Comparison between CFD results and experimental data

In summary, the conversation discusses the comparison of results from a RANS simulation of a round jet of methane and air to experimental data. The question is whether the experimental data for round jets of water can be compared to the simulation results if the Reynolds numbers are the same. The response is that this may not be a good comparison due to differences in behavior between water and air jets. However, it is possible to compare measurements of an air jet in air to a water jet in water after scaling the results properly. There are many papers available on measurements and simulations of round and planar jet flows, some of which have been used to derive constants in turbulence models. The best paper on measurements in turbulent round jets is mentioned for further reference.
  • #1
exergetic
12
0
Hi everybody,
my question is not properly about CFD but mainly about Fluid Dynamics itself. I have performed a RANS simulation of a round jet of methane and air and now I'd like to compare those results to some experiments. Unfortunately there aren't papers about experimental data for methane/air jets but mainly I'm finding a lot of data about round jets of water with different nozzle dimensions. So my question is: can I compare those experimental data with my results if the Reynolds number are the same? Or is it highly incorrect?

Thank you guys!
 
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  • #2
I don't believe that would be a good comparison. Unfortunately, this is the most troubling aspect of CFD: model validation.
 
  • #3
A water jet in air will behave different from a water jet in water. An air jet in stagnant air will behave in the same way as a water jet in stagnant water. This means you can compare measurements of an air jet (in air) with a water jet (in water) after scaling the results properly. There must be hundreds of papers describing measurements and simulations of round and planar jet flows. The constants appearing in turbulence models were derived from such measurements! And you will quickly discover that even for such simple cases, some turbulence models will completely fail to predict even the most basic properties of such jets, like the spreading rate.

The best paper on measurements in turbulent round jets is
Hussein,Cap,George, Velocity measurements in a high-Reynolds-number, momentum conserving,axisymmetric, turbulent jet, Journal of Fluid Mechanics 258 (1994) pp. 31-75
As soon as you ignite your jet, you have to use measurements done in a reacting flow, off course.
 

1. What is the purpose of comparing CFD results and experimental data?

The purpose of comparing CFD results and experimental data is to validate and verify the accuracy of the numerical simulations. By comparing the two, scientists can determine the level of agreement between the simulations and the real-world measurements, and identify any discrepancies or areas for improvement in the CFD model.

2. How do CFD results and experimental data differ?

CFD results are obtained through numerical simulations using mathematical equations, while experimental data is collected through physical experiments and measurements. CFD results are typically faster and more cost-effective to obtain, but may not always accurately reflect real-world conditions. Experimental data, on the other hand, provides direct measurements of the physical system, but can be time-consuming and expensive to obtain.

3. What are the main challenges when comparing CFD results and experimental data?

One of the main challenges when comparing CFD results and experimental data is ensuring that the two are directly comparable. This requires careful consideration of factors such as boundary conditions, model assumptions, and experimental uncertainties. Additionally, there may be discrepancies between the two due to limitations in the CFD model or experimental setup.

4. How can discrepancies between CFD results and experimental data be addressed?

If discrepancies are found between CFD results and experimental data, the CFD model can be refined by adjusting parameters, improving mesh resolution, or using more accurate turbulence models. Alternatively, experimental procedures can be modified to reduce uncertainties and improve accuracy. In some cases, a combination of both approaches may be necessary.

5. What are the limitations of comparing CFD results and experimental data?

Comparing CFD results and experimental data has some inherent limitations. CFD models may not accurately capture all physical phenomena, and experimental data may be affected by uncertainties and external factors. Additionally, the two methods may not always be directly comparable, as certain aspects of the physical system may be difficult to simulate or measure. It is important for scientists to acknowledge and address these limitations when comparing CFD results and experimental data.

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