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24th Symposium on Naval Hydrodynamics Fukuoka, JAPAN, 8-13 July 2002 Wash Waves Generated by Ships Moving on Fairways of Varying Topography Tao Lange, Rupert Henn2 & Som Deo Shaman (~VBD - European Development Centre for Inland and Coastal Navigation, 2Institute of Ship Technology and Transport Systems, Mercator University, Duisburg, Germany) ABSTRACT Using the computer program BEShiWa, based on extended Boussinesq's equations for the far-field flow and slender-body theory for the near-ship flow, the wash waves generated by ships were investigated taking account of varying topographies. It appears that for the time being the shallow-water approxima- tion based on Boussinesq-type equations is a useful method for combining all effects associated with the nonlinear and unsteady nature on the one hand and the large-domain feature on the ether hanr1 of wash problems. A good agreement between computations and measurements was achieved for waves far from the ship in a rectangular channel as well as for 2D wave propagation over an uneven bottom. Further- more, it was found that the propagation of wash waves depends significantly on bottom topography, ship speed, and motion history. INTRODUCTION Recently, increased attention is being paid to wash waves by ferry operators, naval architects, marine and environmental consultants, and port and waterway authorities. Such wash waves can affect the safe operation of other floating bodies near the shoreline or the bank and endanger human life on beaches. Moreover, they can cause environmental damage, for instance, bank erosion. Strong wash waves can be generated by a fast ship at high speeds or by a large ship at moderate speeds, operating on a near-shore fairway or on an inland waterway. The resulting wash-wave system is basically nonlinear due to the wave characteristics in shallow water and usually unsteady due to the non- uniform seabed or inland waterway topography. Moreover, the major part of the wash wave system consists of divergent waves which can travel over a long distance without loss of energy until close to the shore. Due to the great interest in wake-wash ef- fects, a considerable amount of research effort has been devoted to the wash problem during the last few years. In model experimental studies the focus has been on designing low-wash ships and acquiring reliable data for validation, see, e.g., Zibell and Grol- lius (1999), MacLarlane and Renilson (1999), and Koushan et al.~2001~. Also full-scale measurements have been taken, aiming at deriving recommendations for safe ship operation as well as finding possibilities of ship monitoring in sensitive operational areas, see, e.g., Feldtmann and Garner (1999), and Bolt (2001~. In numerical simulations the focus has been on de- veloping efficient methods. For a ship moving on water of uniform depth the linear theory can be ap- plied usefully in the subcritical and the supercritical speed range, see, e.g., Doctors et al.~2001~. For these steady cases a steady nonlinear free-surface panel method can also be used, see, e.g., Raven (20004. The wave generation by a ship moving in a channel at a transcritical speed, on the other hand, can be well predicted using Kadomtsev-Petviashvili (KP) type equations, see, e.g., Chen and Sharma (1995), where the near-ship flow is approximated by an extended slender-body theory. This KP approximation has been extended by Chen and Uliczka (1999) for ships mov- ing in natural waterways with transversally varying water depth. But a basic restriction of the KP equa- tion is that it is not valid for truly unsteady cases, caused, for instance, by varying topography along the ship's track. A more general shallow-water approxi- mation are equations of Boussinesq type, which are valid for almost arbitrarily unsteady cases. In Jiang (1998) a set of modified Boussinesq's equations, which are valid not only for long waves but also for waves of moderate length, was applied to compute ship waves in shallow water, using slender-body theory to approximate the near-ship flow. In Yang et al.~2001) Boussinesq's equations based on a suitable reference level were used for computing ship waves
