Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session U37: Turbulence: Boundary Layers |
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Chair: Scott Salesky, University of Oklahoma Room: 245 |
Tuesday, November 22, 2022 8:00AM - 8:13AM |
U37.00001: Towards the understaing of the role of large-scale structures in the plane turbulent wall-jets Harish M Choudhary, Abhishek Gupta, Shivsai A Dixit, Anandakumar Karipot, Thara Prabhakaran The present study examines the large-scale structures in the turbulent plane wall jet flow and its influence in the region below velocity maximum. To capture these structures in the flow, we use two-dimensional particle image velocimetry (PIV) with four cameras placed side-by-side. Large ensembles of PIV data are collected using this facility to look for the turbulent statistics in spatial fields at three different Reynolds numbers. First, energy maps are plotted using single hot wire time series and PIV spatial fluctuation fields. These plots show the presence of outer and inner turbulent intensity peaks where the outer peak is seen to be more energetic. With an increase in Reynolds number, the energies in the inner peak increase, clearly indicating the influence of large-scales in the near-wall region. This argument is further supported by plotting co-spectra and force spectra fields from PIV data giving the scalewise contributions to Reynolds shear stress. Results from these plots apparantly show the presence of bi-modal structures in the region below velocity maximum, where Jet mode and wall mode appear to be interacting. Next, we carry out Fourier mode decomposition as a tool to look for statistical evidence of these large-scale structures. Here, first we binned PIV fields based on dominant streamwise Fourier modes and then plot the two-point correlation. This study shows the underlying periodicity in the outer region of the wall jet, where the first two dominant modes, $5z_{T}$ and $2.5z_{T}$, occur in almost 50 \% of total fields. Evidences of these dominant modes have also been found in the meso-layer overlap where almost 25 \% of total field shows these modes. This shows that the region below velocity maximum could no longer behave like a boundary layer, in fact, a strong influence of the Jet scale structures increases their activity in the near-wall region with an increase in Reynolds number. |
Tuesday, November 22, 2022 8:13AM - 8:26AM |
U37.00002: Comparison of the genesis and dynamics of Reynolds shear stress within boundary layer transitional spots and near-wall developed turbulence Ahmed Elnahhas, Adrian Lozano-Duran, Parviz Moin, James M Wallace In the late stages of boundary layer transition, localized packets of turbulence emerge, grow and coalesce to form the front of the turbulent boundary layer. These localized packets, or turbulent spots, have long been supposed to be more ordered versions of the structures found in developed wall-bounded turbulence. If true, it should be possible to construct a reduced-order model of the developed turbulence from information learned about the spots. Extensive examination of the spots' conditional first- and second-order statistical properties suggests that the supposition is true (Park et al. 2012, Sayadi et al. 2013, and 2014). Wu et al. 2017 later confirmed the existence of vortex clusters resembling spots deep in a developed turbulent boundary layer. The present work aims to answer this question: Does the similarity between transitional turbulent spots and developed wall-bounded turbulence extend beyond statistical and visual resemblance to their structural and temporal dynamics? We use three-dimensional structure identification and temporal tracking algorithms to identify the momentum-carrying structures within transitional turbulent spots and investigate their dynamical processes. |
Tuesday, November 22, 2022 8:26AM - 8:39AM Author not Attending |
