TY - JOUR

T1 - Turbulent structures, integral length scale and turbulent kinetic energy (TKE) dissipation rate in compound channel flow

AU - Azevedo, Ricardo

AU - Roja-Solórzano, Luís R.

AU - Bento Leal, João

N1 - The authors acknowledge University of Beira Interior for allowing the realization of the experiments. The authors wish to acknowledge the financial support of the Portuguese Foundation for Science and Technology through the project PTDC/ECM/099752/2008. The first author wishes to acknowledge the financial support of the Portuguese Foundation for Science and Technology through the Grant SFRH/BD/33646/2009.

PY - 2017/10/1

Y1 - 2017/10/1

N2 - In the present study, high data rate measurements were obtained for the streamwise and vertical velocity components using 2D Laser Doppler Velocimeter. The turbulent field in a straight compound-channel flow was characterized for three different uniform flow water depths, corresponding to “deep flows”, “intermediate flows” and “shallow flows” conditions. Several methodologies were studied to process the data and to obtain autocorrelation functions, integral length scale and turbulence kinetic energy (TKE) dissipation rate. The Sample and Hold method was adopted to interpolate the unevenly spaced record and calculate the autocorrelation function; the integral-stop-value 1/e was used to estimate the integral length scale; and the TKE dissipation rate was estimated through the velocity energy spectrum. A double shear layer composed of two counter-rotating vertical oriented vortices, interacting with the secondary currents, is observed in the interface region for deep flow conditions. By decreasing the water depth, the interface region becomes dominated by a strong mixing layer of vertical oriented vortices with high TKE dissipation rate and large integral length scale, acting as a vertical wall to the weak secondary currents that develop at the main channel. The determination of the integral length scale permits to confirm the existence and the strength of these turbulence structures, unveiling the strong mixing layer as the origin of the largest integral length scales, even larger than the flow depth, and as the most efficient mechanism to redistribute turbulence generated at the bottom towards upper flow regions. Despite the high complexity of turbulence structures present in the flow, for all water depths, a linear dependence is depicted between integral length scale, TKE dissipation rate, and streamwise turbulence intensity.

AB - In the present study, high data rate measurements were obtained for the streamwise and vertical velocity components using 2D Laser Doppler Velocimeter. The turbulent field in a straight compound-channel flow was characterized for three different uniform flow water depths, corresponding to “deep flows”, “intermediate flows” and “shallow flows” conditions. Several methodologies were studied to process the data and to obtain autocorrelation functions, integral length scale and turbulence kinetic energy (TKE) dissipation rate. The Sample and Hold method was adopted to interpolate the unevenly spaced record and calculate the autocorrelation function; the integral-stop-value 1/e was used to estimate the integral length scale; and the TKE dissipation rate was estimated through the velocity energy spectrum. A double shear layer composed of two counter-rotating vertical oriented vortices, interacting with the secondary currents, is observed in the interface region for deep flow conditions. By decreasing the water depth, the interface region becomes dominated by a strong mixing layer of vertical oriented vortices with high TKE dissipation rate and large integral length scale, acting as a vertical wall to the weak secondary currents that develop at the main channel. The determination of the integral length scale permits to confirm the existence and the strength of these turbulence structures, unveiling the strong mixing layer as the origin of the largest integral length scales, even larger than the flow depth, and as the most efficient mechanism to redistribute turbulence generated at the bottom towards upper flow regions. Despite the high complexity of turbulence structures present in the flow, for all water depths, a linear dependence is depicted between integral length scale, TKE dissipation rate, and streamwise turbulence intensity.

KW - Compound channel flow

KW - Integral length scale

KW - TKE dissipation rate

KW - Turbulence intensity

UR - http://www.scopus.com/inward/record.url?scp=85027551394&partnerID=8YFLogxK

U2 - 10.1016/j.flowmeasinst.2017.08.009

DO - 10.1016/j.flowmeasinst.2017.08.009

M3 - Article

AN - SCOPUS:85027551394

VL - 57

SP - 10

EP - 19

JO - Flow Measurement and Instrumentation

JF - Flow Measurement and Instrumentation

SN - 0955-5986

ER -