Separated and vortex flows




Self-sustained oscillations of turbulent flows over an open cavity 


-      Self-sustained oscillations of velocity and pressure


        Feedback mechanism : acoustic or hydrodynamic

        N coherent structures between leading and trailing edges



-    Experimental set-up

       Open-circuit wind tunnel Microphone / Hot wire measurements

       B&K 4133, TSI 1260(CTA-IFA 300) PIV measurements

       Double-pulse Nd:Yag Laser ( 200mJ/pulse )

       12bit  CCD camera ( 1280 X1024 pixel )

       Iterative deformation algorithm ( 5000 image)




-      Mean velocity and streamlines



-      Instantaneous characteristics


Flow-induced noise



Reduction of flow induced noise in an axisymmetric cavity with lids

-      To reduce the flow-induced noise, reactive resonator has been installed due to its simplicity of application


-  Objective

     Elucidate the hydrodynamic effects of lid on the flow-induced noise


-   Direct numerical simulation

      L=2R, H=R, Le=0.5R, He=0.023R, Lin=Lout=4.54R

      Inflow : Fully-developed turbulent pipe flow at Ret=186.

      Outflow : Convective boundary condition.

      Periodic boundary conditions in the q direction.

      Grid resolution : Dz+min=1.45, Dz+max=17.97, (RDq)+=9.13, D r+min=0.13, Dr+max=4.9

-   Mean velocity

     Relatively strong recirculation in case of the cavity without lid.

     Secondary recirculation is not seen for the cavity with lid.


-  Radial mean velocity profiles

     Mass exchange at the cavity mouth is weakened in case of the cavity with lid


-      Flow-induced noise
      Lighthills equation for acoustic density fluctuations

      From the Lighthill acoustic analogy, acoustic source can be obtained from DNS of incompressible flow.

      Lighthill stress tensor




-  Noise Source Term vs. Pressure Fluctuations

      Distributions of Srms are very similar those of prms

      Flow-induced noise near the downstream edge is significantly reduced by lid


-  3D View of Instantaneous Flow Field

      Vortex identification by -l2 (Jeong and Hussain, 1995).
      Reduced turbulent structure in case of the cavity with lid