Effect of instantaneous stirring process on mixing between initially distant scalars in turbulent obstacle wakes

Abstract

and experimentally (Soltys and Crimaldi 2015). This scalar coalescence occurs even for passive scalars where the dynamics of each scalar field evolves independently of the other; the structure of the shared velocity field is responsible for the coalescence. If the initial separation distance between the two scalars is smaller than the largest coherent length scales of the local flow, the two scalar fields develop similar instantaneous spatial distributions. While it is easier to envision these processes in simple flows, recent numerical simulations (Pratt et al. 2015) have shown that in more complex 2D flows involving multiple interacting vortices, initially distant scalars coalesce in regions of intense mixing and attracting Lagrangian coherent structures.In aquatic ecosystems, structured flows can facilitate stirring and diffusion to enhance reactions or population growth by bringing together species and nutrients (e.g., Richards and Brentnall 2006). Structured flows stir any introduced scalars into thin filament shells (Garrett 1983), enhancing local scalar gradients, increasing the interfacial area of the scalar through chaotic stretching and folding (Hinch 1999), leading finally to increased mixing by molecular diffusion.Structured flow can also contribute directly to the colocation of conspecifics or species competing for a resource (e.g., Károlyi et al. 2000). The presence of benthic aquatic fauna can itself lead to structured flow in wakes downstream of the organisms. Flow structure is also modified by benthic topology such as sea-mounts, and can be dominated by a vortex wake that produces mixing (Richards 1990). As a specific example, these stirring mechanisms may enhance fertilization rates for reproduction by benthic marine invertebrates, which reproduce by broadcast spawning: plumes of egg and sperm are released, at separate locations, into Abstract A two-channel planar laser-induced fluorescence technique is used to study mixing and reactions between two initially distant scalars in the turbulent wake of a cylindrical obstacle. The scalars are released continuously and isokinetically upstream of the cylinder, with a lateral separation that initially impedes mixing between them. The effect of the turbulent wake on mixing and reaction enhancement is determined by measuring the segregation parameter for cases with and without the cylinder obstruction. Results indicate that scalar mixing and reaction rates (in the low-Damkohler limit) increase significantly in the presence of the cylinder wake. The study also shows that the dominant contribution of total reaction derives from the scalar covariance associated with instantaneous flow processes, and depends strongly on streamwise location within the wake. The results have broad implications for mixing processes in engineering and ecology.

Multi-scalar mixing in turbulent coaxial jets

“…Scalar–scalar JPDFs were first reported in homogeneous isotropic turbulence and three-stream scalar mixing layers using direct numerical simulations (DNS) (Juneja & Pope 1996; Sawford & de Bruyn Kops 2008). More recently, experimental techniques enabling simultaneous multi-scalar measurements – such as two-channel laser-induced fluorescence (LIF) (Saylor & Sreenivasan 1998; Lavertu, Mydlarski & Gaskin 2008; Soltys & Crimaldi 2011, 2015; Shoaei & Crimaldi 2017) or planar laser-induced fluorescence (PLIF) and planar laser Rayleigh scattering (Cai et al. 2011; Li et al.…”

“…Scalar-scalar JPDFs were first reported in homogeneous isotropic turbulence and three-stream scalar mixing layers using direct numerical simulations (DNS) (Juneja & Pope 1996;Sawford & de Bruyn Kops 2008). More recently, experimental techniques enabling simultaneous multi-scalar measurements -such as two-channel laser-induced fluorescence (LIF) (Saylor & Sreenivasan 1998;Lavertu, Mydlarski & Gaskin 2008;Soltys & Crimaldi 2011Shoaei & Crimaldi 2017) or planar laser-induced fluorescence (PLIF) and planar laser Rayleigh scattering (Cai et al 2011;Li et al 2017Li et al , 2021 -then made it possible to measure scalar-scalar JPDFs in both parallel and coaxial jets in a coflow (Cai et al 2011;Soltys & Crimaldi 2015;Li et al 2017). These authors additionally measured the conditional scalar diffusion ( γ α ∇ 2 φ α | φα , φβ , γ β ∇ 2 φ β | φα , φβ ), which is said to 'transport' the scalar-scalar JPDF in scalar space, and appears in unclosed form in (2.3) (Cai et al 2011).…”

Quantifying spectral information about source separation in multisource odour plumes

“…Of particular relevance to the work presented here is that, in addition to the factors setting the overall statistics of each individual odour plume, pointwise concentration correlations between odour species will depend strongly on the initial source separation distance. [24, 25, 26, 27, 28]. In particular, the correlations of concentration fluctuations can vary non-monotonically with distance from the source, and they show strong source separation effects with weak Reynolds number effects [27, 29, 30].…”

The Impact of Plant Oscillation on Dispersion in Emergent Aquatic Canopies

“…The vast majority of experimental and numerical studies of mixing in vegetation canopies have used rigid model elements (Abdolahpour et al., 2017; Jamali et al., 2019). Examples are laboratory studies of vertical mixing (Ghisalberti & Nepf, 2006), longitudinal mixing (Murphy, 2006), and lateral dispersion (Serra et al., 2004; Shoaei & Crimaldi, 2017; Tanino & Nepf, 2008b). Many aquatic plants are, however, flexible, a characteristic which has an important influence on water quality through altering light availability (Zimmerman, 2003), nutrient uptake (Huang et al., 2011), oxygen efflux (Mass et al., 2010), and hydrodynamics (Burke, 2007).…”