in shallow water but the near-ship flow was repre- sented by a mean transverse velocity from slender- body theory instead of using the near-field velocity at the reference level. A hybrid approach, comprising the coupling of a steady nonlinear panel method for the near-ship flow to a Boussinesq solver for the far- field wave propagation, has been introduced by Ra- ven (2000~. However, it is useful only for steady problems. It should be noted here that due to the non- linear and unsteady nature as well as the large- domain feature of the wash problems, they can be neither solved well by the linear wave theory nor approximated efficiently by a nonlinear singularity- method, even less by a finite-volume method due to the huge computational domain required. To cover all effects mentioned above an efficient method for the time being seems to be a shallow-water approxima- tion based on Boussinesq-type equations, in which the 3D governing equations for the inviscid fluid are first treated analytically in the vertical direction and the resulting 2D equations then solved numerically in the horizontal plane. In an extensive study by Jiang (2001) a computer program BEShiWa, standing for Boussinesq's Equations for Ship Waves, has been developed with the following features: - extension of the shallow-water equations of Bous- sinesq type to longer and shorter waves over an uneven bottom, inclusion of the near-ship flow into the shallow- water equations either through the law of conser- vation of mass or through a free-surface pressure distribution equal to the hydrostatic pressure on the hull bottom or through a unified shallow-water theory, implementation of suitable boundary conditions, and application of numerically efficient and robust methods. In the present study, we focus on the wash- wave systems generated by a Panmax containership and a fast inland passenger-ferry. Special attention is paid to the interaction between ship waves and bot- tom topography, aiming to find practical criteria for safe ship operation (speed and distance to shoreline) as well as for fairway maintenance (dredging depth and frequency) with regard to the wash effects. BRIEF MATHEMATICAL DESCRIPTION AND NUMERICAL APPROXIMATION Coordinate System For describing the flow generated by a ship sailing in shallow water over a general topography, a right- handed earthbound coordinate system Oxyz is used. The origin O lies on the undisturbed water-plane. The x-axis points in the direction of ship's forward mo- tion; the z-axis, vertically upwards. Field Equations Considering water as incompressible and inviscid, the wave generation by ships in shallow water can be approximated by the well-established shallow-water wave theory, see Jiang (2001) for a review. Assuming that the water depth is small in comparison to the wave length and that the wave amplitude is small in comparison to the water depth, the wave field can be well described by shallow-water equations of Boussi- nesq type. In the present study, Boussinesq's equa- tions based on the mean horizontal depth-averaged velocity for an uneven bottom, without additional terms for correcting the dispersion relation, are ap- plied: +( +hx>)u+(~+h')(ux +vy)+(~<y +hy~v = 0 u, + uux + buy + gjx - 2 (hXxu' + 2hxu~x + hum + hays + hype + hazy + hv~xy>) + 6 (Max + vail) = 0 v, + uvx + wy + guy - 2 (hXyu' + hxU`y + hyU'x + hazy + hays + Shyly + hays) + 6 (it + v~~,y) = 0 These were first derived by Peregrine (1967~. Herein, h(xy) is the water depth, ((x,y,t) the wave elevation, u~x,y,t) and v~x,y,t) the depth- averaged velocity components in the x and y direc- tions, respectively, t the time, and g the acceleration due to gravity. For this set of nonlinear partial differ- ential equations there exists no analytical solution. Therefore, it has to be solved numerically. Boundary Conditions On the truncation boundaries of the computational domain sufficiently far from the ship the Sommerfeld radiation condition q, +~qx = 0 or q' +~qy = 0 is applied, where q stands for each of the variables (, u and v, and ~ ensures the local outgoing characteris- tic of the governing equations on the boundary in question, for instance, ~ = ~ ahead of the ship and ~ = ~ behind the ship. 2
