that the r.m.s. values of acceleration (a'), turbulent fluid velocity (u') and Vs are related by La'/(u'^2) ~ (Vs/u')(L/eta)^(2/3+q) and that Vs/u' ~ (L/eta)^q with q=-1/3 in Kolmogorov turbulence, q=-1/6 in current DNS and q=0 in our KS. The statistical persistence hypothesis is closely related to the Tennekes sweeping hypothesis.

nlin.CD↗

arXiv2003

Kinematic simulation of multi point turbulent dispersion

As three particles are advected by a turbulent flow, they separate from each other and develop non trivial geometries, which effectively reflect the structure of the turbulence. We investigate here the geometry, in a statistical sense, of three Lagrangian particles advected, in 2-dimensions, by Kinematic Simulation (KS). KS is a Lagrangian model of turbulent diffusion that makes no use of any delta correlation in time at any level. With this approach, situations with a very large range of inertial scales and varying persistence of spatial flow structure can be studied. We first show numerically that the model flow reproduces recent experimental results at low Reynolds numbers. The statistical properties of the shape distribution at much higher Reynolds number is then considered. Even at the highest available inertial range, of scale, corresponding to a ratio between large and small scales of $L/η\approx 17,000$, we find that the radius of gyration of the three points does not precisely follow Richardson's prediction. The shapes of the triangles have a high probability to be elongated. The corresponding shape distribution is not found to be perfectly self similar, even for our highest ratio of inertial scales. We also discuss how the parameters of the synthetic flow, such as the exponent of the spectrum and the effect of the sweeping affect our results. Our results suggest that a non trivial distribution of shapes will be observed at high Reynolds numbers, although it may not be exactly self similar. Special attention is given to the effects of persistence of spatial flow structure.

nlin.CD↗

arXiv2002

Turbulent Pair Diffusion

Kinematic Simulations of turbulent pair diffusion in planar turbulence with a -5/3 energy spectrum reproduce the results of the laboratory measurements of Jullien Phys. Rev. Lett. 82, 2872 (1999), in particular the stretched exponential form of the PDF of pair separations and their correlation functions. The root mean square separation is found to be strongly dependent on initial conditions for very long stretches of times. This dependence is consistent with the topological picture of turbulent pair diffusion where pairs initially close enough travel together for long stretches of time and separate violently when they meet straining regions around hyperbolic points. A new argument based on the divergence of accelerations is given to support this picture.

nlin.CD↗

arXiv2001

The Scalings of Scalar Structure Functions in a Velocity Field with Coherent Vortical Structures

In planar turbulence modelled as an isotropic and homogeneous collection of 2-D non-interacting compact vortices, the structure functions S_p(r) of a statistically stationary passive scalar field have the following scaling behaviour in the limit where the Péclet number Pe -> \infty S_p(r) ~ constant+\ln({\frac{r}{LPe^{-1/3}}}) for LPe^{-1/3} << L, S_p(r) ~ ({\frac{r}{LPe^{-1/3}}})^{6(1-D)} for LPe^{-1/2} << LPe^{-1/3}, where L is a large scale and D is the fractal co-dimension of the spiral scalar structures generated by the vortices (1/2 <= D < 2/3). Note that LPe^{-1/2} is the scalar Taylor microscale which stems naturally from our analytical treatment of the advection-diffusion equation. The essential ingredients of our theory are the locality of inter-scale transfer and Lundgren's time average assumption. A phenomenological theory explicitly based only on these two ingredients reproduces our results and a generalisation of this phenomenology to spatially smooth chaotic flows yields (k\ln k)^{-1} generalised power spectra for the advected scalar fields.

nlin.CD↗