Measurements Of Hydrodynamic Forces Research Articles

Measurements of hydrodynamic forces, surface pressure, and wake for obliquely towed tanker model and uncertainty analysis for CFD validation

This article presents hydrodynamic forces and moments, surface pressures, estimated side force distributions, and wakes under oblique towing conditions for a practical tanker model (model KVLCC2M), which was designed by the Korea Research Institute of Ships and Ocean Engineering (KRISO). Ship offset data is readily available and can be obtained from the Internet. The model ship has no appendages and no rudder. Trim and sinkage were adjusted to zero in the static condition and the model ship was constrained against any motion. Although the drift angle β was primarily set to 0°, 6°, and 12°, other settings were used in some experiments. All experimental results were processed using uncertainty analysis. The uncertainty analyzing method follows the ANSI/ASME Performance Test Code (PTC19.1-1985) and the AIAA Standard S-071-1995. Only a few error components were considered here and they were empirically chosen because they had a heavy weighting when used in the uncertainty calculation. The results of these towing tank experiments will contribute to the development of computational fluid dynamics (CFD) research in ship hydrodynamics.

In situ measurements of hydrodynamic forces imposed on Chondrus crispus Stackhouse

Among the hydrodynamic forces experienced by intertidal organisms, drag and the impingement force are thought to have the greatest effect on macroalgae. These forces are modified by biotic factors such as algal morphology, reconfiguration, and the presence of a canopy. However, much of what is known about the hydrodynamics of macroalgae has been garnered from low-velocity laboratory flume studies. Few field studies have measured drag and none have directly measured the effects of the canopy on force. To examine in situ hydrodynamic forces imposed on the turf forming macroalga Chondrus crispus, compact digital force sensors were developed that measure and record the 3-dimensional force imposed on a macroalga without disturbing the surrounding canopy. Sensors were positioned within natural Chondrus beds and the effects of the canopy, algal morphology, and sea state on in situ hydrodynamic force were examined. Additionally, the predictions of a new model for drag on flexible macroalgae were tested by simultaneously measuring force and water velocity. Digital force recordings indicated that Chondrus only experience drag; lift and impingement force were negligible in all combinations of factors. Canopies significantly reduced drag by 15–65%. Morphology and size also influenced drag, such that lower forces were imposed on small planar algae than large arborescent individuals. Further, planar algae experienced low drag in all combinations of sea and canopy state, indicating that these individuals may not be as susceptible to wave disturbance as arborescent individuals. Overall, these data indicate that the ability for Chondrus to grow large, arborescent individuals is dependent on the drag reducing properties of the canopy, while more hydrodynamically harsh habitats may be accessible to planar morphologies. Additionally, these data suggest that drag models for canopy forming macroalgae must incorporate the effects of the canopy to predict drag accurately in situ.

Hydrodynamic force measurements: boundary slip of water on hydrophilic surfaces and electrokinetic effects.

The hydrodynamic drainage force of aqueous medium between smooth hydrophilic surfaces was measured with the colloidal probe technique up to shear rates of typically 10(4) s(-1). Measured force curves were compared to simulations. To reach agreement between experimental and simulated force curves, the hydrodynamic force had to be fitted with a model allowing for boundary slippage. Boundary slip was characterized by a slip length of 8-9 nm. Force measurements with charged surfaces could be simulated taking only hydrodynamic and electrostatic double-layer forces into account.

Acceleration Performance of High-Speed Planing Craft from Rest

In this study, the performance of a high-speed planing craft during rapid acceleration from rest is investigated, following series studies on the performances in steady running condition including ship motions due to instability in calm water.Measurement of running attitude of a model of planing craft during acceleration from rest is carried out to compare with that in running at constant speed. The results show the force component which is proportional to acceleration is important as well as a conventional displacement ship. Measurements of hydrodynamic forces acting on it during acceleration suggest, however, that the added mass component does not play an important role because it is much smaller than the inertia force and the steady drag components which are proportional to square of advanced speed.A computer simulation program to predict this performance is developed using a motion equation with experimental and theoretical hydrodynamic coefficients. The results of the simulation are in fairly good agreement with the experimental ones. It is shown that the procedure can simulate unsteady motions like porpoising in accelerating condition as well as in steady running condition.

A Study on Blade Sections for a Trans-Cavitating Propeller

In this study, a trans-cavitating propeller is defined as a propeller whose design condition is between those of conventional propellers and supercavitating propellers. A propeller for high speed vessel of about 30 knots falls in such a design condition, where a suitable propeller design method has not yet been found. For such a condition, it would be realistic to adopt supercavitating blade sections near the tip and noncavitating ones near the root, since ordinary blade sections can lead to harmful violent cloud cavitation. This study investigates foil sections suitable for a trans-cavitating propeller. The purposes of this study are to investigate the noncavitating and supercavitating foil design methods for such intermediate design condition, and to find a criterion to switch the design between noncavitating and supercavitating foils.By prescribing pressure distributions, one noncavitating foil section and one supercavitating foil section are designed for a condition of r/R =0.7 position of a trans-cavitating propeller. Cavitation performance as well as noncavitating performance are examined by the noncavitating and cavitating foil theories, and by the cavitation tunnel tests such as hydrodynamic force measurement and cavitation observation. From these test results, the foil performance and design method are evaluated as well as the accuracy of the theoretical prediction are discussed. The following results are obtained.(1) The noncavitating foil section did not cavitate at the design condition as expected. High lift/drag ratio of 35.3 was obtained. Wide cavitation bucket of about 2 deg. was also achieved. Theoretical prediction gave slightly higher lift coefficient by 0.25 deg. in the angle of attack and narrower cavitation bucket than the experiment. The theory should be further improved in these points.(2) The supercavitating foil section generated supercavitation at the design point as expected. High lift/drag ratio of 33.1 was obtained. The predictions by a linear supercavitating foil theory showed fine agreement with the experimental results.

