Investigation of Wake behind Finite Cylinder at Critical Reynolds Number

Abstract

Presented research aims on the three dimensional turbulent wake structure at critical Reynolds number Experiment was carried out in 4m×3m close type wind tunnel with a finite circular cylinder at Reynolds number 4×105 long time average measurement was carried out to measure the velocity signal within the near wake region by hot wire anemometer We analyzed turbulence data with statistical approaches like building probability density function (PDF) skewness and flatness These statistics represent different turbulence relevant physics respectively For skewness it indicates the vortex stretching in the free shear layer; flatness shows the intermittence of the turbulence and the feature of entrainment Besides from the turbulence momentum equation the additional term Reynolds stress drives the momentum transport in turbulence flow It indicates two important flow characteristics in finite cylinder wake i e downwash and upwash dominating the whole flow structure in the cylinder wake Consequently we analyzed the turbulence spectrum by both linear and nonlinear analysis First the fast Fourier transform provides linear analysis whose final solution is the sum of all linear solutions of the turbulence data this is Fourier’s view Nevertheless turbulence is nonstationary and nonlinear in this way Hilbert transform (EMD) is applied Processed by Hilbert transform we decompose several modes called IMF in our signal Such nonlinear analysis shows us the local turbulence structure unlike the Fourier’s approach and we successfully discover the Karman vortex shedding hidden in the three dimensional wake Interestingly there are some residuum modes which has its principle frequency but is much lower than Karman vortex This gives a way for future work

Date of Award	2016 Aug 4
Original language	English
Supervisor	Jiun-Jih Miau (Supervisor)