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Paul A. Durbin - One of the best experts on this subject based on the ideXlab platform.
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Direct numerical simulations of transition in a compressor cascade: the influence of free-stream turbulence
Journal of Fluid Mechanics, 2010Co-Authors: Tamer A. Zaki, Jan G. Wissink, Wolfgang Rodi, Paul A. DurbinAbstract:The flow through a compressor passage without and with incoming free-stream grid turbulence is simulated. At moderate Reynolds number, Laminar-to-turbulence transition can take place on both sides of the aerofoil, but proceeds in distinctly different manners. The direct numerical simulations (DNS) of this flow reveal the mechanics of breakdown to turbulence on both surfaces of the blade. The pressure surface boundary layer undergoes Laminar Separation in the absence of free-stream disturbances. When exposed to free-stream forcing, the boundary layer remains attached due to transition to turbulence upstream of the Laminar Separation Point. Three types of breakdowns are observed; they combine characteristics of natural and bypass transition. In particular, instability waves, which trace back to discrete modes of the base flow, can be observed, but their development is not independent of the Klebanoff distortions that are caused by free-stream turbulent forcing. At a higher turbulence intensity, the transition mechanism shifts to a purely bypass scenario. Unlike the pressure side, the suction surface boundary layer separates independent of the free-stream condition, be it Laminar or a moderate free-stream turbulence of intensity T u ~ 3%. Upstream of the Separation, the amplification of the Klebanoff distortions is suppressed in the favourable pressure gradient (FPG) region. This suppression is in agreement with simulations of constant pressure gradient boundary layers. FPG is normally stabilizing with respect to bypass transition to turbulence, but is, thereby, unfavourable with respect to Separation. Downstream of the FPG section, a strong adverse pressure gradient (APG) on the suction surface of the blade causes the Laminar boundary layer to separate. The Separation surface is modulated in the instantaneous fields of the Klebanoff distortion inside the shear layer, which consists of forward and backward jet-like perturbations. Separation is followed by breakdown to turbulence and reattachment. As the free-stream turbulence intensity is increased, T u ~ 6.5 %, transitional turbulent patches are initiated, and interact with the downstream separated flow, causing local attachment. The calming effect, or delayed re-establishment of the boundary layer Separation, is observed in the wake of the turbulent events.
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DNS of Transition to Turbulence in a Linear Compressor Cascade
High Performance Computing in Science and Engineering Garching Munich 2007, 1Co-Authors: Tamer A. Zaki, Paul A. Durbin, Jan G. Wissink, Wolfgang RodiAbstract:A series of direct numerical simulations were carried out of the flow through a compressor passage. The behavior of the flow in the presence and absence of free-stream turbulent fluctuations is contrasted. In the former case, both the pressure and suction surface undergo Separation due to the adverse pressure gradient in the passage. In the presence of free-stream turbulent fluctuation, the pressure surface boundary layer transitions to turbulence upstream of the Laminar Separation Point and, hence, remains attached. The suction surface, however, undergoes Separation independent of the free-stream perturbation. The frequency of shedding, however, and the mechanics of the Separation region, are dependent on the turbulence level in the free-stream. Further simulations at higher turbulence intensities are planned.
Sergio Montelpare - One of the best experts on this subject based on the ideXlab platform.
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a quantitative ir thermographic method to study the Laminar Separation bubble phenomenon
International Journal of Thermal Sciences, 2005Co-Authors: R Ricci, Sergio MontelpareAbstract:Abstract The boundary layer Separation phenomena are present in many application fields: for example the sailplanes, the micro-vehicles, the small wind turbines, the airplanes and the cars. On aerodynamic bodies operating at low Reynolds numbers, lesser than one million, it may happen a boundary layer local Separation defined as: Laminar Separation bubble. This phenomenon induces a body drag increase and an eventual lift decrease; in some situations the cyclical bubble formation and detachment may induce pressure pulses and consequent vibration phenomena. In a previous research work [Internat. J. Thermal Sci. 43 (2004) 315] was verified the possibility to show qualitatively the presence of a Laminar bubble by means of a thermographic observation of the body surface. In this work is verified the possibility to carry out a quantitative study of the Laminar bubble phenomenon by using the same IR technique. Three characteristic Points of the bubble are particularly studied: the Laminar Separation Point, the transition Point and the turbulent reattachment Point. The Laminar bubble behaviour is analysed on a RR3823HL airfoil by varying the angle of attack and the Reynolds number; the adimensional Stanton number, based on the airfoil chord, is obtained in order to individuate the requested Points in a more simple and objective way. This adimensional number is carried out by means of a finite numerical difference approach that makes a balance among the heat fluxes on the airfoil surface.
