Fluid systems have been in existence in the engineering world; from ancient Roman public water systems to modern planes, trains and automobiles. Indeed, it is more correct to say that most modern consumer products are complex systems-of-systems that are a mixture of fluid, mechanical, electrical and control systems with varying degrees of interactions. When considering fluids, the term “Computational Fluid Dynamics” (CFD) has been around for over 40 years and it encompasses a wide range of commercial and in-house 1D, 2D & 3D computer-aided engineering (CAE) simulation software available to engineers today. In an earlier White Paper (1) we dealt with “What is System Level Thermo-Fluid Analysis?” This paper will focus on the 12 “pillars” of thermo-fluid system simulation, or 1D CFD, that are important or critical to good application of these tools.
In the last 15 years there has been huge productivity gains from the use of CFD and Computer-Aided Engineering (CAE) tools and 1D systems simulation software in particular as they have matured. Enabling frameworks such as PLM, Product Lifecycle Management, Mechanical Computer-Aided Design (CAD), and allied simulation tools such as Structural Analysis (or Finite Element Analysis, FEA) have allowed engineers to reach new levels of design and understanding, enabling much better manufactured end products particularly at component and sub-assembly level. A global challenge still exists to extend this knowledge and engineering knowhow to simulate the performance of complete systems and even systems-of-systems at every stage of the design cycle from Upfront Engineering through to Decommissioning.
Engineering thermo-fluid systems simulation products, such as Flowmaster® from Mentor Graphics, take into account the complex combinations of internal flows, thermal and mass transfer effects within a system’s component through 1D empirical or mathematical relationships of pressure drop, flow rate and temperature difference across the component, often based on specific component parameters (e.g. valve position or internal friction). Consequently, such tools are excellent at gaining an understanding of the effect of a single component on a wider system and how it contributes to the performance of the complete system. They will also predict how the complete system will perform under almost any condition, thus reducing down-stream errors, costly retrofits and rework.
We will now look at the first 4 of the pillars. These are the “Foundation” or core that all the other pillars are built around.
1 The New Age of Design Exploration
One of the most powerful aspects of using a 1D simulation tool is the versatility of the tools and their adaptability for use at any stage of the design process to explore options for improving the final design and reduce the design cycle. There are three distinct stages of design exploration where these tools can be utilized.
- Conceptual or Upfront Phase – Exploring Differing Scenarios & Innovating
Once a conceptual thermo-fluid system simulation model is available it can be used to explore a huge range of different scenarios involving pump sizing, pump trip evaluation, surge prediction, flow balancing, component sizing, priming, temperature distribution, flow distribution and pressure drops. Significant system changes may be proposed during the design optimization in order to eliminate potential problems or to optimize performance. In either instance, the system simulation tool enables an engineer to validate his or her proposals and to quantify the potential benefits before making a recommendation to adopt a new design.
Let’s take a typical safety critical 1D thermo-fluid system simulation scenario. Many aircraft fuel systems engineers want to explore the varying pumping options available to transfer fuel between various wing and main body tanks in order to maintain the correct trimming characteristics. Additionally, the ability to examine the capacity of the system to function in an emergency scenario, such as pump failure, needs to be evaluated to allow them to produce designs improved for weight, cost and safety. Ultimately, through 1D CFD, the engineer needs to complete this work before the main aircraft optimization phase where any changes or mistakes are more costly to correct.
- 2. Interaction Phase – Exploring Interactions between Differing Systems
As components interact within a single system, so too can many different types of systems interact within a complex product. Traditionally the effects of these interactions have only been investigated through physical prototype testing later in the design cycle. However, with 1D thermo-fluid system simulation tools that can model the various systems concurrently, it is possible to produce a full digital prototype to understand the effects of these interactions at an early stage. You may see many influences between systems, for example some aircraft manufacturers cool hydraulic fluid by passing the pipes through the fuel tanks. With a 1D CFD tool you can examine these influences between your systems very early on to highlight problems in the design process that might only be found at the test stage.
- 3. Optimization Phase – Design of Components that Meet System Requirements
Understanding the desired performance of the aircraft’s fuel system defines the needs for acceptable component performance; and components can then be selected or designed to meet those needs. There will then be a desire to feed back the performance of the actual components into the 1D system simulation for further verification. This can be done by inputting more detailed parameters and performance curves into the model.
As the design moves through these distinct phases, the 1D thermo-fluid model becomes more refined and provides more complete answers and allows the engineers to make informed decisions earlier in the process resulting in a better, safer, more complete end product.
2. Efficient Workflow Processes
Systems Simulation engineers want to ensure that their chosen 1D CFD simulation product can be used efficiently throughout their complete design process from concept to product operation, and it will integrate with all other aspects of their business, right through to their customers and suppliers on a day-to-day basis.
Having a data management structure within a 1D CFD tool that can support a company’s workflow is critical. The system must be flexible enough to provide the different consumers of the data access to the information they need, when they need it, without the chance of it being inadvertently changed or deleted. It must give the right people the right permissions at the right time.
