Free Surface Flow Simulation with ACUSIM in the Water Industry Tuan Ta Research Scientist, Innovation, Thames Water Kempton Water Treatment Works, Innovation, Feltham Hill Road, Hanworth, TW13 6XH, UK. tuan.ta@thameswater.co.uk Abstract In the Water industry, almost all is free surface flow. A slow varying water surface is found when filling and emptying a water storage tank; still slow turbulent flow is found in the flow distribution channels, and higher turbulent flow is found in the pump sumps. Understanding the free surface flow is important for the engineering design and for the process performance optimisation. Current computational fluid dynamic (CFD) simulations experience difficulties with free surface flow problems. Assumptions have to be made to simplify the problem into single phase calculations. More models using Volume of Fluid method and Eulerian-Eulerian multiphase are becoming available but, in these models, there remains the uncertainty in locating the water surface. ACUSIM s free surface modelling capability is seen as an alternative. This paper presents the results of current investigations. Keywords: Free Surface, Outfall, Pump Station, Vortex, Service Reservoir, Water Jet 1.0 Introduction Many water treatment processes employ an open top tank operating in a steady flow condition. As the feed and discharge are equal, the water surface is flat and horizontal. This steady flow condition ensures the treatment processes to have sufficient time to operate. The flat water surface has little influence on the flow pattern in the tank and, in most cases, only allows the removal of floatable (low density, air bubbles) objects. The inlet and outlet are submerged and as the flow is normally low (velocity <0.1m/s), they do not affect the water surface. The CFD model of the tank simply assumes the water surface as either zero shear stress fixed wall or symmetry plane with zero vertical velocity component. Examples of these treatment plants include the sedimentation tank, the dissolved air flotation tank, the ozone tank and the disinfection contact tank. Tanks used for water storage are subject to variations of water level. A service reservoir is used to store clean water in the water distribution networks. The varying reservoir water level determines both the flow control and the mixing, which are important for reservoir operation. Storm tanks are used to hold excessive storm water also operate with varying water levels. The performance of the storm tank operation can only be evaluated if the water level is simulated accurately. Simulation of the water surface movement using the Volume of Fluid (VOF) method is difficult because the vertical dimension is normally much Altair Engineering 2011 1
smaller than the horizontal dimension. Furthermore, the water level can only be extracted from a selected volume fraction value (e.g. 0.7), and the result can be different from that of other volume fraction values. Tanks used for distribution of flow are normally operated at higher flow (velocity ~ 1m/s) and have varying water surface. The main objective of the distribution channel is to deliver flow to many parallel streams at controlled rates. The feed channel to an array of filters is an open flow channel. The water surface is flat but with gradient sloping downward away from the inlet end. The design requires an even flow distribution (variation < 5%) to each unit, which in turn is determined by the water level. In many cases, engineering formulae cannot be applied because of the complex geometry of the arrangement. The distribution chamber is used to distribute the flow to a number of sedimentation tanks, and again correct feed is essential. The small (~ the inlet dimension) chamber causes the varied water surface, which determines the discharge over the weirs to the units. CFD simulation must be employed for the design calculation. For the distribution of flow in the large pump suction chamber, (> the inlet/outlet dimension), the flow pattern is determined by the geometry of the tank. Each outlet, which is the suction pipe, creates a local downward flow. This downward flow then gives rise to vortices if the horizontal flow circulation due to the local geometry exists. Simulation of the vortices and/or potential to vortices is required to determine the lowest operating water level of the suction chamber. Current CFD simulation assume a flat water surface and simply calculates the potential of vortex formation. For better results, free surface model is desirable. Examples of the suction chambers include the pump chamber of water intake, the pumping station and the sewage transfer station. Higher flow velocity (>2m/s) flow causes a highly turbulent water surface. This flow condition is found in the outfall and the sewage tunnels. A Multiphase Eulerian-Eulerian CFD model is required although recently the VOF model has become more available. In this paper, the following examples will be discussed Design of an Outfall Simulation of the pump suction chamber Simulation for service reservoirs The comparison of the simulation and the experimental results are visual only. Quantitative measurements are presented elsewhere. 2.0 The Design of an Outfall Outfall is used to discharge water into rivers or an open flow area. The concern arises when the discharge flow is high (> 2m/s) as it may cause structural damage, scouring and flooding. Figure 1 shows an example of an outfall. The flow (>8m/s) is fed at the two inlets into a chamber and is subsequently discharged over a weir into the river. Altair Engineering 2011 Free Surface Flow Simulation with ACUSIM in Water the Industry 2
The design of the outfall requires: Figure 1: Geometry of an Outfall Flow velocity over the weir to be sufficiently low. This is determined by the length of the weir when the flow is evenly distributed. Small footprint for low installation cost. Simplicity for low maintenance. Figure 2 shows various designs which were investigated. Free surface simulations of the flow is required to demonstrate the effectiveness of the preferred option. Figure 2: Various Designs of the Outfall Altair Engineering 2011 Free Surface Flow Simulation with ACUSIM in Water the Industry 3