Beverley Mckeon - One of the best experts on this subject based on the ideXlab platform.
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The effect of small-amplitude time-dependent changes to the surface morphology of a sphere
Journal of Fluid Mechanics, 2011Co-Authors: Adam Norman, Eric C. Kerrigan, Beverley MckeonAbstract:Typical approaches to manipulation of flow Separation employ passive means or active techniques such as blowing and suction or plasma acceleration. Here it is demonstrated that the flow can be significantly altered by making small changes to the shape of the surface. A proof of concept experiment is performed using a very simple time-dependent perturbation to the surface of a sphere: a roughness element of 1% of the sphere diameter is moved azimuthally around a sphere surface upstream of the uncontrolled Laminar Separation Point, with a rotational frequency as large as the vortex shedding frequency. A key finding is that the non-dimensional time to observe a large effect on the lateral force due to the perturbation produced in the sphere boundary layers as the roughness moves along the surface is ˆt =tU_(∞)/D ≈4. This slow development allows the moving element to produce a tripped boundary layer over an extended region. It is shown that a lateral force can be produced that is as large as the drag. In addition, simultaneous particle image velocimetry and force measurements reveal that a pair of counter-rotating helical vortices are produced in the wake, which have a significant effect on the forces and greatly increase the Reynolds stresses in the wake. The relatively large perturbation to the flow-field produced by the small surface disturbance permits the construction of a phase-averaged, three-dimensional (two-velocity component) wake structure from measurements in the streamwise/radial plane. The vortical structure arising due to the roughness element has implications for flow over a sphere with a nominally smooth surface or distributed roughness. In addition, it is shown that oscillating the roughness element, or shaping its trajectory, can produce a mean lateral force.
Tamer A. Zaki - One of the best experts on this subject based on the ideXlab platform.
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Direct numerical simulations of transition in a compressor cascade: the influence of free-stream turbulence
Journal of Fluid Mechanics, 2010Co-Authors: Tamer A. Zaki, Jan G. Wissink, Wolfgang Rodi, Paul A. DurbinAbstract:The flow through a compressor passage without and with incoming free-stream grid turbulence is simulated. At moderate Reynolds number, Laminar-to-turbulence transition can take place on both sides of the aerofoil, but proceeds in distinctly different manners. The direct numerical simulations (DNS) of this flow reveal the mechanics of breakdown to turbulence on both surfaces of the blade. The pressure surface boundary layer undergoes Laminar Separation in the absence of free-stream disturbances. When exposed to free-stream forcing, the boundary layer remains attached due to transition to turbulence upstream of the Laminar Separation Point. Three types of breakdowns are observed; they combine characteristics of natural and bypass transition. In particular, instability waves, which trace back to discrete modes of the base flow, can be observed, but their development is not independent of the Klebanoff distortions that are caused by free-stream turbulent forcing. At a higher turbulence intensity, the transition mechanism shifts to a purely bypass scenario. Unlike the pressure side, the suction surface boundary layer separates independent of the free-stream condition, be it Laminar or a moderate free-stream turbulence of intensity T u ~ 3%. Upstream of the Separation, the amplification of the Klebanoff distortions is suppressed in the favourable pressure gradient (FPG) region. This suppression is in agreement with simulations of constant pressure gradient boundary layers. FPG is normally stabilizing with respect to bypass transition to turbulence, but is, thereby, unfavourable with respect to Separation. Downstream of the FPG section, a strong adverse pressure gradient (APG) on the suction surface of the blade causes the Laminar boundary layer to separate. The Separation surface is modulated in the instantaneous fields of the Klebanoff distortion inside the shear layer, which consists of forward and backward jet-like perturbations. Separation is followed by breakdown to turbulence and reattachment. As the free-stream turbulence intensity is increased, T u ~ 6.5 %, transitional turbulent patches are initiated, and interact with the downstream separated flow, causing local attachment. The calming effect, or delayed re-establishment of the boundary layer Separation, is observed in the wake of the turbulent events.
