The Experts below are selected from a list of 14109 Experts worldwide ranked by ideXlab platform
Israel J Wygnanski - One of the best experts on this subject based on the ideXlab platform.
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The Control of Separation from Curved Surfaces and Blunt Trailing Edges
2002Co-Authors: Israel J WygnanskiAbstract:Abstract : The project that was initiated in the fall of 1999 encompasses three exploratory studies: (1) A pulsed wall jet over a curved surface in the absence of an External Stream (2) Pulsation emanating from a circular cylinder in Streaming flow. (3) The application of active flow control to an airfoil having a divergent trailing edge. The report will cover all three experiments although most progress (beyond the exploratory stage) was made on understanding the wall jet flowing over a convex surface. Some of the experimental work on this project was initiated prior to the last funding period but data analysis took more time some of the results have been discussed in interim progress reports. The other investigations are still ongoing at a lower pace.
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Film Cooling by a Pulsating Wall Jet.
1997Co-Authors: Israel J Wygnanski, Alfonso Ortega, Hermann FaselAbstract:Abstract : Turbulent wall jets have many important engineering applications. Much effort has been spent to investigate the plane turbulent wall jet without External Stream (Launder and Rodi 1981,1983, Katz et al 1992, Wygnanski et al 1992) and with a relatively slow External Stream (Zhou and Wygnanski 1993, Zhou et al 1996). However, many engineering applications seem to be described better by a wall jet embedded in a uniform Stream of comparable velocity (the weak wall jet), for example, the cooling turbine blades and the flows over a wing equipped with a slotted flap (Fig. 1) represents such flows. The recently developed technique for separation control by periodic blowing/suction on the flap also belongs to category (Fig.2). Thus, it is important to provide a better understanding of the development of these flows. For example: the possibility of flow similarity, normalization of the mean velocity fields, scaling laws for the governing parameters, as well as the various responses to External excitations. This report represents but a single facet of the general effort endeavoring to use the wall jet for boundary layer control, film cooling and the exertion of force on a body through the use of what is commonly known as the Coanda Effect.
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Film Cooling by a Pulsating Wall Jet.
1997Co-Authors: Israel J Wygnanski, Alfonso Ortega, Hermann FaselAbstract:Abstract : Turbulent wall jets have many important engineering applications. Much effort has been spent to investigate the plane turbulent wall jet without External Stream (Launder and Rodi 1981,1983, Katz et al 1992, Wygnanski et al 1992) and with a relatively slow External Stream (Zhou and Wygnanski 1993, Zhou et al 1996). However, many engineering applications seem to be described better by a wall jet embedded in a uniform Stream of comparable velocity (the weak wall jet), for example, the cooling turbine blades and the flows over a wing equipped with a slotted flap (Fig. 1) represents such flows. The recently developed technique for separation control by periodic blowing/suction on the flap also belongs to category (Fig.2). Thus, it is important to provide a better understanding of the development of these flows. For example: the possibility of flow similarity, normalization of the mean velocity fields, scaling laws for the governing parameters, as well as the various responses to External excitations. This report represents but a single facet of the general effort endeavoring to use the wall jet for boundary layer control, film cooling and the exertion of force on a body through the use of what is commonly known as the Coanda Effect.
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The Turbulent Wall Jet
1993Co-Authors: Israel J Wygnanski, M D ZhouAbstract:Abstract : The flow of a wall jet embedded in an External Stream or in a quiescent surrounding fluid was investigated experimentally. It was determined that the flow scales with the excess momentum injected into the Stream and the viscosity of the fluid rather than the jet efflux velocity and the dimension of the nozzle. In the presence of an External Stream, a velocity ratio parameter R = (U i - U.)/(Uj + U ) had to be added in order to obtain a universal scaling of this flow. Low-amplitude External excitation, of the wall jet resulted in a reduction of skin friction and therefore a reduction of drag. The only possible cause for this behavior observed is the enhancement of the two-dimensionality of the large eddies as expressed by spanwise coherence and correlation measurements.... Turbulence, Control of Shear Flows, Wall Jet, Boundary Layer Control
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Parameters governing the turbulent wall jet in an External Stream
AIAA Journal, 1993Co-Authors: Ming De Zhou, Israel J WygnanskiAbstract:Mean velocity distributions in a plane, turbulent, and fully developed wall jet embedded in a uniform Stream were measured for a variety of initial velocity ratios and Reynolds numbers. It was determined that the bulk of the flow is self-similar, provided the maximum velocity in the jet is twice as large as the freeStream velocity. The normalized velocity profile depends on two velocity scales and on two length scales that, in turn, depend on the momentum flux at the nozzle, the viscosity, and the initial velocity ratio between the jet and the freeStream defined by R≡(U j -U ∞)/(U j +U ∞). The width of the nozzle that was commonly used to reduce these data has no part in the similarity considerations
G N. Abramovich - One of the best experts on this subject based on the ideXlab platform.
