The structural arrangements in the outer layer of turbulent boundary layer flows were explored with large-field time-resolved particle image velocimetry measurements at moderate Reynolds number. The large- and small-scale structures were reconstructed by the modes of multiscale proper orthogonal decomposition. The association between hairpin packets and uniform momentum zones (UMZs) was examined by the conditional averaging results based on the large-scale positive-to-negative/negative-to-positive (PN/NP) zero-crossings. The scale arrangements provided the spatial evidence that the intense small-scale swirling motions are aligned in the confined internal shear layers along the backside of the large-scale, low-speed region, which was characterized by hairpin vortex packets. The uniform momentum zones (UMZs) conditioned on the large-scale PN/NP zero-crossings were detected from the histograms of the instantaneous streamwise velocity. The attached eddy behavior was consolidated based on the conditional events, by presenting the joint probability of UMZs thickness and wall-normal location. A close agreement of the conditional averaging raw velocity and modal velocity was examined. Moreover, the conditional averaging results of the UMZs interface probability exhibited a similar spatial distribution as the small-scale turbulent kinetic energy and swirling strength, which manifests the coincidence between the hairpin heads and the UMZs interfaces. This result was confirmed by the distribution of the wall-normal locations corresponding to the maximum value of interface probability and small-scale representations, which performs a streamwise inclination angle of 15°. The statistical spatial feature demonstrated the association between hairpin packets and uniform momentum as proposed by Adrian et al. [“Vortex organization in the outer region of the turbulent boundary layer,” J. Fluid Mech. 422, 1–54 (2000)].
Based on the idea of local resonance, a class of hybrid acoustic metamaterials with interposed resonant strips is proposed in this article, and the bandgap characteristics and transmission properties are calculated by combining Bloch's theorem and lattice theory. The new structure is verified to have low‐frequency damping and noise reduction capability through the analysis of the stress cloud diagram at specific frequencies in the bandgap range. A comprehensive analysis of vibration modes, phase constant surfaces, group velocity, phase velocity, and wave propagation direction diagram is used to explore the wave propagation and bandgap opening mechanism, providing technical support and theoretical basis for subsequent bandgap optimization and structural improvement. The results show that this structure can open multiple bandgaps in the low‐frequency range below 500 Hz, and the stepped design of the single‐cell structure and the introduction of resonant strips provide a new design idea for low‐frequency vibration and noise reduction.
This study reports the observation of cross term events of scale-decomposed skewness factor in turbulent boundary layer at moderate Reynolds number. The large field-of-view particle image velocimetry was utilized to measure the flow fields. By the approach of multi-scale proper orthogonal decomposition (mPOD), the large- and small-scale structures were reconstructed by the mPOD modes relevant to the predefined frequency bands. Then, the cross term of the scale-decomposed skewness was observed, which was proposed in the previous works by Schlatter and Örlü [Phys. Fluids 22, 051704 (2010)] and Mathis et al. [Phys. Fluids, 23, 121702 (2011)]. The cross term events are featured by both the large and small scales, which were consolidated by the linear fitting of correlation coefficients with different slope angles. The characteristic length of the local intense cross term events is around 0.1δ (δ is the boundary layer thickness), which is comparable with that of the swirling structures related to hairpin vortice in the form of hairpin packets. The conditional averaging results presented the arrangement that the local cross term event appears underneath the hairpin vortex in the statistical viewpoint. Based on the hairpin vortex model, it was proposed that the local intense cross term events are associated with the local low-speed fluids induced by the hairpins through the ejection process. Especially, in the wake region, the cross term events are promoted, and also well-correlated with the swirling structures. This kind of configuration was attributed to the combination of the vortex induction and the entrainment process relative to the turbulent/non-turbulent intermittency.
This study reports the modification of large and small scales in a turbulent boundary layer (TBL) perturbed by a dynamic cylindrical element (DCE). Tomographic particle image velocimetry (Tomo-PIV) was utilized to measure the flow fields downstream of the dynamic perturbation. By the approach of multi-scale proper orthogonal decomposition (mPOD), the coherent modes relevant to the predefined frequency bands were extracted from the Tomo-PIV dataset. Then, a method was developed to construct the large- and small-scale structures and the DCE-perturbed structure based on the mPOD modes. The DCE impact on the large- and small-scale structures was elaborated by comparing with the unperturbed TBL case. The two-point correlation analysis indicated that large-scale structures appear downstream of the DCE perturbation in a short streamwise length scale. More importantly, the scale rearrangements were further examined by presenting the modulation coefficients between the large scales and small-scale energy. It revealed that even though the DCE perturbation alters the level of correlation, three different types of interaction scenario can still be observed. In the near-wall region, the large-scale structures have an amplitude modulation effect on the small-scale energy with the lower positive coefficients. The reversal scale arrangement was observed at the wall-normal height around the DCE amplitude, which could be attributed to the fluid exchange caused by the new-generated turbulent structures. In the log region, it confirmed that the inclined shear layer resides along the low-speed regions, which supported the robustness of the conceptual model of hairpin packets in the current DCE-perturbed TBL.
We experimentally investigate leading-edge separation control effect by bionic coverts with various materials and sawteeth shapes in wind tunnel tests. The artificial flexible coverts, bio-inspired by bird covert feathers on upper wings, are hinged at the trailing-edge of a NACA 0018 airfoil at a constant high angle-of-attack of 15°. The chord-based Reynolds number is 1.0 × 105 in the generic range of bird flight in nature and low-speed fixed-wing unmanned aerial vehicles. The velocity profiles in the wake flow are measured by multi-channel hot-wire anemometer. By comparing the mean velocity profiles and root-mean-square velocities, we find the trailing-edge coverts reduced the thickness of the shear layers by 0.05 chord length. The turbulence intensity of the trailing- and leading-edge shear layers are reduced 34% and 5%, respectively. Further wavelet analysis reveals that the large sizes of vortices are considerably suppressed in the time-frequency spectrum. Based on the hot-wire datasets, we develop a novel multi-dimensional genetic algorithm to analyze the featured ordered structures in the shear layers and quantitatively characterize the amplitude modulation between the large- and small-scale flow structures. As a result, we find that the coverts-generated perturbations induce an increase in the high-frequency ( f = 91.2 Hz) coherence between the leading- and trailing-edge shear layers from 40% to 70%, leading to a reduction of the flow separation bubble on the upper wing. The present work reveals that the artificial bionic coverts have leading-edge flow separation control effectiveness and shows the engineering potential for aircrafts and unmanned aerial vehicles.
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