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Moderately three-dimensional separated and reattaching turbulent flowHardman, Jonathan Ralph January 1998 (has links)
No description available.
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[en] NUMERICAL INVESTIGATION OF THE TURBULENT FLOW SEPARATION BUBBLE OVER INCLINED THIN FLAT PLATE / [pt] ANÁLISE NUMÉRICA DA BOLHA DE SEPARAÇÃO DO ESCOAMENTO TURBULENTO SOBRE PLACA PLANA FINA INCLINADAANDRE LUIZ TENORIO REZENDE 18 November 2009 (has links)
[pt] A estabilização de mísseis e projéteis é normalmente realizada através de
aletas, que podem ser representadas por placas planas finas. O escoamento
sobre placas finas é de difícil previsão por apresentar diversos fenômenos, tais
como transição da camada cisalhante para regime turbulento, recolamento,
relaminarização e geração de bolhas primárias e secundárias. A proposta deste
trabalho é analisar o escoamento turbulento ao longo de uma placa plana com
pequeno ângulo de incidência, e ao mesmo tempo investigar o desempenho de
diferentes modelos para a previsão da turbulência, empregando duas
metodologias. A primeira é baseada nas Equações de Média de Reynolds
(RANS), a qual requer um menor esforço computacional, por considerar um
domínio bi-dimensional e regime permanente. Neste caso, três níveis de
modelagens foram selecionados, os quais envolvem a solução de uma, duas e
cinco equações diferenciais parciais, correspondendo aos modelos de Spalart-
Allmaras (SA), kapa-ômega Shear Stress Tensor (SST) e Reynolds Stress Model (RSM),
respectivamente. No segundo enfoque, investigou-se o desempenho do modelo
Smagorinsky Dinâmico, que é proveniente da metodologia da Simulação de
Grandes Escalas (LES), a qual é tri-dimensional e transiente. Os resultados
foram obtidos para número de Reynolds igual a 2,13 x 10(5) e para três ângulos de
incidência (um, três e cinco graus). A modelagem da turbulência foi validada
através de comparação como dados numéricos e experimentais existentes na
literatura. Os resultados obtidos mostraram que apesar do modelo RSM
conseguir uma melhor previsão dos níveis de turbulência, o mesmo não é
adequado para prever camadas cisalhantes livres. Já o modelo SA é muito
difusivo, e não consegue prever adequadamente as tensões normais
turbulentas, enquanto que o modelo SST foi capaz de prever razoavelmente bem
a bolha de separação. Porém, apesar do custo bem superior, as previsões dos
fenômenos provenientes da bolha de recirculação principal obtidas com a
metodologia LES foram sensivelmente superiores e forneceram maior riqueza de
informações que as apresentadas pelas soluções RANS. / [en] Missiles and projectiles stabilization is usually accomplished through fins,
which can be represented by thin flat plates. The flow field over thin plates is
difficult to predict due to the existence of laminar-to-turbulent transition, boundary
layer separation, leading edge bubble and reattachment. The purpose of this
study is to analyze the flow over a thin flat plate, and at the same time, to
investigate the performance of different models to predict turbulence, by
employing two methodologies. The first one is based on the Reynolds Average
Navier-Stokes Equations (RANS), which requires less computational effort, since
it can be applied to a two-dimensional steady flow. In this case, three levels of
modeling were employed, through the solution of one, two and five differential
equations, corresponding to the Spalart-Allmaras (SA), kapa-ômega Shear Stress Tensor
(SST) and Reynolds Stress Model (RSM) models, respectively. The second
approach corresponds to the Large Eddy Simulation (LES) methodology, and the
performance of the Dynamic Smagorinsky model was investigated. Results were
obtained for Reynolds number equal to 2.13 x 10(5) and for three incidence angles
(one, three and five degrees). The results were validated by comparing with
available numerical and experimental data. It was shown that, in spite of
predicting better turbulence levels, the RSM is not adequate to predict free shear
layers. The SA model is too diffusive and it fails to predict the normal stresses,
while the SST is capable of predicting the separation bubble with reasonable
accuracy. However, in spite of the larger cost, the long separation bubble
predictions obtained with the LES methodology were substantial superior and
more complete than RANS solutions.
