Surface Streamline Research Articles

URANS studies of hydrodynamic performance and slamming loads on high-speed planing hulls in calm water and waves for deep and shallow conditions

URANS capability for hydrodynamic performance and slamming of high speed planing crafts are assessed, using metrics specified for naval applications. Wedge drop validation studies using two different solvers, one single-phase and the other two-phase, shows good agreement with experimental data for time history of pressure gauges. Planing hull validation studies are carried out using the historical benchmark experiments of Fridsma. Simulation conditions include calm water in deep and shallow conditions with fixed and free to sinkage and trim motions, as well as regular and irregular head waves in deep water free to heave and pitch motions. For simulations in calm deep water free to sinkage and trim, grid studies show that refined grid density on the hull, especially over the spray root area, chines, and transom stern are necessary for accurate solutions. Detailed verification and validation studies are carried out with satisfactory results since monotonic convergence and validation are achieved for most of the variables. Additional assessment metrics are studied including wave rise, hull pressure distribution, surface streamlines, existence of porpoising, and main spray trajectory. The results are compared with empirical expressions with reasonably good agreement except for main spray trajectory. Resolving the details of the spray flow is beyond the scope of the current work since finer grid resolution and/or more advanced free surface modeling is required. For simulations in calm deep water with fixed sinkage and trim, the assessment metrics include drag and lift forces, pitch moment, drag-lift ratio, and center of pressure. The results agree well with empirical expressions. For simulations in shallow water, similar studies are carried out and reasonable comparison results are obtained. Seakeeping simulations in regular head waves are carried out and validated with experimental data for response amplitudes and phases, with good agreement. Simulations in irregular head waves are carried out for two different weight configurations and validated against experiments for mean and amplitudes of resistance, heave, pitch, and acceleration. Slamming load analysis is carried out, where both emerging and re-entering slams are detected, although the current experimental data lack slamming pressures for validation. More recent research provides additional studies for calm water motions, slamming pressures, and spray flow trajectory that supports current results.

Flow separation from the smooth-contoured streamlined surfaces

In this article the flow features of liquids and gases in the diffuser channels are analyzed on the basis of Prandtl-averaged equation for the boundary layer. It is shown that in this situation there is a layer of liquid or gas close to the wall where the shear stress τ increases in the direction along the normal to streamlined surface (∂τ / ∂y > 0), which enables the separation-free flow of working fluids in

Barotropic and baroclinic contributions to along-stream and across-stream transport in the Antarctic Circumpolar Current

The Southern Ocean's ability to store and transport heat and tracers as well as to dissipate momentum and energy are intimately related to the vertical structure of the Antarctic Circumpolar Current (ACC). Here the partition between barotropic and baroclinic flow in the time-mean ACC is investigated in a Southern Ocean state estimate. The zonal geostrophic transport is predominantly baroclinic, with at most 25% of the transport at any longitude carried by the barotropic component. Following surface streamlines, changes in vertical shear and near-bottom velocity are large, and result in changes in the local partition of barotropic/baroclinic vertically integrated transport from 10/90% in the center of the basins, to 50/50% near complex topography. The velocity at depth is not aligned with the surface velocity. This nonequivalent barotropic flow supports significant cross-stream transports. Barotropic and baroclinic mass transport across the ACC is, on average, in opposite directions, with the net barotropic cross-stream transport being poleward and the net baroclinic equatorward. The sum partially cancels out, leaving a net geostrophic poleward transport across the different fronts between −5 and −20 Sv. Temperature is also transported across the fronts by the nonequivalent barotropic part of the ACC, with maximum values across the northern ACC fronts equivalent to −0.2 PW. The sign and magnitude of these transports are not sensitive to the choice of stream-coordinate. These cross-stream volume and temperature transports are variable in space, and dependent on the interactions between deep flow and bathymetry, thus difficult to infer from surface and hydrographic observations alone.

Single and Multiple Row Endwall Film-Cooling of a Highly Loaded First Turbine Vane With Variation of Loading

