Reverse Flow in A Converging Channel with An Obstruction at The Entry - A Flow Visualization Study

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1 14 Reverse Flow in A Converging Channel with An Obstruction at The Entry - A Flow Visualization Study B. H. L. Gowda, BTL Institute of Technology, Department of Mechanical Engineering, Bommasandra, Bengaluru, India M. M. Naik, BTL Institute of Technology, Department of Mechanical Engineering, Bommasandra, Bengaluru, India J. C. Reuben, BTL Institute of Technology, Department of Mechanical Engineering, Bommasandra, Bengaluru, India M. Sai Pranith, BTL Institute of Technology, Department of Mechanical Engineering, Bommasandra, Bengaluru, India B. Shashank, BTL Institute of Technology, Department of Mechanical Engineering, Bommasandra, Bengaluru, India ABSTRACT Reverse flow in a parallel walled test channel placed inside a wider channel is observed when an obstruction is placed at the entry of the test channel. When the position of the obstruction at the entry of the test channel is varied relative to the test channel, the flow inside the test channel reverses (opposite to the free stream flow direction), stagnates or flows in the forward direction, that is, in the free stream flow direction. The present experimental investigation is to study the reverse flow phenomenon in converging channels for various g/w ratios at Re = 3500 using flow visualization. The converging angles studied are 0.2, 0.4, 0.6, 0.8 and 1.0 degrees. The reverse flow magnitude increases as the convergence angle is increased till about 0.6 o, and then starts to decrease when the convergence angle is further increased. The results also show the mechanism of fluid pumping due to shear layer interaction at channel exit for the various convergence angles of the channel geometry. Keywords Reverse flow in converging channel; Unsteady laminar flow; Flow visualization INTRODUCTION When an obstruction is placed at the entry to a parallel walled channel located within another wider channel (Fig. 1), the flow within the test channel stagnates or reverses its direction (opposite to the free stream flow direction of the wider channel) for different relative positioning of the obstruction. The flow velocity inside the test channel (U i ) is the measure of flow reversal. Gowda and Tulapurkara [1] observed the reverse flow phenomenon inside a channel when an obstruction is placed at the entry of the channel. They studied the influence of gap (g), the length of the test channel (L), and the Reynolds number (Re) based on the channel width (w) and the free stream velocity U. They observed that the maximum reverse flow occurs for g/w = 1.5 and the stagnant flow condition happens for a g/w = 3.5. Experiments conducted in wind tunnels (for Re = 26000) revealed that the behavior of U i /U and flow pattern at higher Reynolds number exhibits trends similar to those at lower Reynolds numbers. Fig.1.Parallel walled channel (test channel) inside the wider channel Subsequent to their first investigation, they have investigated the influence of the various geometries of the obstruction on the reverse flow of the parallel walled channel [2]; effect of obstructions both at the entry and at the rear end of the test channel [3]; investigated the influence of splitter plates 1) when placed at the front end and 2) when placed at the rear end [4]. Further investigations with flow visualization and pressure measurements by Gowda et al. [5] attempted to gain an understanding of the mechanism that triggers and sustains the reverse flow. The obstruction at the front end of the channel is essential for triggering the reverse flow. Sharp changes in pressure occur at the two ends of the channel due to the relative position of the obstruction, resulting in reverse, stagnant and slow forward flow in the test channel. The delicate balance of pressure along the channel controls the direction and magnitude, with a limitation on the maximum reverse flow achievable. Flow visualization and numerical studies by Kabir et al. [6, 7] cover a range of Reynolds numbers up to 9,000. They confirmed the results found by Gowda and Tulapurkara [1] that the reverse flow is at its maximum value for the ratio of g/w = 1.5. The aim of the present study is to establish this complex unsteady flow, involving reverse flow, through flow

2 15 visualization for converging channels. These studies would provide us a better insight on not only the reverse flow phenomenon but also the effect of convergence angles on the pumping mechanism due to the interaction of the shear layers at the channel exit. Some of the applications where the reverse flow phenomenon can occur or can be employed are: control of flow, especially to obtain low velocities; heat transfer problems where it may be required to locally have different types of flows; interaction of shear layers at varying distances apart; flow past obstructions/constrictions in arterial flows under certain physiological situations. The configuration considered is shown in Fig.3. The width of the test channel at entry, w, and also the width of the obstruction plate, b, are equal to 25 mm. The length of the channel, L, is 600 mm i.e., 24 times w. To study the effect of gap width, g is varied from 12.5 mm to 150 mm giving a ratio of g/b between 0.5 and 6. Experiments are carried out for convergence angles of 0.2, 0.4, 0.6, 0.8 and 1.0 degree. At each value of convergence angle α, results are obtained for g/b = 0.5, 1.0, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5 and 6.0. However, results are presented for each α at typical values of g/b which bring out the essential features of the phenomenon. EXPERIMENTAL ARRANGEMENT Experiments have been conducted using Flow Visualization Facility at the Fluid Mechanics Laboratory, Department of Mechanical Engineering, B.T.L Institute of Technology. This facility consists of an FRP tank with 2.5 m length and breadth of 1.5 m (Fig.2) and a set of aluminum discs, each RESULTS Fig. 3 Configuration considered The flow visualization results for α = at several values of g/b are shown in Fig.4. In each case, the velocity inside the test channel, U i is obtained from the time taken for a tracer particle to cover a fixed distance of 200 mm in the central part of the channel. In all cases U i is measured several times and the mean taken. Each figure is shown in two parts to accommodate all the important features at the exit and entry of the channel. Fig. 2 Experimental Arrangement separated from the other by a small distance are located at one end of the tank. The discs are connected to a three phase induction motor with cooling arrangement, through a set of bevel gears and the flow created from the rotation of the discs is guided into the test section by two guide blocks made of FRP. The test section width is 350 mm. By controlling the speed of the motor the speed in the test section could be varied continuously up to 0.2 m/s. At higher speeds the water surface becomes wavy and for the experiments a suitable speed is chosen where such waves do not occur. Fine aluminum powder is used as the tracer medium, and photographs were taken using a single-lens reflex (SLR) camera which was located at a suitable height above the channel capture the flow field. Two halogen 500 watts lamps were used to obtain proper lighting. At g/b =0.5 (Fig.4a) the reverse flow inside the channel is clearly seen. It is very interesting to observe the interaction of the shear layers at the exit of the channel there is rolling up of the shear layers which pushes the flow into the channel in a direction opposite to the free stream flow. This rolling occurs in an alternate fashion resulting in a continuous reverse flow. At the front end, the shear layers separating from the obstruction attach on to the sides of the channel giving rise to vortices/recirculation regions. More or less, very similar features occur at g/b = 1.0 and 1.5 (Figs. 4b and 4c). The reverse flow sometimes leaves from the top side of channel (Fig.4a) or from either side (Fig.4b) or from the bottom side (Fig.4c). The maximum reverse flow occurs at g/b = 1.5, with U i /U o = The flow becomes stagnant at g/b = 2.5 (Fig.4d). The vortices formed from the shear layer separation at the two ends of the obstruction block the channel entry resulting in stagnant conditions. At the rear end the rolling occurs away from the channel exit. The flow conditions at the exit for this g/b value is highly unsteady. However, at the the center of the channel stagnant conditions occur.

