tip vortices
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2021 ◽  
Vol 933 ◽  
Author(s):  
Yong Cao ◽  
Tetsuro Tamura ◽  
Dai Zhou ◽  
Yan Bao ◽  
Zhaolong Han

This study topologically describes near-wall flows around a surface-mounted cylinder at a high Reynolds number ( $Re$ ) of $5\times 10^4$ and in a very thick boundary layer, which were partially measured or technically approximated from the literature. For complete and rational flow construction, we use high-resolution simulations and critical-point theory. The large-scale near-wake vortex is composed of two connected segments rolled up from the sides of the cylinder and from the free end. Another large-scale side vortex clearly roots on two notable foci on the lower side wall. In the junction region, the side vortex moves upwards with a curved trajectory, which induces the formation of nodes on the ground surface. In the free-end region, the side vortex is compressed, which results in a smaller trailing-edge vortex and its downstream movement. Only tip vortices are observed in the far wake. The origin of the tip vortices and their distinction from the near-wake vortex are discussed. Further analyses suggest that $Re$ independence should be treated with high caution when $Re$ increases from 500 to ${O}(10^4)$ . The occurrence of upwash flow behind the cylinder strongly depends on the increase in $Re$ , the mechanism of which is also provided. The separation–reattachment process in the junction region and the trailing-edge vortices are discovered only at a high $Re$ . The former should significantly affect the strength of the side vortex in the junction region and the latter should cause a sharp drop in pressure near the trailing edge.


2021 ◽  
Vol 6 (6) ◽  
pp. 1413-1425
Author(s):  
David H. Wood ◽  
Eric J. Limacher

Abstract. The flow upwind of an energy-extracting horizontal-axis wind turbine expands as it approaches the rotor, and the expansion continues in the vorticity-bearing wake behind the rotor. The upwind expansion has long been known to influence the axial momentum equation through the axial component of the pressure, although the extent of the influence has not been quantified. Starting with the impulse analysis of Limacher and Wood (2020), but making no further use of impulse techniques, we derive its exact expression when the rotor is a circumferentially uniform disc. This expression, which depends on the radial velocity and the axial induction factor, is added to the thrust equation containing the pressure on the back of the disc. Removing the pressure to obtain a practically useful equation shows the axial induction in the far wake is twice the value at the rotor only at high tip speed ratio and only if the relationship between vortex pitch and axial induction in non-expanding flow carries over to the expanding case. At high tip speed ratio, we assume that the expanding wake approaches the Joukowsky model of a hub vortex on the axis of rotation and tip vortices originating from each blade. The additional assumption that the helical tip vortices have constant pitch allows a semi-analytic treatment of their effect on the rotor flow. Expansion modifies the relation between the pitch and induced axial velocity so that the far-wake area and induction are significantly less than twice the values at the rotor. There is a moderate decrease – about 6 % – in the power production, and a similar size error occurs in the familiar axial momentum equation involving the axial velocity.


2021 ◽  
Author(s):  
Rodrigo Soto-Valle ◽  
Stefano Cioni ◽  
Sirko Bartholomay ◽  
Marinos Manolesos ◽  
Christian Navid Nayeri ◽  
...  

Abstract. This study describes the impact of postprocessing methods on the calculated parameters of tip vortices of a wind turbine model when tested using Particle Image Velocimetry (PIV). Several vortex identification methods and differentiation schemes are compared. The chosen methods are based on two components of the velocity field and its derivatives. They are applied to each instantaneous velocity field from the dataset and also to the calculated average velocity field. The methodologies are compared through the vortex center location, vortex core radius and jittering zone. Results show that the tip vortex center locations and radius have good comparability and can vary only a few grid spacings between methods. Conversely, the convection velocity and the jittering surface, defined as the area where the instantaneous vortex centers are located, vary between identification methods. Overall, the examined parameters depend significantly on the post-processing method and selected vortex identification criteria. Therefore, this study proves that the selection of the most suitable postprocessing methods of PIV data is pivotal to ensure robust results.


