Session G22: Flow Control: Passive - Enclosed Flows and Boundary Layers

The amplitude dependence of the mean velocity distortion for turbulence stabilisation in channel flow

Flattening the mean velocity profile has recently been shown both experimentally and computationaly as an effective means to stabilise turbulence in pipe flow (Kühnen et al, 2018, Nat. Phys.). Yet, a subsequent recent study optmising this distortion with an adjoint-based method has revealed that decreasing the mean shear near the wall may be more effective than flattenning the mean velocity subject to a sufficiently high forcing amplitude used for the mean distortion (Marensi et al., 2020, J. Fluid Mech.). In this study, we systematically investigate the amplitude dependence for the optimal mean distortion using an ensemble variation technique, which enables us to study the regime of a relatively small-amplitude mean distortion that was not able to be studied previously. The mean distortion mechanism is implemented by applying a streamwise localised linear damping force, and the optimisation of its wall-normal profile is subsequently performed. At small amplitudes of the forcing, the most effective approach is to apply the damping in the vicinity of the half-channel height, similar to the original study by Kühnen et al (2018). However, on increasing the amplitude, the optimal damping coefficient profile moves progressively closer to the near-wall region, resulting in a distinctive 'M' shape, consistent with the findings of Marensi et al (2020). These observations suggest that there exist some non-trivial physical mechanisms in turbulence stabilisation, and this will be dicussed in detail in the final presentation.   

Flux enhancement by Shear Free Surfaces in Turbulent Convection

Experimental investigations into the effects of introducing solid inserts termed 'Shear Free Surfaces' (SFS) into an axially homogenous, buoyancy driven zero mean shear turbulent convection in a vertical pipe are reported. These SFSes attenuate the lateral momentum transports due to kinematic wall blocking and modify the large scales of the flow. There is a concurrent increase in the tangential transports. In this work the analogous effects of these SFSes on the scalar transports is studied. The control flow considered for this purpose is inherently shear-free, thus rendering any stationary surface in the flow as a SFS. The suppression of the lateral scalar transport in the presence of the SFS, leads to an increase in the axial transports. Preliminary evidence for this is seen from the enhancement of the salt flux across the pipe. The placement of cylindrical rod inserts acting as SFSes into the flow resulted in flux enhancements disproportionate to the area blocked. Flow field measurements using combined PIV-PLIF techniques revealed that the attenuation of the lateral transports by the SFS, results in reduced mixing which disrupts the decoherence of flow structures, ultimately leading to an increased axial transport and flux enhancement. This phenomenon manifests in the form of reduced lateral velocity fluctuations and increased axial velocity and scalar fluctuations, an increase in the axial spatio-temporal scales of the flow and buoyancy production and decrease of the lateral transports.   

Broadening Applicability of Zero-Stiffness Dampening: Active Pressure Modulation in Hyper-Elastic Dampers

In prior research, a novel pulsation dampening device, labeled as a "hyper-elastic damper," was developed, comprising of hyperelastic materials. The inherent properties of these materials enabled the damper to exhibit a unique zero-stiffness characteristic within a specified deformation range. The nonlinear relationship between stress and strain in the materials facilitated this unique property. Such a feature made it possible for the damper to significantly decrease micro-pulsations in fluid transport systems, with reductions reaching up to 97%. Remarkably, this high level of performance was achieved without the need for external control, additional sensors, or power consumption. However, a limitation became evident - the zero-stiffness property was confined to specific pressure conditions, restricting the damper's broader applicability.

The focus of the current study is to address this limitation. We propose a more advanced damper design that includes an external chamber capable of modulating pressure actively. The introduction of this feature facilitates the damper to leverage zero-stiffness properties across an array of systems, effectively overcoming the pressure limitations of the original design. Our experimental validation provides strong evidence that the redesigned damper retains the high-efficiency pulsation reduction of the original model while demonstrating enhanced adaptability to various system conditions.

The potential impact of these advancements on industries reliant on precision fluid handling, such as semiconductor manufacturing, could be substantial. This research opens up opportunities for notable improvements in manufacturing yields and overall operational efficiency. In essence, our study represents a significant stride towards the broader implementation and application of zero-stiffness dampening technology, with the promise of improved productivity and efficiency across diverse industry sectors.   

Passive mixing enhancement in microchannel flows using flexible structures

In the field of biomedical diagnostics and drug development-related chemical applications, miniaturised systems are employed and are usually termed microfluidic devices. Mixing samples and reagents in these devices must be mixed thoroughly before processing. The available mixing methods like active stirrers, high flow rate, additional pumping and active methods usually used in macro-scale devices cannot be used in microfluidic devices due to the dimensional constraints such as shorter channel length, and low Reynold number flows due to the viscosity-dominated biological flows. So, passive mixing processes are the best choice which solely rely on mass transport phenomena due to molecular diffusion, flow transition and chaotic advection. To assist these phenomena in micro-devices, usually, geometric designs are proposed and experimented which increase the contact surface area and decrease the diffusion path. Some such designs and passive mixing methods are T- and Y- Y-shaped mixers, separation and re-lamination mixers and serpentine-shaped mixers. In this work, we introduce structural flexibility in the channel geometrical design and investigate the flow agitation and its impact on the quality of the mixing process. We found that wall-mounted flexible structures inside a channel flow act as a passive stirrer that causes increased mixing efficacy than that in the case of rigid plates. We systematically investigate the various parametric influences on flow agitation, such as structure flexibility and inlet flow rate. This numerical setup involving flexible structures transforms a steady inlet flow into a periodic flow generator in the channel. This pumping effect, due to the 'passive stirrers' in the design, leads to flow transition and encourages fluid mixing. An efficient mixing device shall significantly advance the state-of-the-art developments in microfluidic devices. Also, similar inspirations can be forwarded to the mixing process across the scales in industries.