Detailed analysis of the flow within the boundary layer and wake of a full-scale ship

TL;DR: A detailed numerical flow assessment of the boundary layer and wake of a full-scale cargo ship was conducted using a sophisticated numerical approach that is able to resolve large turbulent scale vortices contained in the flow as mentioned in this paper.

About: This article is published in Ocean Engineering.The article was published on 2020-12-15 and is currently open access. It has received 5 citations till now. The article focuses on the topics: Boundary layer & Wake.

Summary

1. Introduction

• Experimental fluid dynamics towing tank tests have been traditionally used to evaluate the flow around the ship.
• Also, the aft boundary layer is significantly different in model and full-scale.
• LES resolves turbulent vortices everywhere in the flow domain down to the grid size.
• This method is more computationally affordable in ship hydrodynamics than LES, allowing the study of complex unsteady flows in full-scale and being deemed as the best alternative to calculate wake parameters, especially behind high block coefficient ships (Larsson et al., 2015).
• The authors established that both DES approaches improve the prediction of the total resistance and velocity distribution for most of the propeller plane; however, the authors also revealed that both models showed issues predicting the shear stress in the boundary layer region.

2. Benchmark Case Study

• The 'Regal' is a 138m single screw vessel with the following main particulars (Table 1):.
• Before the sea trials, the vessel was dry-docked, the hull was cleaned, and the propeller surface was polished.
• The scanned geometry was directly imported into the CFD computations, thus ensuring high accuracy of the geometry CAD models.
• These numerical study simulations of the flow around the ship were conducted using the commercial CFD code Siemens Star CCM+.

3. Numerical Approach

• This work uses the Improved Delayed Detached Eddy Simulation turbulence modelling strategy and following the approach described in previous work (Pena et al., 2019; Pena et al., 2020).
• In general, this model switches between the RANS SST k-ω model, which has demonstrated maturity and reliability calculating skin friction coefficients and steady flow features (Wilcox, 1993; Menter, 1994); and LES in away from the wall, where it can capture the larger unsteady eddies such as the bilge vortex.
• This approach follows the ITTC recommended practices (ITTC, 2014c, Lloyds Register, 2016).
• The DFBI (Dynamic Fluid Body Interaction) module was used to simulate the motion of the ship in response to pressure and shear forces exerted by the fluid on the solid body, as well as gravity.

4. Boundary Layer Analysis Approach

• As mentioned in the introduction section, this analysis required a higher definition of the boundary layer of the ship that is required to measure the velocity profiles across the 3D ship boundary layer of the.
• All planes are parallel to each other, and their normal component is parallel to the ship length vector.
• Transversal waterline (WLi) and length planes (LGi) are also defined for reference purposes.
• Note that FR½ corresponds to the propeller plane, where nominal wake velocity measurements are taken.
• Additional probe points are installed at the flat bottom which are built through the intersection of LGi planes, FRi planes and the hull surface.

5. Ship Resistance: Calculated Components

• The bare hull total resistance, viscous resistance and pressure resistance are determined from the converged IDDES simulations.
• The drag forces are made dimensionless and represented by the total resistance coefficient (𝐶𝑇), pressure resistance coefficient (𝐶𝑃) and the viscous resistance coefficient (𝐶𝑉).
• Note that 𝐶𝑉 is calculated using the shear stress tensor and accounts for viscous stresses that the fluid exerts to the hull.
• Ship resistance distribution per unit length is also measured to quantify the drag contribution of each hull segment (i.e. bow, stern).
• Resistance per unit length was plotted, where 1 is at the Aft Perpendicular and 14 is at the Forward Perpendicular (F.P).

6. Computational Set-up

• 1 Time-step and Spatial Discretization The Courant number (Cr) is used to represent the number of cells that the fluid travels through within a time step, and it is defined as Cr= 𝑢∆𝑡 ∆𝑥 (where 𝑢 is the local speed, ∆𝑡 the interval of time (time step) and ∆𝑥 the cell size in the direction of the flow).
• All simulations used a 2nd order spatial and temporal discretisation for all equations.
• A very high-density mesh was generally defined in regions around the stern of the vessel , including the rudder.
• Besides, a second- order spatial resolution scheme was used to discretise the free-surface, with a trimmed mesh aligned with the calm water free-surface.
• With regards to the near-wall cell, the grid was set-up to achieve a mean y+ of 1 and ensuring that no wall functions are applied in the near-wall region.

