| 1. |
- Andersson, Jennie, 1986, et al.
(författare)
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Ship-scale CFD benchmark study of a pre-swirl duct on KVLCC2
- 2022
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Ingår i: Applied Ocean Research. - : Elsevier Ltd. - 0141-1187 .- 1879-1549. ; 123
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Tidskriftsartikel (refereegranskat)abstract
- Installing an energy saving device such as a pre-swirl duct (PSD) is a major investment for a ship owner and prior to an order a reliable prediction of the energy savings is required. Currently there is no standard for how such a prediction is to be carried out, possible alternatives are both model-scale tests in towing tanks with associated scaling procedures, as well as methods based on computational fluid dynamics (CFD). This paper summarizes a CFD benchmark study comparing industrial state-of-the-art ship-scale CFD predictions of the power reduction through installation of a PSD, where the objective was to both obtain an indication on the reliability in this kind of prediction and to gain insight into how the computational procedure affects the results. It is a blind study, the KVLCC2, which the PSD is mounted on, has never been built and hence there is no ship-scale data available. The 10 participants conducted in total 22 different predictions of the power reduction with respect to a baseline case without PSD. The predicted power reductions are both positive and negative, on average 0.4%, with a standard deviation of 1.6%-units, when not considering two predictions based on model-scale CFD and two outliers associated with large uncertainties in the results. Among the variations present in computational procedure, two were found to significantly influence the predictions. First, a geometrically resolved propeller model applying sliding mesh interfaces is in average predicting a higher power reduction with the PSD compared to simplified propeller models. The second factor with notable influence on the power reduction prediction is the wake field prediction, which, besides numerical configuration, is affected by how hull roughness is considered. © 2022 The Authors
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| 2. |
- Abolfazl, Shiri, 1975, et al.
(författare)
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Hydrodynamics of a Displacement Air Cavity Ship
- 2012
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Ingår i: 29th Symposium on Naval Hydrodynamics, Gothenburg, Sweden, Aug. 2012.
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Konferensbidrag (refereegranskat)abstract
- To study the resistance reduction of an air cavity for adisplacement vessel, a simplified model of a single cavityis tested in a cavitation tunnel. The drag force actingon the cavity and the aft plate were measured andthe water-air interface was monitored in different conditions.Behavioural changes to the free surface wereobserved by changing the geometry and flow characteristicslike air pressure, air flow rate and water flow speed.Computational model for different cases with the samegeometry and conditions were simulated using computationalfluid dynamics(CFD).The length of the cavity was designed to include approximately2.5 times the expected wavelength of thewater-air interface with Froude number based on theship length of 0.19 which approximately represents adisplacement ship with multi-wave air cavity at its cruisingspeed.Parameters like cavity pressure play an important rolefor the wave’s shape and the stability of the free surfaceinside the cavity. Both computation and experimentshow that the amplitude of the wave is sensitive to the airpressure in the cavity and the re-attachment of the waterto the rear end of the cavity has a close correlationto the inlet air pressure and water velocity. The ultimategoal of this investigation is to minimize effective powerincluding air supply while reducing resistance in/aroundthe cavity, but equally important is it to gain amore basicunderstanding of the air-cavity’s behaviour and possibleadditional resistance components.
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| 3. |
- Abolfazl, Shiri, 1975, et al.
(författare)
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Study of an Air Cavity in Water Tunnel
- 2011
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Ingår i: Proceedings of the 14th Numerical Towing Tank Symposium, Poole, UK.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- To study the resistance reduction of an air cavity for a displacement vessels, a simplified model of a cavity is simulated using computational fluid dynamics(CFD) and model tests. Initial CFD computation have been performed to design the model tests and the model tests will subsequently be used to validate the CFD simulations. The length of the cavity is approximately 2.5 times the expected wavelength of the water-air interface and the Froude number based on the cavity length is 0.2. Parameters like pressure play important role in the wave’s shape and the stability of the water surface inside the cavity. Computation shows that the amplitude of the wave is sensitive to the air pressure in the cavity and the re-attachment of the water to the rear end of the cavity has a close correlation to the inlet air pressure. The ultimate goal of this investigation is to minimize air supply while reducing resistance in/around the cavity, but equally important is it to gain a more basic understanding of the air-cavity’s behavior and possible additional resistance components.
