scholarly journals The Steady Wake of a Wall-Mounted Rectangular Prism with a Large-Depth-Ratio at Low Reynolds Numbers

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3579
Author(s):  
Arash Zargar ◽  
Ali Tarokh ◽  
Arman Hemmati
Keyword(s):  
Separated Flow ◽  
Leading Edge ◽  
Surface Friction ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Recirculation Region ◽  
Large Depth ◽  
Pressure Distributions ◽  
Depth Ratio ◽  
Drag And Lift

The wakes of wall-mounted small (square) and large (long) depth-ratio rectangular prisms are numerically studied at Reynolds numbers of 50–250. The large depth-ratio significantly alters the dominance of lateral secondary flow (upwash and downwash) in the wake due to the reattachment of leading-edge separated flow on the surfaces of the prism. This changes the wake topology by varying the entrained flow in the wake region and changing the distribution of vorticity. Thus, the magnitude of vorticity significantly decreases by increasing the prism depth-ratio. Furthermore, the length of the recirculation region and the orientation of near wake flow structures are altered for the larger depth-ratio prism compared to the square prism. Drag and lift coefficients are also affected due to the change of pressure distributions on the rear face of the prism and surface friction force. This behavior is consistently observed for the entire range of Reynolds numbers considered here. The wake size is scaled with Re1/2, whereas drag coefficient scaled with Re−0.3.

Vortex-induced rotations of a rigid square cylinder at low Reynolds numbers

2017 ◽  
Vol 813 ◽  
pp. 482-507 ◽  
Author(s):  
Sungmin Ryu ◽  
Gianluca Iaccarino
Keyword(s):  
Cross Flow ◽  
Reynolds Numbers ◽  
Dynamic Responses ◽  
Two Dimensional ◽  
Square Cylinder ◽  
Low Reynolds Numbers ◽  
Pressure Distributions ◽  
Low Reynolds ◽  
Drag And Lift ◽  
Vortex Patterns

A numerical investigation of vortex-induced rotations (VIRs) of a rigid square cylinder, which is free to rotate in the azimuthal direction in a two-dimensional uniform cross-flow, is presented. Two-dimensional simulations are performed in a range of Reynolds numbers between 45 and 150 with a fixed mass and moment of inertia of the cylinder. The parametric investigation reveals six different dynamic responses of the square cylinder (expanding on those reported by Zaki et al. (J. Fluids Struct., vol. 8, 1994, pp. 555–582)) and their coupled vortex patterns at low Reynolds numbers. In each characteristic regime, moment generating mechanisms are elucidated with investigations of instantaneous flow fields and surface pressure distributions at chosen time instants in a period of rotation response. Our simulation results also elucidate that VIRs significantly influence the statistics of drag and lift force coefficients: (i) the onset of a rapid increases of the two coefficients at $Re=80$ and (ii) their step increases in the autorotation regime.

The Functional Role of Wing Corrugations in Living Systems

1986 ◽  
Vol 108 (1) ◽  
pp. 93-97 ◽  
Author(s):  
R. H. Buckholz
Keyword(s):  
Reynolds Number ◽  
Functional Role ◽  
Separated Flow ◽  
Leading Edge ◽  
Flow Region ◽  
Wing Surface ◽  
Living Systems ◽  
Recirculation Region ◽  
Insect Wings

Questions concerning the functional role of spanwise wing corrugation in living systems are experimentally investigated. Attention was initially directed to this problem by observation of the irregular shape of many insect wings as well as other studies indicating higher lift on these wings. First, a flow visualization scheme was used to observe and photograph streamlines around two different wing sections. One of these, a sheet metal model with geometry matching that of a butterfly wing, was studied at a chord Reynolds number of 1500 and at a Reynolds number of 80 based on corrugation depth. A steady-state recirculation region near the model leading edge was found, and the separated flow region above this recirculation zone formed a laminar reattachment to the model. A second thicker wing was corrugated on the upper surface. Closed streamlines inside these upper surface corrugations were photographed at Reynolds numbers of 8000 and 3800 based on chord length, and 200 and 90 based on corrugation depth. Reductions in pressures on the corrugated upper wing surface relative to a smooth upper wing surface were then measured.

