Experiments in Fluids

Experimental data of the flow field over a six-finned configuration are compared with computations from an inviscid Euler code and a parabolized Navier-Stokes (PNS) code to assess their accuracy. Surface oil flow patterns, surface pressures, and flow field pilot surveys are presented. The PNS computations are in good agreement with the measurements in the inviscid regions. Differences are noted in the viscous regions possibly due to nose tip bluntness effects and the associated entropy layer.

An experimental investigation of the interaction of surface radiation and natural convection in an L shaped corner is presented. Parametric studies to explore the influence of surface emissivity and aspect ratio on the total heat transfer rate from the isothermal vertical wall of the L corner, have been carried out. For an L corner with highly polished walls, the present experimental results are compared with results available in literature, and are found to be in good agreement. A new semi-experimental method for solving this class of problems is discussed, and a correlation for estimating the total Nusselt number is proposed. It is found that surface radiation has a significant effect on the total heat transferred from the vertical wall of the L-shaped corner.

The effects of shape and relative submergence (the ratio of flow depth to obstacle height, d/H) are investigated on the wakes around four different low-aspect-ratio wall-mounted obstacles at Re H  = 17,800: semi-ellipsoids with the major axes of the base ellipses aligned in the streamwise and transverse directions, and two cylinders with aspect ratios matching the ellipsoids (H/D = 0.89 and 0.67, where D is the maximum transverse dimension). Particle Image Velocimetry was used to interrogate the flow. Streamwise features observed in the mean wake include counter-rotating distributions of vorticity inducing downwash (tip structures), upwash (base structures), and horseshoe vortices. In particular, the relatively subtle change in geometry produced by the rotation of the ellipsoid from the streamwise to the transverse orientation results in a striking modification of the mean streamwise vorticity distribution in the wake. Tip structures are dominant in the former case, while base structures are dominant in the latter. A vortex skeleton model of the wake is proposed in which arch vortex structures, shed from the obstacle, are deformed by the competing mechanisms of Biot-Savart self-induction and the external shear flow. The selection of tip or base structures in the ellipsoid wakes is caused by tilting of the arch structures either upstream or downstream, respectively, which is governed by ellipsoid curvature. An inverse relationship was observed between the relative submergence and the strength of the base structures for the ellipsoids, with a dominant base structure observed for d/H = 1 in both cases. These results demonstrate a means by which to achieve significant modifications to flow structure and thereby also to transport mechanisms in the flow. Therefore, this work provides insight into the modeling and control of flow over wall-mounted bodies.

The paper reports on the nonintrusive, simultaneous measurement of velocity and temperature fluctuations in a turbulent jet diffusion flame. Velocity fluctuations were measured using laser Doppler anemometry (LDA), whereas coherent anti-Stokes Raman spectroscopy (CARS) was used for temperature measurements. The simultaneous measurements were affected by both density bias and velocity bias because the LDA imposed a form of biased sampling on the CARS. The measured velocity-temperature correlation coefficients indicated that the gradient-diffusion hypothesis is reasonably accurate for the radial direction. However, for the axial direction the gradient diffusion hypothesis is accurate only in the central region of the flame, while countergradient diffusion is found in the outer region.

The wake structure of a vertical-axis wind turbine (VAWT) is strongly dependent on the tip-speed ratio, $$\lambda$$ , or the tangential speed of the turbine blade relative to the incoming wind speed. The geometry of a turbine can influence $$\lambda$$ , but the precise relationship among VAWT geometric parameters and VAWT wake characteristics remains unknown. To investigate this relationship, we present the results of an experiment to characterize the wakes of three VAWTs that are geometrically similar except for the ratio of the turbine diameter (D), to blade chord (c), which was chosen to be $$D/c =$$ 3, 6, and 9. For a fixed freestream Reynolds number based on the blade chord of $$Re_c = 1.6\times 10^3$$ , both two-component particle image velocimetry (PIV) and single-component hot-wire anemometer measurements are taken at the horizontal mid-plane in the wake of each turbine. PIV measurements are ensemble averaged in time and phase averaged with each rotation of the turbine. Hot-wire measurement points are selected to coincide with the edge of the shear layer of each turbine wake, as deduced from the PIV data, which allows for an analysis of the frequency content of the wake due to vortex shedding by the turbine.

The paper describes a novel technique which enables evaluation of sediment fluxes on the upper layer of a granular bed by means of separate measurements of concentration and velocity of the moving particles. Specific elaboration techniques based on digital image processing were applied to films of the solid fluxes, allowing for automatic measurement. Sediment concentration was measured via a technique based on image subtraction and following proper filtering procedures, while grain velocity was measured by Particle Image Velocimetry. The method was applied in laboratory experiments of one dimensional bed load; the solid discharges measured by the proposed image processing technique were compared to those obtained by manual count of the grains passing over a fixed plate used as a sight. After calibration of the automatic technique, dimensionless solid discharges ranging from 1.0 × 10−3 to 1.2 × 10−1 were measured with a maximum error as large as 25%. The technique proposed also enables measurement of the time variation of the quantities and the two dimensional direction of sediment motion, for a complete characterization of grain kinematics in solid transport processes.

The accuracy of the particle image velocimetry technique was investigated using synthetic images having known characteristics. Algorithms were developed to extract two-dimensional velocity information by tracking particles between successive frames of a movie automatically without operator assistance. This allowed to parametrically investigate the influence of the various parameters (image contrast, image noise, particle density, distribution of sizes of particles and particle displacement between frames) on the accuracy of the technique. It was found that as long as the images have a good contrast, particle locations can be determined with sub-pixel accuracy and particle velocities can be determined within a few percent.

A variable angle calibration technique for hot wire and hot film X-probes incorporating a new method of interpolation is described here along with measurements in a fully developed turbulent channel flow. Results based on the new method of calibration include the mean velocity profile, Reynolds stress, and probability density distributions for fluctuating velocity components u and υ and for the flow angle. Also skewness and flatness factors for u and υ are given. Measurement data were also evaluated using the conventional method. A comparison of both techniques shows that the new method does not yield appreciable differences in statistical flow analyses but is more accurate in measuring rare flow events associated with large flow angles. An extended version of the new method of calibration allowing three dimensional measurements in turbulent flow will be discussed.