Three-dimensional Boundary-layer Transition Research Articles

Numerical and Experimental Analysis of Three-Dimensional Boundary-Layer Transition Induced by Isolated Cylindrical Roughness Elements

Numerical and Experimental Analysis of Three-Dimensional Boundary-Layer Transition Induced by Isolated Cylindrical Roughness Elements

Cross-flow transition model predictions of hypersonic transition research vehicle

Hypersonic transition research vehicle (HyTRV) is a customized lifting body for studying hypersonic three-dimensional boundary-layer transition on a complex geometry. In this study, the transition model of an HyTRV lifting body at relatively high angles of attacks is investigated. A hypersonic cross-flow transition correlation is embedded in the γ-Re˜θt transition model on the Chant2.0 computing platform, called the C-γ-Reθ transition model. The transition model is validated in a series of typical three-dimensional aerodynamic configurations. HyTRV's flow characteristics and transition phenomenon are investigated. The model predictions are in good agreement with the stability analysis results and shockwave wind tunnel flow visualizations, exhibiting the model potential for hypersonic cross-flow transition prediction. The results of wind tunnel experiments, stability analysis, and model predictions confirm that transition regions on the upper/lower surface of HyTRV are dominated by cross-flow instabilities. With an increase in the angle of attack, the circumferential pressure gradient on the lower surface decreases and cross-flow intensity reduces; the transition onset location recesses until finally disappears. 2–3 pairs of isolated cross-flow transition zones exist on the surface of the HyTRV at a low angle of attack, which guarantees the HyTRV to be a typical validation case for cross-flow transition modeling.

Quantification technique of transition to turbulence in boundary layers using infrared thermography

The problem of accurate experimental detection of laminar-turbulent transition in three-dimensional boundary layers is discussed. A high-resolving panoramic experimental technique to estimate the onset and length of the transition for aerodynamic applications is proposed and described in detail. The technique includes multistage processing of recorded infrared image sequences to provide a robust detection of the transition in a broad range of base flow parameters. The data processing is fast and virtually hands-free that allows extensive parametric investigation of the transition in experiments. Moreover, an application of statistics procedures to obtain various spanwise-averaged values quantifying different aspects of the transition important for calibrating and validating engineering transition prediction methods is possible.

Predicting crossflow induced transition with laminar kinetic energy transition model

The hypersonic laminar kinetic energy transition model is developed to be appropriate for crossflow induced boundary layer transition prediction. A crossflow timescale is constructed and incorporated in the kT-kL-ω transition model to reflect crossflow effect during three-dimensional boundary layer transition. The stream-wise vorticity is selected as the indicator of crossflow strength. Regarding the inviscid unstable characteristic of crossflow instability, the crossflow timescale is constructed by reference to the second mode timescale. To eliminate inappropriate development of the crossflow timescale where the effective length scale is large enough while the crossflow strength remains at a quite low level, a crossflow velocity limit function is proposed. The revised kT-kL-ω transition model has been applied to HIFiRE-5 and blunt cone with 1°angle of attack test cases. Results show good correspondence with the experimental data and DNS data, which demonstrates that the constructed crossflow timescale makes the revised transition model capable of reproducing crossflow induced transition behavior with a reasonable degree of accuracy.

A combined criteria-based method for hypersonic three-dimensional boundary layer transition prediction

In this paper, a new method for predicting hypersonic three-dimensional (3D) boundary layer transition is developed. It is based on the Reθ/Me criterion for streamwise instability and on the Recf,new criterion for crossflow instability. An intermittency function is also formulated and applied to combine laminar and turbulent flows. Additionally, a computational grid pretreating method, compatible with modern computational fluid dynamics (CFD) techniques based on parallel execution is adopted in order to obtain the boundary layer parameters. Four criteria are compared to define the boundary layer edge. A HIFiRE-5 elliptic cone at different Reynolds numbers is adopted to validate the performance of the criteria-based transition model and the effectiveness of the four criteria for boundary layer edge definition. The results show that the boundary layer edge of complex hypersonic 3D flows could be obtained properly with the computational grid pretreating method and the combination of h0/h0,∞=0.995 and (du/dH)/(du/dH)w=0.1 criteria. Moreover, the computed Reθ/Me and Recf,new distributions in the region between the leading edge and the centerline are similar to the N-factor for streamwise and crossflow instabilities from linear parabolized stability equation (PSE) methods. The shape and trend of the transition onsets predicted by the criteria-based transition model between the centerline and leading edge of HIFiRE-5 agree well with the experiment. However, as for the transition on the centerline, which is dominated by the inflection point in streamwise velocity profiles, using the criteria-based transition model related to boundary layer thickness would predict the transition onset prematurely.