On vertical channel sidewalls, if any, the condition of no-flux or, equivalently, perfect reflec- tion holds. Initial Conditions In compliance with the unsteady nature of the flow, the ship is assumed to start from rest and accelerate uniformly to a final velocity like in a model towing tank. As may be expected, the final wave system is found to be influenced by the acceleration rate, espe- cially in case of trackwise varying topography. This is because the waves caused by the accelerating ship with a rather arbitrarily assumed starting point can be reflected by the bottom topography and then interact with the waves generated by the ship at steady speed. Approximation of the Near-Ship Flow The main interest in the present study lies in the wave propagation far from the ship. So a slender-body theory is applied to approximate the near-ship flow. Starting from the general formulation of the depth- averaged mean transversal velocity for a slender body in Jiang (2001), and additionally taking account of the asymmetric effect caused by nonuniform channel topography, the boundary condition on the longitudi- nal ship-centerline (the mathematical dividing line between the near-field and the far-field) relevant to the far-field flow reads vie - Or = Ah + ~ ~ [(V - u0 );oBX + hBuox + VSx - (unsex ] + vo with the port-starboard mean values of the longitudi- nal velocity component u0= to 2 IN of the transversal velocity component 2C`xy XS,e= (uly~o+ - ammo- Arc, and of the wave elevation `~0= to 2 IN . The hull sectional area is denoted by S(x), and the beam by B(x). The sectional blockage coefficient C(x) can be calculated by a 2D boundary element method in ad- vance, see, e.g., Taylor (1973~. xbOW and xs~errl are the longitudinal positions of the bow and stern, respec- tively. Moreover, the Kutta condition is implemented at the stern through U1 0+ = al o_ Physically, it means that the longitudinal velocities on the two sides of the hull have to be identical at the ship stern to ensure that there is no pressure jump. Numerical Solution Technique To solve this initial-boundary value problem gov- erned by Boussinesq's equations, an implicit Crank- Nicolson scheme is implemented as usual. But it encounters some difficulties arising from the nonlin- ear terms and the linear high-order terms. The devel- oped solution technique comprises: Crank-Nicolson scheme of high-order accuracy for the time and space discretization, approximation of the state values of the nonlinear terms by means of Taylor series expansion, SOR iterative solution of the resulting sparse equation system, overrelaxation to accelerate the convergence, and local and global filtering to suppress numerical oscillations and instabilities. RESULTS AND DISCUSSION Example Ships Two example ships are investigated in the present study. One is an inland passenger ferry that operates on the river Rhine and in a sheltered region of vary- ing water-depth; this aims at finding suitable criteria for fast-ship operations. The other is a Panmax con- tainer ship; this is intended to predict the wash-waves typical of large ships operating on a near-shore fair- way or near a harbor. The main dimensions of both ships are listed in Table 1. Table 1: Main dimensions of investigated ships Inland Passenger Ferry Panmax Container Vessel EWL 39.3 m 280 m B 8.8 m 32.2 m T 1.2m 11 m Representative Waves in a Large Shallow-Water Region To demonstrate the capability of the computer pro- gram BEShiWa to predict ship waves over a huge computational domain, Figure 1 shows three repre- sentative wave systems generated by the subject inland passenger-ferry moving in an unbounded shal- low-water region of uniform depth. The computa- tional domain was of 17.5 ship lengths long and 7.5 ship lengths wide, taking advantage of transverse symmetry. The grid size was lm x lm, yielding a total of 210,000 grid points. The CPU time required for a typical run was about 23.5 hours for 4,500 time steps. At a subcritical speed, Fnh= 0.7 in graph (a), the wave system is steady and close to a Kelvin- Havelock wave pattern with pronounced transverse waves. At critical speed, Fnh= 1.0 in graph (b), the 3
wave system is characterized by significant divergent waves. Long-time simulations showed that no asymp- totic steady state could be reached at transcritical speeds. For the same speed in a (finite-width) chan- nel, so-called solitons, which are perfect transverse waves propagating ahead of the ship, were generated in accordance with observations in model tanks and full-scale, see, e.g., Jiang (2001~. Similar unsteady response to steady excitation has been observed in other nonlinear problems. In fact, for a nonlinear system governed by Boussinesq's equations, there is no guaranty that the asymptotic solution would be steady and independent of the initial conditions. At a supercritical speed, Fnh= 1.5 in graph (c), the final wave system comprises only divergent waves, no initial transverse waves generated during the accel- eration phase could keep up with the ship. The as- ymptotic wave system is again steady relative to the ship. a) Fnh = 0 7 b)Fnh= 1.0 4
