High-Reynolds-number flat plate turbulent boundary layer measurements and skin-friction drag reduction with gas or polymer injection.

摘要

Presented here are measurements of the near-zero-pressure-gradient turbulent boundary layer (TBL) that forms on a flat-plate at free-stream speeds from 3.3 to 20.0 m/s. Measurements of the unmodified TBL were made at momentum-thickness-based Reynolds numbers up to 157,000 and skin-friction drag-reduction was observed with the wall-injection of gas or polymer additives. Experiments were conducted at the U.S. Navy's Large Cavitation Channel (LCC), on a hydraulically-smooth (k+ 0.2) flat-plate test model 12.9 m long and 3.05 m wide. A new skin-friction correlation is generated from direct measurements made with flush-mounted floating-plate force balances. Composite wall-normal profiles of the mean velocity and second-order turbulence statistics measured by conventional laser-Doppler velocimetry (LDV) and near-wall particle tracking Velocimetry (PTV) span the wall-normal coordinate (y) from y + - y/lv below unity to y > delta99. Indirect skin-friction measurements from LDV and (PTV) agree within experimental error.; Bubble-drag-reduction (BDR) was achieved with the injection of gas from the wall of the TBL at three nominal test speeds (6.7, 13.3, and 20.0 m/s). Gas was injected at rates up to 800 cubic feet per minute (0.14 m2 /sec per unit span) from one or both of two injectors located 1.38 and 3.73 m from the leading edge. During BDR, significant levels of drag reduction ( 25%) were not observed beyond the first two meters downstream of injection. However, an abrupt transition to air layer drag reduction (ALDR) was observed at four free-stream speeds up to 13.3 m/s, where an air-layer forms between the liquid and the model surface resulting in drag reductions exceeding 80% over the entire model. A cost-benefit analysis shows that, under the right conditions, ALDR could yield a net power-savings.; Polymer-drag-reduction (PDR) was achieved with the slot-injection of dilute (1000, 2000, and 4000) solutions of three molecular weight (2, 4, and 8 million) polyethylene oxides (PEOs). Polymer solutions were injected at flow-rates from 0.14 to 0.71 liter/sec per meter span. The influence of molecular weight, injection rate, concentration, and free-stream speed on the stream-wise distribution of PDR was investigated. Existing scaling laws for PDR and diffusion are evaluated for a wide range of experimental parameters in a high-Reynolds-number TBL.