scholarly journals Computational Characterization of a Rectangular Vortex Generator on a Flat Plate for Different Vane Heights and Angles

2019 ◽  
Vol 9 (5) ◽  
pp. 995 ◽  
Author(s):  
Iosu Ibarra-Udaeta ◽  
Iñigo Errasti ◽  
Unai Fernandez-Gamiz ◽  
Ekaitz Zulueta ◽  
Javier Sancho
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Stokes Equations ◽  
Vortex Generator ◽  
Vortex Generators ◽  
Local Boundary ◽  
Passive Flow Control ◽  
Vortex Size

Vortex generators (VG) are passive flow control devices used for avoiding or delaying the separation of the boundary layer by bringing momentum from the higher layers of the fluid towards the surface. The Vortex generator usually has the same height as the local boundary layer thickness, and these Vortex generators can produce overload drag in some cases. The aim of the present study was to analyze the characteristics and path of the primary vortex produced by a single rectangular vortex generator on a flat plate for the incident angles of β = 10 ∘ , 15 ∘ , 18 ∘ and 20 ∘ . A parametric study of the induced vortex was performed for six VG heights using Reynolds average Navier–Stokes equations at Reynodls number R e = 27,000 based on the local boundary layer thickness, using computational fluid dynamics techniques with OpenFOAM open-source code. In order to determine the vortex size, the so-called half-life radius was computed and compared with experimental data. The results showed a similar trend for all the studied vortex generator heights and incident angles with small variations for the vertical and the lateral paths. Additionally, 0.4H and 0.6H VG heights at incident angles of β = 18 ∘ and β = 20 ∘ showed the best performance in terms of vortex strength and generation of wall shear stress.

scholarly journals Computational Modelling of Three Different Sub-Boundary Layer Vortex Generators on a Flat Plate

Energies ◽  
2018 ◽  
Vol 11 (11) ◽  
pp. 3107 ◽  
Author(s):  
Ruben Gutierrez-Amo ◽  
Unai Fernandez-Gamiz ◽  
Iñigo Errasti ◽  
Ekaitz Zulueta
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Flow Separation ◽  
Computational Modelling ◽  
Vortex Generator ◽  
Vortex Generators ◽  
Primary Vortex ◽  
Good Efficiency ◽  
Local Boundary ◽  
Passive Flow Control

Flow separation is the source of several problems in a wind turbine including load fluctuations, lift losses, and vibrations. Vortex generators (VGs) are passive flow control devices used to delay flow separation, but their implementation may produce overload drag at the blade section where they are placed. In the current work, a computational model of different geometries of vortex generators placed on a flat plate has been carried out throughout fully meshed computational simulations using Reynolds Averaged Navier-Stokes (RANS) equations performed at a Reynolds number of R e θ = 2600 based on local boundary layer (BL) momentum thickness θ = 2.4 mm. A flow characterization of the wake behind the vortex generator has been done with the aim of evaluating the performance of three vortex generator geometries, namely Rectangular VG, Triangular VG, and Symmetrical VG NACA0012. The location of the primary vortex has been evaluated by the vertical and lateral trajectories and it has been found that for all analyzed VG geometries the primary vortex is developed below the boundary layer thickness δ = 20 mm for a similar vorticity level ( w x m a x ). Two innovative parameters have been developed in the present work for evaluating the vortex size and the vortex strength: Half-Life Surface S 05 and Mean Positive Circulation Γ 05 + . As a result, an assessment of the VG performance has been carried out by all analyzed parameters and the symmetrical vortex generator NACA0012 has provided good efficiency in energy transfer compared with the Rectangular VG.

