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 U32: Flow Instability: Boundary Layers and Transition I |
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Chair: George Papadakis, Imperial College London Room: 240 |
Tuesday, November 22, 2022 8:00AM - 8:13AM |
U32.00001: On the role of laminar/turbulent interface on energy transfer between scales in bypass transition George Papadakis, Hanxun Yao The role of laminar/turbulent interface on the interscale energy transfer in a boundary layer undergoing bypass transition is investigated with the aid of Karman-Howarth-Monin-Hill (KHMH) equation. A binary indicator is used to detect the interface and employed to define two-point intermittencies. These are used to decompose the interscale and interspace energy fluxes into conditionally-averaged components. We find that the inverse cascade in the streamwise direction arises due to events across the downstream or upstream interfaces (head or tail respectively) of a turbulent spot. The three-dimensional energy flux maps reveal significant differences between these two regions: in the downstream interface, inverse cascade is stronger and dominant over a larger range of streamwise and spanwise separations. This is explained by considering a propagating spot as it crosses a fixed streamwise location. The conditionally-averaged KHMH equation is also derived. We compare the three-dimensional maps of the conditionally-averaged production and total energy flux within turbulent spots against the maps of standard-averaged quantities within the fully turbulent region. The results indicate remarkable similarities in the two-point statistics between spots and the fully turbulent region. |
Tuesday, November 22, 2022 8:13AM - 8:26AM |
U32.00002: On the stability of rarefied gas flows over a compression corner calculated using kinetic methods Nicolas Cerulus, Irmak Taylan Karpuzcu, Deborah A Levin, Vassilis Theofilis Two-dimensional supersonic flow over several compression corners have been studied using Direct Simulation Monte Carlo (DSMC) and linear BiGlobal stability analysed. The steady-state base flow is dominated by a large separation bubble, which becomes larger at increasing ramp angles. The maximum recirculation, calculated by reference to the free stream velocity, is found to be around 10% for all cases. Linear stability analysis is performed on the two-dimensional DSMC states by imposing a wide range of spanwise wavelengths in order to understand the transition behaviour of such flows. At long times these flows are shown to be linearly stable despite the high ramp angles used. The dominant stationary least damped mode persists over a range of wavenumbers and its amplitude function is consistent with those shown in many previous separated flow studies. While, at short time behaviour, through the solution of an initial value problem, there are signs of significant energy growth of disturbances that may lead to the bypass of the long-time behaviour causing transition to occur in specific three-dimensional cases. These potential peaks in energy gain are currently being studied more closely to identify the scenarios most at risk of causing a transition to turbulence. |
Tuesday, November 22, 2022 8:26AM - 8:39AM |
U32.00003: Input-Output analysis prediction of hypersonic laminar-turbulent transition over slender sphere-cones Michael C Krygier, Neal P Bitter, Jeffrey A Fike, Ross M Wagnild The transition between laminar-turbulent flow is important to hypersonic re-entry vehicle design due to the associated rise in heat flux and shear stress. In particular, the location along the surface of the vehicle at which transition occurs is critical to know to mitigate large heating loads using thermal protective systems to prevent structural damage. Moreover, if the flow over the vehicle is turbulent, the heating rate can be 4x larger than that of laminar flow. Numerical simulations are unable to reliably predict the transition location a priori. Instead, boundary layer transition predictions are traditionally made using empirical correlations calibrated against historical flight data. But these correlations may be inaccurate when changes to the vehicle geometry or trajectory are made. In this work, we utilize computational fluid dynamics with input-output (IO) analysis to predict the transition location of hypersonic flow over both a sharp and blunt cone. We accomplish this goal by linearizing the compressible Navier-Stokes equations around a baseflow and applying the IO method to extract out the frequency that produces the largest disturbance. We find that slow acoustic waves reproduce the transition to turbulence on the sharp cone. Whereas, for the blunt cone, an optimized disturbance is the trigger to turbulence. Finally, we compare the results we find to those in the literature and experimental wind tunnel data. |
Tuesday, November 22, 2022 8:39AM - 8:52AM |