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| 4. |
- Janson, Carl-Erik, 1957, et al.
(författare)
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Nonlinear computations of heave motions for a generic Wave Energy Converter
- 2018
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Ingår i: NAV International Conference on Ship and Shipping Research. - : Associazione Italiana di Tecnica Navale. - 2282-8397. ; 1 January 2018, s. 283 - 290, s. 283-290
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Konferensbidrag (refereegranskat)abstract
- A benchmarking activity of numerical methods for analysis of Wave Energy Converters (WEC) was proposed under the Ocean Energy Systems (OES) International Energy Agency (IEA) Task 10 in 2015. The purpose of the benchmark is to do a code-2-code comparison of the predicted motions and power take out for a WEC. A heaving sphere was used as a first simple test case. The participants simulated heave decay and regular and irregular wave cases. The numerical methods ranged from linear methods to viscous methods solving the Navier-Stokes equations (CFD). An overview of the results from the first phase of the benchmark was reported in (Wendt et al 2017). The present paper focus on the simulations of the sphere using one fully nonlinear time-domain BEM method one transient RANS method and one transient Direct FE method with no turbulence model. The theory of the three methods as well as the modelling of the sphere are described. Heave decay and heave motions for steep regular waves were selected as test cases in order to study and compare the capability to handle nonlinear effects. Computational efficiency and applicability of the three methods are also discussed.
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| 5. |
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| 6. |
- PERAVALI, SURYA KIRAN, 1991, et al.
(författare)
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AN INVESTIGATION ON ITTC 78 SCALING METHOD FOR UNCONVENTIONAL PROPELLERS
- 2016
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Ingår i: International Conference on Hydrodynamics.
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Konferensbidrag (refereegranskat)abstract
- The effort to find solutions to the environmental and energy saving problems regarding the operation of ships is always a matter of concern. Several new unconventional propeller designs have been introduced in recent years. These unconventional propellers are de- signed with non-planar lifting surfaces and a design approach to improve the energy efficiency by reducing the tip vortex loss and having a better lift/drag ratio. Suspicions have been raised that the standard methods for evaluation of model tests such as the ITTC 78 method does not take the full effect of unconventional propellers in to account [1]. In the present investigation, the performance of two propellers (one conventional and one unconventional) are analyzed us- ing CFD (RANS) in model and full scale in different operating conditions (open-water and behind a hull). The validation studies are performed in model scale and compared with the ex- perimental data. Further the computations are extended to full scale to study the scaling effects on the propulsive efficiency and different propulsive coefficients. The outcome is compared with the predictions from ITTC 78 method applied on different geometries and conditions.
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| 7. |
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| 8. |
- Shiri, Abolfazl, 1975, et al.
(författare)
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Evaluation of Closure Hypotheses Using Recent Experimental Data on the Similarity Region of Swirling Jet Flow
- 2007
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Ingår i: Proceedings of the 4th Ankara International Aerospace Conference.
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Konferensbidrag (refereegranskat)abstract
- An experimental recent study by Shiri et al. using laser Doppler anemometry showed that the growth rate enhancement due to swirl (c.f.\ Gilchrist, Naughton) does not persist in the far-field of a swirling jet flow with moderate swirl numbers (0.15 and 0.25). The results were consistent with the equilibrium similarity theory of Ewing in which the mean swirl velocity was argued to decrease downstream as $1/(x-x_o)^2$, while the mean stream-wise velocity decreased as $1/(x-x_o)$. The investigation included all three velocity components of the turbulence quantities at a swirl number of $S=0.25$, and all moments to third order were obtained (except those involving both the azimuthal and radial components simultaneously). As noted by George, if there were an effect of the source conditions on the similarity profiles, it is in the second and higher moment profiles where it would be expected to appear. The second and third-order moments are quite close to the earlier non-swirling results; some are collapsing perfectly, others have a slight difference. Regardless, the results are of considerable interest, since many of the quantities measured are those which must be modeled in all second-order closure models. This evaluation is of particular interest since both the mean azimuthal and mean radial velocities were obtained directly, meaning that both continuity and momentum balances were possible. The primary goal of this paper was originally to carry out such evaluations, but that work is still in progress. Here the swirl is shown to have a negligible effect on the overall Reynolds normal and shear stress balances.
