Simulation Software

Leveraging advanced 3D CAD-embedded CFD capabilities can add robustness and fidelity to a 1D system model.

By Mike Croegaert, Siemens Digital Industries Software

A common issue with modeling complex 1D systems is adding the required detail for complex components that cannot easily be represented through correlations or empirical data. To get the data for these types of components, engineers have two options: physical testing and 3D computational fluid dynamics (CFD) simulation and analysis. Physical testing can be costly and obtaining the data can be difficult for all possible operating scenarios. Whereas, 3D CFD can be time-consuming and often requires an expert to get reliable results.

Fortunately, we now have the option to use advanced 3D CAD-embedded CFD solutions in conjunction with 1D CFD tools to quickly and accurately characterize complex components without the need for a specialist, while providing an added level of robustness to a complex system model.

The basic fuel system: solving pump limitations
To illustrate the effectiveness of using this combination, a simple passenger aircraft fuel system example will be used. Figure 1 shows a 1D system model drawn on top of the basic fuel system schematic image. It includes source components for the boundary conditions and extra loss components for items such as filters and couplings. The thin link lines represent a direct connection between adjacent components, but they are not pipes themselves. Nodes sit in the middle of the links and serve as convenient points to enter elevation data and interrogate flow results of temperature and total pressure.

During normal operation, the fuel is drawn from the tanks with mechanical fuel pumps. However, most mechanical pumps must be fully wetted to function properly. This means sometimes unusable fuel is left in the bottom of the tank. To extract the residual fuel, jet pumps can be added with the suction side connected to the lowest sump point on the inner wing tank. This pump feeds the collector cell by using the motive energy provided by a small amount of high-pressure fuel bled directly from the mechanical fuel pumps. For this example, the jet pumps are placed in the lowest portion of the tank.

In contrast to most other components, this jet pump does not come pre-supplied with performance data. That leaves the designer with two options to define performance. The first method is to enter detailed geometry for the pump and the 1D CFD tool can apply a built-in empirical correlation for jet pump behavior. The second option is a more rigorous databased approach. The pump requires a curve of flow ratio versus head ratio and a curve of motive flow rate versus pressure difference between the motive and suction arms. This data can come from many sources, including the vendor of the pump, physical testing, or 3D CFD characterization and analysis.

3D-1D characterization
Sophisticated 3D CAD-embedded CFD programs have key technologies that facilitate 1D data generation. These tools can be used by the typical engineer as well as seasoned CFD analysts. They apply automated modified wall functions to capture boundary layer effects properly, regardless of the density of the mesh in the boundary layer. They also have an automated solver to determine the flow regime between laminar, turbulent, or transitional without intervention. The most advanced 3D CAD-embedded CFD software has a unique and automated mesher that is geometry aware. If the CAD geometry changes, the mesh changes automatically, updating as the problem solves, intelligently putting more mesh where it is needed. Because these tools are completely CAD-embedded, the designer can run parametric studies, by not only varying flow conditions, but also changing the actual geometry over the course of a study, feeding data back to the 1D CFD software as seamlessly as possible.

Using an intuitive graphical interface, the designer inputs the key variables—choosing the units, the physics to consider in the simulation (including heat transfer, gravity effects and rotation), the fluids to use in the simulation, and lastly, the initial conditions such as temperature, pressure, and initial velocity. The 3D software then calculates a computational domain surrounding the relevant fluid geometry. The design engineer can resize the domain area or even slice the volume in half and do an axisymmetric simulation to save computational resources. The mesh (Figure 2), although highly automated, can be manually driven to add grid cells where needed and manually refined by selecting specific geometry, adding optional control planes, or even disabled bodies into the CAD model to serve as the mesh structure.

Once the model is prepared, the boundary conditions set, and goals determined, the simulation is ready to run launching the solver. The solver is the only aspect of 3D CAD-embedded CFD that does not operate directly in the CAD environment. Instead, a second window is launched to monitor the progress of the solution. Custom-defined preview plots for parameters such as pressure, velocity, and even the mesh can be created. Goals can also be plotted to have a real-time monitor on current value and their trend toward convergence. Parametric studies can also be performed on multiple machines at once to improve performance.

