Mechanisms of surface pressure distribution within a laminar separation bubble at different Reynolds numbers
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Mechanisms behind the pressure distribution and skin friction within a laminar separation bubble (LSB) are investigated by large-eddy simulations around a 5% thickness blunt flat plate at the chord length based Reynolds number 5.0 × 103, 6.1 × 103, 1.1 × 104, and 2.0 × 104. The characteristics inside the LSB change with the Reynolds number; a steady laminar separation bubble (LSB_S) at the Reynolds number 5.0 × 103 and 6.1 × 103, and a steady-fluctuating laminar separation bubble (LSB_SF) at the Reynolds number 1.1 × 104, and 2.0 × 104. Different characteristics of pressure and skin friction distributions are observed by increasing the Reynolds number, such that a gradual monotonous pressure recovery in the LSB_S and a plateau pressure distribution followed by a rapid pressure recovery region in the LSB_SF. The reasons behind the different characteristics of pressure distributions at different Reynolds numbers are discussed by deriving the Reynolds averaged pressure gradient equation. It is confirmed that the viscous stress distributions near the surface play an important role in determining the formation of different pressure distributions. Depending on the Reynolds numbers, the viscous stress distributions near the surface are affected by the development of a separated laminar shear layer or the Reynolds shear stress. In addition, we show that the same analyses can be applied to the flows around a NACA0012 airfoil.Keywords:
Pressure gradient
Laminar sublayer
Pressure gradient
Adverse pressure gradient
Separation (statistics)
Temperature Gradient
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Flow separation of turbulent boundary layer flow on a smooth surface due to an adverse pressure gradient is still an open issue. One major reason for this is that for turbulent boundary layer flows at a strong adverse pressure gradient, the description of the mean flow using wall-laws, the characterization of the Reynolds stresses and the mechanisms
of momentum exchange in terms of coherent structures (hairpin-hypothesis, sweepstreak-interaction, super-structures) are still unresolved problems, see.
Adverse pressure gradient
Pressure gradient
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Two test cases for the assessment of ZDES mode 3 (WMLES approach) in both pressure gradient conditions and mild boundary layer separation are presented. For both of them, the confinement effects of side wall boundary layers from the reference experiments are non negligible and they are taken into account in two dimensional simulations by means of a top wall geometry modification. A better agreement in the pressure coefficient with experimental data results from this manipulation. Results from the first test case evidence the advantage of resolving turbulence. A more physical flow is predicted in such a case and more in depth analysis of turbulence is possible, for instance spectral analysis as presented in this work. RANS results for the boundary layer separation case are presented showing the improvement on the pressure coefficient prediction thanks to the top wall geometry modification. At the time of the redaction of this manuscript, ZDES results for the boundary layer separation case are not available yet. However, the initial run of the ZDES simulation is very encouraging and suggests that a significant improvement over RANS predictions might be achieved.
Pressure gradient
Adverse pressure gradient
Boundary layer suction
Separation (statistics)
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Adverse pressure gradient
Pressure gradient
External flow
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The reliable prediction of low-speed flow separation of a turbulent boundary layer on a smooth surface due to an adverse pressure gradient is still an open issue. In the literature, there is no agreement on the law-of-the-wall for adverse pressure gradients. The objective of a recent and ongoing research investigation started in the DLR internal project RETTINA is to build up a worldwide unique set of experimental data for a turbulent boundary layer flow at adverse pressure gradient at high Reynolds numbers by a joint PIV campaign of DLR and UniBw Munchen, and to investigate existing and to develop improved proposals for the law-of-the-wall in case of an adverse pressure gradient.
Adverse pressure gradient
Pressure gradient
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The results of experimental investigation of laminar-turbulent transition in three-dimensional flow under the high continuous pressure gradient including the flow with local boundary layer separation are presented. The experimental studies were performed within the Mach number range from 4 to 6 and Reynolds number 10-60×106 1/m, the angles of attack were 0° and 5°. The experiments were carried out on the three-dimensional convergent inlet model with and without sidewalls. The influence of artificial tubulator of boundary layer on transition and flow structure was studied. The conducted researches have shown that adverse pressure gradient increase hastens transition and leads to decrease of transition area length. If pressure gradient rises velocity profile fullness increases and profile transformation from laminar to turbulent occurs. As a result of it the decrease of separation area length occurs. The same effect was reached with Reynolds number increase. These results are compared with the data on two-dimensional model with longitudinal curvature.
Adverse pressure gradient
Pressure gradient
Laminar-turbulent transition
Transition point
Laminar sublayer
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An experimental study of the laminar-to-turbulent transition and resulting hydrodynamic forces on a body of revolution with a long, favorable pressure gradient forebody (i.e., where pressure is dropping and the flow accelerating) is reported. Over a substantial range of body velocity and angle of attack the favorable pressure gradient is shown to postpone transition to the point of laminar separation, and this extended laminar region results in a much lower hydrodynamic drag than is characteristic of an all-turbulent body. The intermittency of the boundary layer and the propagation characteristics of turbulent spots in the extended favorable pressure gradient region are quantified by hot film probes mounted flush with the body surface. The sensitivity of the boundary layer transition to three-dimensional surface roughness elements located in tandem (along a streamline) is also quantified. A number of such elements in tandem causes transition at a lower Reynolds number than would a single element of the same size, this effect becoming more pronounced with increasing number of roughness elements and decreasing space between them.
Pressure gradient
Adverse pressure gradient
Transition point
Intermittency
Laminar sublayer
Laminar-turbulent transition
Surface pressure
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Results of an experimental investigation of the laminar flow of air over a downstream-facing step are presented. The experiments include visual observations of smoke filaments (in the viscous layer), qualitative velocity fluctuation measurements, and mean velocity profiles. Results are reported over a range of 0.36 – 1.02 cm in step height, 0.61 – 2.44 m/sec in free stream velocity at the step, and 0.16 – 0.51 cm in boundary layer displacement thickness at the step. Laminar flow to reattachment of a free shear layer is observed for subsonic flow and two criteria for which transition to turbulence at reattachment exists are presented. The laminar reattachment length is not a constant number of step heights as for turbulent flow, but varies with Reynolds number and boundary layer thickness at the step. The shape of the velocity profile at reattachment is found to be similar to the shape of a laminar boundary layer profile at separation and the boundary layer profiles downstream of reattachment are similar to those in a laminar boundary layer developing toward separation except that they are traversed in the reverse sense.
Laminar sublayer
Laminar flow reactor
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Two attempts were made to develop a three-dimensional laminar boundary layer in the flow over a flat plate in a curved duct, establishing a negligible streamwise pressure gradient and, at the same time, an appreciable crosswise pressure gradient. A first series of measurements was undertaken keeping the free-stream velocity at about 30 ft/s; the boundary layer was expected to be laminar, but appears to have been transitional. As was to be expected, the cross-flow in the boundary layer decreased gradually as the flow became progressively more turbulent. In a second experiment, at a lower free-stream velocity of approximately 10 ft/s, the boundary layer was laminar. Its streamwise profile resembled closely the Blasius form, but the cross-flow near the edge of the boundary layer appears to have exceeded that predicted theoretically. However, there was a substantial experimental scatter in the measurements of the yaw angle, which in laminar boundary layers is difficult to obtain accurately.
Laminar sublayer
Pressure gradient
Adverse pressure gradient
External flow
Boundary layer suction
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Laminar sublayer
Adverse pressure gradient
Incompressible Flow
External flow
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