Thank you, yes that works.
However, I can only use the project method for the expr_bessel and can not use the interpolate function. But the project returns strange results if an radial angle is uses, see: Strange behavior of project vs interpolate for C++/CPP vs UFL Expression (azimuth, radial or polar angle)
Is there a way to use UFL expression such as Constant(1.2345)*bessel_I(0,x[0]) inside the interpolate function?