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Research Papers: Bio-Heat and Mass Transfer

The Role of Pinnae Flapping Motion on Elephant Metabolic Heat Dissipation

[+] Author and Article Information
Moise Koffi

Department of Academic Affairs,
CUNY-Hostos Community College,
475 Grand Concourse,
Bronx, NY 10551
e-mail: mkoffi@hostos.cuny.edu

Yiannis Andreopoulos

Department of Mechanical Engineering,
The City College of New York,
160 Convent Avenue,
New York, NY 10031
e-mail: andre@me.ccny.cuny.edu

Latif M. Jiji

Department of Mechanical Engineering,
The City College of New York,
160 Convent Avenue,
New York, NY 10031
e-mail: jiji@ccny.cuny.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received September 19, 2013; final manuscript received June 9, 2014; published online July 2, 2014. Assoc. Editor: James A. Liburdy.

J. Heat Transfer 136(10), 101101 (Jul 02, 2014) (12 pages) Paper No: HT-13-1496; doi: 10.1115/1.4027864 History: Received September 19, 2013; Revised June 09, 2014

The oscillatory rotational motion of the elephant pinna is considered a key mechanism in metabolic heat dissipation. Limited experimental investigations have revealed that the flapping of the elephant's pinna is responsible for surface heat transfer enhancement. The objective of the present experimental and computational work is to investigate the physics of the flow induced by the pinna's motion and its effects on the heat transfer. This was accomplished by designing, fabricating and testing two full-size laboratory models of elephant pinnae: one rigid and one flexible, both instrumented with small size thermocouples for time-dependent surface temperature measurements. A constant heat flux is applied to both sides of each model which is rotated about a fixed edge with a frequency of 2 rad/s in an infinite domain at ambient conditions. Of interest is the study of the impact of the flexural strength of the model's material on surface heat transfer. Additional computer simulations of the flow and thermal fields revealed a hooked-shape vortex tube around the free edges of the flapping pinna. This result is confirmed by the flow visualization with smoke particles. Both experimental and computational results exhibit local surface temperature profiles characterized by a transient unsteady periodic variation followed by a steady periodic phase. Flow visualization indicated significant interaction between the vortical structures shed off the edge and the flexible model's boundary layer. It has been found that the cooling of the flexible model is enhanced by 30%.

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Figures

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Fig. 1

Sketch of the elephant pinna model with basic dimensions

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Fig. 2

Experimental investigation set up of full-size flapping African elephant pinna. Oscillating mechanism apparatus with elephant pinna model at 90 deg angle with adjacent base-wall.

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Fig. 3

Experimental setup for pinna's surface temperature measurement, with pinna model at 90-deg angle from base wall. J-type thermocouples attached to flexible silicone rubber heaters, cemented on pinna's surface.

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Fig. 4

Forced convection temperature history for a rigid model pinna flapping at 35 flaps /min. Surface temperature profiles recorded at the center of flexible silicone rubber heaters at specific locations on the pinna's surface.

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Fig. 5

Sectional cut of flow domain of elephant pinna; (a) mesh on plane section before motion of pinna at Y = 0; (b) details of tetrahedral cells in the vicinity of pinna; and (c) mesh details when the elephant pinna is located at 10 deg during the upstroke

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Fig. 6

Flow visualization experiment. (a) Flapping elephant model illuminated with laser sheet in the radial direction. (b) Down-stroke flapping motion of elephant pinna model exhibiting clockwise vortex.

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Fig. 7

Pictures of vortical structures viewed from bottom of rigid flapping elephant pinna model undergoing a cycle with a reduced frequency k = 1. Stages of developing vortex from starting down stroke (stage 1) to end stroke (stage 4).

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Fig. 8

Comparison of computational with experimental results; upstroke motion with pinna at 10 deg angle. Left: Tip vorticity evidenced by flow path lines colored by velocity magnitudes. Right: Tip vorticity evidenced by a picture of smoke particles using a high speed camera.

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Fig. 9

Comparison of computational with experimental results during upstroke motion with pinna at 10 deg angle. Left: Side vortex with flow pathlines colored by velocity magnitude. Right: Picture of side vortex described by smoke particles.

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Fig. 10

Flow contours during upstroke motion of elephant pinna model at a 10 deg angle; (a) contours of static pressure in Z = 0.6 m-plane; (b) contours of velocity magnitude in Z = 0.6 m-plane; (c) pressure distribution over the front face; and (d) pressure distribution over the back face

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Fig. 11

Upstroke motion of flexible model of elephant pinna. Picture of developing vortex behind flapping model.

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Fig. 12

Comparative analysis of the flow induced by rigid and flexible elephant pinnae models during upstroke motion. Left: Picture of smoke particles developing a vortex behind rigid model. Right: Picture of smoke particles developing vortex behind flexible model.

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Fig. 13

Flow structure during a down stroke of a flexible pinna at various phases of the cycle: (a) Beginning of the down stroke; (b) core formation; (c) shear layer wrapping into vortical structure; and (d) growth stage of vortical structure

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Fig. 14

Flow structure during the final stages of an up stroke cycle of a flexible pinna flapping. (a) Flow separation and (b) development of clockwise rotating vortex.

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Fig. 15

Comparative analysis of elephant ear's surface temperature: (a) and (b) computational investigation results and (c) experimental data from literature [8]

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Fig. 16

Computational temperature contours of elephant pinna at 10-deg during upstroke: (a) Front face temperature distribution and (b) back face temperature distribution

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Fig. 17

Computational local surface temperature distribution of front versus back faces of the African elephant pinna model

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Fig. 18

Time-dependent surface temperature profile at the center of a flapping African elephant pinna at 2 rad/s (35 flaps/min) obtain from a computational simulation with a wall heat flux of 375 W/m2

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Fig. 19

Time-dependent surface temperature measurement at the center of a flapping elephant pinna at 2 rad/s (35 flaps/min) obtained by a computational simulation. Comparison of front and back faces surface temperature for a wall heat flux of 375 W/m2.

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Fig. 20

Temperature distribution over the surface of a medium size elephant pinna. Comparative analysis of steady periodic surface temperature between flexible and rigid elephant pinnae in rotational oscillation at different frequencies: (a) 20 flaps/min and (b) 35 flaps/min.

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