U37.00003: Free-stream turbulence induced boundary-layer transition: a comparative study of EXP with DNS Santhosh B Mamidala, André Weingärtner, Jens H Fransson To date, very few careful and direct comparisons between experiments (EXP) and direct numerical simulations (DNS) have been published on free-stream turbulence (FST) induced boundary-layer transition, whilst there exist numerous published works on the comparison of canonical turbulent boundary layers. The primary reason being that the former comparison is vastly more difficult to carry out, simply because all known transition scenarios have large energy gradients and are extremely sensitive to surrounding conditions. Direct comparisons between EXP and DNS are important for two reasons. Firstly, the two approaches are frequently used for validation of computational fluid dynamics (CFD) where mathematical models describe complex fluid physics. This type of validation is redundant unless we can certify that we are able to reproduce detailed results from EXP with DNS or vice versa. Secondly, a direct comparison may pin-point the critical parameters that are important to match for a satisfying comparison, and hence can give guidance on how to develop better CFD models. In this paper we present a detailed comparison between our EXP and available DNS data of this complex transition scenario in a flat plate boundary layer at a turbulence intensity level of about Tu = 3% and an FST Reynolds number of about Refst = 67. We identify the leading edge (LE) pressure gradient distribution and the full energy spectrum at the LE as the two most important parameters for a satisfying comparison. Matching the LE characteristic FST parameters are not enough as previously thought. |
Tuesday, November 22, 2022 8:39AM - 8:52AM |
U37.00004: Streak transient growth (STG) and internal shear layers (ISL) in the outer regions of TBL: some new perspectives Edgardo J Garcia, Fazle Hussain The STG mechanism in a TBL, previously studied only near the wall (y+<60), is now extended to the outer region (y+>500) via DNS of a turbulent channel flow (Ret=2000). This is found to be also the underlying mechanism of large-scale motions (LSM) in the outer layer of TBLs. LSM – typically about 3h to 6h long, where 2h is the channel height -- are characterized by uniform momentum zones, vortex clusters, hairpin head vortices, etc. Our study shows that these features are indeed captured in the outer layer STG. We show that the LSM appears due to the Kelvin-Helmholtz instability of the outer-layer ISL – inherently formed during the outer STG – and evolves independently of the near wall vortices. The outer-layer STG is distinguished by a length scale, topology, and dynamics different from those of the near-wall STG. The outer-layer ISL -- inherent to the outer layer STG -- rolls up, forming either localized vortex clusters or a series of approximately equispaced spanwise structures (hairpin heads). The ISL, along with these spanwise sub-structures, then undergoes a rapid transition (avalanche) to turbulence. We will discuss the more realistic scenario where the STG perturbation is concurrently applied to the near-wall and outer streaks and detail the dynamical interaction of these two evolving structures. |
Tuesday, November 22, 2022 8:52AM - 9:05AM |
U37.00005: Effect of Stratification on Near-Wall Streaks Emma Lenz, Jane Bae We study the effect of stratification on near-wall streaks, defined as regions of slowly moving fluid elongated in the direction of the mean flow, in a turbulent boundary layer. Stable stratification suppresses vertical transport of momentum, which affects the number, size, and location of near-wall streaks. Analyzing the effect of stratification on streaks offers insight into the overall dynamics. Streaks are identified using direct numerical simulation data under the Boussinesq approximation at low Reynolds number in neutrally and stably stratified cases, following the method of Bae & Lee, PRF, 2021. Streaks are classified according to their relative distance from the wall and size. The distribution of streaks between these categories is analyzed, and volume, aspect ratio, meandering, and other spatial characteristics are compared between the neutral and stratified cases. We investigate how streaks and the dominant production mechanism change with Richardson number. |
Tuesday, November 22, 2022 9:05AM - 9:18AM |
U37.00006: Experimental Investigation of Turbulent Boundary Layer Dynamics Via Active Manipulation of Large-Scale Structures Mitchell Lozier, Flint O Thomas, Stanislav Gordeyev It has been established that the dynamics of large-scale structures (LSS) in the outer region of turbulent boundary layers (TBL) and the near-wall small-scale turbulence are correlated. In previous experiments using a single hot-wire; it was shown that a synthetic LSS introduced by a plasma-based actuator in the outer region of TBL had a strong modulating effect on the near-wall turbulence. Results showed that for streamwise locations close to the actuator, an actuation frequency comparable to the burst/sweep frequency of the near-wall structure created the strongest modulation effect. Farther downstream, an actuation frequency