# Homework Help: Flow is inviscid?

1. Feb 21, 2010

### shah4u19

Hello, I need help with this homework question.
One of the assumption the idea flow solution uses to solve equation is that flow is inviscid.
What does this assumption mean?
What are the implications of this assumption:
a. on the flow on the surface of the airfoil
b. in calculating drag
c. in determining what happens when the airfoil stalls

I would really appreciate you help.
Thank You.

2. Feb 23, 2010

### shinobiazra

Inviscid means flow of a fluid that is assumed to have no viscosity (i.e no firction forces).

Try this website; http://www.centennialofflight.gov/essay/Theories_of_Flight/Skin_Friction/TH11.htm [Broken]

If we assume drag is skin-fiction drag then the effects of a real fluid flow are the result of the viscosity of the fluid. The viscosity causes a boundary layer and, hence, a skin-friction drag. The flow field is disrupted because of viscosity to the extent that a pressure drag arises. Also, the net pressure lift is reduced.

As for part c, I am not sure. Hope this helps.

Last edited by a moderator: May 4, 2017
3. Feb 24, 2010

### suratpanas

when airfoil stalls occur, there's flow separation at a point on the airfoil. this separation makes the air flow no longer attached to the airfoil surface thus changing the flow trajectory and pressure distribution. the pressure is lowered at airfoil's back thus drag increased significantly; the intended smooth flow at trailing edge to meet the kutta condition is no longer exist thus lift is failed to be generated.

the flow separation occurs because the flow at boundary layer is running out its energy due to the viscous friction between the boundary layer air with the airfoil surface. at the point of separation, the flow cannot maintain its capability to attach to the hump-shaped surface because it doesn't have enough kinetic energy (something dependent to speed) to overcome the pressure gradient on its way to the trailing edge.

CMIIW

suratpanas