Consider an infinitely thin flat plate of chord c at an angle of attack α in a supersonic flow. The pressure on the upper and lower surfaces are different but constant over each surface; that is, pu(s) = C1 and pl(s) = C2, where C1 and C2 are constants and C2 > C1. Ignoring the shear stress, calculate the location of the center of pressure.

Respuesta :

The center of pressure is located at; X_cp = c/2

How to find the location of center of pressure?

We are given;

Chord = c

Angle of attack = α

Pu (s) = c₁

P₁(s) = c₂, and c₂ > c₁

First of all, we need to find the resultant normal force on the plate and the total moment about leading edge.

N' = [tex]\int\limits^T_L ({P_{u}dx - \tau_{u} dy} \,) + \int\limits^T_L ({P_{l}dx - \tau_{l} dy} \,)[/tex]

That is the normal force.

Now the total moment about the leading edge is;

[tex]M_{LE} = \int\limits^T_L [({P_{u}dx - \tau_{u} dy} \,)x - ({P_{u}dy - \tau_{u} dx})] + \int\limits^T_L [({-P_{l}dx - \tau_{l} dy} \,)x + (-{P_{l}dy - \tau_{l} dx})y][/tex]

Now, if we ignore the y-direction and neglect shear stress and put C for T and 0 for L, then we have;

[tex]N' = \int\limits^c_0 ({P_{L} - P_{U}} \,) dx \\and M'_{LE} = \int\limits^c_0 ({P_{U} - P_{L}} \,) x.dx[/tex]

When we integrate the above, we arrive at;

[tex]N' = (P_{L} - P_{U})C \\Also, M'_{LE} = (P_{U} - P_{L})\frac{C^{2}}{2}[/tex]

From the above, we can see that the moment about the leading edge, P_u subtracts P_l which renders it negative.

Thus;

M'_LE can be expressed as the product of the normal force and the length from the leading edge to the location to the center of the pressure. Thus;

M'_LE = N' * X_cp

So, [tex]X_{CP} = \frac{-(P_{u} - P_{l})\frac{C^{2}}{2} }{(P_{L} - P_{U})C} = \frac{(P_{L} - P_{U})\frac{C^{2}}{2} }{(P_{L} - P_{U})C}[/tex]

X_cp = (C²/2) ÷ C

X_cp = C²/2C = C/2

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