Influence of boundary-layer disturbances on the instability of a roughness wake in a high-speed boundary layer

2014 ◽  
Vol 763 ◽  
pp. 136-165 ◽  
Author(s):  
Nicola De Tullio ◽  
Neil D. Sandham
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Boundary Layers ◽  
Boundary Layer Thickness ◽  
High Speed ◽  
Stokes Equations ◽  
Roughness Element ◽  
Transition To Turbulence ◽  
Local Boundary ◽  
Wake Modes

AbstractThe excitation of instability modes in the wake generated behind a discrete roughness element in a boundary layer at Mach 6 is analysed through numerical simulations of the compressible Navier–Stokes equations. Recent experimental observations show that transition to turbulence in high-speed boundary layers during re-entry flight is dominated by wall roughness effects. Therefore, understanding the roughness-induced transition to turbulence in this flow regime is of primary importance. Our results show that a discrete roughness element with a height of about half the local boundary-layer thickness generates an unstable wake able to sustain the growth of a number of modes. The most unstable of these modes are a sinuous mode (mode SL) and two varicose modes (modes VL and VC). The varicose modes grow approximately 17 % faster than the most unstable Mack mode and their growth persists over a longer streamwise distance, thereby leading to a notable acceleration of the laminar–turbulent transition process. Two main mechanisms are identified for the excitation of wake modes: the first is based on the interaction between the external disturbances and the reverse flow regions induced by the roughness element and the second is due to the interaction between the boundary-layer modes (first modes and Mack modes) and the non-parallel roughness wake. An important finding of the present study is that, while being less unstable, mode SL is the preferred instability for the first of the above excitation mechanisms, which drives the wake modes excitation in the absence of boundary-layer modes. Modes VL and VC are excited through the second mechanism and, hence, become important when first modes and Mack modes come into interaction with the roughness wake. The new mode VC presents similarities with the Mack mode instability, including the tuning between its most unstable wavelength and the local boundary-layer thickness, and it is believed to play a fundamental role in the roughness-induced transition of high-speed boundary layers. In contrast to the smooth-wall case, wall cooling is stabilising for all the roughness-wake modes.

Numerical Investigation on Boundary Layer Flow Control with Vortex Generators

2013 ◽  
Vol 432 ◽  
pp. 351-357
Author(s):  
Guang Yuan Liu ◽  
Rui Bo Wang ◽  
Chang Rong Zhang ◽  
Feng Chen ◽  
Jiang Yu Xie ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Side Wall ◽  
Vortex Generator ◽  
Simulation Method ◽  
Vortex Generators ◽  
Layer Flow ◽  
Transonic Wind Tunnel

The numerical simulation method was adopted to analyze the effect on boundary layer thickness reduction of various vortex generator parameters. Results show that vortex generators are capable of reducing boundary layer thickness for about 66 percent, and the influence on centerline Mach number distributions is neglectable. Practicable vortex generators for 2.4m transonic wind tunnel half-model test section side wall are founded. Research results can be used for further applications of vortex generator in wind tunnel tests.

scholarly journals Flow around a Finite Circular Cylinder on a Flat Plate : Cylinder height greater than turbulent boundary layer thickness

1984 ◽  
Vol 27 (232) ◽  
pp. 2142-2151 ◽  
Author(s):  
Takao KAWAMURA ◽  
Munehiko HIWADA ◽  
Toshiharu HIBINO ◽  
Ikuo MABUCHI ◽  
Masaya KUMADA
Keyword(s):  
Boundary Layer ◽  
Circular Cylinder ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Layer Thickness ◽  
Boundary Layer Thickness

scholarly journals Influence of localised smooth steps on the instability of a boundary layer

2017 ◽  
Vol 817 ◽  
pp. 138-170 ◽  
Author(s):  
Hui Xu ◽  
Jean-Eloi W. Lombard ◽  
Spencer J. Sherwin
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Gaussian White Noise ◽  
Random Phase ◽  
Linear Analysis ◽  
Base Flow ◽  
Neutral Stability ◽  
Local Boundary ◽  
Type Transition

