Flow in a meandering channel

2015 ◽  
Vol 770 ◽  
pp. 52-84 ◽  
Author(s):  
J. M. Floryan
Keyword(s):  
Vortex Formation ◽  
Comprehensive Analysis ◽  
Linear Stability Theory ◽  
Vortex Generation ◽  
Centrifugal Forces ◽  
Streamwise Vortices ◽  
Meandering Channel ◽  
Channel Narrowing ◽  
Effective Channel ◽  
Wall Geometry

A comprehensive analysis of the pressure-gradient driven flow in a meandering channel has been presented. This geometry is of interest as it can be used for the creation of streamwise vortices which magnify the transverse transport of scalar quantities, e.g. heat transfer. The linear stability theory has been used to determine the meandering wavelengths required for the vortex formation. It has been demonstrated that reduction of the wavelength results in the onset of flow separation which, when combined with the wall geometry, results in an effective channel narrowing: the stream ‘lifts up’ above the wall and becomes nearly rectilinear, thus eliminating vortex-generating centrifugal forces. Increase of the wavelength also leads to a nearly rectilinear stream, as the slope of the wall modulations becomes negligible. As shear-driven instability may interfere with the formation of vortices, the conditions leading to the onset of such instability have also been investigated. The attributes of the geometry which lead to the most effective vortex generation without any interference from the shear instabilities and with the smallest drag penalty have been identified.

scholarly journals On the formation of streamwise vortices by plasma vortex generators

2013 ◽  
Vol 733 ◽  
pp. 370-393 ◽  
Author(s):  
Timothy N. Jukes ◽  
Kwing-So Choi
Keyword(s):  
Boundary Layer ◽  
Flow Direction ◽  
Vortex Formation ◽  
Vortex Generator ◽  
Streamwise Vortex ◽  
Spanwise Direction ◽  
Formation Mechanisms ◽  
Wall Jet ◽  
Streamwise Vortices ◽  
Barrier Discharge Plasma

AbstractThe streamwise vortices generated by dielectric-barrier-discharge plasma actuators in the laminar boundary layer were investigated using particle image velocimetry to understand the vortex-formation mechanisms. The plasma vortex generator was oriented along the primary flow direction to produce a body force in the spanwise direction. This created a spanwise-directed wall jet which interacted with the oncoming boundary layer to form a coherent streamwise vortex. It was found that the streamwise vortices were formed by the twisting and folding of the spanwise vorticity in the oncoming boundary layer into the outer shear layer of the spanwise wall jet, which added its own vorticity to increase the circulation along the actuator length. This is similar to the delta-shaped, vane-type vortex generator, except that the circulation was enhanced by the addition of the vorticity in the plasma jet. It was also observed that the plasma vortex was formed close to the wall with an enhanced wall-ward entrainment, which created strong downwash above the actuator.

Influence of active control on STG-based generation of streamwise vortices in near-wall turbulence

2012 ◽  
Vol 710 ◽  
pp. 234-259 ◽  
Author(s):  
B.-Q. Deng ◽  
C.-X. Xu
Keyword(s):  
Drag Reduction ◽  
Turbulent Flows ◽  
Active Control ◽  
Phase Control ◽  
Transient Growth ◽  
Vortex Generation ◽  
Streamwise Vortices ◽  
Detection Plane ◽  
Near Wall ◽  
Opposition Control

AbstractNear-wall streamwise vortices are closely related to the generation of high skin friction in wall-bounded turbulent flows. A common feature of controlled, friction-reduced turbulent flows is weakened near-wall streamwise vortices. In the present study, the streak transient growth (STG) mechanism for generating near-wall streamwise vortices by Schoppa & Hussain (J. Fluid Mech., vol. 453, 2002, pp. 57–108) is employed, and the opposition control proposed by Choi, Moin & Kim (J. Fluid Mech., vol. 262, 1994, pp. 75–110) is imposed during the transient growth process of perturbations to determine how active control affects the generation of quasi-streamwise vortices. In the transient growth stage, when the detection plane is located near the wall (${ y}_{d}^{+ } = 15$), the control can suppress the production of streamwise vorticity by weakening the near-wall vertical velocity; when the detection plane moves away from the wall (${ y}_{d}^{+ } = 28$), the control has the opposite effect. In the vortex generation stage, the control cannot change the dominance of the stretching effect. Controls imposed at different stages reveal the importance of the STG stage in vortex generation. Strengthened out-of-phase control and lessened in-phase control are proposed as an extension of the original opposition-control scheme. Application in a fully developed turbulent channel flow shows that strengthened ${ y}_{d}^{+ } = 10$ control can yield an even higher drag reduction rate than the original ${ y}_{d}^{+ } = 15$ control. Moreover, lessened ${ y}_{d}^{+ } = 28$ control can also achieve drag reduction and turbulence suppression.

