Coherent structures in Taylor-Couette flow: Experimental investigation
Başlık çevirisi mevcut değil.
- Tez No: 401592
- Danışmanlar: PROF. JERRY WESTERWEEL, DR. GERRIT E. ELSINGA
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2014
- Dil: İngilizce
- Üniversite: Technische Universiteit Delft (Delft University of Technology)
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 154
Özet
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Özet (Çeviri)
Taylor-Couette flow is defined as the flow confined between two coax-ial cylinders which can rotate independently. Several different flow states can be observed in the gap between the cylinders by changing the rota-tion speeds and the rotating directions of the cylinders. As it is a closed environment, the input and the output of the system can be monitored easily, thus the Taylor-Couette flow becomes quite useful for turbulence studies. The aim of this thesis is to investigate the previously reported change of torque values with the rotation speeds of the cylinders and to study its relation to coherent turbulent flow structures. The flow structures are investigated using tomographic PIV, which is a fully volumetric mea-surement method that resolves all three velocity components. Different flow states, and their contribution to the Reynolds stresses are revealed. Initially the validation of the implementation of tomographic PIV was done using the analytically well-defined laminar Taylor-Couette flow at a shear Reynolds number ReS = 615. The results showed that the mea-sured velocities deviate from the analytical solution by not more than 3.2%. Measurements of turbulent flows should ideally have a resolution of the order of the Kolmogorov microscale to resolve the smallest scales in turbulence. However, as the number of reconstructed particle images is typically less than the number of velocity vectors, the actual spatial reso-lution of tomographic PIV is not well defined. Furthermore, the required resolution to resolve smallest scales in turbulence is not known exactly. Therefore, the Taylor-Couette setup was also used to investigate the spa-tial resolution of tomographic PIV, by exploiting the fact that the power input to the system, as determined from the torque measurements and the cylinder rotation speed, is balanced by the viscous dissipation rate,which can be computed using the measured velocities. The comparison reveals that the dissipation rate was underestimated by tomographic PIV for all turbulent cases studied in this thesis (3800 < ReS < 47000). Application of a large eddy turbulence model to the PIV data showed that the error in the direct estimation of the dissipation rate by tomo-graphic PIV can be ascribed to unresolved scales. It was found that the actual spatial resolution of tomographic PIV is dependent on both the interrogation volume overlap, and the interrogation volume size (Di). Increasing the interrogation overlap at a constant DI decreases the er-ror and results in better spatial resolution (although it might result in higher data density that possibly exceeds the tracer particle density). On the other hand it was found that the distance between two neighboring vectors (i.e. vector spacing öx, which depends on both overlap and DI) is a more suitable parameter to quantify the actual spatial resolution of tomographic PIV (and PIV in general), rather than the interrogation volume size. This seems to apply at least up to 75% overlap. The minimum required vector spacing to resolve small-scale motions in the flow was found as 1.5 — 2.0 times the Kolmogorov length scale (corresponding to interrogation volume size of 6.0 — 8.0 times Kolmogorov length scale at 75% overlap). Therefore interrogation volumes that are bigger than the values suggested in the literature might be used instead, as these were based on 50% overlap. Next, time-resolved tomographic PIV measurements were performed at fully turbulent flow to demonstrate the capability of the measurement system and the flow geometry to study dynamic events in turbulence. Turbulent flow with an approximately zero mean velocity was created by rotating the cylinders in opposite directions with the same wall veloc-ities. Using this idea, the observation times of the flow structures could be increased by an order of magnitude as compared to similar studies in turbulent boundary layers. Examples of observed events, such as azimuthal velocity bursts, stretching and breaking-up of vortical struc-tures, are presented. In the literature it was reported that at a constant shear Reynolds number (ReS), the measured torque values change depending on the rotation number (Rq). In Chapter 5 of this thesis, the connection be-tween turbulent flow structures and the change of the torque was made by using tomographic PIV. It was shown that the large-scale turbu-lent flow structures change significantly with Rq in both the mean and the instantaneous flows, which explains the change in the torque. In order to compute the contribution of the large and the smaller-scale structures to the torque, the instantaneous flow was decomposed into the large and the smaller-scale motions by filtering. It was shown that at a constant turbulent Reynolds number, the instantaneous large-scale structures change their orientation from the azimuthal direction (at only inner cylinder rotation), to blobs (at exact counter rotation), and finally to Taylor-column-like structures elongated in the axial direction (at only outer cylinder rotation). The Reynolds stresses associated with these structures indicate that this orientational change may be the mecha-nism responsible for the reported change of the torque scaling. Close to only inner cylinder rotation the mean flow contribute significantly to the angular momentum transport, and it is ineffective elsewhere. The large-scale turbulent structures are not effective on the angular momentum transport in cases close to only inner and only outer cylinder rotation. However, close to exact counter rotation, inclined large-scale structures induce azimuthal and radial velocities simultaneously, which results in higher Reynolds shear stress, hence torque. The smaller-scales were found only to be significant for the cases close to only outer cylinder rotation.
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