How is CFD used in aircraft design? Qualitative vs Quantitative?

Question

I have learnt how to do CFD analysis but really I have little idea on what to do with the data to improve/optimise the aircraft design. One approach I can think of is to run lots of analysis for different geometries and compare the results. Another approach may be to visualise flow and get insights from it to proceed to the next iteration.

What I want to ask here is how it is done in the industry. Do major companies just make a lot of small changes and compare thousands of analysis or are there human minds involved after each design iteration using their insights to optimise the aircraft.

edit: How do smaller companies do the same?

Answer 1

In the industry CFD is used for both of the purposes you describe.

CFD simulations are run on existing and proposed designs in order to understand the flow over them, and then based on that understanding to propose design changes or make other alterations using "human minds".

Sometimes the same simulation is run on a variety of different designs, in order to establish which is most efficient.

The two approaches can be intermixed. For example a simulation may be run on a variety of designs to identify the few best ones. Then there may be additional analysis on those best designs, including visualization of the flow, analysis at higher resolutions, analysis of the design with different flow conditions. There may be additional analysis on designs that were expected to be good but weren't, in order to understand why they were good. The design of these analyses is part of what makes a good CFD egineer.

Answer 2

It really depends on what your objectives are. For design optimization, the currently available automated optimization methods (Adjoint and other topology methods) are not as good as a well experienced Aerodynamicist. There has been a lot of effort in improving these tools and creating more complex design optimization tools (such as neural net optimizers), however these tools do not "understand" the fluid flow and simply try and chase whatever metric is being targeted. A good aerodynamicist will be able to make informed decisions based on their interpretation of results in order to iteratively improve the design.

Simulating a large number of different designs and trying to compare the results can be an inefficient use of resources. This method can be useful when comparing a range of options on a similar concept (think simulating 5 options of winglet height to find the optimal). If changing multiple things, it is impossible to work out which change was the most beneficial - the general best practice is to just change one thing at a time.

Also, running simulations across a comprehensive range of roll, pitch, yaw & wing configurations is extremely expensive. A good Aerodynamicist will be able to optimize using simulations at one or a small number of operating conditions and be able to understand the impact of their changes across the entire operating envelope. An automated optimization method would have to simulate many more operating conditions in order to properly evaluate and compare design iterations.

Once a design starts to become more mature, simulating more operating conditions is needed to understand performance across the whole envelope. If the design performs poorly at certain conditions, or if any of the performance characteristics transition too abruptly, the design can then be improved to minimize this unwanted behaviour.

Understanding the fluid flow, and knowing how to optimize the design to meet requirements are the hardest skills to develop. This is what sets good and great Aerodynamicists apart.

Answer 3

The quick answer is it depends.

As for the small businesses doing aircraft design generally sweeps or trades will be run on a low to mid fidelity multidisciplinary design code to map the design space. There are many many factors that go into an optimized design, such as what is the purpose/mission for the aircraft, what are design considerations such as it needs to be quiet, carry a certain payload/passenger capacity, fly in specific conditions etc. Once the larger space has been explored for general configurations, designs identified as more optimal will be more closely analyzed. This is where CFD generally comes in. A more refined design will necessitate a higher level of fidelity analysis. Now you may be running CFD to get more accurate aerodynamics for specific conditions - acoustics, drag reduction, complex geometry etc conditions that just can't be captured at a lower level of accuracy.

This is an example of the aerodynamic design process from the small company I spent 6 years doing aircraft design at. It's only one example but it gives you an idea:

CFD is an art, grid generation is an art, aircraft design is really a craft. It is not turn key, there are general rules of thumb and procedures to follow but in my opinion it would be extremely difficult to remove the human factor from this process if you are trying to achieve an optimal design.