We consider a smooth, spanwise-uniform forward-facing step defined by a Gauss error function of height 4 %–30 % and four times the width of the local boundary layer thickness $\unicode[STIX]{x1D6FF}_{99}$. The boundary layer flow over a smooth forward-facing stepped plate is studied with particular emphasis on stabilisation and destabilisation of the two-dimensional Tollmien–Schlichting (TS) waves and subsequently on three-dimensional disturbances at transition. The interaction between TS waves at a range of frequencies and a base flow over a single or two forward-facing smooth steps is conducted by linear analysis. The results indicate that for a TS wave with a frequency ${\mathcal{F}}\in [140,160]$ (${\mathcal{F}}=\unicode[STIX]{x1D714}\unicode[STIX]{x1D708}/U_{\infty }^{2}\times 10^{6}$, where $\unicode[STIX]{x1D714}$ and $U_{\infty }$ denote the perturbation angle frequency and free-stream velocity magnitude, respectively, and $\unicode[STIX]{x1D708}$ denotes kinematic viscosity), the amplitude of the TS wave is attenuated in the unstable regime of the neutral stability curve corresponding to a flat plate boundary layer. Furthermore, it is observed that two smooth forward-facing steps lead to a more acute reduction of the amplitude of the TS wave. When the height of a step is increased to more than 20 % of the local boundary layer thickness for a fixed width parameter, the TS wave is amplified, and thereby a destabilisation effect is introduced. Therefore, the stabilisation or destabilisation effect of a smooth step is typically dependent on its shape parameters. To validate the results of the linear stability analysis, where a TS wave is damped by the forward-facing smooth steps direct numerical simulation (DNS) is performed. The results of the DNS correlate favourably with the linear analysis and show that for the investigated frequency of the TS wave, the K-type transition process is altered whereas the onset of the H-type transition is delayed. The results of the DNS suggest that for the perturbation with the non-dimensional frequency parameter ${\mathcal{F}}=150$ and in the absence of other external perturbations, two forward-facing smooth steps of height 5 % and 12 % of the boundary layer thickness delayed the H-type transition scenario and completely suppressed for the K-type transition. By considering Gaussian white noise with both fixed and random phase shifts, it is demonstrated by DNS that transition is postponed in time and space by two forward-facing smooth steps.

Some Boundary Layer Measurements on a Slender Wing at Supersonic Speeds

1963 ◽  
Vol 67 (629) ◽  
pp. 291-295
Author(s):  
R. T. Griffiths
Keyword(s):  
Boundary Layer ◽  
Mach Number ◽  
Aspect Ratio ◽  
Flat Plate ◽  
Layer Thickness ◽  
Incompressible Flow ◽  
Boundary Layer Thickness ◽  
Static Pressure ◽  
Leading Edge ◽  
Slender Wing

SummaryBoundary layer measurements have been made at four positions on a slender gothic wing of aspect ratio 0·75. Test's were made over a range of incidence at M=1·42 and 1·82. With transition fixed by roughness near the leading edge the boundary layer thickness varied little with small positive or negative incidence but was reduced at larger incidences, this being most marked at positive incidence for positions nearest the leading edge due to the influence of the wing vortex. With the exception of positions in the vicinity of the vortex, a good estimate of the boundary layer thickness was given by the theory for incompressible flow over a flat plate and an excellent estimate of the variation of local static pressure and Mach number with incidence was given by not-so-slender wing theory.

Fluid-Acoustics Interaction in Self-Sustained Oscillations Over a Cavity in a Turbulent Boundary Layer

2008 ◽  
Author(s):  
H. Yokoyama ◽  
C. Kato
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Layer Thickness ◽  
Turbulent Flows ◽  
Acoustic Waves ◽  
Boundary Layer Thickness ◽  
Stokes Equations ◽  
Practical Applications ◽  
Sustained Oscillations ◽  
Mach Numbers

Self-sustained oscillations with fluid-acoustics interaction over a cavity can radiate intense tonal sound and fatigue nearby components of industrial products. The prediction and the suppression of these oscillations are very important for many practical applications. However, the fluid-acoustics interaction has not been thoroughly clarified in particular for the oscillations in turbulent flows. We investigate the mechanism of the oscillations over a rectangular cavity with a length-to-depth ratio of 2:1 by directly solving the compressible Navier-Stokes equations. The boundary layer over the cavity is turbulent and the freestream Mach numbers are M = 0.4 and 0.7. The results clarify that the self-sustained oscillations occur in the shear layer of the cavity and the oscillations are reinforced by the streamwise acoustic mode in the cavity for both Mach numbers. The shear layer of the cavity undulates. This undulation causes the deformation of fine vortices in the shear layer and radiates acoustic waves from the downstream edge of the cavity. Also, we clarify by the conditional identification of longitudinal vortices that the acoustic waves cause the undulation of the shear layer and a feedback loop is formed. Moreover, the comparison of the flow field over the cavity with that over a simple backstep shows that the shear layer in the cavity becomes two-dimensional by the acoustic feedback. Finally, we show that the oscillations become weaker particularly at M = 0.4 and the frequencies of the oscillations become lower as the boundary layer thickness at the upstream edge of the cavity increases. Considering this effect of the boundary layer thickness, the peak frequencies predicted by our computations are in good agreement with those measured in a past experiment.