U32.00004: Effects of roughness spacing on boundary-layer transition due to distributed surface roughness Rong Ma, Krishnan Mahesh The influence of roughness distribution with different streamwise and spanwise spacings on laminar-turbulent transition in wall-bounded flows is investigated using direct numerical simulation and global linear stability analysis. The results are compared to an isolated roughness element with aspect ratio \eta=1. The small spanwise spacing (\lambda_z=2.5h) inhibits the formation of hairpin vortices and prevents transition to occur. For \lambda_z=5h, the hairpin vortices are induced by the first-row roughness, perturbing the downstream shear layer and cause transition. The periodicity of the primary hairpin vortices is independent on the streamwise spacing and the distributed surface roughness leads to lower critical Re_h for instability to occur. When the streamwise spacing is comparable to the region of flow separation (\lambda_x=5h), the high-momentum fluid hardly moves downward into the cavities and the wake flow has little impact on the following roughness elements. The leading unstable varicose mode is associated with the central low-speed streaks along the aligned roughness elements. For larger streamwise spacing ($\lambda_x=10h$), two distinct modes are obtained from global stability analysis. The first one presents varicose symmetry, corresponding to the primary hairpin vortex shedding induced by the first-row roughness. The high-speed streaks form in the longitudinal grooves are also found to be unstable and modulated with the varicose mode. The second one is a sinuous mode with lower frequency, induced as the wake flow of the first-row roughness runs onto the second row, and extracts energy from the spanwise shear between the high- and low-speed streaks. |
Tuesday, November 22, 2022 8:52AM - 9:05AM |
U32.00005: Travelling and Stationary Crossflow Instability and Control Experiments Involving a Swept Fin-Cone Model in Mach 6 Flow John B Middlebrooks, Thomas C Corke, Eric H Matlis, Michael T Semper The harshness of the hypersonic environment presents numerous technical challenges. The stability of the boundary layer over a vehicle in this regime is still a major issue in flight vehicle design. If the boundary layer becomes turbulent, aerothermodynamic heating and viscous effects such as skin friction dramatically increase. One method that a 3-D boundary layer may become turbulent is through an inviscid mechanism known as the crossflow instability. The crossflow instability has been well studied at subsonic speeds, but comparatively few crossflow experiments have been done in hypersonic flow. Even fewer hypersonic crossflow studies have been conducted where explicit attempts at influencing the location of boundary layer transition (flow control) have been made. |
Tuesday, November 22, 2022 9:05AM - 9:18AM |
U32.00006: Hypersonic Boundary Layer Stability of Local Cooling Strip and Porous Surface Furkan Oz, Kursat Kara Hypersonic boundary layer transition is one problem the modern aviation world tries to overcome for sustained hypersonic flight. The early laminar to turbulent transition over the air vehicle's body leads to increased heat transfer and aerodynamic drag, which decreases the vehicle's performance. Although thermal protection systems can alleviate the drawbacks, they increase the cost and weight of the aircraft and require frequent maintenance. In the literature, researchers proposed promising solutions to stabilize the instabilities that lead to hypersonic boundary layer transition from laminar to turbulent flow. Local wall cooling is one method that stabilizes Mack's first mode. However, second mode instabilities are the dominant waves in two-dimensional boundary layer flow at hypersonic speeds. The porous surface on the solid wall may stabilize the second mode instabilities. The drawback of porous surfaces is the destabilization in the first mode instabilities. Stabilizing the first mode with the local cooling strip and the second mode with the porous surface may delay the transition significantly. This study developed a high-order linear stability code (HLST) to solve the eigenvalue problem for the combined local cooling and porous surface (LCPS) application. Flow over a flat plate, wedge and cone are solved in hypersonic flow conditions to validate the results with the literature. The similarity solution and a high-order accurate steady flow solver are used to obtain the boundary layer profiles. The effect of the local cooling strip and porous surface on the hypersonic boundary layer is investigated. |
Tuesday, November 22, 2022 9:18AM - 9:31AM |
U32.00007: Instability of flow over a supersonic ramp Henry Broadley, Richard Hewitt
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Tuesday, November 22, 2022 9:31AM - 9:44AM |
U32.00008: Laminar-turbulent transition mechanisms of separated flows using the Harmonic Balance method Flavio Savarino, Denis Sipp, Georgios Rigas Flow separation affects the performance of aerodynamic designs due to the increased drag and unsteadiness arising from the laminar to turbulence transition of the separated shear layer. To date, most numerical techniques to analyse the stability of laminar boundary layers subject to external disturbances are linear and thus fail to describe the nonlinear transitional and turbulent regimes. In this study, we apply a recently proposed computational framework (Rigas et al., JFM 2021) that solves the nonlinear Navier-Stokes equations in truncated frequency/wavenumber space using the Harmonic Balance method. Nonlinear optimisation is employed to calculate the optimal forcing-response mechanisms that maximise the skin friction coefficient of wall-bounded flows with localised separation, so-called laminar separation bubbles. We show that an efficient path to transition is initiated by the Kelvin-Helmholtz (KH) instability which then nonlinearly transfers energy to streamwise streaks and KH super-harmonic waves. The interacting multi-modal instabilities cause spanwise deformation of the mean bubble and earlier reattachment of the shear layer. |