On hairpin vortex generation from near-wall streamwise vortices

2015 ◽  
Vol 31 (2) ◽  
pp. 139-152 ◽  
Author(s):  
Yinshan Wang ◽  
Weixi Huang ◽  
Chunxiao Xu
Keyword(s):  
Hairpin Vortex ◽  
Vortex Generation ◽  
Streamwise Vortices ◽  
Near Wall

Vortex formation in a free boundary layer according to stability theory

1965 ◽  
Vol 22 (2) ◽  
pp. 371-383 ◽  
Author(s):  
A. Michalke
Keyword(s):  
Boundary Layer ◽  
Free Boundary ◽  
Stability Theory ◽  
Vortex Formation ◽  
Linear Stability Theory ◽  
Critical Layer ◽  
Disturbed Flow ◽  
Vorticity Distribution ◽  
Linearized Stability ◽  
Non Linear

An attempt is made to explain the formation of vortices in free boundary layers by means of stability theory using a hyperbolic-tangent velocity profile. The vorticity distribution of the disturbed flow, as obtained by the inviscid linearized stability theory, is discussed. The path lines of particles which are initially placed along straight lines parallel to thex-axis are calculated. Lines connecting the positions of these particles give an impression of the instant shape of the disturbed flow. With increasing time the boundary layer becomes thinner in certain regions and thicker in others. A special line—originally positioned at the critical layer—shows in the thicker region a tendency to roll up. Also extrema of the vorticity are located there. Finally, these results are compared with those which can be expected from the non-linear Helmholtz equation. Disagreement is found in the neighbourhood of the critical layer. Using the non-linear stability theory of Stuart up to the third-order terms, the vorticity distribution shows the tendency expected from the non-linear equation.

Designing of Micro Gravitational Vortex Turbine’s Vortex Pool

2017 ◽  
Author(s):  
Wajiha Rehman ◽  
Masooma Ijaz ◽  
Asma Munir
Keyword(s):  
Cross Section ◽  
Flow Rate ◽  
Turbine Blades ◽  
Vortex Formation ◽  
Vortex Generation ◽  
Geometrical Characteristics ◽  
Novel Technology ◽  
Hydropower Plants ◽  
Low Head ◽  
Micro Level

Water is one of the major sources of renewable energy and many hydropower plants are working across the world but they require specific values of head and flow rate for their operation and optimum results. There are many sites where limited head and flow rate is available but these resources cannot be exploited due to inefficient technologies. Gravitational vortex turbine (GVT) is a novel technology that is suitable for micro-level power production where low head and flow rate is available. It consists of two main parts: vortex pool for vortex generation and turbine blades. This paper focuses on parametrical analysis of GVT to determine the geometrical characteristics which gives the best performance. These parameters would address; effect of velocity and symmetry of vortex with the ratio of upper diameter of funnel (D) to outlet diameter (d), effect of the angle of rectangular inlet passage on the vortex formation. It will also analyze flow in rectangular passage with constant cross section vs. converging cross section. All of these parameters have major impact on the velocity and symmetry of flow. Results show that outlet of the funnel should be 40% of the upper diameter while highest velocity was achieved when rectangular passage was at 60 degrees with pre-rotational plate at 30 degrees.

Transition to turbulence over convex surfaces

2018 ◽  
Vol 855 ◽  
pp. 1208-1237
Author(s):  
Michael Karp ◽  
M. J. Philipp Hack
Keyword(s):  
Opposite Sign ◽  
Transition To Turbulence ◽  
Transient Growth ◽  
Centrifugal Forces ◽  
Streamwise Vortices ◽  
Boundary Layer Flows ◽  
Convex Surfaces ◽  
Exponentially Stable ◽  
Opposing Effects ◽  
Secondary Instabilities

Although boundary-layer flows over convex surfaces are exponentially stable, non-modal mechanisms may enable significant disturbance growth which can make the flow susceptible to secondary instabilities. A parametric investigation of the transient growth and secondary instabilities in flows over convex surfaces is performed. The optimal disturbance in the steady case corresponds to alternating streaks and streamwise vortices of opposite sign that reinforce one another due to lift-up and centrifugal forces, respectively. The process repeats with a constant (naturally appearing) streamwise wavelength which is proportional to the square root of the radius. Unsteady disturbances achieve a higher optimal gain, compared to the steady case, as a result of the opposing effects of the lift-up and centrifugal mechanisms. Linear analysis shows that the curvature has a negligible effect on secondary instabilities. Direct numerical simulations of transient growth with and without secondary instabilities confirm the predictions obtained by the local stability theory. It is found that the presence of a secondary instability is not sufficient, on its own, to ensure transition to turbulence. Only sufficiently long and energetic streaks trigger the breakdown to turbulence.

