Effect Of Wall Suction Research Articles

Effect of wall suction on leading edge contamination

This paper is devoted to an experimental study of swept wing leading edge contamination by the turbulence emanating from the wing-wall junction. The main objective is to delay the contamination onset by applying surface suction along the attachment line. Two series of experiments are described; the first one was performed in a small wind tunnel at CERT ONERA, the second one was carried out in the F2 wind tunnel at Le Fauga Mauzac centre. Hot film measurements showed that leading edge contamination could be delayed up to very large Reynolds numbers. We also studied the behaviour of the relaminarized boundary layer downstream of the sucked region, along the span as well as in the chordwise direction.

Fluid flow and heat transfer in a pipe with wall suction

The effects of wall suction on turbulent fluid flow and heat transfer in a pipe were numerically studied using the Reynolds-averaged Navier-Stokes equations in conjunction with the temperature equation. Linear and non-linear κ-ε or κ-ω low-Re models of turbulence are used for ‘closing’ the governing equations. Computed mean velocity and temperature profiles were compared with analytical solutions and experimental measurements for suction rates between 0.46 and 2.53%. Analytical results, based on boundary layer assumptions and mixing length concepts, were found to be in satisfactory agreement with computed and experimental data for the lowest suction rate examined. However, for the highest rate, they did not follow the computed and experimental ones, especially in the near-wall region. Near-wall velocities and temperatures increased with increasing suction rate. Computed and experimental velocities and temperatures fell below the corresponding logarithmic law of the wall with no suction. Computed and experimental turbulent shear stress and heat flux were reduced by the presence of wall suction. The excess skin friction coefficient and Nusselt number, resulting from suction, were found to be up to 9 times the respective ones with no suction.

Wall Suction Effects on the Structure of Fully Developed Turbulent Pipe Flow

The effects of wall suction on the structure of fully developed pipe flow are studied numerically by solving the Reynolds averaged Navier-Stokes equations. Linear and nonlinear k-ε or k-ω low-Re models of turbulence are used for “closing” the system of the governing equations. Computed results are compared satisfactorily against experimental measurements. Analytical results, based on boundary layer assumptions and the mixing length concept, provide a law of the wall for pipe flow under the influence of low suction rates. The analytical solution is found in satisfactory agreement with computed and experimental data for a suction rate of A = 0.46 percent. For the much higher rate of A = 2.53 percent the above assumptions are not valid and analytical velocities do not follow the computed and experimental profiles, especially in the near-wall region. Near-wall velocities, as well as the boundary shear stress, are increased with increasing suction rates. The excess wall shear stress, resulting from suction, is found to be 1.5 to 5.5 times the respective one with no suction. The turbulence levels are reduced with the presence of the wall suction. Computed results of the turbulent shear stress uv are in close agreement with experimental measurements. The distribution of the turbulent kinetic energy k is predicted better by the k-ω model of Wilcox (1993). Nonlinear models of the k-ε and k-ω type predict the reduction of the turbulence intensities u’, v’, w’, and the correct levels of v’ and w’ but they underpredict the level of u’.

Dean vortices with wall flux in a curved channel membrane system: 2. The velocity field

AbstractThe velocity and pressure fields and the effect of wall flux on these fields in a spiral channel are presented. As fluid flows inward through a spiral channel with constant gap and permeable walls, the streamwise flux decreases while the curvature increases. Thus, by balancing the stabilizing effect of wall suction with the destabilizing effect of increasing curvature, established vortices can be maintained along the spiral channel. This approach is used to prescribe spiral geometries with different wall fluxes. Using a weakly nonlinear stability analysis, the influence of wall flux on the characteristics of Dean vortices is obtained. The critical Dean number is reduced when suction is through the inner wall only, is slightly reduced when suction is equal through both walls, and is increased when suction is through the outer wall only. The magnitude of change is proportional to a ratio of small numbers that measures the importance of the effect of curvature. In membrane filtration applications the wall flux is typically 2 to 5 orders of magnitude less than the streamwise flow. If the radius of curvature of the channel is of the order of 100 times the channel gap, the effect on the critical Dean number is within 2% of the no‐wall flux case. If the radius of curvature is sufficiently large, however, it is possible to observe effects on the critical Dean number that approach O(1) in magnitude for certain parameter ranges.

Heat transfer between a fluid-saturated porous medium and a permeable wall with fluid injection or withdrawal

The present paper addresses heat and mass transfer between a permeable wall and a fluid-saturated porous medium. To assess the effect of wall suction or injection on sensible heat transfer, a stagnant film model is developed. The model yields a thermal correction factor accounting for the effect of wall transpiration on heat transfer. In order to apply this correction factor, neutral (or zero mass transfer) heat transfer rates must be specified. In the past, for a large number of practical situations heat transfer correlations have been obtained with the boundary layer model, which are summarized here. Subsequently, the derived correction factor is applied to free, mixed and forced convection flows along vertical and horizontal permeable walls embedded in a porous medium. The predictions of the film model are compared and found to be in good agreement with corresponding results obtained with the boundary layer model (also provided by the literature). Hence, the film model approach appears to be a compact and adequate description of the processes considered.

Transition correlation in subsonic flow over a flat plate

A 2D subsonic flow over a flat plate with a freestream Mach number M(infinity) up to 0.8 is considered. The flow can experience continuous uniform suction through the wall and the wall can be heated or cooled continuously with a fixed wall temperature. For a specific combination of M(infinity), suction velocity, and level of heat transfer, the mean flow problem is solved and linear stability calculations are performed to compute the location on the flat plate where the factor representing the integration of growth rates reaches nine. These calculations are repeated for several combinations of flow parameters and the theoretically predicted transition location is presented in the form of a correlation that can account for the effect of wall suction, heat transfer, and Mach number.