Tuesday, November 22, 2022 9:44AM - 9:57AM |
U32.00009: Computational Study of Transition on a Flared Cone Using Random Forcing Andrew J Shuck, Jonathan Poggie, Gregory A Blaisdell Incorporating boundary layer transition prediction in the design of high speed vehicles is necessary to ensure safe and efficient operation. The heat transfer to the surface of the vehicle increases significantly when the boundary layer transitions from laminar to turbulent flow. To study boundary layer transition, DNS is needed to resolve the large range of length scales required to accurately model the nonlinear interactions that occur in the boundary layer. Furthermore, forcing is required to lead the flow to laminar-turbulent transition. In this study, randomly generated traveling plane waves are used to perturb the flow and promote transition. To more closely align with experiments, the amplitude vector of these plane waves is set as the freestream noise profiles of a wind tunnel. Using this approach, the DNS successfully achieved transition and resulted in similar findings as those in previous experimental and computational studies. |
Tuesday, November 22, 2022 9:57AM - 10:10AM |
U32.00010: One-Way Navier-Stokes (OWNS) Approach for Nonlinear Analysis of Instability and Transition in Boundary-Layer Flows Michael Sleeman, Matthew T Lakebrink, Tim Colonius This work extends the One-Way Navier Stokes (OWNS) approach so that it supports nonlinear interactions between waves of different frequencies, which will enable nonlinear analysis of instability and transition in boundary-layer flows. The OWNS approach has previously been used for linear analysis of boundary-layer flows. In linear OWNS, the linearized Navier-Stokes equations are modified such that all upstream propagating modes are removed. The resulting equations can be solved efficiently in the frequency domain as a spatial initial-value problem, where an initial perturbation is specified at the domain inlet and evolved in the downstream direction using spatial integration. In nonlinear OWNS, the fully nonlinear Navier-Stokes equations are marched in the downstream direction. At each step of the march, a projection operator (based on the linearized Navier-Stokes equations), is applied to the fully nonlinear equations to remove modes with upstream group velocity. We examine the spatial stability of two-dimensional boundary layers, corresponding to Blasius flows. The method is validated against results from the literature, including results based on direct numerical simulation (DNS) and the nonlinear parabolized stability equations (PSE). |
Tuesday, November 22, 2022 10:10AM - 10:23AM |
U32.00011: Effect of surface imperfections on the transition of a Blasius boundary layer Ming TENG, Ugo Piomelli The evolution to turbulence of a boundary-layer with surface imperfection was investigated. Three geometries were considered: backward and forward-facing steps (BFS and FFS), and a cavity. The step-sizes and cavity depth were a small fraction of the boundary-layer thickness. Transition was initiated using a narrow ribbon upstream of the surface imperfection, which generated small and monochromatic perturbations by periodic blowing and suction. A direct numerical simulation was performed. The transition was delayed for the small-step FFS case, whereas in all other cases it was accelerated. The BFS caused early transition, promoted by the instability of the separated shear layer. In the FFS a small region of local stabilisation was present immediately downstream of the step, where the Reynolds shear stress was negative. For the cavity, transiton was also accelerated, and the evolution of the flow features resembled that of the backward-facing step with a corresponding step height. Although the imperfections resulted, in most cases, in early transition, the features of a standard K-type transition were observed in all cases. |
Tuesday, November 22, 2022 10:23AM - 10:36AM |
U32.00012: Boundary layer receptivity to periodically passing vortical disturbances Jiangsheng Wang, Jun Wu, Guosheng He, Jinjun Wang The boundary layer receptivity to periodically passing vortical disturbances is investigated with rod/airfoil and rod/flat-plate configurations using time-resolved particle image velocimetry and hydrogen bubble visualization techniques. Compared to the canonical free-stream turbulence (FST), the periodically passing vortical disturbances introduced by the rod wakes own larger length scale, higher intensity, stronger inhomogeneity and anisotropy, leading to the boundary layer receptivity different from that in FST-induced bypass transition. It is found that the receptivity is characterized by the formation of spanwise either single-secondary vortex or double-secondary vortices, depending on whether the disturbances related to the high-order harmonics of the periodically passing frequency could penetrate the boundary layer and are significantly amplified. The formation of these secondary vortices possesses good periodicity, which makes them different from the shed vortices of a separated boundary layer. After combining previous investigations with a multi-element airfoil configuration and current investigations, it is confirmed that the formations of single-secondary vortex and double-secondary vortices are typical behaviors of boundary layer receptivity to periodically passing vortical disturbances. |
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