scholarly journals An Experimental-Numerical Investigation of the Wake Structure of a Hovering Rotor by PIV Combined with a Γ2 Vortex Detection Criterion

Energies ◽  
2021 ◽  
Vol 14 (9) ◽  
pp. 2613
Author(s):  
Fabrizio De Gregorio ◽  
Antonio Visingardi ◽  
Gaetano Iuso
Keyword(s):  
Numerical Simulations ◽  
Large Data ◽  
Tip Vortex ◽  
Fundamental Aspect ◽  
Small Scale ◽  
Mean Velocity ◽  
Rotor Wake ◽  
Tip Vortices ◽  
Swirl Velocity ◽  
Rotor Disk

The rotor wake aerodynamic characterization is a fundamental aspect for the development and optimization of future rotary-wing aircraft. The paper is aimed at experimentally and numerically characterizing the blade tip vortices of a small-scale four-bladed isolated rotor in hover conditions. The investigation of the vortex decay process during the downstream convection of the wake is addressed. Two-component PIV measurements were carried out below the rotor disk down to a distance of one rotor radius. The numerical simulations were aimed at assessing the modelling capabilities and the accuracy of a free-wake Boundary Element Methodology (BEM). The experimental and numerical results were investigated by the Γ2 criterion to detect the vortex location. The rotor wake mean velocity field and the instantaneous vortex characteristics were investigated. The experimental/numerical comparisons show a reasonable agreement in the estimation of the mean velocity inside the rotor wake, whereas the BEM predictions underestimate the diffusion effects. The numerical simulations provide a clear picture of the filament vortex trajectory interested in complex interactions starting at about a distance of z/R = −0.5. The time evolution of the tip vortices was investigated in terms of net circulation and swirl velocity. The PIV tip vortex characteristics show a linear mild decay up to the region interested by vortex pairing and coalescence, where a sudden decrease, characterised by a large data scattering, occurs. The numerical modelling predicts a hyperbolic decay of the swirl velocity down to z/R = −0.4 followed by an almost constant decay. Instead, the calculated net circulation shows a gradual decrease throughout the whole wake development. The comparisons show discrepancies in the region immediately downstream the rotor disk but significant similarities beyond z/R = −0.5.

Phase-Averaged Particle Tracking Velocimetry Measurements of Propeller Wake

2005 ◽  
Vol 49 (01) ◽  
pp. 43-54
Author(s):  
Sang-Joon Lee ◽  
Bu-Geun Paik ◽  
Choung Mook Lee
Keyword(s):  
Particle Tracking ◽  
Particle Tracking Velocimetry ◽  
Flow Characteristics ◽  
Computation Time ◽  
Velocity Fields ◽  
Tip Vortex ◽  
Wake Flow ◽  
Mean Velocity ◽  
Tip Vortices ◽  
Propeller Wake

The objective of the present paper is to apply an adaptive hybrid two-frame particle tracking velocimetry (PTV) technique for measuring the flow characteristics of turbulent wake behind a marine propeller with five blades. Compared to the conventional particle image velocimetry method, the hybrid PTV technique increases the spatial resolution and measurement accuracy significantly while reducing the computation time. For each of four different blade phases of 0, 18, 36, and 54 deg, 400 instantaneous velocity fields were measured. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the region ranging from the trailing edge to a two propeller diameter (D) downstream location. The phase-averaged mean velocity fields show that the trailing vorticity and the viscous wake are formed by the boundary layers developed on the blade surfaces. The vorticity contours at each phase angle show that the tip vortices are produced periodically. The slipstream contraction occurs in the near-wake region up to about x= 0.5 D downstream. Thereafter the unstable oscillation occurs due to the separation of tip vortex from the wake sheet behind the maximum contraction point. As the tip vortex evolves downstream, its strength is reduced due to turbulent diffusion, viscous dissipation, and active mixing between tip vortices and adjacent wake flow. The technique presented here can be readily extended to investigate the nominal and effective wake distribution as well as the details of the flow field fore and aft of a rotating propeller behind a ship model.

Effect of Upstream Rotor Vortical Disturbances on the Time-Averaged Performance of Axial Compressor Stators: Part 1—Framework of Technical Approach and Wake–Stator Blade Interactions

1999 ◽  
Vol 121 (3) ◽  
pp. 377-386 ◽  
Author(s):  
T. V. Valkov ◽  
C. S. Tan
Keyword(s):  
Total Pressure ◽  
Axial Compressor ◽  
Tip Vortex ◽  
Technical Approach ◽  
Tip Vortices ◽  
Tip Leakage ◽  
Unsteady Interaction ◽  
Flow Approximation ◽  
Stator Blade ◽  
The Impact

