Starting, Travelling & Colliding Vortices: DBD Plasma in Quiescent Air

Development and interaction of starting vortices initiated by Dielectric Barrier Discharge (DBD) plasma actuators in quiescent air are illustrated in the attached fluid dynamics videos. These include a series of smoke flow visualisations, showing the starting vortices moving parallel or normal to the wall at several different actuator configurations.Comment: 1 page, hyperlinked to two fluid mechanics videos MPEG fomat

BUTERLI D3.14 Report

Final actuator hardware producing a spanwise row of wall-normal jet

Turbulent boundary-layer control with plasma spanwise travelling waves

Arrays of dielectric-barrier-discharge plasma actuators have been designed to generate spanwise travelling waves in the turbulent boundary layer for possible skin-friction drag reductions. Particle image velocimetry was used to elucidate the modifications to turbulence structures created by the plasma spanwise travelling waves. It has been observed that the plasma spanwise travelling waves amalgamated streamwise vortices, lifting low-speed fluid from the near-wall region up and around the peripheries of their cores to form wide ribbons of low-speed streamwise velocity within the viscous sublayer

BUTERFLI D3.19 Report

D3.19 Report on the experimental results on transition delay in the “Juju” TRIN1 wind tunnel by the VR DBD actuators based on wall-normal jet

Resistance of velocity slip flow in pipe/channel with a sudden contraction

© 2020 Author(s). A novel approach based on the local entropy generation rate, also known as the second law analysis (SLA), is proposed to compute and visualize the flow resistance in mass transfer through a pipe/channel with a sudden contraction component (SCC) at low Reynolds number (Re) featuring velocity slip. The linear Navier velocity slip boundary condition is implemented using the explicit scheme. At small Reynolds number, i.e., Re ≤ 10.0, the flow resistance coefficient of the SCC, KSCC, is found to be a function of the dimensionless velocity slip length Lslip∗ and Re-1, and gradually increase to a constant value at contraction ratio Rarea ≥ 8, reaching a formula KSCC=(0.4454Lslip∗ 3-1.894Lslip∗ 2+2.917Lslip*+8.909)/Re. Over this range of Re, the equivalent length of the flow resistance is almost independent of Re, while out of this range, the equivalent length increases monotonically with Re. Moreover, the dimensionless drag force work around the SCC is negative and reaches a minimum at a critical Lslip*. The SLA reveals that the regions affected by the SCC mainly concentrate around the end section of the upstream pipe/channel rather than the initial partition of the downstream section reported in large Re turbulent flow, and this non-dimensional affected upstream length increases with Lslip*. The fluid physics are further examined using SLA to evaluate the energy loss over the entire domain, decomposed as the viscous dissipation inside the domain and the drag work on the wall boundary

Plasma virtual-roughness elements for cross-flow instability control

Recent experiments and numerical simulations demonstrated that discrete roughness elements can be used to control cross-flow instability over a swept wing. Here, the application of this passive technique requires a row of thin cylindrical elements of a few microns high immediately downstream of the leading edge to excite the subcritical modes of cross-flow instability. By properly choosing the spanwise spacing of these roughness elements, one can suppress the growth of most unstable modes, thereby delay transition. However, this passive technique of controlling cross-flow instability is very sensitive to the size (diameter and height), shape and location of discrete roughness. To mimic the discrete roughness elements and to be able to adjust the roughness parameters dynamically, virtual roughness elements based on dielectric-barrier-discharge plasma actuators have been developed and tested. In this paper, we show the plasma-induced flow field of several different prototype virtual-roughness elements for cross-flow instability control, by describing the mechanisms of vortex generation from the virtual roughness elements through an interaction with the incoming laminar boundary layer

BUTERLI D3.14 Report

Final actuator hardware producing a spanwise row of wall-normal jet

An analytical solution of convective heat transfer in microchannel or nanochannel

The two-dimensional energy equation with a first-order velocity slip model and a temperature jump model is studied analytically and a solution consisting of an infinite series is obtained. Impacts of viscous dissipation, axial conduction and rarefied effect on the local Nusselt number, the asymptotic Nusselt number and the bulk temperature profile of fluid are investigated. Results show that the cooling effect of the fluid benefits from the higher rarefied effect and axial conduction effect, as well as the lower viscous dissipation. The asymptotic dimensionless bulk temperature of fluid converges to a constant value that is higher than the wall temperature at a given set of Brinkman number, Péclet number and Knudsen number regardless of the inlet conditions. When neglecting axial conduction and the rarefied effect, the asymptotic Nusselt number with or without viscous dissipation is 17.5 or 7.54, respectively. Effects of axial conduction on the asymptotic Nusselt number are negligible when the Péclet number is greater than 10, while its influence on the non-dimensional bulk temperature of fluid and local Nusselt number can be neglected only when Pe > 100