The influence of wall roughness on bubble drag reduction in Taylor–Couette turbulence

Abstract

We experimentally study the influence of wall roughness on bubble drag reduction in turbulent Taylor-Couette flow, i.e. the flow between two concentric, independently rotating cylinders. We measure the drag in the system for the cases with and without air, and add roughness by installing transverse ribs on either one or both of the cylinders. For the smooth wall case (no ribs) and the case of ribs on the inner cylinder only, we observe strong drag reduction up to DR = 33% and DR = 23%, respectively, for a void fraction of α = 6%. However, with ribs mounted on both cylinders or on the outer cylinder only, the drag reduction is weak, less than DR = 11%, and thus quite close to the trivial effect of reduced effective density. Flow visualizations show that stable turbulent Taylor vortices -large scale vortical structures -are induced in these two cases, i.e. the cases with ribs on the outer cylinder. These strong secondary flows move the bubbles away from the boundary layer, making the bubbles less effective than what had previously been observed for the smooth-wall case. Measurements with counter-rotating smooth cylinders, a regime in which pronounced Taylor rolls are also induced, confirm that it is really the Taylor vortices that weaken the bubble drag reduction mechanism. Our findings show that, although bubble drag reduction can indeed be effective for smooth walls, its effect can be spoiled by e.g. biofouling and omnipresent wall roughness, as the roughness can induce strong secondary flows.

Bubbly drag reduction using a hydrophobic inner cylinder in Taylor–Couette turbulence

“…Further downstream, turbulent diffusion causes bubbles to move away from the wall (Murai 2014). A similar mechanism was observed in Taylor-Couette flow, where strong secondary flows transport bubbles away from the inner cylinder, resulting in a decrease of DR (van den Berg et al 2007;Fokoua et al 2015;Verschoof et al 2018a).…”

Controlling the number of vortices and torque in Taylor–Couette flow

“…The flow between two counter-rotating coaxial circular cylinders known as Taylor–Couette (TC) flow is a paradigm in fluid mechanics for studies of instability, spatiotemporal chaos, pattern formation and turbulence (Grossmann, Lohse & Sun 2016). On the other hand, due to its simple geometry, and experimental accessibility with high precision, the TC system also becomes an ideal test bed for various drag reduction methods, such as adding drag reducers (Groisman & Steinberg 1996; Nakken, Tande & Elgsaeter 2001; Guersoni et al 2015; Van Buren & Smits 2017), injection of bubbles (Van den Berg et al 2005; Murai, Oiwa & Takeda 2008; Sugiyama, Calzavarini & Lohse 2008; Verschoof et al 2018), using the Leidenfrost effect (Saranadhi et al 2016; Ayan, Entezari & Chini 2019) or modification of solid surfaces (Greidanus, Delfos & Westerweel 2011; Srinivasan et al 2015; Rosenberg et al 2016; Hu et al 2017; Naim & Baig 2019). In TC flow, pairs of counter-rotating vortices arise when the Reynolds number () exceeds a critical value (Andereck, Liu & Swinney 1986; Maretzke, Hof & Avila 2014; Grossmann et al 2016).…”

Towards boiling Taylor-Couette turbulence

“…If we were to conduct boiling experiments in this configuration, we would expect the vapor bubbles to be trapped by the rolls because of the lower pressure there. However, as it was also pointed out before [23,34,182], the interaction of the bubbles with the boundary layers is required to achieve drag reduction, so it is not clear a priori what would be the effect of this reallocation of the bubbles. Further experimental work would shed more light into this effect; and finally, what would happen to the morphology of the bubbles?…”