Mechanisms of evolution of the propeller wake in the transition and far fields

2011 ◽  
Vol 682 ◽  
pp. 5-53 ◽  
Author(s):  
M. FELLI ◽  
R. CAMUSSI ◽  
F. DI FELICE
Keyword(s):  
Transition Region ◽  
Mutual Inductance ◽  
Transitional Region ◽  
Far Field ◽  
Time Resolved ◽  
Tip Vortices ◽  
Far Fields ◽  
Propeller Wake ◽  
Wake Instability ◽  
Blade Geometry

In the present study the mechanisms of evolution of propeller tip and hub vortices in the transitional region and the far field are investigated experimentally. The experiments involved detailed time-resolved visualizations and velocimetry measurements and were aimed at examining the effect of the spiral-to-spiral distance on the mechanisms of wake evolution and instability transition. In this regard, three propellers having the same blade geometry but different number of blades were considered. The study outlined dependence of the wake instability on the spiral-to-spiral distance and, in particular, a streamwise displacement of the transition region at the increasing inter-spiral distance. Furthermore, a multi-step grouping mechanism among tip vortices was highlighted and discussed. It is shown that such a phenomenon is driven by the mutual inductance between adjacent spirals whose characteristics change by changing the number of blades.

Large eddy simulation of propeller wake instabilities

2017 ◽  
Vol 814 ◽  
pp. 361-396 ◽  
Author(s):  
Praveen Kumar ◽  
Krishnan Mahesh
Keyword(s):  
Large Eddy Simulation ◽  
Dominant Mode ◽  
Operating Conditions ◽  
Computational Domain ◽  
Tip Vortices ◽  
Eddy Simulation ◽  
Propeller Wake ◽  
Actual Mechanism ◽  
Large Eddy ◽  
Wake Instability

The wake of a five-bladed marine propeller at design operating condition is studied using large eddy simulation (LES). The mean loads and phase-averaged flow field show good agreement with experiments. Phase-averaged and azimuthal-averaged flow fields are analysed in detail to examine the mechanisms of wake instability. The propeller wake consisting of tip and hub vortices undergoes streamtube contraction, which is followed by the onset of instabilities as evident from the oscillations of the tip vortices. Simulation results reveal a mutual-induction mechanism of instability where, instead of the tip vortices interacting among themselves, they interact with the smaller vortices generated by the roll-up of the blade trailing edge wake in the near wake. It is argued that although the mutual-inductance mode is the dominant mode of instability in propellers, the actual mechanism depends on the propeller geometry and the operating conditions. The axial evolution of the propeller wake from near to far field is discussed. Once the propeller wake becomes unstable, the coherent vortical structures break up and evolve into the far wake, composed of a fluid mass swirling around an oscillating hub vortex. The hub vortex remains coherent over the length of the computational domain.

Hydrodynamic and Hydroacoustic Phenomena in the Propeller Wake-Rudder Interaction

2014 ◽  
Author(s):  
Mario Felli ◽  
Silvano Grizzi ◽  
Massimo Falchi
Keyword(s):  
Pressure Fluctuation ◽  
Holistic Approach ◽  
Wall Pressure ◽  
Marine Propeller ◽  
Flow Measurements ◽  
Time Resolved ◽  
Propeller Wake ◽  
Wall Pressure Fluctuation ◽  
Free Running

The present paper describes the major mechanisms underlying the hydroacoustic and hydrodynamic perturbations in a rudder operating in the wake of a free running marine propeller. The study was based on a holistic approach which concerned time resolved visualizations and detailed flow measurements around the rudder as well as wall-pressure fluctuation measurements over the rudder surface, at different deflection angles.

Doubly-Charged Xe Ions Evidenced by Time Resolved RPA Measurement in the Far Field Plume of a Low-Power HET

2015 ◽  
Vol 55 (7) ◽  
pp. 529-537 ◽  
Author(s):  
T. Gibert ◽  
L. Balika ◽  
F. Diop ◽  
A. Bouchoule
Keyword(s):  
Low Power ◽  
Far Field ◽  
Time Resolved ◽  
Doubly Charged

Analysis of the flow field around a rudder in the wake of a simplified marine propeller

2017 ◽  
Vol 814 ◽  
pp. 547-569 ◽  
Author(s):  
Roberto Muscari ◽  
Giulio Dubbioso ◽  
Andrea Di Mascio
Keyword(s):  
Flow Field ◽  
Leading Edge ◽  
The Body ◽  
Tip Vortex ◽  
Tip Vortices ◽  
Propeller Wake ◽  
Instantaneous Flow Field ◽  
The Mean ◽  
Flow Variables ◽  
Interaction Problem

The vortex–body interaction problem, which characterizes the flow field of a rudder placed downstream of a single-blade marine rotor, is investigated by numerical simulations. The particular topology of the propeller wake, consisting of a helicoidal vortex detached from the blade tips (tip vortex) and a longitudinal, streamwise oriented vortex originating at the hub (hub vortex), embraces two representative mechanisms of vortex–body collisions: the tip vortices impact almost orthogonally to the mean plane, whereas the hub vortex travels in the mean plane of the wing (rudder), perpendicularly to its leading edge. The two vortices evolve independently only during the approaching and collision phases. The passage along the body is instead characterized by strong interaction with the boundary layer on the rudder and is followed by reconnection and merging in the middle and far wake. The features of the wake were investigated by the $\unicode[STIX]{x1D706}_{2}$-criterion (Jeong & Hussain, J. Fluid Mech., vol. 285, 1995, pp. 69–94) and typical flow variables (pressure, velocity and vorticity) of the instantaneous flow field; wall pressure spectra were analysed and related to the tip and hub vortices evolution, revealing a non-obvious behaviour of the loading on the rudder that can be related to undesired unsteady loads.

