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Student Submission for the 4th OpenFOAM User Conference 2016, Cologne - Germany
Numerical Investigation of a Fish-friendly Weir withOpenFOAM
Stephanie Müller
Otto-von-Guericke Universität Magdeburg, Universitätspl. 2 39106 Magdeburg,[email protected]
The fish-friendly weir combines fish protection and local renewable energy generation by deceleratingthe stream velocity in a vortex pool by the means of a dedicated turbine. Hence, fish can ascend anddescend through the slowly rotating blades without damage. A first pilot plant was built in Saxony in thegrayling fish region. This plant was used to replace a conventional fish ladder and produce energy for upto ten households. In addition it promotes the renaturation of landscapes with a concern for ecology andthe regional economy [1]. In this way the fish-friendly weir contributes to a sustainable development.
The Laboratory of Fluid Dynamics and Technical Flows of the University of Magdeburg “Otto-vonGuericke” has started the numerical investigation of the fish-friendly weir as part of the research action“Wachstumskern Flussstrom Plus”. This research action deals with energy generation using stream waterpower plants with low ecological impact [2].
The analysis of the fish-friendly weir was carried out using a multiphase simulation. Among differentsolutions, the open-source toolbox OpenFOAM 2.3.1 has been used as part of the work carried outduring a project at Master level.
The fish-friendly weir was simulated without turbine. One objective of the work was to investigate thevortex position for different volume flow rates. To do this, two simulations were performed; one with 700liters per second volume flow rate, and the other with 850 liters per second. The maximum velocity ofthe inlet is limited to 1.08 m/s. The initial values are defined by site-specific values of the pilot plant.
In order to obtain a constant water level a ramp function for the volumetric flow rate of the water inletwas included into the boundary conditions. Within the first ten seconds the volume flow rate rises fromzero to the given value. At the outlet a reversed condition was used. Additionally, a horizontal pad wasincluded at the inlet to control the water level and limit the return flow.
To allow the integration of an adaptive mesh, the multiphase solver interDyMFoam was selected. Themesh was refined by defined values of alpha.water; thus, by the phase boundary. The domain geometry,which reproduces an experimental model of the weir, was re-worked using SALOME. The computationmesh was then constructed with snappyHexMesh and contains around 2 million cells before meshrefinement; after the refinement, this number reaches 4 million cells. The k-omega-SST turbulence modelwas chosen and an adaptive time step was used for computation. The simulation was computed carried
Student Submission for the 4th OpenFOAM User Conference 2016, Cologne - Germany
out on SuperMuc with two node of 56 processors each. The simulation run on Haswell. Thecomputational time amounts to around 10 seconds of physical time per computing day; without adaptivetime step the computational time increased to around 2 seconds of physical time per computing day.
Results
The evaluation was performed at the time of 120 seconds (for volume flow rate 850 liters per second)and 90 seconds (for volume flow rate 700 liter per second) with the open-source tool ParaView. Duringthe evaluation, the maximum swimming speed of the grayling fish was found in the domain. This fishtype was chosen because of the region where the pilot plant is situated. Important swimming speedswhich were tracked are the minimal swimming velocity, the maximum swimming velocity and the sprintvelocity, which is the velocity the grayling fish can overcome for only a limited time.
Figure 1 and 2: left: minimal swimming velocity and right: maximal swimming velocity of grayling (volume flow rate: 850 liters per second)
Figure 1 to 3 represent the free surface and athreshold with all cells within the specifiedvelocity range. At the core of the vortex, theangular velocity grows with increasing radius. Itreaches a maximum, and then decreases in thedirection of the pool edge.
Figure 3: sprint swimming velocity (volume flow rate: 850liters per second)
Student Submission for the 4th OpenFOAM User Conference 2016, Cologne - Germany
Afterwards the tangential velocities were compared with experimental measurements from the diplomathesis of P. Lippitsch [3] and with the theory of Einstein-Li [4].
