Adverse pressure gradient on a wing

[hob63]

For a symmetrical wing (NACA 0012 - due to wide data avaialble) at 0 deg inclination the following Cp to x/c relationship exists:

The upper and lower surfaces produce the same Cp and hence a symmetric wing with no inclination doesn't produce a result force (i'm happy with this).

Now at an inclination angle (10 deg) the following happens:

My question is for the lower surface we have Cp = 1.0 (stagnation point, this is ok), however, this decays to freestream (Cp = 0) close to trailing edge.

Is the lower surface actually an unfavourable pressure gradient? it is a symmetric wing and so I have a hard time seeing the lower as unfavourable (in terms of geometry). Why is it that curving upwards = favourable pressure gradient and curving downwards = unfavourable pressure gradient? there is a small contribution (ρg(Δh)) in terms of pressure due to the actual height, is it simply due to this? Or is it that the stagnation points dominates the lower flow resulting in a high->low gradient and thus an unfavourable gradient (not resulting from the actual geometry but due to the location of the stagnation point).
Cheers for any help :)

 

[rcgldr]

Is the lower surface actually an unfavorable pressure gradient?

Why do you call the lower surface gradient unfavorable? As the flow approaches the trailing edge, it should be approaching ambient pressure. Also, the lowest pressure above the wing in the graph seems to occur unusually close to the leading edge.

 

[NatSusan]

Hi there,

The lower surface of the wing does indeed experience an unfavourable pressure gradient at an inclination angle of 10 degrees. This is due to the fact that the flow is being pushed downwards by the upper surface, causing the air to accelerate and create a low pressure region. This decrease in pressure is what causes the Cp to decay towards the freestream value close to the trailing edge.

In terms of geometry, it is true that curving upwards is generally associated with a favourable pressure gradient and curving downwards with an unfavourable pressure gradient. This is because the flow naturally wants to follow the curvature of the surface, and when the surface is curving upwards, the flow is being pushed towards the surface, resulting in a higher pressure. When the surface is curving downwards, the flow is being pushed away from the surface, resulting in a lower pressure.

The small contribution of ρg(Δh) does play a role in the overall pressure distribution, but it is not the main reason for the unfavourable pressure gradient on the lower surface. It is mainly due to the effects of the upper surface and the flow being pushed downwards.

I hope this helps to clarify your question. Let me know if you have any further questions.