Direct Numerical Simulations of Wake-Perturbed Separated Boundary Layers

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Ayse G. Gungor ◽  
Mark P. Simens ◽  
Javier Jiménez
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Plate Boundary ◽  
Maximum Velocity ◽  
Adverse Pressure Gradient ◽  
Single Experiment ◽  
Regeneration Time ◽  
Wake Passing

A wake-perturbed flat plate boundary layer with a streamwise pressure distribution similar to those encountered on the suction side of typical low-pressure turbine blades is investigated by direct numerical simulation. The laminar boundary layer separates due to a strong adverse pressure gradient induced by suction along the upper simulation boundary, transitions, and reattaches while still subject to the adverse pressure gradient. Various simulations are performed with different wake passing frequencies, corresponding to the Strouhal number 0.0043< fθb/ΔU <0.0496 and wake profiles. The wake profile is changed by varying its maximum velocity defect and its symmetry. Results indicate that the separation and reattachment points, as well as the subsequent boundary layer development, are mainly affected by the frequency, but that the wake shape and intensity have little effect, and that the forcing is effective as long as the wake-passing period is shorter than the bubble-regeneration time. Moreover, the effect of the different frequencies can be predicted from a single experiment in which the separation bubble is allowed to reform after having been reduced by wake perturbations. The stability characteristics of the mean flows resulting from the forcing at different frequencies are evaluated in terms of local linear stability analysis based on the Orr-Sommerfeld equation.

Direct Simulations of Wake-Perturbed Separated Boundary Layers

2011 ◽  
Author(s):  
Ayse G. Gungor ◽  
Mark P. Simens ◽  
Javier Jime´nez
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Plate Boundary ◽  
Maximum Velocity ◽  
Adverse Pressure Gradient ◽  
Suction Side ◽  
Single Experiment ◽  
The Stability

A wake-perturbed flat plate boundary layer with a stream-wise pressure distribution similar to those encountered on the suction side of typical low-pressure turbine blades is investigated by direct numerical simulation. The laminar boundary layer separates due to a strong adverse pressure gradient induced by suction along the upper simulation boundary, transitions and reattaches while still subject to the adverse pressure gradient. Various simulations are performed with different wake passing frequencies, corresponding to the Strouhal number 0.0043 < fθb/ΔU < 0.0496 and wake profiles. The wake profile is changed by varying its maximum velocity defect and its symmetry. Results indicate that the separation and reattachment points, as well as the subsequent boundary layer development, are mainly affected by the frequency, but that the wake shape and intensity have little effect. Moreover, the effect of the different frequencies can be predicted from a single experiment in which the separation bubble is allowed to reform after having been reduced by wake perturbations. The stability characteristics of the mean flows resulting from the forcing at different frequencies are evaluated in terms of local linear stability analysis based on the Orr-Sommerfeld equation.

Turbine Cascade Flow Control Using a “Wake Filling” Pulsed Plasma Actuator

2005 ◽  
Author(s):  
H. Perez-Blanco ◽  
Robert Van Dyken ◽  
Aaron Byerley ◽  
Tom McLaughlin
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Laminar Boundary Layer ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Suction Side ◽  
Plasma Actuator ◽  
Pulsed Plasma ◽  
Power Cost ◽  
Hot Film

Separation bubbles in high-camber blades under part-load conditions have been addressed via continuous and pulsed jets, and also via plasma actuators. Numerous passive techniques have been employed as well. In this type of blades, the laminar boundary layer cannot overcome the adverse pressure gradient arising along the suction side, resulting on a separation bubble. When separation is abated, a common explanation is that kinetic energy added to the laminar boundary layer speeds up its transition to turbulent. In the present study, a plasma actuator installed in the trailing edge (i.e. “wake filling configuration”) of a cascade blade is used to excite the flow in pulsed and continuous ways. The pulsed excitation can be directed to the frequencies of the large coherent structures (LCS) of the flow, as obtained via a hot-film anemometer, or to much higher frequencies present in the suction-side boundary layer, as given in the literature. It is found that pulsed frequencies much higher than that of LCS reduce losses and improve turning angles further than frequencies close to those of LCS. With the plasma actuator 50% on time, good loss abatement is obtained. Larger “on time” values yield improvements, but with decreasing returns. Continuous high-frequency activation results in the largest loss reduction, at increased power cost. The effectiveness of high frequencies may be due to separation abatement via boundary layer excitation into transition, or may simply be due to the creation of a favorable pressure gradient that averts separation as the actuator ejects fluid downstream. Both possibilities are discussed in light of the experimental evidence.

