Experimental Investigation of Turbulence under the Influence of Confinement and Rotation.
Strömungslehre und Aerodynamik
[Ph.D. Thesis], (2010)
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|Item Type:||Ph.D. Thesis|
|Title:||Experimental Investigation of Turbulence under the Influence of Confinement and Rotation|
In this thesis shear-free turbulence under the influence of confinement and ro- tation has been analyzed experimentally via Particle Image Velocimetry (PIV) and three-dimensional Particle Tracking Velocimetry (3D-PTV). The measure- ments have been carried out in a water tank placed on a rotating table in which the flow is mechanically forced from the top by an oscillating grid. 3D-PTV measurements were conducted in two different volume sizes, namely a large ob- servation volume allowing to capture the large scale motions of the flow and a small observation volume, in which the spatial resolution was sufficient to assess all nine components of the velocity gradient tensor. One of the main achievements of this work is the simultaneous measurement in the large and small observation volume using two synchronized PTV systems and two types of seeding particles. The spreading of turbulence has been investigated experimentally for three different cases, i.e. (i) free turbulent diffusion and spreading of turbulence under (ii) the influence of confinement and (iii) rotation and compared to the theoretical results of Oberlack and Günther (2003). In particular, for case (i) we observe that the turbulent/non-turbulent interface propagates according to a power law, H ∝ t^n, where n is estimated to be n=0.6 ± 0.1. For case (ii) we confirm that the behavior changes to a logarithmic law with H ∝ ℓ_c ln(t−t_0)+y_0. Finally, for case (iii) we confirm the theory only in the sense that turbulence remains confined within a finite domain. The flow was observed to change drastically under the influence of rotation. In particular it was found that the 3D turbulent flow remains confined to a distances y < y∗ and becomes quasi-2D for larger distances from the grid. The analysis of the region between the 3D and quasi-2D flow shows that the transition from one flow state to the other is more gradual and not as abrupt as for the turbulent/non-turbulent interface, which occurs in the free turbulent diffusion without rotation. The study of the alignments, which are formed by the rotation confirms that the quasi-2D regime is dominated by columnar vortices. A closer examination has demonstrated that this region can be divided into three parts: (i) the core of the vortex columns, (ii) the outer region of the vortex columns and (iii) the flow in-between the columnar structures. Fluid is transported upwards in cyclones and downwards in anticyclones due to Ekman pumping, while the velocity is primarily orientated normal to the rotation axis in the outer regions of the vortex columns and hence exhibits a quasi-2D flow pattern. In the intermediate region the flow remains three-dimensional. Both enstrophy and strain were found to be depleted under the influence of rotation. High levels of enstrophy seem to be confined to the regions of the vortex cores while regions of high strain are predominately located in the intermediate region. Vortex stretching also occurs less frequently and the ratio of vortex stretching to compression changes and can become even smaller than one within the columnar vortices. Also, the positive skewness of the Λ2-PDF, which is characteristic for 3D-turbulence is nearly vanished in the quasi-2D regime. It was also found that generally these effects become stronger with angular velocity.
|Uncontrolled Keywords:||turbulence, confinement, rotation, Particle Tracking Velocimetry|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 500 Naturwissenschaften
500 Naturwissenschaften und Mathematik > 530 Physik
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften
|Divisions:||Fachbereich Maschinenbau > Strömungslehre und Aerodynamik|
|Date Deposited:||25 May 2010 08:39|
|Last Modified:||07 Dec 2012 11:57|
|Referees:||Tropea, Prof. Dr.- Cameron and Kinzelbach, Prof. Dr.- Wolfgang|
|Refereed:||31 March 2010|
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