Cross-correlation analysis of synchronized PIV and microphone measurements of an oscillating airfoil. - In: Journal of visualization, ISSN 1875-8975, Bd. 21 (2018), 3, S. 381-395
The present study focuses on the correlation between the flow structures evolving during the dynamic stall processes of a two-dimensional NACA64-618 airfoil, which performs a sinusoidal movement about its quarter chord axis, and their aeroacoustic response in the far field. Experiments are conducted in an anechoic wind tunnel at a Reynolds number of 8 × 10^5 based on the chord length and include simultaneous velocity field measurements in the vicinity of the airfoil and microphone measurements in the acoustic far field. A causality correlation method based on phase locked snapshots of the velocity field allows for the identification of specific structures at different phases of the dynamic stall life cycle that contribute to the sound generation process. The sound emission during the stall development and flow reattachment phases is attributed to coherent structures evolving downstream of the trailing edge. When the flow is fully stalled, the region that contributes to the sound emission increases. The position of the sound emitting coherent structures also fluctuates stronger between oscillation cycles during full stall.
https://doi.org/10.1007/s12650-018-0473-7
Computational domain length and Reynolds number effects on large-scale coherent motions in turbulent pipe flow. - In: Journal of turbulence, ISSN 1468-5248, Bd. 19 (2018), 3, S. 274-295
We present results from direct numerical simulations (DNS) of turbulent pipe flow at shear Reynolds numbers up to Re T = 1500 using different computational domains with lengths up to L/R = 42. The objectives are to analyse the effect of the finite size of the periodic pipe domain on large flow structures in dependency of Re T and to assess a minimum L/R required for relevant turbulent scales to be captured and a minimum Re T for very large-scale motions (VLSM) to be analysed. Analysing one-point statistics revealed that the mean velocity profile is invariant for L/R ≥ 7. The wall-normal location at which deviations occur in shorter domains changes strongly with increasing Re T from the near-wall region to the outer layer, where VLSM are believed to live. The root mean square velocity profiles exhibit domain length dependencies for pipes shorter than 14R and 7R depending on Re T. For all Re T, the higher-order statistical moments show only weak dependencies and only for the shortest domain considered here. However, the analysis of one- and two-dimensional pre-multiplied energy spectra revealed that even for larger L/R, not all physically relevant scales are fully captured, even though the aforementioned statistics are in good agreement with the literature. We found to be sufficiently large to capture VLSM-relevant turbulent scales in the considered range of Re T based on our definition of an integral energy threshold of 10%. The requirement to capture at least 1/10 of the global maximum energy level is justified by a 14% increase of the streamwise turbulence intensity in the outer region between Re T = 720 and 1500, which can be related to VLSM-relevant length scales. Based on this scaling anomaly, we found Re T 1500 to be a necessary minimum requirement to investigate VLSM-related effects in pipe flow, even though the streamwise energy spectra does not yet indicate sufficient scale separation between the most energetic and the very long motions.
https://doi.org/10.1080/14685248.2017.1418086
A local sensor for joint temperature and velocity measurements in turbulent flows. - In: Review of scientific instruments, ISSN 1089-7623, Bd. 89 (2018), 1, S. 015005, insges. 12 S.
We present the principle for a micro-sensor aimed at measuring local correlations of turbulent velocity and temperature. The operating principle is versatile and can be adapted for various types of flow. It is based on a micro-machined cantilever, on the tip of which a platinum resistor is patterned. The deflection of the cantilever yields an estimate for the local velocity, and the impedance of the platinum yields an estimate for the local temperature. The velocity measurement is tested in two turbulent jets: one with air at room temperature which allows us to compare with well-known calibrated reference anemometers, and another one in the GReC jet at CERN with cryogenic gaseous helium which allows a much larger range of resolved turbulent scales. The recording of temperature fluctuations is tested in the Barrel of Ilmenau which provides a controlled turbulent thermal flow in air. Measurements in the wake of a heated or cooled cylinder demonstrate the capability of the sensor to display the cross correlation between temperature and velocity correctly.
https://doi.org/10.1063/1.4989430
Subgrid scale modelling of relaminarization effects in a differentially heated channel. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 499-508
https://doi.org/10.1007/978-3-319-64519-3_45
Aerodynamic investigations of the effects of virtual coupling on two next generation trains. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 695-704
https://doi.org/10.1007/978-3-319-64519-3_62
Revisiting the higher-order statistical moments in turbulent pipe flow using direct numerical simulations. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 75-84
https://doi.org/10.1007/978-3-319-64519-3_7
Direct numerical simulation of turbulent mixed convection in a vertical channel for the assessment of relaminarization effects on the hot wall. - In: Turbulence and interactions, (2018), S. 271-277
https://doi.org/10.1007/978-3-319-60387-2_30
Experimental study of turbulent Rayleigh-Bénard convection using large-scale Tomo-PIV and high-density PTV. - In: Turbulence and interactions, (2018), S. 225-231
https://doi.org/10.1007/978-3-319-60387-2_24
On the convergence and scaling of high-order statistical moments in turbulent pipe flow using direct numerical simulations. - In: Physics of fluids, ISSN 1089-7666, Bd. 29 (2017), 12, 125105, insges. 13 S.
https://doi.org/10.1063/1.4996882
Acoustic particle velocity investigations in aeroacoustics synchronizing PIV and microphone measurements. - In: Towards a quieter future, (2016), S. 5845-5855