In a two-part paper, key computed results from a set of first-of-a-kind numerical simulations on the unsteady interaction of axial compressor stators with upstream rotor wakes and tip leakage vortices are employed to elucidate their impact on the time-averaged performance of the stator. Detailed interrogation of the computed flow field showed that for both wakes and tip leakage vortices, the impact of these mechanisms can be described on the same physical basis. Specifically, there are two generic mechanisms with significant influence on performance: reversible recovery of the energy in the wakes/tip vortices (beneficial) and the associated nontransitional boundary layer response (detrimental). In the presence of flow unsteadiness associated with rotor wakes and tip vortices, the efficiency of the stator under consideration is higher than that obtained using a mixed-out steady flow approximation. The effects of tip vortices and wakes are of comparable importance. The impact of stator interaction with upstream wakes and vortices depends on the following parameters: axial spacing, loading, and the frequency of wake fluctuations in the rotor frame. At reduced spacing, this impact becomes significant. The most important aspect of the tip vortex is the relative velocity defect and the associated relative total pressure defect, which is perceived by the stator in the same manner as a wake. In Part 1, the focus will be on the framework of technical approach, and the interaction of stator with the moving upstream rotor wakes.

2D and 2.5D Numerical Simulations for Vortex-Induced Vibration (VIV) of a 1.5:1 Rectangular Cylinder

2021 ◽  
Author(s):  
Bowen Yan ◽  
Yangjin Yuan ◽  
Dalong Li ◽  
Ke Li ◽  
Qingshan Yang ◽  
...  
Keyword(s):  
Wind Speed ◽  
Numerical Simulations ◽  
Phase Difference ◽  
Vortex Shedding ◽  
Lift Force ◽  
Small Scale ◽  
Aerodynamic Damping ◽  
Displacement Response ◽  
Wind Speeds ◽  
Rectangular Cylinder

The semi-periodic vortex-shedding phenomenon caused by flow separation at the windward corners of a rectangular cylinder would result in significant vortex-induced vibrations (VIVs). Based on the aeroelastic experiment of a rectangular cylinder with side ratio of 1.5:1, 2-dimensional (2D) and 2.5-dimensional (2.5D) numerical simulations of the VIV of a rectangular cylinder were comprehensively validated. The mechanism of VIV of the rectangular cylinder was in detail discussed in terms of vortex-induced forces, aeroelastic response, work analysis, aerodynamic damping ratio and flow visualization. The outcomes showed that the numerical results of aeroelastic displacement in the cross-wind direction and the vortex-shedding procedure around the rectangular cylinder were in general consistence with the experimental results by 2.5D numerical simulation. In both simulations, the phase difference between the lift and displacement response increased with the reduced wind speed and the vortex-induced resonance (VIR) disappeared at the phase difference of approximately 180∘. The work done by lift force shows a close relationship with vibration amplitudes at different reduced wind speeds. In 2.5D simulations, the lift force of the rectangular cylinder under different wind speeds would be affected by the presence of small-scale vortices in the turbulence flow field. Similarly, the phase difference between lift force and displacement response was not a constant with the same upstream wind speed. Aerodynamic damping identified from the VIV was mainly dependent on the reduced wind speed and negative damping ratios were revealed at the lock-in regime, which also greatly influenced the probability density function (PDF) of wind-induced displacement.

Metal Flow and Die Filling in Coining of Micro Structures with and without Flash

2005 ◽  
Vol 6-8 ◽  
pp. 631-638 ◽  
Author(s):  
M. Thome ◽  
Gerhard Hirt ◽  
B. Rattay
Keyword(s):  
Numerical Simulations ◽  
Sheet Metal ◽  
Metal Forming ◽  
Production Systems ◽  
Metal Flow ◽  
Small Scale ◽  
Die Filling ◽  
Metal Plates ◽  
Micro Structures ◽  
Channel Structures

The continuing miniaturization of production systems and products poses a challenge for metal forming technologies to produce precise small scale products with microscopic geometric details. Thin metal plates with channel structures are considered to be typical examples for microfluidic applications [1,2]. In this study the coining process of sheet metal to produce channel and rib structures is examined in terms of geometrical die parameters and tool design. For this reason extensive experimental series and numerical simulations have been realized and evaluated.

The Fully Developed Wake of a Circular Cylinder

1949 ◽  
Vol 2 (4) ◽  
pp. 451 ◽  
Author(s):  
AA Townsend
Keyword(s):  
Circular Cylinder ◽  
Diffusion Coefficients ◽  
Large Scale ◽  
Turbulent Transport ◽  
Turbulent Energy ◽  
Small Scale ◽  
Mean Velocity ◽  
Turbulent Fluid ◽  
Air Stream ◽  
Scale Motion

