Flow resistance behavior of pulsating flow in circular pipes

An experimental study investigated pulsating air flow in circular pipes using acoustic excitation, with a focus on pressure drop-based flow resistance. Time-averaged pressure drop (ΔPta) measurements were analyzed by varying the Womersley number (Wo = 10–75), the pulsation amplitude (A = 20–50%), pipe diameter (d = 15–25 mm), and the Reynolds number (Re = 5000–8000). A peak increase in ΔPta of up to 260% was observed for Wo = 61, while a reduction of up to 77% was recorded for Wo > 66 at A = 35%. The lowest pulsation amplitude (A = 20%) had a negligible effect on flow resistance, while higher amplitudes amplified pressure drop in the mid-Wo range. When comparing diameters at constant Wo (∼31), smaller pipes (d = 15 mm) exhibited higher spectral energy in both primary and secondary peaks and resulted in greater resistance than larger pipes (d = 25 mm). Spectral analysis using fast Fourier transform (FFT) and continuous wavelet transform (CWT) revealed that Wo = 30–61 corresponds to chaotic flow structures with multiple dominant frequencies, whereas flows at Wo > 61 showed stronger coherence. The findings suggest that selecting specific combinations of pulsation parameters can optimize flow resistance for various industrial needs such as heat transfer enhancement, drag reduction, or internal pipe cleaning.