A pressure jump method for modeling waterjet/hull interaction Arash Eslamdoost n , Lars Larsson, Rickard Bensow Department of Shipping and Marine Technology, Chalmers University of Technology, 412 96 Gothenburg, Sweden article info Article history: Received 13 September 2013 Accepted 22 June 2014 Keywords: Waterjet propulsion Potential flow Free-surface Waterjet/hull interaction Gross thrust Thrust deduction fraction abstract A fast and robust method for the simulation of waterjet hull interaction is presented. Balancing the thrust force of the waterjet unit with the hull resistance, a method is developed for the prediction of the flow rate through the unit. The method is called the Pressure Jump Method and may be used in combination with both potential flow/boundary layer methods and more advanced viscous flow methods, for instance of the RANS type. In the present work the potential flow/boundary layer approach has been used. Validation of the method is accomplished through comparison of predicted results with measured data. The inlet velocity ratio, nozzle velocity ratio, gross thrust and thrust deduction are all within the experimental scatter. For the case studied the force and moment created by the waterjet unit cause the hull to sink deeper and attain a bow down trim compared to the bare hull case. The thrust deduction fraction is positive both in the computations and the measurements. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Waterjet propulsion is presently very common for high-speed vehicles. In particular, due to the high manoeuvrability achievable by means of waterjet systems, these propulsion units are being installed on craft, which require high manoeuvrability. The key point in the operation of waterjet systems is the momentum increment of the water drawn through a ducting channel by the action of an internal pump. The difference between the low energy flow at the system intake and high-energy flow expelled out of the nozzle generates the required thrust force for propelling the craft. In the early 1990s, the ITTC Waterjet Specialist Committee initiated a campaign for studying the waterjet propulsion system and its interaction with the hull. The outcome of this campaign is published in multiple ITTC proceedings, (ITTC 21 st 1996; ITTC 22nd 1998; ITTC 24th 2004; ITTC 24th 2005a,b; ITTC 25th 2008). A test procedure of a waterjet-propelled hull is defined and some measured data regarding both the bare hull and self-propelled hull are reported in these series of proceedings. van Terwisga (1996) carried out a very comprehensive study on waterjet/hull interaction, employing different tools, such as analytical, numer- ical and experimental methods. More recently Bulten (2006) analysed the flow inside a waterjet unit employing CFD tools. At higher ship speeds, he reported a clear deviation of the forces obtained from the integration of the axial force component on the solid wall with the thrust obtained from a simplified version of the integral momentum balance equation. This was against the obser- vation of van Terwisga (1996) that these forces are almost the same except at the hump speed of the craft. Wilson et al. (2005) performed some numerical investigations by means of a potential flow code capable of capturing the free-surface. Waterjet intakes were represented by a flat rectangular segment of hull surface having a uniform normal velocity and sucking the flow inside the hull. A downward force was reported on the aft-body of the hull due to the suction of the waterjets. Neglecting the detailed flow modelling inside the ducting channels, Kandasamy et al. (2012) derived an integral force/moment waterjet model and applied it to a CFD code to predict ship local flow and powering. Although the predicted resistance error for the bare hull and self-propelled hull was less than 5%, the calculated thrust deduction fraction showed a larger deviation from the data measured. Through the use of a Reynolds-Averaged Navier–Stokes (RANS) approach and overset grids, Takai (2010) solved and analysed flow fields for the bare hull and self-propelled flow fields of a high-speed sea lift hull over a range of speeds. A body force was used to model the pump effect. They state that RANS captures the important trends of force and motions, but the application of overset grids causes a mass loss inside the ducting channel that affects the prediction of the waterjet unit thrust force. In the present paper a fast and robust potential flow method for the simulation of waterjet/hull interaction is presented. Balancing the thrust force of the waterjet unit with the hull resistance, a method is developed for the prediction of the flow rate through the waterjet unit. In the following sections first the general terms Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/oceaneng Ocean Engineering http://dx.doi.org/10.1016/j.oceaneng.2014.06.014 0029-8018/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: arash.eslamdoost@chalmers.se (A. Eslamdoost). Ocean Engineering 88 (2014) 120–130