In some nonlinear unsteady inverse problems, the inverse solution will
oscillate violently in the whole time domain due to the sharp change of the
sensitivity coefficients. To deal with this problem, a new sequential
function specification method with variable future time steps is proposed in
this paper. The future time steps are adjusted by the error amplification
coefficients which are defined as the reciprocal of the square sum of the
sensitivity coefficients. When the error amplification coefficients are
small, a small number of future time steps is used to reduce the
deterministic error; while in the period with large error amplification
coefficient, a large number of future time steps is used to reduce
stochastic error. Finally, the total error of estimated heat flux is
reduced. Avoid the sharp fluctuation of estimated heat flux in time domain
due to the sharp change of sensitivity coefficients. The variable
future-time-steps method is applied to the estimation of one-dimensional
nonlinear unsteady heat flux without and with ablation through numerical
experiments. Numerical experiments show that the proposed method can not
only estimate various forms of heat flux, but also its inversion results are
significantly better than those of the fixed future time steps method based
on the discrepancy principle, and also better than those of the fixed future
time step method based on the minimum relative error of heat flux.