Hydrodynamic stability of a boundary layer with a pressure or suction flow

The possibility of detecting transition through the very small laser drilled perforations in panels representing the suction surface of a hybrid laminar flow aircraft is examined. The method uses miniature microphones to detect changes to the noise received from the boundary layer. Tests using a flat plate rig in a low-turbulence wind tunnel at Reynolds numbers up to 3.8 million per metre, demonstrate that the boundary layer state can be defined in this manner, most simply through measurement of the root mean square (r.m.s.) of the microphone signal. It is shown that the r.m.s. reaches a peak in the transition zone and that when the boundary layer is fully turbulent the value is still significantly higher than it was before transition. Porosity in the range 0.8-6.4 percent was examined, with nominal hole diameters of 0.06 and 0.10 mm in 0.9 mm thick laser drilled suction surface specimens. Suction flow through the surface was found not adversely to affect the operation of the system. The experiment was limited to low Reynolds numbers because the high background noise in the wind tunnel made detection of the boundary layer element of the signal increasingly difficult to define as speed increased. It is considered that test in flight will be needed to prove fully the validity of the method. A preliminary design of an installation for this purpose is suggested that allows the suction flow to be maintained over the measuring region.

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Swirling Flow in a Fixed Container: Generation and Attenuation of a Vortex Column

Journal of Fluids Engineering ◽

10.1115/1.4002773 ◽

2010 ◽

Vol 132 (11) ◽

Author(s):

A. Nahas ◽

A. Calvo ◽

M. Piva

Keyword(s):

Boundary Layer ◽

Swirling Flow ◽

Vortex Formation ◽

Fluid Motion ◽

Bottom Boundary ◽

Global Circulation ◽

Columnar Vortex ◽

Central Nozzle ◽

Suction Flow ◽

Vessel Bottom

The development of a columnar vortex and its attenuation using radial rods at the bottom boundary of a stationary container are experimentally studied. The fluid motion is achieved combining two independent flows: a global circulation around the cylinder axis and a meridian flow generated by recirculating fluid through a central nozzle located at the vessel bottom. The resulting velocity field is analyzed under two conditions: with and without the meridian or suction flow. It is shown that in the second condition a columnar vortex merges and that its intensity increases when the suction flow rate is increased. The key role played by the bottom boundary layer in the vortex formation is demonstrated. In the last part of the work, the attenuation of the vortex intensity produced by a set of rods located at the vessel bottom is investigated. It is found that obstacles with heights of the order of the boundary layer thickness are enough to produce the total annihilation of the vortex column.

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Aerodynamic Investigation on Performance Enhancement of NACA 2412 Airfoil with Suction Assistance

International Journal of Vehicle Structures and Systems ◽

10.4273/ijvss.12.1.16 ◽

2020 ◽

Vol 12 (1) ◽

Author(s):

S. Venkatesh ◽

S. Rakesh Vimal ◽

S. Manigandan ◽

P. Gunasekar

Keyword(s):

Boundary Layer ◽

Flow Rate ◽

Performance Enhancement ◽

Boundary Layer Separation ◽

Boundary Layer Control ◽

Wing Model ◽

Suction Control ◽

Suction Flow ◽

Suction Hole ◽

Layer Control

Advanced transport aircraft concept has active boundary-layer control by slot suction which reduces drag by stabilizing the laminar boundary. Thus, the prevention of transition and delaying the boundary layer separation will lead to a higher lift co-efficient. The influence of location and position of suction, suction flow rate and suction hole width on aerodynamic performance have greater influence. These examine the potential payoff for boundary-layer control as applied to the advanced-concept wings. An experimental work deals with the continuous normal suction from the wing upper surface effects on the aerodynamic forces. The wing model with NACA-2412 has been made to achieve normal suction from the wing upper surface by means of four slot channels. The results showed that the continuous normal suction can significantly increase the lift to drag force ratio and this ratio is increasing as the strength of suction increases. There is a convincing decrease in drag and pressure loss and an increase in max lift, which in turn improves the overall performance of the aircraft. While multi-hole suction control can reduce drag much more efficiently than single hole suction control, the position of the suction hole has a greater effect on reducing pressure losses than the suction flow rate.

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An experimental investigation of turbulent spots and breakdown to turbulence

Journal of Fluid Mechanics ◽

10.1017/s0022112060001079 ◽

1960 ◽

Vol 9 (2) ◽

pp. 235-246 ◽

Cited By ~ 80

Author(s):

J. W. Elder

Keyword(s):

Boundary Layer ◽

Experimental Investigation ◽

Reynolds Number ◽

Hydrodynamic Stability ◽

Free Stream ◽

Stream Velocity ◽

Turbulent Spot ◽

Turbulent Spots ◽

The Impact ◽

Critical Intensity

The theory of hydrodynamic stability and the impact on it of recent work with turbulent spots is discussed. Emmons's (1951) assumptions about the growth and interaction of turbulent spots are found experimentally to be substantially correct. In particular it is shown that the region of turbulent flow on a flat plate is simply the sum of the areas that would be obtained if all spots grew independently.An investigation of the conditions required for breakdown to turbulence near a wall, that is, to initiate a turbulent spot, suggests that regardless of how disturbances are generated in a laminar boundary layer and independent of both the Reynolds number and the spatial extent of the disturbances, breakdown to turbulence occurs by the initiation of a turbulent spot at all points at which the velocity fluctuation exceeds a critical intensity. Over most of the layer this intensity is about 0·2 times the free-stream velocity. The Reynolds number is important merely in respect of the growth of disturbances prior to breakdown.

