Local Suction Research Articles

Experiments on control of streamwise streaks

Control of streamwise velocity streaks are studied experimentally in a plane channel flow. High and low-velocity streaks are created by suction through streamwise slots and, further downstream, the secondary instability of the streaks is forced by speakers. The streaks are controlled by localized suction downstream of the disturbance generation. In a modified setup, reactive control is used in order to delay transition of low-velocity regions appearing at known spanwise positions randomly in time. As expected, the growth rate of the secondary instability decreases when localized suction is applied below a low-velocity streak. With control applied, transition is substantially delayed. The suction position and, in the case of reactive control of randomly appearing disturbances, the time instants at which control suction was turned on/off, were varied. The parameter study shows that the control suction has to be applied within a narrow region (10% of a streak width) around the centre of a low-velocity streak. The timing of the control suction is seen to be less critical.

Venomous snakes of Saudi Arabia and the Middle East: a keynote for travellers

Geographically Saudi Arabia and the Middle East include Asian Turkey, Syria, Lebanon, Jordan, Israel, Palestine, the Arabian Peninsula, Iraq, Iran and the previous Southern Asiatic Soviet Republics. The snake fauna contains species in common with northern Africa, Europe and central Asia and towards the east there is infiltration of species characteristic of tropical Asia. A classification of the venomous snakes of this area together with their distribution in the different countries is presented. The epidemiology of snake bites, pathophysiology of toxicity and the clinical features of envenoming by the different species of snakes are discussed. Management of snake bite victims including first aid, treatment at the hospital, clues from signs and symptoms that can help in the identification of the causative snake and antivenom treatment is stressed. Traditional manipulations like local incision and suction, use of caustics, oxidizing agents and cryotherapy and the injudicious use of tourniquets are evaluated and criticized. Finally the preventive control of snake bites for travellers in areas infested with venomous snakes is presented and discussed.

Local peak pressure and conical vortex on building

Characteristics and cause of local peak suction are discussed through a relation with the conical vortex. The peak suction is induced by the large and strong conical vortex formed on the roof and the wall when the gust attacks a building instantly from a particular direction in any mean wind directions. The large-scale gust is responsible for the formation of the vortex and the occurrence of the peak suction, because the development of the strong vortex takes time at least when the fluid particle travels a distance 4–5 times the roof size. The modification of the edge is very effective to reduce the peak suction, as it controls effectively the separation of the shear layer. When the eaves of the flat roof are recessed by 1 10 of the height of the building, the peak suction can be reduced to 2 3 of that on the roof without the recession.

Assessment of Local Blowing and Suction in a Turbulent Boundary Layer

Effects of local blowing or suction from a spanwise slot on a turbulent boundary layer flow are investigated using the direct numerical simulation technique. Three different blowing or suction velocities are imposed on the slot keeping blowing or suction flow rate constant. The recoveries of mean wall pressure for the different blowing velocities collapse well on the same recovery line. However, each wall pressure recovers just after the slot for suction. In the blowing case, the relaxation of rms wall pressure fluctuations and pressure gradient after the slot is seen and the relaxation distance from the slot center is nearly constant for three different blowing velocities. In the suction case, the flow recovers from the immediate rear of the slot. These features are also observed in three-dimensional views of the near-wall vortices.

Assessment of local blowing and suction in a turbulent boundary layer

Effects of local blowing or suction from a spanwise slot on a turbulent boundary layer flow are investigated using the direct numerical simulation technique. Three different blowing or suction velocities are imposed on the slot keeping blowing or suction flow rate constant. The recoveries of mean wall pressure for the different blowing velocities collapse well on the same recovery line. However, each wall pressure recovers just after the slot for suction. In the blowing case, the relaxation of rms wall pressure fluctuations and pressure gradient after the slot is seen and the relaxation distance from the slot center is nearly constant for three different blowing velocities. In the suction case, the flow recovers from the immediate rear of the slot. These features are also observed in three-dimensional views of the near-wall vortices.

