Session L02: Turbulence: Wake

Experimental study of the stratified wake of a sphere: Internal Waves

Wakes in density-stratified fluids radiate internal waves due to the random turbulent motions in the core of the wake (Meunier et al. JFM 2018; Rowe, Diamessis, and Zhou JFM 2020).  To investigate the relationship between the core turbulent wake and the resulting internal waves, experiments with a dimpled sphere at Reynolds number of ~100,000 and Froude number of ~64 were conducted using.  The experiments extend prior unstratified wake results of Saunders et al. (PRF 2020; Exp. Fluids 2022) to the stratified regime.  This presentation will focus on the radiated internal waves extending to a dimensionless time of Nt ~ 50, where N is the buoyancy frequency.   

Experimental study of the stratified wake of a sphere: Core Turbulence

Wakes in density-stratified fluids radiate internal waves due to the random turbulent motions in the core of the wake (Meunier et al. JFM 2018; Rowe, Diamessis, and Zhou JFM 2020).  To investigate the relationship between the core turbulent wake and the resulting internal waves, experiments with a dimpled sphere at Reynolds number of ~100,000 and Froude number of ~64 were conducted using.  The experiments extend prior unstratified wake results of Saunders et al. (PRF 2020; Exp. Fluids 2022) to the stratified regime.  This presentation will focus on the core turbulent wake immediately behind the sphere.              

Momentum entrainment of turbulent/turbulent interface in a cylinder wake

This study explores the spatial evolution of the momentum entrainment of the

turbulent/turbulent interface of a cylinder wake, in the context of the

diminishing of the coherent vortices from the near to the far field. The wake

was marked with a high-Schmidt-number fluorescent dye, and measurements combined

planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV)

were carried out from x/d = 5 to 40, where x is the streamwise distance

from the cylinder center and d is the cylinder diameter. The cylinder is

placed at various downstream distances from various grids which are used to

generate the background turbulence, such that the the parameter space of

turbulence intensity and integral length scale was explored as widely as

possible. It is found that the momentum flux across the interfaces decays

substantially beyond x/d = 10. This is highly likely associated with the

rapidly decay of the coherent vortices when x/d >10, since the

strong coherent vortices in the near field (x/d < 10) can induce

intense engulfment. The scaling of the momentum entrainment while the wake

evolves downstream is also explored, the effect of the coherent motions on the

scaling will be reported in the meeting presentation.   

Anisotropic eddy diffusivity field associated with a two-dimensional vortex shedding flow

Eddy viscosity models are often used to close turbulent momentum flux terms in the RANS equations. Analogously, eddy diffusivity models close turbulent scalar flux terms in Reynolds-averaged scalar transport equations. Such models commonly invoke the Boussinesq hypothesis, which assumes isotropy – that the magnitude of eddy diffusivity is agnostic to direction of diffusion of the mean scalar field – and locality – that turbulent fluxes are sensitive only to the local mean scalar gradient. In this work, we use the macroscopic forcing method (Mani and Park, Phys. Rev. Fluids, 2021) to compute anisotropic eddy diffusivity for scalar flux of a passive scalar subject to a two-dimensional vortex shedding flow. Specifically, macroscopic forcings that result in polynomial mean fields are used to quantify the leading coefficients of the Kramers-Moyal expansion of the eddy diffusivity operator. The leading order eddy diffusivity is found to exhibit substantial anisotropy in the wake flow with non-orthogonal principal axes. Results from this study provide insights into challenges of steady RANS modeling of shedding-dominated wake flows.   

Causality analysis of large-scale structures in the flow around a wall-mounted square cylinder

The aim of this work is to analyse the formation mechanisms of large-scale coherent structures in the flow around a wall-mounted square cylinder, due to their impact on pollutant transport within cities. To this end, we assess causal relations between the modes of a reduced-order model obtained by applying proper orthogonal decomposition to high-fidelity simulation data of the flow case under study. The causal relations are identified using conditional transfer entropy, which is an information-theoretical quantity that estimates the amount of information contained in the past of one variable about another. This allows for an understanding of the origins and evolution of different phenomena in the flow, with the aim of identifying the modes responsible for the formation of the main vortical structures. Our approach unveils that vortex-breaker modes are the most causal modes, in particular, over higher-order modes, and no significant causal relationships were found for vortex-generator modes.   