Figure 3 shows the result of the simulation for the option using wedges. The simulation was carried out using ACUSIM software, part of the HyperWorks suite of CAE software. Figure 3: Simulation result and Installation 1. The model was transient turbulent (Spalart-Allmaras) flow and arbitrary mesh movement option. 2. The time increment is auto between 10-5 0.1s. 3. Gravity was included. 4. The top surface was specified as free surface option with surface tension. Initially this surface was flat. 5. Side walls and wedges were specified wall and slip as mesh displacement type. 6. Outlet was outflow and slip as mesh displacement type. In comparison with VOF model, the free surface option of ACUSIM software gives a clear indication of the water surface, which is required to determine the height of the outfall walls. At low flow, however, the free surface hits the outlet weir causing severe mesh distortion error. Only recently VOF modelling is becoming robust so that the solution will converge. Eulerian-Eulerian modelling has also developed. The advantage of this method is that the flow into an empty outfall could be simulated. In both models, water surface can only be obtained by assuming a value of volume fraction. Altair Engineering 2011 Free Surface Flow Simulation with ACUSIM in Water the Industry 4
3.0 Pumping Suction Chamber: Figure 4 shows the tank being used as the pump suction chamber. Water was delivered into the chamber via four penstocks, and water was pumped out of the tank via seven 90 o downward pipes with a bellmouth. Not all the penstocks were opened, and not all the pumps were in operation all the time. Figure 4: Pump Suction Chamber The ACUSIM model was set up using the setting as shown in the outfall model (section 2). The outlets (the bellmouth openings) were specified as fixed mesh displacement types. Because the flow entered the tank in various directions, horizontal flow circulation was expected. At low water level (<1D pipe above the bellmouth), the downward flow caused the vortex to form from the flow circulation in the vicinity. The result of the simulation indicated that the water surface was lowered in some areas but the vortex was not observed. Figure 5 shows the formation of the vortex in a different arrangement. The vortex caused the air to enter the pump. It is observed that the water surface is essentially flat and, for this experiment, the flow velocity is low. The local flow circulation could hardly be seen without the vortex. Once the air was sucked into the outlet pipe, the vortex disappeared and then formed later. Altair Engineering 2011 Free Surface Flow Simulation with ACUSIM in Water the Industry 5
Figure 5: Vortex Formation The current ACUSIM model showed clear indication of the water levels and the flow circulation but simulation of the vortex has not been achieved yet by the author. To investigate further, an experiment has been set up (Figure 6). The cylindrical tank was used with the inlet pipe being tangential to the circumference and the outlet was an opening at the bottom. Initially the tank was fed from partially full to full with closed outlet. The feed was then stopped to leave the circulation to slow down with flat water surface. The outlet was then opened to observe the vortex formation. In the early stage, the vortex was simulated, the visual agreement with the observation is shown the figure. In the later stage, the simulation was stopped because of severe mesh distortion error. The vortex, however, was observed to continue to develop (Figure 6 B) to form an air column at the centre of the tank. Figure 6: Simple Arrangement to check the Vortex Simulation Altair Engineering 2011 Free Surface Flow Simulation with ACUSIM in Water the Industry 6
4.0 Service Reservoir A service reservoir is used by the water industry for the storage of drinking water. The arrangement is simply a tank with a few inlet and outlet pipes (Figure 7). The water level in the tank rises when the tank is filled and falls when the tank is emptied. Figure 7: Geometry of a Service Reservoir The tank must be mixed to ensure that there is no flow stagnation zone causing excessive water age. Mixed condition is also required during the disinfection boosting. The mixing of the reservoir depends on the inlet arrangement and is encouraged by the movement of the water level. For most tanks, the inlets and outlets are submerged and the water surface is a moving (up and down) flat surface. CFD modelling of these tanks is carried out either using the deforming mesh approach or the ACUSIM free surface model. For some tanks, the inlet is at the high level (Figure 7). The inlet with a ball valve delivers flow as a downward jet onto the bulk water. For other tanks, the inlet is a vertical pipe with a bellmouth, which causes the flow to cascade downward along the pipe when the inlet is not submerged. These inlet geometries are to be modelled using the free surface simulation. For design calculation purposes, single phase modelling can only be used by assuming the inlet of appropriate area at the water surface. Figure 8 shows the ACUSIM simulation of a downward water jet on a water tank. Initially the jet was a cylindrical pipe with a pipe wall specified as free surface. The setting was the same as in sections 2 and 3. Visual agreement with simple experimental arrangement (not shown) was observed. Validation experiment is in progress. Altair Engineering 2011 Free Surface Flow Simulation with ACUSIM in Water the Industry 7
Figure 8: Simulation of Jet onto a Water Tank 5.0 Conclusions In comparison with VOF model, the free surface option of the ACUSIM software gives a clear indication of the water surface, which is required to determine the height of the outfall walls. It has been demonstrated that ACUSIM is a solution for simulating free surface flow problems. Altair Engineering 2011 Free Surface Flow Simulation with ACUSIM in Water the Industry 8