Lateral inertial migration of a small sphere in fast laminar flow through a membrane duct

The hydrodynamic response of a small suspended sphere to laminar flow is studied for relatively fast flow rates in a duct with porous walls. There is a lift effect on the sphere which competes with a wall suction effect. The details of this balance determine whether the sphere will reach the membrane. An expression is developed, from first principles, to predict conditions under which a sufficiently long cross-flow plate and frame membrane module exposed to dilute suspensions of essentially spherical particles will not foul, assuming nonhydrodynamic attractions between a particle and the membrane are negligible. This is the first expression which might be properly tested experimentally to determine if the elevated permeation rates observed with colloidal suspensions (Porter, 1972) are due to particle lift or to some other phenomena such as a flowing cake. The strongest particle lift effect occurs when the sphere is much closer to one wall than the other and is due to convective interaction of the disturbance caused by the sphere and the undisturbed flow, in the presence of the nearby wall. The lift velocity has the same dependence on parameters as the previous results derived for slow laminar flow (Cox and Brenner, 1968; Ho and Leal, 1974). However, the maximum lift velocity is smaller than that found by Vasseur and Cox (1976) by roughly a factor of 2.6. These results agree with those of Schonberg and Hinch (1989) in the following features: the equilibrium position moves with increasing Reynolds number towards the wall, the magnitude of the velocity near the wall is in good agreement, and our results extend to higher Reynolds number. Also, the results are compared with the experiments of Segrè and Silberberg (1962a, b).

Effect of wall suction on bursting in a turbulent boundary layer

The effect of suction on the wall region of a turbulent boundary layer over a slightly heated wall has been quantified in terms of several aspects of the motion that are related to bursts and sweeps. These events are detected by the modified u-level method (with alignment on the ends of bursts), the uv-quadrant 2 method, and the uv-quadrant 4 method. The mean periods of bursts and sweeps are increased by suction. The regions upstream from u-level detected bursts are more coherent than the bursts themselves. Suction substantially increases the relative contributions from the organized motion to momentum and heat fluxes, ∼(uv), ∼(uθ), and ∼(vθ), and has a lesser effect on contributions to ∼(u2), ∼(v2), and ∼(θ2).

Influence of wall suction on the organized motion in a turbulent boundary layer

The effect of wall suction on the organized motion of a tubulent boundary layer is examined experimentally both in a wind tunnel and in a water tunnel. In the windtunnel boundary layer, which developed over a slighly heated surface, temperature fluctuations were simultaneously obtained at several points, aligned in either the x (streamwise) or y (normal to the wall) direction. The temperature traces reveal the existence of two spatially coherent events, characterized either by a sudden decrease (cooling) or by a sudden increase (heating) of temperature. Estimates are presented for the average convection velocity, and average frequency of these events. The average convection velocity of ‘coolings’ is about 15% larger than that of ‘heatings’, the velocity of both events exhibiting an important local maximum in the buffer region. Near the wall, the convection velocity of both events is increased slightly by suction while their average frequency is reduced by suction. Away from the wall, the average inclination of ‘coolings’ and ‘heatings’ is about 40° without suction; suction does not alter the inclination of ‘coolings’ but increases that of ‘heatings’ to about 50°. Visualizations in the water tunnel indicate that suction increases the stability and the longitudinal coherence of low-speed streaks. They also show that suction reduces the average frequency of dye ejections into the outer layer.

The effect of wall suction and thermophoresis on aerosol particle deposition from a laminar boundary layer on a flat plate

The effect of wall suction and thermophoresis on aerosol particle deposition from a laminar boundary layer on a flat plate

The Effect of Wall Suction on the Apparent Roughness of Porous Tubes

The Effect of Wall Suction on the Apparent Roughness of Porous Tubes

Experimental Study of Turbulent Flow in a Tube With Wall Suction

The effect of wall suction on the turbulent flow of air in a porous tube has been studied. Measurements of the radial distribution of the turbulent velocity fluctuations were obtained over a range of Reynolds numbers from 104 to 2 × 105. Various suction rates were employed, for both local suction over a short length of tube and continuous suction over various lengths. The results obtained for local suction (step reduction in Reynolds number) show that approximately 40 dia are required for the turbulent velocity fluctuations to reach flow equilibrium at the lower downstream value of the Reynolds number. The results for the case of continuous suction show that after a short suction length, there is an apparent increase in the turbulence level compared with that found at the same Reynolds number with no suction. This appears to be due to the greater turbulence level which exists at the higher (presuction) Reynolds number. Longer suction lengths, above 40 dia, always result in a decrease in the turbulence level compared with turbulent flow with no suction at the same Reynolds number. The present results suggest that simple mixing-length models, incorporating local flow parameters, may be inadequate to describe the turbulent momentum transport in a tube with surface suction. Certainly, the existing mixing-length models should be re-examined in the light of this new experimental data.

Turbulent Flow in a Tube With Wall Suction

The effect of wall suction on turbulent flow in a tube is analyzed, using a mixing-length model with an extended form of van Driest’s damping factor. It is shown that an empirical damping factor similar to that proposed by Kays, Moffat, and Thielbahr, when incorporated in a method of solution developed by Kinney and Sparrow, predicts that the turbulence level in tube flow decreases with suction, and yields excellent correlation with the experiments of Weissberg and Berman.