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| 9. |
- Shiri, Abolfazl, 1975, et al.
(författare)
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Experimental Study of the Far Field of Incompressible Swirling Jets
- 2008
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Ingår i: AIAA Journal. - 1533-385X .- 0001-1452. ; 46:8, s. 2002-2009
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Tidskriftsartikel (refereegranskat)abstract
- The far field of an incompressible swirling jet has been studied using two-component laser Doppler anemometry.Three pairs of symmetric injectors were used to produce weak-to-moderate swirling jets. Velocity profiles of themean and fluctuating streamwise and azimuthal velocity components were measured in jets with two swirl numbers(S = 0.15 and 0.25) at axial locations up to 50 jet exit diameters. The velocity and turbulence intensity profiles,centerline decay, and growth rates for the various swirling jets have been compared with those obtained in the samefacility without swirl (S = 0). Like the previous observations for the near jet, there was no observable effect on the properly scaled far jet for the S = 0.15 case. The results were virtually identical to the non-swirling jet. For the S = 0.25 case, the only statistically significant effect was a shift in the virtual origin (from x/D* = 0.75 to -2.9). Therecent predictions of equilibrium similarity theory were found to be in excellent agreement with the experimentalresults. In particular, the mean azimuthal component of velocity falls off as the inverse square of the downstreamdistance. By contrast, the mean stream-wise velocity and turbulence intensities fall off with the inverse of thedownstream distance. As a consequence, the mean azimuthal equation uncouples from the rest, and so the asymptoticswirling jet behaves like the non-swirling jet.
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| 10. |
- Shiri, Abolfazl, 1975
(författare)
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Turbulence Measurements in a Natural Convection Boundary Layer and a Swirling Jet
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two sets of experimental measurement were carried out on an axisymmetric swirling jet flow and a natural convection boundary layer. In the first experiment, the far field of an incompressible swirling jet, discharged into a quiescent ambient has been studied using laser Doppler anemometry. The effect of low to moderate swirl (below vortex breakdown) was studied by measuring velocity profiles of the mean and fluctuating streamwise, radial and azimuthal velocity components at different streamwise locationsup to 50 jet exit diameters. The scaled velocity and turbulence intensity profiles, centerline decay and growth rates for two swirling jets with the strength of(S=0.15 and 0.25) have been compared to those obtained in the same facility without swirl (S=0). Like the previous observations for the near jet, there was no observable effect on the properly scaled far jet for the S=0.15case. For the S=0.25 case, the only statistically significant effect was a shift in the virtual origin. The results were in excellent agreement with the equilibrium similarity theory of Ewing[1999] in which the mean azimuthalcomponent of velocity falls off as the inverse square of the downstream distance.By contrast, the mean streamwise velocity and turbulence intensities fall off with the inverse of the downstream distance. As a consequence, the mean azimuthal equation uncouples from the rest, so the asymptotic swirling jet behaves like the non-swirling jet. The swirl is also shown to have a negligible effect on the overall Reynolds normal and shear stress balances.Measurements are also presented for the boundary layer flow of air along a heated vertical cylinder. The flow was entirely driven by natural convection: there was no co-flow. The cylinder was 4.5 m in height, had a diameter of 0.15 m, and was maintained at a temperature of 70 C (approximately 40 C above ambient). The cylinder was heated by water flowing through it, and mounted inside a 1.2 m in diameter cylindrical tunnel through which the ambient flowcould be controlled. Detailed measurements of temperature and velocity statistics were taken at heights of 1.5 m, 3m , 4 m height, the latter corresponding to a Rayleigh number based on length, Ra = g \beta \Delta T x^4 / \alpha \nu = 1.7 \times 10^{11}. Two-component burst-mode LDA was used for measuring the instantaneous velocity, while the fluctuating temperature was measured simultaneously using 1-micron platinum wire. Arrays of thermocouples were used to monitor the ambient and wall conditions, as well as the mean profile. Particular attention was given to the buoyancy and momentum differential and integral equations in order to evaluate the residual effects of stratification and co-flow. The strong temperature gradients and end conduction effects on the temperature probes adverselyaffected unsteady temperature results, as did the development with increasing height of the flow between the concentric cylinders.
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