Author(s): Kimberley Kasischke; Mario Oertel
Linked Author(s): Mario Oertel
Keywords: Hydraulic structure block ramp turbulent processes CFD simulations
Abstract: Turbulent profiles of flows on a block ramp via numerical CFD simulations Kimberley KASISCHKE1, Mario OERTEL2 1,2 Hydraulic Engineering Section, Helmut-Schmidt-University, Hamburg, Germany email: kimberley. kasischke@hsu-hh. de email: mario. oertel@hsu-hh. de Abstract The present study investigates turbulent processes around block ramps within a basin structure. To examine these processes, CFD simulations are conducted on a ramp with a slope of 3.33 % for different discharge events. The results of the simulations are employed to provide qualitative assessments of the turbulence profiles within the selected geometry. In particular, the flow velocity in the x-, y-, and z-directions over the length of the ramp are considered. Correlations are identified for the turbulence intensity and turbulent kinetic energy, comparisons with previous formulations found in the literature were carried out. This included the trend of longitudinal turbulence intensities, vertical turbulence intensities, and the turbulent kinetic energies along the block ramp. Furthermore, the results also indicate that the analysis of turbulence parameters facilitate a more accurate determination of turbulence characteristics around block ramps. Keywords: hydraulic structure; block ramp; turbulent processes; CFD simulations. Introduction The maintenance of ecological continuity in watercourses with cross structures or other obstacles hinges on the effective implementation of the requisite measures. Therefore, it is essential to develop a comprehensive model of the prevailing hydraulic processes in order to guarantee a design with minimum uncertainty. Potential measures include the construction of fishways with basin structures, which are designed to enable the migration of fish. For a considerable number of installations, near-natural channels, such as block ramps, represent an optimal solution, as this approach minimizes alterations to the natural watercourse and the surrounding nature while ensuring continuity. In addition to the maintenance of river continuity, block ramps are employed in the stabilization of river beds. This structure is formed by installing boulders in rows upstream, creating a basin structure. Among other things, the boulders serve to stabilize the river bed and also create a pronounced roughness of the surface (Oertel 2012; Oertel and Schlenkhoff 2012). The generation of excessive turbulence is a consequence of the dissipation of flow energy over rough surfaces. Turbulences are an individual property of flows and can be described as a temporal and spatial flow phenomenon (Ahmad et al. 2013). In detail, the dissipation of flow energy is affected by surface roughness, with the alteration in the mean velocity profile in the vicinity of the wall exerting a considerable influence. This results in a corresponding alteration to the coefficient of friction. Provided that the maximum roughness height in the system is not negligible, the flow change not only extends to the viscous layers and buffer layers, but also includes the logarithmic layer, in which at least half of the turbulent energy is generated (Jalalabadi 2022). In the context of fish migration, various of turbulence parameters have been identified as influencing behavior and movement. These parameters have been the subject of numerous studies. This includes turbulence intensity, turbulent kinetic energy, Reynolds shear stress, eddy strength, and eddy sizes (Baki et al. 2015; Dizabadi 2019). The turbulence of the flow can be evaluated using the parameters described. Nevertheless, turbulent processes are distinguished by a considerable degree of unpredictability. It is therefore important to determine the processes using numerical models, as basic mathematical dimensioning alone is not always sufficient to guarantee functionality. In addition, experimental models are often not feasible due to the presence of considerable scale effects. The study uses CFD simulations to identify and investigate the turbulent processes occurring on around block ramps. The velocity profiles in the x-, y-, and z-directions, the turbulence intensities, and the turbulent kinetic energies are of paramount importance in this context. The aim is to evaluate the feasibility of determining turbulence profiles with numerical simulations and to compare the results with formulations of previous studies. Methods The parameters that influence fish migration can be observed in turbulent flows. To analyze the turbulent flow, a near-steady state must be achieved. The type of any flow regime can be determined based on the Reynolds number. As the Reynolds number continuously rises, the flow becomes fully turbulent. However, it should be noted that the size of the Reynolds number depends on the roughness of the model, and thus a fully turbulent flow can also occur at low Reynolds numbers (in the case of a rough surface). In a fully turbulent flow, turbulent eddies generate velocity fluctuations in all three velocity components (longitudinal u, lateral v, vertical w) (Fig. 2). In turbulent flows, a time series velocity contains mean and fluctuating components (Eq. 1). This phenomenon is known as the Reynolds decomposition. u (t) = ¯u+ u^' (1) The mean