Methods, systems, and apparatus, including medium-encoded computer program products, for computer aided design of physical structures using generative design processes. A method includes obtaining one or more design criteria for a modeled object including at least one design constraint; calculating a series of target values for the at least one design constraint, from an initial target value to a final target value; iteratively modifying a generatively designed three dimensional shape of the modeled object in the design space, wherein the iteratively modifying comprises performing numerical simulation of the modeled object, computing shape change velocities for an implicit surface in a level-set representation of the three dimensional shape in accordance with respective ones of target values in the series of target values, starting from the initial target value and ending with the final target value, and updating the level-set representation using the shape change velocities.