Measurements Within Go¨rtler Vortices

Local measurements of stream wise velocity component have been made in the laminar boundary layer on the concave surface of a water channel, supported by flow visualization. Details of the naturally-occurring Go¨rtler vortex pattern are presented.

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Control of Synthetic Hairpin Vortices in Laminar Boundary Layer for Skin-Friction Reduction

Journal of Marine Science and Engineering ◽

10.3390/jmse8010045 ◽

2020 ◽

Vol 8 (1) ◽

pp. 45

Author(s):

Bonguk Koo ◽

Yong-Duck Kang

Keyword(s):

Boundary Layer ◽

Flow Visualization ◽

Skin Friction ◽

Laminar Boundary Layer ◽

Water Channel ◽

Friction Reduction ◽

Control Technique ◽

Hairpin Vortices ◽

Local Skin ◽

Hot Film

The results of flow visualization and hot-film measurement in a water channel are presented in this paper, in which the effectiveness of controlling synthetic hairpin vortices in the laminar boundary layer is examined to reduce skin friction. In this study, hairpin vortices were generated by periodically injecting vortex rings into a cross flow through a hole on a flat plate. To control the hairpin vortices, jets were issued from a nozzle directly onto the head of the hairpins. The results of the flow visualization demonstrated that the jets destroyed the hairpins by disconnecting the heads from their legs, after which the weakened hairpin vortices could not develop. Therefore, the circulation around the legs was reduced, which suggests that the direct intervention on the hairpin heads resulted in the reduction of streamwise stretching. Data obtained by a hot-film sensor showed that the high-speed regions outside the hairpin legs were reduced in speed by this control technique, leading to a decrease in the associated local skin friction.

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Experimental observation of frequency lock-in of roughness-induced instabilities in a laminar boundary layer

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.290 ◽

2019 ◽

Vol 870 ◽

pp. 680-697

Author(s):

Dominik K. Puckert ◽

Ulrich Rist

Keyword(s):

Boundary Layer ◽

Aspect Ratio ◽

Laminar Boundary Layer ◽

Roughness Element ◽

Water Channel ◽

Three Dimensional ◽

Boundary Layer Theory ◽

Linear Stability Theory ◽

Mode Decomposition ◽

Lock In

The interaction of disturbance modes behind an isolated cylindrical roughness element in a laminar boundary layer is investigated by means of hot-film anemometry and particle image velocimetry in a low-turbulence laminar water channel. Both sinuous and varicose disturbance modes are found in the wake of a roughness with unit aspect ratio (diameter/height $=$ 1). Interestingly, the frequency of the varicose mode synchronizes with the first harmonic of the sinuous mode when the critical Reynolds number from three-dimensional global linear stability theory is exceeded. The coupled motion of sinuous and varicose modes is explained by frequency lock-in. This mechanism is of great importance in many aspects of nature, but has not yet received sufficient attention in the field of boundary-layer theory. A Fourier mode decomposition provides detailed analyses of sinuous and varicose modes. The observation is confirmed by a second experiment with the same aspect ratio at a different position in the laminar boundary layer. When the aspect ratio is increased, the flow is fully governed by the varicose mode. Thus, no frequency lock-in can be observed in this case. The significance of this work is to explain how sinuous and varicose modes can co-exist behind a roughness and to propose a mechanism which is well established in physics but not encountered often in boundary-layer theory.

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The Flow Structure and Statistics of a Passive Mixing Tab

Journal of Fluids Engineering ◽

10.1115/1.2910133 ◽

1993 ◽

Vol 115 (2) ◽

pp. 255-263 ◽

Cited By ~ 44

Author(s):

W. J. Gretta ◽

C. R. Smith

Keyword(s):

Boundary Layer ◽

Flow Visualization ◽

Flow Structure ◽

Water Channel ◽

Symmetry Plane ◽

Streamwise Vortices ◽

Hairpin Vortices ◽

Mean Velocity ◽

Velocity Statistics ◽

Passive Mixing

Water channel flow visualization and anemometry studies were conducted to examine the flow structure and velocity statistics in the wake of a passive mixing tab designed for enhancement of cross-stream mixing by generation of flow structures characteristic of turbulent boundary layers. Flow visualization reveals that the mixing tab generates a wake comprising a combination of counter rotating, streamwise vortices enveloped by distinct hairpin vortex structures. The counter rotating streamwise vortices are observed to stimulate a strong ejection of fluid along the symmetry plane, which results in very rapid cross-stream mixing. The hairpin vortices are found to undergo successive amalgamation and coalescence downstream of the device, which aids in the streamwise mixing and outward penetration of ejected fluid. After an initially intense mixing process, the mixing tab wake rapidly develops mean velocity, turbulence intensity, and boundary layer integral properties characteristic of a significantly thickened turbulent boundary layer.

