Near-wake Patterns Research Articles

Spanwise modulation of a three-dimensional wake using distributed forcing

Synthetic jets that are spatially distributed in the spanwise direction are used to control the flow over a circular cylinder. The Reynolds number based on the cylinder diameter and the free-stream velocity is Re = 500. To reveal how the finite spanwise forcing is coupled with the flow around the cylinder, three spanwise forcing wavelengths are considered, namely one, two, and three times of the cylinder diameter. Through distributed control, both the morphologic and dynamic features of the near wake are changed. The vortex shedding mode downstream of the forcing section can be converted from the original asymmetric form to a symmetric form, whereas the wake downstream of the no-forcing section is less influenced by the synthetic jets. Thus, the near-wake patterns and vortex dynamics vary in the spanwise direction according to the distributed forcing. In particular, the original large-scale Kármán vortex and streamwise vortex become smaller-scale vortices, and their spanwise characteristic length scale resembles the distributed forcing wavelength. This spanwise modulation by the proposed control strategy demonstrates that the lift fluctuations could be suppressed and the mean drag could be reduced, though this will be affected by the spanwise forcing wavelength.

Vortex-induced vibration of a long flexible cylinder in uniform cross-flow

Numerical simulations are performed of a long flexible cylinder undergoing vortex-induced vibration at a Reynolds number of 500. The cylinder is pinned at both ends, having an aspect ratio of 100 (cylinder length to cylinder diameter) and a mass ratio of 4.2 (structural mass to displaced fluid mass). Temporal and spatial information on the cross-flow (CF) and in-line (IL) vibrations is extracted. High modal vibrations up to the 6th in the CF direction and the 11th in the IL direction are observed. Both the CF and IL vibrations feature a multi-mode mixed pattern. Mode competition is observed. The 2nd mode with a low frequency dominates the IL vibration and its existence is attributed to a wave group propagating back and forth along the span. Distributions of fluid force coefficients are correlated to those of the CF and IL vibrations along the span. Histograms of the x-y motion phase difference are evaluated from the total simulation time and a complete vibration cycle representing the standing or travelling wave pattern. Correlations between the phase difference and the vibrations are discussed. Vortex structures behind the cylinder show an interwoven near-wake pattern when the standing wave pattern dominates, but an oblique near-wake pattern when the travelling wave pattern prevails.

Flow-induced vibrations of two side-by-side circular cylinders: Asymmetric vibration, symmetry hysteresis and near-wake patterns

The immersed boundary method was utilised to numerically investigate the flow-induced vibrations (FIV) of two elastically mounted side-by-side circular cylinders in a uniform flow with low Reynolds numbers. Six distinct near-wake patterns were observed; the irregular (IR) pattern, the in-phase synchronized (IS) pattern, the anti-phase synchronized (AS) pattern, the biased anti-phase synchronized (BAS) pattern, the out-of-phase flip-flopping (OFF) pattern, and the hybrid (HB) pattern. A detailed analysis on the asymmetric vibration and symmetry hysteresis phenomena was conducted by focusing on the near-wake patterns and the interaction between the cylinders. Results show that the asymmetric vibrations of the cylinders are closely related with the stably biased gap flow and the resulting narrow-wide near-wake pattern. While the symmetry hysteresis is caused by the coexistence of two distinct near-wake patterns – the IS and the BAS patterns. The transition processes of BAS to IS and IS to BAS were illustrated by using the long-time histories of the lift coefficients, the combined lift coefficient and the phase differences of lift and displacement. Results on hydrodynamic forces and the vibration responses show that the HB pattern is a combination of IS, OFF and AS patterns with a very long period.

Response and wake patterns of two side-by-side elastically supported circular cylinders in uniform laminar cross-flow

Vortex-induced vibrations (VIV) of two side-by-side elastically supported circular cylinders in a uniform flow with the Reynolds number of 100 are numerically investigated by using the immersed boundary method. The cylinders are constrained to oscillate in the cross-flow direction with a center-to-center spacing ratio T/D ranging from 2 to 5. The structural damping is set to zero to enable large vibration amplitudes in the range of reduced velocity Ur=3−10. It is found that the proximity of the cylinders does not have a significant impact to the lock-in region and cylinder responses, except at a small spacing ratio of T/D=2. The critical spacing ratio is determined as T/D=4 and beyond that the interaction between the cylinders is negligible. The following six near-wake patterns are observed; the irregular pattern, in-phase flip-flopping pattern, out-of-phase flip-flopping pattern, in-phase-synchronized pattern, antiphase-synchronized pattern and the biased antiphase-synchronized pattern. These patterns are plotted in a plane of Ur and T/D, together with approximate borderlines to distinguish one region from the others. The time histories, spectral features and wavelet transform contours of drag and lift forces are presented to elucidate the mechanisms of the in-phase and out-of-phase flip-flopping phenomena. It is established that the in-phase flip-flopping stems from the long-short near-wake pattern and its low-frequency flip-over, whereas the out-of-phase pattern originates from the large vortex shedding from the fictitious bluff-body with an augmented characteristic length.

Time-resolved measurements of total temperature and pressure in the vortex street behind a cylinder

Recent theoretical and numerical investigations revealed the prospect that the instantaneous total temperature becomes separated around vortices; once time averaged, however, the wake behind a bluff body takes the guise of a colder wake, displaying the so-called Eckert–Weise effect. For low Mach number flows, the instantaneous total pressure was also shown to become separated in such a similar manner that the near-wake patterns of instantaneous total pressure exhibit almost exact facsimiles of those of total temperature. In this paper the results from the time-accurate measurements of fluctuating total temperature and pressure are presented. The data were obtained by placing a time-resolved aspirating probe in the vortex street behind a cylinder. The free-stream Mach number was 0.4 and the Reynolds number based on the cylinder diameter was 2.3×105. The maximum magnitude of the rms fluctuations of total temperature are of the order of 15 ° K—about 5% of the upstream total temperature. Maximum fluctuations in total pressure are approximately 10% of the upstream total pressure. Not only are the time traces of the total temperature and pressure taken in the near-wake qualitatively similar, but also quantitatively they agree with the predictions.

Flow past two-dimensional ribbon parachute models

Aerodynamic characteristics of two-dimensional, slotted bluff bodies are experimentally investigated. The models consist of a series of thin flat plates arranged to form flat and curved geometries. Flow visualizations, base pressure measurements, mean velocity vector measurements, arid drag force measurements are conducted to analyze the effects of spacing ratio (i.e., porosity), curvature, and a center opening (i.e., vent). Low-porosity model configurations produced stable near-wake patterns with enhanced vortex sheddings downstream. Model curvature reduced drag forces and weakened the vortex sheddings. Stabilizing effect of curvature on the nearwake patterns was also found. A vent combined with large model curvature was found to reduce drag force significantly as well as suppress the vortex sheddings among models with equivalent porosity.