velocity profiles are employed to illustrate the velocity distribution within the individual basins (see Fig. 3). Thereby the distinctive characteristics of the velocity profiles are identified and discussed. The mean flow velocity ¯u, the turbulent fluctuations u_i^', the turbulence strength u_rms and the turbulence intensity u_rms/u^* are described by equations 2 to 4. In these equations, u^* denotes the shear stress in terms of a velocity scale. The turbulence intensity is a measure of the degree of turbulence. A large value for u_rms indicates a high level of turbulence. Where root-mean-square (rms) represents the standard deviation of the random velocity fluctuations u_i^' (Baki et al. 2015). u_i^'= (u_i - ¯u) (2) u_rms= √ (1/N ∑_ (i=1) ^N▒〖 (u_i - ¯u) 〗^2) (3) u_rms/u^* (4) In addition to the turbulence intensity, the kinetic turbulence energy along the ramp is determined using the following equation. The kinetic energy can be used to describe and analyze the energy evolution. The turbulence parameters can be determined equivalently for the v and w components (Baki et al. 2015; Ahmad et al. 2013). TKE=1/2 (¯ ((u_i^') ^2) + ¯ ((v_i^') ^2) + ¯ ((w_i^') ^2) ) (5) The setup of the numerical model was based on an experimental model scale. This model comprises a ramp with a length (L) of approximately 6.0 m and a width (W) of 1.6 m. Additionally, it incorporates an inlet basin and an outlet area. The ramp is composed of eight rows of blocks, forming seven basins (Fig. 1). To ensure accuracy, the mesh is refined in the area of the ramp and near the boundary layer. Velocities are tracked individually in each basin using history probes. The simulation incorporates multiple discharges. Fig. 1. Model structure of a block ramp. Results and discussion Figure 2 shows an example of the aforementioned velocity fluctuations generated by the turbulent eddies of the rough surface. It can be seen that the velocity component u (Fig. 2 left) is the largest and therefore has a significant influence on the system. The velocity u indicates that the system is near-steady, exhibiting only minor deviations from the mean value. Fig. 2. Velocity fluctuations of the individual velocity components u, v, w in basin 4 at a discharge of 400 ls-1. Fig. 3. Velocity profiles along the ramp for a discharge of 400 ls-1. Figure 3 illustrates the velocity profiles along the ramp at a discharge of 400 ls-1, with z representing the distance of the probes from the bottom. A profile is presented for each basin. It can be observed that the highest flow velocities u occur in basin 7. The results for basins 1 - 5 are close to each other and show a similar progression of fittings. However, the results for basins 6 and 7 exhibit significant discrepancies in comparison to the remaining basins. This is attributed to the distinct flow pattern observed in these basins that differs from the subsequent basins. Figure 4 shows the water surface above the ramp where the wave structures of basins 7 and 6 are easily identifiable. At a close distance from the bottom, recirculation (negative velocities) can be observed between the boulders (see Fig. 5). Furthermore, it is essential to highlight that two distinct velocity profiles are present within each basin. One is observed within the basins up to a boulder height of 0.2 m, and the other is observed after the boulders are submerged. This will be investigated in further analysis. Additionally, it can be seen that the velocities decrease with increasing proximity of the probes to the bottom, and that the results in the different basins become increasingly similar. This effect can be attributed to the boundary layer. Fig. 4. Water surface at the centre line of the ramp for a discharge of 400 ls-1 at 320 s. Fig. 5. Velocity streamlines in basins 7 - 4 with a discharge of 400 ls-1 at 320 s. Outlook For further and more detailed investigation, other different discharges are simulated. The velocity profiles are then determined and compared. The objective is to compare not only the profiles in the individual basins, but also those of the different discharges. Further history probes will be added to increase the resolution of data points in the results. The turbulence intensity and the kinetic energy along the ramp will be calculated and evaluated. Further insights into turbulent flows on block ramps will be provided. References Ahmad, Z. ; Sharma, H. ; Westrich, B. (2013). Turbulence Characteristics of Flow over a Block Ramp. In: Journal of Water Resource and Hydraulic Engineering, 2 (1), pp. 21–29. Baki, A. B. M. ; Zhu, D. Z. ; Rajaratnam, N. (2015). Turbulence Characteristics in a Rock-Ramp-Type Fish Pass. In: Journal of Hydraulic Engineering, 141 (2). Dizabadi, S. ; Azimi, A. H. (2019). Hydraulic and turbulence structure of triangular labyrinth weir-pool fishways. In: River Research and Applications, 36, pp. 280–295. Jalalabadi, R. ; Stoesser, T. (2022). Reynolds and dispersive shear stress in free-surface turbulent channel flow over square bars. In: Physical Review, 105. Oertel, M. (2012). Cross-Bar Block Ramps: Flow Regimes, Flow Resistance, Energy Dissipation, Stability. Professorial dissertation. Bergische Universitat Wuppertal. Oertel, M. ; Schlenkhoff, A. (2012a). Crossbar Block Ramps: Flow Regimes, Energy Dissipation, Friction Factors, and Drag Forces. Journal of Hydraulic Engineering. 138 (5), pp. 440–448.
Year: 2025