This paper reports endwall film-cooling investigations with single and multiple rows of fan-shaped film holes using temperature-sensitive paint (TSP). The experiments are carried out in a six-bladed linear cascade based on the geometry of a highly loaded gas turbine first vane. The film effectiveness performance of the cooling rows is investigated under the influence of enhanced near-wall secondary flow. Tests are conducted at three different loading conditions changing the profile incidence. Film-cooling injection is established at elevated coolant density ratios of 1.4 using heated carbon dioxide. Due to the finite thermal conductivity of the wall material, the heat conduction effects observed in the measured temperature fields are assessed by a newly developed data analysis based on a finite element thermal analysis and tracking algorithms along CFD-computed near-wall surface streamlines. The results showed that the coolant trajectories are visibly influenced revealing the intense interaction between the film jets and the near-wall flow field. These effects are certainly enhanced with higher incidence leading to increased streamwise coolant consumption and reduced wall coverage. At the cascade inlet, the film-cooling injection is significantly affected by the near-wall flow field showing distinct over- and undercooled regions. Due to the enhanced deflection and mixing of the film jets injected from a single row, area-averaged film effectiveness and wall coverage decreases about 9 and 11%, respectively. With adding more cooling holes to this endwall area, the influence of the enhanced secondary flow becomes more pronounced. Hence, larger reduction in film effectiveness of 23% and wall coverage with 28% is observed. For single row injection at the airfoil pressure side, the stronger secondary flow motion with intensified streamwise mixing leads to a visibly decreased endwall coverage ratio of about 38% and maximum flow path reduction of about 41%. In this case, film effectiveness is found to be reduced up to 47% due to the small amount of coolant injected through this row. This effect is significantly smaller when more cooling rows are added showing an almost constant cooling performance for all incidence cases.

Splash jet generated by collision of two liquid wedges

AbstractA complete nonlinear self-similar solution that characterizes the impact of two liquid wedges symmetric about the velocity direction is obtained assuming the liquid to be ideal and incompressible, with negligible surface tension and gravity effects. Employing the integral hodograph method, analytical expressions for the complex potential and for its derivatives are derived. The boundary value problem is reduced to two integro-differential equations in terms of the velocity modulus and angle to the free surface. Numerical results are presented in a wide range of wedge angles for the free surface shapes, streamline patterns, and pressure distributions. It is found that the splash jet may cause secondary impacts. The regions with and without secondary impacts in the plane of the wedge angles are determined.

An experimental study of stall delay on the blade of a horizontal-axis wind turbine using tomographic particle image velocimetry

Volumetric velocity fields were measured, for the first time, using Tomographic Particle Image Velocimetry (Tomo-PIV) on a model of the rotating blade of a 5kW horizontal-axis wind turbine (HAWT) to study the stall delay phenomenon at two different global tip speed ratios (TSR) of 3 and 5 with Reynolds number (Re) ≈5000. Static pressures were also measured and results illustrated higher suction peaks on the rotating blade than those on the static airfoil, which is typically observed for stall delay. Rather than the recirculation bubbles with strong reversed flows for the static airfoil at stall, attached flows were observed on the suction surface of the rotating blade. Radial flows from blade's root to tip were also found with strong spanwise velocity component, located in the vicinities of the vortices and close to the blade's suction surface. In contrast to the airfoil case, the vortices shed from the blade's edges were not found to break down into smaller ones. Blade's surface streamlines were also presented. At large angles of attack (AOAs), Coriolis forces were found to be larger than centrifugal forces in chordwise direction in all three measurement volumes, which contributes to the reduction of the adverse pressure gradient.

Lagrangian blocking in highly viscous shear flows past a sphere

An analytical and computational study of Lagrangian trajectories for linear shear flow past a sphere or spheroid at low Reynolds numbers is presented. Using the exact solutions available for the fluid flow in this geometry, we discover and analyse blocking phenomena, local bifurcation structures and their influence on dynamical effects arising in the fluid particle paths. In particular, building on the work by Chwang & Wu, who established an intriguing blocking phenomenon in two-dimensional flows, whereby a cylinder placed in a linear shear prevents an unbounded region of upstream fluid from passing the body, we show that a similar blocking exists in three-dimensional flows. For the special case when the sphere is centred on the zero-velocity plane of the background shear, the separatrix streamline surfaces which bound the blocked region are computable in closed form by quadrature. This allows estimation of the cross-sectional area of the blocked flow showing how the area transitions from finite to infinite values, depending on the cross-section location relative to the body. When the sphere is off-centre, the quadrature appears to be unavailable due to the broken up-down mirror symmetry. In this case, computations provide evidence for the persistence of the blocking region. Furthermore, we document a complex bifurcation structure in the particle trajectories as the sphere centre is moved from the zero-velocity plane of the background flow. We compute analytically the emergence of different fixed points in the flow and characterize the global streamline topology associated with these fixed points, which includes the emergence of a three-dimensional bounded eddy. Similar results for the case of spheroids are considered in Appendix B. Additionally, the broken symmetry offered by a tilted spheroid geometry induces new three-dimensional effects on streamline deflection, which can be viewed as effective positive or negative suction in the horizontal direction orthogonal to the background flow, depending on the tilt orientation. We conclude this study with results on the case of a sphere embedded at a generic position in a rotating background flow, with its own prescribed rotation including fixed and freely rotating. Exact closed-form solutions for fluid particle trajectories are derived.

