Experiments on identification and control of inflow disturbances in contracting streams

Vorticity from all surfaces and isolated objects in the vicinity of the fan intake, including the outside surfaces of the fan housing, were identified as the major sources for disturbances leading to blade passing frequency noise. The previously proposed mechanism based on atmospheric turbulence is refuted. Flow visualization and hot wire techniques were used in three different facilities to document the evolution of various types of disturbances, including the details of the mean flow and turbulence characteristics. The results suggest that special attention must be devoted to the design of the inlet and that geometric modeling may not lead to adequate simulation of the in flight characteristics. While honeycomb type flow manipulators appear to be effective in reducing some of the disturbances, higher pressure drop devices that generate adequate turbulence, for mixing of isolated nonuniformities, may be necessary to suppress the remaining disturbances. The results are also applicable to the design of inlets of open return wind tunnels and similar flow facilities

Effects of axisymmetric contractions on turbulence of various scales

Digitally acquired and processed results from an experimental investigation of grid generated turbulence of various scales through and downstream of nine matched cubic contour contractions ranging in area ratio from 2 to 36, and in length to inlet diameter ratio from 0.25 to 1.50 are reported. An additional contraction with a fifth order contour was also utilized for studying the shape effect. Thirteen homogeneous and nearly isotropic test flow conditions with a range of turbulence intensities, length scales and Reynolds numbers were generated and used to examine the sensitivity of the contractions to upstream turbulence. The extent to which the turbulence is altered by the contraction depends on the incoming turbulence scales, the total strain experienced by the fluid, as well as the contraction ratio and the strain rate. Varying the turbulence integral scale influences the transverse turbulence components more than the streamwise component. In general, the larger the turbulence scale, the lesser the reduction in the turbulence intensity of the transverse components. Best agreement with rapid distortion theory was obtained for large scale turbulence, where viscous decay over the contraction length was negligible, or when a first order correction for viscous decay was applied to the results

Visualization of Unsteady Flow Over Oscillating Airfoils

This study is concerned primarily with the complex nature of leading edge flow separation occurring on airfoils oscillating in a uniform flow field at low Reynolds numbers. The flow field past an oscillating airfoil and a fixed airfoil with an oscillating flap were investigated using various visualization techniques in water. The role of mean flow velocity, instantaneous angle of attack, mean angle of attack and amplitude and frequency of oscillation as well as the location of the support point are examined. The results which were obtained over a range of parameters substantially beyond previous studies include new information regarding the effect of these parameters on the nature and onset of separation. Two basic forms of leading edge separation have been observed. At low values of reduced frequency (ω* \u3e 0.5) the separation resembles leading edge separation on stationary airfoils with the separated flow remaining detached from the upper surface. At higher values, beyond ω*cr, a strong vortex (roller) is formed at the leading edge with the flow reattaching downstream from it. In addition to these two flow regimes, a third regime made its appearance at extremely high values of frequency and amplitude where the flow around the airfoil has collapsed. The frequency of oscillation is found to govern the angle, αcr, at which leading edge separation occurs at low reduced frequencies (ω* \u3c 0.2); at higher frequencies αcr occurs at the portion of the cycle where d2 α/dt2 is near its maximum

Wall-bounded turbulent flows at high Reynolds numbers: Recent advances and key issues

Wall-bounded turbulent flows at high Reynolds numbers have become an increasingly active area of research in recent years. Many challenges remain in theory, scaling, physical understanding, experimental techniques, and numerical simulations. In this paper we distill the salient advances of recent origin, particularly those that challenge textbook orthodoxy. Some of the outstanding questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the principal model parameters such as the von Kármán “constant,” the parametrization of roughness effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that may provide answers to these questions, notably the improvement of measuring techniques and the construction of new facilities, are identified. We also highlight aspects where differences of opinion persist, with the expectation that this discussion might mark the beginning of their resolution

On the Interpretation of the Output of Hot-Film Anemometers and a Scheme of Dynamic Compensation for Water Temperature Variation

