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Differential Diffusion: Often a Finite-Mixing Length Effect

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Differential Diffusion: Often a Finite-Mixing Length Effect 15th Australasian Fluid Mechanics Conference The University of Sydney, Sydney Australia 13-17 December 2004 Differential Diffusion: Often a Finite-Mixing Length Effect Peter Nielsen & Ian A L Teakle Department of Civil Engineering, The University of Queensland Brisbane, AUSTRALIA 4072, [email protected] Abstract Nielsen & Teakle [11] reconsidered the simple mixing length Many instances of differential diffusion, i e, different species scenario of Figure 1, where parcels with vertical spacing lm, the having different turbulent diffusion coefficients in the same flow, mixing length, are swapping positions traveling with vertical speed can be explained as a finite mixing length effect. That is, in a wm, the mixing velocity. simple mixing length scenario, the turbulent diffusion coefficient z c(z) lm 2 has the form KwlO=+1()where, w is the mixing lm mmm c z+ Lc 2 velocity, lm the mixing length and Lc the overall distribution scale for a particular species. The first term represents the familiar l gradient diffusion while the second term becomes important when z+ m 2 lm/Lc is finite. This second term shows that different species will have different diffusion coefficients if they have different overall distribution scales. Such different Lcs may come about due to z lm wm different boundary conditions and different intrinsic properties wm (molecular diffusivity, settling velocity etc) for different species. For momentum transfer in turbulent oscillatory boundary layers the l z- m second term is imaginary and explains observed phase leads of 2 shear stresses ahead of velocity gradients. l Introduction c z- m 2 Understanding and modelling natural and industrial processes requires in very many cases understanding of turbulent mixing. The c(z) present paper addresses an aspect of turbulent mixing, which is still not well understood namely, differential diffusion. The term Figure 1: A simple mixing length scenario involving a single mixing length differential diffusion refers to the observation of different turbulent lm and corresponding mixing velocity wm. diffusion coefficients K defined by The resulting flux density is then turbulent mixing flux density K = − (1) lm lm concentration gradient qm = wm[c(z − ) − c(z + )] 2 2 (2) for different species in a given flow. dc lm 2 = − wmlm 1+ O( ) dz L In physical oceanography, the species in focus are usually, c momentum, heat and salt, and differential diffusion of these has been argued by Gargett [4] to be an important player in many which, by the definition (1), gives the diffusion coefficient oceanographical processes. Laboratory experiments, e g, Turner lm 2 [13], Nagata & Komori [9] and Jackson & Rehmann [7] have K = wmlm 1+O( ) of which the second term offers an shown differential diffusion of heat and salt. Turner [13] and Lc Jackson & Rehmann [7] found Kheat>Ksalt, and this has been explanation for differential diffusion as a finite mixing length effect. rationalized, by Turner [13] and others, as being due to heat having Examples are given below. a greater molecular diffusivity than salt. However, Nagata & Komori [9] found the opposite, i e Kheat<Ksalt, in experiments which Steady Sediment Suspensions had a different geometry. Hence, differential diffusion cannot be Consider sediment particles with a range of settling velocities ws totally due to differences in molecular diffusivity. suspended in the same turbulent flow, and adopting velocity fluctuations with the same statistics as those of the fluid. The The idea that species with greater molecular diffusivity should turbulent swapping process in Figure 1 then creates an upward flux diffuse more rapidly in turbulence has also been applied to various density given by (2), which in a steady situation is balanced by the species in flame combustion by Kronenburg & Bilger [8]. Chanson settling flux –ws c(z) i e, ([1], [2]) has reported data, which show differential diffusion of lm lm momentum and bubbles in supercritical free surface flows. wm[c(z − ) −c(z + )]− ws c(z) = 0 (3) Differential diffusion of momentum and different sediment sizes 2 2 has been documented for steady flows by Coleman [3] and Graf & In general (wm, lm) may be functions of z. However, the essence of Cellino [5] and for oscillatory flows by Nielsen [10] p 220. differential diffusion as a finite mixing length effect is captured by the simple case of homogeneous turbulence, i e, constant (wm, lm). where ρww is the autocorrelation function for w(t). This standard In this case Equation (3) has solutions of the form deviation, or cloud size, is independent of ws under the assumption −z / L of identical fluctuation statistics. There is thus no differential c(z) = C e s (4) o diffusion in Taylor’s infinite, unsteady scenario. Differential with diffusion only occurs when a boundary condition, e g, c(0,t) ≡ Co, lm as in the example above, is enforced. Ls = −1 ws 2 sinh ( ) 1.2 2wm (5) lm wm 1 ws 2 = 1+ ( ) + ... 