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Calibri 83ffff̙̙3f3fff3f3f33333f33333.ʏTU Delft Repositoryg Zuuidrepository linktitleauthorcontributorpublication yearabstract
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departmentresearch group programmeprojectcoordinates)uuid:cd6fe4c6-c420-4e88-b711-efd5d86714dbDhttp://resolver.tudelft.nl/uuid:cd6fe4c6-c420-4e88-b711-efd5d86714db`Three-dimensional vortex analysis and aeroacoustic source characterization of jet core breakdownViolato, D.; Scarano, F.
The three-dimensional behavior of jet core breakdown is investigated with experiments conducted on a free water jet at Re = 5000 by time-resolved tomographic particle image velocimetry (TR-TOMO PIV). The investigated domain encompasses the range between 0 and 10 jet diameters. The characteristic pulsatile motion of vortex ring shedding and pairing culminates with the growth of four primary in-plane and out-of-plane azimuthal waves and leads to the formation of streamwise vortices. Vortex ring humps are tilted and ejected along the axial direction as they are subjected to higher axial velocities. By the end of the potential core, this process causes the breakdown of the vortex ring regime and the onset of streamwise filaments oriented at 30-45 to the jet axis and C shaped peripheral structures. The latter re-organize further downstream in filaments oriented along the azimuthal direction at the jet periphery. Instead, in the vicinity of the jet axis the filaments do not exhibit any preferential direction resembling the isotropic turbulent regime. Following Powell's aeroacoustic analogy, the instantaneous spatial distribution of the acoustic source term is mapped by the second time derivative of the Lamb vector, revealing the highest activity during vortex ring breakdown. A three-dimensional modal analysis of velocity, vorticity, Lamb vector, and Lamb vector second time derivative fields is conducted by proper orthogonal decomposition (POD) within the first 10 modes. The decomposed velocity fluctuations describe a helical organization in the region of the jet core-breakdown and, further downstream, jet axis flapping and precession motions. By the end of the potential core, vorticity modes show that vortex rings are dominated by travelling waves of radial and axial vorticity with a characteristic 40-45 inclination to the jet axis. The Lamb vector and the Lamb vector second time derivative modes exhibit similar patterns for the azimuthal component, whereas the vortex ring coherence is described by the radial and the axial components. While velocity, vorticity, and Lamb vector modes are typically associated with Strouhal numbers (St) smaller than 0.9, the modes of the Lamb vector second time derivative are also related to higher frequencies (1.05 ? St ? 1.9) ascribed to the three-dimensional travelling waves. Far-field acoustic predictions are obtained on the basis of direct evaluation of Powell's analogy with TR-TOMO PIV data. The spectral analysis returns peaks at pairing (St = 0.36) and shedding (St = 0.72) frequency. A broader distribution with a hump between St = 1 and 2.25 is observed, which corresponds to the breakdown of ring vortices.Zaeroacoustics; flow visualisation; jets; pulsatile flow; turbulence; vortices; water wavesenjournal articleAmerican Institute of PhysicsAerospace Engineering&Aerodynamics, Wind Energy & Propulsion)uuid:bbd816bb-3f8c-4e5e-b956-7ab9eee328f3Dhttp://resolver.tudelft.nl/uuid:bbd816bb-3f8c-4e5e-b956-7ab9eee328f3XThe three-dimensional flow organization past a micro-ramp in a supersonic boundary layer<Sun, Z.; Schrijer, F.F.J.; Scarano, F.; Van Oudheusden, B.W.&The three-dimensional instantaneous flow organization in the near wake of a micro-ramp interacting with a Mach 2.0 supersonic turbulent boundary layer is studied using tomographic particle image velocimetry. The mean flow reveals a wake with approximately circular cross section dominated by a pair of counter-rotating streamwise vortices generating a focused upwash motion at the symmetry plane. In the instantaneous flow organizat< ion a flow instability of Kelvin-Helmholtz (K-H) type is observed in the shear layer between the wake and outer flow. Intermittent arc-shaped vortices are visualized that locally accelerate the outer fluid and further decelerate the inner region. The streamwise vortex pair displays an undulating behavior. Their interaction with the K-H vortices considerably increases the overall complexity of the wake. It appears that the streamwise vortex filaments under the K-H vortex train approach each other due to the focused ejection activity resulting from the K-H vortex. The statistical properties of turbulent fluctuations yield maximum activity at the core of the streamwise vortex and within the upwash region, and the Reynolds stresses peak within the shear layer. The topological organization of the wake vortices is formulated through a conditional average over the vorticity field.boundary layer turbulence; external flows; flow visualisation; fluctuations; Kelvin-Helmholtz instability; shear turbulence; statistical analysis; supersonic flow; vortices; wakesAerodynamics & Wind Energy)uuid:9474c415-f726-48ed-ae85-91cfe103b5bbDhttp://resolver.tudelft.nl/uuid:9474c415-f726-48ed-ae85-91cfe103b5bb9An experimental study of transitional pulsatile pipe flow0Trip, R.; Kuik, D.J.; Westerweel, J.; Poelma, C.The transitional regime of a sinusoidal pulsatile flow in a straight, rigid pipe is investigated using particle image velocimetry. The main aim is to investigate how the critical Reynolds number is affected by different pulsatile conditions, expressed as the Womersley number and the oscillatory Reynolds number. The transition occurs in the region of Re?