3 results
Periodic forcing of a large turbulent separation bubble
- Abdelouahab Mohammed-Taifour, Julien Weiss
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- Journal:
- Journal of Fluid Mechanics / Volume 915 / 25 May 2021
- Published online by Cambridge University Press:
- 11 March 2021, A24
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The response of a pressure-induced turbulent separation bubble (TSB) to periodic forcing by pulsed-jet actuators (PJAs) positioned in the upstream boundary layer is investigated experimentally in an attempt to elucidate the mechanism of low-frequency contraction and expansion (‘breathing’) already documented in this flow by Mohammed-Taifour & Weiss (J. Fluid Mech., vol. 799, 2016, pp. 383–412). The TSB is generated on a flat test surface by a combination of adverse and favourable pressure gradients and the free-stream velocity is $25\ \textrm {m}\,\textrm {s}^{-1}$. The results indicate that periodic forcing artificially reduces the size of the TSB by moving separation downstream and reattachment upstream. The smaller TSB is associated with narrower streamwise distributions of average pressure and forward-flow fraction, as well as smaller turbulent stresses in the shear layer bounding the recirculation region. Transient forcing experiments further demonstrate that the TSB responds to upstream forcing with a characteristic time scale that is of the same order of magnitude as that of the breathing motion. Overall, the results of this study support a mechanism whereby the low-frequency breathing motion is a response of the TSB to upstream perturbations that affect the position of separation first and, indirectly, the position of reattachment through a global redistribution of the pressure and velocity fields. The low-frequency behaviour of the TSB appears to be well illustrated by a first-order low-pass filter model that converts the broadband fluctuations of the incoming turbulent boundary layer into a low-frequency, large-scale oscillation of the separation and reattachment fronts, thus leading to a contraction and expansion of the TSB. The results of the continuous forcing experiments also offer new insights into active separation control with PJAs by showing that such actuators generate strong starting vortices that, when convected within an adverse pressure gradient, are associated with a downstream shift of the separation front.
Measurements of pressure and velocity fluctuations in a family of turbulent separation bubbles
- Arnaud Le Floc'h, Julien Weiss, Abdelouahab Mohammed-Taifour, Louis Dufresne
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- Journal:
- Journal of Fluid Mechanics / Volume 902 / 10 November 2020
- Published online by Cambridge University Press:
- 07 September 2020, A13
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Measurements of wall-pressure and velocity fluctuations are performed in a family of three incompressible, pressure-induced, turbulent separation bubbles (TSBs) of varying sizes, with an emphasis on the energetic low and medium frequencies. In all three cases the streamwise distribution of wall-pressure fluctuations shows a bi-modal character, with a first local maximum close to the position of maximum adverse pressure gradient and a second local maximum at the very end of the region of intermittent back flow. The first maximum is shown to be caused by the superposition of two separate phenomena occurring at approximately the same streamwise position: first, the pressure signature of a low-frequency contraction and expansion (breathing) of the TSBs, whose amplitude is shown to increase with the size of the separation bubble, and second, the effect of the adverse pressure gradient on the turbulent structures responsible for the pressure fluctuations in the attached boundary layer. The second maximum of the wall-pressure fluctuation coefficient also increases with the size of the TSB and is associated with the convection of large structures within the shear layer. Possible scaling laws are examined to show that both the local maximum Reynolds shear stress ${-\rho \overline {u'v'}_{max}}$ and the local maximum wall-normal stress ${\rho \overline {v'v'}_{max}}$ are adequate to scale the pressure fluctuations along the TSBs, with a better match when low frequencies are removed. Furthermore, a comparison with existing data from the literature illustrates the effects of Reynolds number and TSB size on the wall-pressure and velocity fluctuations. Finally, measurements in the spanwise direction demonstrate that, although corner effects strongly distort the average flow, the scaling of wall-pressure fluctuations with the turbulent stresses remains relatively unaffected. The present results provide new insights into the unsteady character of pressure-induced turbulent separation bubbles and their associated wall-pressure fluctuations.
Unsteadiness in a large turbulent separation bubble
- Abdelouahab Mohammed-Taifour, Julien Weiss
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- Journal:
- Journal of Fluid Mechanics / Volume 799 / 25 July 2016
- Published online by Cambridge University Press:
- 23 June 2016, pp. 383-412
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The unsteady behaviour of a massively separated, pressure-induced turbulent separation bubble (TSB) is investigated experimentally using high-speed particle image velocimetry (PIV) and piezo-resistive pressure sensors. The TSB is generated on a flat test surface by a combination of adverse and favourable pressure gradients. The Reynolds number based on the momentum thickness of the incoming boundary layer is 5000 and the free stream velocity is $25~\text{m}~\text{s}^{-1}$. The proper orthogonal decomposition (POD) is used to separate the different unsteady modes in the flow. The first POD mode contains approximately 30 % of the total kinetic energy and is shown to describe a low-frequency contraction and expansion, called ‘breathing’, of the TSB. This breathing is responsible for a variation in TSB size of approximately 90 % of its average length. It also generates low-frequency wall-pressure fluctuations that are mainly felt upstream of the mean detachment and downstream of the mean reattachment. A medium-frequency unsteadiness, which is linked to the convection of large-scale vortices in the shear layer bounding the recirculation zone and their shedding downstream of the TSB, is also observed. When scaled with the vorticity thickness of the shear layer and the convection velocity of the structures, this medium frequency is very close to the characteristic frequency of vortices convected in turbulent mixing layers. The streamwise position of maximum vertical turbulence intensity generated by the convected structures is located downstream of the mean reattachment line and corresponds to the position of maximum wall-pressure fluctuations.