Computation of CL CD from history.csv and flow.vtu differ by a factor of 2.

[atharvaaalok]

I am doing the following:
Given an airfoil shape using coordinates X (which is a (N, 2) matrix of x, y pairs) I first run it through SU2 to generate the different output files: history.csv, flow.vtu, surface_flow.csv.

Step 1: I first parse the history.csv file and read the last value of CL (that is the one obtained after convergence).

Step 2: Then I read the surface coordinate points from surface_flow.csv and use the other data rho_u, rho_v etc to compute the pressure coefficient cp. Another way to get cp is to simply read the pressure coefficient values from flow.vtu at the surface points.

Step 3: Compute CL and CD using CP distribution on the surface. I use the function described below.

Step 4: Compare the CL compute as in step 3 with the one read from history.csv in step 1.

Observation: The CL computed from Step 3 is for all shapes 2 times the CL reported in history.csv.

def compute_lift_drag(X, CP):
    # Segment vectors
    dX = X[1:] - X[:-1]
    dx = dX[:, 0]
    dy = dX[:, 1]

    # Segment lengths
    lengths = np.sqrt(dx**2 + dy**2)

    # Outward normals: rotate 90 deg clockwise (dy, -dx) / length
    nx = dy / lengths
    ny = -dx / lengths

    nx_times_length = dy
    ny_times_length = -dx

    # Average pressure per segment: shape (N-1,)
    CP_avg = 0.5 * (CP[1:] + CP[:-1])

    # Pressure force per segment: -CP_avg * normal * length
    fx = -CP_avg * nx_times_length
    fy = -CP_avg * ny_times_length

    # Total forces
    CD = fx.sum()
    CL = fy.sum()

    return CL, CD

I have raised an issue for this before:
#2549
But the code links provided in the reply are very very hard to parse and I am not able to pin point what exactly the problem is.