Simulating a fish like body and choosing reference values

quarkz · November 6, 2015, 12:05

Hi,

I need to simulate a fish like body with the mechanical tail fin attached to its back, as shown in the attachment.

Its fin is pitching up and down at 1 Hz and it moving slowly at a constant speed. Body length is 24cm and it's moving at about 4cm/s.

So I need to choose reference length and velocity

I chose its steady forward velocity (U) as the reference velocity.
For the length, I initially chose the body length (L).
However, when I tried to compute its reduced frequency rf（fL/U), it's extremely high, at around 6.

The moment I run my solver, the solution diverges.

Did I choose a scale which is not suitable? Is there a "better" reference length? I thought of using the fin's horizontal length, something like the chord of a wing. However, fr is still high at around 3.

Alternatively, I can choose the lateral displacement amplitude of the fin, or change the reference velocity to the average fin velocity.

So what is a appropriate way to choose?

quarkz · November 18, 2015, 23:38

Well, after thinking for a while, I figure out that the most appropriate choice would be the average tip velocity and chord of the fin. This is because unlike a fish which moves using its entire body, the current setup only has the fin moving, so I thought the reference focus should be on the fin, not body.

November 6, 2015, 12:05	Simulating a fish like body and choosing reference values	#1
quarkz Senior Member TWB Join Date: Mar 2009 Posts: 414 Rep Power: 19	Hi, I need to simulate a fish like body with the mechanical tail fin attached to its back, as shown in the attachment. Its fin is pitching up and down at 1 Hz and it moving slowly at a constant speed. Body length is 24cm and it's moving at about 4cm/s. So I need to choose reference length and velocity I chose its steady forward velocity (U) as the reference velocity. For the length, I initially chose the body length (L). However, when I tried to compute its reduced frequency rf（fL/U), it's extremely high, at around 6. The moment I run my solver, the solution diverges. Did I choose a scale which is not suitable? Is there a "better" reference length? I thought of using the fin's horizontal length, something like the chord of a wing. However, fr is still high at around 3. Alternatively, I can choose the lateral displacement amplitude of the fin, or change the reference velocity to the average fin velocity. So what is a appropriate way to choose?

November 18, 2015, 23:38		#2
quarkz Senior Member TWB Join Date: Mar 2009 Posts: 414 Rep Power: 19	Well, after thinking for a while, I figure out that the most appropriate choice would be the average tip velocity and chord of the fin. This is because unlike a fish which moves using its entire body, the current setup only has the fin moving, so I thought the reference focus should be on the fin, not body.