An Experimental Study of Turbulent Boundary Layers Subjected to High Free-Stream Turbulence Effects PDF Download
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Languages : en
Pages : 109
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The work presented in this thesis was on nominally two-dimensional turbulent boundary layers at zero pressure gradient subjected to high free-stream turbulent intensities of up to 7.9% in preparations for high free-stream turbulence studies on three-dimensional boundary layers. The two-dimensional turbulent flow that will impinge three-dimensional bodies needed to be characterized, before the three-dimensional studies can be made. An active turbulence generator designed to create high free-stream turbulence intensities in the wind tunnel was tested and modified in order to obtain the lowest possible mean flow non-uniformities. A seven-hole pressure probe was used to obtain planes of mean velocity measurements. A three-component state of the art laser-Doppler velocimeter (LDV) was used to obtain mean and fluctuating velocities. Previous high free-stream turbulence studies have been reviewed and are discussed, and some of the previously published data of other authors have been corrected. Based on the measurements obtained with the LDV, it was also determined that the semi-log law of the wall is valid for high free-stream turbulence cases, but with different constants than the ones proposed by Coles, where the constants for the high free-stream cases may be dependent on the turbulence intensity. For the first time, the skin friction coefficient (CO was deduced from the viscous sublayer. The difference between the Utau obtained in the viscous sublayer mean velocity profile and the Utau obtained in the semi-log layer was 1.5%. The skin friction coefficient was determined to increase by 10.5% when the two-dimensional turbulent boundary layer was subjected to high free-stream turbulence effects. Spectral data were compared to the von Karmen and Pope's model spectra; the von Karman spectrum was proven to fit slightly better. Finally the Hancoc-Bradshaw-Blair parameter agreed very well with previously published data.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 109
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Book Description
The work presented in this thesis was on nominally two-dimensional turbulent boundary layers at zero pressure gradient subjected to high free-stream turbulent intensities of up to 7.9% in preparations for high free-stream turbulence studies on three-dimensional boundary layers. The two-dimensional turbulent flow that will impinge three-dimensional bodies needed to be characterized, before the three-dimensional studies can be made. An active turbulence generator designed to create high free-stream turbulence intensities in the wind tunnel was tested and modified in order to obtain the lowest possible mean flow non-uniformities. A seven-hole pressure probe was used to obtain planes of mean velocity measurements. A three-component state of the art laser-Doppler velocimeter (LDV) was used to obtain mean and fluctuating velocities. Previous high free-stream turbulence studies have been reviewed and are discussed, and some of the previously published data of other authors have been corrected. Based on the measurements obtained with the LDV, it was also determined that the semi-log law of the wall is valid for high free-stream turbulence cases, but with different constants than the ones proposed by Coles, where the constants for the high free-stream cases may be dependent on the turbulence intensity. For the first time, the skin friction coefficient (CO was deduced from the viscous sublayer. The difference between the Utau obtained in the viscous sublayer mean velocity profile and the Utau obtained in the semi-log layer was 1.5%. The skin friction coefficient was determined to increase by 10.5% when the two-dimensional turbulent boundary layer was subjected to high free-stream turbulence effects. Spectral data were compared to the von Karmen and Pope's model spectra; the von Karman spectrum was proven to fit slightly better. Finally the Hancoc-Bradshaw-Blair parameter agreed very well with previously published data.
Author: Edgar Orsi Filho
Publisher:
ISBN:
Category :
Languages : en
Pages : 97
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Book Description
The work presented in this thesis was on nominally two-dimensional turbulent boundary layers at zero pressure gradient subjected to high free-stream turbulent intensities of up to 7.9% in preparations for high free-stream turbulence studies on three-dimensional boundary layers, which will be done in the future in the Aerospace and Ocean Engineering Boundary Layer Wind Tunnel at Virginia Tech. The two-dimensional turbulent flow that will impinge three-dimensional bodies needed to be characterized, before the three-dimensional studies can be made. An active turbulence generator designed to create high free-stream turbulence intensities in the wind tunnel was tested and modified in order to obtain the lowest possible mean flow non-uniformities. A seven-hole pressure probe was used to obtain planes of mean velocity measurements. A three-component state of the art laser-Doppler velocimeter (LDV) was used to obtain mean and fluctuating velocities. Previous high free-stream turbulence studies have been reviewed and are discussed, and some of the previously published data of other authors have been corrected. Based on the measurements obtained with the LDV, it was also determined that the semi-log law of the wall is valid for high free-stream turbulence cases, but with different constants than the ones proposed by Coles, where the constants for the high free-stream cases may be dependent on the turbulence intensity. For the first time, the skin friction coefficient (Cf) was deduced from the viscous sublayer. The difference between the U_tau obtained in the viscous sublayer mean velocity profile and the U_tau obtained in the semi-log layer was 1.5%. The skin friction coefficient was determined to increase by 10.5% when the two-dimensional turbulent boundary layer was subjected to high free-stream turbulence effects. Spectral data obtained with the LDV, were compared to the von Kármán model spectrum and to the Pope's model spectrum, where the von Kármán spectrum was proven to fit the spectral data slightly better than the Pope's spectrum. Finally, the Hancock-Bradshaw-Blair parameter obtained for this experiment agreed very well with previously published data.
