Vortex-Excited Response of Large-Scale Cylinders in Sheared Flow

1988 ◽  
Vol 110 (3) ◽  
pp. 272-277 ◽  
Author(s):  
J. A. Humphries ◽  
D. H. Walker
Keyword(s):  
Large Scale ◽  
Shear Velocity ◽  
Reynolds Numbers ◽  
Hydrodynamic Drag ◽  
Flow Induced Vibration ◽  
Sheared Flow ◽  
Drag Forces ◽  
Linear Shear ◽  
Series Of Experiments ◽  
The Mean

A series of experiments were performed to measure the vortex-excited response of a 0.168-m-dia slender circular cylinder in a range of linear shear velocity profiles. Reynolds numbers of up to 2.5 × 105 were achieved. The results clearly showed that regular large-amplitude cylinder vibrations occurred well within the critical drag transition region. It was found that increasing the linear shear profile decreased the peak amplitude response but broadened the range of lock-on over which large oscillations occurred. The flow-induced vibration of the cylinder caused amplification of the mean hydrodynamic drag forces acting on the cylinder when compared with those expected for a similar rigid cylinder.

LDV and PIV Velocity Results in a Thermosiphon

2003 ◽  
Author(s):  
J. Kulman ◽  
D. Gray ◽  
S. Sivanagere ◽  
S. Guffey
Keyword(s):  
Large Scale ◽  
Single Phase ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Flow Patterns ◽  
Laser Doppler Velocimeter ◽  
Mean Velocity ◽  
Developed Turbulence ◽  
The Mean ◽  
Good Agreement

Heat transfer and flow characteristics have been determined for a single-phase rectangular loop thermosiphon. The plane of the loop was vertical, and tests were performed with in-plane tilt angles ranging from 3.6° CW to 4.2° CCW. Velocity profiles were measured in one vertical leg of the loop using both a single-component Laser Doppler Velocimeter (LDV), and a commercial Particle Image Velocimeter (PIV) system. The LDV data and PIV data were found to be in good agreement. The measured average velocities were approximately 2–2.5 cm/s at an average heating rate of 70 W, and were independent of tilt angle. Significant RMS fluctuations of 10–20% of the mean velocity were observed in the test section, in spite of the laminar or transitional Reynolds numbers (order of 700, based on the hydraulic diameter). These fluctuations have been attributed to vortex shedding from the upstream temperature probes and mitre bends, rather than to fully developed turbulence. Animations of the PIV data clearly show these large scale unsteady flow patterns. Multiple steady state flow patterns were not observed.

scholarly journals Physical and Aerodynamic Properties of Lavender in relation to Harvest Mechanisation

2014 ◽  
Vol 2014 ◽  
pp. 1-8
Author(s):  
Christos I. Dimitriadis ◽  
James L. Brighton ◽  
Mike J. O’Dogherty ◽  
Maria I. Kokkora ◽  
Anastasios I. Darras
Keyword(s):  
Surface Area ◽  
Drag Force ◽  
Aerodynamic Drag ◽  
Reynolds Numbers ◽  
Ultimate Tensile Stress ◽  
Flower Head ◽  
Drag Forces ◽  
Aerodynamic Properties ◽  
The Mean ◽  
Harvest Moisture

A laboratory study evaluated the physical and aerodynamic properties of lavender cultivars in relation to the design of an improved lavender harvester that allows removal of flowers from the stem using the stripping method. The identification of the flower head adhesion, stem breakage, and aerodynamic drag forces were conducted using an Instron 1122 instrument. Measurements on five lavender cultivars at harvest moisture content showed that the overall mean flower detachment force from the stem was 11.2 N, the mean stem tensile strength was 36.7 N, and the calculated mean ultimate tensile stress of the stem was 17.3 MPa. The aerodynamic measurements showed that the drag force is related with the flower surface area. Increasing the surface area of the flower head by 93% of the “Hidcote” cultivar produced an increase in drag force of between 24.8% and 50.6% for airflow rates of 24 and 65 m s−1, respectively. The terminal velocities of the flower heads of the cultivar ranged between 4.5 and 5.9 m s−1, which results in a mean drag coefficient of 0.44. The values of drag coefficients were compatible with well-established values for the appropriate Reynolds numbers.

