Experimental investigation of Coandã flow attachment in automotive windshield defrosting

Abstract

Abstract This paper examines the occurrence of the Coandã effect in the context of automotive windshield defrosting. A generic, parameterized pseudo-2D test rig representing the main windshield defrosting components of a passenger vehicle is introduced. By externally controlling key geometric features such as the defrost duct angle and dashboard position, attachment of the defrost jet to the dashboard due to the Coandã effect is induced, which is incompatible with the functional requirements of passenger vehicle HVAC systems. By exploiting the tendency of Coandã flows to exhibit hysteresis, steady mono- and bistable states are identified in which the turbulent defrost jet remains attached either to the windshield or the dashboard over extended time periods, as well as unsteady configurations characterized by approximately memoryless switching between states. Two-dimensional two-component particle image velocimetry (2D2C PIV) is used to analyze the mean flow fields of the identified jet states in terms of velocity and turbulent kinetic energy contours, maximum velocity decay, and half-width growth rates. In addition, pressure coefficients measured along the windshield centerline demonstrate how proximity to the dashboard alters the pressure distribution to resemble that of a shallower or steeper duct angle through deflection of the defrost jet. Proper orthogonal decomposition is applied to the PIV data to extract the switching modes and their associated temporal coefficients in unsteady states, with subsequent statistical analysis indicating that the observed state switching exhibits Poissonian characteristics. Beyond the present study, the experimental data are intended to serve as a benchmark for the validation and assessment of CFD results.