Measurements Of Boundary Layer Flow Research Articles

Hydrodynamic Function of the Slimy and Scaly Surfaces of Teleost Fishes.

The scales and skin mucus of bony fishes are both proposed to have a role in beneficially modifying the hydrodynamics of water flow over the body surface. However, it has been challenging to provide direct experimental evidence that tests how mucus and fish scales change the boundary layer in part due to the difficulties in working with live animal tissue and difficulty directly imaging the boundary layer. In this manuscript, we use direct imaging and flow tracking within the boundary layer to compare boundary layer dynamics over surfaces of fish skin with mucus, without mucus, and a flat control surface. Our direct measurements of boundary layer flows for these three different conditions are repeated for two different species, bluegill sunfish (Lepomis macrochirus) and blue tilapia (Oreochromis aureus). Our goals are to understand if mucus and scales reduce drag, shed light on mechanisms underlying drag reduction, compare these results between species, and evaluate the relative contributions to hydrodynamic function for both mucus and scales. We use our measurements of boundary layer flow to calculate shear stress (proportional to friction drag), and we find that mucus reduces drag overall by reducing the velocity gradient near the skin surface. Both bluegill and tilapia showed similar patterns of surface velocity reduction. We also note that scales alone do not appear to reduce drag, but that mucus may reduce friction drag up to 50% compared to scaled surfaces without mucus or flat controls.

Detailed Measurements of Boundary Layer Flow under Swash with High-resolution PIV

Detailed Measurements of Boundary Layer Flow under Swash with High-resolution PIV

Determination of hot-wire position from a solid wall in an opaque channel

Hot-wire experiments sometimes require the position of the hot-wire probe with respect to a solid wall to be known, as is often the case with boundary layer flow measurements. However, optical access to determine the position of the wall visually may not always be available. The current paper describes a procedure to allow a hot wire to determine accurately the location of the solid wall with respect to any arbitrary reference position using the response of the hot wire when it approaches a solid wall under no flow conditions. An accurate probe-positioning system can then be used to position the probe with respect to this wall. The method is applied to both an aluminum wall and a Perspex wall, and both give consistent measurements with an accuracy of better than ±0.005 mm.

Turbulent boundary layer measurements over flat surfaces coated by nanostructured marine antifoulings

Whilst recent developments of nanotechnology are being exploited by chemists and marine biologists to understand how the completely environmentally friendly foul release coatings can control marine biofouling and how they can be developed further, the understanding of the hydrodynamic performances of these new generation coatings is being overlooked. This paper aims to investigate the relative boundary layer, roughness and drag characteristics of some novel nanostructured coatings, which were developed through a multi-European and multi-disciplined collaborative research project AMBIO (2010), within the framework of turbulent flows over rough surfaces. Zero-pressure-gradient, turbulent boundary layer flow measurements were conducted over flat surfaces coated with several newly developed nanostructured antifouling paints, along with some classic reference surfaces and a state-of-the-art commercial coating, in the Emerson Cavitation Tunnel (ECT) of Newcastle University. A large flat plane test bed that included interchangeable flat test sections was used for the experiments. The boundary layer data were collected with the aid of a two-dimensional DANTEC Laser Doppler Velocimetry (LDV) system. These measurements provided the main hydrodynamic properties of the newly developed nanostructured coatings including local skin friction coefficients, roughness functions and Reynolds stresses. The tests and subsequent analysis indicated the exceptionally good frictional properties of all coatings tested, in particular, the drag benefit of some new nanostructured coatings in the Reynolds number range investigated. The rapidly decreasing roughness function trends of AKZO19 and AKZO20 as the \( k_{s}^{ + } \) increases were remarkable along with the dissimilar roughness function character of all tested coatings to the well-known correlation curves warranting further research at higher Reynolds numbers. The wall similarity concept for the Reynolds stresses was only validated for the transitionally rough surfaces from \( (y + \varepsilon)^{ + } \approx 100 \) up to the end of the boundary layer.

