Fluid flows in nature and applications are frequently subject to periodic velocity modulations. Surprisingly, even for the generic case of flow through a straight pipe, there is little consensus regarding the influence of pulsation on the transition threshold to turbulence: while most studies predict a monotonically increasing threshold with pulsation frequency (i.e. Womersley number, α), others observe a decreasing threshold for identical parameters and only observe an increasing threshold at low α. In the present study we apply recent advances in the understanding of transition in steady shear flows to pulsating pipe flow. For moderate pulsation amplitudes we find that the first instability encountered is subcritical (i.e. requiring finite amplitude disturbances) and gives rise to localized patches of turbulence ("puffs") analogous to steady pipe flow. By monitoring the impact of pulsation on the lifetime of turbulence we map the onset of turbulence in parameter space. Transition in pulsatile flow can be separated into three regimes. At small Womersley numbers the dynamics are dominated by the decay turbulence suffers during the slower part of the cycle and hence transition is delayed significantly. As shown in this regime thresholds closely agree with estimates based on a quasi steady flow assumption only taking puff decay rates into account. The transition point predicted in the zero α limit equals to the critical point for steady pipe flow offset by the oscillation Reynolds number (i.e. the dimensionless oscillation amplitude). In the high frequency limit on the other hand puff lifetimes are identical to those in steady pipe flow and hence the transition threshold appears to be unaffected by flow pulsation. In the intermediate frequency regime the transition threshold sharply drops (with increasing α) from the decay dominated (quasi steady) threshold to the steady pipe flow level.
An experimental investigation was carried out on the flapping motion of a turbulent reattaching shear layer downstream of a two-dimensional backward-facing step. The Reynolds number was 2.0 × 104, based on the free-stream velocity and the step height. The aim of this study is to analyze the flapping motion, which is featured unsteadiness of the reattaching shear layer, and its interaction with the recirculation region. High-resolution planar particle image velocimetry was used to measure the separated and reattaching shear layer in a horizontal-vertical plane. The velocity vector fields have shown the reattaching shear layer considerably flaps upwards and downwards as much in scale as approximately one step height from the middle part of recirculation region to the reattachment area. As a result, the recirculation region varies in size and the reattachment point shifts upstream and downstream. By applying singular value decomposition and proper orthogonal decomposition, the flapping motion is decomposed into multiple spatial modes, each of which represents interactions between the reattaching shear layer and recirculation region. In particular, the unsteady movement of the reattachment point is highly correlated with the flapping motion, and so is the maximum reverse flow. As a result, the flapping motion contributes substantial parts of the Reynolds shear stress and turbulent kinetic energy within the shear layer in the latter half of the reattachment length.
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.
We experimentally investigate the pulsating circular jet flow at moderate Reynolds numbers. By applying time-resolved particle image velocimetry in the axial-radial plane, we measure the near-field velocity fields with the jet source temporally modulated by sinusoidal pulsations. As a baseline, the steady jet flow with the same mean Reynolds number is tested. The direct comparisons of the mean and fluctuating velocity fields show that the whole potential core as well as the axisymmetric shear layer is modulated by the pulsation effect. Meanwhile, larger-scale vortices are formed in the shear layer with phase correlation of the pulsation cycle. As a result, the pulsation increases the turbulent mixing in the latter half of the potential core, and it extends the fluid entrainment further in the radial direction. The increased fluid entrainment of the ambient quiescent fluid is clearly identified by the attracting Lagrangian coherent structures as the bounds of the growing vortices within the shear layer. By analyzing the dynamic modes, we find that the low-frequency off-the-axis helical structures, which are dominant in the steady jet flow, are inhibited. The axisymmetric jet column mode and its harmonics along the axis are strengthened by the pulsation effect. Furthermore, the vortex formation mainly takes place particularly in the deceleration phase, whereas a shock-like wave front is formed during the acceleration, indicating the distinct roles of the pulsation phases in the jet instability.
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