Reducing drag on a bluff body using dimples

BBA · April 19, 2017, 13:30

I am performing simulations on an Ahmed body (bluff body) having significant amount of pressure drag and trying to control it using dimples on its surface(Top as well as slant face).

I understand that the problem is one with laminar flow over the surface which transitions to turbulent conditions under the effect of dimples as irregularities and theoretically should result in a reduction of drag value.

But the problem is that we are not able to achieve any drag reduction at all. Drag is always a bit higher than the nominal value, always a bit HIGHER( We have exhausted all the k-e wall treatment options scalable as well as enhanced wall treatment, the low Re number k-w approach with yplus=1 and the 4 eqn transition SST model for our case and still unable to get a reduced drag value).

I would like to know how to tackle the above problem in order to get tangible results.

Just to give you an idea, the dimple dimensions are like 30mm dia with 6mm depth incorporated over a body having a length of 1044mm.

Mesh is about 5.3 million.
Orthogonality is less due to high aspect ratio cells near the wall.
Turbulence intensity at inlet is 1%
Turbulence intensity at Outlet is 5%
Turbulent viscosity ratio is kept at its default value 10 for both inlet and outlet.

Which turbulence modelling to choose (RANS) and with what settings ?

Qkarl · January 27, 2018, 14:37

Hi, can i know do you solve this problem already? I am having this trouble too.

BBA · January 29, 2018, 09:09

Hi there !

As far as I know, small scale turbulence effect in dimples which we are trying to capture does not lie in the scope of RANS models.
I would rather go for DNS.

Regards

Qkarl · January 29, 2018, 11:09

Hi,

But mine is just a student project to study how the dimples will affect the drag of the golf ball. I could not run DNS with the limited resources. Is there any way to solve it?

April 19, 2017, 13:30	Reducing drag on a bluff body using dimples	#1
BBA New Member Grover Join Date: Apr 2017 Location: New Delhi Posts: 2 Rep Power: 0	I am performing simulations on an Ahmed body (bluff body) having significant amount of pressure drag and trying to control it using dimples on its surface(Top as well as slant face). I understand that the problem is one with laminar flow over the surface which transitions to turbulent conditions under the effect of dimples as irregularities and theoretically should result in a reduction of drag value. But the problem is that we are not able to achieve any drag reduction at all. Drag is always a bit higher than the nominal value, always a bit HIGHER( We have exhausted all the k-e wall treatment options scalable as well as enhanced wall treatment, the low Re number k-w approach with yplus=1 and the 4 eqn transition SST model for our case and still unable to get a reduced drag value). I would like to know how to tackle the above problem in order to get tangible results. Just to give you an idea, the dimple dimensions are like 30mm dia with 6mm depth incorporated over a body having a length of 1044mm. Mesh is about 5.3 million. Orthogonality is less due to high aspect ratio cells near the wall. Turbulence intensity at inlet is 1% Turbulence intensity at Outlet is 5% Turbulent viscosity ratio is kept at its default value 10 for both inlet and outlet. Which turbulence modelling to choose (RANS) and with what settings ?

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January 27, 2018, 14:37		#2
Qkarl Member Join Date: Apr 2017 Posts: 32 Rep Power: 9	Hi, can i know do you solve this problem already? I am having this trouble too.

January 29, 2018, 09:09		#3
BBA New Member Grover Join Date: Apr 2017 Location: New Delhi Posts: 2 Rep Power: 0	Hi there ! As far as I know, small scale turbulence effect in dimples which we are trying to capture does not lie in the scope of RANS models. I would rather go for DNS. Regards

January 29, 2018, 11:09		#4
Qkarl Member Join Date: Apr 2017 Posts: 32 Rep Power: 9	Hi, But mine is just a student project to study how the dimples will affect the drag of the golf ball. I could not run DNS with the limited resources. Is there any way to solve it?