Experimental Study of Turbulence Response in a Slowly Accelerating Turbulent Channel Flow

An investigation of flow acceleration from initial statistically steady turbulent flow to final statistically steady turbulent flow is conducted experimentally using particle image velocimetry (PIV) and constant temperature anemometry (CTA). The turbulence response is investigated as the acceleration periods and acceleration rates are varied in a controlled fashion. This work expands the research by Mathur et al. (2018, “Temporal Acceleration of a Turbulent Channel Flow,” J. Fluid Mech., 835, pp. 471–490.) studying slower and longer transient flows. It also complements the numerical studies of a step increase in the flowrate of (He and Seddighi, 2013, “Turbulence in Transient Channel Flow,” J. Fluid Mech., 715, pp. 60–102. and He and Seddighi, 2015, “Transition of Transient Channel Flow After a Change in Reynolds Number,” J. Fluid Mech., 764, pp. 395–427.). The results obtained from the current investigations are qualitatively similar to those obtained previously. Consistent with previous studies, the response of turbulence in the current slow transient flow is again characterized by a laminar-turbulent transition. The initial increase of the flow development among the cases investigated can be categorized as faster, medium, and slower responses. Modifications are made to the equivalent Reynolds number and the initial turbulence intensity proposed earlier in order to account for the slow accelerating flow rates and the continuous change of the bulk velocities of the cases investigated. It has been shown that the critical equivalent Reynolds number based on these modifications and the initial turbulence intensity are well correlated for all cases investigated and a power-law relation is established.