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DNS of Transition to Turbulence in a Linear Compressor Cascade
High Performance Computing in Science and Engineering Garching Munich 2007, 1Co-Authors: Tamer A. Zaki, Paul A. Durbin, Jan G. Wissink, Wolfgang RodiAbstract:A series of direct numerical simulations were carried out of the flow through a compressor passage. The behavior of the flow in the presence and absence of free-stream turbulent fluctuations is contrasted. In the former case, both the pressure and suction surface undergo Separation due to the adverse pressure gradient in the passage. In the presence of free-stream turbulent fluctuation, the pressure surface boundary layer transitions to turbulence upstream of the Laminar Separation Point and, hence, remains attached. The suction surface, however, undergoes Separation independent of the free-stream perturbation. The frequency of shedding, however, and the mechanics of the Separation region, are dependent on the turbulence level in the free-stream. Further simulations at higher turbulence intensities are planned.
Yousef M. F. El Hasadi - One of the best experts on this subject based on the ideXlab platform.
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On the Existence of Logarithmic Terms in the Drag Coefficient and Nusselt Number of a Single Sphere at High Reynolds Numbers
arXiv: Fluid Dynamics, 2020Co-Authors: Yousef M. F. El Hasadi, Jt Johan PaddingAbstract:In the beginning of the second half of the twentieth century, Proudman and Pearson (JFM,2(3), 1956, pp.237-262) suggested that the functional form of the drag coefficient of a single sphere subjected to uniform fluid flow consists of a series of logarithmic and power terms of the Reynolds number. In this paper, we will explore the validity of the above statement for Reynolds numbers up to $ 2\times 10^{5}$, by using a symbolic regression machine learning method.The algorithm is trained by using available experimental data, as well as data from a well-known correlation from the literature.The symbolic regression method finds the following expression for the drag coefficient $C_D = a+\frac{24}{Re}+f(\log(Re))$, where $Re$ is the Reynolds number, and the constituents of $f(\log(Re))$ are integer powers of $\log(Re)$. Interestingly, the value of $a$ resembles the value of $C_D$, at the Point where Laminar Separation Point occurs. We did the same analysis for the problem of heat transfer under forced convection around a sphere, and found that the logarithmic terms of $Re$ and Peclect number $Pe$ play an essential role in the variation of the Nusselt number $Nu$. The machine learning algorithm independently found the asymptomatic solution of Acrivos and Goddard (JFM, 23(2),pp.273-291).
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On the Existence of Logarithmic Terms in the Drag Coefficient and Nusselt Number of a Single Sphere at High Reynolds Numbers
2020Co-Authors: Yousef M. F. El Hasadi, Padding JohanAbstract:In the beginning of the second half of the twentieth century, Proudman and Pearson (JFM,2(3), 1956, pp.237-262) suggested that the functional form of the drag coefficient of a single sphere subjected to uniform fluid flow consists of a series of logarithmic and power terms of the Reynolds number. In this paper, we will explore the validity of the above statement for Reynolds numbers up to $ 2\times 10^{5}$, by using a symbolic regression machine learning method.The algorithm is trained by using available experimental data, as well as data from a well-known correlation from the literature.The symbolic regression method finds the following expression for the drag coefficient $C_D = a+\frac{24}{Re}+f(\log(Re))$, where $Re$ is the Reynolds number, and the constituents of $f(\log(Re))$ are integer powers of $\log(Re)$. Interestingly, the value of $a$ resembles the value of $C_D$, at the Point where Laminar Separation Point occurs. We did the same analysis for the problem of heat transfer under forced convection around a sphere, and found that the logarithmic terms of $Re$ and Peclect number $Pe$ play an essential role in the variation of the Nusselt number $Nu$. The machine learning algorithm independently found the asymptomatic solution of Acrivos and Goddard (JFM, 23(2),pp.273-291).Comment: 31 pages, 3 Figure
