A comparison of entrainment in turbulent line plumes adjacent to and distant from a vertical wall

Vertically distributed wall sources of buoyancy. Part 1. Unconfined

“…Our findings support their observation that the distributed wall-source plume has a significantly lower entrainment coefficient, α = 0.068 ± 0.006, than the free plume, α f = 0.135 ± 0.010, however, we find only a small reduction compared to the wall plume, α w = 0.076 ± 0.006 (Parker et al 2020). The relative difference between α and α w is noticeably smaller than previous findings, where α ranges from 0.014 to 0.08 (Cooper & Hunt 2010;McConnochie & Kerr 2015;Caudwell et al 2016;Gayen et al 2016;Bonnebaigt et al 2018) and α w = 0.1 (Bonnebaigt et al 2018), which merited such an in depth discussion by Kaye & Cooper (2018).…”

“…The experiments were designed to create vertically distributed wall plumes with a uniform buoyancy flux which could be examined by performing simultaneous 10 -1 10 0 10 1 10 0 10 1 measurements of the buoyancy and velocity field. The experiments were performed in a Perspex acrylic tank of horizontal cross-section 1.20 m × 0.40 m filled with dilute saline solution of uniform density ρ a to a depth of 0.75 m. The buoyant wall source was created by forcing relatively dense sodium nitrate solution, which enabled refractive indices of the source fluid and ambient to be approximately matched (for full details see Parker et al (2020), and appendices therein) through two porous stainless steel plates, with a very low permeability of P 0 = 2.20 × 10 −13 m 2 , of dimensions 0.48 m × 0.23 m and thickness d = 5 mm, connected in series via two source chambers. The source fluid was supplied using a Cole-Parmer Digital Gear Pump System, 0.91 ml rev −1 .…”

“…Both calibration images were captured with the polyamide particles within the tank, at the seeding density used for the experiment, to account for differences in the laser intensity due to the presence of the particles. Parker et al (2020) used the same laser, ambient tank and camera to perform LIF on a wall and free plume. They performed a detailed analysis of the effect of the attenuation of the laser on the LIF measurements by measuring the extinction coefficient of the Rhodamine 6G and added salts and calculating the effective dye concentration that the laser beam rays pass through.…”

“…Similar to stereo PIV calibration, e.g. Willert (1997), these particle images were then cross-correlated to determine a disparity map and shift the coordinate mappings to compensate for any small misalignment between the calibration target and the light sheet, an identical procedure to that described in Parker et al (2020).…”

“…However, the slightly lower value, compared to the wall plume, suggests that vertically distributed buoyancy also makes a minor contribution to reducing entrainment. Based on the conclusions of Parker et al (2020), it is reasonable to conclude here that the significant reduction in entrainment observed in the distributed wall-source plume relative to free-shear buoyancy-driven flows (also compare to α p ≈ 0.11 in an axisymmetric plume, Burridge et al 2017) is primarily a result of the suppression of large-scale coherent structures (which have been shown to dominate the flow, e.g. Burridge, Partridge & Linden 2016), and the wall inhibiting meandering of the plume both leading to reduced turbulent mixing by the plume.…”