scholarly journals Decomposition of the mean friction drag on an NACA4412 airfoil under uniform blowing/suction

2021 ◽  
Vol 932 ◽  
Author(s):  
Yitong Fan ◽  
Marco Atzori ◽  
Ricardo Vinuesa ◽  
Davide Gatti ◽  
Philipp Schlatter ◽  
...  
Keyword(s):  
Large Scale ◽  
Control Strategies ◽  
Reference Conditions ◽  
Adverse Pressure Gradient ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Friction Drag ◽  
Mode Decomposition ◽  
Control Scheme ◽  
The Mean

The application of drag-control strategies on canonical wall-bounded turbulence, such as periodic channel and zero- or adverse-pressure-gradient boundary layers, raises the question on how to distinguish consistently the origin of control effects under different reference conditions. We employ the RD identity (Renard & Deck, J. Fluid Mech., vol. 790, 2016, pp. 339–367) to decompose the mean friction drag and investigate the control effects of uniform blowing and suction applied to an NACA4412 airfoil at chord Reynolds numbers $Re_c=200\,000$ and $400\,000$ . The connection of the drag reduction/increase by using blowing/suction with the turbulence statistics (including viscous dissipation, turbulence kinetic energy production and spatial growth of the flow) across the boundary layer, subjected to adverse or favourable pressure gradients, is examined. We found that the inner and outer peaks of the contributions associated with the friction-drag generation show good scaling with either inner or outer units, respectively. They are also independent of the Reynolds number, control scheme and intensity of the blowing/suction. The small- and large-scale structures are separated with an adaptive scale-decomposition method, namely the empirical mode decomposition (EMD), which aims to analyse the scale-specific contribution of turbulent motions to friction-drag generation. Results unveil that blowing on the suction side of the airfoil is able to enhance the contribution of large-scale motions and to suppress that of small scales; however, suction behaves contrarily. The contributions related to cross-scale interactions remain almost unchanged with different control strategies.

Structure of Turbulent Flows Over Forward Facing Steps With Adverse Pressure Gradient

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
Hassan Iftekhar ◽  
Martin Agelin-Chaab
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Step Height ◽  
Reattachment Length ◽  
Mean Velocity ◽  
The Mean

This paper reports an experimental study on the effects of adverse pressure gradient (APG) and Reynolds number on turbulent flows over a forward facing step (FFS) by employing three APGs and three Reynolds numbers. A particle image velocimetry (PIV) technique was used to conduct velocity measurements at several locations downstream, and the flow statistics up to 68 step heights are reported. The step height was maintained at 6 mm, and the Reynolds numbers based on the step height and freestream mean velocity were 1600, 3200, and 4800. The mean reattachment length increases with the increase in Reynolds number without the APG whereas the mean reattachment length remains constant for increasing APG. The proper orthogonal decomposition (POD) results confirmed that higher Reynolds numbers caused the large-scale structures to be more defined and organized close to the step surface.

scholarly journals Overcoming Stopovers in Cycloidal Rotor Propulsion Integration on Air Vehicles

2012 ◽  
Author(s):  
José C. Páscoa ◽  
Galina I. Ilieva
Keyword(s):  
Large Scale ◽  
Reynolds Numbers ◽  
Rotational Axis ◽  
Major Drawback ◽  
Drag Forces ◽  
Flow Mechanisms ◽  
Weight Penalty ◽  
Thrust Vector ◽  
Lift And Drag ◽  
Rotating Wing