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Visualization of transition

Journal of Fluid Mechanics ◽

10.1017/s0022112069000280 ◽

1969 ◽

Vol 38 (3) ◽

pp. 473-480 ◽

Cited By ~ 48

Author(s):

F. X. Wortmann

Keyword(s):

Boundary Layer ◽

Experimental Study ◽

Flow Visualization ◽

Three Dimensional ◽

Vortex Pair ◽

Instability Mode ◽

Vortex Pattern ◽

Görtler Vortices ◽

Instability Modes ◽

Gortler Vortices

In an experimental study the development of transition downstream of Görtler vortices was investigated. With the tellurium method it was possible to distinguish beyond the Görtler vortices to successive instability modes. The first deforms the vortex pattern in a steady way and produces between each vortex pair boundary-layer profiles with two points of inflexion. When this has been established another instability mode starts, consisting of regular three-dimensional oscillations. By detailed flow visualization a nearly complete picture of the different flow patterns can be obtained.

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Predicting Transition on Concave Surfaces

Volume 3: Heat Transfer, Parts A and B ◽

10.1115/gt2006-90455 ◽

2006 ◽

Author(s):

Mark W. Johnson

Keyword(s):

Boundary Layer ◽

Flat Plate ◽

Boundary Layers ◽

Laminar Boundary Layer ◽

Rapid Development ◽

Current Work ◽

Concave Surface ◽

Transition Model ◽

Surface Boundary ◽

Flat Plates

Boundary layers on concave surfaces differ from those on flat plates due to the presence of Taylor Goertler (T-G) vortices. These vortices cause momentum transfer normal to the blade’s surface and hence result in a more rapid development of the laminar boundary layer and a fuller profile than is typical of a flat plate. Transition of boundary layers on concave surfaces also occurs at a lower Rex than on a flat plate. Concave surface transition correlations have been formulated previously from experimental data, but they are not comprehensive and are based on relatively sparse data. The purpose of the current work was to attempt to model the physics of both the laminar boundary layer development and transition process in order to produce a transition model suitable for concave surface boundary layers. The development of the laminar boundary layer on a concave surface was modeled by considering the profiles at the upwash and downwash locations separately. The profiles of the boundary layers at these two locations were then combined to successfully approximate the spanwise averaged profile. The ratio of the boundary layer thicknesses at the two locations was found to be as great as 50 and this leads to laminar boundary layer shape factors as low as 1.3 and skin friction coefficients up to 12 times the value for a flat plate laminar boundary layer. Boundary layers therefore grow much more rapidly on concave surfaces than on flat plates. The transition model assumed that transition commenced in the upwash location boundary layer at the same transition inception Reθ observed on a flat plate. Transition at the downwash location then results from the growth of turbulent spots from the upwash location rather than through the initiation of spots. The model showed that initially curvature promotes transition because of the thickened upwash boundary layer, but for strong curvature the T-G vortices effectively stabilise the boundary layer and transition then occurs at a higher Reθ than on a flat plate. Results from the transition model were in broad agreement with experimental observations. The current work therefore provides a basis for the modeling of transition on concave surfaces.

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The laminar horseshoe vortex

Journal of Fluid Mechanics ◽

10.1017/s0022112079001506 ◽

1979 ◽

Vol 95 (2) ◽

pp. 347-367 ◽

Cited By ~ 265

Author(s):

C. J. Baker

Keyword(s):

Boundary Layer ◽

Flow Visualization ◽

Laminar Boundary Layer ◽

Horseshoe Vortex ◽

Oscillatory Behaviour ◽

Vortex System ◽

Smoke Flow ◽

Pressure Distributions ◽

Visualization Techniques ◽

Vortex Systems

The horseshoe vortex formed around the base of a cylinder by a separating laminar boundary layer has been investigated experimentally. Smoke flow visualization shows that both steady and unsteady vortex systems exist. Pressure distributions beneath both types of vortex system have been measured and the variation of the horseshoe vortex position on the plane of symmetry upstream of the cylinder has been determined. Unsteady horseshoe vortex systems are shown to have a complex oscillatory behaviour and the nature of this oscillatory behaviour is described. Using smoke flow visualization techniques some measurements have been made of the velocity distributions within horseshoe vortex systems.

