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BRIDGING THEORY, EXPERIMENT, AND SIMULATION: CFD-DRIVEN INSIGHTS INTO THE FREE JETS LABORATORY**

Abstract

The classical “Free Jets” fluid mechanics laboratory experiment assesses the force exerted by a water jet on deflector plates (flat or angled) using measured flow rate and balance of applied weights and verifies the theoretical prediction from the linear momentum equation for different deflection angles. However, such experiments can suffer from various sources of measurement error — including nonideal inflow profiles, jet spreading, finite discharge geometry, and unaccounted turbulent fluctuations — which may lead to deviations between measured and theoretical forces. In this work, we employ computational fluid dynamics (CFD) to simulate the free-jet and impingement patterns of the experimental setup, reconstruct “virtual experiments,” and systematically quantify the impact of potential error sources on the force measurements. Using 3D CFD models with controlled inflow conditions, we examine how variations in nozzle exit profile, flow turbulence intensity, deflector geometry, and virtual “probe” (deflector) insertion affect both mean and instantaneous momentum flux. Our simulations reproduce the theoretical relationship between applied weight and square of exit velocity, when idealized conditions are assumed; but when realistic conditions are modeled, systematic biases emerge that are comparable to typical experimental discrepancies. We further perform a sensitivity analysis to identify which factors (jet spreading, turbulence, deflector geometry, unsteady velocity fluctuations) contribute most to measurement error. The combined CFD + experimental-data approach provides a more rigorous quantification of uncertainty in classical momentum-jet experiments, enhancing their pedagogical value and offering guidance for improved laboratory design and measurement protocols.

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