c) F~,h = 1.5 Figure 1: Representative wave systems generated by the subject inland passenger-ferry in shallow water Validation of Waves at a Large Distance from the Ship To validate the computational results from BEShiWa, especially at a large distance from the ship, Figure 2 compares the computed wave records (dashed lines) with those measured (solid lines) in the Duisbur~ Shallow-Water Towing Tank (VBD) for the inland ferry model. At the design speed of Fnh = 0.873 the following observations can be made: (i) The agree- ment is quite satisfactory near the ship (y = 6 m) and pretty good ahead of the ship. (ii) The consistently improving agreement (in both amplitude and phase) with increasing proximity to the channel sidewall demonstrates the usefulness of the present solution method for predicting far-field wash-waves. (iii) The relatively large discrepancy in the ship's wake may have been caused by the running submergence of the transom stern which was not explicitly accounted for in the present computer program. Luckily, transverse waves on the ship's track are not relevant to the wash problem. (iv) Multiple upstream solitons ahead of the ship are not visible. However, other computations have shown that shape and speed of the wave ahead of the ship do depend on the initial acceleration pat- tern. Influence of Channel Section Shape on Wash Waves Figure 3 shows wave patterns generated by the inland passenger-ferry moving at critical speed (referred to the water depth along the channel centerline). Three different channel section shapes are presented to display the influence of transversally varying bottom topography on wash waves, namely, a rectangular 5 channel section in graph (a), a trapezoidal channel section in graph (b), and a polygonal section consist- ing of a deepened fairway in a shallow channel in graph (c), all three of the same depth at the centerline and the same width overall. In all cases the ferry runs along the channel centerline. It is seen that unlike the rectangular channel (a) perfect solitary waves could be generated neither in the trapezoidal channel (b) nor in the deepened fairway (c), although a perfect- reflection condition was implemented on the side- walls in all cases. An important observation is that the highest waves occur either near the sidewall in the trapezoidal channel (b) or in the shallow region be- side the deepened fairway (c). These high waves could affect the safe operation of other floating bod- ies near the bank or possibly cause bank-erosion. Effects of Initial Ship Acceleration Since wave elevation and wave energy both propa- gate in shallow water at the same velocity, an initial disturbance would also travel at the same speed. This leads to an interaction of waves generated in the initial acceleration phase with those later generated by the ship at its asymptotic speed. Due to the possi- ble scattering of initial waves by the longitudinal bottom-topography, the resulting wave pattern could depend on the ship acceleration or deceleration. To examine this effect, two simulations were performed for the subject container ship moving on a near-shore fairway. The contour plot of the investigated fairway is given in Figure 5. The wave patterns generated are shown in Figure 6 (a) and (b) for the case of slow and fast acceleration, respectively. As expected, the larger
the acceleration, the higher the initial waves observed lated wave probes (locations marked in Figure 5~. ahead of the main wave system generated by the Moreover, an interaction of waves generated in the steady ship motion. Within the main wave system, acceleration phase with those later generated by ship the initial acceleration influences strongly the so- at constant speed can be noticed in the transition called primary wave but only weakly the trailing between the deepened and shallow regions, see graph waves. This phenomenon can be clearly observed in 7(a), where the higher harmonic waves are missing Figure 7 showing wave records taken by four simu- totally in the slow-acceleration case. [m] ~ I ~ ~ ~ ~ I ~ ~ y=6m ~ -0.5 ~ <~ ~ ~ NX /~ ___ -1 1 ----------- -----1-------- -1-------- --------- I -200 -150 -100 -50 Stern O Bow 50 100 150 [m] 200 [m] ~ ~ ~ / ~ ~ ~ I y=15m 1 .0.5--~ ~ ~ N!