Analysis of streaks in unstratified and stratified wakes

Abstract

A turbulent circular disk wake [ 1] is investigated through visualizations, conditionalaveraging, and spectral analysis to analyze the presence of large-scale streaks. The near wakeand intermediate wake are dominated by the vortex shedding mode residing at 𝑚 = 1, St = 0.135in the unstratified wake (Fr = ∞). Once the influence of the 𝑚 = 1 mode is filtered, large-scalestreaks become apparent in the flow. These streaks predominantly reside at 𝑚 = 2, 𝑆𝑡 → 0.The effect of stratification on streaks is investigated through the analysis of two body-basedFroude numbers Fr = 2 and 10. Streaks are present and the lift-up mechanism is active inthe near wake of the Fr = 10 wake. However, as the Fr = 10 wake evolves downstream, thesignature of these streaks get weaker. The Fr = 2 wake is significantly different from the othertwo cases from the very beginning. Large-scale streaks are significantly attenuated and theenergy at St → 0 is notably lower than the other two cases throughout the course of its evolution.Conditionally averaged streamwise vorticity fields reveal that streaks are generated throughthe lift-up mechanism. Higher stratification diminishes the strength of the streamwise vorticesthereby diminishing the lift-up effect.

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