|Mustafa Umut Sarac
Wohnort: Istanbul - Turkey
|Verfasst am: 12.12.2019, 20:27 Titel: Tip Hub Vortex, Wake, Near Far Field Evolution FINAL WORDS
|J. Fluid Mech., page 1 of 49 c Cambridge University Press 2011
Mechanisms of evolution of the propeller
wake in the transition and far fields
M. FELLI†1, R. CAMUSSI2 AND F. DI FELICE1
1CNR-INSEAN, Italian Ship Model Basin, Via di Vallerano 139, 00128 Rome, Italy
2Dipartimento di Ingegneria Meccanica e Industriale, Universit’a Roma Tre,
Via della Vasca Navale 79, 00146, Italy
(Received 4 December 2009; revised 4 March 2011; accepted 22 March 2011)
i) Rotor wake transition to instability. Recent experimental works
provide a description of some typical features of the propeller wake in the transition and far fields, as the occurrence of a precession motion of the propeller streamtube around the hub vortex spiral coupled with the energy transfer from the blade to the shaft harmonic (Felli et al. 2006 Di Felice et al. (2004) hypothesized a strict correlation between the transition to instability and the interaction between the trailing wake of the actual blade and the tip vortex of the previous one which occurs as the consequence of the different pitch angle of their helical paths. Pressure measurements (Felli et al.2006) and flow visualizations (Stella et al. 2000) support this thesis, which revealing dependence of some typical features of the wake instability (e.g. phase shift of the tip vortex, blade-to-shaft harmonic energy transfer, destabilization of the hub vortex, precession of the streamtube) on the aforementioned mechanism of mutual interaction between the tip vortex and the trailing wake of the successive blade. Therefore, the Mechanisms of evolution of the propeller wake in the transition and far fields 3 distance between two consecutive blade wakes seems to play a role in the slipstream instability mechanism, especially at the starting point of the tip and hub vortex destabilization.
(ii) Wake evolution in the transition and the far regions. The dynamics of the propeller vortices is a complex phenomenon in which the effect of the viscosity, the mechanisms of auto and mutual induction (Widnall 1972), the effect of torsion (Ricca 1994) have to be accounted for and described during the evolution down to the breakdown. This is a difficult task for both a theoretical and an experimental approach, justifying the lack of studies dealing with this aspect in the literature,especially for what concerns the transition and the far wakes. Indeed, the state of the art on the problem of the rotor wake evolution is limited to the near wake and to the development of simplified models. On the theoretical side, the state of the art has progressed from the early momentum and vortex theories (Rankine 1865; Froude 1889; Glauert 1927; Goldstein 1929; Mangler & Squire 1953; Pizali & Duwaldt 1962;Miller, Tang & Perlmutter 1968) to advanced models with a distorted geometry of the wake, in which each blade releases a sheet of vortex filaments free to interact and distort until converging to a deformed geometry. These models include the tip vortex roll-up and the distortion of the wake shed from the inboard sections (Landgrebe 1972; Kerwin 1986). On the experimental side, the support of advanced optical techniques and flow visualizations allowed a quite detailed reconstruction of the rotor structure dynamics in the near wake of marine propellers, helicopter rotors and wind turbines. The mechanism of the tip vortex rolls up was investigated by
Kobayashi (1982), Cenedese, Accardo & Milone (1985), Jessup (1989), Chesnakas &Jessup (1998), Stella et al. (2000). In these papers, the problem of wake evolution was analysed through velocity measurements along transversal planes in the near wake
of a marine propeller. Di Felice et al. (2004) studied the propeller wake evolution along a diametral plane of the wake for different swirl numbers. Felli et al. (2006) reproposed the experiment of Di Felice et al. (2004) investigating the phase correlations between velocity and pressure signals. Vermer et al. (2003) tackled the problem of the aerodynamics of a wind turbine. The problem of the rotor wake instability was tackled experimentally by Ortega (2001) for a two-bladed propeller and re-proposed
by Felli et al. (2008) that analysed the power spectra of the velocity signals streamwise down to the far wake.
(iii) Tip and hub vortex breakdown. The basic underlying mechanism leading to tip and hub vortex breakdown in a rotor wake is still not known completely. Flow visualizations have provided some global information on the location where the hub vortex breaks down (Stella et al. 2000). However, the behaviour of the tip vortices as well as the physical mechanism or mechanisms leading to the propeller wake breakdown is still unknown. This is mainly due to the fact that measurements, whether invasive (e.g. hot wires) or non-invasive (e.g. LDV), and flow visualizations are difficult to obtain and interpret in such a complex, unsteady, three-dimensional (3D) flow.
Mustafa Umut Sarac