Passive control of transition in three-dimensional boundary layers, with emphasis on discrete roughness elements

A brief review of laminar flow control techniques is given and a strategy for achieving laminarization for transonic transport aircraft is discussed. A review of some flight-test results on swept-wing transition is presented. It is also shown that polished leading edges can create large regions of laminar flow because the flight environment is relatively turbulence free and the surface finish reduces the initial amplitude of the stationary crossflow vortex.

Basic experiments on transition in three-dimensional boundary layers dominated by crossflow instability

The objective of this review is to summarize the results of basic experiments on the transition process in three-dimensional boundary layers governed by crossflow instability and to inform on the present state of knowledge. In particular, the intent is to give a detailed description of the essential physical features rather than a complete survey on all the experiments known so far. For convenience, emphasis is placed on the experiments carried out at DLR in Göttingen, however, essential contributions of other experiments which complete the physical understanding of the problem or lead to different conclusions are also discussed. The DLR experiments were specifically designed to study the transition process dominated by crossflow instability and to create the database for theoretical and numerical analyses of the disturbance development up to the final breakdown of laminar flow and for transition prediction. These analyses should take into account the physics of transition in a more comprehensive manner than the e N-method allows. It will be shown why, in contrast to transition dominated by Tollmien–Schlichting instability, details of the upstream conditions and the related nonlinear development cannot be ignored. The possibilities for their inclusion are discussed. In this regard, it is of special interest to verify that modern stability analyses enable the quantitative description of the linear and nonlinear disturbance development. High emphasis is placed on comparisons between experiment and nonlinear PSE and DNS approaches.

The nonlinear evolution of centrifugal instabilities in curved, compressible mixing layers

The nonlinear development of Görtler vortices in a curved compressible mixing layer is studied. It has been shown both experimentally and theoretically that the curved mixing layer can support a centrifugal mode, which is believed to be similar to the Görtler vortex. This study follows the corresponding incompressible study of Seddougui & Otto [Proceedings of the IUTAM Symposium on Nonlinear Instability and Transition in Three-Dimensional Boundary Layers, edited by P. W. Duck and P. Hall (Kluwer Academic, Amsterdam, 1995)] and attempts to demonstrate the effect compressibility has on the growth of such modes. The formulation follows that of Hall & Lakin [Proc. R. Soc. London, Ser. A 415, 421 (1988)] for the corresponding incompressible boundary-layer problem, and utilises a stream function formulation to solve the governing equations. We will present the nonlinear structure of the so-called thermal modes, whose linear counterparts were first discovered in Owen, Seddougui & Otto [Phys. Fluids 9, 2506 (1997)]. A brief discussion will be made concerning the effect of the nonlinear modes on the inherent Kelvin–Helmholtz modes of instability, which may have significant ramifications in flows dominated by this form of instability.

Stability Analysis of Boundary Layer on a Sphere Rotating in Still Fluid.

Theoretical study of three-dimensional boundary layer transition on a sphere rotating in still fluid has been carried out by a linear stability analysis. It is shown that the neutral stability curve shifts toward the lower Reynolds number and the lower spiral vortex number, either closer to the equator or with the decrease in the vortex angle. The critical Reynolds numbers obtained in the analysis are close to but smaller than the experimental results. The rotational speed ratio between the spiral vortices and the surface agrees with the experiments. It is also shown that the critical Reynolds number becomes insensitive to the vortex angle near the equator. On the other hand, the critical wave number of spiral vortices becomes larger with the increase in the vortex angle and decreases near the equator. Computation including the streamline-curvature effects in general gives the critical reynolds numbers closer to the experiments.

The Stability of Boundary Layer on a Rotating Sphere in Still Fluid.

