Measurements with a flow direction boundary-layer probe in a two-dimensional laminar separation bubble

2000 ◽  
Vol 28 (3) ◽  
pp. 236-242 ◽  
Author(s):  
C. P. Häggmark ◽  
A. A. Bakchinov ◽  
P. H. Alfredsson
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Flow Direction ◽  
Laminar Separation Bubble ◽  
Two Dimensional ◽  
Laminar Separation

Instability and low-frequency unsteadiness in a shock-induced laminar separation bubble

2016 ◽  
Vol 798 ◽  
pp. 5-26 ◽  
Author(s):  
Andrea Sansica ◽  
Neil D. Sandham ◽  
Zhiwei Hu
Keyword(s):  
Boundary Layer ◽  
Linear Stability ◽  
Flow Instability ◽  
Separation Bubble ◽  
Low Frequency ◽  
Boundary Layer Transition ◽  
Separation Point ◽  
Oblique Shock Wave ◽  
Laminar Separation Bubble ◽  
Laminar Separation

Three-dimensional direct numerical simulations (DNS) of a shock-induced laminar separation bubble are carried out to investigate the flow instability and origin of any low-frequency unsteadiness. A laminar boundary layer interacting with an oblique shock wave at $M=1.5$ is forced at the inlet with a pair of monochromatic oblique unstable modes, selected according to local linear stability theory (LST) performed within the separation bubble. Linear stability analysis is applied to cases with marginal and large separation, and compared to DNS. While the parabolized stability equations approach accurately reproduces the growth of unstable modes, LST performs less well for strong interactions. When the modes predicted by LST are used to force the separated boundary layer, transition to deterministic turbulence occurs near the reattachment point via an oblique-mode breakdown. Despite the clean upstream condition, broadband low-frequency unsteadiness is found near the separation point with a peak at a Strouhal number of $0.04$, based on the separation bubble length. The appearance of the low-frequency unsteadiness is found to be due to the breakdown of the deterministic turbulence, filling up the spectrum and leading to broadband disturbances that travel upstream in the subsonic region of the boundary layer, with a strong response near the separation point. The existence of the unsteadiness is supported by sensitivity studies on grid resolution and domain size that also identify the region of deterministic breakdown as the source of white noise disturbances. The present contribution confirms the presence of low-frequency response for laminar flows, similarly to that found in fully turbulent interactions.

scholarly journals An Experimental Study of Boundary-Layer Transition Induced Vibrations on a Hydrofoil

2010 ◽  
Author(s):  
Antoine Ducoin ◽  
Jacques Andre´ Astolfi ◽  
Marie-Laure Gobert
Keyword(s):  
Boundary Layer ◽  
Vortex Shedding ◽  
Separation Bubble ◽  
Operating Conditions ◽  
Boundary Layer Transition ◽  
Laminar Separation Bubble ◽  
Naval Academy ◽  
Layer Transition ◽  
Laminar Separation ◽  
Layer Flow

In this paper, we investigate through an experimental approach the laminar to turbulent transition in the boundary-layer flow along a hydrofoil at a Reynolds number of 7.5 × 105, together with the vibrations of the hydrofoil induced by the transition. The latter is caused by a Laminar Separation Bubble (LSB) resulting from a laminar separation of the boundary-layer. The experiments, conducted in the hydrodynamic tunnel of the Research Institute of the French Naval Academy, are based on wall pressure and flow velocity measurements along a rigid hydrofoil, which enable a characterization of the Laminar Separation Bubble and the identification of a vortex shedding at a given frequency. Vibrations measurements are then carried out on a flexible hydrofoil in the same operating conditions. The results indicate that the boundary-layer transition induces important vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies.

Laminar separation bubble development on an airfoil emitting tonal noise

2015 ◽  
Vol 780 ◽  
pp. 167-191 ◽  
Author(s):  
S. Pröbsting ◽  
S. Yarusevych
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Quantitative Description ◽  
Suction Side ◽  
Pressure Side ◽  
Laminar Separation Bubble ◽  
Tonal Noise ◽  
Trailing Edge ◽  
Laminar Separation ◽  
Trailing Edge Noise

The subject of this experimental study is the feedback effects due to tonal noise emission in a laminar separation bubble (LSB) formed on the suction side of an airfoil in low Reynolds number flows. Experiments were performed on a NACA 0012 airfoil for a range of chord-based Reynolds numbers $0.65\times 10^{5}\leqslant \mathit{Re}_{c}\leqslant 4.5\times 10^{5}$ at angle of attack ${\it\alpha}=2^{\circ }$, where laminar boundary layer separation is encountered on both sides of the airfoil. Simultaneous time-resolved, two-component particle image velocimetry (PIV) measurements, unsteady surface pressure and far-field acoustic pressure measurements were employed to characterize flow development and acoustic emissions. Amplification of disturbances in separated shear layers on both the suction and pressure sides of the airfoil leads to shear layer roll-up and shedding of vortices from separation bubbles. When the vortices do not break up upstream of the trailing edge, the passage of these structures over the trailing edge generates tonal noise. Acoustic feedback between the trailing edge noise source and the upstream separation bubble narrows the frequency band of amplified disturbances, effectively locking onto a particular frequency. Acoustic excitation further results in notable changes to the overall separation bubble characteristics. Roll-up vortices forming on the pressure side, where the bubble is located closer to the trailing edge, are shown to define the characteristic frequency of pressure fluctuations, thereby affecting the disturbance spectrum on the suction side. However, when the bubble on the pressure side is suppressed via boundary layer tripping, a weaker feedback effect is also observed on the suction side. The results give a detailed quantitative description of the observed phenomenon and provide a new outlook on the role of coherent structures in separation bubble dynamics and trailing edge noise generation.

