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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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TL;DR: In this article, the authors performed a conditional sampling on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=(v/U 2 ∞)(dU∞/dx) as high as 9×10 -6 ) acceleration.
Abstract: Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=(v/U 2 ∞)(dU∞/dx) as high as 9×10 -6 ) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts
46 citations
TL;DR: In this article, the effects of freestream turbulence intensity and integral length scale on the drag coefficient of a sphere were experimentally investigated in a closed circuit wind tunnel and the results showed that over the range of conditions studied, the drag always decreases with increasing Tu and, the critical Reynolds number at which the Drag coefficient is dramatically reduced is decreased by increasing Tu.
46 citations
TL;DR: An optical method for generating localized, controlled perturbations has been developed for use in supersonic and hypersonic flowfields as discussed by the authors, where the thermal spot disturbance is generated when the pulsed beam from a laser is focused at the desired origin and a small region of the gas is ionized after recombination.
Abstract: An optical method for generating localized, controlled perturbations has been developed for use in supersonic and hypersonic flowfields The thermal spot disturbance is generated when the pulsed beam from a laser is focused at the desired origin and a small region of the gas is ionized After recombination, the thermal spot persists as a region of heated gas that convects with the local flow velocity The perturbation is approximately spherical and several millimeters in diameter, with the size dependent on the flow density Optimal formation of the perturbation is achieved with large-F-number focusing systems To date, the disturbance has been used as a freestream perturbation for supersonic receptivity experiments as well as forward-facing cavity and blunt-body shock oscillation studies
46 citations
TL;DR: In this paper, the authors studied the details of the horseshoe vortex formation around a cylinder to determine the flow parameters that effect the flow separation in front of the cylinder, using a wind tunnel test section.
Abstract: Details of the horseshoe vortex formation around a cylinder were studied to determine the flow parameters that effect the flow separation in front of the cylinder. An experimental setup, consisting of a circular cylinder vertically mounted on the floor of a wind tunnel test section, was assembled. The approaching turbulent boundary layer was 4cm thick. Pressures were measured on the cylinder surface and the tunnel floor with surface static pressure taps.Surface flow visualisations were accomplished to locate singular points and the size of separation region on the end wall surface. Other details of the study are given.
46 citations
TL;DR: In this article, a comparison between supersonic combustion in two commonly used, but fundamentally different, facilities for scramjet research, a vitiation-heated blowdown tunnel and a free-piston shock tunnel was made.
Abstract: A comparison has been made between supersonic combustion in two commonly used, but fundamentally different, facilities for scramjet research, a vitiation-heated blowdown tunnel and a free-piston shock tunnel. By passing the shock-tunnel freestream flow through a normal shock and then expanding it to Mach 2.5, combustor inlet conditions and geometries were nominally replicated between the two facilities. A constant-area rectangular duct and a diverging duct, both employing central-strut hydrogen injection, were used. Boundary-layer separation and choking in the constant-area duct limited supersonic combustion comparisons up to a fuel equivalence ratio of the order of 0.3. The experimental results also show that the onset of boundary-layer separation occurs at the same combustor pressure loads and that it behaves similarly in the different facilities. With the diverging duct, comparisons were made up to an equivalence ratio of 1.05. Agreement between the results obtained in the two facilities is within experimental error when the different freestream and boundary layers are accounted for.
46 citations