/_ or ~ ~ ~ -1 r--------~---------l------__l____ ~~~r~ ~~ ~~ It l -200 -150 -100 -50 0 50 100 150 [m] 200 1 r--------l---------l------__l_____ ~~~r~-- ~ It lo lo l [m] ~ ~ ~ ~ _ ~ ~ ~ ~ y=25m 0.5 ~ f ;~ A__, __ _ ~ ~ ,._ I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -200 -150 -100 -50 0 50 100 150 [m] 200 [m] _ ~ ~ 3Sm 1 1 1 1 1 1 1 1 1 -200 -150 -100 -50 0 50 100 150 [m] 200 1 r--------l---------l------__l_____ ~~~r--- ~ It lo lo l m| 1 1 1 1 1 1 1 1 1 -200 -150 -100 -50 0 50 100 150 [m] 200 Figure 2: Comparison of wave records (replotted as profiles in shipbound coordinates) of the subject ferry at Fnh = 0.873 as measured in the VBD ~ ~ and calculated by the computer program BEShiWa (-- -) 6
a) Contour plot of the wave pattern in a rectangular channel with uniform water-depth h = 5 m b) Contour plot of the wave pattern in a trapezoidal channel with maximum water-depth h = 5 m c) contour plot of the wave pattern in a channel with a deepened fairway (h = 5 m) and shallow banks (h = 3 m) Figure 3: Influence of transversally varying bottom topography on the wave pattern generated by the subject inland passenger-ferry moving at constant speed As = 7 m/s (corresponding to local Fnh = 1 on the channel centerline) z _} ~ Y- 1 LL: . 'i l - '1 50 m ~ - Figure 4: Channel section shapes for cases (b) and (c) 7
Figure 5: Contour plot of the bottom topography (locations of the four wave probes (a)~d) marked) a) Slow acceleration b) High acceleration Figure 6: Contour plots of wave patterns generated by the subject containership at a speed V=8.35 m/s in a near- shore fairway (Note: Here the direction of motion is from right to left) 8
0.4 - 0.3 - 0.1 - O - -0.1 -0.2 -0.3 0.3 0.2 0.1 ,_ O -0.1 -0.2 -0.3 -0.4 - 0.3 0.25 0.2 0.15 0.1 - ,= 0.05 or -0.05 -0.1 -0.15 -0.2 0.5 0.4 0.3 0.2 0.1 o -0.1 -0.2 -0.3 - . ..... . .. _~ . . 0.2- / ~ , . WN 1 \K wave probe (a) ... 0 50 100 150 200 250 300 350 t [s] 1 ~ ~ 1 M7' wave probe (b) 0 50 100 150 1\ 1~ ~ _~ ~ ~ ~ \\ ~ 200 250 300 .. 350 t[s] wave probe (c) 0 50 100 150 200 250 300 . 1 1\ 1 1 ~ ~ , _- ~ 1 ~ ' 1 . ..... .............. 1 t [s] wave probe (d) ~ I 1 1 1 0 50 100 150 200 250 300 t [s] Figure 7: Wave records showing the influence of initial ship acceleration ~ fast, ----slow acceleration) 9
Interaction of Wave Generation and Propagation with Bottom Topography As discussed by Feldtmann and Garner (1999), a suitable artificial ramp in a wash-sensitive region could reduce the wash. The main idea is to minimize the passing time from a supercritical speed in a shal- low region to a subcritical speed in a deeper region or vice versa so that the large wave generation in the transcritical speed range may be avoided or at least reduced. Figure 9 demonstrates the wave generation and propagation over a fairway with a ramp such as shown in Figure 8. The ferry speed was assumed to be constant at 8 m/s. Before the subject ferry arrives at the ramp it moves at a supercritical speed. So the wave pattern has pronounced divergent waves ac- companied by transverse waves caused by the initial acceleration, see graph (a). As soon as the ferry crosses over the ramp a strong interaction of the su- percritical wave-pattern occurs with the ramp, and the wave pattern changes its form as seen in graph (b). This interaction continues until the ferry has moved far beyond the ramp, see graphs (c) to (d). Finally, there is a wave pattern typical of the subcritical speed in the deeper region, and the wake waves over the ramp almost disappear from the calculation domain due to the no-reflection condition implemented on the open truncation-boundaries to each side, see graph (e). Of period 2.02 s and amplitude 0.02 m propagate over the upward slope of the bar, the nonlinear effect in- creases and, hence, higher harmonics are generated, see records of wave probes located at x = 26.04 m and 28.04 m. These higher harmonics become quickly free over the downward slope, see records of wave probes located at x= 30.44 m and 37.04 m. There is remarkable agreement between calculation and measurement as long as the higher harmonics do not get free. Thereafter, a strong interaction between the primary waves and the free waves makes the latter equally important. Since the free wave has approximately half the wave length of the primary one, the dispersion relation of the classical set of Boussinesq's equations needs to be corrected, as discussed by Dingemans (1997~. Similar results were obtained for