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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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Journal ArticleDOI
TL;DR: In this paper, the spreading rate and mixing of a transverse jet in high-speed crossflow were modified using a swirling injector with a central control jet, which could be used to affect mixing both in the core and the shear layer of the jet.
Abstract: The spreading rate and mixing of a transverse jet in high-speed crossflow were modified using a swirling injector with a central control jet. The controlled supersonic swirling injector (CSSI) could be used to affect mixing both in the core and the shear layer of the jet. Rayleigh/Mie scattering from flowfield ice crystals and planar laser-induced fluorescence of the NO molecules were used to characterize penetration and mixing of the CSSI for six different cases. Instantaneous images were used to study the dynamical structures in the jet, whereas ensemble images provided information regarding the jet trajectory. Standard deviation images revealed information about the large-scale mixing/entrainment. Probability density functions were used to evaluate the probability and location of freestream, mixed, and jet fluid. They were also used to track the centerline and jet boundary on a dynamic scale. Side- (streamwise)-view images showed that the injector was capable of providing high penetration when compared to circular and swirling baseline injectors. An increase of 16% in mixing area was observed with the optimal case as compared with the other control cases. End- (spanwise)-view images show a maximum of 78 % increase in total area contained within the jet boundary for the optimal case when compared to the circular injector. Higher spanwise extent of the jet boundary was also observed with controlled cases, which could provide higher interfacial area for better mixing between the jet and the cross stream when compared to their baseline counterparts.

20 citations

Proceedings ArticleDOI
09 Jan 2006
TL;DR: In this article, a set of DES simulations were run with an unsteady inflow boundary layer, where the perturbations are a statistically meaningful representation of a series of randomly placed hairpin eddies.
Abstract: Numerical simulations of transverse injection through low-angled injector ports into a supersonic freestream are performed using a hybrid, unstructured solver. Two cases are investigated: air injected into a M=2.9 air freestream with a 25◦ injection angle, and heated helium injected into a M=4.0 air freestream with a 30◦ injection angle. Simulations were run in RANS and DES modes. A set of the DES simulations were run with an unsteady inflow boundary layer, where the perturbations are a statistically meaningful representation of a series of randomly placed hairpin eddies. This boundary condition was fed periodically into the domain, and was reused multiple times over the course of a simulation. The RANS and DES simulations are found to capture the salient features of the flow, though discrepancies with experimental data are found. While the DES simulations were found to give steady-state solutions for the flow fields, the addition of the unsteady inflow boundary layer was found to greatly impact the downstream flow field and to improve the overall agreement with experiment. In the case of the helium injection, it was found that the predicted mass fraction distributions of the DES simulations with the unsteady inflow boundary layer was far less dependent on the value of the turbulent Schmidt number. This result shows that the mixing found in DES simulation with the unsteady inflow boundary layer is a result of the large-scale turbulent motion of the flow, rather than because of the gradient diffusion term. However, the ‘box of eddies’ used to create the unsteady inflow boundary layers were not long enough to ensure that no bias was introduced into the flow field, and future simulations will be run with larger boxes. The results of the study show a great deal of promise for the use of DES simulations in conjunction with unsteady inflow boundary layers for simulation of SCRAMjet fuel injection.

20 citations

Journal ArticleDOI
TL;DR: In this paper, the aerodynamic performance of an oscillating heaving and pitching foil operating in the energy harvesting mode was experimentally investigated at reduced frequencies (k = f c ∕ U ∞ ) of 0.04 to 0.08.

20 citations

Journal ArticleDOI
Roger L. Simpson1
TL;DR: The time-dependent structure of the wall region of separating, separated, and reattaching flows is considerably different than that of attached turbulent boundary layers as mentioned in this paper, whose frequency of passage scales on the freestream velocity and shear layer thickness, produce large Reynolds shearing stresses and most of the turbulence kinetic energy in the outer region of the shear layers and transport it into the low velocity reversed flow next to the wall.
Abstract: The time-dependent structure of the wall region of separating, separated, and reattaching flows is considerably different than that of attached turbulent boundary layers. Large-scale structures, whose frequency of passage scales on the freestream velocity and shear layer thickness, produce large Reynolds shearing stresses and most of the turbulence kinetic energy in the outer region of the shear layer and transport it into the low velocity reversed flow next to the wall. This outer flow impresses a near wall streamwise streaky structure of spanwise spacing λ z simultaneously across the wall over a distance of the order of several λ z . The near wall structures produce negligible Reynolds shear stresses and turbulence kinetic energy.

20 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used planar laser-induced fluorescence (PLIF) of the hydroxyl (OH) and nitric oxide (NO) molecules and complementary 3-D nonreacting CFD simulations to visualize the structure of the lateral counter-rotating vortex pair (LCVP) and assess reactivity within it.
Abstract: IMPROVING fuel–air mixing and flame holding are current research areas for combined-cycle engines for hypersonic propulsion [1–6]. The scramjet mode is particularly difficult because fuel and air residence times within the engine are of the order of milliseconds; during this time, the fuel must penetrate into and mix with the freestream air and then substantially react to completion. Ideally, penetration and then mixing of the fuel should be done with minimal pressure loss, and thus there is an incentive, especially for small-scale engines, to accomplish fuel injection through so-called nonintrusive injection ports, the most basic of which is the circular, flush-wall, normal injector. Furthermore, while penetration and mixing with the crossflow are typically of interest, other considerations may be important too, such as entrainment into a flameholding device. In the present study, the focus is on flush-wall injection through a diamond-shaped orifice [7,8]. Recently, Srinivasan and Bowersox [9,10] investigated with computational fluid dynamics (CFD) the possibility of tailoring the flow structure to enhance mixing and produce a stable vortex for gas-dynamically induced flame holding. The goal was to control the shape of the interaction barrel shock to produce a flow structure that resembled a blunt bodywith a truncated transverse plane at the trailing edge. A diamond-shaped port was found to produce the desired flow structure. Boundary-layer and injector fluid would then be entrained into this recirculation zone, termed the lateral counter-rotating vortex pair (LCVP), because the structure contains a vortex pair spanning the width of the barrel shock. To determine the robustness of the LCVP, parametric studies were performed for freestream Mach numbers from 2 to 5; it was found that by controlling the injector pressure, the LCVP could be created. The residence time within the LCVP was estimated to be an order of magnitude longer than the flow time through the solution domain, indicating that LCVP may be sufficient for flame holding under some circumstances. The objective for the present studywas to visualize the structure of the LCVP and assess reactivity within it. As with the previous study [11], the tools employed include planar laser-induced fluorescence (PLIF) of the hydroxyl (OH) and nitric oxide (NO) molecules and complementary 3-D nonreacting CFD simulations.

20 citations


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