We introduced a smartphone schlieren imaging system in our advanced undergraduate fluid mechanics laboratory. The smartphone-based schlieren imaging system provides a low-cost, portable alternative for project-based learning. The system is based on a single-mirror schlieren setup, with the smartphone flash serving as the light source and the smartphone camera as the detector (see the figure below). The light source is positioned near the center of curvature of a concave mirror, which focuses the light on the detector. Adjustable mounts for the smartphone and mirror are 3D printed and secured to a common base, allowing the apparatus to be relocated as a single, portable system. Schlieren imaging is achieved by cutting off approximately 50 percent of the focused light and the image is sharpened by placing a small aperture in front of the flash. Schlieren images can be recorded by the camera for gas flows with spatial density variations and are the result of the refraction of light between the camera and mirror.