Three-Dimensional Structure of a Nominally Planar Turbulent Boundary Layer

1979 ◽  
Vol 101 (3) ◽  
pp. 326-330 ◽  
Author(s):  
Y. Furuya ◽  
I. Nakamura ◽  
H. Osaka
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Three Dimensional ◽  
Leading Edge ◽  
Turbulent Boundary Layers ◽  
Main Flow ◽  
The Other ◽  
Dimensional Structure ◽  
Spanwise Direction ◽  
Streamwise Vortices

This research is concerned with detailed experiments on spanwise nonuniformity of nominally planar turbulent boundary layers. Two procedures for eliminating spanwise nonuniformity are studied. One method is to remove the original, natural vortices by introducing additional ones arising from protuberances attached to the leading edge of a flat plate, and the other technique is by making the main flow entirely uniform. Effects of artificially controlled streamwise vortices on spanwise nonuniformity are examined. From these experiments, the process by which induced vortices cause nonuniformity of turbulent boundary layer characteristics in the spanwise direction is discussed.

Three-Dimensional Structure of Pressure–Velocity Correlations in a Turbulent Boundary Layer

2015 ◽  
pp. 103-113
Author(s):  
Yoshitsugu Naka ◽  
Michel Stanislas ◽  
Jean-Marc Foucaut ◽  
Sebastien Coudert ◽  
Jean-Philippe Laval
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Three Dimensional ◽  
Dimensional Structure ◽  
Three Dimensional Structure

0517 Three-dimensional structure of the linear disturbance in a turbulent boundary layer

2013 ◽  
Vol 2013 (0) ◽  
pp. _0517-01_-_0517-02_
Author(s):  
Masanari NAGASAKI ◽  
Taiki MISHIBA ◽  
Konosuke MATSUMOTO ◽  
Masaharu MATSUBARA
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Three Dimensional ◽  
Dimensional Structure ◽  
Three Dimensional Structure ◽  
Linear Disturbance

Study on Vortex Generators for Control of Attached Cavitation

2017 ◽  
Author(s):  
Bangxiang Che ◽  
Dazhuan Wu
Keyword(s):  
Boundary Layer ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Main Flow ◽  
Vortex Generators ◽  
Streamwise Vortices ◽  
Fluid Machinery ◽  
Cavitation Inception ◽  
Sheet Cavitation ◽  
Cavitation Phenomenon

Attached cavitation is a type of common cavitation phenomenon in fluid machinery. It is important to develop methods to control its generation. From the view of cavitation inception, the generation of attached cavitation is greatly influenced by the separated boundary layer upstream of cavitation detachment. In this research, a row of microscopic delta-shaped counter-rotating vortex generators (VGs) was applied on the leading edge of the NACA0015 hydrofoil in order to suppress the boundary layer separation and then suppress the generation of attached cavitation. The application of VGs fixed the position of cavitation inception on hydrofoil thus the sheet cavitation became more stable and the cloud cavity shed from hydrofoil with trim trailing edge more regularly. It was found that cavitation inception always appeared adjacent to VGs due to the low pressure in the corner of streamwise vortices induced by VGs. Hydrofoil with VGs showed an entirely different cavitation morphology on the leading edge. A row of separate microscopic vortex cavitation was induced by the counter-rotating vortices firstly. With the lower the height of VGs, the longer the length of these vortex cavitation due to the weaker interaction between vortices and main flow. Following the vortex cavitation, the attached cavitation was developing, but without typical “finger” structure anymore.

Buffer layer structures associated with extreme wall stress events in a smooth wall turbulent boundary layer

2009 ◽  
Vol 633 ◽  
pp. 17-60 ◽  
Author(s):  
J. SHENG ◽  
E. MALKIEL ◽  
J. KATZ
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Buffer Layer ◽  
Inclination Angle ◽  
Three Dimensional ◽  
Wall Stress ◽  
Streamwise Vortices ◽  
Smooth Wall ◽  
Velocity Deficit ◽  
Layer Structures