A cyclorotor (also known as a cyclocopter or cyclogiro) is a rotating-wing system where the span of the blades runs parallel to the axis of its rotation. The pitch angle of each of the blades is varied cyclically by mechanical means such that the blades experiences positive angles of attack at both the top and bottom positions of the azimuth cycle. The resulting time-varying lift and drag forces produced by each blade can be resolved into the vertical and horizontal directions. Varying the amplitude and phase of the cyclic blade pitch can be used to change the magnitude and direction of the net thrust vector produced by the cyclorotor. Compared to a conventional rotor, each spanwise blade element of a cyclorotor operates at similar aerodynamic conditions (i.e., at similar flow velocities, Reynolds numbers, and angles of incidence), and so the blades can be optimized to achieve the best aerodynamic efficiency. Moreover, because the blades are cyclically pitched once per revolution (1/rev), unsteady flow mechanisms may delay blade stall onset and in turn may augment the lift produced by the blades. Albeit proposed to MAV-scale, its use on large scale vehicles turns problematic, and we proposed in this paper to address their stopovers. Furthermore, since the thrust vector of a cyclorotor can be instantaneously set to any direction perpendicular to the rotational axis, a cyclorotor-based air vehicle may ultimately show better maneuverability and agility as compared to a classical powered conventional rotor system. One major drawback of a cyclorotor is its relatively large rotating structure which might offer a weight penalty when compared to a conventional rotor.

scholarly journals Scale interactions in turbulent rotating planar Couette flow: insight through the Reynolds stress transport

2019 ◽  
Vol 879 ◽  
pp. 255-295 ◽  
Author(s):  
Takuya Kawata ◽  
P. Henrik Alfredsson
Keyword(s):  
Couette Flow ◽  
Reynolds Stress ◽  
Coriolis Force ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Wall Structures ◽  
Rotation Numbers ◽  
Small Scales ◽  
The Mean ◽  
Scale Turbulence

In turbulent planar Couette flow under anticyclonic spanwise system rotation, large-scale roll-cell structures arise due to a Coriolis-force-induced instability. The structures are superimposed on smaller-scale turbulence, and with increasing angular velocity ($\unicode[STIX]{x1D6FA}_{z}$) such roll cells dominate the flow field and small-scale turbulence is instead suppressed in a certain rotation number range $0<Ro\lesssim 0.1$ ($Ro=2\unicode[STIX]{x1D6FA}_{z}h/U_{w}$, where $h$ is the channel half-width, $U_{w}$ the wall velocity). At low rotation numbers around $Ro\approx 0.02$ both large-scale roll cells and smaller-scale turbulence coexist. In the present study, we investigate interaction between these structures through a scale-by-scale analysis of the Reynolds stress transport. We show that at low rotation numbers $Ro\approx 0.01$ the turbulence productions by the mean flow gradient and the Coriolis force occur at different scales and thereby the turbulent energy distribution over a wide range of scales is maintained. On the other hand at higher rotation numbers $Ro\gtrsim 0.05$, a zero-absolute-vorticity state is established and production of small scales from the mean shear disappears although large-scale turbulence production is maintained through the Coriolis force. At high enough Reynolds numbers, where scale separation between the near-wall structures and the roll cells is relatively distinct, transition between these different $Ro$ regimes is found to occur rather abruptly around $Ro\approx 0.02$, resulting in a non-monotonic behaviour of the wall shear stress as a function of $Ro$. It is also shown that at such an intermediate rotation number the roll cells interact with smaller scales by moving near-wall structures towards the core region of the channel, by which the Reynolds stress is transported from relatively small scales near the wall towards larger scales in the channel centre. Such Reynolds stress transport by scale interaction becomes increasingly significant as the Reynolds number increases, and results in a reversed mean velocity gradient at the channel centre at high enough Reynolds numbers.

