Effect of background turbulence on an axisymmetric turbulent jet

Khorsandi, Babak; Gaskin, Susan; Mydlarski, Laurent

doi:10.1017/jfm.2013.465

Cited by 38 publications

(47 citation statements)

References 61 publications

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“…The evolution of the jet half-width, R U , for the present jet is explored in Darisse et al (2013b) and yielded a value of B R U = 0.091, which is very close to the values obtained by HCG (LDV) and PL, as shown in table 1. A similarly good agreement between measured and calculated values of V/U 0 was obtained by Khorsandi, Gaskin & Mydlarski (2013) in a water jet using acoustic doppler velocimetry. Figure 5 shows radial profiles of the square root of the Reynolds stresses u 2 , v 2 and w 2 , normalized by the mean centreline velocity, along with the turbulent kinetic energy k, normalized by the square of the mean centreline velocity.…”

Section: Momentum Integral and Mean Velocity Resultssupporting

confidence: 72%

Budgets of turbulent kinetic energy, Reynolds stresses, variance of temperature fluctuations and turbulent heat fluxes in a round jet

2015

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The self-preserving region of a free round turbulent air jet at high Reynolds number is investigated experimentally (at x/D = 30, Re D = 1.4 × 10 5 and Re λ = 548). Air is slightly heated (20 • C above ambient) in order to use temperature as a passive scalar. Laser doppler velocimetry and simultaneous laser doppler velocimetry-cold-wire thermometry measurements are used to evaluate turbulent kinetic energy and temperature variance budgets in identical flow conditions. Special attention is paid to the control of initial conditions and the statistical convergence of the data acquired. Measurements of the variance, third-order moments and mixed correlations of velocity and temperature are provided (including vw 2 , uθ 2 , vθ 2 , u 2 θ, v 2 θ and uvθ ). The agreement of the present results with the analytical expressions given by the continuity, mean momentum and mean enthalpy equations supports their consistency. The turbulent kinetic energy transport budget is established using Lumley's model for the pressure diffusion term. Dissipation is inferred as the closing balance. The transport budgets of the u i u j components are also determined, which enables analysis of the turbulent kinetic energy redistribution mechanisms. The impact of the surrogacy vw 2 = v 3 is then analysed in detail. In addition, the present data offer an opportunity to evaluate every single term of the passive scalar transport budget, except for the dissipation, which is also inferred as the closing balance. Hence, estimates of the dissipation rates of turbulent kinetic energy and temperature fluctuations ( k and θ ) are proposed here for use in future studies of the passive scalar in a turbulent round jet. Finally, the budgets of turbulent heat fluxes (u i θ) are presented.

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Section: Momentum Integral and Mean Velocity Resultssupporting

confidence: 72%

Budgets of turbulent kinetic energy, Reynolds stresses, variance of temperature fluctuations and turbulent heat fluxes in a round jet

2015

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“…A random jet array (RJA) is a relatively new approach to generate Quasi-HIT with zero mean flow ( (Variano et al 2004), (Lavertu et al 2006), (Variano & Cowen 2008), (Delbos et al 2009), (Khorsandi et al 2013)). This device consist of a planar configuration of jets that are turned on and off, randomly and independently, to produce turbulence.…”

Section: Generation Of Homogeneous Turbulencementioning

confidence: 99%

Laboratory experiments on the temporal decay of homogeneous anisotropic turbulence

2019

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We experimentally investigate the temporal decay of homogeneous anisotropic turbulence, monitoring the evolution of velocity fluctuations, dissipation and turbulent length scales over time. We employ an apparatus in which two facing random jet arrays of water pumps generate turbulence with negligible mean flow and shear over a volume that is much larger than the initial characteristic turbulent large scale of the flow. The Reynolds number based on the Taylor microscale for forced turbulence is $Re_{\unicode[STIX]{x1D706}}\approx 580$ and the axial-to-radial ratio of the root mean square velocity fluctuations is $1.22$. Two velocity components are measured by particle image velocimetry at the symmetry plane of the water tank. Measurements are taken for both ‘stationary’ forced turbulence and natural decaying turbulence. For decaying turbulence, power-law fits to the decay of turbulent kinetic energy reveal two regions over time; in the near-field region ($t/t_{L}<10$, $t_{L}$ is the integral time scale of the forced turbulence) a decay exponent $m\approx -2.3$ is found whereas for the far-field region ($t/t_{L}>10$) the value of the decay exponent was found to be affected by turbulence saturation. The near-field exhibits features of non-equilibrium turbulence with constant $L/\unicode[STIX]{x1D706}$ and varying $C_{\unicode[STIX]{x1D716}}$ (dissipation constant). We found a decay exponent $m\approx -1.4$ for the unsaturated regime and $m\approx -1.8$ for the saturated regime, in good agreement with previous numerical and experimental studies. We also observe a fast evolution towards isotropy at small scales, whereas anisotropy at large scales remains in the flow over more than $100t_{L}$. Direct estimates of dissipation are obtained and the decay exponent agrees well with the prediction $m_{\unicode[STIX]{x1D716}}=m-1$ throughout the decay process.