Three-dimensional velocity distributions and corresponding wall stresses are measured concurrently in the inner part of a turbulent boundary layer over a smooth wall using digital holographic microscopy. The measurements are performed in a square duct channel flow atReδ= 50000 andReτ= 1470. A spatial resolution of 3–8 wall units (δυ= μm) in streamwise and spanwise directions and 1 wall unit in the wall-normal direction are sufficient for resolving buffer layer structures and for measuring the instantaneous wall shear stresses from velocity gradients in the viscous sublayer. Mean velocity and Reynolds stress profiles agree well with previous publications. Rudimentary observations classify the buffer layer three-dimensional flow into (i) a pair of counter-rotating inclined vortices, (ii) multiple streamwise vortices, some of them powerful, and (iii) no apparent buffer layer structures. Each appears in about one third of the realizations. Conditional sampling based on local wall shear stress maxima and minima reveals two types of three-dimensional buffer layer structures that generate extreme stress events. The first structure develops as spanwise vorticity lifts from the wall abruptly and within a short distance of about 10 wall units, creating initially a vertical arch. Its only precursors are a slight velocity deficit that does not involve an inflection point and low levels of vertical vorticity. This arch is subsequently stretched vertically and in the streamwise direction, culminating in formation of a pair of inclined, counter-rotating vortices with similar strength and inclination angle exceeding 45°. A wall stress minimum exists under the point of initial lifting. A pair of stress maxima develops 35δυdownstream, on the outer (downflow) sides of the vortex pair and is displaced laterally by 35–40δυfrom the minimum. This flow structure exists not only in the conditionally averaged field but in the instantaneous measurement as well and appears in 16.4% of the realizations. Most of the streamwise velocity deficit generated by this phenomenon develops during this initial lifting, but it persists between the pair of vortices. Distribution of velocity fluctuations shows that spanwise transport of streamwise momentum plays a dominant role and that vertical transport is small under the vortices. In other regions, e.g. during initial lifting, and between the vortices, vertical transport dominates. The characteristics of this structure are compared to early experimental findings, highlighting similarities and differences. Abundance of pairs of streamwise vortices with similar strength is inconsistent with conclusions of several studies based on analysis of direct numerical simulation (DNS) data. The second buffer layer structure generating high wall stresses is a single, predominantly streamwise vortex, with characteristic diameter of 20–40δυand inclination angle of 12°. It generates an elongated, strong stress maximum on one side and a weak minimum on the other and has been observed in 20.4% of the realizations. Except for a limited region of sweep above the high-stress region, this low-lying vortex mostly induces spanwise momentum transport. This structure appears to be similar to those observed in several numerical studies.

Turbulent Boundary Layer and Flow Resistance on Plates Roughened by Wires

1976 ◽  
Vol 98 (4) ◽  
pp. 635-643 ◽  
Author(s):  
Y. Furuya ◽  
M. Miyata ◽  
H. Fujita
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Surface Resistance ◽  
Flow Resistance ◽  
Flow Direction ◽  
Roughness Element ◽  
Three Dimensional ◽  
Dimensional Structure ◽  
Secondary Currents ◽  
Pressure Drag

The flow resistance in a plate roughened by equally spaced wires at right angles to the flow direction was investigated experimentally by measuring the turbulent boundary layer developing along it. Measurements of pressure distribution around a roughness element revealed that the pressure drag accounts for a large portion of the surface resistance and remaining skin frictional part is almost equal to that of a smooth plate. Measurements were also made for plates having three-dimensional roughness. These plates were roughened by short wires in a staggered manner. In this case, the boundary layer was found to have a three-dimensional structure due to accompanying secondary currents.

Turbulent structure of high-amplitude pressure peaks within the turbulent boundary layer

2013 ◽  
Vol 735 ◽  
pp. 381-426 ◽  
Author(s):  
S. Ghaemi ◽  
F. Scarano
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Shear Layer ◽  
Pressure Field ◽  
Three Dimensional ◽  
High Amplitude ◽  
Shear Layers ◽  
Turbulent Structure ◽  
Streamwise Vortices ◽  
Ejection Event

AbstractThe positive and negative high-amplitude pressure peaks (HAPP) are investigated in a turbulent boundary layer at $R{e}_{\theta } = $ 1900 in order to identify their turbulent structure. The three-dimensional velocity field is measured within the inner layer of the turbulent boundary layer using tomographic particle image velocimetry (tomo-PIV). The measurements are performed at an acquisition frequency of 10 000 Hz and over a volume of $418\times 149\times 621$ wall units in the streamwise, wall-normal and spanwise directions, respectively. The time-resolved velocity fields are applied to obtain the material derivative using the Lagrangian method followed by integration of the Poisson pressure equation to obtain the three-dimensional unsteady pressure field. The simultaneous volumetric velocity, acceleration, and pressure data are conditionally sampled based on local maxima and minima of wall pressure to analyse the three-dimensional turbulent structure of the HAPPs. Analysis has associated the positive HAPPs to the shear layer structures formed by an upstream sweep of high-speed flow opposing a downstream ejection event. The sweep event is initiated in the outer layer while the ejection of near-wall fluid is formed by the hairpin category of vortices. The shear layers were observed to be asymmetric in the instantaneous visualizations of the velocity and acceleration fields. The asymmetric pattern originates from the spanwise component of temporal acceleration of the ejection event downstream of the shear layer. The analysis also demonstrated a significant contribution of the pressure transport term to the budget of the turbulent kinetic energy in the shear layers. Investigation of the conditional averages and the orientation of the vortices showed that the negative HAPPs are linked to both the spanwise and quasi-streamwise vortices of the turbulent boundary layer. The quasi-streamwise vortices can be associated with the hairpin category of vortices or the isolated quasi-streamwise vortices of the inner layer. A bi-directional analysis of the link between the HAPPs and the hairpin paradigm is also conducted by conditionally averaging the pressure field based on the detection of hairpin vortices using strong ejection events. The results demonstrated positive pressure in the shear layer region of the hairpin model and negative pressure overlapping with the vortex core.