=?2250-3000 and is characterized by an increasing number of isolated turbulence structures. Based on velocity fields and flow visualizations, these structures can be identified as puffs, similar to those observed in steady flow transition. Measurements at different Womersley numbers yield similar transition behavior, indicating that pulsatile effects do not play a role in the regime that is investigated. Variations of the oscillatory Reynolds number also appear to have little effect, so that the transition here seems to be determined only by the mean Reynolds number. For larger mean Reynolds numbers, a second regime is observed: here, the flow remains turbulent throughout the cycle. The turbulence intensity varies during the cycle, but has a phase shift with respect to the mean flow component. This is caused by a growth of kinetic energy during the decelerating part and a decay during the accelerating part of the cycle. Flow visualization experiments reveal that the flow develops localized turbulence at several random axial positions. The structures quickly grow to fill the entire pipe in the decelerating phase and (partially) decay during the accelerating phase.oflow visualisation; fluid oscillations; laminar to turbulent transitions; pipe flow; pulsatile flow; turbulence.Mechanical, Maritime and Materials EngineeringProcess and Energy)uuid:36a6086e-ced5-4396-8161-42e50a775a9eDhttp://resolver.tudelft.nl/uuid:36a6086e-ced5-4396-8161-42e50a775a9eXThree-dimensional evolution of flow structures in transitional circular and chevron jetsThe three-dimensional behavior of flow transition in circular and 6-chevron jets at Re?=?5000 is investigated with experiments conducted on a free water jet by time-resolved tomographic particle image velocimetry. The emphasis is on the unsteady organization of coherent flow structures, which play a role in the generation of acoustic noise. Shedding and pairing of vortices are the most pronounced phenomena observed in the near field of the circular jet. The first and second pairing amplify the axial pulsatile motion in the jet column and lead to the growth of azimuthal waves culminating in the breakup of the vortex ring. Streamwise vortices of axial and radial vorticity are observed in the outer region and move inward and outward under the effect of the vortex rings. In the jet with chevrons, the axisymmetric ring-like coherence of the circular jet is not encounter< ed. Instead, streamwise flow structures of azimuthal vorticity emanate from the chevron apices, and counter-rotating streamwise vortices of axial and radial vorticity develop from the chevron notches. The decay of streamwise vortices is accompanied by the formation of C-shaped structures. The three-dimensional analysis allows quantifying the vortex stretching and tilting activity, which, for the circular jet exit, is related to the azimuthal instabilities and the streamwise vortices connecting the vortex rings. In the chevron jet, stretching and tilting peak during the formation of C-structures. Following Powell s aeroacoustic analogy, the spatial distribution of the source term is mapped, evaluating the temporal derivative of the Lamb vector. The spatio-temporal evolution of such source term is visualized revealing that the events of highest activity are associated with the processes of vortex-ring pairing and vortex-ring disruption for the circular jet, and with the decay of streamwise instabilities and the formation of C-shaped structures for the chevron case.yacoustic noise; aeroacoustics; flow instability; flow visualisation; jets; spatiotemporal phenomena; turbulence; vortices)uuid:9fd7c4dc-50f6-4b41-a9a8-3a6a131f20f8Dhttp://resolver.tudelft.nl/uuid:9fd7c4dc-50f6-4b41-a9a8-3a6a131f20f8WA study on the application of two different acoustic analogies to experimental PIV data-Koschatzky, V.; Westerweel, J.; Boersma, B.J.The aim of the present study is to compare two different acoustic analogies applied to time-resolved particle image velocimetry (PIV) data for the prediction of the acoustic far-field generated by the flow over a rectangular cavity. We consider the model problem of sound radiating from an open, two-dimensional, shallow cavity with an aspect ratio of 2 at a Reynolds number of 3.0??104 (based on the cavity length). The study is carried out by simultaneous high-speed two-dimensional PIV and sound measurements. The instantaneous flow field is obtained from the PIV measurements. The emitted sound is then calculated using Curle s analogy and Vortex Sound Theory. To our knowledge, Vortex Sound Theory is used here for the first time in combination with time-resolved PIV data. The acoustic analogies are derived through rather different pathways, and the mathematical schemes used to solve the equations are sensitive in a different way to factors such as data resolution, noise level, and complexity of the geometry. Both methods indicate that the trailing edge of the cavity is the main sound source. The predictions of the acoustic field obtained by applying the two methods are analyzed and compared with the measured sound. For the presented case, the results show that both analogies estimate the overall sound pressure level quite well and that they give very similar results, both in total intensity and in the spectral distribution of the emitted sound.acoustic emission; acoustic field; acoustic intensity; acoustic measurement; acoustic noise; aeroacoustics; boundary layers; confined flow; external flows; flow visualisation; laminar