Effects of bed roughness on boundary layer mixing and mass flux across the sediment‐water interface

Fine‐scale measurements of boundary layer flow and solute transport were conducted within a laboratory flume to determine how bed topography influences mixing and mass transport at the sediment‐water interface. Three different bed topographies were examined, with roughness composed of sand, gravel, or cobbles. Fluorescein dye, used as a dissolved tracer, was injected into the flow, and concentration and velocity were simultaneously measured using a combined planar laser‐induced fluorescence and particle image velocimetry technique. Enhanced turbulent mixing due to bed roughness increased mass flux across the sediment‐water interface between 1.3 times greater for the sand and 7.5 times greater for the cobble bed compared to estimates over a hydraulically smooth surface. As bed roughness increased, mass exchange became spatially more heterogeneous, coinciding with a transition from a boundary layer to a shear‐dominated mixing layer in the near‐wake region behind individual roughness elements. The enhanced flux increased pore water concentrations. However, for the same bed geometry, increased mean flow within the water column locally reduced pore water concentrations, likely due to greater downstream transport and dispersion occurring within the bed.

1646 新しい熱線プローブによる境界層流れの測定

It is necessary to measure longitudinal and lateral velocity components for the study of boundary layer of boundary layer in a transitional region. However, it is difficult to measure simultaneously these velocity components because a transitional boundary-layer thickness is very thin. Therefore, we manufactured a new hot-wire anemometer with two wires placed parallel and close to each other. The main feature of this probe is extremely small compared with hot-wire probe existed, such as X- and V-type probes, in vertical size, and can be easily treated similar to I-type probe. We measured a turbulent boundary layer flow of low Reynolds number using this probe. As a result, we found that this probe is very useful for the boundary layer flow measurements because the velocity profiles are obtained with sufficient accuracy using the simple correction terms.

1009 近接 2 線式熱線風速計

It is necessary to measure horizontal and vertical velocity components for the research of boundary layer in the transition region. However, bondary layer thickness is very thin in this region. It is difficult to measure simultaneously these two velocity components. Therefore we manufacture a new hot-wire anemometer with two close wires. The main feature of this probe is extremely small compared with hot-wire probe existed, such as X- and V-configuration probe, in vertical size, and can be easily treated like I-configuration probe. We explore the near wake flow filed of hemispherical roughness using this probe. As a result, we find that this probe is very useful for the boundary layer flow measurements and velocity profiles data obtained are satisfied qualitatively.

Acoustic measurements of boundary layer flow and sediment flux.

Results are reported on an assessment of the application of coherent Doppler and cross-correlation techniques to measure nearbed boundary layer flow. The approaches use acoustic backscattering from sediments entrained into the water column from the bed, to obtain high-resolution profiles of the nearbed hydrodynamics. Measurements are presented from a wave tunnel experiment in which sediment was entrained by unidirectional, oscillatory, and combined flows. The data collected have been used to evaluate the capability of the two flow techniques to measure boundary layer mean, turbulent, and intrawave velocity profiles. Further, the backscattered signal has been used to measure suspended sediment concentration profiles, which have been combined with the velocity profiles to obtain high-resolution measurements of boundary layer sediment flux.

A study of the dynamics of a marine sandwave

ABSTRACTThe movement of the crest of a sandwave was studied using cross‐sectional profiles obtained from lines of sea‐bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest.The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea‐bed by storm waves, caused major reductions in crestal heights.Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.

An evaluation of Bagnold's dimensionless conefficient of proportionality using measurements of sandwave movement

Only a few observations are currently available from the tidal marine environment which can be used to evaluate sediment transport formulae. Research carried out on sandwave movement indicates that most of the sediment transport is restricted to the crestal area. Mean rates of sediment transport were calculated from volumetric changes, between successive slack waters, over a nine day period spanning Spring tides. The results were used, in conjunction with boundary-layer flow measurements, to calculate the threshold velocity for the movement of sediment and also to evaluate Bagnold's (1963) dimensionless coefficient of proportionality, K. Comparisons are made with the flume data of Guy et al. (1966) for the appropriate sediment grain size. The dependence of K, upon normalised excess shear stress was found to be weaker than that suggested by Sternberg (1972). Within the range of flow velocities considered there was no indication that K tended towards a constant at high flow velocities.

Measurements of boundary-layer flow and boundary roughness over Campeche Bank, Yucatan

Aseries of boundary-layer measurements was made at three anchor stations on Campeche Bank, Yucatan. Measurements consisted of current speed at six levels within 1.5 m of the bed, current direction, and stereophotographs of boundary. Analysis of the drag coefficient revealed that the boundary-layer flow was hydrodynamically transitional and the magnitude of the drag coefficient varied between 2·10 −3 and 6·10 −2 as a function of the flow conditions and bed configuration. These data appear to be representative of flows of an intermediate nature, not only in the observed variations of the drag coefficient with respect to Reynolds number but in the way that the drag coefficient correlates with other measurements made in low- and high-velocity environments, respectively.