Predicting Transitional Separation Bubbles

2004 ◽  
Author(s):  
John A. Redford ◽  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Transition Process ◽  
Reynolds Numbers ◽  
Test Cases ◽  
Transition Model ◽  
Flow Transition ◽  
Attached Flow

This paper describes the modifications made to a successful attached flow transition model to produce a model capable of predicting both attached and separated flow transition. This transition model is used in combination with the Fluent CFD software, which is used to compute the flow around the blade assuming that it remains entirely laminar. The transition model then determines the start of transition location and the development of the intermittency. These intermittency values weight the laminar and turbulent boundary layer profiles to obtain the resulting transitional boundary layer parameters. The ERCOFTAC T3L test cases are used to validate the predictions. The T3L blade is a flat plate with a semi-circular leading edge, which results in the formation of a separation bubble the length of which is strongly dependent on the transition process. Predictions were performed for five T3L test cases for differing freestream turbulence levels and Reynolds numbers. For the majority of these test cases the measurements were accurately predicted.

On the steady separated flow around an inclined flat plate

1997 ◽  
Vol 333 ◽  
pp. 403-413 ◽  
Author(s):  
W. W. H. YEUNG ◽  
G. V. PARKINSON
Keyword(s):  
Flat Plate ◽  
Bluff Body ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Source Model ◽  
Pressure Distributions ◽  
Wide Range ◽  
Trailing Edges ◽  
Edge Separation

An inviscid analytic model is proposed for the steady separated flow around an inclined flat plate. With the plate normal to the stream, the model reduces to the wake-source model of Parkinson & Jandali originally developed for flow external to a symmetrical two-dimensional bluff body and its wake. At any other inclination, the Kutta condition is satisfied at both leading and trailing edges of the plate, and, in the limit that the angle of attack approaches zero, classical airfoil theory is recovered. A boundary condition is formulated based on some experimental results of Abernathy, but no additional empirical information is required. The predicted pressure distributions on the wetted surface for a wide range of angle attack are found to be in good agreement with experimental data, especially at smaller angles of attack. An extension to include a leading-edge separation bubble is explored and results are satisfactory.

The wake behind a cylinder rolling on a wall at varying rotation rates

2010 ◽  
Vol 648 ◽  
pp. 225-256 ◽  
Author(s):  
B. E. STEWART ◽  
M. C. THOMPSON ◽  
T. LEWEKE ◽  
K. HOURIGAN
Keyword(s):  
Reynolds Number ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Wake Flow ◽  
Recirculation Region ◽  
Two Dimensional ◽  
Number Range ◽  
Rotation Rates ◽  
Transition Mechanism ◽  
Three Dimensional Simulations

A study investigating the flow around a cylinder rolling or sliding on a wall has been undertaken in two and three dimensions. The cylinder motion is specified from a set of five discrete rotation rates, ranging from prograde through to retrograde rolling. A Reynolds number range of 20–500 is considered. The effects of the nearby wall and the imposed body motion on the wake structure and dominant wake transitions have been determined. Prograde rolling is shown to destabilize the wake flow, while retrograde rotation delays the onset of unsteady flow to Reynolds numbers well above those observed for a cylinder in an unbounded flow.Two-dimensional simulations show the presence of two recirculation zones in the steady wake, the lengths of which increase approximately linearly with the Reynolds number. Values of the lift and drag coefficient are also reported for the steady flow regime. Results from a linear stability analysis show that the wake initially undergoes a regular bifurcation from a steady two-dimensional flow to a steady three-dimensional wake for all rotation rates. The critical Reynolds number Rec of transition and the spanwise wavelength of the dominant mode are shown to be highly dependent on, but smoothly varying with, the rotation rate of the cylinder. Varying the rotation from prograde to retrograde rolling acts to increase the value of Rec and decrease the preferred wavelength. The structure of the fully evolved wake mode is then established through three-dimensional simulations. In fact it is found that at Reynolds numbers only marginally (~5%) above critical, the three-dimensional simulations indicate that the saturated state becomes time dependent, although at least initially, this does not result in a significant change to the mode structure. It is only at higher Reynolds numbers that the wake undergoes a transition to vortex shedding.An analysis of the three-dimensional transition indicates that it is unlikely to be due to a centrifugal instability despite the superficial similarity to the flow over a backward-facing step, for which the transition mechanism has been speculated to be centrifugal. However, the attached elongated recirculation region and distribution of the spanwise perturbation vorticity field, and the similarity of these features with those of the flow through a partially blocked channel, suggest the possibility that the mechanism is elliptic in nature. Some analysis which supports this conjecture is undertaken.