Measurement of Stern Flow Field of a Ship in Oblique Towing Motion

In this paper, we describe on the results of our investigation on a stern flow field of a ship in oblique towing motion. The purpose of this investigation is to get basic data necessary and available for developing, improving and verifying an estimation method of hydrodynamic forces acting on a ship in manoeuvring motion.We used three VLCC model ships. They have almost same principal dimensions but different stern shapes. One ship has a V shaped stern, another ship has an U shaped stern, and the other ship has a medium shape stern between the V and the U shaped stern.Kinds of experiments that we did are measurement of flow field by 5 hole Pitot tube, observation of flow field by tuft and measurement of hydrodynamic forces acting on a model ship. By these experiments, we investigated characteristics of flow field around a ship in oblique towing motion, and discussed relation among stern shape, flow field and hydrodynamic forces.There is no big difference qualitatively among stern flow field of these three model ships in oblique towing motion, characteristics of which are represented by two groups of separating vortices. However, there are clear differences in strength, concentration and range of distribution of wake and vorticity among three model ships.Concerning to hydrodynamic forces acting on a model ship in oblique towing motion, there is little difference in magnitude itself of hydrodynamic forces, but there is a clear difference in center of pressure that has a serious effect on a ship manoeuvrability.These results will be useful data for framing a flow model, developing, improving and verification of estimation method of flow field and hydrodynamic forces, and for examination of ship stern shape.

234 斜航する船体の船尾流場計測

In this paper, we describe on the results of our investigation of a stern flow field around a ship in oblique towing motion. The purpose of this inveitigation is to get basic data available for developing, improving and verifying an estimation method of hydrodynamic forces acting on a ship in manoeuvring motion. Kinds of experiments which we carried out on three VLCC model ships are measurements of flow field by 5 hole Pitot tube, observation of flow field by tuft and measurements of hydrodynamic forces acting on a model ship. By these experiments, we investigated characteristics of flow field around a ship in oblique towing motion, and discussed relation between stern shape, flow field and hydrodynamic forces.

WAVE FORCES ON ARMOR BLOCKS

Experimental measurements of hydrodynamic forces on a cubic block near the bottom under solitary waves were carried out. Horizontal and vertical forces were recorded and instantaneous and averaged values of hydrodynamic coefficients CD, C,, and C,_ for different boundary conditions, gap between block and bottom, e, and two or three-dimensional flow, were obtained. Horizontal and vertical forces were found to depend strongly on e/D, where D is the block side . Instantaneous values of hydrodynamic coefficients vary considerably during the wave passage and differ appreciably from the averaged coefficients.

Interaction forces between red cells agglutinated by antibody. II. Measurement of hydrodynamic force of breakup

The expressions derived in the previous paper for the respective normal, F3, and shear forces, Fshear, acting along and perpendicular to the axis of a doublet of rigid spheres, were used to determine the hydrodynamic forces required to separate two red cell spheres of antigenic type B crosslinked by the corresponding antibody. Cells were sphered and swollen in isotonic buffered glycerol containing 8 X 10(-5) M sodium dodecyl sulfate, fixed in 0.085% glutaraldehyde, and suspended in aqueous glycerol (viscosity: 15-34 mPa s), containing 0.15 M NaCl and anti-B antibody from human hyperimmune antiserum at concentrations from 0.73 to 3.56 vol%. After incubating and mixing for 12 h, doublets were observed through a microscope flowing in a 178-micron tube by gravity feed between two reservoirs. Using a traveling microtube apparatus, the doublets were tracked in a constantly accelerating flow and the translational and rotational motions were recorded on videotape until breakup occurred. From a frame by frame replay of the tape, the radial position, velocity and orientation of the doublet were obtained and the normal and shear forces of separation at breakup computed. Both forces increased significantly with increasing antiserum concentration, the mean values of F3 increasing from 0.060 to 0.197 nN, and Fshear from 0.023 to 0.072 nN. There was no significant effect of glycerol viscosity on the forces of separation. It was not possible to determine whether the shear or normal force was responsible for doublet separation. Measurements of the mean dimensionless period of rotation, TG, of doublets in suspensions containing 0.73 and 2.40% antiserum undergoing steady flow were also made to test whether the spheres were rigidly linked or capable of some independent rotation. A fairly narrow distribution in TG about the value 15.64, predicted for rigidly-linked doublets, was obtained at both antiserum concentrations.