2021 ◽  
Vol 62 (9) ◽  
Author(s):  
Johannes N. Braukmann ◽  
Andreas Goerttler ◽  
C. Christian Wolf ◽  
Clemens Schwarz ◽  
Markus Raffel

Abstract An investigation into blade tip vortices of a sub-scale rotor under cyclic pitch conditions is carried out. Background oriented schlieren (BOS), particle image velocimetry (PIV), and computational fluid dynamics (CFD) are applied to the same test cases. This approach allows to combine the velocity data from PIV in a measurement plane, the density related data from BOS in a measurement volume, and the comprehensive set of flow variables provided by unsteady detached eddy simulations. Vortices up to an age of $$\varPsi _{{\text {v}}}= {70}^\circ$$ Ψ v = 70 ∘ in case of PIV and CFD, and up to $$\varPsi _{{\text {v}}}= {200}^\circ$$ Ψ v = 200 ∘ in case of BOS are considered. The vortex locations are obtained through all three techniques. The unsteadiness of the vortices was obtained by the experimental results, whereas CFD provides an average solution. An increased position scatter was observed during the downstroke of the pitch cycle with both experimental methods and was found to be in good agreement. In the second part, the PIV velocity data are compared to common vortex models. An approach to link the density distribution and the swirl velocity is applied to the measured data. Based on the CFD results, it is shown that the assumption of isothermal flow yields better agreement between velocity and density than isentropic flow. Graphic abstract


Author(s):  
R. Asad Ahmed ◽  
S. Syam Narayanan ◽  
Anandh C.M. Joseph

The application of bio-mimetic in Micro Aerial Vehicles (MAV) is a developing field and has enormous applications in the aviation industry. In this work, experimental investigation of the insect like MAV having flapping wing is discussed. The iphiclus-egensis-aile flexible wing and the nearctic-ceratopogonidae flexible wing were chosen for this study. Values of the coefficient of lift for the unsteady aerodynamics were converted to normalized frequency domain using Welch power spectral density method at various flapping frequencies and Reynolds number. The wing tip vortices were captured using laser visualization technique and further the images were processed using MATLAB. Experimental investigations were carried out in wind tunnel for various velocities and Angle of Attack (AoA) and the results were obtained using digital six component balancing system. Detailed study of both the results shows that the nearctic-ceratopogonidae flexible wing has better lifting properties than the iphiclus-egensis-aile flexible wing at low flapping frequencies and low AoA. For higher flapping frequencies and high AoA, the latter shows better lifting properties than the former.


2021 ◽  
Vol 10 (1) ◽  
Author(s):  
Azhim Asyratul Azmi ◽  
Satriawan Dini Hariyanto ◽  
Arif Hidayat

A telescopic wing is a shape-changing method of the aircraft wing known as the morphing wing system. Wingspan extends capability on telescopic wing increasing the aspect ratio to get a high lift force. The telescopic wing on a flying wing configuration as an external wing and glider wing as an internal wing can be used to increase the coefficient lift (CL) when carrying out special missions. The aerodynamic characteristics using the Computational Fluid Dynamic (CFD) simulation approach is presented. For the 40% internal wingspan, the highest CL increment was 12.9% at a 10o angle of attack. For the 50% internal wingspan, the highest CL increment was 14.9% at a 10o angle of attack. on the 40% internal wing, the highest coefficient drag (CD) increment was 4.7%, and the largest CD increment on 50% internal was 9.5% at the angle of attack of 20o. The pressure distribution along the internal wingspan was uneven from an angle of attack of 15o due to the wing tip vortices of the external wing. Streamline pattern shown a bubble separation from the leading edge at an internal wing root by external wing tip vortices.Keywords: Morphing wing, telescopic wing, flying wing, glider


2021 ◽  
Vol 920 ◽  
Author(s):  
Antonio Posa ◽  
Riccardo Broglia ◽  
Elias Balaras
Keyword(s):  