Illustrative Stream Surfaces

Stream surfaces are an intuitive approach to represent 3D vector fields. In many cases, however, they are challenging objects to visualize and to understand, due to a high degree of self-occlusion. Despite the need for adequate rendering methods, little work has been done so far in this important research area. In this paper, we present an illustrative rendering strategy for stream surfaces. In our approach, we apply various rendering techniques, which are inspired by the traditional flow illustrations drawn by Dallmann and Abraham \& Shaw in the early 1980s. Among these techniques are contour lines and halftoning to show the overall surface shape. Flow direction as well as singularities on the stream surface are depicted by illustrative surface streamlines. ;To go beyond reproducing static text book images, we provide several interaction features, such as movable cuts and slabs allowing an interactive exploration of the flow and insights into subjacent structures, e.g., the inner windings of vortex breakdown bubbles. These methods take only the parameterized stream surface as input, require no further preprocessing, and can be freely combined by the user. We explain the design, GPU-implementation, and combination of the different illustrative rendering and interaction methods and demonstrate the potential of our approach by applying it to stream surfaces from various flow simulations. ;

Numerical Investigation of the Performance of Three Hinge Designs of Bileaflet Mechanical Heart Valves

Thromboembolic complications (TECs) of bileaflet mechanical heart valves (BMHVs) are believed to be due to the nonphysiologic mechanical stresses imposed on blood elements by the hinge flows. Relating hinge flow features to design features is, therefore, essential to ultimately design BMHVs with lower TEC rates. This study aims at simulating the pulsatile three-dimensional hinge flows of three BMHVs and estimating the TEC potential associated with each hinge design. Hinge geometries are constructed from micro-computed tomography scans of BMHVs. Simulations are conducted using a Cartesian sharp-interface immersed-boundary methodology combined with a second-order accurate fractional-step method. Leaflet motion and flow boundary conditions are extracted from fluid–structure-interaction simulations of BMHV bulk flow. The numerical results are analyzed using a particle-tracking approach coupled with existing blood damage models. The gap width and, more importantly, the shape of the recess and leaflet are found to impact the flow distribution and TEC potential. Smooth, streamlined surfaces appear to be more favorable than sharp corners or sudden shape transitions. The developed framework will enable pragmatic and cost-efficient preclinical evaluation of BMHV prototypes prior to valve manufacturing. Application to a wide range of hinges with varying design parameters will eventually help in determining the optimal hinge design.

Eddies in the southwestern East/Japan Sea

Closed loop mesoscale eddies were identified and tracked in the Ulleung Basin of the southwestern Japan/East Sea (JES) using the winding-angle (WA) methodology, for mapping the absolute geostrophic currents into surface streamlines of flow. The geostrophic velocity used here was the sum of the Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO), time variable velocity and the 1992–2007 mean geostrophic velocity. Local sampling bias was removed using the drifter observations. This WA methodology of deriving the Lagrangian path lines that drifters followed over a 7-day period was validated by individual drifter tracks and it demonstrated closed looping eddy motions. The WA method demonstrated that less than 6% of the closed streamlines appeared when drifters did not show a closed loop in their vicinity, compared to 30% of the excess detection rate by the Okubo–Weiss method of locating closed loop structures. Three groups of eddies were identified: (1) Coastal Cold and Warm Eddies, which appeared in the area between the coast of southern Korea and the East Korean Warm Current (EKWC), when a southward coastal current was present, (2) Frontal Cold and Warm Eddies, which were formed in the region of the seaward extension of the meandering EKWC, north of Ulleung Island and (3) Ulleung Warm Eddies (UWE) and Dok Cold Eddies (DCE), which appeared during meanders of the EKWC, in the Ulleung Basin. No seasonal concentration for eddy generation and eddy population was found. The average radius of eddies was about 38–60 km. These were born, moved in an erratic pattern and then died in the vicinity where the EKWC separated from the coast and formed a large meander. The time-mean large meander formed meridionally concentrated bands of positive and negative relative vorticity. The cyclonic (cold) eddies tend to reside within the band of positive time-mean relative vorticity, and the anticyclonic (warm) eddies reside within the bands of negative relative vorticity. Six UWE and four warm eddies, in the Yamato Basin (about 10% of warm eddies), were sustained longer than a year. Because the large meander of the EKWC appeared to be controlled by topography, and the JES is a nearly enclosed basin with rapid flow-out to the east through the narrow Tsugaru Strait, there was little eddy energy propagation to the west. The warm eddies in the southwestern part of the JES appeared to be interacting very locally with the mean flow.

