A novel phase-averaging method based on vortical structure correlation

Aerodynamic

Asymmetric vortex shedding in the wake of an abruptly expanding annular jet

Abstract: In this paper, the structure of the turbulent wake behind the inner tube of a suddenly expanding annular jet flow is studied. The flow field is measured using tomographic particle image velocimetry and analyzed using proper orthogonal decomposition (POD). It was found that both the instantaneous and time-averaged central wakes behind the inner pipe are highly asymmetric despite the axisymmetric structure of the geometry. This asymmetry is the result of a bifurcation at low Reynolds numbers which persists up to the turbulent regime. The asymmetry induces a pair of counter-rotating vortices in the jet which are aligned with the main flow direction. Moreover, the asymmetry also induces a highly dynamical flow field. Analyzing the flow structures using POD shows that the wake oscillates around the asymmetric equilibrium position at a very low Strouhal number in the order of 0.01. On top of this motion, the inner shear layer oscillates with Strouhal numbers in the range of 0.1-0.3. This oscillation causes an asymmetric shedding of vortices of the hairpin type in the inner shear layer. As such, a local asymmetric region of very intensive mixing is induced near the stagnation point. Graphical Abstract: [Figure not available: see fulltext.]Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerodynamic

Visualization of the structure of vortex breakdown in free swirling jet flow

In this paper we investigate the three dimensional flow structures in a free annular swirling jet flow undergoing vortex breakdown. The flow field is analyzed by means of time-resolved Tomographic Particle Image Velocimetry measurements. Both time-averaged and instantaneous flow structures are discussed, including a detailed analysis of the first and second order statistical moments. A Reynolds decomposition of the flow field shows that the time averaged flow is axisymmetric with regions of high anisotropic Reynolds stresses. Two recirculation zones exist with regions of very intensive mixing around them. Despite the axisymmetric nature of the time-averaged flow, a non-axisymmetric structure of the instantaneous flow is revealed, including a central vortex core which breaks up into a double helix. The winding sense of this double helix is opposite to the swirl direction and it is wrapped around the vortex breakdown bubble. The double helix precesses around the central axis of the flow with a precessing frequency corresponding to a Strouhal number of 0.13. To the authors’ knowledge, this structure of vortex breakdown has not been previously reported in the literature to occur for turbulent jet flow and it suggests that the well-known Precessing Vortex Core (PVC) found in swirling jets corresponds to the helical mode of vortex breakdown.Aerodynamic

Double helix vortex breakdown in a turbulent swirling annular jet flow

In this paper, we report on the structure and dynamics of double helix vortex breakdown in a turbulent annular swirling jet. Double helix breakdown has been reported previously for the laminar flow regime, but this structure has rarely been observed in turbulent flow. The flow field is investigated experimentally by means of time-resolved tomographic particle image velocimetry. Notwithstanding the axisymmetric nature of the time-averaged flow, analysis of the instantaneous three-dimensional (3D) vortical structures shows the existence of a vortex core along the central axis which breaks up into a double helix downstream. The winding sense of this double helix is opposite to the swirl direction (m=-2) and it is wrapped around a central vortex breakdown bubble. This structure is quite different from double helix breakdown found in laminar flows where the helix is formed in the wake of the bubble and not upstream. The double helix precesses around the central axis of the jet with a precessing frequency corresponding to a Strouhal number of 0.27.B.W. van OudheusdenAerodynamic

Analysis of the pressure fields in a swirling annular jet flow

In this paper, we investigate the flow structures and pressure fields of a free annular swirling jet flow undergoing vortex breakdown. The flow field is analyzed by means of time-resolved tomographic particle image velocimetry measurements, which enable the reconstruction of the three-dimensional time-resolved pressure fields using the governing flow equations. Both time-averaged and instantaneous flow structures are discussed, including a characterization of the first- and second-order statistical moments. A Reynolds decomposition of the flow field shows that the time-averaged flow is axisymmetric with regions of high anisotropic Reynolds stresses. Two recirculation zones exist that are surrounded by regions of very intense mixing. Notwithstanding the axisymmetric nature of the time-averaged flow, a non-axisymmetric structure of the instantaneous flow is revealed, comprising a central vortex core which breaks up into a precessing vortex core. The winding sense of this helical structure is opposite to the swirl direction and it is wrapped around the vortex breakdown bubble. It precesses around the central axis of the flow at a frequency corresponding to a Strouhal number of 0.27. The precessing vortex core is associated with a low-pressure region along the central axis of the jet and the maximum pressure fluctuations occur upstream of the vortex breakdown location, where the azimuthal velocity component also reaches peak values as a result of the inward motion of the fluid and the conservation of angular momentum. The POD analysis of the pressure fields suggests that the precessing helical vortex formation is the dominant coherent structure in the instantaneous flow.Aerodynamic