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Effects of a flexible boundary on hydrodynamic stability

Journal of Fluid Mechanics ◽

10.1017/s0022112060001286 ◽

1960 ◽

Vol 9 (4) ◽

pp. 513-532 ◽

Cited By ~ 180

Author(s):

T. Brooke Benjamin

Keyword(s):

Boundary Layer ◽

Surface Waves ◽

Hydrodynamic Stability ◽

Stability Problem ◽

Resonance Effect ◽

Wall Friction ◽

Stream Velocity ◽

Rigid Boundary ◽

Flexible Material ◽

Rigid Plane

The theoretical study presented in this paper was inspired by the recent report (Krämer 1960) of experiments showing that considerable reductions in the drag of an underwater solid body were achieved by covering it with a skin of flexible material; apparently this effect was due to the boundary layer being stabilized in the presence of the skin, so that transition to a turbulent condition of flow was prevented or at least delayed. The stability problem for flow past a flexible boundary is here formulated in a general way which allows a full exploration of the possibility of a stabilizing effect without the need to assign specific properties to the flexible medium; the collective properties of possible boundaries are represented by a ‘response coefficient’ α (a sort of ‘effective compliance’) measuring the deflexion of the surface under a travelling sinusoidal distribution of pressure.A remarkably simple analytical connexion is established between the present general problem and the corresponding stability problem for the boundary layer on a rigid plane wall, and hence many details of the existing theory of hydrodynamic stability are immediately useful. However, the presence of the flexible boundary admits possible modes of instability additional to those which already exist when the boundary is rigid, and clearly every mode must be considered with regard to practical measures for stabilization—that is to say, it might be useless to inhibit one mode by a device which lets in another. What is believed to be an essentially complete interpretation of the over-all possibilities is deduced on recognizing three more or less distinct forms of instability. The first comprises waves resembling the unstable waves which can arise in the presence of a rigid boundary, but now being modified by the effects of flexibility. These waves tend to be stabilized when the boundary has a compliant response to them, which means the respective wave velocity has to be less than the velocity of free surface waves on the boundary; but it is found that the effect of internal friction in the flexible medium is actually destabilizing. The second form of instability is essentially a resonance effect and comprises waves travelling at very nearly the velocity of free surface waves. These waves can only be excited when the latter velocity falls below the free-stream velocity; they are scarcely affected by the viscosity of the fluid since the ‘wall friction layer’ is largely cancelled, so that damping due to the medium itself becomes the only stabilizing factor. The third form is akin to Kelvin–Helmholtz instability.This interpretation of the theoretical results seems to point to the essential factors in the operation of a flexible skin as a stabilizing device, and accordingly in the concluding secttion of the paper two alternative sets of criteria are proposed each of which would provide a logical basis for designing such a device. The principle of the first alternative explains the success of Gamer's invention, but the second appears equally promising and the relative advantages of the two can really be proved only by further experiment.

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Shock train control by boundary layer suction in a scramjet isolator

Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering ◽

10.1177/0954410020967537 ◽

2020 ◽

pp. 095441002096753

Author(s):

Vignesh Ram Petha Sethuraman ◽

Tae Ho Kim ◽

Heuy Dong Kim

Keyword(s):

Boundary Layer ◽

Flow Field ◽

Pressure Ratio ◽

Critical Role ◽

Pressure Rise ◽

Flow Ratio ◽

Shock Train ◽

Boundary Layer Suction ◽

Scramjet Engine ◽

Suction Flow

The isolator plays a critical role in the scramjet engine situated between the inlet and the combustion chamber. The flow field is more complex with shock–shock interaction and shock boundary layer interaction result in a series of compression waves reffered to as “shock train”. The presence of such flow inside the isolator can degrade the performance of the scramjet engine. The present study focus on the characteristic of the shock train flow field in an isolator and its control by partial removal of the boundary layer. The results examine the variation of the inlet to outlet pressure ratio along with different suction flow ratio. Numerical results indicate that boundary layer suction will cause the slight downstream movement of shock train location and the length of the shock train is reduced. Also when the suction flow gets choked, the transformation of shock train into a single curved normal shock is observed. The effect of varying the upstream boundary layer plays a major role in the suction flow ratio. Furthermore, a significant improvement in the total pressure loss and static pressure rise is obtained by boundary layer suction. The location of the shock train has a greater impact on the performance of the isolator.

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Ventilated oscillatory boundary layers

Journal of Fluid Mechanics ◽

10.1017/s002211209400193x ◽

1994 ◽

Vol 273 ◽

pp. 261-284 ◽

Cited By ~ 57

Author(s):

Daniel C. Conley ◽

Douglas L. Inman

Keyword(s):

Boundary Layer ◽

Boundary Layers ◽

Half Cycle ◽

Injection Flow ◽

Oscillatory Boundary Layer ◽

Bed Stress ◽

Layer Velocity ◽

Time Required ◽

Suction Flow ◽

Oscillatory Boundary Layers

Boundary layers arising from flows which oscillate parallel to a permeable bed, and are subject to oscillating percolation of the same frequency as the bed parallel flow, referred to here as ‘ventilated oscillatory boundary layers’, are the subject of this laboratory study. These boundary layers are intended to approximate naturally occurring wave boundary layers over permeable beds. Measurements of boundary-layer velocities, bed stress and turbulent flow properties are presented. It is observed that suction (flow into the bed) enhances the near-bed velocities and bed stress while injection (flow out of the bed) leads to a reduction in these quantities. As the ventilated oscillatory boundary layer experiences both these phenomenon in one full cycle, the result is a net stress and a net boundary-layer velocity in an otherwise symmetric flow. While production of turbulence attributable to injection is enhanced, the finite time required for this to occur leads to a greater vertically averaged turbulence in the suction half-cycle. Turbulence generated in the suction half-cycle is maintained in a compact layer much closer to the bed. These effects appear to hold for$\widetilde{Re}$ranging from 105to 106and for oscillations other than sinusoidal.

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