Evolution of the streamwise vortices generated between leading edge tubercles

2016 ◽  
Vol 788 ◽  
pp. 730-766 ◽  
Author(s):  
Kristy L. Hansen ◽  
Nikan Rostamzadeh ◽  
Richard M. Kelso ◽  
Bassam B. Dally
Keyword(s):  
Boundary Layer ◽  
Performance Enhancement ◽  
Numerical Study ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Surface Flux ◽  
Flow Visualisation ◽  
Streamwise Vortices ◽  
Momentum Exchange

Sinusoidal modifications to the leading edge of a foil, or tubercles, have been shown to improve aerodynamic performance under certain flow conditions. One of the mechanisms of performance enhancement is believed to be the generation of streamwise vortices, which improve the momentum exchange in the boundary layer. This experimental and numerical study investigates the formation and evolution of these streamwise vortices at a low Reynolds number of $Re=2230$, providing insight into both the averaged and time-dependent flow patterns. Furthermore, the strength of the vortices is quantified through calculation of the vorticity and circulation, and it is found that the circulation increases in the downstream direction. There is strong agreement between the experimental and numerical observations, and this allows close examination of the flow structure. The results demonstrate that the presence of strong pressure gradients near the leading edge gives rise to a significant surface flux of vorticity in this region. As soon as this vorticity is created, it is stretched, tilted and diffused in a highly three-dimensional manner. These processes lead to the generation of a pair of streamwise vortices between the tubercle peaks. A horseshoe-shaped separation zone is shown to initiate behind a tubercle trough, and this region of separation is bounded by a canopy of boundary-layer vorticity. Along the sides of this shear layer canopy, a continued influx of boundary-layer vorticity occurs, resulting in an increase in circulation of the primary streamwise vortices in the downstream direction. Flow visualisation and particle image velocimetry studies support these observations and demonstrate that the flow characteristics vary with time, particularly near the trailing edge and at a higher angle of attack. Numerical evaluation of the lift and drag coefficients reveals that, for this particular flow regime, the performance of a foil with tubercles is slightly better than that of an unmodified foil.

Experimental investigation of coherent structures of a three-dimensional separated turbulent boundary layer

2018 ◽  
Vol 859 ◽  
pp. 1-32 ◽  
Author(s):  
Mohammad Elyasi ◽  
Sina Ghaemi
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Turbulent Boundary Layer ◽  
Saddle Point ◽  
Turbulent Kinetic Energy ◽  
Coherent Structures ◽  
Three Dimensional ◽  
Spanwise Direction ◽  
Mean Flow ◽  
Elliptical Cross

Coherent structures of a three-dimensional (3D) separation due to an adverse pressure gradient are investigated experimentally. The flow set-up consists of a flat plate to develop a turbulent boundary layer upstream of an asymmetric two-dimensional diffuser with one diverging surface. The diffuser surface has an initial mild curvature followed by a flat section where flow separation occurs. The top and the two sidewalls of the diffuser are not equipped with any flow control mechanism to form a 3D separation. Planar particle image velocimetry (PIV) using four side-by-side cameras is applied to characterize the flow with high spatial resolution over a large streamwise-wall-normal field of view (FOV). Tomographic PIV (tomo-PIV) is also applied for volumetric measurement in a domain flush with the flat surface of the diffuser. The mean flow obtained from averaging instantaneous velocity fields of this intermittent unsteady flow appears as a vortex with an elliptical cross-section. The major axis of the ellipse is tilted with respect to the streamwise direction. As a result, the average velocity in the mid-span of the diffuser has an upstream forward flow and a downstream backward flow, separated by a point of zero wall shear stress. Sweep motions mainly carry out transport of turbulent kinetic energy upstream of this point, while ejections dominate at the downstream region. In the instantaneous flow fields, forward and backward flows have equivalent strength, and the separation front is extended in the spanwise direction. The conditional average of the separation instants forms a saddle-point structure with streamlines converging in the spanwise direction. Proper orthogonal decomposition (POD) of the tomo-PIV data demonstrates that about 42 % of the turbulent kinetic energy is present in the first pair of modes, with a strong spanwise component. The spatial modes of POD also show focus, node and saddle-point structures. The average of the coefficients of the dominant POD modes during the separation events is used to develop a reduced-order model (ROM). Based on the ROM, the instantaneous 3D separation over the diffuser is a saddle-point structure interacting with focus-type structures.

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