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“…However, noting that classical plume theory predicts the virtual source correction for infinitely lazy plumes to be zero (Hunt & Kaye 2001), we would associate an effective virtual source with the indirect effect that background turbulence has in modifying the near-field development of the plume. In view of the reduction in entrainment with respect to background turbulence reported by (Khorsandi et al 2013;Lai et al 2019), attribution of α * = 0.2 solely to direct effects of background turbulence would therefore be incorrect and misleading, not least because it would also ignore possible modifications to entrainment arising from the mean ambient flow discussed in § 3.…”

Section: Entrainmentmentioning

confidence: 99%

“…Key questions in this regard relate to the effect of background turbulence, stratification, mean co-or cross-flow and entrainment or detrainment from nearby plumes (see e.g. Khorsandi, Gaskin & Mydlarski 2013;Gladstone & Woods 2014;Bonnebaigt, Caulfield & Linden 2018;Lai, Law & Adams 2019). All of these effects have the potential to influence the buoyancy structure of a space and the transport of pollutants.…”

Section: Introductionmentioning

confidence: 99%

“…This makes it difficult to develop models using reductionism and might preclude simple explanations of observed phenomena. A case in point that has received attention recently is the observation and prediction that background turbulence reduces entrainment into turbulent jets (Hunt, Eames & Westerweel 2006;Khorsandi et al 2013;Lai et al 2019), in spite of contrary suggestions elsewhere (Hubner 2006). A further example from the present study is our identification of a hierarchy of mean-flow structures (see Baines & Turner 1969, figure 10) induced by confined plumes, which are necessarily accompanied by background turbulence and a stable, layered stratification.…”

Section: Introductionmentioning

confidence: 99%

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The entrainment and energetics of turbulent plumes in a confined space

Craske

Wykes

2019

J. Fluid Mech.

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We analyse the mixing and energetics of equal and opposite axisymmetric turbulent plumes in a confined space using theory and direct numerical simulation. On domains of sufficiently large aspect ratio the steady-state flow and buoyancy structure consists of turbulent plumes penetrating an interface between two layers of uniform buoyancy. On penetrating the interface the plumes become jets, because their mean buoyancy is reduced to zero. Domains of relatively small aspect ratio produce a single primary meanflow circulation that is maintained by entrainment into the plumes. At larger aspect ratios the mean flow bifurcates and exhibits secondary and tertiary circulation cells within each layer and in the vicinity of the interface, respectively. Our results suggest that the strength of the primary circulation is maximised by a domain aspect ratio that is as large as it can be without producing a sustained secondary circulation in the mean flow.To study the flow's mixing and energetics we isolate the behaviour of the plumes from the behaviour of the jets by obtaining statistics over sub-domains using a local definition of available and background potential energy. In contrast to the global energy budgets, the local energy budgets predicted by theory are determined by the assumed buoyancy and velocity profiles in the plumes, which feed the jets. For plumes with Gaussian velocity and buoyancy profiles, theory suggests that the dissipation of kinetic energy is split equally between the jets and the plumes and, collectively, accounts for almost half of the input of available potential energy at the boundaries. In contrast, 1/4 of the dissipation of available potential energy occurs in the jets, with the remaining 3/4 occurring in the plumes. These theoretical predictions agree with the simulation data to within 1% and form the basis of a similarity solution that models the vertical dependence of available potential energy dissipation Φ d in the jets. The model predicts that Φ d scales in proportion to the fourth power of the vertical distance from the source and exhibits a reasonably good agreement with the simulation data.

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Effect of background turbulence on an axisymmetric turbulent jet

Cited by 38 publications

References 61 publications

Budgets of turbulent kinetic energy, Reynolds stresses, variance of temperature fluctuations and turbulent heat fluxes in a round jet

Budgets of turbulent kinetic energy, Reynolds stresses, variance of temperature fluctuations and turbulent heat fluxes in a round jet

Laboratory experiments on the temporal decay of homogeneous anisotropic turbulence

The entrainment and energetics of turbulent plumes in a confined space

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