Hydrofoil Drag Reduction by Partial Cavitation

[+] Author and Article Information
Eduard Amromin

 Mechmath LLC, Prior Lake, MN 55372

Jim Kopriva, Roger E. A. Arndt, Martin Wosnik

Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55414

One issue with the current force balance was sealing. During the study two different seals were tested. This issue affected the ability to compare drag data from different versions of the balance due to a different drag offset with each seal due to pressure differential effects. The drag accuracy is based on the worst-case scenario between the two seals used. This error is found to be much less by limiting comparisons to within data sets associated with each version of the balance.

J. Fluids Eng 128(5), 931-936 (Feb 07, 2006) (6 pages) doi:10.1115/1.2234787 History: Received February 28, 2005; Revised February 07, 2006

Partial cavitation reduces hydrofoil friction, but a drag penalty associated with unsteady cavity dynamics usually occurs. With the aid of inviscid theory a design procedure is developed to suppress cavity oscillations. It is demonstrated that it is possible to suppress these oscillations in some range of lift coefficient and cavitation number. A candidate hydrofoil, denoted as OK-2003, was designed by modification of the suction side of a conventional NACA-0015 hydrofoil to provide stable drag reduction by partial cavitation. Validation of the design concept with water tunnel experiments has shown that the partial cavitation on the suction side of the hydrofoil OK-2003 does lead to drag reduction and a significant increase in the lift to drag ratio within a certain range of cavitation number and within a three-degree range of angle of attack. Within this operating regime, fluctuations of lift and drag decrease down to levels inherent to cavitation-free flow. The favorable characteristics of the OK-2003 are compared with the characteristics of the NACA-0015 under cavitating conditions.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Schematic of cavitating flow

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Figure 2

Computed cavities on the developed hydrofoil OK-2003. The fictitious body of the initial NACA-0015 hydrofoil at design conditions is transformed into the afterbody of the hydrofoil OK-2003.

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Figure 3

Pressure distribution over a basic contour

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Figure 4

Contours of the designed hydrofoil OK-2003 (solid line), the initial hydrofoil NACA-0015 (dashed line), and the basic contour (transparent line) that is a solution of Eqs. 1,2,3,4,5 for the initial hydrofoil

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Figure 5

Comparison of pressure distributions over the suction side of the designed OK-2003 hydrofoil and the initial NACA-0015 hydrofoil computed for an ideal fluid

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Figure 6

Comparison of lift and coefficient recorded at SAFL and at the Sandia National Laboratories (SNL)

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Figure 7

Comparison of drag coefficient recorded at SAFL and at the Sandia National Laboratories

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Figure 8

View of OK-2003 in the SAFL water tunnel

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Figure 9

Coefficient of lift and drag versus cavitation number for OK-2003 and NACA-0015 at the same free-stream speed

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Figure 10

Lift to drag ratio of OK-2003 for different angles of attack

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Figure 11

Effect of angle of attack on the drag of OK-2003

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Figure 12

Comparison of normalized drag of hydrofoils OK-2003 and NACA-0015

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Figure 13

Comparison of normalized lift to drag ratio for the NACA-0015 and OK-2003. All data is normalized to it respective non-cavitating condition. Thick vertical lines separate a range where partial cavitation inflicts an opposite performance response.

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Figure 14

Comparison of lift to drag ratios of the hydrofoil OK-2003 at α=6deg and NACA-0015

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Figure 15

Correlation between average and fluctuating (rms) drag coefficient for the NACA-0015 and OK-2003. CDRMS is scale along the right while CD is scaled on the left.

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Figure 16

Correlation between average and fluctuating (rms) lift coefficient for NACA-0015 and OK-2003. The top plot features α=7deg, the bottom plot features α=5deg and α=6deg.

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Figure 17

Comparison in the variation in cavity length on the NACA-0015 and the OK-2003 based one shedding cycle at peak conditions. NACA-0015 left (U=7.65m∕s, α=8deg, σ=1.3, σ∕2α=4.66) and OK-2003 right (U=8m∕s, α=7deg, σ=1.14, σ∕2α=4.67). Note the stable reattachment on the OK 2003 while the NACA-0015 foil experiences large fluctuations in cavity length.

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Figure 18

Sideview of a low-drag cavity for the OK-2003 hydrofoil. The foil’s profile and the cavity shape with ideal reattachment from computation are superimposed onto the HSV.




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