Computational and Experimental study of Flow Past Axially Aligned Spinning Cylinders

Computational and experimental investigations are performed for flow past a spinning cylinder aligned along its axis with a focus on effective control, stability and target guidance of projectiles with free spinning segments. The computational investigations utilize both RANS and LES. The RANS model is based on modified energy-spectrum to incorporate the effects of swirl and rotation using a parametric characterization of the model coefficients. The LES approach is based on a continuous turbulence model that is suitable for representing both the subgrid scale stresses in large eddy simulation and the Reynolds stresses in the Reynolds averaged Navier-Stokes formulation. The experimental investigation was performed in a low-intensity subsonic wind tunnel for Reynolds number range of up to 60,000 and rotation numbers of up to 2 (based on cylinder diameter). Full measurement of the wake is possible since a forward mounted sting is used. Comparison of comptations findings with the experimental results will be presented.   

Dynamic response of a cantilevered cylinder exposed to a turbulent cross-flow and freestream turbulence

The flow over bluff bodies is typically characterized as a multi-scale flow, in which several turbulent length/time scales develop in their respective wakes, imprinting these multi-scale physics into the structure's response. Meanwhile, the presence of freestream turbulence has a direct and indirect impact on the fluctuating loads of such bodies exposed to cross-flows, the later by modifying the vortex shedding process within the bluff body wake. Experiments are carried out in a water channel, combining simultaneous measurements of particle image velocimetry of the upstream and downstream flow conditions of the cantilevered cylinder cross-flow, and structural strain measurements using Rayleigh Backscattering distributed fibre optic sensors, distributed over the surface of a rigid cylinder. Different "flavours" of background turbulence on the inflow section of the cylinder are produced by a set of turbulence-generating grids placed upstream of the body, designed to encourage the development of ${L, TI}$ turbulence parameters. The objective of this work is to explore the effect of freestream turbulence on the developing strain/loads over a cantilevered cylinder, aiming to provide more insight on the influence of the explored freestream turbulence parameters on the development of the flow induced loading events, to further optimize and possibly develop new concepts of structural design/layout of arrays of structures.   

Modeling contaminant spread in the wake of moving persons and vehicles in various environments

Moving persons and vehicles create aerodynamic wakes that contain and distribute contaminants to the environment through which the object moves.  Characterizing the contamination spread from a moving object is important to understand the impact that persons and vehicles have on environments through which they move.  This work is developing analytical and experimental based models to understand wake contamination footprints in atmospheric environments for Earth and planetary exploration applications.  A literature review is used to document wake and plume propagation in the near field behind moving objects, with a specific focus on appropriate non-dimensional scaling of all parameters to accommodate widely varied atmospheric conditions.  Contamination transport from the vehicle through the wake and to the atmosphere is then estimated based on theoretical and empirical models of the plume behaviors and notional contaminant particle size and mass.  Contamination source characterization is considered for its impact on the resulting contamination footprint.  The results are parameterized contamination maps that are scalable through dimensional analysis for a given environment and source.  This ongoing study is also developing supplemental experimental measurements to build verification of the models for notional moving vehicle geometries.    

Assimilation of under-resolved velocity observations in the wake of a towed sphere

Reconstruction of the wake behind a towed sphere from sparse data is performed using an adjoint-variational data assimilation.  As the sphere is towed at the free-stream speed, under-resolved velocity measurements are collected in a stationary sub-volume of the wake, at moderate Reynolds number.  The forward-adjoint Navier-Stokes solver reconstructs the flow state within the volume (Wang, Wang & Zaki, J. Compu. Phys. 396, 2019) and has been extended to also estimate the time-dependent boundary conditions on the surfaces of the sub-volume. Tests using independent simulations as surrogate for experimental measurements are used to highlight important considerations of the data assimilation procedure. The dispersion properties of the numerical solver must be taken into consideration when performing these numerical tests. We demonstrate that data assimilation can accurately predict the full flow state of the wake within the observation sub-domain. The error reduction is assessed for the velocity fields which are observed but under-resolved in the data, as well as for unobserved quantities such as vorticity and pressure. The results highlight the potential of infusing measurements from laboratory experiments with numerical simulations to overcome limitations of both techniques.