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The Theory of Turbulent Jets
2003Co-Authors: G N. AbramovichAbstract:This chapter contains sections titled: Fundamental Concepts, Submerged Jet, Velocity Profiles in a Submerged Jet, Spread of a Turbulent Submerged Jet, Lines of Constant Velocity in a Submerged Jet, Velocity Variation Along the Axis of a Submerged Jet, Heat Transfer in a Submerged Jet, Diffusion of Constituents in a Submerged Jet, Velocity, Temperature, and Concentration Profiles in a Turbulent Jet Spreading into an External Stream of Fluid, Spread of a Turbulent Jet into a Coflowing or Counterflowing External Stream, Turbulence Characteristics in a Free Jet
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Jet of an Incompressible Fluid in a Coflowing External Stream
2003Co-Authors: G N. Abramovich, Leon SchindelAbstract:This chapter contains sections titled: Initial Region of a Two-Dimensional Jet, Initial Region of an Axially Symmetric Jet, General Relations Characterizing the Main Region of a Jet with an External Flow, Main Region of a Two-Dimensional Jet, Main Region of an Axially Symmetric Jet, Transition Region of a Jet, Turbulent Wake Behind a Body, The Influence of an Initial Nonuniformity of the Stream on the Initial Region of the Jet, The Influence of an Initial Nonuniformity of Flow on the Main Part of a Jet, Comparison of the Theory of a Jet with Experimental Data, Approximate Theory of a Jet in an External Stream
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Turbulent Gas Streams
2003Co-Authors: G N. Abramovich, Leon SchindelAbstract:This chapter contains sections titled: General Properties of Turbulent Jets of a Compressible Gas, Turbulent Mixing Zone at the Boundary Between Parallel Nonisothermal Streams of Gas, Zone of Turbulent Mixing on the Boundary Between Parallel Streams of High-Velocity Gas, Basic Relations for Computing the Initial Part of a Jet of Gas in an External Stream, Principal Region of a Jet of Heated Gas, Principal Region of High-Velocity Gas Jet, Discharge of a Supersonic Gas Jet from a Nozzle at Off-Design Conditions
Martin Wosnik - One of the best experts on this subject based on the ideXlab platform.
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a similarity theory for the turbulent plane wall jet without External Stream
Journal of Fluid Mechanics, 2000Co-Authors: William K George, Lennart Löfdahl, Rolf Karlsson, J. Eriksson, Hans Abrahamsson, Martin WosnikAbstract:A new theory for the turbulent plane wall jet without External Stream is proposed based on a similarity analysis of the governing equations. The asymptotic invariance principle (AIP) is used to require that properly scaled profiles reduce to similarity solutions of the inner and outer equations separately in the limit of infinite Reynolds number. Application to the inner equations shows that the appropriate velocity scale is the friction velocity, u ∗, and the length scale is v / u ∗. For finite Reynolds numbers, the profiles retain a dependence on the length-scale ratio, y + 1/2 = u ∗ y 1/2 / v , where y 1/2 is the distance from the wall at which the mean velocity has dropped to 1/2 its maximum value. In the limit as y + 1/2 → ∞, the familiar law of the wall is obtained. Application of the AIP to the outer equations shows the appropriate velocity scale to be U m , the velocity maximum, and the length scale y 1/2 ; but again the profiles retain a dependence on y + 1/2 for finite values of it. The Reynolds shear stress in the outer layer scales with u 2 * , while the normal stresses scale with U 2 m . Also U m ∼ y n 1/2 where n < −1/2 and must be determined from the data. The theory cannot rule out the possibility that the outer flow may retain a dependence on the source conditions, even asymptotically. The fact that both these profiles describe the entire wall jet for finite values of y + 1/2 , but reduce to inner and outer profiles in the limit, is used to determine their functional forms in the ‘overlap’ region which both retain. The result from near asymptotics is that the velocity profiles in the overlap region must be power laws, but with parameters which depend on Reynolds number y + 1/2 and are only asymptotically constant. The theoretical friction law is also a power law depending on the velocity parameters. As a consequence, the asymptotic plane wall jet cannot grow linearly, although the difference from linear growth is small. It is hypothesized that the inner part of the wall jet and the inner part of the zero-pressure-gradient boundary layer are the same. It follows immediately that all of the wall jet and boundary layer parameters should be the same, except for two in the outer flow which can differ only by a constant scale factor. The theory is shown to be in excellent agreement with the experimental data which show that source conditions may determine uniquely the asymptotic state achieved. Surprisingly, only a single parameter, B 1 = ( U m v / M o )/ ( y + 1/2 M o / v 2 ) n = constant where n ≈ −0.528, appears to be required to determine the entire flow for a given source.