Abstract


2021 ◽  
Author(s):  
David Wood ◽  
Eric Limacher

Abstract. Upwind of an energy-extracting horizontal-axis wind turbine, the flow expands as it approaches the rotor, and the expansion continues in the vorticity-bearing wake behind the rotor. The upwind expansion has long been known to influence the axial momentum equation through the axial component of the pressure, although the extent of the influence has not been quantified. Starting with the impulse analysis of Limacher & Wood (2020), but making no further use of impulse techniques, we demonstrate that the expansion redistributes momentum from the external flow to the wake and derive its exact expression when the rotor is circumferentially uniform. This expression, which depends on the radial velocity and the axial induction factor, is added to the thrust equation containing the pressure on the back of the disk. Removing the pressure to obtain a practically useful equation shows the axial induction in the far-wake is twice the value at the rotor only at high tip speed ratio and only if the relationship between vortex pitch and axial induction in non-expanding flow carries over to the expanding case. At high tip speed ratio, we assume that the expanding wake approaches the "Joukowsky'' model of a hub vortex on the axis of rotation and tip vortices originating from each blade. The additional assumption that the helical tip vortices have constant pitch, allows a semi-analytic treatment of their effect on the rotor flow. Expansion modifies the relation between the pitch and induced axial velocity so that the far-wake area and induction are significantly less than twice the values at the rotor. There is a moderate decrease – about 6 % – in the power production and a similar size error occurs in the familiar axial momentum equation involving the axial velocity.


2021 ◽  
Vol 2 (396) ◽  
pp. 13-36
Author(s):  
A. Pustoshny ◽  
◽  
A. Koval ◽  

Object and purpose of research. The object of the research is the cavitation noise of transport vessels propellers. The purpose is to analyze and generalize modern research and methods for predicting broadband noise induced by the operation of the transport vessels cavitating propellers. Materials and methods. A review of the research materials (published in the XXI century) on the cavitation noise of transport vessels propellers, in particular broadband noise, as well as the author's own research on the causes and physical aspects of the occurrence of broadband noise is carried out. Based on the Fourier analysis of the time functions of velocities and pressures in the flow near the propeller, the conditions for the occurrence of broadband noise at (4–10)-blade harmonics and the parameters that relate this noise to various types of cavitation on the propeller are found. Main results. A review of recent studies results devoted to the cavitation noise of transport vessels propellers has shown that the levels of the so-called broadband noise that occurs when the cavitating propeller operates in nonuniform flow at high (4–10)-blade harmonics are used as the basis for modern noise predicting methods. At the same time, the authors of the methods consider broadband noise only as a result of the tip vortices development. The paper considers and generalizes an alternative hypothesis of the broadband noise occurrence caused by the occurrence and collapse of cavities within the blade with the formation of a double-headed peak on the time function of pressures. The analysis of publications on the development of the vortex wake behind the propeller and the modeling of pressures at the buckling failure of the tip vortex spirals allowed to show that double pressure peaks can be formed in the flow during the development of the vortex wake behind the propeller. This made it possible to combine both hypotheses of the broadband noise formation – from the development of the tip vortex and from the occurrence and collapse of cavities on the blades – and to link the assessment of the growth possibility of high blade harmonics with the cavitation parameters. Conclusions. It is shown that modern research methods have made it possible to obtain new data on the mechanism of the occurrence and collapse of cavitation both on the blade and in the tip vortex. At the same time, the currently used methods for predicting cavitation and, in particular, broadband noise are very approximate and require refinement to assess the effect of various cavitation characteristics on broadband noise. The hypotheses of broadband noise occurrence and the proposed physical model covering both vortex cavitation and the formation and collapse of cavities can serve as a basis for such clarification.


Energies ◽  
2021 ◽  
Vol 14 (9) ◽  
pp. 2613
Author(s):  
Fabrizio De Gregorio ◽  
Antonio Visingardi ◽  
Gaetano Iuso

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.


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