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An Experimental Investigation Of Airfoils With Laminar Separation Bubbles And Effects Of Distributed SuctionWahidi, Redha 11 December 2009 (has links)
In an effort to understand the behavior of the laminar separation bubbles on NACA 0012 and Liebeck LA2573a airfoils at different Reynolds numbers and angles of attack, the boundary layers on the solid airfoils were investigated by measuring the mean and fluctuating components of the velocity profiles over the upper surfaces of the airfoils. Surface pressure measurements were carried out to complete the mapping of the laminar separation bubble and to calculate the lift generated by the airfoils. The experiments were carried out at Reynolds numbers of 150,000 and 250,000. The locations of separation, transition and reattachment were determined as functions of angle of attack and Reynolds number for the two airfoils. The drag was estimated from wake pressure measurements and was based on the momentum deficit generated by the airfoil. The size and location of the laminar separation bubble did not show significant changes with Reynolds number and angle of attack for values of the angle of attack between 0 and 6 d grees. The baseline results of the size and location of the laminar separation bubble on the LA2573a airfoil were used to design a suction distribution. This suction distribution was designed based on Thwaites’ criterion of separation. The effects of applying suction on the size and location of the laminar separation bubble were investigated. The results showed that the suction distribution designed in this work was effective in controlling the size of the laminar separation bubble, maintaining an un-separated laminar boundary layer to the transition point, and controlling the location of transition. The effects of different suction rates and distributions on the drag were investigated. Drag reductions of 14-24% were achieved. A figure of merit was defined as drag reductions divided by the equivalent suction drag to assess the worthiness of the utilizing suction on low Reynolds number flows. The values of the figure of merit were around 4.0 which proved that the penalty of using suction was significantly less than the gain obtained in reducing the drag.
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Experimental Testing of Low Reynolds Number Airfoils for Unmanned Aerial VehiclesLi, Leon 04 December 2013 (has links)
This work is focused on the aerodynamics for a proprietary laminar flow airfoil for Unmanned Aerial Vehicle (UAV) applications. The two main focuses are (1) aerodynamic performance at Reynolds number on the order of 10,000, (2) the effect of a conventional hot-wire probe on laminar separation bubbles. For aerodynamic performance, pressure and wake velocity distributions were measured at Re = 40,000 and 60,000 for a range of angles of attack. The airfoil performed poorly for these Reynolds numbers due to laminar boundary layer separation. 2-D boundary layer trips significantly improved the lift-to-drag ratio. For probe effects, three Reynolds numbers were investigated (Re = 100,000, 150,000, and 200,000), with three angles of attack for each. Pressure and surface shear distributions were measured. Flow upstream of the probe tip was not affected. Transition was promoted downstream due to the additional disturbances in the separated shear layer.
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Experimental Testing of Low Reynolds Number Airfoils for Unmanned Aerial VehiclesLi, Leon 04 December 2013 (has links)
This work is focused on the aerodynamics for a proprietary laminar flow airfoil for Unmanned Aerial Vehicle (UAV) applications. The two main focuses are (1) aerodynamic performance at Reynolds number on the order of 10,000, (2) the effect of a conventional hot-wire probe on laminar separation bubbles. For aerodynamic performance, pressure and wake velocity distributions were measured at Re = 40,000 and 60,000 for a range of angles of attack. The airfoil performed poorly for these Reynolds numbers due to laminar boundary layer separation. 2-D boundary layer trips significantly improved the lift-to-drag ratio. For probe effects, three Reynolds numbers were investigated (Re = 100,000, 150,000, and 200,000), with three angles of attack for each. Pressure and surface shear distributions were measured. Flow upstream of the probe tip was not affected. Transition was promoted downstream due to the additional disturbances in the separated shear layer.
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Implicit Large Eddy Simulation of Low-Reynolds-Number Transitional Flow Past the SD7003 AirfoilGalbraith, Marshall Chistopher 27 July 2009 (has links)
No description available.
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Experimental Studies on the Effect of an Upstream Periodic Wake on a Turbulent Separation BubbleSuneesh, S S January 2016 (has links) (PDF)
The object of the present work is to experimentally study the case of a turbulent boundary layer subjected to an Adverse Pressure Gradient (APG) with separation and reattachment. The effect of unsteadiness on turbulent boundary layer separation by means two different methods were explored viz. the effect of local forcing by acoustic waves and effect of wakes on separation bubble.