scholarly journals Turbulent Spot Development Under a Moderate Adverse Pressure Gradient

1993 ◽  
Author(s):  
J. P. Gostelow ◽  
G. Hong ◽  
N. Melwani ◽  
G. J. Walker
Keyword(s):  
Boundary Layer ◽  
Wave Packet ◽  
Pressure Gradient ◽  
Plate Boundary ◽  
Adverse Pressure Gradient ◽  
Spreading Rate ◽  
Lateral Spreading ◽  
Data Sampling ◽  
Turbulent Spot ◽  
Included Angle

Triggered turbulent spots are under investigation in a wind tunnel. A turbulent spot was initiated in a flat plate boundary layer under a moderate adverse pressure gradient. The spot was traversed at four streamwise locations using conventional hot-wire anemometry techniques. A triggering jet provided a phase reference for data sampling. Phase-averaged velocity traces, boundary layer integral properties and contours of velocity perturbation and disturbance level are presented. The central region resembles a zero pressure gradient spot but much of the span is dominated by the different behavioral stages of a stongly-amplified wave packet. The celerities of the spot leading and trailing edges under an adverse pressure gradient are significantly lower than those associated with zero pressure gradient spots but the lateral spreading rate is much higher. This combination of turbulent spot and wave packet, which spreads at an included angle as high as 60° is quite different from the well-documented zero pressure gradient spot spreading at an included angle of about 20°. Improvements in transition region predictions are therefore dependent on further detailed measurements of spots and wave packets under adverse pressure gradients.

Flow Separation Control in an Annular to Conical Diffuser Using Two-Dimensional and Three-Dimensional Wall Steps

2013 ◽  
Vol 135 (4) ◽  
Author(s):  
Kin Pong Lo ◽  
Christopher J. Elkins ◽  
John K. Eaton
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Flow Control ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Boundary Layer Separation ◽  
Core Flow ◽  
Layer Separation ◽  
Wall Boundary Layer ◽  
Wall Boundary

Conical diffusers are often installed downstream of a turbomachine with a central hub. Previous studies showed that nonstreamlined hubs had extended separated wakes that reduced the adverse pressure gradient in the diffuser. Active flow control techniques can rapidly close the central separation bubble, but this restores the adverse pressure gradient, which can cause the outer wall boundary layer to separate. The present study focuses on the use of a step-wall diffuser to stabilize the wall boundary layer separation in the presence of core flow control. Three-component mean velocity data for a set of conical diffusers were acquired using magnetic resonance velocimetry. The results showed the step-wall diffuser stabilized the wall boundary layer separation by fixing its location. An axisymmetric step separation bubble was formed. A step with a periodically varying height reduced the reattachment length of the step separation and allowed the diffuser to be shortened. The step-wall diffuser was found to be robust in a range of core flow velocity profiles. The minimum distance between the core flow control mechanism and the step-wall diffuser as well as the minimum length of the step were determined.

The form drag of two-dimensional bluff-plates immersed in turbulent boundary layers

1968 ◽  
Vol 31 (3) ◽  
pp. 547-582 ◽  
Author(s):  
M. C. Good ◽  
P. N. Joubert
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Gradient Flows ◽  
Mean Flow ◽  
Plate Height ◽  
Form Drag ◽  
Law Of The Wall ◽  
History Of