flow; vortices)uuid:0f9c7c00-a527-4d2b-8788-c940b70ca7d6Dhttp://resolver.tudelft.nl/uuid:0f9c7c00-a527-4d2b-8788-c940b70ca7d6JA stereo vision method for tracking particle flow on the weld pool surfaceCZhao, C.X.; Richardson, I.M.; Kenjeres, S.; Kleijn, C.R.; Saldi, Z.The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained< using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool./flow visualisation; fluid oscillations; welding!Materials Science and Engineering)uuid:da77cd4c-6fe8-41cb-87c5-0e274401559dDhttp://resolver.tudelft.nl/uuid:da77cd4c-6fe8-41cb-87c5-0e274401559d<Unsteady flow organization of compressible planar base flows/Humble, R.A.; Scarano, F.; Van Oudheusden, B.W.The unsteady flow features of a series of two-dimensional, planar base flows are examined, within a range of low-supersonic Mach numbers in order to gain a better understanding of the effects of compressibility on the organized global dynamics. Particle image velocimetry is used as the primary diagnostic tool in order to characterize the instantaneous near wake behavior, in combination with data processing using proper orthogonal decomposition. The results show that the mean flowfields are simplified representations of the instantaneous flow organizations. Generally, each test case can be characterized by a predominant global mode, which undergoes an evolution with compressibility, within the Mach number range considered. (The term global mode is defined herein as an organized global dynamical behavior of the near wake region, recognizing that the near wake dynamics may be describable in terms of several global modes.) At Mach 1.46, the predominant global mode can be characterized by a sinuous or flapping motion. With increasing compressibility, this flapping mode decreases, and the predominant global mode evolves into a pulsating motion aligned with the wake axis at Mach 2.27. These global modes play an important role in the distributed nature of the turbulence properties. The turbulent mixing processes become increasingly confined to a narrower redeveloping wake with increasing compressibility. Global maximum levels of the streamwise turbulence intensity and the kinematic Reynolds shear stress occur within the vicinity of the mean reattachment location, and show no systematic trend with compressibility. In contrast, the global maximum level of the vertical turbulence intensity moves upstream from the redeveloping wake toward the mean reattachment location. The vertical turbulence intensity decays thereafter more slowly than the other turbulence quantities. Overall, the local maximum levels of the turbulence properties decrease appreciably with increasing compressibility.Icompressible flow; supersonic flow; flow visualisation; wakes; turbulence)uuid:115c96a3-6c4e-48c1-a698-f831829d184cDhttp://resolver.tudelft.nl/uuid:115c96a3-6c4e-48c1-a698-f831829d184cSA linear approach for the evolution of coherent structures in shallow mixing layers&Van Prooijen, B.C.; Uijttewaal, W.S.J.rThe development of large coherent structures in a shallow mixing layer is analyzed. The results are validated with experimental data obtained from particle tracking velocimetry. The mean flow field is modeled using the self-similarity of the velocity profiles. The characteristic features of the down-stream development of a shallow mixing layer flow, like the decrease of the velocity difference over the mixing layer, the decreasing growth of the mixing layer width, and the transverse shift of the center of the mixing layer layer are fairly well represented. It turned out that the entrainment coefficient could be taken constant, equal to a value obtained for unbounded mixing layers: ? = 0.085. Linearization of the shallow water equations leads to a modified Orr Sommerfeld equation, with turbulence viscosity and bottom friction as dissipative terms. Growth rates are obtained for each position downstream, using the model for the mean flow field. For a given energy density spectrum at the inflow boundary, integration of the growth rates along the downstream direction yields the spectra at various downstream positions. These spectra provide a measure for the intensity and the length scale of the coheren<ht structures (the dominant mode). The length scales found are in good agreement with the measured ones. The length scale of the most unstable mode appears much larger than the length scale of the dominant mode. Obviously, the longevity of the coherent structures plays a significant role. Three growth regimes can be distinguished: in the first regime the dominant mode is growing, in the second regime the dominant mode is dissipating, but other modes are still growing, and in the third regime all modes are dissipating. It is concluded that the development of the coherent structures in a shallow mixing layer can fairly well be described and interpreted by the proposed linear analysis.crivers; channel flow; mixing; turbulence; flow instability; flow visualisation; hydrology; fractals!Civil Engineering and GeosciencesHydraulic Engineering)uuid:788b804b-f2fe-435f-a360-4eb83e7a5fa7Dhttp://resolver.tudelft.nl/uuid:788b804b-f2fe-435f-a360-4eb83e7a5fa7Progress report of the development of a system for the numerical simulation of Euler flows, with results of preliminary 3D propellersllpstream/exhaust-jet calculationsBoerstoel, J.W.Euler equations of motion; Computational Fluid Dynamics; Supercomputers; Body-wing and tall configurations; Computational grids; Numerical flow visualisation; Propeller slipstreams; Computerized simulation; Topology; Blocks; Three dimensional bodiesreport+Nationaal Lucht- en RuimtevaartlaboratoriumCampus only
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