primary harmonic waves of period 2.525 s and amplitude 0.029 m, see Figure 11. CONCLUSION For predicting the wash waves generated by ships a method based on Boussinesq-type equations for the far-field flow and on slender-body theory for the near-ship flow yields satisfactory results. It covers all relevant effects associated with the nonlinear and unsteady nature as well as with the large-domain feature of the wash problems. Any neglect of these effects would lead to a poorer approximation. Since the propagation of wash waves sig- Validation of Wave Propagation over Uneven nifir~nt]~~ ~~ ~~ i-- ~ Bottom To validate the computation of the wave propagation over an uneven bottom, a measurement performed at Delft Hydraulics (Dingemans, 1994) with a 2D bar- type bottom topography as shown in Figure 12 was numerically simulated. A wavemaker generates a harmonic wave train propagating from left to right in a wave channel. Figure 10 compares the cal~,l~.~1 .~ . _ ~ . At .C ~ _ ~~$ ·t,J~. _= ~1_ ~~4A~ ~~ wavy; It;~;~lUb WItIt ~r~e measured by wave probes at 6 different locations. As the harmonic primary waves = /~///////~/////~) Lit Cal milcantly depends on bottom topography, ship speed and motion history, any measures for reducing wash waves deduced from computational predictions need to be validated by experiments. At the same time, however, this strong dependence opens up possibili- ties of formulating suitable criteria for safe ship op- eration (speed and distance to shoreline or river bank) , management (con- struction and maintenance). as well as for effective fairway I//// 50 20 _ _ 30 _ Figure 8: Schematic of the subject ramp (all dimensions in m) 10
(a) wave pattern generated by the subject ferry in the shallow region before the ferry moves over the ramp (b) interaction of the supercritical wave-pattern with the ramp as the ferry crosses over the ramp (c) wave pattern as the ferry leaves the ramp behind (d) evolution of the wave pattern while the ferry moves beyond the ramp (e) wave pattern of the ferry at subcritical speed in the deeper region Figure 9: Evolution of the wave pattern generated by the subject inland passenger-ferry moving at a constant speed of 8 m/s over a fairway with a ramp as shown in Figure 8. 11
0,03 0,02 0,01 0,00 -0,01 -0,02 -0,03 o 0,04 0,03 0,02 E 0,01 0,00 -0,01 -0,02 A AA 0,06 0,04 E 0,02 0,00 -0,02 -0,04 _ o 5 10 15 20 0.04 E 0,02 0,00 -0,02 1 -0,04 0,04 E 0,02 0,00 -0,02 -0,04 0.04 0.02 -0.02 20 25 30 35 40 45 50 Y = ;^~A ndm 10 15 20 0 5 0,06 -0.04 0 5 25 30 35 40 x = 26.04 m 45 50 10 15 20 15 20 25 x = 37.04m . ~\ ~ . ~ it, it' An 25 30 35 40 45 50 -30.44 m 30 35 40 45 50 10 15 20 25 30 35 40 45 t [s] 50 Figure 10: Records of wave elevation at 6 different wave-probe locations for an initially harmonic wave of period 2.02 s and amplitude 0.02m propagating over an uneven bottom (see Figure 12) as measured by Dingemans and calculated using BEShiWa (- - -) 12
x = 9.44 m O,04 0,03 0,02 0,01 0,00 -0,01 -0,02 -0,03 -0,04 0,10 O,08 0,06 E 0~04 0,02 0,00 -0,02 _ -0,04 o 0,12 0,10 0,08 _ 0,06 E 0,04 0,02 0,00 -0,02 -0,04 _ o 0,10 0,08 0,06 E 0~04 0,02 0,00 -0,02 -0,04 _ o 0,08 0.06 15 20 25 30 5 10 ...... .- :~ ~ V ~ r ~ it' ~ . 15 it [~ As on ' 1 0,04 E 0,02 0,00 -0,02 -0,04 -0,06 0 5 10 0,08 0,06 0,04 0,02 E BOO -O,02 -O,04 -0,06 -0,08 o 15 20 25 30 35 40 x = 37.04m Figure 11: Records of wave elevation at 6 different wave-probe locations for an initially harmonic wave of period 2.525 s and amplitude 0.029 m propagating over an uneven bottom (see Figure 12) as measured by Dingemans ~ and calculated using BEShiWa (- - -)
o -0.2 _ -0.4 - -0.6 ens \ _ -1 _ 0.00 5.00 10.00 15.00 20.00 25.00 x [m] 30.00 35.00 40.00 45.00 Figure 12: The two-dimensional bar-type bottom topography investigated by Dingemans REFERENCES Bolt, E.: "Fast ferry wash measurement and criteria," Proceedings of the FAST 2001, Southhampton, UK, 2001. Chen, X.-N. and Sharma, S.D.: "A slender ship moving at a near-critical speed in a shallow channel," Journal of Fluid Mechanics 291 (1995, S. 263-285. - Presented at the 18th Int. Congress of Theoretical and Applied Mech., Haifa, Israel, 1992. Chen, X.