Improvement of the Performance of Conical Diffusers by Vortex Generators

1974 ◽  
Vol 96 (1) ◽  
pp. 4-10 ◽  
Author(s):  
Y. Senoo ◽  
M. Nishi
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Area Ratio ◽  
Vortex Generators ◽  
Pressure Recovery ◽  
Divergence Angle ◽  
Recovery Coefficient ◽  
Conical Diffuser ◽  
Pressure Recovery Coefficient

Vortex generators, which consist of small blades, are applied to conical diffusers the divergence angles of which are 8, 12, 16, 20, and 30 deg, respectively. The area ratio of each diffuser is four. The experiment covers the influence of various parameters, such as the arrangement of blades, inlet boundary layer thickness, and location of vortex generators relative to the conical diffuser, on the pressure-recovery coefficient. The experiment shows that the vortex generators prevent the flow in a conical diffuser from separating up to a divergence angle of 16 deg, and that the pressure-recovery coefficient is approximately equal to that of conventional best conical diffusers.

A Normal Flat Plate Close to a Large Plane Surface

1982 ◽  
Vol 33 (1) ◽  
pp. 90-104 ◽  
Author(s):  
K.W. Everitt
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Layer Thickness ◽  
Vortex Shedding ◽  
Boundary Layer Thickness ◽  
Plane Surface ◽  
Surface Boundary ◽  
Plate Height ◽  
Surface Boundary Layer ◽  
Large Plane

SummaryThe flow around a normal flat plate close to a large plane surface has been investigated for a range of boundary-layer thickness on the plane surface of 0.72 to 2.53 times the plate height, h. As the gap between the plate and the plane surface is reduced below about 0.55h vortex shedding from the plate is inhibited by the presence of the plane surface. Boundary layer thickness has little effect on the gap at which this occurs, but does affect the strength and frequency of vortex shedding at greater gaps. As the gap between the plate lower edge and the plane surface is decreased towards the critical value of 0.55h, both vortex strength and Strouhal number are more significantly affected when the approaching boundary layer is thin. It is not clear whether this is a result of shear or increased turbulent intensity.

scholarly journals AN EXPERIMENTAL STUDY OF THE EFFECT OF VORTEX GENERATOR ON THE FLAT-PLATE BOUNDARY LAYER

2021 ◽  
Vol 13 (2) ◽  
pp. 68-78
Author(s):  
عباس فاضل محمود ◽  
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Flat Plate ◽  
Average Distance ◽  
Plate Boundary ◽  
Vortex Generator ◽  
Vortex Generators ◽  
Stream Velocity ◽  
Drag Coefficients ◽  
Flat Plate Boundary Layer

This paper is dealing with an experimental study to show the influence of the geometric characteristics of the vortex generators VG son the thickness of the boundary layer (∂) and drag coefficients (CD) of the flat plate. Vortex generators work effectively on medium and high angles of attack, since they are "hidden" under the boundary layer and practically ineffective at low angles. The height of VGs relative to the thickness of the boundary layer enables us to study the efficacy of VGs in delaying boundary layer separation. The distance between two VGs also has an effect on the boundary layer if we take into account the interference between two pairs of VGs. The effect of the changing in (h- the height of vortex generator, d- the average distance between tow vortex generators) on the thickness of the flat plate boundary layer and the drag coefficients has been studied for triangular vortex generator. The measurements of the vortex generator have been changed to determine the optimum boundary layer thickness and the change in drag coefficients. An experiment was done at an average free stream velocity, (U∞,) of 28 m/s. The experiment was conducted in the wind tunnel UTAD-2 University (NAU) Kiev, Ukraine.

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