Measurements of the distributions of pressure on a bluff flat plate (fence) have been correlated with the characteristics of the smooth-wall boundary layer in which it is immersed. For zero pressure-gradient flows, correlations are obtained for the variation of form drag with plate heighthwhich are analogous in form to the ‘law of the wall’ and the ‘velocity-defect law’ for the boundary-layer velocity profile. The data for adverse pressure-gradient flows is suggestive of a ‘law of the wake’ type correlation. Pressures on the upstream face of the bluff-plate are determined by a wall-similarity law, even forh/δ > 1, and are independent of the pressure-gradient history of the flow; the separation induced upstream is apparently of the Stratford-Townsend type. The effects of the history of the boundary layer are manifested only in the flow in the rear separation bubble, and then only forh/δ > ½. The base pressure is also sensitive to free-stream pressure gradients downstream of the bluff-plate. The relative extent of upstream influence of the bluff-plate on the boundary layer is found to increase rapidly ash/δ decreases. One set of measurements of the mean flow field is also presented.

An Experimental Study of the Laminar Flow Separation on a Low-Reynolds-Number Airfoil

2008 ◽  
Vol 130 (5) ◽  
Author(s):  
Hui Hu ◽  
Zifeng Yang
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Laminar Boundary Layer ◽  
Flow Separation ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Laminar Separation Bubble ◽  
Airfoil Surface ◽  
Laminar Separation ◽  
Turbulence Transition

An experimental study was conducted to characterize the transient behavior of laminar flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds number of 70,000. In addition to measuring the surface pressure distribution around the airfoil, a high-resolution particle image velocimetry (PIV) system was used to make detailed flow field measurements to quantify the evolution of unsteady flow structures around the airfoil at various angles of attack (AOAs). The surface pressure and PIV measurements clearly revealed that the laminar boundary layer would separate from the airfoil surface, as the adverse pressure gradient over the airfoil upper surface became severe at AOA≥8.0deg. The separated laminar boundary layer was found to rapidly transit to turbulence by generating unsteady Kelvin–Helmholtz vortex structures. After turbulence transition, the separated boundary layer was found to reattach to the airfoil surface as a turbulent boundary layer when the adverse pressure gradient was adequate at AOA<12.0deg, resulting in the formation of a laminar separation bubble on the airfoil. The turbulence transition process of the separated laminar boundary layer was found to be accompanied by a significant increase of Reynolds stress in the flow field. The reattached turbulent boundary layer was much more energetic, thus more capable of advancing against an adverse pressure gradient without flow separation, compared to the laminar boundary layer upstream of the laminar separation bubble. The laminar separation bubble formed on the airfoil upper surface was found to move upstream, approaching the airfoil leading edge as the AOA increased. While the total length of the laminar separation bubble was found to be almost unchanged (∼20% of the airfoil chord length), the laminar portion of the separation bubble was found to be slightly stretched, and the turbulent portion became slightly shorter with the increasing AOA. After the formation of the separation bubble on the airfoil, the increase rate of the airfoil lift coefficient was found to considerably degrade, and the airfoil drag coefficient increased much faster with increasing AOA. The separation bubble was found to burst suddenly, causing airfoil stall, when the adverse pressure gradient became too significant at AOA>12.0deg.

On the origin of the inflectional instability of a laminar separation bubble

2009 ◽  
Vol 629 ◽  
pp. 263-298 ◽  
Author(s):  
SOURABH S. DIWAN ◽  
O. N. RAMESH
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Maximum Height ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Separated Shear Layer ◽  
The Mean