scholarly journals Turbulent Taylor–Couette flow with stationary inner cylinder

2016 ◽  
Vol 799 ◽  
Author(s):  
R. Ostilla-Mónico ◽  
R. Verzicco ◽  
D. Lohse
Keyword(s):  
Angular Momentum ◽  
Large Scale ◽  
Outer Cylinder ◽  
Reynolds Numbers ◽  
Law Of The Wall ◽  
Large Scale Structures ◽  
Large Gradient ◽  
The Mean ◽  
Taylor Couette ◽  
Cylinder Rotation

A series of direct numerical simulations were performed of Taylor–Couette (TC) flow, the flow between two coaxial cylinders, with the outer cylinder rotating and the inner one fixed. Three cases were considered, where the Reynolds number of the outer cylinder was $Re_{o}=5.5\times 10^{4}$, $Re_{o}=1.1\times 10^{5}$ and $Re_{o}=2.2\times 10^{5}$. The ratio of radii ${\it\eta}=r_{i}/r_{o}$ was fixed to ${\it\eta}=0.909$ to mitigate the effects of curvature. Axially periodic boundary conditions were used, with the aspect ratio of vertical periodicity ${\it\Gamma}$ fixed to ${\it\Gamma}=2.09$. Being linearly stable, TC flow with outer cylinder rotation is known to have very different behaviour than TC flow with pure inner cylinder rotation. Here, we find that the flow nonetheless becomes turbulent, but the torque required to drive the cylinders and level of velocity fluctuations was found to be smaller than those for pure inner cylinder rotation at comparable Reynolds numbers. The mean angular momentum profiles showed a large gradient in the bulk, instead of the constant angular momentum profiles of pure inner cylinder rotation. The near-wall mean and fluctuation velocity profiles were found to coincide only very close to the wall, showing large deviations from both pure inner cylinder rotation profiles and the classic von Karman law of the wall elsewhere. Finally, transport of angular velocity was found to occur mainly through intermittent bursts, and not through wall-attached large-scale structures as is the case for pure inner cylinder rotation.

A Novel Look at the Performance of the Cyclorotor Propulsion System for Air Vehicles

2012 ◽  
Author(s):  
José C. Páscoa ◽  
Antonio Dumas ◽  
Michele Trancossi
Keyword(s):  
Large Scale ◽  
Reynolds Numbers ◽  
Unsteady Motion ◽  
Rotational Axis ◽  
Rotor Systems ◽  
Drag Forces ◽  
Blade Optimization ◽  
Weight Penalty ◽  
Lift And Drag ◽  
Rotating Wing

A system in which a rotating-wing device, comprising several pitching blades, turns around an axis along the span of the blades is called a cyclorotor. During the azimuthal rotation of the blades they also experience a change in the pitch angle. For each of the blades its pitch is varied cyclically by mechanical means such that the blades experience positive angles of attack at both the top and bottom positions of the azimuth cycle. The resultant unsteady motion of each blade can then be summed up into a resultant lift and drag forces. An almost instantaneous variation of magnitude and direction of the total cyclo rotor thrust can be obtained by changing the amplitude and phase of the cyclic blade pitch. In this rotor, conversely to classical propellers, each spanwise blade element operates at similar flow velocities, Reynolds numbers and incidence, this allows an easier blade optimization to achieve the best aerodynamic efficiency. Further, the cyclorotor is based on using dynamic pitching in order to delay stall and in this way increase the lift produced by the blades. Realistic flying vehicles have only be presented for the MAV-scale, its use on large scale vehicles turns problematic, herein we will analyze its stopovers. Finally, a very advantageous characteristic is the possibility to achieve almost instantaneous thrust variation in any direction perpendicular to the rotational axis, this will result in an air vehicle with a much better maneuverability, as compared with vehicles powered by classical rotor systems. This comes at a cost of a larger structure which might lead to a weight penalty.

