Experimental investigation on drag reduction in a turbulent boundary layer with a submerged synthetic jet

This work investigates the active control of a fully developed turbulent boundary layer by a submerged synthetic jet actuator.The impacts of the control are explored by measuring the streamwise velocities using particle image velocimetry,and reduction of the skin-friction drag is observed in a certain range downstream of the orifice.The coherent structure is defined and extracted using a spatial two-point correlation function,and it is found that the synthetic jet can efficiently reduce the streamwise scale of the coherent structure.Proper orthogonal decomposition analysis reveals that large-scale turbulent kinetic energy is significantly attenuated with the introduction of a synthetic jet.The conditional averaging results show that the induction effect of the prograde vortex on the low-speed fluid in a large-scale fluctuation velocity field is deadened,thereby suppressing the bursting process near the wall.

Effects of single synthetic jet on turbulent boundary layer

The turbulent boundary layer(TBL)is actively controlled by the synthetic jet generated from a circular hole.According to the datasets of velocity fields acquired by a time-resolved particle image velocimetry(TR-PIV)system,the average drag reduction rate of 6.2%in the downstream direction of the hole is obtained with control.The results of phase averaging show that the synthetic jet generates one vortex pair each period and the consequent vortex evolves into hairpin vortex in the environment with free-stream,while the reverse vortex decays rapidly.From the statistical average,it can be found that a low-speed streak is generated downstream.Induced by the two vortex legs,the fluid under them converges to the middle.The drag reduction effect produced by the synthetic jet is local,and it reaches a maximum value at x^(+)=400,where the drag reduction rate reaches about 12.2%.After the extraction of coherent structure from the spatial two-point correlation analysis,it can be seen that the synthetic jet suppresses the streamwise scale and wall–normal scale of the large scale coherent structure,and slightly weakens the spanwise motion to achieve the effect of drag reduction.

Effects of single circular synthetic jet on turbulent boundary layer

The periodic synthetic jet emerging from a circular orifice actively controls the turbulent boundary layer(TBL).A time-resolved particle image velocimetry(TR-PIV)system was designed to capture the velocity field database and based on the single-pixel ensemble correlation(SPEC)algorithm,an average drag reduction rate of 6.2%was obtained.The results show that the synthetic jet causes a wide range of low momentum zones and a low-speed streak in the downstream flow field.And the places where the disturbance intensity is strong are often accompanied by a larger velocity deficit.The instantaneous flow fields are visualized with the Finite-Time Lyapunov Exponent(FTLE),and the hairpin vortex packet composed of five hairpin vortices and the generation of new hairpin vortices are observed when there is no control.Under the action of the synthetic jet,the hairpin vortices are continuously generated from the jet orifice.The synthetic jet mainly achieves the drag reduction effect mainly by modulating the mean convection term cC and the spatial development term c^(D).The drag reduction effect appears in the region of x/δ0>0.38,and the maximum drag reduction rate is 12.2%at x/δ0=0.75,and then gradually decreased.Using proper orthogonal decomposition(POD),it is found that the synthetic jet reduces the energy proportion of the large-scale energetic structures.After the conditional average,the synthetic jet limits the influence range of bursting events at various scales in the near-wall region,and weakens the normal transport of momentum and energy brought about by large-scale ejection events(Q2 events)and the wall friction resistance caused by large-scale sweep events(Q4 events).