H. E. Fiedler - One of the best experts on this subject based on the ideXlab platform.
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Experimental Investigations of a Jet in Counterflow
Fluid Mechanics and Its Applications, 1998Co-Authors: S. Bernero, H. E. FiedlerAbstract:The interaction of a jet with an External Stream is of great importance in many practical applications, both in nature and in technology. Although jets in a coflow or crossflow have been widely investigated over the past years, relatively few studies are available on a jet flowing into a uniform Stream of opposite direction, since this presents additional experimental and theoretical difficulties, related to flow reversal and to pronounced instability.
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The Structure of Round Turbulent Jets in Counterflow: A Flow Visualization Study
Advances in Turbulence 3, 1991Co-Authors: O. König, H. E. FiedlerAbstract:The round free jet in a counterflowing External Stream is an interesting configuration both from the practical point of view of mixing enhancement as well as scientifically, pertaining e.g. to stability. Only little is found about this flow in the literature, presumably owing to its inherent experimental difficulties. Two different states of flow are observed: a stable case at low velocity ratios U jet /|U|≲1.4, and an unstable situation at high velocity ratios U jet /|U| > 1.4, which is dominated by random motions of high amplitude.
William K George - One of the best experts on this subject based on the ideXlab platform.
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a similarity theory for the turbulent plane wall jet without External Stream
Journal of Fluid Mechanics, 2000Co-Authors: William K George, Lennart Löfdahl, Rolf Karlsson, J. Eriksson, Hans Abrahamsson, Martin WosnikAbstract:A new theory for the turbulent plane wall jet without External Stream is proposed based on a similarity analysis of the governing equations. The asymptotic invariance principle (AIP) is used to require that properly scaled profiles reduce to similarity solutions of the inner and outer equations separately in the limit of infinite Reynolds number. Application to the inner equations shows that the appropriate velocity scale is the friction velocity, u ∗, and the length scale is v / u ∗. For finite Reynolds numbers, the profiles retain a dependence on the length-scale ratio, y + 1/2 = u ∗ y 1/2 / v , where y 1/2 is the distance from the wall at which the mean velocity has dropped to 1/2 its maximum value. In the limit as y + 1/2 → ∞, the familiar law of the wall is obtained. Application of the AIP to the outer equations shows the appropriate velocity scale to be U m , the velocity maximum, and the length scale y 1/2 ; but again the profiles retain a dependence on y + 1/2 for finite values of it. The Reynolds shear stress in the outer layer scales with u 2 * , while the normal stresses scale with U 2 m . Also U m ∼ y n 1/2 where n < −1/2 and must be determined from the data. The theory cannot rule out the possibility that the outer flow may retain a dependence on the source conditions, even asymptotically. The fact that both these profiles describe the entire wall jet for finite values of y + 1/2 , but reduce to inner and outer profiles in the limit, is used to determine their functional forms in the ‘overlap’ region which both retain. The result from near asymptotics is that the velocity profiles in the overlap region must be power laws, but with parameters which depend on Reynolds number y + 1/2 and are only asymptotically constant. The theoretical friction law is also a power law depending on the velocity parameters. As a consequence, the asymptotic plane wall jet cannot grow linearly, although the difference from linear growth is small. It is hypothesized that the inner part of the wall jet and the inner part of the zero-pressure-gradient boundary layer are the same. It follows immediately that all of the wall jet and boundary layer parameters should be the same, except for two in the outer flow which can differ only by a constant scale factor. The theory is shown to be in excellent agreement with the experimental data which show that source conditions may determine uniquely the asymptotic state achieved. Surprisingly, only a single parameter, B 1 = ( U m v / M o )/ ( y + 1/2 M o / v 2 ) n = constant where n ≈ −0.528, appears to be required to determine the entire flow for a given source.