Magnetohydrodynamic compressible laminar boundary-layer adiabatic flow with adverse pressure gradient and continuous or localized mass transfer

The problem of magnetohydrodynamic compressible boundary-layer flow over a flat plate, in the presence of an adverse pressure gradient, is studied numerically. The fluid is assumed to be Newtonian, electrically conducting and the magnetic field is constant and applied transversely to the direction of the flow. The fluid flow is subjected to a constant velocity of suction and (or) injection, continuous or localized, and there is no heat transfer between the plate and the fluid (adiabatic flow). The system of partial differential equations, describing the problem under consideration, is solved numerically by applying a modification of the Keller box technique. Numerical calculations are carried out for different values of the free-stream Mach number and the magnetic parameter for continuous or localized suction and (or) injection imposed at the wall. The results obtained are shown in the figures and their analysis shows that the flow field can be controlled by the application of a magnetic field as well as by continuous or localized suction and (or) injection. PACS Nos.: 51.00, 52.00

Effect of local forcing on a turbulent boundary layer

An experimental study is performed to analyze flow structures behind local suction and blowing in a flat-plate turbulent boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about Re θ =1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011 ≤ f +≤ 0.044). The forcing amplitude is fixed at A 0=0.4. It is found that a small local forcing reduces the skin friction and the skin friction reduction increases with the forcing frequency. A phase-averaging technique is employed to capture the large-scale vortex evolution. An organized spanwise vortical structure is generated by the local forcing. The cross-sectional area of vortex and the time fraction of vortex are examined by changing the forcing frequency. An investigation of the random fluctuation components reveals that turbulent energy is concentrated near the center of vortical structures.

Particle image velocimetry studies of a boundary layer perturbed by localized suction

Difference experiments were carried out in a low-Reynolds number (490 based on momentum thickness) flat-plate boundary layer. Particle image velocimetry (PIV) was used to measure the two velocity components parallel to the plate in the boundary layer with and without weak localized suction through two holes at suction speeds of less than 1% of the free-stream value under otherwise identical conditions. The results from the preliminary PIV data show that suction does not affect the mean streamwise velocity profile, but decelerates the boundary layer in the vicinity of the suction holes. This deceleration is on average 6% of the mean speed at 13 viscous length scales above the plate and persists for at least 10 hole diameters downstream of the suction holes. Two-dimensional correlations of streamwise velocity fluctuations imply that even these low suction levels reduce the cross-stream and streamwise length scales in the boundary layer.

Numerical simulation of capillary-induced flow in a powder-embedded porous matrix

Capillary-induced fluid motions in an isotropic powder-embedded porous matrix are studied by the Monte-Carlo simulation method. The concept of random walk and a two-block hexagonal network model are employed to accomplish the simulation procedures. The pressure field, intrinsically representing the internal collision effects among fluid particles, is correlated by the Laplace equation and can facilitate the determination of the pressure-orienteddisplacing strength, Γ, of the random walk. Also, a set of normalized random numbers on the interval (0, 1) is used to modify the displacing strength for ‘chance’ variation. Simulated conditions are selected to coincide with the practical capillary-wick debinding process in metal powder injection molding. The numerical results agree well with the theoretical and experimental ones in the literature. This shows that capillary fingering in the wicking material tends to lift the potential of the local suction malfunction of the leading front. Methods that shorten the length of the liquid column in the wick will reduce the debinding time.

Flow of a viscous incompressible fluid of temperature dependent viscosity past a permeable wedge with uniform surface heat flux

The effect of uniform suction on the steady two-dimensional laminar forced flow of a viscous incompressible fluid of temperature dependent viscosity past a wedge with uniform surface heat flux is considered. The governing equations for the flow are obtained by using suitable transformations and are solved by using an implicit finite difference method. Perturbation solutions are also obtained near the leading edge and in the downstream regime. The results are obtained in terms of the local skin friction coefficient and the rate of heat transfer for various values of the pertinent parameters, such as the Prandtl number, Pr, the velocity gradient parameter, m, the local suction parameter, ξ, and the viscosity variation parameter, ɛ. Perturbation solutions are compared with the finite difference solutions and are found to be in excellent agreement. The effect of ξ, m and ɛ on the dimensionless velocity profiles and viscosity distribution are also presented graphically for Pr = 0.7 and 7.0, which are the appropriate values for gases and water respectively.

