How can I define a vector function?

Dear Community:

I am trying to define a vector function (function vi as below)

my code is

vel_H = Expression(“0.51 * pow(L_n, 3) * pow(w, 1.5) * x[0] * x[0] / pow(vis, 0.5) / Di”,degree = 1, L_n = L_n, w = w, vis = vis, Di = diff_coeff[‘H’])

dot(vel_H, grad(u_H)) * v_H *dx

When I run the code, it throw the error:

"Dot product requires non-scalar arguments, got arguments with ranks 0 and 1"

I think the function vel_H should be a vector function, but I don’t konw how to define it. Could you please guide me/direct me to useful resources?

Thank You
Warm Regards

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