-N. and Uliczka, K.: "On ships in natural waterways," Proceedings of the RINA International Conference on Coastal Ships and Inland Waterways London 1999. ~ , Dingemans, M.W.: "Comparison of computations with Boussinesq-like models and laboratory measurements," MAST-G8M note, H1684, Delft Hydraulics, 1994. Dingemans, M.W.: "Water Wave Propagation over Uneven Bottoms", Advanced Series on Ocean Engineering, Vol. 13, 1997, Part II, pp. 635. Doctors, L.J., Philipps, S.J. and Day, A.H.: "Focussing the wave-wake system of a hi~h-sneed marine ferry " Pro- ceedings of the FAST 2001, Southhampton, UK, 2001. ---I-- -I - - ~ ~ ~ _ _ Doyle, R., Whittaker, T.J.T. and Elsasser, B.: "A study of fast ferry wash in shallow water," Proceedings of the FAST 2001, Southhampton, UK, 2001. Feldtmann, M. and Garner, J.: "Seabed modifications to prevent wake wash from fast ferries," Proceedings of the RINA International Conference on Coastal Ships and Inland Waterways, London, 1999. Henn, R., Sharma, S. D. and Jiang, T. "Influence of Canal Topography on Ship Waves in Shallow Water," Proceed- ings of the 1 6th Int. Workshop on Water Waves and Floating Bodies, Hiroshima, Japan, 2001. Jiang, T.: "Ship Waves in Shallow Water," Fortschritt-Berichte VDI, Series 12, No. 466 with ISBN 3-18-346612-0, 2001. Jiang, T.: "Investigation of waves generated by ships in shallow water," Proceedings of the 22n~ Symposium On Naval Hydrodynamics Washington, D.C., USA, 1998. Koushan, K., Werenskiold, P., Zhao, R. and Lawless, J. "Experimental and theoretical investigation of wake wash," Proceedings ofthe FAST 2001, Southhampton, UK, 2001. MacLarlane, G.J. and Renilson, M.R.: "Wake wave - a rational method for assessment," Proceedings of the RINA International Conference on Coastal Ships and Inland Waterways London 1999. , , 14
Peregrine, D.H.: "Long waves on a beach," Journal of Fluid Mechanics, Vol. 27, 1967, pp. 815-827. Raven, H.C. "Numerical Wash Prediction Using A Free-Surface Panel Code," Proceedings of the RINA Interna- tional Conference on Hydrodynamics of High-Speed Craft - Wake Wash and Motion Control, London, 2000. Taylor, P.J.: "The Blockage Coefficient for Flow About an Arbitrary Body Immersed in a Channel," Journal of Ship Research Vol. 17, 1973, pp. 97-105. Yang, G.-Q., Faltinsen, O.M. and Zhao, R.: "Wash of ships in finite water depth," Proceedings of the FAST 2001, Southhampton, UK, 2001. Zibell, H.G. and Grollius, W.: "Fast vessels on inland waterways," Proceedings of the RINA International Confer- ence on Coastal Ships and Inland Waterways, London, 1999. 15
DISCUSSION Stephane Cordier Bassin d'Essais des Carenes, France Ships inland waterway are confronted with changes in maneuvering behavior in shallow or restricted waters. Could you please tell us how this method can be used or extended to improve the prediction of maneuvering forces for ships in restricted water? AUTHORS' REPLY We thank Dr. Cordier for his question. For predicting the maneuvering forces on ships in restricted water, we need only to improve our approximation for the near-ship flow. Currently, we examine the possibility of coupling the BEShiWa program with different methods, such as with a panel program or a Euler solver or a RANSE solver. We hope to present our new results in the near future. DISCUSSION L.J. Doctors University of New South Wales, Australia I would like to express my appreciation to the three authors for a most interesting paper on the subject of wave generation, a matter of interest to many researchers who are aiming to reduce the potential damage done by high-speed ferries as well as traditional vessels, travelling near coastlines and river banks. The plots in Figure 1, in particular, are excellent for displaying the wave patterns created by the vessel at the various depth Froude numbers. It is encouraging, also, to observe the good comparison between the measured and calculated wave profiles in Figure 2. It is particularly impressive to see the calculations for the non- uniform bottom topography in Figure 3 and Figure 1 1. Referring specifically to Figure 2, could the authors comment on the likely relative accuracy of the BeShiWa (Boussinesq's Equations for Ship Waves) program, compared with, say, a more traditional linearized-free-surface method, in which no depth averaging is effected? That is, what sacrifice has been made in losing the details of the vertical distribution of the transverse velocities within the flow domain, in order to obtain the very impressive capabilities of BeShiWa? Secondly, can the authors verify that the effects of sinkage and trim are not included in their work? No doubt this would require a full near- field calculation (presumably not done here). The discusser feels that the effects of sinkage and trim are probably