If you're going ahead into this venture with a view to tick a box, that's probably too thin a motivation and won't survive the test of both time and cost of opportunity. The latter will hit you hard when you find yourself stuck in a rut.

My advice to you, should you wish to embark on this risky journey, is to be absolutely clear about your goals and carefully ponder whether you want your thesis to be in engineering or in science. Fluid dynamics is broad spectrum with its ends reaching wide into both fields. I mention this because there's a chance you'd find yourself with the wrong audience if you came up with something absolutely brilliant from an engineering viewpoint but that would only carry marginal weight to more basic scientists. And vice-versa.

Of course I don't know you at all but the description of your current situation seems to suggest that you are currently in comfortable albeit no longer challenging position. Perhaps this is actually an opportunity for you to flip this around and start exploring potential research projects while retaining your day job.

Also this may or may not be an option for you but consider doing a PhD in Switzerland or Norway. They have indecently competitive salaries and their research ecosystem is excellent plus you can easily get by speaking English alone.

Norway has many fluid-related research projects linked to their oil and gas platforms activities.

With respect to your seniority, the only flag I'd raise is not to underestimate finding yourself surrounded with younger fellows. If you've gotten accustomed (and perhaps rightly so) to certain life standards and have achieved the status and maturity levels that I suspect, do ask yourself if being back to school with ``kids'' is not going to take an unsuspected toll on your morale.

I think most views expressed here will be helpful. At the end of the day, everyone's different and a happy research experience rests on so many factors that no single opinion can possibly accurately summarize.

All the best, I wish you well.

If you're going ahead into this venture with a view to tick a box, that's probably too thin a motivation and won't survive the test of both time and cost of opportunity. The latter will hit you hard when you find yourself stuck in a rut.

My advice to you, should you wish to embark on this risky journey, is to be absolutely clear about your goals and carefully ponder whether you want your thesis to be in engineering or in science. Fluid dynamics is broad spectrum with its ends reaching wide into both fields. I mention this because there's a chance you'd find yourself with the wrong audience if you came up with something absolutely brilliant from an engineering viewpoint but that would only carry marginal weight to more basic scientists. And vice-versa.

Of course I don't know you at all but the description of your current situation seems to suggest that you are currently in a comfortable albeit no longer challenging position. Perhaps this is actually an opportunity for you to flip this around and start exploring potential research projects while retaining your day job.

If the financials of it is your main concern, this may or may not be an option for you but consider doing a PhD in Switzerland or Norway. They have indecently competitive salaries and their research ecosystem is excellent plus you can easily get by speaking English alone. In Switzerland semester fees are so (ridiculously) low that student debts are virtually unheard of. I suspect the same can be said of Scandinavian countries.

Norway has many fluid-related research projects linked to their oil and gas platforms activities.

With respect to your seniority, the only flag I'd raise is not to underestimate finding yourself surrounded with younger fellows. If you've gotten accustomed (and perhaps rightly so) to certain life standards and have achieved the status and maturity levels that I suspect, do ask yourself if being back to school with ``kids'' is not going to take an unsuspected toll on your morale. That might sound arrogant and haughty but you'll know what I mean if you get there.

I think most views expressed here will be helpful. At the end of the day, everyone's different and a happy research experience rests on so many factors that no single opinion can possibly accurately summarize.

All the best, I wish you well.