Spanwise flow development within a laminar separation bubble under natural and forced transition

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.expthermflusci.2018.02.032 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The variation of streamwise and spanwise characteristic wavelengths of a NACA 0018 laminar separation bubble under natural and periodic excitation conditions is investigated experimentally. Periodic forcing is applied with an AC-DBD plasma actuator, and the response of the bubble is characterised in two orthogonal planes by means of time-resolved particle image velocimetry. Periodic excitation results in substantial time-averaged size reduction of the bubble. Linear stability analysis is used to establish that the most notable flow deformation is achieved when excitation is applied at the most unstable frequency, which does not significantly vary (<4%) for the range of excitation parameters investigated. At excitation frequencies well below the unstable frequency band, the shear layer does not lock to the excitation and is, instead, modulated. Lock-in is achieved at higher forcing frequencies, which are within the unstable band. For the case of modulated shedding, spanwise deformations become more significant than in the natural case; whereas when shedding becomes locked to the excitation frequency, the coherence of the rollers along the span increases. Characteristic streamwise and spanwise wavelengths are statistically quantified by means of spatial wavelet analysis, demonstrating that spanwise deformations attain wider range of wavelengths than the respective streamwise rollers. Analysis of these results suggests that spanwise deformation is associated to both the incoming boundary layer and shear layer stability characteristics

Opposition flow control for reducing skin-friction drag of a turbulent boundary layer

This work explores the dynamic response of a turbulent boundary layer to large-scale reactive opposition control, at a friction Reynolds number of $Re_\tau \approx 2\,240$. A hot-film is employed as the input sensor, capturing large-scale fluctuations in the wall-shear stress, and actuation is performed with a single on/off wall-normal blowing jet positioned $2.4\delta$ downstream of the input sensor, operating with an exit velocity of $v_{\rm j} = 0.4U_\infty$. Our control efforts follow the work by Abbassi et al. [Int. J. Heat Fluid Fl. 67, 2017], but includes a control-calibration experiment and a performance assessment using PIV- and PTV-based flow field analyses. The controller targets large-scale high-speed zones when operating in ``opposing" mode and low-speed zones in the ``reinforcing" mode. An energy-attenuation of about 40% is observed for the opposing control mode in the frequency band corresponding to the passage of large-scale motions. This proves the effectiveness of the control in targeting large-scale motions, since an energy-intensification of roughly 45% occurs for the reinforcing control mode instead. Skin friction coefficients are inferred from PTV data to yield a direct measurement of the wall-shear stress. Results indicate that the opposing control logic can lower the wall-shear stress by about 3% with respect to a desynchronised control strategy, and by roughly 10% with respect to the uncontrolled flow. A FIK-decomposition of the skin-friction coefficient was performed, revealing that the off-the-wall turbulence follows a consistent trend with the PTV-based wall-shear stress measurements, although biased by an increased shear in the wake of the boundary layer given the formation of a plume due to the jet-in-crossflow actuation

Active control of turbulent convective heat transfer with plasma actuators

We study an array of streamwise-oriented Dielectric Barrier Discharge (DBD) plasma actuators as an active control technique in turbulent flows. The analysis aims at elucidating the mechanism of interaction between the structures induced by the DBD-plasma actuators and the convective heat transfer process in a fully developed turbulent boundary layer. The employed flush-mounted DBD-plasma actuator array generates pairs of counter-rotating, stationary, streamwise vortices. The full three-dimensional, velocity field is measured with stereoscopic PIV and convective heat transfer at the wall is assessed by infrared thermography. The plasma actuator forcing diverts the main flow, yielding a low-momentum region that grows in the streamwise direction. The suction effect promoted on top of the exposed electrodes confines the vortices in the spanwise direction. Eventually, the pair of streamwise vortices locally reduces the convective heat transfer with a persistence of several outer lengthscales downstream of the actuation.Rodrigo Castellanos, Stefano Discetti and Andrea Ianiro have been supported by the project ARTURO, ref. PID2019-109717RB-I00/AEI/10.13039/501100011033, funded by the Spanish State Research Agency. Theodoros Michelis and Marios Kotsonis are supported by the European Research Council under StG project GloWing (#803082)

Tollmien-Schlichting waves over forward-facing steps: An experimental and numerical study

This work presents an experimental and numerical investigation jointly conducted by TU Delft and DLR on Tollmien-Schlichting (TS) waves interaction with a Forward-Facing Step (FFS). Experiments are conducted at the TU Delft low turbulence anechoic wind tunnel on an unswept flat plate model. Single-frequency disturbances are introduced using controlled acoustic excitation. The temporal response of the flow in the vicinity of the step is measured using Hot-Wire Anemometry (HWA). In addition, the global effect of the step on laminar-turbulent transition is captured using Infrared Thermography (IR). Two-dimensional (2-D) Direct Numerical Simulations (DNS) performed at DLR provide detailed information at the step. Experimental and DNS results in clean and step case conditions present very good agreement. Both methods predict large distortion of the TS waves downstream of the step, where DNS results present different growth trends between velocity components of the TS waves. Furthermore, negative and positive regions of the production term are observed to correlate with streamwise positions where the disturbances appear tilted in and against the mean shear, respectively. These findings point towards the presence of different growth mechanisms triggered by the step which could modify the level of amplification of disturbances far downstream

DNS of the interaction of crossflow instabilities with forward-facing steps

Previous studies on the interaction between stationary crossflow (CF) vortices and a forward-facing step (FFS) have shown a significant influence on the laminar-turbulent transition (e.g. [1, 2]). In a recent experimental investigation, Rius-Vidales & Kotsonis [3] found that the effect of the step height on the transition location is non-monotonic. An unprecedented transition delay (w.r.t to the case without FFS) occurs when the incoming stationary CF vortices interact with a shallow FFS. Instead, the interaction with a higher FFS leads to an upstream advancement of the transition front location. The present work aims to numerically reproduce the experimental setup in [3] through direct numerical simulation (DNS). The current investigation’s final goal is to understand further the flow physics behind the observed behaviour in the experiments