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Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.


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Proceedings ArticleDOI
01 Jan 2007
TL;DR: In this article, a comparison of the profile and secondary flow performance of a family of three highly loaded low-pressure (LP) turbine airfoils operating over a wide range of Reynolds numbers (25,000 to 150,000 based on the axial chord and inlet velocity), and for values of freestream turbulence intensity of 1.5% and 4%.
Abstract: At the 2006 ASME-IGTI Turbo-Expo, low-speed cascade results were presented for the midspan aerodynamic behaviour of a family of three highly loaded low-pressure (LP) turbine airfoils operating over a wide range of Reynolds numbers (25,000 to 150,000 based on the axial chord and inlet velocity), and for values of freestream turbulence intensity of 1.5% and 4%. All three airfoils have the same design inlet and outlet flow angles. The baseline cascade has a Zweifel coefficient of 1.08 and the two additional blade rows have values of 1.37. The new, more highly-loaded blade rows differ mainly in their loading distributions: one is front-loaded while the other is aft-loaded. The new front-loaded airfoil was found to have particularly attractive profile performance. Despite its exceptionally high value of Zweifel coefficient, it was found to be free of a separation bubble on its suction side at Reynolds numbers as low as 50,000, and this was reflected in very good profile loss behaviour. However, it was also noted in the earlier paper that the choice of a particular loading level and loading distribution would be influenced by more than its profile performance at design incidence. The present two-part paper extends the midspan aerodynamic comparison of the three airfoils to the secondary flow performance. The first part of the paper discusses both the profile and secondary flow performance of the three cascades at their design Reynolds number of 80,000 (or ∼ 125,000 based on exit velocity) for two freestream turbulence intensities of 1.5% and 4%. The secondary flow behaviour was determined from detailed flowfield measurements made at 40% axial chord downstream of the trailing edge using a seven-hole pressure probe. In addition to providing total pressure losses, the seven-hole probe measurements were also processed to give the downstream vorticity distributions. As has been found in other secondary flow investigations in turbine cascades, the present front-loaded airfoil showed higher secondary losses than the aft-loaded airfoil with the same value of Zweifel coefficient.Copyright © 2007 by ASME

63 citations

Proceedings ArticleDOI
TL;DR: In this article, high freestream turbulence along a gas turbine airfoil and strong secondary flows along the endwall have both been reported to significantly increase convective heat transfer.
Abstract: High freestream turbulence along a gas turbine airfoil and strong secondary flows along the endwall have both been reported to significantly increase convective heat transfer. This study superimposes high freestream turbulence on the naturally occurring secondary flow vortices to determine the effects on the flowfield and the endwall convective heat transfer. Measured flowfield and heat transfer data were compared between low freestream turbulence levels (0.6%) and combustor simulated turbulence levels (19.5%) that were generated using an active grid. These experiments were conducted using a scaled-up, first stage stator vane geometry. Infrared thermography was used to measure surface temperatures on a constant heat flux plate placed on the endwall surface. Laser Doppler velocimeter (LDV) measurements were performed of all three components of the mean and fluctuating velocities of the leading edge horse-shoe vortex. The results indicate that the mean flowfields for the leading edge horseshoe vortex were similar between the low and high freestream turbulence cases. High turbulence levels in the leading edge-endwall juncture were attributed to a vortex unsteadiness for both the low and high freestream tubulence cases. While, in general, the high freestream turbulence increased the endwall heat transfer, low augmentations were found to coincide with the regions having the most intense vortex motions.Copyright © 2000 by ASME

63 citations

Journal ArticleDOI
TL;DR: In this paper, surface temperatures and pressures were measured on an elliptic cone lifting body in a hypersonic e owe eld using thin-e lm (5πm) temperature and pressure-sensitive paints (TSPs and PSPs ).
Abstract: Surface temperatures and pressures were measured on an elliptic cone lifting body in a hypersonic e owe eld using thin-e lm (» 5πm) temperature- and pressure-sensitive paints (TSPs and PSPs ). The tests were conducted in the 48-inch hypersonic shock tunnel (48-inch HST) at Calspan‐University of Buffalo Research Center and were part of a more comprehensive experimental study examining the three-dimensional characteristics of laminar, transitional, and turbulent e ow over the model. Measurement opportunity in the 48-inch HST was limited by the short duration of steady freestream conditions of the driven gas; image acquisition times were » 3 ms. Images of the coatings applied to the broad side of the symmetric elliptic cone were calibrated with in situ static pressure and surface-e lm temperature measurements. The TSP results illustrate the higher heat transfer rates and change in boundary-layer transition over the model surface caused by the nose geometry, and the PSP results show a mild pressure gradient over the interrogated surface region. Submillisecond TSP acquisition using a high-speed imager demonstrated the feasibility of measuring the surface temperature rise.

63 citations

Journal ArticleDOI
TL;DR: In this article, a laminar separation bubble is created by imposing a streamwise adverse pressure gradient at the freestream boundary of the integration domain and different steady and unsteady boundary layer disturbances are then introduced at a disturbance strip upstream of separation and their effects on the separation bubble are studied.
Abstract: This paper presents detailed investigations related to active transition control in laminar separation bubbles. The investigations rely on direct numerical simulations based on the complete Navier-Stokes equations for a flat-plate boundary layer. A laminar separation bubble is created by imposing a streamwise adverse pressure gradient at the freestream boundary of the integration domain. Different steady and unsteady boundary layer disturbances are then introduced at a disturbance strip upstream of separation and their effects on the separation bubble are studied. It is shown that the size of the separated region can be controlled most efficiently by very small periodic oscillations, which lead to traveling instability waves that grow to large levels by the hydrodynamic instability of the flow. Indications for the preferred frequency of these waves can be obtained from linear stability theory, but since the problem is nonlinear, only direct numerical simulations can really qualify or disqualify the predictions. Overall, it turns out that unsteady two- or three-dimensional disturbances have a stronger impact on the size of the bubble than steady disturbances, because they directly provide initial amplitudes for the laminar-turbulent transition mechanism.

62 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigated the broadband noise generated by the scattering of turbulent flow at the trailing edge of a NACA 0018 airfoil with trailing edge serrations, varying both the angle of attack and serration flap angle.

62 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023195
2022350
2021108
2020113
201986
2018118