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Predicting Transition on Concave Surfaces

Journal of Turbomachinery ◽

10.1115/1.2720865 ◽

2006 ◽

Vol 129 (4) ◽

pp. 750-755 ◽

Cited By ~ 1

Author(s):

Mark W. Johnson

Keyword(s):

Boundary Layer ◽

Flat Plate ◽

Boundary Layers ◽

Laminar Boundary Layer ◽

Rapid Development ◽

Current Work ◽

Concave Surface ◽

Transition Model ◽

Surface Boundary ◽

Flat Plates

Boundary layers on concave surfaces differ from those on flat plates due to the presence of Taylor-Goertler (T-G) vortices. These vortices cause momentum transfer normal to the blade’s surface and hence result in a more rapid development of the laminar boundary layer and a fuller profile than is typical of a flat plate. Transition of boundary layers on concave surfaces also occurs at a lower Rex than on a flat plate. Concave surface transition correlations have been formulated previously from experimental data, but they are not comprehensive and are based on relatively sparse data. The purpose of the current work was to attempt to model the physics of both the laminar boundary layer development and transition process in order to produce a transition model suitable for concave surface boundary layers. The development of the laminar boundary layer on a concave surface was modeled by considering the profiles at the upwash and downwash locations separately. The profiles of the boundary layers at these two locations were then combined to successfully approximate the spanwise averaged profile. The ratio of the boundary layer thicknesses at the two locations was found to be as great as 50 and this leads to laminar boundary layer shape factors as low as 1.3 and skin friction coefficients up to 12 times the value for a flat plate laminar boundary layer. Boundary layers therefore grow much more rapidly on concave surfaces than on flat plates. The transition model assumed that transition commenced in the upwash location boundary layer at the same transition inception Reθ observed on a flat plate. Transition at the downwash location then results from the growth of turbulent spots from the upwash location rather than through the initiation of spots. The model showed that initially curvature promotes transition because of the thickened upwash boundary layer, but for strong curvature the T-G vortices effectively stabilize the boundary layer and transition then occurs at a higher Reθ than on a flat plate. Results from the transition model were in broad agreement with experimental observations. The current work therefore provides a basis for the modeling of transition on concave surfaces.

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On the growth of turbulent regions in laminar boundary layers

Journal of Fluid Mechanics ◽

10.1017/s002211208100061x ◽

1981 ◽

Vol 110 ◽

pp. 73-95 ◽

Cited By ~ 155

Author(s):

Mohamed Gad-El-Hak ◽

Ron F. Blackwelderf ◽

James J. Riley

Keyword(s):

Boundary Layer ◽

Laminar Boundary Layer ◽

Roughness Element ◽

Water Channel ◽

Lateral Spreading ◽

Lateral Growth ◽

Turbulent Spots ◽

Salinity Stratification ◽

Laminar Boundary Layers ◽

Probe Measurements

Turbulent spots evolving in a laminar boundary layer on a nominally zero pressure gradient flat plate are investigated. The plate is towed through an 18 m water channel, using a carriage that rides on a continuously replenished oil film giving a vibrationless tow. Turbulent spots are initiated using a solenoid valve that ejects a small amount of fluid through a minute hole on the working surface. A novel visualization technique that utilizes fluorescent dye excited by a sheet of laser light is employed. Some new aspects of the growth and entrainment of turbulent spots, especially with regard to lateral growth, are inferred from the present experiments. To supplement the information on lateral spreading, a surbulent wedge created by placing a roughness element in the laminar boundary layer is also studied both visually and with probe measurements. The present results show that, in addition to entrainment, another mechanism is needed to explain the lateral growth characteristics of a turbulent region in a laminar boundary layer. This mechanism, termed growth by destabilization, appears to be a result of the turbulence destabilizing the unstable laminar boundary layer in its vicinity. To further understand the growth mechanisms, the turbulence in the spot is modulated using drag-reducing additives and salinity stratification.

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