The experiments were conducted in a low speed open circuit blower type wind tunnel. The turbulent separation bubble was created on the test plate by a contoured ceiling which created the adverse pressure gradient. The velocities were measured using single element hot wire and X-wire. Limited studies on quasi shear stress were also conducted using surface mounted hot film probes. Static pressure was measured using a projection manometer. Boundary layer is tripped near the leading edge of the flat plate to ensure a turbulent boundary layer. Surface pressure distribution and flow visualization were conducted as part of diagnostics.
In the case of laminar separation bubble, lot of investigations have been done on the effect of unsteady wake and the most important conclusion was that the wake induces `bypass' transition to turbulence and since the turbulent boundary layer is more resistant to separation, it remains attached. In the case of turbulent separation bubble, laminar-turbulent transition is not relevant and if the bubble is suppressed, it should be by some other mechanism. This is what we seek to unravel in this study.
A closer look at the mean velocity profiles reveal the occurrence of inflection point before separation as in the case of laminar separation bubble and the peak values of turbulence intensities correspond to the location of point of inflection. Turbulent separation correlations proposed by various investigators were compared with the present results and are found to be in good agreement. Surface flow visualization pictures are used to get qualitative information.
The wall forcing on the separation bubble was done using a speaker which blows a small amount of air when the diaphragm moves up and sucks in when the diaphragm moves down. The blowing effect seems to be more effective in suppressing the separation compared to suction.
The interaction with wake is studied using an unsteady bar which is moving up and down. The inflection point in the mean velocity distribution seems to move closer to the wall with the impingement o the wake. Also the turbulence intensities have increased and seem to move closer to the wall. The displacement and momentum thickness have increased and the shape factor has decreased which indicates suppression of the bubble. The quasi shear stress in the separated region also increased which indicates suppression of separation.
While the oncoming unsteady wake might be a parcel of fluid with defect velocity when seen in isolation, in comparison to the velocity defect in the separation bubble, it is a region of velocity excess. As a result, one can expect the impingement of the unsteady wake on the TSB to transport momentum thereby contributing to separation reduction. But the mechanism of separation is different from laminar separation bubble affected by wakes. The suppression in the case of turbulent separation bubble is partly due to the entrainment of turbulence and partly due to the kinematic impact of the wake on the bubble.
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[en] LARGE EDDY SIMULATIONS OF THE THIN PLATE SEPARATION BUBBLE AT SHALLOW INCIDENCE / [pt] SIMULAÇÃO DE GRANDES ESCALAS DA BOLHA DE SEPARAÇÃO EM PLACAS FINAS A PEQUENO ÂNGULO DE INCIDÊNCIALUIZ EDUARDO BITTENCOURT SAMPAIO 26 January 2007 (has links)
[pt] Escoamentos aerodinâmicos externos sobre membranas e
aerofólios finos representam
um enorme desafio para simulações numéricas, tendo em
vista os
diversos e complexos regimes de escoamento presentes, que
incluem separa
ção da camada limite, transição da camada de mistura para
regime turbulento,
recolamento, relaminarização da camada limite, e formação
de bolhas
de recirculação primárias e secundárias. Uma maior
compreensão sobre estas
estruturas é obtida através da simulação numérica de
grandes escalas
(LES) do escoamento sobre placas planas e finas, com
ângulos de incidência
entre um e três graus e número de Reynolds superior a 105.
A necessidade
do emprego de malhas não uniformes, geralmente imposta por
escoamentos
externos, provoca instabilidades numéricas em esquemas não
dissipativos,
sendo duas possíveis soluções apresentadas nesse trabalho.