Using a special calibration tunnel developed during the course of this study, the static and dynamic response of several kinds of commercially available hot-film probes with single and multiple sensors of the cylindrical-fiber type are examined. The effects of different parameters, including those of the anemometer bridge, on the output and performance of the probes are evaluated. In particular, the consequences of variations in water temperature on the hot-film anemometer output are determined. The results reveal a large effect of the water temperature on the calibration curves (in an extreme case a change in temperature of only 5.5°F can result in a 100% error in the mean velocity reading). In general, the Fourier components are inclined to the wall - the lower frequencies making smaller angles with the wall than the higher frequencies. The higher frequency disturbances became more nearly perpendicular to the wall in the central region of the pipe. For points very near the wall the disturbances appear to be very obliquely inclined. A scheme which utilizes a temperature sensing probe immersed in the working fluid is used to compensate for the water temperature variation. Several possible circuit configurations for this scheme, including an optimum circuit design, are investigated and the results from some of them are presented and discussed. The circuit has a frequency response to temperature variations which depends on the thermal time constant of the temperature probe (up to several cycles per second can be obtained using commercially available probes) and can be used to compensate for temperature variations of more than 20°F with an accuracy better than + 0.2%. By using an effective value (much smaller than EQ) instead of the zero- velocity bridge voltage (E0) in exponential-type linearizers, a constant exponent is found useful in linearizing the anemometer output over a wider range of velocities, especially the very low ones. Finally, a linearized hot-film anemometer compensated for temperature variation by utilizing the present scheme is successfully used to obtain precision measurements in a standard laminar flow- field where the water temperature varied. The results compare favorably with classical theory which is quite encouraging in view of the low overheat ratio used with hot-films and the large effects of temperature on water density and viscosity

Interpretation of 2-probe turbulence measurements in an axisymmetric contraction

Simultaneous measurements of the streamwise and radial velocity components at two points, one on and one off the centerline with variable radial separation, were digitally recorded and processed at several stations along a four to one contraction with controlled upstream turbulence conditions. Various statistical quantities are presented including spectra and coherence functions. The integral L sub ux, L sub um, L sub vx, L sub vm were also estimated and their variation along the contraction is examined

Histopathologic and Radiologic Assessment of Chemotherapeutic Response in Ewing's Sarcoma: A Review

Ewing's sarcoma is a highly malignant tumor that metastasizes rapidly and is thus associated with a low survival rate. The intensification of chemotherapy has been shown to improve the overall survival of patients with Ewing's sarcoma. However, intensified chemotherapy can lead to increased toxicity or even the development of secondary malignancies. The stratification of patients with Ewing's sarcoma into “good” and “poor” responders may help guide the administration of progressively more intensified chemotherapy. Thus, an accurate assessment of the chemotherapeutic response, as well as the extent of chemotherapy-induced tumor necrosis, is critical for avoiding potential treatment-related complications in these patients. This paper reviews the methods currently used to evaluate chemotherapeutic response in Ewing's sarcoma, focusing specifically on histopathologic and imaging analyses, and discusses novel therapies and imaging methods that may help improve the overall survival of these patients

Obtaining accurate mean velocity measurements in high Reynolds number turbulent boundary layers using Pitot tubes

This article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from Re_θ = 11 100 to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here

Scaling in Wall Turbulence: Scale Separation and Interaction (Invited Paper)

High Reynolds number pipe flow data are used to demonstrate the importance of several conditions related to scale separation that are either assumed in the classical theories or may be used in light of recent results in wall turbulence to infer a minimum Reynolds number condition above which scaling results may be suitable for extrapolation. Results from the Princeton Superpipe have suggested Re_τ > 5000 as the minimum Reynolds number for which key properties of pipe flow reach a “fully-developed” condition, based on observations of streamwise mean and turbulent velocity structure. Additional values related to finer constraints on the structural development are also discussed. A “skeleton” of wall turbulence is introduced, based on structural components identified as having a dominant role in the dynamics of near-wall turbulence in recent experiments by a variety of authors. Possible interaction mechanisms between these components are described alongside some outstanding questions concerning scale separation and interaction