Experimental and Numerical Study of Heat Transfer and Pressure Loss in a Swirl Multi-pass Channel with Convergent Jet Slots

This paper presents a comparative experimental and numerical study of the heat transfer and pressure loss in a swirl multi-pass channel with tangential jet slots, and another baseline multi-pass channel with 180-deg U-bends as comparison baseline has also been investigated. Transient liquid crystal thermography is used to obtain the detailed heat transfer distribution on the internal surfaces of the multi-pass serpentine channels. The heat transfer patterns in the swirl multi-pass channel are quite different from that of the baseline multi-pass channel. Compared with the baseline multi-pass channel, the experimental globally averaged Nusselt number ratios of the last two passes in the swirl multi-pass channel can be increased by up to 82.9%, 104.8% and 124.6% for the Reynolds numbers 20,000, 40,000 and 60,000, respectively. The high and circumferentially uniform heat transfer is mainly due to the large-scale swirling flow induced by the tangential slots. More detailly, the large-scale swirling flow impinges onto the surface and further induces high tangential velocity near the wall, which destroys the boundary layer flow and thus improves the heat transfer rates at the wall. However, the notable pressure loss of the swirl multi-pass channel should be further controlled reasonably, which is about 5.4 times that of the baseline multi-pass channel. As supplements to the experiments, three-dimensional numerical computations provide more insights into the turbulent flow structure in the two kinds of multi-pass serpentine channels.

Turbulent Flow Structure in a Confluence: Influence of Tributaries Width and Discharge Ratios

River channel confluences are rather important interfaces where intense changes in physical, mixing and sediment transport processes occur. Following an experimental campaign, the main flow mechanisms in confluences and the development of the shear layer formed between the two tributary flows are presented. As the experimental flow cases comprised changes in the flow discharge and channel widths of the tributaries, the influence of width and discharge ratios on the turbulent flow structure and shear layer is also evaluated. Main findings indicate that changes in the difference between momentum ratio in the tributaries have a significant effect on the magnitude and location of flow mechanisms.

Role of Transient Characteristics in Fish Trajectory Modeling

In this paper, we analyzed the live fish trajectory recorded from an experiment in an experimental vertical slot fishway. Combined with a numerical simulation, we demonstrated that randomness shown in fish trajectory might not merely be attributed to fish's random choices in its swimming, also could be an adaption consequence to the bulk unsteady turbulent flow structure. Simple superposing the fish trajectory on the time-averaged flow field obtained either by interpolating on discrete point measurements or numerical simulation is not an ideal method for fish movement description in fishway engineering. How to model the fish paths in transient flow and the necessity of simultaneous recording of the flow field and the fish locomotion are challenging topics. The suggested spectrum analysis of the flow field may provide a new general method to reproduce the fish trajectory in a complex turbulent flow.

Turbulent characteristics of flow in the vicinity of mid-channel braid bar

The impact of a mid-channel bar on the turbulent flow structure has been investigated in this research. A new Dominance Function S_(i,H) is proposed in this study as a measure of the relative dominance of ejection and sweep events in turbulent flow structure. Occurrence of the kolk-boil phenomenon is observed due to interaction of ejection and sweep events.. A new parameter Movement Ratio is formulated in this study which is found to faithfully reflect the fluvial processes of sedimentation and scouring on the channel bed.. Acceleration of flow is seen to occur at adjoining regions close to the upstream end of the bar. Due to the presence of the bar, the flow area in its proximity decreases which has caused increment in the velocity at sections located near the upstream end of the mid-channel bar. For model runs with bars, a distinct bulge in the turbulent intensity graph is observed.

Directional Surface Roughness Influence on Turbulent Flow Structure

A series of experiments have been conducted to investigate turbulent flow structures when it is exposed to a highly directional riblet-type surfaces roughness (converging-diverging/herringbone pattern) at a relatively low Reynolds number (Reτ). These experiments show that even at a low Reτ, the surface pattern is able to modify the turbulent boundary layer. Over the diverging region, we observe a decrease in drag penalty, while over the converging region there is an increase of drag penalty, which is indicated by the shift in the mean velocity profiles. The surface roughness also influences the turbulence production, indicated by the elevated turbulence intensities profiles for both the converging and diverging regions. The result seems to deviate from early investigations that show an increase in turbulence intensities above the converging region and a lowered turbulence intensities above the diverging region. The discrepancy may be caused by the lower Reτ Ret in the current report. Other important statistics such as skewness and flatness are also reported.