Extending previous work on turbulent diffusion in the wake of a circular-cylinder, a series of measurements have been made of the turbulent transport of mean stream momentum, turbulent energy, and heat in the wake of a cylinder of 0.169 cm. diameter, placed in an air-stream of velocity 1280 cm. sec.-1. It has been possible to extend the measurements to 960 diameters down-stream from the cylinder, and it 1s found that, at distances in excess of 600 diameters, the requirements of dynamical similarity are very nearly satisfied. To account for the observed rates of transport of turbulent energy and heat, it is necessary that only part of this transport be due to bulk convection by the slow large-scale motion of the jets of turbulent fluid emitted by the central, fully turbulent core of the wake, which had been supposed previously to perform most of the transport. The remainder of the transport is carried out by the small-scale diffusive motion of the turbulent eddies within the jets, and may be described by assigning diffusion coefficients to the turbulent fluid. It is found that the diffusion coefficients for momentum and heat are approximately equal, but that for turbulent energy is considerably smaller. On the basis of these hypotheses, it is possible to calculate $he form of the mean velocity distribution in good agreement with experiment, and to give a qualitative explanation of the apparently more rapid diffusion of heat.

scholarly journals On the structure of the self-sustaining cycle in separating and reattaching flows

2018 ◽  
Vol 857 ◽  
pp. 907-936 ◽  
Author(s):  
A. Cimarelli ◽  
A. Leonforte ◽  
D. Angeli
Keyword(s):  
Shear Layer ◽  
Rectangular Plate ◽  
Large Scale ◽  
Reverse Flow ◽  
Leading Edge ◽  
Streamwise Velocity ◽  
The Self ◽  
Small Scale ◽  
Spectral Evolution ◽  
Mean Velocity

The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.

scholarly journals Computation of vortex sheet roll-up in the Trefftz plane

1987 ◽  
Vol 184 ◽  
pp. 123-155 ◽  
Author(s):  
Robert Krasny
Keyword(s):  
Mesh Size ◽  
Vortex Sheet ◽  
Smoothing Parameter ◽  
Tip Vortex ◽  
Tip Vortices ◽  
Time Calculation ◽  
Long Time ◽  
Principal Value ◽  
Different Levels ◽  
Good Agreement

Two vortex-sheet evolution problems arising in aerodynamics are studied numerically. The approach is based on desingularizing the Cauchy principal value integral which defines the sheet's velocity. Numerical evidence is presented which indicates that the approach converges with respect to refinement in the mesh-size and the smoothing parameter. For elliptic loading, the computed roll-up is in good agreement with Kaden's asymptotic spiral at early times. Some aspects of the solution's instability to short-wavelength perturbations, for a small value of the smoothing parameter, are inferred by comparing calculations performed with different levels of computer round-off error. The tip vortices’ deformation, due to their mutual interaction, is shown in a long-time calculation. Computations for a simulated fuselage-flap configuration show a complicated process of roll-up, deformation and interaction involving the tip vortex and the inboard neighbouring vortices.

Whistler Waves' Propagation in Plasmas With Systems of Small‐Scale Density Irregularities: Numerical Simulations and Theory

2019 ◽  
Vol 124 (6) ◽  
pp. 4739-4760 ◽  
Author(s):  
I. Yu. Zudin ◽  
T. M. Zaboronkova ◽  
M. E. Gushchin ◽  
N. A. Aidakina ◽  
S. V. Korobkov ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Small Scale ◽  
Whistler Waves ◽  
Waves Propagation

Transient growth in the near wake region of the flow past a finite span wing

2019 ◽  
Vol 866 ◽  
pp. 399-430 ◽  
Author(s):  
Navrose ◽  
V. Brion ◽  
L. Jacquin
Keyword(s):  
Periodic Structures ◽  
Time Integration ◽  
Tip Vortex ◽  
Wake Region ◽  
Wind Tunnel Experiments ◽  
Near Wake ◽  
Optimal Perturbation ◽  
Tip Vortices ◽  
Naca0012 Airfoil ◽  
Flow Past

We investigate optimal perturbation in the flow past a finite aspect ratio ($AR$) wing. The optimization is carried out in the regime where the fully developed flow is steady. Parametric study over time horizon ($T$), Reynolds number ($Re$), $AR$, angle of attack and geometry of the wing cross-section (flat plate and NACA0012 airfoil) shows that the general shape of linear optimal perturbation remains the same over the explored parameter space. Optimal perturbation is located near the surface of the wing in the form of chord-wise periodic structures whose strength decreases from the root towards the tip. Direct time integration of the disturbance equations, with and without nonlinear terms, is carried out with linear optimal perturbation as initial condition. In both cases, the optimal perturbation evolves as a downstream travelling wavepacket whose speed is nearly the same as that of the free stream. The energy of the wavepacket increases in the near wake region, and is found to remain nearly constant beyond the vortex roll-up distance in nonlinear simulations. The nonlinear wavepacket results in displacement of the tip vortex. In this situation, the motion of the tip vortex resembles that observed during vortex meandering/wandering in wind tunnel experiments. Results from computation carried out at higher $Re$ suggest that, even beyond the steady flow regime, a perturbation wavepacket originating near the wing might cause meandering of tip vortices.

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