Observations of flow repeatability and secondary circulation in an oscillating grid-stirred tank

Abstract

An oscillating grid-stirred tank was studied for flow repeatability and the existence of secondary circulations. For the particular tank studied, results indicate that mean flow values may vary by up to 25% and turbulent fluctuations may vary by up to 15% from one run to another. This result was observed to exhibit a potential grid geometry dependence. More importantly, there is evidence of significant flow field sensitivity to initial conditions. Particle image velocimetry results were used to reveal secondary mean flows in the grid-stirred tank. Because these characteristics are believed to be intrinsic to grid-stirred tanks, studies using such tanks must recognize and consider these effects.Oscillating grid-stirred tanks have long been used in experimental studies of turbulent mixing and transport processes in a variety of fluid systems (e.g., Refs. 1-5). The flow produced by an oscillating grid is the result of interactions between the individual jets and wakes created by the motion of the grid bars. At sufficient distance from the grid, these jets and wakes interact and break down into turbulence that is carried away from the grid by the jet motions. In concept, no mean flow exists and the turbulence is considered near isotropic and homogeneous in planes parallel to the driving grid.The oscillating grid-stirred tank examined here was used as part of a larger study of free-surface air-water gas transfer in the presence of subsurface turbulence. 6 As part of that investigation, several diagnostic experiments were conducted with the grid tank to quantify its particular characteristics. Among these were flow repeatability experiments and studies of secondary circulations. The findings from these investigations revealed some subtle characteristics of grid-stirred tanks that may have important consequences.The tank studied had dimensions 45ϫ 45ϫ 57 cm 3 and was fitted with a vertically oscillating grid, making it similar to tanks used by those cited above. Two stirring grids were used. Grid A was a 7 ϫ 7 square grid of solid bars. The cross section of each bar was d = 1.3 cm square, and the bars were spaced equally in both directions by M = 6.4 cm. Grid A closely resembled the grid used by Brumley and Jirka. 2 Grid B was a 5ϫ 5 square array of the same bars, spaced 8.9 cm apart in both directions. The grid was rigidly attached at its center point to a stainless steel shaft that passed through the tank floor with a watertight seal. A gearmotor-driven reciprocating mechanism beneath the tank vertically oscillated the shaft and grid. All experiments were performed with a water depth of 51 cm. The mean vertical position of the grid was located midway between the tank bottom and the free surface. For both grids, the distance from the extreme edge of the grid bar ends to the tank side wall was 4 mm. Several different grid stroke S and frequency f combinations were explored and are summarized in Table I. Other parameters appearing in Table I were determined using the empirical expressions of Hopfinger and Toly, 4 who fou...

Direct numerical simulation of turbulent scalar transport across a flat surface

“…Laboratory measurements have indeed shown β rms to be a good measure for mass transfer as it implicitly takes the surface conditions into account (Turney & Banerjee 2013). Figure 9 shows the dependency of K L on the surface divergence supporting the surface divergence model of McCready et al (1986), K L ∝ √ β rms D. It can be seen that our numerical results predict that K L = 0.525 √ β rms D which matches the trend of the experimental results in grid-stirred tanks performed by Herlina & Jirka (2008); McKenna & McGillis (2004a). The advantage of the present numerical simulation is that surface contamination is not an issue so that unbiased investigations of the effect of Sc and the turbulence levels (here described in terms of the turbulent Reynolds number R T ) can be performed.…”

Experimental study of oscillating-grid turbulence interacting with a solid boundary

“…The mean flow within OGT has been studied in detail in numerous previous studies (see for example De Silva & Fernando 1994;McKenna & McGillis 2004;McDougall 1979;Dohan & Sutherland 2002), and the qualitative characteristics of the mean flow observed with our apparatus are little different to those reported in these studies. In particular, the mean flow is not symmetrical, being most intense close to the grid (and tank walls) and decaying with distance below the grid.…”

“…In particular, the mean flow is not symmetrical, being most intense close to the grid (and tank walls) and decaying with distance below the grid. Plots showing typical mean-flow fields observed in OGT can be found, for example, in McKenna & McGillis (2004).…”

Mussel larval responses to turbulence are unaltered by larval age or light conditions

“…Isotropy ratios were slightly more variable than those in tanks with turbulence generated by a single grid (Hopfinger and Toly 1976; De Silva and Fernando 1994) or jet actuators (Hwang and Eaton 2004; Webster et al 2004) but no greater than those in other double‐grid tanks (Srdic et al 1996; Shy et al 1997). Some anisotropy can be attributed to unavoidable secondary circulations (e.g., McKenna and McGillis 2004). Dissipation rates were estimated without assuming isotropy and were relatively homogeneous both horizontally and vertically (Fig.…”