7. Mesh Performance Analysis

• A mesh independence study was conducted for the simulation set-up on four different grid resolutions by varying the mesh size input parameter while holding all other parameters constant and following recommended practices (ITTC, 2017).
• The uniform parameter refinement ratio was established as √2.
• Also, mesh convergence is monitored by plotting the resolved Turbulent Kinetic Energy (TKE) convergence for the four grids at FR1 as shown in Figure 8.
• The comparison demonstrates that as desired, the core of the bilge vortex falls in the LES region and confirms that the present grid will resolve most of the vortex turbulence.
• Looking back at Figure 11, it could be seen that the LES region is already close to the near-wall region, so it could be risky to significantly reduce the mesh size, since there could be an incursion of the LES region into the RANS region and thus trigger the undesired MSD phenomenon.

8. Results Analysis

• The numerical results of the flow around the hull are shown and discussed in terms of ship resistance coefficients, limiting streamlines, nominal wake and the boundary layer at the stern of the ship.
• Overall, the pressure resistance displays a significant pressure imbalance between the aft and bow ends of the ship.
• Downstream at FR7 , the boundary layer considerably thickens, and stronger crossflows appear.
• The FR10 plot and WL1 series represent the velocity profiles measured on the probe point located at the intersection of the frame 'FR10' and the waterline 'WL1' (or the point PFR10WL1).
• The side vortex sheet is formed due to the change in curvature of the hull and is found to be weak when compared to the bilge vortex.

8. Discussion and Conclusions

• The present research has numerically investigated the ship hydrodynamic performance of a full-scale general cargo ship using extensive flow data on the boundary layer and wake field.
• This negative pressure gradient, together with the bilge vortex create the perfect environment for a local flow recirculation region near the propeller hub.
• As the flow advances, the near-wall region flow particles retard to a point where it can no longer counteract the pressure gradient, thus separating from the surface.
• The detriment on the ship efficiency is triggered by the loss of momentum that takes place on the aft end boundary layer.
• In general, the analysis conducted within this work has demonstrated to be essential to a full understanding and explanation of the underlying physical structure of the full-scale aft end flow.

Figures

Table 3 CFD simulation ship parameters

Figure 2 Reference planes for boundary layer numerical measurements

Table 5 Mesh independence study for simulation at 8 knots

Figure 8 Mean resolved TKE for G1, G2, G3 and G4

Figure 26 Flow separation on the hull represented by isosurfaces of negative shear stress

Figure 25 Streamwise y+ u+ velocity profile and hull flow separated regions

Figure 27 Vorticity fields in x, y and z directions

Table 6 Integral Hydrodynamic Resistance Coefficients

Figure 12 Pressure and viscous resistance coefficient comparison per unit length

Figure 21 Mean velocity field at FR3

Figure 20 Mean velocity field at FR7

Table 4 Reference planes

Figure 3 Reference planes for boundary layer numerical measurements

Figure 23 Turbulent kinetic energy distribution. Dark isolines show the boundary layer extension at that crosssection.

Figure 22 Mean velocity field at FR1

Figure 6 Fluid domain (left) and bow refinements (right)

Figure 7 Mesh in the region of free-surface and wake (right plan view, left represents a side view of the mesh)

Figure 11 IDDES blending function vs resolved TKE at FR1

Table 1 Regal main particulars (Lloyds Register, 2016)

Figure 1 Regal general cargo ship (Lloyds Register, 2016)

Figure 15 Mean Nominal Wake Field at FR1/2 (x/L=0.02)

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Detailed analysis of the flow within the boundary layer and wake of a full-scale ship" ?

This article presents a detailed numerical flow assessment of the boundary layer and wake of a full-scale cargo ship. The analysis method followed during this work has been a determinant factor for fast and efficient design of energy saving devices, propellers or rudders that work within the limits of the boundary layer of a ship. In particular, this thorough analysis avoided the necessity to use the commonly used practice of trial and error that is typically followed in the maritime industry.