On wall pressure fluctuations and their coupling with vortex dynamics in a separated–reattached turbulent flow over a blunt flat plate

Tenaud, Christian; Podvin, Bérengère; Fraigneau, Yann; Daru, Virginie

doi:10.1016/j.ijheatfluidflow.2016.08.002

Cited by 19 publications

(21 citation statements)

References 52 publications

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“…Then, transition to turbulence takes place very quickly. Indeed, the largescale spanwise rolls develop into hairpin-like vortices further downstream (Sasaki & Kiya 1991;Hourigan et al 2001;Tenaud et al 2016). As shown by Lasheras & Choi (1988), this three-dimensional pattern is at the basis of the observed presence of streamwise velocity streaks which result from the interaction of counter-rotating pairs of streamwise vortices with the mean shear in analogy with plane free shear layers.…”

Section: Introductionmentioning

confidence: 89%

“…Subsequently, transition to turbulence sets in very rapidly for x > 0.5 (Winant & Browand 1974;Spalart & Strelets 2000). Under the effect of the mean shear, still strong at these streamwise locations, perturbations of the flow field lead to the lift up and stretching of the primary spanwise vortices thus forming hairpin-like structures (Hourigan et al 2001;Langari & Yang 2013;Tenaud et al 2016) arranged in a staggered manner (Soria et al 1993;Sasaki & Kiya 1991), see the enlargement of figure 3(a). By moving downstream, these hairpin-like structures are stretched and, as shown in figure 4 where the pattern taken by the streamwise vorticity, ω x = ∂w/∂y − ∂v/∂z is reported, the flow motion develops streamwise vortices (Kiya & Sasaki 1985;Bernal & Roshko 1986), which are known to induce entrainment and high-and low-speed streaks (Jiménez 1983).…”

Section: Instantaneous Flow Topologymentioning

confidence: 97%

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On the structure of the self-sustaining cycle in separating and reattaching flows

2018

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The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.

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Section: Introductionmentioning

confidence: 89%

Section: Instantaneous Flow Topologymentioning

confidence: 97%

On the structure of the self-sustaining cycle in separating and reattaching flows

2018

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“…Following Eaton & Johnston (1982), they proposed that the origin of the lowfrequency unsteadiness may be the change of the spanwise coherence of the vortices and the variation in their ability in entraining momentum. Some high-fidelity simulations of similar configurations also show the presence of the low-frequency process whose cause was not elucidated (Tafti & Vanka 1991;Tenaud et al 2016).…”

Section: Unsteadiness Of Separating Flowsmentioning

confidence: 99%

“…In most of these applications, the incoming boundary layer is turbulent and separates either because of an adverse pressure gradient (APG) or a geometric discontinuity (e.g., backward facing step). The separated flow typically exhibits unsteadiness across a broad range of time-scales (Eaton & Johnston (1982); Cherry et al (1984); Kiya & Sasaki (1985); Tenaud et al (2016); Najjar & Balachandar (1998), among others). These unsteady modes can dominate the dynamics of the separation bubble and they have significant implications for the performance of the flow device/system at hand.…”

Section: Introductionmentioning

confidence: 99%

Spatio-temporal dynamics of turbulent separation bubbles

Meneveau

Mittal

2019

J. Fluid Mech.

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The spatio-temporal dynamics of separation bubbles induced to form in a fully-developed turbulent boundary layer (with Reynolds number based on momentum thickness of the boundary layer of 490) over a flat plate are studied via direct numerical simulations. Two different separation bubbles are examined: one induced by a suction-blowing velocity profile on the top boundary and the other, by a suction-only velocity profile. The latter condition allows reattachment to occur without an externally imposed favorable pressure gradient and leads to a separation bubble more representative of those occurring over airfoils and in diffusers. The suction-only separation bubble exhibits a range of clearly distinguishable modes including a high-frequency mode and a low-frequency "breathing" mode that has been observed in some previous experiments. The high-frequency mode is well characterized by classical frequency scalings for a plane mixing layer and is associated with the formation and shedding of spanwise oriented vortex rollers. The topology associated with the low-frequency motion is revealed by applying dynamic mode decomposition to the data from the simulations and is shown to be dominated by highly elongated structures in the streamwise direction. The possibility of Görtler instability induced by the streamwise curvature on the upstream end of the separation bubble as the underlying mechanism for these structures and the associated low frequency is explored.

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“…The implementation of Proper Orthogonal Decomposition (POD) to analyze the extensive data using advanced diagnostics has provided valuable insights. POD analysis of Podvin et al (2014) and Tenaud et al (2016) showed that although the flow exhibited 2 characteristic frequencies corresponding to the flapping and shedding modes, the most energetic motions corresponded to the shedding of large-scale vortices. A quasi-invariance in the spanwise direction was also noted.…”

Section: Introductionmentioning

confidence: 99%

Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

2020

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Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number. Graphic abstract

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On wall pressure fluctuations and their coupling with vortex dynamics in a separated–reattached turbulent flow over a blunt flat plate

Abstract: DARU-On wall pressure fluctuations and their coupling with vortex dynamicsin a separate d-reattache d turbulent flow over a blunt flat plate

Cited by 19 publications

References 52 publications

On the structure of the self-sustaining cycle in separating and reattaching flows

On the structure of the self-sustaining cycle in separating and reattaching flows

Spatio-temporal dynamics of turbulent separation bubbles

Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

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