4 results
A study of the Blasius wall jet
- ORI LEVIN, VALERY G. CHERNORAY, LENNART LÖFDAHL, DAN S. HENNINGSON
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- Journal:
- Journal of Fluid Mechanics / Volume 539 / 25 September 2005
- Published online by Cambridge University Press:
- 05 September 2005, pp. 313-347
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A plane wall-jet flow is numerically investigated and compared to experiments. The measured base flow is matched to a boundary-layer solution developing from a coupled Blasius boundary layer and Blasius shear layer. Linear stability analysis is performed, revealing high instability of two-dimensional eigenmodes and non-modal streaks. The nonlinear stage of laminar-flow breakdown is studied with three-dimensional direct numerical simulations and experimentally visualized. In the direct numerical simulation, an investigation of the nonlinear interaction between two-dimensional waves and streaks is made. The role of subharmonic waves and pairing of vortex rollers is also investigated. It is demonstrated that the streaks play an important role in the breakdown process, where their growth is transformed from algebraic to exponential as they become part of the secondary instability of the two-dimensional waves. In the presence of streaks, pairing is suppressed and breakdown to turbulence is enhanced.
Experiments on secondary instability of streamwise vortices in a swept-wing boundary layer
- VALERY G. CHERNORAY, ALEXANDER V. DOVGAL, VICTOR V. KOZLOV, LENNART LÖFDAHL
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- Journal:
- Journal of Fluid Mechanics / Volume 534 / 10 July 2005
- Published online by Cambridge University Press:
- 21 June 2005, pp. 295-325
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A detailed experimental study on the formation of crossflow vortex mode packets and their high-frequency secondary instability in a swept-wing boundary layer was carried out. Stationary vortex packets are most likely to be generated under natural flight conditions and transition to turbulence is quickest within these disturbances. In the present experiments, different methods of controlled excitation are used so that the crossflow vortex packets are generated by surface-roughness elements and by localized continuous suction. It is found that as the stationary disturbance reaches a significant amplitude, of about 10% of the free-stream velocity, while being below the saturation level, high-frequency secondary instabilities start to grow. Influence of the crossflow vortex packet magnitude on the development of the secondary instability is investigated in detail and below its threshold the crossflow vortex packet was found to be nearly neutrally stable. By studying the unstable packets, the frequency of natural secondary perturbations was identified and the travelling disturbances were forced in a controlled manner by periodic blowing–suction applied locally under the stationary vortex. Two modes of secondary instability were found to develop and the preferred mode was dependent on the properties of the primary stationary disturbance. Additionally, the underlying physics of the process of nonlinear formation and development of the vortices in the boundary layer is clarified. It was observed that the large-amplitude co-rotating vortices may interact, thus reducing their amplitude. Also a large-scale excitation by an isolated roughness element produced two individual stationary crossflow vortex packets at its tips, each with different preferred secondary instability modes.
A similarity theory for the turbulent plane wall jet without external stream
- WILLIAM K. GEORGE, HANS ABRAHAMSSON, JAN ERIKSSON, ROLF I. KARLSSON, LENNART LÖFDAHL, MARTIN WOSNIK
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- Journal:
- Journal of Fluid Mechanics / Volume 425 / 25 December 2000
- Published online by Cambridge University Press:
- 01 December 2000, pp. 367-411
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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∗y1/2/v, where y1/2 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 Um, the velocity maximum, and the length scale y1/2; 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 u2*, while the normal stresses scale with U2m. Also Um ∼ yn1/2 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, B1 = (Umv/Mo)/ (y+1/2Mo/v2)n = constant where n ≈ −0.528, appears to be required to determine the entire flow for a given source.
Shear-free turbulence near a wall
- DAG ARONSON, ARNE V. JOHANSSON, LENNART LÖFDAHL
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- Journal:
- Journal of Fluid Mechanics / Volume 338 / 10 May 1997
- Published online by Cambridge University Press:
- 10 May 1997, pp. 363-385
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The mean shear has a major influence on near-wall turbulence but there are also other important physical processes at work in the turbulence/wall interaction. In order to isolate these, a shear-free boundary layer was studied experimentally. The desired flow conditions were realized by generating decaying grid turbulence with a uniform mean velocity and passing it over a wall moving with the stream speed. It is shown that the initial response of the turbulence field can be well described by the theory of Hunt & Graham (1978). Later, where this theory ceases to give an accurate description, terms of the Reynolds stress transport (RST) equations were measured or estimated by balancing the equations. An important finding is that two different length scales are associated with the near-wall damping of the Reynolds stresses. The wall-normal velocity component is damped over a region extending roughly one macroscale out from the wall. The pressure–strain redistribution that normally would result from the Reynolds stress anisotropy in this region was found to be completely inhibited by the near-wall influence. In a thin region close to the wall the pressure–reflection effects were found to give a pressure–strain that has an effect opposite to the normally expected isotropization. This behaviour is not captured by current models.