Investigation of laminar separation bubble over a supercritical airfoil in an incompressible flow

2021 ◽  
pp. 095440622110207
Author(s):  
Massoud Tatar ◽  
Mehran Masdari ◽  
Mojtaba Tahani
Keyword(s):  
Boundary Layer ◽  
Surface Pressure ◽  
Separation Bubble ◽  
Second Derivative ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Supercritical Airfoil ◽  
Time Frequency ◽  
Transition Points ◽  
Hot Film

Supercritical airfoils have an unknown behavior at incompressible flow regime and Reynolds numbers lower than those related to their design point at transonic condition. In this work, boundary layer transition is studied over a supercritical airfoil by means of hot-film and pressure measurements completed with numerical simulations. The experiments are performed at chord-based Reynolds number of [Formula: see text]and Mach number of [Formula: see text] at different angles of attack. Hot-film measurement over the upper surface of the supercritical airfoil is carried out and the transition points are computed using the standard deviation of the signals. The upper surface pressure is also recorded and a peak in its second derivative is presented as the transition point generated by the laminar separation bubble mechanism. Moreover, an appropriate time-frequency analysis is applied to the hot-film signals to get an insight into the spectral content and development of the transitional boundary layer structures. On the other hand, two numerical codes are employed and the transition points obtained from numerical simulations are compared with the experimental outcomes. Results express a rapid change of the bubble position over the upper surface, as the angle of attack is increased to the value of [Formula: see text]. Laminar separation bubble is observed in the surface pressure distribution data and is well identified using its second derivative along the streamwise direction. The spectral characteristics of the boundary layer are satisfactorily explored including the streamwise fluctuations within the laminar flow, intermittent behavior of the transitional zone and the wide range of the spectrum in turbulent flow, thanks to the time-frequency analysis. A promising agreement is observed between the transition points computed by both the numerical and experimental studies and confirms the accuracy of findings achieved by the second derivative of surface pressure data, hot-film measurements and the reliability of the employed numerical transition models for optimization studies.

Structure and dynamics of a laminar separation bubble near a wingtip

2021 ◽  
Vol 929 ◽  
Author(s):  
Connor E. Toppings ◽  
Serhiy Yarusevych
Keyword(s):  
Vortex Shedding ◽  
Vortex Dynamics ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Surface Flow ◽  
Laminar Separation Bubble ◽  
Two Dimensional ◽  
Flow Topology ◽  
Laminar Separation ◽  
The Mean

The three-dimensional flow topology of a laminar separation bubble forming on the suction surface of a semispan wing with an aspect ratio of $2.5$ and NACA 0018 airfoil section is characterised experimentally using surface pressure measurements and particle image velocimetry at a chord Reynolds number of $125\ 000$ . In the inboard region of the wing, the separation bubble is essentially two-dimensional, and the transition process in the separated shear layer leads to periodic vortex shedding, which dominates the bubble dynamics, similar to two-dimensional separation bubbles. However, progressive spanwise changes in the mean structure and vortex dynamics occur near the wingtip, leading to an open separation and eventual suppression of the bubble. In the immediate proximity of the wingtip, the boundary layer remains attached, no vortex shedding occurs and the flow remains laminar, terminating separation bubble formation. Despite variations in the mean separation bubble topology and vortex dynamics along the span, the fundamental shedding characteristics remain nearly invariant across the portion of the wing where vortex shedding occurs, and the flow appears to lock onto a common instability mode across the span, leading to minimal changes in the mean bubble characteristics despite notable changes in the effective angle of attack along the span. A comparison with available surface flow visualisations from previous studies indicates that the observed changes to the mean bubble footprint along the span of the wing are similar across different geometries and flow characteristics, suggesting similarities in the three-dimensional bubble topology and dynamics on finite wings.

The Pervasive Effect of the Calmed Region

2005 ◽  
Author(s):  
R. L. Thomas ◽  
J. P. Gostelow
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Laminar Separation Bubble ◽  
Free Shear Layer ◽  
Hot Wire ◽  
Strong Suppression ◽  
Velocity Fluctuations ◽  
Laminar Separation

Experiments have been conducted relating to the interaction of imposed freestream wakes upon a flat plate laminar separation bubble under an adverse pressure gradient. Controlled wakes, representative of those seen in turbomachinery environments, were used to investigate unsteadiness effects upon a separating boundary layer that undergoes natural transition in the free shear layer under steady conditions. Hot-wire anemometry using a single hot-wire has shown leading edge boundary layer disturbances induced under each passing wake, which grow steadily via by-pass and natural transition methods into turbulent strips that convect with the flow. These disturbances are of such strength that the separated region is resisted and effectively swept away by the passing turbulence, momentarily giving rise to a wholly attached laminar boundary layer. Controlling the chord-wise proximity of neighboring wakes allowed for the investigation of the effect and extent of the calmed region behind each induced turbulent strip. Measurements have shown that a strong suppression of velocity fluctuations is seen related to the proximity of the turbulent strips. Turbulence level reductions of up to 40% have been demonstrated as wake spacing is reduced. Even for those cases where systematic wakes are sufficiently close together to prevent the development of a visible calmed region, very strong calming influences are seen in the wake induced turbulent domain that would have normally been occupied by the calmed flow.

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