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Research Papers: Multiphase Flows

# Synergy of Resistance Reduction Effects for a Ship With Bottom Air Cavity

[+] Author and Article Information
E. L. Amromin

Mechmath LLC, Prior Lake, MN 55372amromin@aol.com

B. Metcalf, G. Karafiath

NSWCCD, W. Bethesda, MD 20817

J. Fluids Eng 133(2), 021302 (Feb 17, 2011) (7 pages) doi:10.1115/1.4003422 History: Received October 27, 2009; Revised December 18, 2010; Published February 17, 2011; Online February 17, 2011

## Abstract

Friction on a surface covered by an air cavity is much less than friction in water but there is a resistance penalty caused by the cavity tail oscillations. Nevertheless, there is a method for designing the ship bottom form for suppressing these oscillations. This study describes the design method and calm water towing tank tests for a ship with a bottom ventilated air cavity operating at Froude range $0.45, where both Fr and cavitation number influence the cavity shape. At this Fr range, wave resistance significantly contributes to the total ship resistance. Model experiments were conducted in the NSWCCD linear tow tank at three diverse drafts. The attained resistance reduction ratio was up to 25%, which is significantly greater than the calculated water friction resistance of the unwetted area of the air cavity. This is a result of the increased ship elevation over the water level due to cavity buoyancy. This contributes to the resistance reduction by decreasing the side wetted surface area and by reducing the submerged volume; thus, there is a synergy of resistance reduction effects. The power spent on air supply is under 2% of the propulsion power.

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## Figures

Figure 1

A vertical section of the hull. An initial buttock includes an initial approach to Sc. In the process of iterative design, a part of this buttock downstream of cavity is deflected up for the smooth cavity closure. This deflection is indeed the locker design.

Figure 2

Effect of Froude number on the air-water interfaces (dashed curves) within a niche

Figure 3

Cavitation number effect on air cavity lengths as a function of Fr, σ<0 here because of excessive air pressure in the cavity

Figure 4

Body plan of model 5694 (x=constant sections) illustrating the niche height, side walls, and air supply slot

Figure 17

Normalized resistance reduction versus Froude number for various model drafts. Flat plate friction coefficient is calculated for the model test Re value.

Figure 18

Estimated reduction of wetted surface area under cavitation impact versus Fr for T=L/20

Figure 19

Percentage of resistance reduction by cavitation. Numbers near curves indicate the model displacement (in kilograms).

Figure 20

Views of a breaking wave at the model bow

Figure 21

Estimation of power spent by the air supply line made for L/T=20 and Q(Fr) corresponding to the maximum resistance reduction

Figure 22

Example of relative losses on air supply

Figure 5

View of the overturned model 5694 with a niche and a cavity locker behind it in the bottom

Figure 6

Experiment setup in David Taylor Model Basin linear tow tank

Figure 7

The baseline model with the flat bottom cover on the niche. One can also better see the stern shape here.

Figure 8

Resistance to displacement ratio versus the dimensionless volumetric air supply with the model at L/T=20 and various Fr. Triangles at abscissa axis represent the limiting range of Q/(LBU∞) in water tunnel tests with air cavity at large Reynolds numbers (6).

Figure 9

Cavity tails under towed model during towing tank tests. Flow goes from left to right. Turbid zones cover the cavity tail with air-water mixture. Upper photo shows the cavity at air supply below optimum. Medium photo shows the cavity at the optimum air supply. Lower photo shows the cavity at air oversupply.

Figure 10

The resistance to displacement ratio versus Fr for the model with the niche at various drafts

Figure 11

Resistance to displacement ratios versus Fr for the baseline hull and the model 5694 with the wetted niche at various drafts

Figure 12

Resistance to displacement ratio versus Fr (a) for the model with cavity and (b) for the baseline model for various displacements (in kilograms)

Figure 13

Pitch versus Fr for towed models

Figure 14

The hull dimensionless elevation from water dz/L versus dimensionless air supply with the model at L/T=20 and various Fr values

Figure 15

Hull dimensionless elevation dz/L versus Fr of model 5694 with the air cavity at Q of saturation and as the baseline hull at various L/T

Figure 16

Computed cavity volume versus Fr for T/L=20

## Errata

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