scholarly journals Separating atrial near fields and atrial far fields in simulated intra-atrial electrograms

2021 ◽  
Vol 7 (2) ◽  
pp. 175-178
Author(s):  
Leonie Schicketanz ◽  
Laura Anna Unger ◽  
Jorge Sánchez ◽  
Olaf Dössel ◽  
Axel Loewe
Keyword(s):  
Atrial Flutter ◽  
Spectral Power ◽  
Near Field ◽  
Energy Operator ◽  
Far Field ◽  
Maximum Slope ◽  
Far Fields ◽  
Atrial Electrograms ◽  
Complex Forms ◽  
Feature Values

Abstract The detailed characterization of complex forms of atrial flutter relies on the correct interpretation of intra-atrial electrograms. For this, the near field components, which represent the local electrical activity, are decisive. However, far field components arising from distant electrical sources in the atria can obscure the diagnosis. We developed a method to separate and characterize atrial near field and atrial far field components from bipolar intra-atrial electrograms. First, a set of bipolar electrograms was created by simulating different propagation scenarios representing common clinical depolarization patterns. Second, near and far fields were detected as active segments using a non-linear energy operator-based approach. Third, the maximum slope and the spectral power were extracted as features for all active segments. Active segments were grouped accounting for both the timing and the location of their occurrence. In a last step, the active segments were classified in near and far fields by comparing their feature values to a threshold. All active segments were detected correctly. On average, near fields showed 15.1x larger maximum slopes and 40.4x larger spectral powers above 100 Hz than far fields. For 135 active segments detected in 72 bipolar electrograms, 5.2% and 6.7% were misclassified using the maximum slope and the spectral power, respectively. All active segments were classified correctly if only one near field segment was assumed to occur per electrogram. The separation of atrial near and atrial far fields was successfully developed and applied to in silico electrograms. These investigations provide a promising basis for a future clinical study to ultimately facilitate the precise clinical diagnosis of atrial flutter.

Phase-Averaged Particle Tracking Velocimetry Measurements of Propeller Wake

2005 ◽  
Vol 49 (01) ◽  
pp. 43-54
Author(s):  
Sang-Joon Lee ◽  
Bu-Geun Paik ◽  
Choung Mook Lee
Keyword(s):  
Particle Tracking ◽  
Particle Tracking Velocimetry ◽  
Flow Characteristics ◽  
Computation Time ◽  
Velocity Fields ◽  
Tip Vortex ◽  
Wake Flow ◽  
Mean Velocity ◽  
Tip Vortices ◽  
Propeller Wake

The objective of the present paper is to apply an adaptive hybrid two-frame particle tracking velocimetry (PTV) technique for measuring the flow characteristics of turbulent wake behind a marine propeller with five blades. Compared to the conventional particle image velocimetry method, the hybrid PTV technique increases the spatial resolution and measurement accuracy significantly while reducing the computation time. For each of four different blade phases of 0, 18, 36, and 54 deg, 400 instantaneous velocity fields were measured. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the region ranging from the trailing edge to a two propeller diameter (D) downstream location. The phase-averaged mean velocity fields show that the trailing vorticity and the viscous wake are formed by the boundary layers developed on the blade surfaces. The vorticity contours at each phase angle show that the tip vortices are produced periodically. The slipstream contraction occurs in the near-wake region up to about x= 0.5 D downstream. Thereafter the unstable oscillation occurs due to the separation of tip vortex from the wake sheet behind the maximum contraction point. As the tip vortex evolves downstream, its strength is reduced due to turbulent diffusion, viscous dissipation, and active mixing between tip vortices and adjacent wake flow. The technique presented here can be readily extended to investigate the nominal and effective wake distribution as well as the details of the flow field fore and aft of a rotating propeller behind a ship model.

scholarly journals On high-frequency sound generated by gust–aerofoil interaction in shear flow

2015 ◽  
Vol 766 ◽  
pp. 297-325 ◽  
Author(s):  
Lorna J. Ayton ◽  
N. Peake
Keyword(s):  
Shear Flow ◽  
Transition Region ◽  
High Frequency ◽  
Outer Region ◽  
Far Field ◽  
Radiated Power ◽  
Surface Transition ◽  
Trailing Edges ◽  
Parallel Shear Flow ◽  
Field Sound

AbstractA theoretical model is constructed to predict the far-field sound generated by high-frequency gust–aerofoil interaction in steady parallel shear flow, including the effects of aerofoil thickness. Our approach is to use asymptotic analysis of the Euler equations linearised about steady parallel shear flow, in the limits of high frequency and small, but non-zero, aerofoil thickness and Mach number. The analysis splits the flow into various regions around the aerofoil; local inner regions around the leading and the trailing edges where sound is generated and scattered; a surface transition region accounting for the curvature of the aerofoil; a wake transition region downstream of the aerofoil; and an outer region through which the sound propagates to the observer. Solutions are constructed in all regions, and matched using the principle of matched asymptotic expansions to yield the first two terms in the expansion of both the amplitude and the phase of the far-field pressure. Result are computed for the particular case of scattering of a gust by a symmetric Joukowski aerofoil placed in symmetric Gaussian parallel shear flow. The introduction of mean shear is shown to have a significant effect on the far-field directivity and on the total radiated power.

Time-resolved detection of far-field THz-radiation patterns: spatially restricted coherence of surface field THz emitters

1999 ◽  
Author(s):  
F. Hilbk-Kortenbruck ◽  
Peter H. Bolivar ◽  
Rainer Martini ◽  
Henricus P. M. Pellemans ◽  
Heinrich Kurz
Keyword(s):  
Thz Radiation ◽  
Far Field ◽  
Radiation Patterns ◽  
Time Resolved ◽  
Surface Field
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