## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### How to input a periodic excitation to a flow or jet

• Kauê Castro
Subscriber

Im a beginner in Ansys, Im trying to get the hang of all it's features.. I need to learn on how to input an sinusoidal excitation to a jet, perhaps someone could help me?

Best regards
Kauê C.

• Rob
Ansys Employee

Have a look at profiles, expressions and DEFINE_PROFILE UDFs. I'd typically favour the latter, but expressions are possibly a better tool for you to learn.

• Kauê Castro
Subscriber

Hey Rob, I couldn't find my last post so I tried reposting it (sorry for the complication).
Anyway, I'll have a look at all three of those cited to implement them.
As you've asked on the other post, I want to excite the jet in the range of 10-200 Hz. The Model is really really simple as in the image im uploading below

It consists of a velocity profile coming out generated by an other internal flow simulation.

Basically, my Idea was to excite the inlet region in blue in the top and bottom ends aiming to deflect it, similar to this Ilustration without the diffusers:

(Out of the article "Control of a Plane Jet by Fluidic Wall Pulsing" By J. C. Béra, Et al)

I'll post as soon as possible as a reply to this post the contours of velocity & velocity vectors. You've previously asked if I was running transient, and actually I'm initially trying to run steady, and eventually progressing to transient. Also, I'll have a look for fluidic switches as you had said.

Take care..

• Rob
Ansys Employee

The vibrations are a little more complicated. You may need to look at moving mesh as you won't have a flow boundary.

• Kauê Castro
Subscriber

Hey Rob, as you previously asked, the velocity contours map:

As you can see, a recurring problem I'm having is the reversing of the flow on the BC (using outflow), one solution I was thinking of is making the second derivative 0 for the cross-stream directions, however I didn't find where I could change that, in the case it's possible.

• Rob
Ansys Employee

Don't use outflow, it's a really bad idea generally, and even worse with backflow. You want a pressure outlet.

Now, backflow is also going to happen with that bc, and I'll leave you to explain why (in reality) it's happening.

• Kauê Castro
Subscriber

Im sorry, I actually don't know haha, is it due to eddies formed curling inwards to the jet?

• Rob
Ansys Employee

Sort of. You're sending higher speed flow into a confined space. That will entrain additional fluid, and that fluid has to come from somewhere. In this case the fluid comes from outside the domain.

• Kauê Castro
Subscriber

For me it still doesn’t make sense, intuitively I’d think that the development of the jet would seem something like this:

I tried prolongating the domain and it didn’t change anything, so probably the problem is due to turbulence intensity aaa. Nevertheless, thank you for the help Rob, you’ve been extremely helpful and im very grateful.

• Rob
Ansys Employee

Not so much turbulence, the same phenomena occurs in laminar jets. Plot vectors on the above model, and look at the flow around the jet. If you block that flow (from the upstream) how will the flow around the jet behave?

There are a couple of modules on Learning that may help explain.