Receptivity of three-dimensional boundary-layers to localized wall roughness and suction

The receptivity of three-dimensional, incompressible, boundary-layer flows to localized roughness or suction is studied theoretically and numerically. Three-dimensional boundary-layer flows have a strong “nonparallel” component due to the curvature of the potential-flow streamlines. Our theoretical model extends the Fourier-transform methodology to nonparallel flows using a Taylor expansion of the laminar mean-flow at the location of the roughness. Both the near-field and the far-field solutions are contained in the model. Additionally, the use of MacLaurin expansions in the complex plane leads quickly to the receptivity factors of eigenmodes. The theoretical results are validated with solutions to the linearized Navier–Stokes equations efficiently obtained by replacing the actual wall forcing with a smoother, but equivalent, forcing. We find that the far-field response is proportional to both H̃(αe) and dH̃(αe)/dα, where H̃(α) is the Fourier transform of the forcing distribution, and αe is the eigenmode’s wave number. The receptivity coefficient for H̃(αe) decreases when nonparallelism is included in our models, while that for dH̃(αe)/dα increases.

Shock control on a swept wing

Shock and boundary layer control by contour bumps and local boundary layer suction have been investigated experimentally and numerically on a transonic swept wing. Additional 2-D numerical investigations were performed for the airfoil, corresponding to the wing. The investigations were primarily stimulated by the question concerned with the influence of sweep on the bump effectiveness. This influence has been found to be rather small; the drag reduction by the bump is slightly lower for the swept wing than for the airfoil. A location of the bump in the shock region has shown its effectiveness for reducing shock strength and hence wave drag. A position of the bump downstream of the shock wave has been shown to reduce viscous drag and to postpone buffet-onset to higher lift coefficients. Furthermore, the results indicate that boundary layer suction is a powerful device for drag reduction, but the effectiveness decreases with increasing Reynolds number. Higher effectiveness of suction can be attained, when it is coupled with a contour bump. The parameters height and position (relative to the shock) of the bump, optimized in terms of drag, depend on the shock strength; an influence of the boundary layer thickness upstream of the shock on the optimal bump parameters has not been found. A possibility to control an adaptive bump, mounted on an aircraft wing, is to employ the trailing edge pressure.

Numerical prediction of the formation of Goertler vortices on a concave surface with suction and blowing

SUMMARY This paper presents a numerical prediction of the formation of Goertler vortices on a concave surface with suction and blowing. Suction stabilizes the boundary layer flow on the surface, whereas blowing destabilizes the flow. The criterion on the position marking the onset of Goertler vortices is defined in the present paper. For facilitating the numerical study, the computation is carried out in the transformed x ‐h plane. The results show that the onset position characterized by the Goertler number depends on the local suction:blowing parameter, the Prandtl number and the wavenumber. The value of the critical Goertler number increases with the increase in suction, while the value of the Goertler number decreases with the increase in blowing. Both the experimental and the numerical data can be correlated by Gu10.2(a%u)* 3:2 without suction and blowing and by a simple relation G* (G* )g 0 e g with suction and blowing. The obtained critical Goertler number and wavenumber are in good agreement with the previous experimental data. Copyright © 1999 John Wiley & Sons, Ltd.

Mean wind pressures on flat roof corners affected by parapets: field and wind tunnel studies

This paper describes the experimental data obtained in a full-scale wind-pressure study on a low-rise building, 3.3 m high, located beside a football field on the Loyola Campus of Concordia University in Montreal. Mean wind-induced pressures have been measured on 12 roof corner points for several wind directions. A series of building models were also tested in the boundary layer wind-tunnel of the Centre for Building Studies at three different geometric scales. In particular, the effect of parapets on roof corner local suctions (mean values) has been examined in detail. Both full-scale and wind-tunnel data show that high parapets generally reduce the high suctions on roof corners. On the other hand, low parapets may increase the high values of mean suctions on roof edges and corners. Extensive experimentation has shown that significant suction increases occur only for a particular range of parapet height to building length ratio, namely 0.01< h/ L<0.02. The paper compares the results with those of previous studies on wind loads on flat roofs with and without parapets.

