Numerical Study on the Gap Flow of a Rudder System with Bisymmetric Blocking Bar

Abstract

In recent practice a half round prismatic bar has fillet welded or formed through foundry work along the centerline on rear concave surface of the horn to mitigate gap flow between fixed and movable part of the rudder system. When the gap clearance has been blocked with this practice, numerical simulations indicate that the practices are not only effective in reducing the gap flow but also in mitigating the cavitation. The blocking effects are remarkably improved when a pair of blocking bar is bisymmetrically attached with respect to centerline on the opposite convex surface of the movable part. The blocking bar could be placed on the exposed surface under maximum rudder angle. This implies that the blocking bar could be easily adopted not only in a design stage but also in a maintenance stage for mitigating rudder cavitation. In addition, the numerical simulations imply that more improvements could be anticipated through the selection of section shape of prismatic bar for gap flow blocking.

Practically applicable devices for blocking the gap flow of a horn rudder to reduce rudder cavitation and their verification through numerical simulations

“…However, it is not possible to block the gap flow completely, even with blocking bars, since the gap clearance varies with the rudder angle and oversized bars may cause jamming of the rudder during operation. Accordingly, it is a common practice that blocking bars of semicircular cylindrical shapes not higher than 0.4 times the gap clearance are fillet-jointed to the concave surface of the fixed portion that faces the gap during the casting stage of horns [9]. Figure 8 shows that the negative pressure peaks around the tip of the fixed part are considerably retarded in order of (a) without bar, (b) single bar and (c) symmetric bars.…”

“…This is to keep the bars inside the gap during cruising under small rudder angles while exposing them outside the gap at the maximum rudder angle. Cavitation near the horizontal gap becomes more intense as the vertical gap flow is successfully retarded by the blocking bars [9][10][11]. It has been speculated that the phenomenon may arise from an increased flow through the horizontal gap; hence, the flow through the horizontal gap of a horn rudder should be blocked simultaneously to successfully reduce rudder cavitation.…”

“…He found that modest reductions in gap cavitation are achievable with the device, but it was also hard to apply in existing rudder systems. Oh et al [9] and Seo et al [10,11] found that cavitation near the upper corner of the pintle becomes more intense even when the vertical gap flow has been successfully retarded through blocking bars. It can be speculated that the phenomenon may arise from increased flow through the horizontal break.…”

A Study on Optimized Rudder Design by Comparison and Analysis of Design Process of Rudder Device.

“…간극주위에 서 발생되는 캐비테이션 현상을 감소시키기 위해 간극 사이에 차단봉 (Bu et al 2004, Paik et al 2008, Oh et al 2009a, 2009b)이나 유동분사 장 치 (Seo et al 2009), 틈새유동 차단 장치 (Oh et al 2008) …”

A Numerical Study on the Control of the Gap Flow Using a Fluid Supply Device

“…1 (D))과 간극유동과 주유동이 만나면서 흡입면의 간극 주위에서 캐비테이션( Fig. 1 (A (Bu et al 2004, Paik et al 2004, Oh et al 2009a, 2009b)이나 유동분사 장 치 (Seo et al 2009), 틈새유동 차단 장치 (Oh et al 2008), baffle plate등을 장착하여 간극을 통해 드나드는 유량을 감소시켜 간극 주위의 캐비테이 션을 감소시킨 연구가 수행된 바 있다. 또한 프로펠러 후류에 있는 타 캐비테이션을 효 과적으로 모사하기 위해 포텐시얼 이론을 이용한 수치적 연구 (Kong et al 2002, Kim et al 2006)와 이상화한 프로펠러 후류의 점성유동 계산 을 이용한 수치적 연구 (Bu et al 2004, Choi and) 등에서 프로펠러 후류의 유동장을 고려하여 타 캐비테이션을 감소시키기 위한 연구 가 수행되었다.…”