Once complete, the results are loaded back into the CAD interface, the first example of a result is a cut plot that shows the contours of velocity with overlaid streamlines (Figure 3). The cut plane can be manually adjusted with a live preview. 3D CAD-embedded CFD can also generate three dimensional flow trajectories inside of the fluid region.

Typically, for characterizing a component using 1D CFD software, multiple simulations are needed to generate data over a range of flow conditions. Once the study is complete, the results can be saved to a file that acts as a raw record of the flow conditions at each boundary at each experiment point. Each point contains data for flow rate, pressure, temperature, density, viscosity, enthalpy, and heat capacity. From this data, non-dimensional curves can be created so that the software can extrapolate performance of the part over a range of fluids, temperature, and pressures.

After the curves are created, they can be imported into the 1D CFD tool either as an individual data curve or as a complete component. The software will parse the data and automatically determine what type of component to create. Once this step is done, the 1D CFD software adds the component to the catalog and automatically populates it with the data just generated.

Before using the component in the fuel system model, it will first be verified in a unit test. Unlike most components, components sourced from the 3D CAD-embedded CFD program require no further data to use because relevant data such as the curves are pre-applied. All that is required to run a unit test is to add boundary conditions and run an analysis, and the component should perform exactly as the software predicted during the characterization step. After the unit test has verified the component was created correctly, the jet pump can be added to the fuel system model (Figure 4), and the designer can run a series of analyses.

In this example, two transient simulation scenarios will be examined. In the first simulation, the jet pump has been included but blocked the motive flow, so the tanks will only feed into the collector cell via gravity. The connection between each section of the tanks is through a series of holes in the ribs that separate each compartment. These holes span almost the entire height of the tank, but the bottom 2-in. of the tank is blocked by structure.

As shown in Figure 5a with the blue line, the outer tank is draining briefly into the inner tank marked in red. The outer tank stops flowing as the level of fuel drops below the holes in the rib. Rendering the remaining 2-in. of fuel in that tank unusable. The inner tank in red, and the collector cell in green both drain together. The collector cell is drained via the mechanical fuel pumps, and the inner tank drains via gravity into the cell until it too reaches about 2 in. of height and can no longer drain. The tank is exhausted of all usable fuel in about 125 seconds.

The second case shown in Figure5b has the jet pump enabled. Here, the jet pump is moving fuel from the inner tank to the collector cell at a rate of about 10 gal per minute. In this case, the inner tank drains much faster, but that fuel is being transferred to the collector cell to feed the pumps. Because the suction inlet to the jet pump can be placed at the lowest point in the tank, it can drain almost all of the fuel out of it before it stops providing flow. At this point, the fuel level in the collector cell starts to drop quickly until it is completely exhausted. In this case, because more fuel is available, the tank does not drain for 320 seconds, a significant improvement.

Leveraging advanced 3D CAD-embedded CFD capabilities can add robustness and fidelity to a 1D system model. The 3D CFD solution does this by characterizing complex components within the CAD environment and then automatically importing the results into the 1D model. Here, a common issue with 1D system models of aircraft fuel systems was examined: how to add the required detail to the model for complex components that cannot easily be represented through correlations or empirical data. Siemens 3D CFD software, Simcenter FLOEFD, runs inside of several CAD packages including Siemens NX and Solid Edge, CATIA V5, PTC Creo, and a standalone version. This example also uses Siemens Flomaster 1D CFD software for the system simulation.

Until recently, designers had to choose between costly physical testing and time-consuming 3D CFD that usually requires an expert to obtain reliable results. This approach that combines 3D CAD-embedded CFD and 1D CFD can be used to quickly and accurately characterize complex components without the need for a CFD analyst and provide an added level of robustness to complex models such as the aircraft fuel system.

Siemens Digital Industries Software
www.sw.siemens.com