related to the streamwise wavelength of the naturally occurring LSS resulted in the strongest modulation effect. In the study reported here, an improved plasma-based active flow control device was placed in a similar region of the TBL to introduce periodic synthetic LSS. Planar PIV was used to measure the time-resolved two-dimensional velocity field downstream of the actuator at various streamwise locations. Detailed analysis of the PIV data, including phase-locked measurements of various Reynolds stresses at each streamwise location, was used to quantify the TBL response to the periodic spanwise-uniform actuation. The results are discussed and compared with previous experimental measurements. |
Tuesday, November 22, 2022 9:18AM - 9:31AM |
U37.00007: Characterisation of flow and wall-pressure statistics of permeable-rough surfaces Dea D Wangsawijaya, Prateek Jaiswal, Bharathram Ganapathisubramani Permeable and rough walls are known to affect the turbulent boundary layers (TBLs) developing over them differently. The effect of roughness on a developing TBL can be characterised by the log-law shift (the Hama roughness function). The effect of permeability, on the other hand, depends on the pore size and the surface thickness. In terms of turbulent structures, near-wall cycles are replaced by coherent roughness-sized structures, while permeability weakens them. Independently, the effects of these wall conditions on their flow characteristics and wall-pressure statistics are relatively well-established. Their combined effects, however, are still largely unknown. |
Tuesday, November 22, 2022 9:31AM - 9:44AM |
U37.00008: A universal velocity transformation for attached turbulent boundary layers subjected to favorable or adverse pressure gradients Xiang F Yang, Peng Chen, Kevin P Griffin, Salvador R Gomez, Yipeng Shi A velocity transformation is derived that transforms the mean flow in a boundary layer with streamwise pressure gradients to the equilibrium log law. The derivation assumes incompressible flow, two-dimensional mean flow, attached thin boundary layer, and arbitrary streamwise pressure gradient. The derived transformation accounts for the effects of flow history, turbulent mixing, and viscosity. The scaling is applied to spatially and temporally developing boundary layer flows subjected to favorable and adverse pressure gradients. The log law fails in these flows when scaled with viscous wall scaling, but the transformed velocity profiles collapse and follow the equilibrium log law. This is the first time the idea of velocity transformation is applied in the domain of incompressible flows and to non-equilibrium flows. The previous applications of velocity transformations, i.e., the van Driest transformation, are limited to equilibrium, high-Mach-number flows. Also, unlike prior studies on the topic that focused on adjusting the log law coefficients or tuning empirical damping or wake functions, this work corrects deviations of the untransformed velocity profiles by accounting for history effects. |
Tuesday, November 22, 2022 9:44AM - 9:57AM |
U37.00009: Manipulation of a turbulent boundary layer using a large-eddy break-up device Abhishek Tyagi, A Chandankumar, Sourabh S Diwan Large eddy break-up (LEBU) device has been used in the past as means to reduce skin friction drag associated with a turbulent boundary layer (TBL). In this work we report wind-tunnel measurements on a flat-plate TBL perturbed by a LEBU placed at the edge of the logarithmic layer. Particle image velocimetry (PIV) measurements show that the LEBU alters the wall-normal velocity gradient and Reynolds shear stress not only near its location but also close to the wall, thereby directly affecting the near-wall turbulence cycle; this is also seen from the hotwire measurements downstream of LEBU. An internal layer formed over the surface of the LEBU interacts with the oncoming eddies and introduces synthetic scales inside the TBL (typically three times the boundary-layer thickness) whose signature can be found in the perturbed boundary layer. Proper orthogonal decomposition is carried out on PIV frames to understand the dominant modes with and without the presence of LEBU. These results show that a LEBU can affect the structure of the oncoming TBL in a complex manner and act as an effective scale manipulator for the TBL. More results on the effect of LEBU will be presented during the talk. |
Tuesday, November 22, 2022 9:57AM - 10:10AM |