not important in most cases of practical interest. AUTHORS' REPLY We greatly appreciate Professor Doctors's comments and questions. The accuracy of predicting ship-wave propagation in shallow water by using the BEShiWa program is generally remarkable or at least practically acceptable in comparison with model tests. Till now, no attempt has been done by us to compare with a traditional linearized- free-surface method. A simple answer here would be that we do not have such a linear code. However, we would emphasize again our statement that due to the nonlinear and unsteady nature of ship waves in shallow water the linear theory remains to be a restricted approximation. Furthermore, it should be clarified that the vertical distribution of the transversal velocity components is explicitly described as an analytical function of the averaged horizontal velocity in the Boussinesq's shallow-water theory. So the vertical effects are not neglected, but analytically approximated in the BEShiWa program. Coming now to the second question, the effect of the sinkage and trim as well as the free surface elevation are simultaneously included in our near-field solution, see paragraph "Approximation of the Near-Ship Flow". As shown by Jiang (1998), the sinkage and trim could be well predicted by the BEShiWa program. The agreement of our calculations with model measurements was good not only in the subcritical speed range, but also in the transcritical and supercritical one.
DISCUSSION H.C. Raven MARIN, The Netherlands This is an interesting paper on a topical subject. The extensive results illustrate the richness of wave phenomena occurring in practical situations; and show how strongly the particulars of the waterway determine which wave effects dominate and whether any wash problems will occur. In order to predict these phenomena, there is a need for a computational tool that incorporates the essential features and has reasonable efficiency. Boussinesq-type models seem to go a long way toward that objective, as the applications illustrate. My question is on the boundary condition at the ship hull; which is the one that generates the waves. In the present work, a 'slender-body' type condition is used: the passage of the ship imposes a lateral velocity distribution, which is averaged over the entire water depth. This is consistent with Boussinesq theory; but intuitively one would expect that this is less accurate for higher water depth / draught ratio's. Could the authors comment on their experience in this regard, and mention the water depth / draught ratio for the good results in Fig. 2? Secondly, is there a way to compute and incorporate the dynamic trim and sinkage in this method? AUTHORS' REPLY We thank Dr. Raven for his comments, particularly for his indication of our consistent approximation in using the Boussinesq's equations for the far-field flow and an extended slender-body theory for the near-ship flow. We agree with his presumption that our method is less accurate for higher ratios of water-depth to ship draught in the absolute sense of the increased water depth, but not in the relative sense of the ratio. For instance, the ratio for the good agreement in Figure 2 was approximately 4. The crucial parameter for using the BEShiWa program is the depth Froude number which should not be below the associated lower limit defined by Jiang (2001~. For the answer to the second question we refer to our reply to Professor Doctors on the previous page. DISCUSSION H. S. Choi Seoul National University, Korea In this paper, you have used the depth-averaged Boussinesq equations to describe wave field generated by ships moving on Fairways. Have you ever compared your numerical results with those obtained by FEM based on ON equations, which, for example, we presented at the 1 8th SNH in Ann Arbor, 1990? AUTHORS' REPLY We thank Professor Choi for the reference of his work with the generalized Green-Naghdi (GN) equations. In comparison with the Boussinesq's equations the Green-Naghdi theory takes account of the fully nonlinear effects. As discussed by Jiang (2001), the application of the classical Boussinesq's equations for most practical cases is not limited by the nonlinear treatment but by the dispersion treatment. Various methods are derived in the work cited for the improvement of the dispersion relation of the Boussinesq's equations. Numerically we prefer the numerical more efficient Boussinesq approximation.
