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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 aerodynamic behavior of an inverted airfoil in ground effect under heaving motion was investigated numerically and three fundamental features that define these regimes have been identified: 1) ground effect, 2) incidence effect, and 3) added mass effect.
Abstract: The aerodynamic behavior of an inverted airfoil in ground effect under heaving motion was investigated numerically. A sinusoidal vertical movement perpendicular to a uniform freestream was imposed on the airfoil. The response showed a periodic pattern that repeated with the same frequency as the heaving motion. The effect of reduced frequency at several mean distances from the ground was examined. By analyzing the lag of the aerodynamic coefficients with respect to the vertical motion, the flow can be classified into different regimes. Three fundamental features that define these regimes have been identified: 1) ground effect, 2) incidence effect, and 3) added mass effect. At low frequencies, the airfoil can be assumed to be in a quasi-stationary motion and the ground effect governs the flow. The contribution of incidence effects becomes apparent at medium frequencies and can be explained by defining an effective angle of attack. The maximum downforce is obtained at high effective angles of attack. As the frequency is increased even further, the only apparent effect is that of added mass. In this regime, downforce is shown to be related to the vertical acceleration of the airfoil. These mechanisms are independent of Reynolds number.

64 citations

Proceedings ArticleDOI
01 Jan 1988
TL;DR: In this article, the direct-simulation Monte Carlo method incorporating a dissociating and ionizing gas model for air with thermal radiation is used to characterize the hypersonic flow about an axisymmetric representation of an aero-assist flight experiment (AFE) vehicle, whose freestream conditions correspond to selected points along the entry, aerobraking, and exit phases of the trajectory.
Abstract: The direct-simulation Monte Carlo method incorporating a dissociating and ionizing gas model for air with thermal radiation is used to characterize the hypersonic flow about an axisymmetric representation of an aeroassist flight experiment (AFE) vehicle, whose freestream conditions correspond to selected points along the entry, aerobraking, and exit phases of the trajectory. Calculations for two trajectory conditions indicate that the radiative heating of the AFE forebody is lower than the convective heating, but becomes significant as the maximum convective heating rate condition is approached.

64 citations

Proceedings ArticleDOI
28 Jun 2004
TL;DR: In this paper, a direct numerical simulation database of supersonic and hypersonic turbulent boundary layers is presented and the systematic procedure for initializing the turbulent flow fields at controlled Mach number and Reynolds number conditions is described.
Abstract: We present a direct numerical simulation database of supersonic and hypersonic turbulent boundary layers. The systematic procedure for initializing the turbulent flow fields at controlled Mach number and Reynolds number conditions is described. It is shown that simulation transients are less than 10% of the time required for gathering statistical data of the turbulent flows. The experimental conditions of Debieve and Elena are simulated. The magnitude of velocity and temperature fluctuations, as well as the turbulent shear stresses given by the direct numerical simulations are in excellent agreement with the experimental data. Using the direct numerical simulation database we perform parametric studies varying freestream Mach number in the range of 3 to 8 and wall-temperature condition for wall-to-freestream-temperature ratio of 2 to 5.5.

63 citations

01 May 1988
TL;DR: In this article, the boundary layer transition process in a low intensity disturbance environment to that in an environment in which the disturbances are initially nonlinear in amplitude was conducted using a flat plate model.
Abstract: A detailed investigation to compare the boundary layer transition process in a low intensity disturbance environment to that in an environment in which the disturbances are initially non-linear in amplitude was conducted using a flat plate model. The transition mechanism based on linear growth of Tollmien Schlichting (T-S) waves was associated with a freestream turbulence level of 0.3 percent; however, for a freestream turbulence intensity of 0.65 percent and higher, the bypass transition mechanism prevailed. The results of detailed measurements acquired to study and compare the two transition mechanisms indicate that there exists a critical value for the peak rms of the velocity fluctuations within the boundary layer of approximately 3 to 3.5 percent of the freestream velocity. Once the unsteadiness within the boundary layer reached this critical value, turbulent bursting initiated, regardless of the transition mechanism. The two point correlations and simultaneous time traces within the transition region illustrate the features of a turbulent burst and its effect on the surrounding flowfield.

63 citations

Proceedings ArticleDOI
TL;DR: In this article, the effects of high freestream turbulence on the boundary layer development of a stator vane airfoil were examined, showing that the mean velocity profiles appeared to be more consistent with laminar profiles.
Abstract: High freestream turbulence levels have been shown to greatly augment the heat transfer on a gas turbine airfoil. To better understand these effects, this study has examined the effects elevated freestream turbulence levels have on the boundary layer development along a stator vane airfoil. Low freestream turbulence measurements (0.6 percent) were performed as a baseline for comparison to measurements at combustor simulated turbulence levels (19.5 percent). A two-component LDV system was used for detailed boundary layer measurements of both the mean and fluctuating velocities on the pressure and suction surfaces. Although the mean velocity profiles appeared to be more consistent with laminar profiles, large velocity fluctuations were measured in the boundary layer along the pressure side at the high freestream turbulence conditions. Along the suction side, transition occurred further upstream due to freestream turbulence.

63 citations


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