U37.00010: Quantification of anisotropic eddy viscosity in a separated turbulent boundary layer with a spanwise sweep Danah Park, Ali Mani We study anisotropic effects of a separated boundary layer flow with a spanwise sweep at Reθ=350. Following the similar canonical settings in the literature, a separation bubble is induced on a fully turbulent boundary layer over a flat plate using a suction and blowing boundary condition. Furthermore, the spanwise sweep is applied at a 35° angle to the chordwise direction to generate a three-dimensional separation effect. We use the macroscopic forcing method (MFM) to quantify the key components in the leading-order anisotropic eddy viscosity. Finally, we compare the eddy viscosity with the no-sweep separated boundary layer to understand the missing pieces in the current turbulence models for separated flows. |
Tuesday, November 22, 2022 10:10AM - 10:23AM |
U37.00011: Non-intrusive sensing of wall-temperature fluctuations beneath a turbulent boundary layer for wall-based estimation of turbulent structures Firoozeh Foroozan, Woutijn J Baars, Andrea Ianiro, Stefano Discetti An experimental study was designed to capture the time-resolved wall-footprint of velocity fluctuations within a turbulent boundary layer. A central facet of this proof-of-concept study involved a non-intrusive film-based sensor at the wall, which was imaged using high-speed infrared thermography (IRT) to obtain a 2D field of the convective heat transfer fluctuations. Simultaneously, velocity fluctuations within the boundary layer were measured using planar 2D2C PIV in both wall-parallel and wall-normal-streamwise planes above the heat transfer sensor. Experiments were carried out in an open-loop wind tunnel facility at the Faculty of Aerospace Engineering of the Delft University of Technology, and comprised a flat plate of 60cm in width and 360cm in length, at a zero-pressure-gradient condition (achieved using a flexible ceiling). A turbulent boundary layer was generated with the aid of a P40 sandpaper trip and measurements were performed at 300cm downstream of the trip at two friction Reynolds numbers: 990 and 1800, at a free-stream velocity of 5m/s and 10m/s, respectively. For the heat transfer measurements, a heated-thin-foil sensor was flush-mounted within the wall. The sensor was made of a 10µm thin stainless-steel foil, and was heated by a direct current that was uniformly applied to the foil. Time-resolved heat transfer measurements yielded the instantaneous wall-temperature field and the convective heat transfer coefficient. The fluctuations of the thermal quantities at the wall depict the footprint of the wall-attached flow structures. Also, the synchronized temperature and velocity measurements allowed for a direct investigating of the correlation between the off-the-wall velocity fluctuations and the heat transfer fluctuations at the wall. The demonstration of the sensor proofs it to be particularly valuable for realizing real-time, wall-based flow control methodologies aimed at heat transfer enhancement or turbulent drag reduction. |
Tuesday, November 22, 2022 10:23AM - 10:36AM |
U37.00012: Wall Curvature and Thermal Effects on 3-D Lagrangian Coherent Structures in a Supersonic, Turbulent Boundary Layer Guillermo Araya, Christian J Lagares Compressible, spatially-evolving turbulent boundary layers are of great importance across civilian and military applications. Understanding transport phenomena is critical to efficient high-speed vehicle designs. Although at any instantaneous point in time a flow field may seem random, regions within the flow can exhibit coherency across space and time. These coherent structures play a key role in momentum and energy transport within the boundary layer. In this work, we focus on three-dimensional Lagrangian Coherent Structure (LCS) based on the finite-time Lyapunov exponent (FTLE) for three wall thermal conditions (cooling, quasi-adiabatic and heating). The flow is subject to a strong concave curvature (δ/R ≈ -0.083, δ is the boundary layer thickness and R is the curvature radius) followed by a very strong convex curvature (δ/R ≈ 0.17). A GPU-accelerated particle simulation forms the basis for the 3-D FTLE where particles are advected over flow fields obtained via Direct Numerical Simulation (DNS) with high spatial/temporal resolution. We have observed a growing anisotropic character in the coherent structures as wall cooling is applied. A high correlation is observed between ejection events, Q2, and attracting manifolds (backward time integration of particle's trajectories). |
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