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 KarmanHowarthMoninHill (KHMH) equation. A binary indicator is used to detect the interface and employed to define twopoint intermittencies. These are used to decompose the interscale and interspace energy fluxes into conditionallyaveraged 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 threedimensional 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 conditionallyaveraged KHMH equation is also derived. We compare the threedimensional maps of the conditionallyaveraged production and total energy flux within turbulent spots against the maps of standardaveraged quantities within the fully turbulent region. The results indicate remarkable similarities in the twopoint 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 Twodimensional supersonic flow over several compression corners have been studied using Direct Simulation Monte Carlo (DSMC) and linear BiGlobal stability analysed. The steadystate 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 twodimensional 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 longtime behaviour causing transition to occur in specific threedimensional 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: InputOutput analysis prediction of hypersonic laminarturbulent transition over slender spherecones Michael C Krygier, Neal P Bitter, Jeffrey A Fike, Ross M Wagnild The transition between laminarturbulent flow is important to hypersonic reentry 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 inputoutput (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 NavierStokes 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 boundarylayer transition due to distributed surface roughness Rong Ma, Krishnan Mahesh The influence of roughness distribution with different streamwise and spanwise spacings on laminarturbulent transition in wallbounded 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 firstrow 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 highmomentum 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 lowspeed 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 firstrow roughness. The highspeed 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 firstrow roughness runs onto the second row, and extracts energy from the spanwise shear between the high and lowspeed streaks. 
Tuesday, November 22, 2022 8:52AM  9:05AM 
U32.00005: Travelling and Stationary Crossflow Instability and Control Experiments Involving a Swept FinCone 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 3D 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 twodimensional 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 highorder 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 highorder 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

Tuesday, November 22, 2022 9:31AM  9:44AM 
U32.00008: Laminarturbulent 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 NavierStokes equations in truncated frequency/wavenumber space using the Harmonic Balance method. Nonlinear optimisation is employed to calculate the optimal forcingresponse mechanisms that maximise the skin friction coefficient of wallbounded flows with localised separation, socalled laminar separation bubbles. We show that an efficient path to transition is initiated by the KelvinHelmholtz (KH) instability which then nonlinearly transfers energy to streamwise streaks and KH superharmonic waves. The interacting multimodal 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 laminarturbulent 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: OneWay NavierStokes (OWNS) Approach for Nonlinear Analysis of Instability and Transition in BoundaryLayer Flows Michael Sleeman, Matthew T Lakebrink, Tim Colonius This work extends the OneWay 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 boundarylayer flows. The OWNS approach has previously been used for linear analysis of boundarylayer flows. In linear OWNS, the linearized NavierStokes 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 initialvalue 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 NavierStokes equations are marched in the downstream direction. At each step of the march, a projection operator (based on the linearized NavierStokes equations), is applied to the fully nonlinear equations to remove modes with upstream group velocity. We examine the spatial stability of twodimensional 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 boundarylayer with surface imperfection was investigated. Three geometries were considered: backward and forwardfacing steps (BFS and FFS), and a cavity. The stepsizes and cavity depth were a small fraction of the boundarylayer 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 smallstep 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 backwardfacing step with a corresponding step height. Although the imperfections resulted, in most cases, in early transition, the features of a standard Ktype 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/flatplate configurations using timeresolved particle image velocimetry and hydrogen bubble visualization techniques. Compared to the canonical freestream 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 FSTinduced bypass transition. It is found that the receptivity is characterized by the formation of spanwise either singlesecondary vortex or doublesecondary vortices, depending on whether the disturbances related to the highorder 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 multielement airfoil configuration and current investigations, it is confirmed that the formations of singlesecondary vortex and doublesecondary vortices are typical behaviors of boundary layer receptivity to periodically passing vortical disturbances. 
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