A primeira delas
é baseada num modelo sub-malha tradicional, onde a
estabilidade numérica
é alcançada através de um esquema numérico misto, no qual
o esquema
de diferenças centrais é empregado em regiões com intensas
atividades turbulentas,
enquanto que um esquema dissipativo é empregado nas regiões
onde a malha sofre grandes variações espaciais e a
atividade turbulenta
é desprezível. Uma segunda solução baseia-se num termo de
forçamento
idealizado para atenuar apenas as menores escalas. Quando
comparadas a
estudos prévios utilizando médias de Reynolds (RANS),
ambas as alternativas
se mostraram adequadas, disponibilizando resultados bem
mais precisos
para perfis de velocidade, flutuações turbulentas e
pressões médias. Em particular,
o comprimento da bolha de recirculação foi previsto com
menos de
5% de discrepância em relação a dados experimentais,
contrastando com
valores maiores que 20%, obtidos com o modelo RANS K - W / [en] Aerodynamic flows around thin airfoils and membranes are
very challenging
to simulate accurately because of complex flow structures,
including
geometry-induced separation of the boundary layer, shear
layer transition to
turbulent behavior, reattachment, relaminarization of the
boundary layer,
and formation of primary and secondary recirculation
bubbles. A physical
insight on these structures can be obtained through the
numerical Large
Eddy Simulation (LES) of the flow around a simpler
geometry, the thin flat
plate, at shallow incidences of one and three degrees and
Reynolds number
above 105, which is the focus of this investigation. In
order to avoid the numerical
instabilities associated with the mesh spreading generally
required
by such external flow, two solutions have been developed
and tested. The
first one consists of the traditional sub-grid model used
along with a mixed
numerical scheme, in which a stable but dissipative part
is active only
in turbulence-free zones where mesh is highly non-regular,
while an unstable
but non-dissipative scheme is employed in turbulence-
crytical zones,
where the mesh is as regular as possible. The second
solution, developed
and validated in the current investigation, is based on a
damping force, aimed
to eliminate the smaller scales while preserving as much
as possible
all other structures. Compared to previous investigations
using Reynolds
Average (RANS) equations, both solutions provided more
accurate and detailed
information about the flow, including velocity, pressure
and turbulent
fluctuations mean profiles, allowing a deeper physical
understanding. In particular,
the main bubble reattachement lenght was predicted within
5% of
the experimental data, while K - W RANS results were found
to disagree in
more than 20%.
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Characterizing the Separation and Reattachment of Suction Surface Boundary Layer in Low Pressure Turbine Using Massively Parallel Large Eddy SimulationsJagannathan, Shriram 2010 December 1900 (has links)
The separation and reattachment of the suction surface boundary layer in a low pressure turbine is characterized using large-eddy simulation at Re=68,000 based on freestream velocity and suction surface length. A high pass filtered Smagorinsky model is used for modeling the sub-grid scales. The onset of time mean separation is at s=so = 0:61 and reattachment at s=so = 0:81, extending over 20% of the suction surface. The boundary layer is convectively unstable with a maximum reverse flow velocity of about 13% of freestream. The breakdown to turbulence occurs over a very short distance of suction surface which is followed by reattachment. Detailed investigations into the structure and kinematics of the bubble and turbulence statistics are presented. The vortex shed from the bubble, convects downstream and interacts with the trailing edge vortices increasing the turbulence intensity. On the suction side, dominant hairpin structures near the transitional and turbulent flow regime are observed. These hairpin vortices are carried by the freestream even downstream of the trailing edge of the blade with a possibility of reaching the next stage. Longitudinal streaks that evolve from the breakdown of hairpin vortices formed near the leading edge are observed on the pressure surface.
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Experimental Investigation Of Boundary Layer Separation Control Using Steady Vortex Generator Jets On Low Pressure TurbinesDogan, Eda 01 June 2012 (has links) (PDF)
This thesis presents the results of an experimental study that investigates the effects of steady vortex generator jets (VGJs) integrated to a low pressure turbine blade to control the laminar separation bubble occurring on the suction surface of the blade at low Reynolds numbers. The injection technique involves jets issued from the holes located near the suction peak of the test blade which is in the middle of a five-blade low speed linear cascade facility. Three injection cases are tested with different blowing ratio values ranging from low to high. Surface pressure and particle image velocimetry (PIV) measurements are performed. The results show that steady VGJ is effective in eliminating the laminar separation bubble. Also it is observed that to have fully developed attached boundary layer, blowing ratio should be chosen accordingly since a very thin separation zone still exists at low blowing ratios.
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