A Study of Dean Vortex Development and Structure in a Curved Rectangular Channel with Aspect Ratio of 40 at Dean Numbers up to 430

Flow in a curved channel with mild curvature, an aspect ratio of 40 to 1, and an inner to outer radius ratio of 0.979 is studied at Dean numbers De ranging from 35 to 430. For positions from the start of curvature ranging from 85 to 145 degrees, the sequence of transition events begins with curved channel Poiseuille flow at De less than 40-64. As the Dean number increases, observations show initial development of Dean vortex pairs, followed by symmetric vortex pairs which, when viewed in spanwise/radial planes, cover the entire channel height (De=90-100). At De from 40 to 125-130, the vortex pairs often develop intermittent waviness in the form of vortex undulations. Splitting and merging of vortex pairs is also observed over the same experimental conditions as well as at higher De. When Dean numbers range from 130 to 185-200, the undulating wavy mode is replaced by a twisting mode with higher amplitudes of oscillation and shorter wavelengths. The twisting wavy mode results in the development of regions where turbulence intensity is locally augmented at Dean numbers from 150 to 185-200, principally in the upwash regions between the two individual vortices which make up each vortex pair. These turbulent regions eventually increase in intensity and spatial extent as the Dean number increases further, until individual regions merge together so that the entire cross section of the channel contains chaotic turbulent motions. When Dean numbers then reach 400-435, spectra of velocity fluctuations then evidence fully turbulent flow

Development of subminiature multi-sensor hot-wire probes

Limitations on the spatial resolution of multisensor hot wire probes have precluded accurate measurements of Reynolds stresses very near solid surfaces in wind tunnels and in many practical aerodynamic flows. The fabrication, calibration and qualification testing of very small single horizontal and X-array hot-wire probes which are intended to be used near solid boundaries in turbulent flows where length scales are particularly small, is described. Details of the sensor fabrication procedure are reported, along with information needed to successfully operate the probes. As compared with conventional probes, manufacture of the subminiature probes is more complex, requiring special equipment and careful handling. The subminiature probes tested were more fragile and shorter lived than conventional probes; they obeyed the same calibration laws but with slightly larger experimental uncertainty. In spite of these disadvantages, measurements of mean statistical quantities and spectra demonstrate the ability of the subminiature sensors to provide the measurements in the near wall region of turbulent boundary layers that are more accurate than conventional sized probes

Wake Turbulence Structure Downstream of a Cambered Airfoil in Transonic Flow: Effects of Surface Roughness and Freestream Turbulence Intensity

The wake turbulence structure of a cambered airfoil is studied experimentally, including the effects of surface roughness, at different freestream turbulence levels in a transonic flow. As the level of surface roughness increases, all wake profile quantities broaden significantly and nondimensional vortex shedding frequencies decrease. Freestream turbulence has little effect on the wake velocity profiles, turbulence structure, and vortex shedding frequency, especially downstream of airfoils with rough surfaces. Compared with data from a symmetric airfoil, wake profiles produced by the cambered airfoils also have significant dependence on surface roughness, but are less sensitive to variations of freestream turbulence intensity. The cambered airfoil also produces larger streamwise velocity deficits, and broader wakes compared to the symmetric airfoil

Structure of transitionally rough and fully rough turbulent boundary layers

The article of record as published may be found at 0.1017/S0022112086001933Structural characteristics of transitionally rough and fully rough turbulent boundary layers are presented. These were measured in flows at different roughness Reynolds numbers developing over uniform spheres roughness. Inner regions of the longitudinal component of normal Reynolds stress profiles and log regions of mean profiles continuously change in the transitionally rough regime, as the roughness Reynolds number, Rek, varies. These properties asymptotically approach fully rough behaviour as Rek increases, and smooth behaviour at low Rek Profiles of other Reynolds-stress tensor components, turbulence kinetic energy, turbulence-kinetic-energy production, and the turbulence-kinetic-energy dissipation are also given, along with appropriate scaling variables. Fully rough, one-dimensional spectra of longitudinal velocity fluctuations from boundary-layer inner regions are similar to smooth-wall results for k1 y > 0.2 when non-dimensionalized using distance from the wall y as the lengthscale, and (τ/ρ)½ as the velocity scale, where τ is local shear stress, ρ is static density, and k1 is one-dimensional wavenumber in the flow direction

Development of subminiature multi-sensor hot-wire probes

Limitations on the spatial resolution of multisensor hot wire probes have precluded accurate measurements of Reynolds stresses very near solid surfaces in wind tunnels and in many practical aerodynamic flows. The fabrication, calibration and qualification testing of very small single horizontal and X-array hot-wire probes which are intended to be used near solid boundaries in turbulent flows where length scales are particularly small, is described. Details of the sensor fabrication procedure are reported, along with information needed to successfully operate the probes. As compared with conventional probes, manufacture of the subminiature probes is more complex, requiring special equipment and careful handling. The subminiature probes tested were more fragile and shorter lived than conventional probes; they obeyed the same calibration laws but with slightly larger experimental uncertainty. In spite of these disadvantages, measurements of mean statistical quantities and spectra demonstrate the ability of the subminiature sensors to provide the measurements in the near wall region of turbulent boundary layers that are more accurate than conventional sized probes.RTOP 505-60-31Approved for public release; distribution is unlimited

Flow structure and heat transfer in a square passage with offset mid-truncated ribs

The enhancement of heat transfer attributed from rib turbulators relative to the increased pressure drop in the channel is a crucial design parameter. Thus, the design of the truncated ribs (whose length is less than the passage width) provides options to address such cooling requirements when the pressure loss is a critical factor. Considering different types of truncated ribs, mid-truncated ribs (which are truncated in the central part of the rib) have been proved to show better thermal performance than other types of truncated ribs. A numerical study of mid-truncated ribs with different offset placements or angles on turbulent heat transfer inside a non-rotating cooling passage of a gas turbine blade is performed for inlet Reynolds number ranging from 10,000 to 50,000. The overall performance characteristics of six different types of mid-truncated ribbed arrangements are also compared: Cases A1, A2, A3 utilize mid-truncated spanwise ribs with different relative rib stagger positions at different spanwise locations; Cases B, C, D utilize mid-truncated angled ribs with the ribs oriented at +/- 45 degrees (Case B), +45 degrees/+45 degrees (Case C), and +/- 135 degrees (Case D). In all cases, a gap is present between the ribs placed on opposite spanwise parts of the channel, to provide the mid-truncation. It is found that the 135 degrees mid-truncated rib (Case D) has the highest heat transfer coefficient, while the 90 degrees mid-truncated ribs with no offset placement (Case A1) behave best in reducing pressure loss penalty. Although Case A shows larger heat transfer augmentation, Case D is advantageous for augmenting side-wall heat transfer when the pressure loss is considered and the Reynolds number is comparatively large. Case C exhibits the best overall thermal performance over the largest range of experimental conditions when the pressure drop is also considered. Staggered arrangement for 90 degrees mid-truncated ribs can enhance heat transfer efficiently and makes a good overall performance at low Reynolds numbers. Case A1 can be used in practical operation because of reduced weight and good thermal performance at high Reynolds numbers. This is the first study on various offset mid-truncated ribs aiming to improve the heat transfer of turbine blade internal cooling passages with reduced pressure loss penalty. (C) 2013 Elsevier Ltd. All rights reserved