Learning Forum

[abd fs]

Hello, dear committee,
To control the recirculation zone at a diverging channel, backward-facing step geometry. I used a synthetic jet where his position is in this stage.
I found in publications that 30 periods were used for the blowing-suction of this jet and that I had also respected this condition. My problem is that in this publication, the synthetic jet takes only 0.1s so that the results converge around a gain of -24% over the distance.
the initial attachment after this gain has remained fixed. Contrary to my situation, I find that the response time of my plane is too long close to 0.3 s and that the gain always increases. I averaged the CF after each period.
Do you have an idea to solve this problem?

[DrAmine]

I do not understand your question? Please add more details and details about the software you are using.

[Rob]

I think it's a fluidic switch? If so check your geometry against the paper too: the dimensions and angles are critical.

[abd fs]

Dear experts,

My work area is aerodynamics, I use active control with the help of an actuator, This actuator is a synthetic jet which is part of the fluidic actuators, it sucks on half a period then it blows on the other half period. To model this membrane,

a C code will be made and injected into FlUENT as a velocity profile. To simplify the problem, the synthetic jet is considered as an input velocity (positive then negative) placed on a surface representing in the attached image.

 

My Flow is unsteady, I use RANS and K-W SST for closing. I carried out a comparative study between the case without control (without jet) which gave me Xr / h = 5,92 and a case with control (with a jet) which gave me Xr / h = 4,7.

My problem :

The response time of my synthetic jet is late. It takes almost 15 periods (15 blows and 15 aspirations) for the reattachment distance to decrease by -20%, but they continue to increase. on the other hand, in the scientific articles, it takes only five periods for the distance of attachment to converge towards -20% compared to its position in the case without control.

I averaged the coefficient of friction in each period to see the evolution of the point of attachment.

- What are the parameters that influence the response time of the jet?

- Do you have any tips to speed up the response time of my jet?

Best regards,

 

[Rob]

We can't help much with the engineering side of things: shame as my PhD was in power fluidics.

Check around the jet, if you're blowing through an inflation layer you need to review the aspect ratio of the mesh.

[abd fs]

Dear rwoolhou,

Thanks for your quick answer if you allow, can you explain this sentence to me:

"Check around the jet, if you're blowing through an inflation layer you need to review the aspect ratio of the mesh"

Inflation

 

Best regards,

SER

[Rob]

I think you need to review the mesh quality at the inflation-tri mesh position.

If you look in the various books on meshing you'll see aspect ratio mentioned. In the inflation zone we have a very high aspect ratio, but as the flow is aligned this is permissible. Where the flow crosses the inflation zone (separation point etc) we need to shorten the cells to give a good solution. 

In the image above the aspect ratio looks fine, but the cells adjacent aren't, and the jump in cell size isn't recommended. 

[abd fs]

Dear rwoolhou,

Following your answer, I want to introduce my mesh quality.

if there are any errors, please give me some tips to resolve these errors.

I received this message always.

 

Does this message influence the results? If yes, how can I solve this problem? 

I would be very grateful for your attention.

Best regards,

SER

[Rob]

Possibly: it depends where the high aspect ratio cells are. Click on "Controls" in the graph and alter the scale. Then pick the highest few columns and see what's going on.