I am investigating the flow and heat transfer over a cylinder in a wind tunnel. I want to record the data that I need to calculate the flow characteristics, But how can I make sure that the steady-state condition is achieved? There are very small changes in measured parameters like velocity and we know that there is no change anywhere with time in steady-state condition.
The main issue you have is not the wind tunnel air flow but the heat transfer.
Generally the air flow in a wind tunnel can very quickly reach a steady state (for both open and closed wind tunnels). However, the problem I believe is with the heat transfer in the cylinder.
Depending on how you setup your experiment it will depend.
For example, if you are using an open wind tunnel, then the temperature of the air during the day will fluctuate. That will change the heat transfer gradient, and you will not (trully) have a steady state.
Another parameter that affects is if the cylinder is heated internally to a temperature or if its just idly sitting there. In the first case (internally heated) you will have a steady state when the external temperature of the cylinder stops to change temperature. On the other hand (not heated cylinder) you will have a steady state, when the cylinder internally has the same temperature with the air flowing around it.
As you can see, there are many variations, and you need to provide more information for a more useful answer.