On the internal structure of weakly nonlinear bores in laminar high Reynolds number flow

The paper deals with the internal structure of hydraulic jumps in near-critical single-layer flows which replaces the discontinuities predicted by hydraulic theory if viscous effects acting inside a thin laminar boundary layer are properly accounted for. In the limit of large Reynolds number this leads to a structure problem formed by the classical triple-deck equations supplemented with a novel nonlinear interaction relationship which allows for the passage through the critical state. Hydraulic jumps are shown to represent eigensolutions of the structure problem where this passage is achieved by the local thickening of the boundary layer which acts as a viscous hump. The effects of detuning and dispersion due to streamline curvature and surface tension on the internal structure of hydraulic jumps are studied in detail. In addition, the interaction of hydraulic jumps with surface mounted obstacles is investigated.

Streak line pattern of supersonic transverse jet into Mach 6 freestream over a sharp cone

An oil flow technique for obtaining good quality streakline patterns around the transverse injection into a Mach 6 freestream over a sharp cone of nose angle 20 degree(1) has been demonstrated. The freestream Reynolds number based on cone diameter is 0.53 million and total pressure of Mach 2.5 transverse jet is 12 bar. The oil viscosity should be low enough for its streaks to align with flow without being swept away completely during start of the blowdown. After many trials, it is found that, 1:5 mixture (by volume) of titanium dioxide powder (as white pigment) and paraffin oil gives good result. Three drops of oleic acid in 0.5 cc of this mixture give good coagulation properties. This mixture is sprayed as sparsely distributed fine droplets on the model painted black. A still camera is used from outside the tunnel to capture the pictures through the tunnel window (quartz glass) during the blowdown. Six pictures from starting to shutdown of the tunnel are shown in Fig. 1. Frame-A is before tunnel starting, Frame-B is immediately after start of the tunnel, Frames-C and D are during the test, Frame-E is after 4 seconds of run time and Frame-F is after shutdown of the tunnel. The time difference between Frames-C and D is 1 ±0.3 s. It is observed that, the oil streaks take sufficient time to align with surface streamlines, also a good amount of oil is washed away during the 4 seconds run time of tunnel and oil is partially disturbed during the shutdown. Frame-D shows full details of the streaklines around the transverse jet on surface of the cone. The present results reveal that, the oil flow pattern has to be captured during the run.

Design of propeller geometry using streamline-adapted blade sections

We refine the geometry of a propeller by modifying the blade sections to align them with surface streamlines, obtained by the panel method. Redefinition of the blade sections aligned with the streamlines is provided together with surface modeling scheme by which model propellers were built. Numerical simulations and open-water tests on models suggest a possible increase of 1% in propeller efficiency.

Tollmien–Schlichting waves in a transonic boundary layer: Excitation from the outer flow and from the surface

Perturbations in a boundary layer when there is unsteady free viscous-inviscid interaction at transonic velocities are investigated using a modified “triple-deck” model. Two problems are considered: the effect of acoustic perturbations arriving from outside and of perturbations excited from a streamline surface with a vibrator situated on it. The modification of the model consists of taking into account singular terms of the transonic expansion, which enables the Lin–Reissner–Tsien equation to be improved and enables non-stationary and non-linear phenomena in the flow to be described more correctly. It is shown that the modified model enables additional perturbations, overlooked when using the classical triple-deck model, to be taken into account.

Transitional surface modeling of flat extrusion container

For improving the flow status of extrusion metal, the boundary condition of transitional surface between flat extrusion container cavity and the inlet of die is established based on the flow function method. With conformal mapping theory, the mapping functions from the profiles of cavity and die to the unit circle are acquired, and the complex 3D model of transitional surface is transformed to a quasi 3D symmetric model. According to the above methods(flow funwtion method and conformal mappirg Theory), flow function of each point in the transitional surface is obtained, and the mathematical representation of transitional surface is established. In order to verify the rationality of new surface, the forming process of a rectangular flat-plate is simulated by finite-element method. Simulation results show that streamline surface can improve the flow of metal, reduce the die pressure and die abrasion, and increase the die life.