1.0 ws 24 wm The corresponding diffusion coefficient is given by 0.8 l l w [c(z − m ) − c(z + m )] q m 0.6 K = m = 2 2 s dc dc − − dz dz (6) 0.4 elevation [m] 1 ws 2 = wmlm 1+ ( ) + ..... 0.2 24 wm This result quantifies differential diffusion for different sand sizes 0.0 in homogeneous turbulence. It is however also in general 0 5 10 15 20 qualitative agreement with data from river flows, where the turbulence is not homogeneous, see e g, Coleman [3], Graf & -0.2 Cellino [5]. Indeed, van Rijn [14] suggested the empirical concentration [-] correction factor K w Figure 2: Successive concentration profiles c(z,t), after 5, 10 and 20 seconds β = s = 1+ 2( s )2 (7) v u of particles with ws=1cm/s (♦), and 3cm/s () respectively settling with t * statistically identical velocity fluctuations after being released as point for dealing with the observed differential diffusion of momentum injections at z=1m at t=0. At any time the concentration profiles have the (with diffusion coefficient vt) and different sand fractions in natural same shape, irrespective of ws. rivers. Van Rijn’s formula is seen to have the same dependence upon ws/wm ~ ws/u* as the finite mixing length result (6). Finite Mixing Length Effects on Eddy Viscosities Steady current profiles, e g, the log-profile can be understood in We acknowledge that the example, which leads to (6), does not terms of a mixing length model where only the first term (the deal with all details, i e, in a natural scenario, particles with gradient term) of (2) is retained. That is however not possible for different settling velocities, and hence different response times ws/g, oscillatory turbulent flows. The most obvious feature of oscillatory do not get identical turbulent velocity statistics (~ same wm). turbulent boundary layers, which requires finite mixing length However, this is usually totally overshadowed by the finite-mixing- terms, is the observed phase lead of local shear stresses ahead of length–effect. That is, the two act in opposite directions, but all the the local velocity gradients, cf Figure 3. experimental data show Ks to be an increasing function of ws as predicted by the finite-mixing-length model. In a formalism with real-valued eddy viscosities, shear stresses τ (z, t) being out of phase with the local velocity gradients ∂u/∂z The importance of boundary conditions The above example, of sediment particles with different w but τ / ρ s leads, through the usual definition, vt = to wildly statistically identical velocity fluctuations in homogeneous ∂u / ∂z turbulence, can also be used to illustrate the importance of variable [-∞; ∞] eddy viscosities. Alternatively one can (for the boundary conditions for the occurrence of differential diffusion. simple harmonic case) use constant, but complex-valued eddy Consider point injections of such sediments into an infinite, viscosities with argument equal to the phase lead of homogeneous turbulence field, Figure 2. Each sediment type will τ (z,t) relative to ∂u/∂z. The complex-valued eddy viscosities are then form a cloud, which sinks at an average rate of ws. At the same time, each cloud grows in accordance with the theory of Taylor however just nominal and rather ad hoc tools. In the following, we make the case that the phase lead of τ (z,t) relative to ∂u/∂z is in [12], i e, the vertical extent σz of each cloud grows in accordance with fact a finite-mixing length effect. In this case the finite mixing length term in (2) is imaginary corresponding to the observed phase t θ shift between τ (z,t) relative to ∂u/∂z. 2 σ z (t) = 2 w′ ∫∫ρww (τ ) dτ dθ (8) 00 1 lm vt = wmlm (1+ i + ...) (16) 24 wm / ω where the second, imaginary term shows that the phase lead of τ ahead of ∂u/∂z develops as lm becomes large compared with the vertical scale Lω= wm/ω ~ u*/ω defined by the mixing (friction) velocity and the wave frequency. For very slow oscillations the second term vanishes and the classical von Karman-Prandtl mixing length theory suffices. Differential Diffusion at an Interface Consider mixing at an interface as in Figure 4. The density variation may be due to temperature, salinity or suspended Figure 3: Velocity gradient and shear stress in a turbulent oscillatory sediment. Horizontal fluid momentum may vary in a similar boundary layer. The stress leads the velocity gradient. Measurements by fashion at the interface. Jonsson & Carlsen [6]. To show this we first note that, in the simple mixing length scenario in Figure 1 the upward flux of x-momentum (= -τ) is lm lm −τ = wm ρu(z + )− ρ u(z − ) (9) 2 2 The velocity in a simple harmonic oscillatory boundary layer flow is often expressed in terms of the free stream velocity u∞(t) and the complex velocity defect function D(z) defined by iωt u(z, t) = Aω e []1− D(z) = u∞ (t)[1− D(z)] (10) in terms of which the shear stress is τ (z, t) ∂ ∞∞ = ∫∫[ u∞ (t) − u(z′, t)]dz′ = iω D(z′) dz′ (11) ρ ∂t zz In combination with (9) this leads to the finite mixing length Figure 4: Assumed density distribution at an interface where turbulent momentum equation mixing can be described by Equation (2) in terms of a single mixing length lm and a single mixing velocity wm.
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