This is an experimental and theoretical study of a laminar separation bubble and the associated linear stability mechanisms. Experiments were performed over a flat plate kept in a wind tunnel, with an imposed pressure gradient typical of an aerofoil that would involve a laminar separation bubble. The separation bubble was characterized by measurement of surface-pressure distribution and streamwise velocity using hot-wire anemometry. Single component hot-wire anemometry was also used for a detailed study of the transition dynamics. It was found that the so-called dead-air region in the front portion of the bubble corresponded to a region of small disturbance amplitudes, with the amplitude reaching a maximum value close to the reattachment point. An exponential growth rate of the disturbance was seen in the region upstream of the mean maximum height of the bubble, and this was indicative of a linear instability mechanism at work. An infinitesimal disturbance was impulsively introduced into the boundary layer upstream of separation location, and the wave packet was tracked (in an ensemble-averaged sense) while it was getting advected downstream. The disturbance was found to be convective in nature. Linear stability analyses (both the Orr–Sommerfeld and Rayleigh calculations) were performed for mean velocity profiles, starting from an attached adverse-pressure-gradient boundary layer all the way up to the front portion of the separation-bubble region (i.e. up to the end of the dead-air region in which linear evolution of the disturbance could be expected). The conclusion from the present work is that the primary instability mechanism in a separation bubble is inflectional in nature, and its origin can be traced back to upstream of the separation location. In other words, the inviscid inflectional instability of the separated shear layer should be logically seen as an extension of the instability of the upstream attached adverse-pressure-gradient boundary layer. This modifies the traditional view that pegs the origin of the instability in a separation bubble to the detached shear layer outside the bubble, with its associated Kelvin–Helmholtz mechanism. We contend that only when the separated shear layer has moved considerably away from the wall (and this happens near the maximum-height location of the mean bubble), a description by the Kelvin–Helmholtz instability paradigm, with its associated scaling principles, could become relevant. We also propose a new scaling for the most amplified frequency for a wall-bounded shear layer in terms of the inflection-point height and the vorticity thickness and show it to be universal.

The structure of wall-pressure fluctuations in turbulent boundary layers with adverse pressure gradient and separation

1998 ◽  
Vol 377 ◽  
pp. 347-373 ◽  
Author(s):  
Y. NA ◽  
P. MOIN
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Separation Bubble ◽  
Reynolds Shear Stress ◽  
Pressure Fluctuations ◽  
Adverse Pressure Gradient ◽  
Wall Pressure ◽  
Spanwise Direction ◽  
Frequency Spectra ◽  
Wall Pressure Fluctuations

Space–time correlations and frequency spectra of wall-pressure fluctuations, obtained from direct numerical simulation, are examined to reveal the effects of pressure gradient and separation on the characteristics of wall-pressure fluctuations. In the attached boundary layer subjected to adverse pressure gradient, contours of constant two-point spatial correlation of wall-pressure fluctuations are more elongated in the spanwise direction. Convection velocities of wall-pressure fluctuations as a function of spatial and temporal separations are reduced by the adverse pressure gradient. In the separated turbulent boundary layer, wall-pressure fluctuations are reduced inside the separation bubble, and enhanced downstream of the reattachment region where maximum Reynolds stresses occur. Inside the separation bubble, the frequency spectra of wall-pressure fluctuations normalized by the local maximum Reynolds shear stress correlate well compared to those normalized by free-stream dynamic pressure, indicating that local Reynolds shear stress has more direct influence on the wall-pressure spectra. Contour plots of two-point correlation of wall-pressure fluctuations are highly elongated in the spanwise direction inside the separation bubble, implying the presence of large two-dimensional roller-type structures. The convection velocity determined from the space–time correlation of wall-pressure fluctuations is as low as 0.33U0 (U0 is the maximum inlet velocity) in the separated zone, and increases downstream of reattachment.

scholarly journals Direct numerical simulation of a separated turbulent boundary layer

2002 ◽  
Vol 471 ◽  
pp. 107-136 ◽  
Author(s):  
MARTIN SKOTE ◽  
DAN S. HENNINGSON
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Direct Numerical Simulation ◽  
Pressure Gradient ◽  
Scaling Laws ◽  
Separation Bubble ◽  
Turbulence Models ◽  
Adverse Pressure Gradient ◽  
Theoretical Work

Direct numerical simulation of two turbulent boundary layer flows has been performed. The boundary layers are both subject to a strong adverse pressure gradient. In one case a separation bubble is created while in the other the boundary layer is everywhere attached. The data from the simulations are used to investigate scaling laws near the wall, a crucial concept in turbulence models. Theoretical work concerning the inner region in a boundary layer under an adverse pressure gradient is reviewed and extended to the case of separation. Excellent agreement between theory and data from the direct numerical simulation is found in the viscous sub-layer, while a qualitative agreement is obtained for the overlap region.

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