Laminar hypersonic leading edge separation – a numerical study

2017 ◽  
Vol 821 ◽  
pp. 624-646 ◽  
Author(s):  
Amna Khraibut ◽  
S. L. Gai ◽  
L. M. Brown ◽  
A. J. Neely
Keyword(s):  
Wall Temperature ◽  
Numerical Study ◽  
Separated Flow ◽  
Leading Edge ◽  
Slip Boundary Condition ◽  
Navier Stokes ◽  
Recirculation Region ◽  
Slip Boundary ◽  
Limiting Case ◽  
Edge Separation

This paper describes laminar hypersonic leading edge separation. Such a configuration of separated flow was originally studied by Chapman et al. (NACA Tech. Rep. 1356, 1958) at supersonic Mach numbers as it is particularly amenable to theoretical analysis and assumes no pre-existing boundary layer. It can be considered as a limiting case of much studied generic configurations such as separation at a compression corner and separated flow behind a base. A numerical investigation is described using a compressible Navier–Stokes solver assuming perfect gas air, no slip boundary condition and a non-catalytic surface. A moderate enthalpy flow of $3.1\times 10^{6}~\text{J}~\text{kg}^{-1}$ with a unit Reynolds number of $1.34\times 10^{6}~\text{ m}^{-1}$ and a Mach number of 9.66 was considered. The resulting separated flow is analysed in the context of viscous–inviscid interaction and interpreted in terms of ‘triple-deck’ concepts. Particular emphasis is given to wall temperature effects. The effects of strong to moderate wall cooling on flow in the separated region as well as on processes of separation, reattachment and separation length, are highlighted. The numerical simulations have also shown the existence of a secondary eddy embedded within the primary recirculation region, with its size, shape and position, being strongly affected by the wall temperature.

Genesis of the Airload Variations on Cylinders of Small Aspect Ratios

2015 ◽  
Author(s):  
Dhwanil Shukla ◽  
Nandeesh Hiremath ◽  
Nicholas Motahari ◽  
Narayanan Komerath
Keyword(s):  
Leading Edge ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Aerodynamic Load ◽  
Force Coefficient ◽  
Side Force ◽  
Rich Mixture ◽  
Pressure Distributions ◽  
Aspect Ratios ◽  
Stereo Particle Image Velocimetry

A cylinder of aspect ratio 1 experiences a rich mixture of phenomena when yawed through 360 degrees. Understanding this variation is crucial to aerodynamic load definition for objects of practical shape. This paper uses several diagnostics to explain the detailed airload map of cylinders with length to diameter ratios 0.25 to 4 in steady incompressible flow, at Reynolds numbers from 50,000 to 450,000. A sharp linear variation of side force coefficient with yaw is bounded by apparent stall. Drag and pitching moment depend more on the curved surfaces, while side force is dominated by flow over the end plates. Tuft visualization and stereo Particle Image Velocimetry complement 6-DOF loads. Pressure distributions are obtained from velocimetry as well as from computations. The flow over the suction side shows a curved leading edge vortex followed by reattachment and an aft separation. The lift at low yaw is largely due to suction from the zones near the front corner, while at higher yaw it comes from the pressure difference between the two sides. The presence of helical vortices differentiates the lift generation from models based purely on separation.

