A study of the nonsimilar nature of the laminar boundary layer in a region of adverse pressure gradient

1969 ◽  
Author(s):  
W. GALLO ◽  
A. GNOS
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Laminar Boundary Layer ◽  
Adverse Pressure Gradient

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 Some Observations of the Behaviour of an Adverse Pressure Gradient Laminar Boundary Layer under Wake Impingement

Fluids ◽  
2021 ◽  
Vol 6 (6) ◽  
pp. 199
Author(s):  
Vasudevan Kanjirakkad ◽  
Thomas Irps
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Laminar Boundary Layer ◽  
Adverse Pressure Gradient ◽  
Transition Process ◽  
Transition Mechanism ◽  
Separated Shear Layer ◽  
Tollmien Schlichting ◽  
The Time Domain

The problem of laminar to turbulent transition in a boundary layer flow subjected to an adverse pressure gradient is relevant to many engineering applications. Under such conditions, the initially laminar flow within the boundary layer can undergo separation and then become turbulent upon reattachment, as transition is triggered by instabilities within the separated shear layer. In turbomachinery blades with high loading, the transition mechanism is further complicated by the presence of periodic wake disturbances shed by blades that move relatively in the upstream flow. The paper reports an experimental study of the effect of wake disturbances generated upstream on the development of a laminar boundary layer over a flat plate imposed with an adverse pressure gradient that is typical of a highly loaded front-stage compressor blade. Detailed velocity measurements using a hotwire are performed along the plate and the results are analysed both in the time domain and the frequency domain. Description of the major features identified is provided and the leading mechanisms that trigger the transition process are identified to be a possible combination of amplified Tollmien–Schlichting waves and the roll-up of vortices due to the Kelvin–Helmholtz instability of the separated shear layer.

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 turbulent spots in a laminar boundary layer subjected to a self-similar adverse pressure gradient

1995 ◽  
Vol 296 ◽  
pp. 185-209 ◽  
Author(s):  
Avi Seifert ◽  
Israel J. Wygnanski
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Laminar Boundary Layer ◽  
Three Dimensional ◽  
Adverse Pressure Gradient ◽  
Spreading Rate ◽  
Major Influence ◽  
Spanwise Direction ◽  
Rate Of Spread ◽  
Self Similar

The characteristics of a turbulent spot propagating in a laminar boundary layer subjected to a self-similar adverse pressure gradient (defined by a Falkner–Skan parameter β = -0.1) were investigated experimentally. It was observed that some small differences in the normalized shape of the undisturbed velocity profile caused by the pressure gradient had a major influence on the spreading rate of the spot at comparable Reδ*. The rate of spread of the spot in the spanwise direction was affected most dramatically by the pressure gradient where the average angle at which the tips of the spots moved outward relative to the plane of symmetry was 21°. It was noted that the strength and duration of the disturbance initiating the spots had an effect on their spanwise rate of spread. For example, a strong impulsive disturbance and a disturbance caused by a stationary three-dimensional roughness generated spots which spread at a much smaller rate. The interaction of the spot with the wave packet existing beyond its tip was enhanced by the adverse pressure gradient because the Reynolds number of the surrounding boundary layer was everywhere supercritical. Thus, the maximum linear amplification rate in this case is approximately four times larger than in Blasius flow. Some features of the breakdown and their relationship to the shape and the perturbation velocities in the spot are discussed. The normalized length of the calmed region relative to the length of the spot is enhanced by the adverse pressure gradient and by an increase in the intensity of the disturbance.

Self-induced interactions

1982 ◽  
Vol 379 (1776) ◽  
pp. 217-230 ◽  
Keyword(s):  
Boundary Layer ◽  
Partial Differential Equations ◽  
Supersonic Flow ◽  
Differential Equations ◽  
Pressure Gradient ◽  
Skin Friction ◽  
Laminar Boundary Layer ◽  
Numerical Solutions ◽  
Adverse Pressure Gradient ◽  
Series Expansions

A laminar boundary layer in supersonic flow can evolve spontaneously from an undisturbed form to a separated state, where an adverse pressure gradient thickens the boundary layer, thus displacing the external streamlines, which leads to the original pressure gradient. A linearized study by Lighthill was later generalized by Stewartson in a triple-deck analysis, in which the equations for the main deck are ∂ u /∂ x + ∂ v /∂ y ═ 0, u ∂ u /∂ x + v ∂ u /∂ y ═ – d p /d x + ∂ 2 u /∂ y 2 , subject to u ═ v ═ 0 at y ═ 0, u → y as x → – ∞, and u → y – ∫ x -∞ p(t) d t as y → ∞. The problem is here studied by means of the series expansions u ═ y – ∑ ∞ n ═ 1 a n e nkx f' n (y) , v ═ ∑ ∞ n ═ 1 nka n e nkx f n (y) , p ═ ∑ ∞ n ═ 1 a n e nkx . This gives a sequence of equations for the f n (y) , of which the first 15 have been solved. Appropriate series for the pressure p and the skin-friction т have been derived and analysed, and previous numerical solutions of the partial differential equations by Williams have been well confirmed, in some instances to greater accuracy. Among the conclusions reached are the following, (i) The value of p at separation is calculated to be 1.02594744. (ii) As x → ∞, p tends to a constant value p 0 ═ 1.7903, compared with the value 1.800 given by Williams. (iii) In the separated region, the most negative value taken by т is –0.1494081.

Sign in / Sign up

Export Citation Format

Share Document