Experimental study of laminar-turbulent transition on a swept wing with local boundary-layer suction at subsonic velocities

The effect of local suction of the boundary layer gas near the leading edge of the upper surface of a swept wing (with the sweep anglex=35°) on the extent of the laminar flow region is studied on a large half-model at subsonic velocities. Various experimental methods were used. The data are subjected to a comparative analysis.

Local severe suctions on the side of a prism model on a field

The wind pressures on a three-dimensional prism model on an outdoor field were measured with multi-simultaneous data acquisition system. From the results of the outdoor measurements, it was shown that the local severe suctions on the side of the prism model were mainly caused by two typical types of conical vortices. These local severe suctions were indicated in the similar wind pressure distributions to the local severe suctions caused by two types of conical vortices on the side, that is, a standing conical vortex and an inverted conical vortex The occurrence conditions of the conical vortices in the natural wind were not a little different from those of the conical vortices in the wind tunnel. The differences were expected to be mainly caused by non-stationary phenomena in natural wind.

A combined experimental and computational study of pressure-driven three-dimensional separation in a turbulent boundary layer

Experimental data and computational results are compared for a pressure-driven three-dimensional separation in a two-dimensional turbulent boundary layer. The comparisons include surface-flow visualizations, wall static pressures, and skin-friction coefficients. With the use of an open-circuit low-speed wind tunnel, the separation pattern was generated by a streamwise adverse pressure gradient combined with localized overhead suction that represented 6% of the test section entrance flow. This separation pattern was characterized by its topological critical points: a saddle point of separation, a nodal point of attachment, two additional saddle points, and two foci. Calculations of the experiment were made by using the INS3D computer code with the Spalart-Allmaras one-equation turbulence model. The topology of the computed near-wall particle traces was similar to the experimental separation pattern having the same geometric aspect ratio and the same number and types of critical points seen in the experiment. Comparisons between experimental data and computed results for static-pressure coefficient and skin-friction coefficient also showed good agreement. Results from the use of another turbulence model, Baldwin-Barth, had a much larger separated region. This experiment provides new data that isolate a three-dimensional separation pattern driven by pressure gradient and the amount of turbulent mixing.

95/04309 Heat transfer enhancement in a vertical channel with asymmetric isothermal walls by local blowing or suction

95/04309 Heat transfer enhancement in a vertical channel with asymmetric isothermal walls by local blowing or suction

A study of non-parallel and nonlinear effects on the localized receptivity of boundary layers

The acoustic receptivity due to localized surface suction in a two-dimensional boundary layer is studied using a finite-Reynolds-number theory and direct numerical simulation of the Navier-Stokes equations. Detailed comparisons between the two methods are used to determine the bounds for application of the theory. Results show a 4% difference between the methods for receptivity in the neighbourhood of branch I with low suction levels, low acoustic levels, and a moderate frequency; we attribute this difference to non-parallel effects, not included in the theory. The difference is larger for receptivity upstream of branch I, and smaller for receptivity downstream of branch I. As the peak suction level is increased to 1% of the free-stream velocity, the simulations show a nonlinear deviation from the theory. Suction levels as small as 0.1% are shown to have a significant effect on the instability growth between branch I and branch II. Increasing the acoustic amplitude to 1% of the steady free-stream velocity produces no significant nonlinear effect.

Heat transfer enhancement in a vertical channel with asymmetric isothermal walls by local blowing or suction

In this article, local heat transfer enhancement in a vertical parallel channel with asymmetric isothermal walls is numerically investigated. A porous wall segment is embedded in the hot wall to achieve this enhancement. By assuming the reference pressure and temperature on the other side of the hot wall, the heat transfer is enhanced as hot fluid is sucked from the channel. The heat transfer improvement is a local effect that becomes more effective with the increase of porosity and the distance measured from the entrance to the position of porous wall segment. It is shown that the local cooling enhancement is effective in low Reynolds number cases (Re ≤ 100) but is less impressive when the Reynolds number is higher. The numerical results also show the possibility of embedding multiple porous segments in a single channel, provided that the mass fraction sucked out of the channel is limited.