Theoretical study of three-dimensional boundary layer transition on a sphere rotating in still fluid has been carried out by a linear stability analysis. It was determined from experimental results that the relative location of spiral vortices which appear in the transition region is fixed against the rotating sphere surface when the rotating effect is large. However as the rotating effect decreases, these vortices begin to move. So far, this phenomenon has not been explained theoretically. In this study, the boundary layer instability and the characteristics of spiral vortices are theoretically investigated by calculating the behavior of the small disturbances and the critical Reynolds number of the boundary layer on a rotating sphere using the linear stability theory.

Three-dimensional boundary-layer transition on a swept wing at Mach 3.5

Transition on a swept-wing leading-edge model at Mach 3.5 is investigated. Surface pressure and temperature measurements are obtained in the NASA Langley Research Center Supersonic Low-Disturbance Tunnel. For one case, temperature-sensitive paint and a sublimating chemical are used to visualize surface flow features such as transition location. The experimental data are compared with 1) mean-flow results computed as solutions to the thin-layer Navier-Stokes equations and 2) N-factors obtained using the envelope e N method. The experimental and computational results compare favorably in most cases. In particular, N ≃ 13 correlates best with the observed transition location over a range of freestream unit Reynolds numbers and angles of attack. Computed traveling crossflow disturbances with frequencies of 40-60 kHz have the largest N factors, and the surface flow visualizations reveal smooth transition fronts with only faint evidence of stationary crossflow vortices. These results suggest that transition is probably dominated by traveling, rather than stationary, crossflow disturbances for the present model.

Review: Laminar-to-Turbulent Transition of Three-Dimensional Boundary Layers on Rotating Bodies

The laminar-turbulent transition of three-dimensional boundary layers is critically reviewed for some typical axisymmetric bodies rotating in still fluid or in axial flow. The flow structures of the transition regions are visualized. The transition phenomena are driven by the compound of the Tollmien-Schlichting instability, the crossflow instability, and the centrifugal instability. Experimental evidence is provided relating the critical and transition Reynolds numbers, defined in terms of the local velocity and the boundary layer momentum thickness, to the local rotational speed ratio, defined as the ratio of the circumferential speed to the free-stream velocity at the outer edge of the boundary layer, for the rotating disk, the rotating cone, the rotating sphere and other rotating axisymmetric bodies. It is shown that the cross-sectional structure of spiral vortices appearing in the transition regions and the flow pattern of the following secondary instability in the case of the crossflow instability are clearly different than those in the case of the centrifugal instability.

Direct numerical simulation of passive control of three-dimensional phenomena in boundary-layer transition using wall heating

A numerical model is presented for investigating control of the three-dimensional boundary-layer transition process. Control of a periodically forced, spatially evolving boundary layer in water is studied using surface heating techniques. The Navier–Stokes and energy equations are integrated using a fully implicit finite difference/spectral method. The Navier–Stokes equations are used in vorticity–velocity form and are coupled with the energy equation through the viscosity dependence on temperature. Passive control of small amplitude two-dimensional waves and three-dimensional oblique waves is numerically simulated with either uniform or non-uniform wall heating applied. Both amplitude levels and amplification rates are strongly reduced with heating applied. Comparison is made with parallel and non-parallel linear stability theory and experiments. Control of the early stages of the nonlinear breakdown process is also investigated using uniform wall heating. Both control of the fundamental and subharmonic routes to turbulence are investigated. For both breakdown processes, a strong reduction in amplitude levels and growth rates results. In particular, the high three-dimensional growth rates that are characteristic of the secondary instability process are significantly reduced below the uncontrolled levels.

Laminar-Turbulent Transition of Three-Dimensional Boundary Layers.

Laminar-Turbulent Transition of Three-Dimensional Boundary Layers.

Three-dimensional boundary-layer transition on a cone at Mach 3.5

A boundary-layer transition study on a sharp, 5° half-angle cone at various angles of attack was conducted at Mach 3.5. Transition data were obtained with and without significantly reduced freestream acoustic disturbance levels. A progressive downstream and upstream motion of the transition front on the windward and leeward rays, respectively, of the cone with angle of attack was observed for the high noise level data in agreement with data trends obtained in conventional (“noisy”) wind tunnels. However, the downstream movement was not observed to the same degree for the low noise level data in the present study. Transition believed to be crossflow dominated was found to be less receptive to freestream acoustic disturbances than first-mode (Tollmien-Schlichting) dominated transition. The previously-developed crossflow transition Reynolds number criterion, χtr,max≈200, was found to be inadequate for the current case. An improved criterion is offered, which includes compressibility and flow-geometry effects.