As part of the diploma thesis, P. Lippitsch performed measurements on the pilot plant. The tangentialvelocities were measured at 12 different water depths and 9 measuring points from the pool edge to thevortex center, for a flow rate of 700 liters per second.
Figure 4: slice through the vortex pool with the tangential velocities (volume flow rate: 850 liters per second, time step: 120seconds)
To compare the experimental results with simulation output, the tangential velocities at fixed waterdepth have been extracted with ParaView and plotted over the distance from the pool edge to the vortexcenter. The mean of those samples was taken as a single curve. First, this was done for the volume flowrate of 700 liter per second. The results are displayed as the green curve in Figure 5. The yellow curvedisplays experimental data reported in the diploma thesis. The red curve shows a fit function of theexperimental data. The trend shows a good agreement between 1 to 2.5 meters. Between 0 to 1 meter,significant divergences can be seen. The yellow and the red curve represent mathematical trends basedon experimental measurements. The trend functions have been extrapolated from the measurementpoints; no raw data were given in the diploma thesis. It is thus uncertain, if the curves correspond to thedirect measurements in the vortex core or to an extrapolation.
In addition a further simulation with a volume flow rate of 850 liters per second was done. For this caseno experimental data exists, but a comparison with the theory of Einstein-Li [4] was carried out. A goodagreement between the theory and the simulation values could be observed, as shown in the Figure 6.
Student Submission for the 4th OpenFOAM User Conference 2016, Cologne - Germany
Figure 5: plot of the tangential velocities over the radius of the vortex pool (volume flow rate: 700 liters per second, timestep: 120 seconds)
Figure 6: plot of the tangential velocities over the radius of the vortex pool (volume flow rate: 850 liters per second, timestep: 120 seconds) and comparison with Einstein-Li [4]
Student Submission for the 4th OpenFOAM User Conference 2016, Cologne - Germany
Finally, the position of the vortex was evaluated. The experimental data were compared with thesimulation data at the time of 120 seconds with a volume flow rate of 870 liters per second, retainedbecause of the given site-specific values. The comparison showed a difference in the location of thevortex between experiment and simulation. The experimental location was reported as (0.115m, 0.125m)and the position of the simulated vortex is (-0.166m, 0.648m) relative to the center of the vortex pool. Itmeans that the simulated vortex is located in the left maxillary quadrant and the measured vortex islocated on the right side. It results in a relative deviation of around 12 percent between the vortexpositions. The same comparison for a volume flow rate of 700 liters per second also resulted in a similardeviation. A reason might be that the vortex in both simulations is not fully developed yet, as suggestedbe Figure 7, showing the height of the vortex versus time.
Figure 7: plot of the vertical vortex position versus time (volume flow rate: 850 liters per second)
Student Submission for the 4th OpenFOAM User Conference 2016, Cologne - Germany
Conclusion and Perspectives
A simulation of the fish-friendly weir without turbine was conducted and the velocity fieldscorresponding to the grayling fish and the vortex position were investigated with OpenFOAM. The vortexposition differs from the experimental results of P. Lippitsch [3]. This might be due to insufficientconvergence, so that the simulation still has to run further until a final position of the vortex can bereached. Until now it is unclear how deep the vortex will reach into the outlet of the weir and if there willbe a final vortex position. A new experimental plant is planned that will enable reliable measurements ofthe velocity field and the vortex at different volume flow rates. Further work will include a variation ofthe weir geometry.
Bibliography
[1] http://www.fischfreundlicheswehr.de/de/kontakt-presse/presse/262-innovationswettbewerb.html
[2] http://www.flussstrom.eu/
[3] diploma thesis P.Lippitsch: “Funktionskontrolle des “Fisch-freundlichen Wehrs” im Bezug auf die Fischdurchgängigkeit; University of Görlitz/Zittau; Ecoligent; February 2013
[4] http://www.shf-lhb.org/articles/lhb/pdf/1955/06/lhb1955047.pdf