Hydrodynamic Forces on Intermittently Spanning Pipelines in Steady Currents

2019 ◽  
Author(s):  
Yunfei Teng ◽  
Liang Cheng ◽  
Hongwei An ◽  
Feifei Tong ◽  
Terry Griffiths ◽  
...  
Keyword(s):  
Reynolds Numbers ◽  
Hydrodynamic Forces ◽  
Experimental Investigations ◽  
Hydrodynamic Drag ◽  
Hydrodynamic Behavior ◽  
Subcritical Region ◽  
Gap Ratio ◽  
The Mean ◽  
Drag And Lift ◽  
Gap Height

Abstract Experimental investigations on the hydrodynamic forces on an intermittently spanning pipeline exposed to steady currents were carried out. The effect of intermittent local spanning sections on the global hydrodynamic behavior was studied by changing the ratio between the non-spanning length (B) and the total length (L), namely the blocking ratio B / L. A range of gap height (G) to diameter (D) ratios, i.e. gap ratio G / D, and 4 different Reynolds numbers (Re) in the subcritical region were tested in the experiments. The results show: i) for a certain gap ratio, the mean drag increases gently with the decreasing blocking ratio at Re = 5.5 × 104, whereas the mean lift decreases significantly with the decreasing blocking ratio at all values of Re tested; and ii) for a certain blocking ratio, increasing the gap ratio leads to an increase in mean drag and decrease in mean lift. Further, simple approaches are proposed based on the present dataset for estimating the global effects on hydrodynamic drag and lift forces due to local spanning geometry.

scholarly journals A statistical state dynamics-based study of the structure and mechanism of large-scale motions in plane Poiseuille flow

2016 ◽  
Vol 809 ◽  
pp. 290-315 ◽  
Author(s):  
Brian F. Farrell ◽  
Petros J. Ioannou ◽  
Javier Jiménez ◽  
Navid C. Constantinou ◽  
Adrián Lozano-Durán ◽  
...  
Keyword(s):  
Dynamical System ◽  
Poiseuille Flow ◽  
Large Scale ◽  
Dynamic Properties ◽  
Reynolds Numbers ◽  
Wall Turbulence ◽  
Plane Poiseuille Flow ◽  
Statistical State ◽  
The Mean ◽  
High Reynolds

The perspective of statistical state dynamics (SSD) has recently been applied to the study of mechanisms underlying turbulence in a variety of physical systems. An SSD is a dynamical system that evolves a representation of the statistical state of the system. An example of an SSD is the second-order cumulant closure referred to as stochastic structural stability theory (S3T), which has provided insight into the dynamics of wall turbulence, and specifically the emergence and maintenance of the roll/streak structure. S3T comprises a coupled set of equations for the streamwise mean and perturbation covariance, in which nonlinear interactions among the perturbations has been removed, restricting nonlinearity in the dynamics to that of the mean equation and the interaction between the mean and perturbation covariance. In this work, this quasi-linear restriction of the dynamics is used to study the structure and dynamics of turbulence in plane Poiseuille flow at moderately high Reynolds numbers in a closely related dynamical system, referred to as the restricted nonlinear (RNL) system. Simulations using this RNL system reveal that the essential features of wall-turbulence dynamics are retained. Consistent with previous analyses based on the S3T version of SSD, the RNL system spontaneously limits the support of its turbulence to a small set of streamwise Fourier components, giving rise to a naturally minimal representation of its turbulence dynamics. Although greatly simplified, this RNL turbulence exhibits natural-looking structures and statistics, albeit with quantitative differences from those in direct numerical simulations (DNS) of the full equations. Surprisingly, even when further truncation of the perturbation support to a single streamwise component is imposed, the RNL system continues to self-sustain turbulence with qualitatively realistic structure and dynamic properties. RNL turbulence at the Reynolds numbers studied is dominated by the roll/streak structure in the buffer layer and similar very large-scale structure (VLSM) in the outer layer. In this work, diagnostics of the structure, spectrum and energetics of RNL and DNS turbulence are used to demonstrate that the roll/streak dynamics supporting the turbulence in the buffer and logarithmic layer is essentially similar in RNL and DNS.

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