표면 유선에 정렬된 날개 단면을 이용한 프로펠러 형상 설계

In this paper, we suggest a design concept of defining the propeller geometry by stacking up the blade sections aligned with propeller surface streamlines. Numerical and experimental propeller open water(P.O.W.) characteristics of a newly designed propeller are presented. The surface streamlines for a propeller are obtained by using the panel method. Redefinition of the blade sections aligned with the streamlines is provided together with 8-spline modeling, by which we manufacture model propellers. We carried out the P.O.W, tests in a towing tank in order to show the effect of the present method on P.O.W. characteristics.

Incipient separation in shock wave/boundary layer interactions as induced by sharp fin

The incipient separation induced by the shock wave turbulent boundary layer interaction at the sharp fin is the subject of present study. Existing theories for the prediction of incipient separation, such as those put forward by McCabe (1966) and Dou and Deng (1992), can have thus far only predicting the direction of surface streamline and tend to over-predict the incipient separation condition based on the Stanbrook's criterion. In this paper, the incipient separation is firstly predicted with Dou and Deng (1992)'s theory and then compared with Lu and Settles (1990)' experimental data. The physical mechanism of the incipient separation as induced by the shock wave/turbulent boundary layer interactions at sharp fin is explained via the surface flow pattern analysis. Furthermore, the reason for the observed discrepancy between the predicted and experimental incipient separation conditions is clarified. It is found that when the wall limiting streamlines behind the shock wave aligning with one ray from the virtual origin as the strength of shock wave increases, the incipient separation line is formed at which the wall limiting streamline becomes perpendicular to the local pressure gradient. The formation of this incipient separation line is the beginning of the separation process. The effects of Reynolds number and the Mach number on incipient separation are also discussed. Finally, a correlation for the correction of the incipient separation angle as predicted by the theory is also given.

Evaluating a Hybrid Prognostic–Diagnostic Model That Improves Wind Forecast Resolution in Complex Coastal Topography

Abstract The results from a hybrid approach that combines the forecasts of a mesoscale model with a diagnostic wind model to produce high-resolution wind forecasts in complex coastal orography are evaluated. The simple diagnostic wind model [Winds on Critical Streamline Surfaces (WOCSS)] was driven with forecasts (on a 9-km grid) from the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) to obtain detailed near-surface wind forecasts with 3-km horizontal spacing. Forecasts were produced by this hybrid model for four cold-season cases—two frontal and two nonfrontal—over the central California coastal region. They were compared with 3-km forecasts from the innermost COAMPS full physics model nest and with winds observed at 35 surface sites scattered throughout the study domain. The evaluation sought to determine the conditions for which the hybrid approach performs well and those for which it does not. The performance (in terms of bias and root-mean-square error) was evaluated 1) when there were and were not fronts and 2) for the early (6–18 h) and late (21–36 h) periods of the mesoscale model forecasts. The geographic distribution of performance was also examined to see if forecasts were affected by mountains and oceans. The hybrid approach performed best during stable, nonfrontal conditions. There were no clearly defined geographic effects on hybrid performance. The computation requirements of the full physics mesoscale model nested down to 3 km are substantially greater than those of the hybrid approach. Suggestions are given for further improvements.

Numerical flow simulation of flush type intake duct of waterjet

Waterjet propulsion system is widely used to thrust high speed marine vessels in excess of 30–35 knots by virtue of high propulsive efficiency, good maneuverability, and less vibration. Since, however, approximately 7–9% of the total power is lost in intake duct due to flow separation, nonuniformity, etc., detail understanding of flow phenomena occurring within intake duct is essential to reduce the power loss. The present work solved 3D incompressible RANS equations on multiblocked grid system of the flush type intake duct of waterjet. The numerical results of surface pressure distributions, velocity vectors, and streamlines were compared with experiments and good agreements were obtained for three jet velocity ratios. Strong suction flow through the inlet was shown and the vortex induced by the separation along the corner of the side wall was clearly shown. Flow separation on the lip was also observed. The location of stagnation point on the lip was well predicted, in accordance with PIV measurement.

Study on Internal Supersonic Flows with Pseudo shock Wave Using Liquid Crystal Flow Visualization Method

The flow visualization technique using shear-sensitive liquid crystal is applied to the investigation of a Mach 2 internal supersonic flow with pseudo-shock wave (PSW) in a pressure vacuum supersonic wind tunnel. It provides qualitative information mainly concerning the overall flow structure, such as the turbulent boundary layer separation, reattachment locations and the dimensionalities of the flow. Besides, it can also give understanding of the surface streamlines, vortices in separation region and the corner effect of duct flow. Two kinds of crystals with different viscosities are used in experiments to analyze the viscosity effect. Results are compared with schlieren picture, confirming the effectiveness of liquid crystal in flow-visualization.