Laminar-Turbulent Transition on a Swept Wing.

Three-dimensional boundary-layer transition experiments are currently being conducted on a 45° swept wing in the Arizona State University Unsteady Wind Tunnel. Crossflow-dominated transition is produced via a model with contoured end liners to simulate infinite swept-wing flow. Fixed-wavelength, stationary and traveling crossflow vortices are observed. The frequencies of the most amplified traveling waves are in agreement with linear-stability theory; however, traveling waves at frequencies an order of magnitude higher than predicted are also observed near transition. Boundary-layer profiles measured at several spanwise locations show streamwise disturbance profiles characteristic of the crossflow instability. Near the transition location, severe distortions of the steady boundary-layer profiles are observed in the form of multiple inflection points. This distorted boundary layer is due to the stationary crossflow vortex and is subject to a Rayleigh-type instability in the stream direction. As a result, a high-frequency secondary instability is detected in the transition region, and spatial relations of the process are well documented by the coupled use of flow visualization and hot-wire measuremests. In all cases, we observe a transition due to this high-frequency Rayleigh instability induced by the stationary crossflow vortex.

Relation Between Three-Dimensional Boundary-Layer Transition and Cavitation on a Yawed Cylinder.

The relation between cavitation phenomena and three-dimensional boundary-layer transition on a 45-degree yawed cylinder was investigated using both wind tunnel and water tunnel facilities. Regularly arranged appearance of the cavitating bubbles on the cylinder surface seems to be caused by the crossflow vortices in the transition region of the same boundary layer. Physical properties like wavelengths are compared, and the results are in good agreement. Details of the two different phenomena are compared and discussed.

Three-dimensional boundary layer transition on a concave surface

Measurements of streamwise mean and r.m.s. velocity, intermittency factor and energy spectra are reported for a laminar boundary layer undergoing transition on a concave wall in the presence of a naturally occurring Görtler vortex system, concentrating on a particular vortex pair. The ratio of boundary layer thickness to wall radius at the start of curvature was 0.01. Nonlinear amplification of the vortices, followed by spanwise meandering and a secondary instability, preceded transition, which was defined in terms of the intermittency factor after filtering out low frequencies associated with this vortex behavior. Transition was initiated in the vortex upwash region and apparently completed within a streamwise distance of only a few boundary layer thickness at both upwash and downwash locations, implying rapid lateral spreading of turbulence. With considerable velocity profile ditortion prior to transition, values of momentum thickness Reynolds number and Görtler number at start of transition, either upwash-localized or upwash-downwash averaged, did not correspond well with established correlations for flat surfaces or with earlier concave-wall studies.

Some expectation on the mechanism of cross-flow instability in a swept wing flow

Three-dimensional boundary layer transition on axisymmetric rotating bodies is the subject of a comprehensive experimental study. Based on this study, hypotheses are made on the mechanism of cross-flow instability for swept wing flow. These new results are combined with past explanations to provide a rough sketch for the entire flow field over the swept wing. From this new viewpoint there appears the mechanism of traveling waves, being induced by a stationary disturbance. Some uncertainties appearing in recent papers concerning this flow field are discussed. Among these uncertainties for which an explanation is provided, is the discrepancy of frequencies between the hot wire signal and the visualized flow pattern.

The boundary-layer transition on rotating cones in axial flow with free-stream turbulence.

This paper is concerned with the laminar-turbulent transition of three-dimensional boundary layers on cones rotating in external axial flow. The experiment was carried out for three cone angles of 15°, 30° and 60° in a range of free-stream turbulence (0.04% to 3.5%) using a hot-wire anemometer and a flow-visualization technique. The transition region was determined with the critical Reynolds number and the transition Reynolds number in relation to the rotational speed ratio. The spiral vortices were found to fix relative to the rotating surface. The relation of the direction of the spiral vortices to the rotational speed ratio remains unchanged as the cone angle becomes larger, while the number of vortices increases with increased cone angle. These experimental results were compared with the present numerical results based on the linear stability theory. The free-stream turbulence has no effect on the transition Reynolds number, while the effect becomes more sensitive to the critical Reynolds number as the cone angle is increased.