E. G. Tulapurkara

Stability characteristics of wavy walled channel flows

The linear temporal stability characteristics of converging–diverging, symmetric wavy walled channel flows are numerically investigated in this paper. The basic flow in the problem is a superposition of plane channel flow and periodic flow components arising due to the small amplitude sinusoidal waviness of the channel walls. The disturbance equations are derived within the frame work of Floquet theory and solved using the spectral collocation method. Two-dimensional stability calculations indicate the presence of fast growing unstable modes that arise due to the waviness of the walls. Neutral stability calculations are performed in the disturbance wavenumber–Reynolds number (αs−R) plane, for the wavy channel with wavenumber λ1=0.2 and the wall amplitude to semi-channel height ratio, ew, up to 0.1. It is also shown that the two-dimensional wavy channel flows can be modulated by a suitable frequency of wall excitation ωg, thereby stabilizing the flow.

A NUMERICAL STUDY OF FLOW THROUGH WAVY-WALLED CHANNELS

SUMMARY A numerical procedure is developed for the analysis of flow in a channel whose walls describe a travelling wave motion. Following a perturbation method, the primitive variables are expanded in a series with the wall amplitude as the perturbation parameter. The boundary conditions are applied at the mean surface of the channel and the first-order perturbation quantities are calculated using the pseudospectral collocation method. Although limited by the linear analysis, the present approach is not restricted by the Reynolds number of the flow and the wave number and frequency of the wavy-walled channel. Using the computed wall shear stresses, the positions of flow separation and reattachment are determined. The variations in velocity and pressure with frequency of excitation are also presented.

INFLUENCE OF SPLITTER PLATE ON THE REVERSE FLOW IN A CHANNEL

Reverse flow inside a parallel plate channel can be achieved by placing an obstruction at the entry. The present work is carried out to study the influence of (i) splitter plates behind the flat plate obstruction at the front end and (ii) splitter plates placed at the rear end of the channel in tandem with the obstruction at the front. The results show that the splitter plate at the front end has a slight unfavourable effect due to a little reduction in the reverse flow. However, when the splitter plate is placed at the rear end, it interferes with the alternate formation and shedding of vortices near the rear end. This interference depends upon the length of the splitter plate and also on its position relative to the exit of the test channel. The resulting flow field causes an increase in the magnitude of the reverse flow which remains constant over a wide range of splitter plate positions.

ON THE MECHANISM OF REVERSE FLOW IN A CHANNEL WITH AN OBSTRUCTION AT THE ENTRY

The occurrence of reverse flow in a channel when an obstruction is placed at the entry is already known. In this article an attempt is made to bring out the mechanism that governs this phenomenon. Flow visualisation and pressure measurements are employed for this purpose. The obstruction at the front end of the channel is essential for triggering the reverse flow. For reverse, stagnant and slow forward flow, sharp changes in pressure occur at the two ends of the channel. The resulting flow field gives rise to a delicate balance of pressure along the channel that controls the flow direction and magnitude. It is argued that there is a limitation on the maximum reverse flow achievable.

Experimental investigation of turbulent flow inside a rectangular enclosure with a central partition

Abstract Flow through a rectangular enclosure with a central partition provides a convenient means of understanding the flow through passenger car cabins, clean rooms, operation theaters and other enclosed spaces. Flow through such a system is characterised by three-dimensionality and turbulence. The present experimental investigation involves measuring the complete flow field by the use of a Conrad probe and is aimed at studying the flow field in the presence of a fixed central partition. Vector plots in vertical planes perpendicular to the longitudinal and transverse axes and contour plots in transverse planes are presented and discussed. It is found that the fully developed turbulent flow issuing out of the inlet pipe shifts upwards from the centreline and elongates as it approaches the partition. The jet entrains the surrounding air and induces velocity in the other regions of the enclosure. In some regions, reverse flow is also observed. The flow enters the exit pipe with a swirl.

Prediction of aerofoil wake subjected to the effects of curvature and pressure gradient

Experimental data on the development of wakes in a straight duct, a curved duct, a curved diffuser and a straight diffuser are compared with computations based on a finite volume scheme incorporating the k– e model of turbulence. The results show that the computations based on the standard k–e model are able to satisfactorily capture only the mean velocity profiles. To improve the predictions, several modifications to the model are tried out. Close agreement between experiment and computation as regards the velocity profiles, wake parameters and profiles of the turbulent kinetic energy k and Reynolds shear stress ⌅{uv} is obtained when modification to the model constant Cμ, based on the curvature parameter and the ratio of the production of turbulent kinetic energy to its rate of dissipation, is incorporated. The modified model is also able to capture the asymmetry in the profiles of k and ⌅{uv} caused by the curvature and its enhancement due to the additional presence of an adverse pressure gradient.

Boundary layer studies over an S-blade

Double cambered S-blades find applications in turbines and pumps used in tidal power plants and textile machinery. Measurements of aerodynamic characteristics like lift, drag, and lift/drag have been reported by other investigators. To gain a better understanding of the flow over such blades, the velocity profiles and turbulence quantities have been measured on the upper and lower surfaces and are reported in this paper. The flow over the upper surface is subjected to convex curvature first and then to concave curvature, whereas flow on the lower surface encounters the concave surface first and convex later. Small regions of separated flow exist even at zero degree angle of attack. The results are compared with those investigation in which the effects of pressure gradient, curvature, flow separation, and reattachment were studied separately. It is found that for the first half on both surfaces the variation of skin friction coefficient and shape parameter are dominated by the influence of surface curvature. In the last quarter the curvature and pressure gradient have cumulative effect on these parameters. In the third quarter (x/c = 0.5 & 0.75) no clear trend is seen as the curvature changes from convex to concave on the upper surface and from concave to convex on the lower surface.

A numerical study on the mechanism of reverse flow in a channel without obstruction at the entry

It is known that reverse flow occurs in a channel when there is an obstruction at the entry. However, it has been recently shown that the reverse flow can be realised even without an obstruction. This is achieved when the two sides of the channel have a stagger and are kept at an angle of attack to the free stream. The features of the computed reverse flow agree with the experimental results. The computations show that the pumping mechanism of reverse flow in the present case can be explained by the relatively lower pressure near the entry to the channel and the slightly higher pressure near the exit of the channel. The low pressure region, near the entry to the channel, having staggered walls and kept at an angle of attack, is generated by the flow separation at the leading edge of the bottom wall of the channel.