© THE FLOW OVER A BACKWARD-FACING STEP UNDER CONTROLLED PERTURBATION - LAMINAR SEPARATION HASAN, MAZ CAMBRIDGE UNIV PRESS, JOURNAL OF FLUID MECHANICS; pp: 73-96; Vol: 238 King Fahd University of Petroleum & Minerals Summary The flow over a backward-facing step with laminar separation was investigated experimentally under controlled perturbation for a Reynolds number of , based on a step height h and a free-stream velocity U0.The reattaching shear layer was found to have two distinct modes of instability:the 'shear layer mode' of instability at St-theta almost-equal-to (St-theta = f-theta/U0, theta being the momentum thickness at separation and f the natural roll-up frequency of the shear layer); and the 'step mode' of instability at St(h) almost-equal-to (St(h) = fh/U0).The shear layer instability frequency reduced to the step mode one via one or more stages of a vortex merging process.The perturbation increased the shear layer growth rate and the turbulence intensity and decreased the reattachment length compared to the unperturbed flow.Cross-stream measurements of the amplitudes of the perturbed frequency and its harmonics suggested the splitting of the shear layer.Flow visualization confirmed the shear layer splitting and showed the existence of a low- frequency flapping of the shear layer. References: BAKER S, 1977, THESIS U SURREY BHATTACHARJEE S, 1986, AIAA J, V24, P623 BRADSHAW P, 1972, J FLUID MECH, V52, P113 BROWAND FK, 1966, J FLUID MECH, V26, P281 BROWN GL, 1974, J FLUID MECH 4, V64, P775 CASTRO IP, 1981, J WIND ENG IND AEROD, V7, P253 CASTRO IP, 1987, J FLUID MECH, V179, P439 CASTRO IP, 1988, J FLUID MECH, V192, P577 Copyright: King Fahd University of Petroleum & Minerals;

© CHANDRSUDA C, 1975, THESIS IMPERIAL COLL CHERRY NJ, 1984, J FLUID MECH, V144, P13 CROW SC, 1971, J FLUID MECH, V48, P547 EATON JK, 1980, MD39 STANF U DEP MEC EATON JK, 1981, AIAA J, V19, P1093 FREYMUTH P, 1966, J FLUID MECH, V25, P683 GOLDSTEIN ME, 1988, J FLUID MECH, V197, P295 HASAN MAZ, 1983, THESIS U HOUSTON HO CM, 1982, J FLUID MECH, V119, P443 HUSAIN ZD, 1983, AIAA J, V21, P1512 HUSSAIN AKM, 1981, J FLUID MECH, V110, P39 HUSSAIN AKMF, 1983, J FLUID MECH, V134, P431 HUSSAIN AKMF, 1985, J FLUID MECH, V150, P159 JAROCH MP, 1989, J FLUID MECH, V205, P523 KELLY RE, 1967, J FLUID MECH, V27, P657 KHAN AS, 1990, THESIS MECH ENG DEP KIBENS V, 1980, AIAA J, V18, P434 KIM J, 1978, MD37 STANF U THERM D KIYA M, 1983, J FLUID MECH, V137, P83 MATTINGLY GE, 1972, J FLUID MECH, V51, P233 MCGUINNESS M, 1978, THESIS CAMBRIDGE U MICHALKE A, 1965, J FLUID MECH, V22, P371 MIKSAD RW, 1972, J FLUID MECHANICS 4, V56, P695 PFIZENMAIER E, 1973, INSTABILITY SOUND IN PRONCHICK SW, 1983, MD42 STANF U THERM D ROOS FW, 1986, AIAA J, V24, P1956 ROSHKO A, 1965, 1965 P HEAT TRANSF F ROTHE PH, 1979, J FLUIDS ENG, V101, P117 RUDERICH R, 1986, J FLUID MECH, V163, P283 SATO H, 1960, J FLUID MECH, V7, P53 TROUTT TR, 1984, J FLUID MECH, V143, P413 WINANT CD, 1974, J FLUID MECH, V63, P237 ZAMAN KBM, 1980, J FLUID MECH, V101, P449 ZAMAN KBM, 1981, J FLUID MECH, V103, P133 For pre-prints please write to: Copyright: King Fahd University of Petroleum & Minerals;