Effect Of Induced Hairpin Vortex On Structures In Turbulent Boundary Layers

Mechanism of the large scale low-speed streaks (>20δ) found in logarithmic layer is experimentally investigated, by inducing a single synthetic hairpin vortex into turbulent boundary layers.It is proved by both flow visualizations and single hot-wire measurements that a single synthetic hairpin is generated by 4.8Hz pulsing jet normal to the wall with a maximum velocity of 1.1ms^-1 (Rjet≈55%) in laminar boundary layers with a thickness of around 12mm at a freestream velocity of about 2ms^-1. The distribution of phase-averaged u fluctuations and marker function in three dimensional area (by using Taylor's hypothesis) at each x-location 10mm downstream the hole, demonstreates that low-speed region is formed inside and upstream of the synthetic hairpin, which is surrounded by high-speed region. The combination of the phase-averaged signals shows the dynamics process of synthetic hairpin within a cycle (209ms), which is in consistent with the flow visualization in laminar boundary layers. Moreover, streaks similar to those found in logarithmic layer in turbulence flows by Hutchins et al. (2007) are observed in horizontal slices of the instantaneous u fluctuations.By introducing a normal jet at 4.8Hz with a maximum velocity of 3.45ms^-1 (Rjet≈172%), phase-averaged u fluctuations and marker function indicate a single synthetic hairpin vortex is formed 40mm downstream the exit of the jet, in a turbulent boundary layer with 50mm thickness at around 2ms^-1. Moreover, low-speed streaks resemble those found in logarithmic layer of turbulence flows by Hutchins et al. (2007). In comparion with the u fluctuations associated with a single hairpin vortex generated by a relatively weak normal jet (Rjet≈55%), dominant passive spanwise vorticity in wake region is responsible for formation of the large scale streaks in logarithmic layer.Due to relationship between a second derivative of mean streamwise velocity and existence of streaks, a strong enough boundary-layer shear is necessary for formation of steaks. By comparion of second derivatives of mean velocity at two Reynolds number, it is figured out the reason why large scale low-speed streaks in logarithmic layer are stronger and easier to be observed.