Concluding Comments

Due to the largely confidential nature of military and commercial applications of ducted propulsors, it has proved very difficult to obtain detailed full-scale design data for comparison with numerical results from the inverse design method. However, the author is confident that the generic test cases presented in Sections 6.1 and 6.2 are representative of existing full-scale propulsors. With respect to these results, the author offers the following concluding comments:-

1) In the numerical solution of the actuator duct and three-dimensional mean

shear runs it is the determination of propulsor mass flow that dominates the computational procedure in terms of CPU time requirements. As a result the computational times for the three-dimensional mean shear runs presented in Figures 6.15, 6.36 and 6.37 are little different from the corresponding actuator duct runs that do not include three-dimensional effects.

However, for the fully three-dimensional runs it is the solution of the block tridiagonal systems of equations (equations A6.5 and A6.6, Appendix VI) that dominates the solution process, resulting in the much greater computational times required for the fully three-dimensional runs when compared to the simplified runs. Additionally, storing the three-dimensional rothalpy and drift function fields is memory-intensive.

When solving the block tridiagonal equations, storage requirements vary roughly in proportion to the number of Fourier harmonics taken, N, with solution times varying approximately in proportion to IsP. Thus, due to the limitations of the available computing facilities it was found necessary to limit N to no greater than 16, resulting in relatively poor resolution of the jumps in rothalpy at the blade (Figures 6.14, 6.32 and 6.33).

2) When compared to the three-dimensional runs, the actuator duct runs produce a very good estimate of overall propulsor performance (i.e. propulsor thrust, torque, mass flow, etc.), with reasonable estimates of such detailed characteristics such as blade shapes and blade pressure jumps being obtained.

3) The fully three-dimensional runs seem to yield little improvement in propulsor

flow field and blade shape estimates over the three-dimensional mean shear runs which neglect the circumferential variations in rothalpy and drift function.

The only noticeable differences between the predictions of the two three- dimensional approaches are in the vicinity of the hub and duct, and would seem to be more a result of forcing the normal derivative of rothalpy to zero here (see Section 5.13 and Appendix VII), than due to any three-dimensional shear phenomena.

4) Bearing in mind the above comments, it might reasonably be concluded that for

practical marine ducted propulsor design three-dimensional mean shear runs of the inverse design method generally suffice.

Alhed to this, the effects of circumferential variations in shear vorticity suggested by the theoretical work of Hawthorne [2], Hawthorne and Armstrong [3], and Horlock and Lakshminarayana [4] would seem to be sufficiently small to be neglected in ducted propulsor blade design, at least for cases of axisymmetric onset flow.

It should be noted, however, that no conclusions can be drawn regarding the significance of three-dimensional shear phenomena at off-design conditions.

5) For submarine propulsors restrictions on propulsor weight together with the

desire to utilise boundary layer inflow to best effect [6] mean that duct diameter is limited, and as a consequence the mass flow rate is relatively low for the shaft power. However, minimising acoustic signature is of prime concern for this type of application, and therefore a relatively large number of lightly loaded blades is usually adopted.

One might also reach similar conclusions for surface warships, where although duct diameter is restricted by both the relatively shallow draught of these

vessels and the twin shaft configuration usually adopted, limiting propulsor noise is again of key importance.

For large merchant ships of deep draught, restrictions on propulsor diameter

and weight are less pressing. Consequently large duct diameters and

correspondingly high propulsor mass flows are desirable on grounds of propulsive efficiency (i.e. minimising "jet wake loss") [7]. Whilst usually only a single blade row configuration with a maximum of six blades is adopted on grounds of cost, avoidance of cavitation is a key factor in the choice of blade loading. Thus for large merchant ship applications one might assume that a relatively small number of large area, relatively lightly-loaded blades are adopted.

The adoption of relatively light blade loadings in each of these instances in some way helps to explain the relative insignificance of three-dimensional flow phenomena in the ducted propulsor designs outlined in this Chapter.

However, it should be noted that ducted propulsors for towing vessels such as a tugs or trawlers may be designed for a static ("bollard pull") condition (the extension of the inverse design method to deal with such cases is considered in Chapter 8). Under such circumstances the undisturbed onset flow is zero, and very high blade loadings may result. Thus, three-dimensional shear effects may well be significant in such instances, although it is quite likely that flow cavitation would be more of a concern.

6) It is generally found that using the procedure outlined in Section 5.7 to

explicitly satisfy the Kutta condition at the blade trailing edge involves only

slight modifications to the specified blade rV^ distribution provided that:-

(a) The specified rV^ distribution satisfies the condition =0 at the

blade trailing edge;

(b) The mesh quasi-streamlines align approximately with the final meridional velocity at the blade trailing edge.

7) It should be noted that, taken in isolation, circumferential variations in drift

[2], [3] and [4], as they occur even in cases where shear is absent (i.e. V // = 0).

8) Whilst for brevity the duct pressure distributions presented in this Chapter are

based on mean flow variables, it should be noted that the three-dimensional methods enable the circumferential variation of duct pressure to be evaluated if required.

9) Unfortunately little information on the avoidance of flow separation on

propulsor ducts was found in the available literature (Chapter 2). As a

consequence no assessment of the suitability of the duct profiles adopted in this

Chapter can be made. However, the duct pressure distributions for the

merchant ship propulsor (Figures 6.8 and 6.9) compare well in overall form to those presented by Gibson and Lewis [8] and Falcao de Campos [5] for ducted propellers mounted on afterbodies in the absence of shear flow.

10) It should also be stressed that the results presented in this Chapter are based on an assumption of inviscid flow. Unfortunately no means were available for assessing the effect flow viscosity and the associated duct, hub and blade boundary layers would have on the predicted results, although it would certainly be wrong to assume that these effects are negligible.

References

1) Andersen,0., Tani,M. "Experience with S.S. Golar Nichu", Symposium On

Ducted Propellers, RINA, Summer 1973

2) Hawthorne,W.R. "Rotational Flow Through Cascades. Part I : The

Components of Vorticity", pp266-279. Quart. Joum. of Mech. & Appl. Math., Vol.8, 1955

3) Hawthorne,W.R., Armstrong,W.D. "Rotational Flow Through Cascades. Part

II : The Circulation About the Cascade", pp280-292. Quart. Joum. of Mech. & Appl. Math., Vol.8, Part 3, 1955

4) Horlock,J.H., Lakshminarayana,B. "Secondary Flows: Theory, Experiment, and Application in Turbomachinery Aerodynamics", Annual Review of Fluid Mechanics, Vol.5, 1973

5) Falcao de Campos,J.A.C., "On the Calculation of Ducted Propeller

Performance in Axisymmetric Flows", Technical Report 696, Netherlands Ship Model Basin, Wageningen, The Netherlands, 1983

6) Friedman,N. "Submarine Design and Development", p.88, Conway Maritime

Press, 1984

7) Clayton,B.R, Bishop,R.E.D. "Mechanics of Marine Vehicles", p.358, E. & F.N.

Spon Ltd., London, 1982

8) Gibson,I.S., Lewis,R.I. "Ducted Propeller Analysis by Surface Vorticity and

Actuator Disc Theory", Symposium on Ducted Propellers, RINA, Summer 1973

Run Type: Actuator Duct 3D Mean Shear Fully 3D

Mass Flow Rate 556.09 556.23 556.52

(Tonnes/Second) Shaft Torque 1805.0 1806.8 1807.1 (kNm) Shaft Power 25.518 25.543 25.547 (MW) Total Blade 1748.8 1744.3 1744.3 Thrust (kN) Duct Thrust (kN) 847.90 847.57 847.91 NB