Tools like Flowmaster bridge the experience gap between “Analysts” and “Designers” by allowing fluid networks to be “locked down” by Analysts and then passed to less specialized Design Engineers who only have access to certain aspects of the design. Parametric and View Only capabilities are available allowing non-analysts users to examine “what-if” scenarios in a secure and controlled environment.
This data management structure gives companies the confidence to allow designers to use analysis tools to make decisions earlier in the process through parametric studies and what-if scenarios on secure models. This frees up the analysts to concentrate on critical scenarios only, and reduces the bottleneck that can occur in simulation departments.
3. Leveraging Upfront Analysis
At the early conceptual phase of designing a system, product or process, you will typically have very little geometric data available. The detail of a given component is likely unavailable and probably unnecessary. Your major requirement is to have some measure (i.e. prediction) of how one or more systems will perform, thereby indicating the performance of the proposed product. Ideally you will want libraries of components populated with pressure loss data, allowing you to build a model of the complete system, which means that you can begin to simulate likely operating conditions with respect to pressure, flow and temperature. This system model actually requires very little geometric data and will, over time, gradually be refined as the design process develops. This is System Simulation with Minimal Data. The cost savings involved with upfront engineering simulation can be three orders of magnitude higher than those seen at the manufacturing stage as witnessed by this graph below from Ricoh:
The Cost of Reliability – fixing mistakes in product design at the early design stage is preferable
The benefits of understanding system requirements early, and upfront, are many and manifest. For instance, process industry companies and ship building organizations usually want to model their piping systems as best they can early in the design process; this will allow them to size specialist pumps and valves etc. enabling the purchasing department to order long lead-time components quickly and accurately.
This is where utilizing 1D CFD can lead to significant savings. These tools use predefined components that allow the user to construct the fluid systems being designed long before CAD geometry is available. With 1D CFD you can also be in a position to run powerful sensitivity studies at this early design stage to determine the effects of proposed design possibilities if you have a good system model. For example, tools like
Similarly, in the automotive industry, 1D CFD models of existing automotive cooling systems and those for proposed concept cars are typically created and run in 1D CFD Software toolsin order to troubleshoot problems and learn more about how the systems perform upfront of committing to the design. Such simulations can show that a chosen cooling strategy would not perform acceptably in all operating conditions thus allowing for changes before the first component is tooled. Without thermo-fluid system simulation these problems may not have been identified until late in the vehicle’s design, prototype testing, or even in operation – meaning an expensive recall with all its attendant costs and adverse publicity. This validation of concepts quickly and upfront with Flowmaster networks for instance that are not CAD geometry dependent is very valuable. 1D CFD simulation will reduce the number of physical prototypes and minimize costly test scenarios
4. ROI is King!
Justifying expenditure to purchase engineering simulation tools is sometimes a challenge; and producing a detailed Return on Investment (ROI) can be challenging at times. The preference being not to change processes that have worked in the past, processes such as overdesigning pipe systems by 20% for example. Of course, systems designers can do this, but there’s always a cost associated with doing nothing or overdesigning equipment. As manufacturing costs escalate, energy use comes under increased scrutiny, and safety needs to be taken into account more, this is no longer a viable way of working.
Typically thermo-fluid system simulation is one area of a company’s business where detailed knowledge has been difficult to assess in the past, yet it can be critical to improving the design, specification, performance, and cost control of products. This means a simulation tool that enables engineers to broaden and deepen their understanding of their system or process in order to act upon the information delivered, thereby creating increased business benefits, competitive advantage, and thus helping to maximize potential.
- A thermo-fluid simulation tool should aim to
- Improve time-to-market and design time;
- Enhance quality;
- Reduce product or plant’s risk of malfunction or failure;
- Increase product performance;
- Accelerate innovation;
- Lower design, production, and operational costs;
- Cut the need for physical testing; and
- Realizing productivity gains.
There are numerous case studies that demonstrate the cost savings of effective CFD design processes. In a paper co-authored by Wayne State University and Ford Motor Company, it is stated that the cost of a single prototype vehicle can cost in excess of $250,000 and a complex vehicle development program can require over 100 prototypes. If through use of a 1D thermo-fluid system simulation tool, a single prototype iteration (a 1% reduction) can be eliminated, the company can realize 2 to 5 times return on the investment in the simulation software. In another study by the University of Birmingham, researchers showed that Boeing reduced physical testing costs of the 787 by 30% as compared to the 777 development program through the use of simulation tools.
Using the example of a marine diesel engine lubrication system, one problem facing design engineers would be to understand the oil pump capacity required to supply specific components under differing loads and different temperatures as the viscosity of the oil changes. 1D thermo-fluid system simulation can be used to establish critical information, in particular what will the performance be like under known extremes of manufacturing tolerances or states of wear. Other factors can be dealt with such as, variations in oil flow the bearings will experience as the oil filter becomes blocked, and whether this acceptable for the chosen bearing. Consequently it will aid in determining how frequently the filter have to be changed or if an alternative filter will extend the service interval. The value of the knowledge gained from answers to such questions can be high for these everyday problems for shipbuilders and designers.
You can download the pdf version please follow this link 12 Pillars of 1D CFD – The Foundations