3 16 As g/b is further increased (Figs 4e and f), there is enough space between the obstruction and the entry to the channel for the rolling up of the shear layers and forward flow within the channel starts, initially weak and strengthening as g/b increases. However, the maximum value of U i /U o obtained is about 0.55 at g/b = 6. The results at other values of α = 0.4, 0.6,0.8 and 1 0 (Figs.5 to 8) show features very similar to what is described above. But the value of g/b at which the maximum reverse flow, stagnant conditions and forward flow occur, vary with α. In Fig.9 is shown the magnitude of the reverse and forward flow velocities for different values of α as g/b varies. The result for the channel with parallel side is also shown for comparison. It is seen that convergent channels result in higher values of reverse flow nearly up to α = and then decrease. The reverse flow magnitudes are higher than for the case α = 0. However, the forward flow magnitudes are higher for the channel with parallel sides. Reverse flow is generated because of the low pressure created behind the obstruction which triggers the flow in the reverse direction. The shear layers start rolling up and there will be a delicate balance between the pressure behind the obstruction and the pressure created at the rear across the vortices formed. The detailed mechanism for the generation of reverse flow is described in Gowda et al [5]. In the case of the convergent channels, the increase in the reverse flow magnitude compared to the channel with parallel sides appears to be the nearness of the shear layers due to convergence. However, when convergence increases beyond about 0.6 or 0.8 0, this interaction seems to decrease resulting in reduced reverse flow magnitude. The flow visualization study has revealed the interesting features at entry and exit, their correspondence with the generation of flow within the channel, in reverse, stagnant or forward direction. Fig.4 Angle of convergence (0.2)

4 17 Fig. 5 Angle of convergence (0.4)

5 18 Fig.6 Angle of convergence (0.6)

6 19 Fig.7 Angle of convergence 0.8

7 20 Fig. 8 Angle of convergence 1.0 CONCLUSIONS The magnitude of the reverse flow in a convergent channel is seen to increase with the convergence angle compared to the case with parallel walled channel, up to an angle of about beyond which it reduces. The flow visualization study has revealed the changes in the flow field at the entry to the channel and the exit of the channel as g/b changes. The interaction between the shear layers which result in rolling up and pumping of the fluid in the reverse direction into the channel for certain values of g/b, the formation of the vortical regions away from the channel exit for certain other values of g/b when stagnant flow occurs, and the shear layers being restricted along the sides when forward flow occurs are brought out by the study. It is also very interesting to see the corresponding changes in the vortical patterns at the entry to the channel. Thus the study has brought out both quantitative and qualitative picture of the reverse flow phenomenon in convergent channels with an obstruction at the entry. ACKNOWLEDGEMENTS The authors express their sincere thanks to the Management of BTL IT for their support and encouragement. REFERENCES [1] Gowda, B.H.L., Tulapurkara, E.G., Reverse flow in a channel with an obstruction at the entry. J. Fluid Mech. 204, [2] Gowda, B.H.L., Tulapurkara, E.G., Swain, S.K., Reverse flow in a channel-influence of obstruction geometry. Exp. Fluids 16, [3] Tulapurkara, E.G., Gowda, B.H.L., Swain, S.K., Reverse flow in channel-effect of front and rear obstructions. Phys.Fluids 6, 3847 to [4] Gowda, B.H.L., Tulapurkara, E.G., Swain, S.K., Influence of splitter plates on the reverse flow in a channel. Fluid Dyn. Res. 21, [5] Gowda, B.H.L., Tulapurkara, E.G., Swain, S.K., On the mechanism of reverse flow in a channel with an obstruction at the entry. Fluid Dyn. Res. 23, [6] Kabir, M.A., Khan, M.M.K., Bhuiyan, M.A., A study of the flow phenomenon of water in a channel with flat plate obstruction geometry at the entry. KSME Int. J. 17, [7] Kabir, M.A., Khan, M.M.K., Bhuiyan, M.A., Flow phenomena in a channel with different shaped obstructions at the entrance. Fluid Dyn. Res. 35, NOMENLATURE b g L U i U 0 w α width of obstruction gap between obstruction and test channel length of the test channel velocity inside test channel free stream velocity width of the test channel angle of convergence