Investigation on the effect of leading edge tubercles of sweptback wing at low reynolds number

2020 ◽  
Vol 21 (6) ◽  
pp. 621
Author(s):  
Veerapathiran Thangaraj Gopinathan ◽  
John Bruce Ralphin Rose ◽  
Mohanram Surya
Keyword(s):  
Leading Edge ◽  
Reynolds Numbers ◽  
Sweep Angle ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Flow Energy ◽  
Airplane Wing ◽  
Low Reynolds ◽  
Naca 0015 ◽  
Wing Performance

Aerodynamic efficiency of an airplane wing can be improved either by increasing its lift generation tendency or by reducing the drag. Recently, Bio-inspired designs have been received greater attention for the geometric modifications of airplane wings. One of the bio-inspired designs contains sinusoidal Humpback Whale (HW) tubercles, i.e., protuberances exist at the wing leading edge (LE). The tubercles have excellent flow control characteristics at low Reynolds numbers. The present work describes about the effect of tubercles on swept back wing performance at various Angle of Attack (AoA). NACA 0015 and NACA 4415 airfoils are used for swept back wing design with sweep angle about 30°. The modified wings (HUMP 0015 A, HUMP 0015 B, HUMP 4415 A, HUMP 4415 B) are designed with two amplitude to wavelength ratios (η) of 0.1 & 0.24 for the performance analysis. It is a novel effort to analyze the tubercle vortices along the span that induce additional flow energy especially, behind the tubercles peak and trough region. Subsequently, Co-efficient of Lift (CL), Co-efficient of Drag (CD) and boundary layer pressure gradients also predicted for modified and baseline (smooth LE) models in the pre & post-stall regimes. It was observed that the tubercles increase the performance of swept back wings by the enhanced CL/CD ratio in the pre-stall AoA region. Interestingly, the flow separation region behind the centerline of tubercles and formation of Laminar Separation Bubbles (LSB) were asymmetric because of the sweep.

Wake Flow of Single and Multiple Yawed Cylinders

2004 ◽  
Vol 126 (5) ◽  
pp. 861-870 ◽  
Author(s):  
A. Thakur ◽  
X. Liu ◽  
J. S. Marshall
Keyword(s):  
Computational Study ◽  
Three Dimensional ◽  
Side Wall ◽  
Wake Flow ◽  
Upstream Region ◽  
Two Dimensional ◽  
Cylinder Wake ◽  
Dimensional Approximation ◽  
The Face ◽  
Drag And Lift

An experimental and computational study is performed of the wake flow behind a single yawed cylinder and a pair of parallel yawed cylinders placed in tandem. The experiments are performed for a yawed cylinder and a pair of yawed cylinders towed in a tank. Laser-induced fluorescence is used for flow visualization and particle-image velocimetry is used for quantitative velocity and vorticity measurement. Computations are performed using a second-order accurate block-structured finite-volume method with periodic boundary conditions along the cylinder axis. Results are applied to assess the applicability of a quasi-two-dimensional approximation, which assumes that the flow field is the same for any slice of the flow over the cylinder cross section. For a single cylinder, it is found that the cylinder wake vortices approach a quasi-two-dimensional state away from the cylinder upstream end for all cases examined (in which the cylinder yaw angle covers the range 0⩽ϕ⩽60°). Within the upstream region, the vortex orientation is found to be influenced by the tank side-wall boundary condition relative to the cylinder. For the case of two parallel yawed cylinders, vortices shed from the upstream cylinder are found to remain nearly quasi-two-dimensional as they are advected back and reach within about a cylinder diameter from the face of the downstream cylinder. As the vortices advect closer to the cylinder, the vortex cores become highly deformed and wrap around the downstream cylinder face. Three-dimensional perturbations of the upstream vortices are amplified as the vortices impact upon the downstream cylinder, such that during the final stages of vortex impact the quasi-two-dimensional nature of the flow breaks down and the vorticity field for the impacting vortices acquire significant three-dimensional perturbations. Quasi-two-dimensional and fully three-dimensional computational results are compared to assess the accuracy of the quasi-two-dimensional approximation in prediction of drag and lift coefficients of the cylinders.

Sign in / Sign up

Export Citation Format

Share Document