Inner-scaled Helmholtz resonators with grazing turbulent boundary layer flow

Response details are presented of small-scale Helmholtz resonators excited by grazing turbulent boundary layer flow. A particular focus lies on scaling of the resonance, in relation to the spatio-temporal characteristics of the near-wall velocity and wall-pressure fluctuations. Resonators are tuned to different portions of the inner-spectral peak of the boundary-layer wall-pressure spectrum, at a spatial scale of $\lambda_x^+ \approx 250$ (or temporal scale of $T^+ \approx 25$). Following this approach, small-scale resonators can be designed with neck-orifice diameters of minimum intrusiveness to the grazing flow. Here we inspect the TBL response by analysing velocity data obtained with hot-wire anemometry and particle image velocimetry measurements. This strategy follows the earlier work by Panton and Miller (J. Acoust. Soc. Am. 526, 800, 1975) in which only the change in resonance frequency, due to the grazing flow turbulence, was examined. Single resonators are examined in a boundary layer flow at $Re_\tau \approx 2\,280$. Two neck-orifice diameters of $d^+ \approx 68$ and 102 are considered, and for each value of $d^+$ three different resonance frequencies are studied (targeting the spatial scale of $\lambda_x^+ \approx 250$, as well as sub- and super-wavelengths). Passive resonance only affects the streamwise velocity fluctuations in the region $y^+ \lesssim 25$, while the vertical velocity fluctuations are seen in a layer up to $y^+ \approx 100$. A narrow-band increase in streamwise turbulence kinetic energy at the resonance scale co-exists with a more than 20% attenuation of lower-frequency (larger scale) energy. Current findings inspire further developments of passive surfaces that utilize the concept of changing the local wall-impedance for boundary-layer flow control, using miniature resonators as a meta-unit

Wall-pressure--velocity coupling in high-Reynolds number wall-bounded turbulence

Wall-pressure fluctuations are a practically robust input for real-time control systems aimed at modifying wall-bounded turbulence. The scaling behaviour of the wall-pressure--velocity coupling requires investigation to properly design a controller with such input data, so that the controller can actuate upon the desired turbulent structures. A comprehensive database from direct numerical simulations of turbulent channel flow is used for this purpose, spanning a Reynolds-number range $Re_\tau \sim 550 - 5200$. A spectral analysis reveals that the streamwise velocity is most strongly coupled to the linear term of the wall-pressure, at a wall-scaling of $\lambda_x/y \approx 14$ (and $\lambda_x/y \approx 8.5$ for the wall-normal velocity). When extending the analysis to both homogeneous directions in $x$ and $y$, the peak-coherence is centred at $\lambda_x/\lambda_z \approx 2$ and $\lambda_x/\lambda_z \approx 1$ for $p_w$ and $u$, and $p_w$ and $v$, respectively. A stronger coherence is retrieved when the quadratic term of the wall-pressure is concerned, but there is only weak evidence for a wall-attached-eddy type of scaling. Experimental data are explored in the second part of this work: wall-pressure data are denoised and subsequently used for predicting the binary-state of the streamwise velocity fluctuations in the logarithmic region. A binary estimation accuracy of up to 72% can be achieved by including both the linear and quadratic terms of the wall-pressure. This study demonstrates that a controller for wall-bounded turbulence (solely relying on wall-pressure data) has merit in terms of a sufficient state estimation capability, even in the presence of significant facility noise

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

The interaction between a sonic transverse jet and an oblique shock wave in a supersonic crossflow: An experimental study

The development of air-breathing propulsion systems as a mean to propel high-altitude high-speed transport and single-stage-to-orbit (SSTO) vehicles promises to be a viable alternative to chemical rocket propulsion. In order to enhance mixing, an oblique shock wave is introduced into the combustion chamber as a source of baroclinic vorticity as well as a static temperature and pressure augmenter. An experimental campaign was conducted in the ST-15 supersonic test facility at Mach 2 to analyse the effect of three main control variables: the jet momentum flux ratio, the flow deflection angle and the impingement position of the shock on the jet plume. Measurements were acquired with Schlieren photography and stereo PIV techniques. Results suggest that, while near-field momentum-driven mixing remains unaltered following the introduction of the impinging shock wave, mid-to-far-field mixing mechanisms do change. An increase in the jet plume elevation was observed together with one in lateral expansion as a consequence of the introduction of a shock wave. Also, the formation of a strong shear layer downstream of the jet was observed, which acts as a source of vorticity to promote entrainment towards the jet mid-field. A stronger wave was noticed to produce more optimistic results for the mixing performance. This effect was seen to decrease with the introduction of a weaker shock or by shifting the strong shock downstream.Aerospace Engineering | Aerodynamic

Best Winglet of Minimum Induced Drag: Viscous and Compressible Flow Predictions

The aerodynamic performance of a wing mounting a simple unconventional box winglet are analyzed in detail by CFD simulations. The decomposition of the computed drag in its irreversible (viscous) and reversible (lift-induced) components by a far field drag breakdown method allows for the determination of the span efficiency, not a trivial task in case of viscous flow. The performance are compared with the ones obtained in case of simple standard winglet and without any tip appendices

Aerodynamic Analysis of a Box Winglet: Viscous and Compressible Flow Predictions

The aerodynamic performance of a wing mounting a simple unconventional box winglet are analyzed in detail by CFD simulations. The decomposition of the computed drag in its irreversible (viscous) and reversible (lift-induced) components by a far field drag breakdown method allows for the determination of the span efficiency, not a trivial task in the case of viscous flow. The aerodynamic performance are compared with the ones obtained in case of a simple standard winglet and without any tip appendices