PhD Defense - Zhenrong JING - LHEEA/ED SPI

Committee

• Reviewer: Jean-Christophe ROBINET, Professeur, laboratoire Dynfluid, Arts et Métiers ParisTech
• Reviewer: Alois SCHAFFARCZYK, Professeur,  Kiel University of Applied Sciences
• Examinator: Jean-Yves BILLARD, Professeur, laboratoire IRENav, Ecole Navale
• Examinator: Mostafa SHADLOO, Maitre de Conférences, laboratoire CORIA, INSA  Rouen
• Supervisor: Antoine DUCOIN, Maître de Conférences HDR, - laboratoire  LHEEA, Centrale Nantes
• Co-Supervisor: Caroline BRAUD, Chargée de Recherche HDR, CNRS - laboratoire  LHEEA, Centrale Nantes
   

Protocole

The thesis defense will take place as usual. The president of the jury will open the defense, followed by a 45-minute presentation. The president will then allow the jury members to ask questions, one by one. After this session, the jury will retire to a separate conference room for discussion; the main conference room will remain open to the public (cameras/microphones allowed). The committee will then return to the main room to announce its decision. Afterwards, the room will remain open for a general discussion where, depending on the result, we will be able to congratulate Zhenrong.

Visioconference

The thesis defense will take place on Zoom:
https://ec-nantes.zoom.us/j/96917201200
Meeting room : 96917201200

The defense will start at 9:00 am, the videoconference will be open from 8:30 am so that you can test your connection.
In order to lighten the bandwidth, all participants will be asked to switch off their camera and microphone throughout the presentation. Finally, we thank you to connect around 8:45 am or earlier in order to have time to manage any technical problems before starting the defense. Thanks to all of you!

Abstract

The Horizontal Axis Wind Turbines (HAWTs) and marine propellers are two types of important fluid machineries. The boundary layer transition on them is nonetheless not fully understood yet.   Equipped with modern cluster computers, Under-resolved Direct Numerical Simulations (UDNS) are performed on both of them in this thesis. The main objective is to study the effect of rotating on the boundary layer transition.
The boundary layers on HAWTs and marine propellers share an apparent common point with von  Kármán swirling flow, which is created by a rotating disk in the otherwise still fluid.  The von Kármán swirling flow is the prototype of cross-flow transition. Therefore one focus of the present study is the possibility of the cross-flow transition on HAWTs and marine propellers.
The present study shows that the natural boundary layer transitions on the HAWT and the marine propeller are induced by distinctively different mechanisms. The numerical result of a HAWT blade shows that the boundary layer profile on it is very close to the 2-Dimensional (2D) airfoil flow. On the blade, the velocity in spanwise direction is small in the attached boundary layer. As a result, the natural transition on HAWT blade is very similar to the 2D airfoil flow and is due to Tollmien–Schlichting (T-S) waves.
On the marine propeller blade, the boundary layer flow is fully 3-Dimensional (3D) due to the rotation. The cross-flow instability and transition are clearly observed. The shapes of the cross-flow vortices are in good agreement with the prediction of Linear Stability Theory (LST). Although it has been long assumed that cross-flow transition should be important for propellers, this is the first direct observation of such phenomena as far as we know. Because the propeller does not have infinite rotational symmetry, our result suggests the boundary layer on the marine propeller is convectively unstable. This is different with the von Kármán boundary layer flow, which is absolutely unstable.
The difference in boundary layer flows and therefore transitions between the HAWT and the marine propeller is likely caused by their shapes. The HAWT blade has a very large aspect ratio, consequently, the cross-flow does not have enough distance to develop from the leading edge to the trailing edge. On the contrary, the cross-flow velocity is able to grow large enough to lead flow transition on the propeller, where the aspect ratio is small.
We also argue that it is necessary to work in the rotating reference frame in order to evaluate the effect of rotations. In the rotating reference frame, the centrifugal force at one position is a constant, while the Coriolis force depends on the local velocity. The magnitude of the spanwise (cross-flow) velocity is closely related to the relative strength of centrifugal and Coriolis forces. For example, cross-flow is usually the largest around separation bubbles, where Coriolis force changes its direction and acts in the same direction as centrifugal force.