Force generation and wing deformation characteristics of the ’DelFly II’ MAV in hovering flight conditions

The study investigates the relation between wing deformation and unsteady force generation of the flapping-wing MAV DelFly II in hovering flight configuration. The three-dimensional wing deformation was measured with a stereo-vision system, while fluid forces were acquired simultaneously with a force sensor. Comparison of the results for different flapping frequencies reveals different wing kinematics and deformation characteristics. The high flapping frequency case produces higher forces throughout the complete flapping cycle. Moreover, a phase difference occurs in the variation of the forces, such that the low flapping frequency case precedes the high frequency case. A similar phase lag is observed in the temporal evolution of the wing deformation characteristics, suggesting that there is a direct link between the two phenomena.AerodynamicsControl & Simulatio

Determination of pressure and load characteristics of flexible revolving wings by means of tomographic PIV

This study explores the flow field and fluid-dynamic loads generated by revolving low-aspect-ratio wings. The pressure field and load characteristics are successfully reconstructed from the phase-locked tomographic measurements in three independently measured volumes along the span of the wing. The vortical structures encompass a low pressure region and the spatial gradient information of the pressure field provides greater insights in their stability mechanisms. The low pressure region associated with the leading edge vortex and its close position to the wing surface are responsible for the high resultant forces acting on the wing. Simultaneous force measurements show a reasonable agreement with the reconstructed loads. The sectional lift and drag characteristics provide greater insights into the distributed load mechanisms along the span.Aerodynamic

Three-dimensional flow and load characteristics of flexible revolving wings

The flow field and fluid-dynamic loads of revolving low-aspect-ratio chordwise-flexible wings are studied experimentally at a Reynolds number of 10,000. The investigation involves phase-locked tomographic particle image velocimetry (PIV) complemented with force measurements. The pressure fields are reconstructed from the three-dimensional velocity fields in a complete volume around the wing. For decreasing flexural stiffness, the coherence of this vortex system and spanwise transport of vorticity along the axis of the leading edge vortex (LEV) increase, which contribute to the stability and retention of the LEV. As the LEV low-pressure region becomes smaller with increasing flexibility, the total force on the wing is reduced, while it is tilted towards the lift direction due to the wing deformation. As a result, the drag is significantly suppressed, while the lift remains relatively high. Consequently, the lift-to-drag ratio increases with increasing flexibility and correlates well with the geometric angle of attack. While the sectional lift along the full span is comparable for the different wings, the sectional drag is significantly reduced at the outboard wing for increasing flexibility. The centroids of lift and drag are located at approximately 70% of the span for all wings throughout the complete revolving motion. Finally, the process of vortex breakdown is found to be related to the formation of a positive spanwise pressure gradient.Aerodynamic

Three-dimensional flow and load characteristics of flexible revolving wings at low Reynolds number

This study explores the flow field and fluid-dynamic loads generated by revolving low-aspect-ratio flat plate wings undergoing a revolving motion starting from rest. Three wings with different degree of chordwise flexural stiffness (i.e., rigid, moderate flexibility and high flexibility) have been tested in order to investigate the influence of the wing flexibility. The wings have an angle of attack of 45 deg in their undeformed condition. The measurements have been performed in a water tank at a Reynolds number of 10,000 based on the chord length and terminal velocity at the 75% span position. The experimental campaign consists of phase-locked tomographic particle image velocimetry measurements complemented with simultaneous force measurements The three-dimensional velocity fields are captured in three measurement volumes positioned side-by-side along the span of the wing for different phases of the revolving motion, generating a time-resolved volumetric velocity field data set. Subsequently, from the velocity data the pressure fields are reconstructed as well as the loads acting on the wing.Aerodynamic

Unsteady flow through a sharp-edged single-hole orifice placed in a pipe

Orifice plates or flow restrictions are key components used in industry for flow measurement and control. They find application in gas and liquid circuits of, e.g., lithography machines, nuclear power plants and aerospace propulsion systems [1]. They are used typically either for measuring flow-rate or to introduce a pressure drop for purposes of flow balancing. It is widely acknowledged in literature that the turbulent, unsteady nature of the flow through an orifice can be a source of structural vibration [Aerodynamic