Keyur Kanade
Ansys Employee

Hi @prajput ,

The lift coefficient is a non-dimensional value that is commonly used to analyze the performance of a wing or its section (airfoil). The lift coefficient can be calculated as: 

Cl = L / (q * S)    for a 3D body

cl = L / (q * c)    for a 2D body

where:

L is the lift force

q = (1/2)*rho*V^2 is the dynamic pressure

S is the reference surface (for wings the plan-view surface)

c is the reference length (for airfoils the chord)

Now analyzing this equation, we can see that area, density and velocity cant be negative. So for the Lift coefficient to be negative the lift should be negative.