Periodic forcing of a large turbulent separation bubble

Mohammed-Taifour, Abdelouahab; Weiss, Julien

doi:10.1017/jfm.2021.77

Cited by 17 publications

(9 citation statements)

References 73 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Thus detachment appears when , and several studies have shown that TD and D take place at the same location (see, for instance, the work of Simpson 1989; Na & Moin 1998). The parameter is a classical quantity very often analysed in turbulent boundary layer separation studies (Simpson 1989; Na & Moin 1998; Driver, Seegmiller & Marvin 1987; Mohammed-Taifour & Weiss 2016, 2021; Elyasi & Ghaemi 2019), and according to Simpson (1989), it is a feature that should be documented in this kind of work. It may also be a quantity of practical interest for the experimental detection of separation onset and reattachment (see, for instance, Mohammed-Taifour et al.…”

Section: Analysis Of Boundary Layer Relaxationmentioning

confidence: 99%

Outer layer turbulence dynamics in a high-Reynolds-number boundary layer up to recovering from mild separation

2022

View full text Add to dashboard Cite

The outer layer dynamics of a high-Reynolds-number boundary layer recovering from non-equilibrium is studied utilising the multi-resolution approach of zonal detached eddy simulation mode 3. The non-equilibrium conditions are obtained from a boundary layer separation over a rounded step enhancing the turbulent production, and recovery happens during redevelopment after reattachment at high Reynolds numbers ( $Re_{\theta,max}\approx 24{,}000$ ). Most of the outer layer turbulence is resolved by the simulation, which reproduces accurately the experimental boundary layer relaxation. The spectral analysis of streamwise velocity fluctuations and turbulent kinetic energy (TKE) production evidences the different turbulent content distribution at separation and within the redevelopment region, at which very large-scale motions are identified with streamwise wavelengths up to $\lambda _x = 9\delta$ , where $\delta$ is the boundary layer thickness. The redevelopment of the boundary layer is analysed in terms of the persistence of a secondary peak in the TKE production and the evolution of the wall-shear stress statistics. The skewness and probability density function of the skin friction show a slower relaxation than the downstream flow fraction. This confirms the long-lasting impact of perturbations of the outer layer in high-Reynolds-number wall-bounded flows. This persistent non-equilibrium state is suggested to be the reason for the reported lack of accuracy of the considered Reynolds-averaged Navier–Stokes models in the relaxation region.

show abstract

Section: Analysis Of Boundary Layer Relaxationmentioning

confidence: 99%

Outer layer turbulence dynamics in a high-Reynolds-number boundary layer up to recovering from mild separation

2022

View full text Add to dashboard Cite

show abstract

“…The basic idea of separation control with PJA is to inject high momentum fluid into a separated flow where the momentum is low in order to transfer momentum between the high and low speed regions of the flow and by that reattach the flow to the surface [31]. The pulsed jet flow control mechanism is driven by complex three-dimensional vortex structures [34,35], which are generated by the starting vortex ring of the pulsed jets injected in the main flow [36]. The reattachment process with a pulsed jet is sketched in Figure 1.…”

Section: Pja Working Principlementioning

confidence: 99%

Fluidic-Oscillator-Based Pulsed Jet Actuators for Flow Separation Control

Löffler

Ebert

Weiss

2021

Fluids

Self Cite

View full text Add to dashboard Cite

The control of flow separation on aerodynamic surfaces remains a fundamental goal for future air transportation. On airplane wings and control surfaces, the effects of flow separation include decreased lift, increased drag, and enhanced flow unsteadiness and noise, all of which are detrimental to flight performance, fuel consumption, and environmental emissions. Many types of actuators have been designed in the past to counter the negative effects of flow separation, from passive vortex generators to active methods like synthetic jets, plasma actuators, or sweeping jets. At the Chair of Aerodynamics at TU Berlin, significant success has been achieved through the use of pulsed jet actuators (PJA) which operate by ejecting a given amount of fluid at a specified frequency through a slit-shape slot on the test surface, thereby increasing entrainment and momentum in a separating boundary layer and thus delaying flow separation. Earlier PJAs were implemented using fast-switching solenoid valves to regulate the jet amplitude and frequency. In recent years, the mechanical valves have been replaced by fluidic oscillators (FO) in an attempt to generate the desired control authority without any moving parts, thus paving the way for future industrial applications.

show abstract

“…2009). Although there are fundamental differences between supersonic and subsonic flows, Weiss, Mohammed-Taifour & Schwaab (2015) and Mohammed-Taifour & Weiss (2021) noted that their breathing motion may share a similar mechanism.…”

Section: Introductionmentioning

confidence: 99%

Unsteady motions in the turbulent separation bubble of a two-dimensional wing

Wang

Ghaemi²

2022

J. Fluid Mech.

View full text Add to dashboard Cite

The unsteadiness of a turbulent separation bubble (TSB) formed close to the trailing edge of a two-dimensional wing was investigated using time-resolved particle image velocimetry. The angle of attack was set to 9.7° and the chord-based Reynolds number was 720 000. The TSB consisted of two shear layers and formed a triangular shape in the streamwise–wall-normal plane. The vertices of this triangle consisted of an intermittent detachment point, a fixed corner close to the airfoil trailing edge and an intermittent endpoint in the wake region. The velocity field had three energetic regions each with different Strouhal numbers (Stl): (a) an upstream turbulent boundary layer (TBL) with Stl = 0.1 to 4, (b) a TSB with Stl = 0.03 to 0.08 and (c) two shear layers with Stl = 0.4 to 0.8. The low-frequency motions in the TSB consisted of large zones of positive and negative streamwise velocity fluctuation that were several times wider than the large-scale structures of the upstream TBL. These zones forced an undulation of the separation line and were attributed to Görtler structures. They were also correlated with the velocity fluctuations between the two shear layers. The breathing motion of the TSB occurred at Stl = 0.05. This breathing correlated with the location of the TSB endpoint and the flapping of the upper shear layer. The detachment point of the TSB featured broad fluctuations and did not demonstrate a strong correlation with the breathing motion.

show abstract

Periodic forcing of a large turbulent separation bubble

Abstract: Abstract

Cited by 17 publications

References 73 publications

Outer layer turbulence dynamics in a high-Reynolds-number boundary layer up to recovering from mild separation

Outer layer turbulence dynamics in a high-Reynolds-number boundary layer up to recovering from mild separation

Fluidic-Oscillator-Based Pulsed Jet Actuators for Flow Separation Control

Unsteady motions in the turbulent separation bubble of a two-dimensional wing

Contact Info

Product

Resources

About