The Effects of Blocking Ratio and Atmospheric Altitude on Starting Annular Jets

2010 ◽  
Author(s):  
Emad Abdel-Raouf ◽  
John Baker ◽  
Muhammad Sharif
Keyword(s):  
Numerical Model ◽  
Vortex Formation ◽  
Comprehensive Analysis ◽  
The Other ◽  
Strong Function ◽  
Air Jets ◽  
Near Space ◽  
Annular Jet ◽  
Starting Jet ◽  
Annular Jets

The effects of the blocking ratio and atmospheric altitude on starting annular air jets at a low Reynolds number are investigated by examining the velocity profiles, jet entrainment and vortex formation. A 2D axisymmetric numerical model is developed to perform the analysis. The numerical model is validated with theoretical and experimental results from other studies. In order to achieve a comprehensive analysis, the annular jet is tested for blocking ratios with the values of 0.00 (i.e. a round jet), 0.50 and 0.75. Air properties at altitudes of 0.00 km, 18.90 km and 33.75 km are tested to simulate atmospheric environments at sea level, edge of near space and near space, respectively. The results showed that the starting jet velocity profile, entrainment and vortex formation is a strong function of blocking ratio. On the other hand, the same parameters are shown to be nearly independent, strong and weak functions of atmospheric altitude, respectively.

Turbulent flow through a rectangular duct with a partially blocked exit

2007 ◽  
Vol 592 ◽  
pp. 51-78
Author(s):  
T. Y. HSU ◽  
H. ELORANTA ◽  
P. SAARENRINNE ◽  
T. WEI
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Vector Fields ◽  
Vortex Formation ◽  
Direct Interaction ◽  
Paper Machine ◽  
Two Dimensional ◽  
Duct Flow ◽  
Streamwise Vortices ◽  
Paper Machines

This paper contains data on and insights into the origins of turbulence associated with a partial blockage at the exit of a two-dimensional, laminar, horizontal duct flow. In essence, this is the upstream approach region of the forward-facing step problem. This work was motivated by the need to identify and control unsteady streamwise vortices generated in the headbox (i.e. contraction section) of an industrial paper machine. The duct was 57.2 cm wide × 10.16 cm high, with up to a 50 % blockage. Experiments were scaled to match Reynolds numbers found in paper machines; exit velocities were as large as 200 cm s−1. The goal of the research was to map the flow at the exit and to examine the response of the flat-plate turbulent boundary layer on the opposing wall under the partial blockage. Laser-induced fluorescence (LIF) and digital particle image velocimetry (DPIV) were used to examine flow in three orthogonal planes at various stations upstream of the duct exit. Mean and instantaneous DPIV vector fields clearly show that an unsteady spanwise vortex forms in the corner formed by the top nozzle wall and partial blockage which, in turn, gives rise to turbulent streamwise vortices.A turbulent boundary layer was initiated on the duct wall opposite the blockage, upstream of a two-dimensional contraction. Results show that even though the acceleration parameter, K, exceeded the nominal critical level of 3.0 × 10−6 for relaminarization beneath the blockage, the flow did not reach a quasi-laminar state. In addition, there did not appear to be direct interaction between unsteady vortex formation at the partial blockage on the upper wall and bottom-wall turbulent boundary layer structures.

Passive flow control application for rotorcraft in transonic conditions

2018 ◽  
Vol 28 (5) ◽  
pp. 1080-1095 ◽  
Author(s):  
Fernando Tejero Embuena ◽  
Piotr Doerffer ◽  
Pawel Flaszynski ◽  
Oskar Szulc
Keyword(s):  
Flow Control ◽  
Vortex Formation ◽  
Strong Shock ◽  
Aerodynamic Performance ◽  
Rotor Blades ◽  
Vortex Generators ◽  
Helicopter Rotor ◽  
Streamwise Vortices ◽  
Content Type ◽  
Helicopter Rotor Blades

Purpose Helicopter rotor blades are usually aerodynamically limited by the severe conditions present in every revolution: strong shock wave boundary layer interaction on the advancing side and dynamic stall on the retreating side. Therefore, different flow control strategies might be applied to improve the aerodynamic performance. Design/methodology/approach The present research is focussed on the application of passive rod vortex generators (RVGs) to control the flow separation induced by strong shock waves on helicopter rotor blades. The formation and development in time of the streamwise vortices are also investigated for a channel flow. Findings The proposed RVGs are able to generate streamwise vortices as strong as the well-known air-jet vortex generators. It has been demonstrated a faster vortex formation for the rod type. Therefore, this flow control device is preferred for applications in which a quick vortex formation is required. Besides, RVGs were implemented on helicopters rotor blades improving their aerodynamic performance (ratio thrust/power consumption). Originality/value A new type of vortex generator (rod) has been investigated in several configurations (channel flow and rotor blades).

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