Author: S. J. Kline
Publisher:
ISBN:
Category : Fluid dynamics
Languages : en
Pages : 70
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Author: Giles John Brereton
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ISBN:
Category : Fluid mechanics
Languages : en
Pages : 304
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An experimental study of the effects of forced, free-stream unsteadiness on turbulent boundary layers was undertaken in an effort to characterize the nature of such flows and to provide guidance for turbulence modelers. A turbulent boundary layer, which had developed under steady, constant-pressure conditions, was locally subjected to a range of frequencies of sinusoidal free-stream unsteadiness. The response of the boundary layer to these free-stream effects was studied through simultaneous measurements of the u and v components of the velocity field, using a two-color laser-doppler anemometer. To focus on the distinct behaviors of the deterministic and time-averaged fields of flow, the equations of fluid motion were reformulated according to a triple decomposition of velocity and pressure into; (i) a time-averaged measure, (ii) a deterministic or organized, unsteady measure, and (iii) a turbulent measure.
Author: Alexander Vladimir Lisin
Publisher:
ISBN:
Category : Boundary layer
Languages : en
Pages : 164
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Author: M. F. Blair
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ISBN:
Category :
Languages : en
Pages : 73
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Experimental research has been conducted to determine the influence of free-stream turbulence on zero pressure gradient, incompressible, fully turbulent boundary layer flow. During this period convective heat transfer coefficients, boundary layer mean velocity and temperature profile and wall static pressure distribution data were obtained for two flow conditions of constant free-stream velocity and low free-stream turbulence intensity and for one flow condition of constant free-stream velocity and higher free-stream turbulence. Documentation of the free-stream turbulence for these flows is currently in progress. The conclusion reached from the low free-stream turbulence test results is that these data are in excellent agreement with classic two-dimensional, low free-stream turbulence, turbulent boundary layer correlations, thus establishing the absolute accuracy of the experiment. The data obtained for the higher free-stream turbulence test case indicates that free-stream turbulence does have a significant effect on fully turbulent boundary layer skin friction and heat transfer. A quantitative assessment of this influence will emerge as data is obtained for additional free-stream turbulence levels and as the turbulence distributions are documented.
Author: Stanford University. Thermosciences Division
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ISBN:
Category :
Languages : en
Pages : 80
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Author: Peter Sten Andersen
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ISBN:
Category : Boundary layer
Languages : en
Pages : 230
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Author: Jack Medzorian
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ISBN:
Category :
Languages : en
Pages : 0
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Wall-bounded tip vortices appear in a variety of aerodynamic applications, such as, aircraft engines, inlet S-ducts, turbomachinery tip leakage, and vortex generators used for flow separation control. Studies show that vortex flows with high swirl that are exposed to a sufficient adverse pressure gradient will trigger the onset of vortex instability. However, it is not known whether the addition of freestream turbulence or the presence of a wall will stabilize a vortex due to momentum entrainment or trigger early bursting via amplification of an instability pathway. In this study, a wall-bounded vortex is analyzed in a low-speed wind tunnel to evaluate the effects that adverse pressure gradient and freestream turbulence have on vortex stability boundaries. Experimental techniques such as high-speed stereoscopic particle image velocity is used to capture three-dimensional flow fields of the wall bounded vortex. In addition to the PIV measurements, point measurements are taken using Laser Doppler Velocimetry to obtain turbulent characterization of the freestream and boundary layer flow. The results in this study predict an increase in vortex stability with freestream turbulence. Near a wall, further increases in vortex stability is depicted due to increased diffusion and reduction in rotational momentum. Furthermore, turbulent kinetic energy in a vortex core is decreased in high freestream turbulence when the vortex is near a wall due to a reduction in roll up and entrainment of high turbulence that exists in turbulent boundary layers near the wall. Turbulent boundary layers exposed to high freestream turbulence and adverse pressure gradients appear in a variety of aerodynamic application such as the suction side of airfoils within gas turbine engines and on aircraft wings. Non-equilibrium turbulent boundary layers exposed to a non-constant adverse pressure gradient have been shown to influence the outer region of the mean boundary layer profile due to the resilience of larger scale superstructures. However, it is not known whether the addition of freestream turbulence will increase the effect of history in a turbulent boundary layer due to increased entrainment of larger scales or decrease in effects from breakdown of these superstructures in the freestream. In this study, an experimental technique known as laser Doppler velocimetry is applied to obtain 2-dimensional, high-speed velocity measurements used for capturing mean profiles, turbulent statistics, and turbulent characterization of the freestream flow. The results in this study predict that pressure gradient history effects can have influence on Reynolds stress distributions past the outer region and into the log-region of the boundary layer due to diffusion of turbulence towards the wall. Additionally, pressure gradient effects, and thus the effects of a boundary layer's history, are significantly reduced when subjected to freestream turbulence due to dominant influence of turbulence within the wake region. Lastly, turbulent statistics show that a straight contoured wall, containing a smaller local pressure gradient but sustained over longer distance, has a greater adverse pressure gradient effect on a boundary relative to concave or convex contouring.
Author: David Michael Driver
Publisher:
ISBN:
Category : Boundary layer
Languages : en
Pages : 268
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