Bird Strike experiment reproduction by RADIOSS

Altair Forum User

 

Hi

 

I'm now working on BirdStrike Analysis using RADIOSS140.

Like many paper on birdstrike analysis reports, I've done a reproduction of birdstrike experiment carried out by Wilbeck in 1977 [1].

The resultant pressure PH(*1) and PS(*2) meets with theoretical value very well.
Except that,if you look at the pressure curve, there is a 'ZERO Pressure' period just after the first contact.
Taking closer look into the animation, the SPH bounce back just after the contact.

Such blank period doesn't appear in the experiment result,or analysis results of other researchers report.

 

[Question] : Is there a way to correct this ?

 

Model setting is follows.

-bird is using SPH. Material law is MLAW6 (Equation of State).

-Using contact TYPE7.
  Istf = 1
  StFac = 550 … recommended value by Altair Support
  Igap = 0
  gapmin = h/2 (h = SPH radius) … recommended value by Altair Support

I've varied the value of these parameters which didn't make any difference on 'ZERO Pressure' period.

 

*******************

  *1 … PH: the peek pressure just after the contact(Hugoniot Pressure)
  *2 … PS: the constant pressure when bird deformation can be seen as constant flow.(Stagnation Point Pressure)

  [1]:J. Wilbeck,AFML-TR-77-134 1977

Altair Forum User

 

Hi Yoshida,

The SPH model seems to be bouncing in this case. What is the initial velocity applied(specify units also)?. As a workaround can you vary the Gapmin value and try. 

I just created a model as seen in the image and tried (Units followed is Tonnes,mm,sec). And I don' t see any bouncing behavior in the model. Model is attached along for your reference. I didn't run the model for complete run time, but I was just checking the behavior. 

Altair Forum User

 

Dear Johnson

 

Thanks for the quick reply.

 

Initial velocity is 200m/sec,as the experiment is done in 100 to 300m/sec range.
Unuits used is [ton/mm/sec] ,so the same as your model.
I've attached my .hm file. If you find any suspicious or incorrect settings,Please Let me know.

 

I've varied some parameters before (including gapmin) but it didn't resulted in major differences.
But I'll re-check in wider range of value.

 

Other difference I noticed in your model is follows.

 

■1.Initial Velocity

Large difference(3m/s VS 200m/s) in InVel might affect the result.
I'm running your model with 200m/sec InVel to see if 'bounce' be observed.

 

■2.ANIM_DT

In order to draw a very smooth curve in HyperView, I set ANIM_DT as short as 5e-008 sec.
If the animation is corse, you might be missing the phase of bouncing.

 

■3.DTIX

The same reason as 2. ,I'm using '/DTIX' card to limit timestep smaller than 5e-009.

 

Anyway, I'll change Invel,ANIM_DT,DTIX in your model and see what happens.

Thanks.

Altair Forum User

 

Hi,

I just made the sphere component rigid and applied the same initial velocity (200m/sec). Attached is the .h3d file