Dead-air Region Research Articles

An Experimental Database for the Analysis of Bursting of a Laminar Separation Bubble

The bursting phenomenon consists in the switch of a laminar separation bubble from a short to a long configuration. In the former case, reduced effects on profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations in the loading coefficient upstream of the separation position, with increased risk of stall of the lifting surfaces. The present work presents an experimental database describing separated boundary layers evolving under different Reynolds numbers, adverse pressure gradients and free-stream turbulence levels. Overall, more than 80 flow conditions were tested concerning short and long bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast-response Particle Image Velocimetry (PIV) system. For each flow case, two sets of 6000 flow records were acquired with an acquisition frequency equal to 300 and 1000 Hz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long bubble states. Additionally, the kind of instability (i.e., convective or absolute) developing into the separated boundary layer was identified based on flow statistics. The present data captures the existing link between the bursting of a laminar separation bubble and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region.

The Influence of Boundary Layer State and Trailing Edge Wedge Angle on the Aerodynamic Performance of Transonic Turbine Blades

Abstract Trailing edge loss typically accounts for a third of the profile loss of a transonic turbine blade. In this paper, two dominant influencing factors—boundary layer state and trailing edge wedge angle—are quantified using a combined experimental (single airfoil transonic tests) and computational (wall-resolved LES) approach. Three key findings are reported. First, the switch between two flow regimes—detached vortex shedding and transonic vortex shedding—as the Reynolds number is increased, is identified and explained. It is shown that the transition of the pressure surface boundary layer drives the switch from the former to the latter; this change is associated with a jump in loss of close to 100%. The switch is also shown to affect the base pressure distribution around the trailing edge, changing it from uniform, with an isobaric dead-air region, to non-uniform. Second, the influence of the trailing edge wedge angle is explored. It is found that higher wedge angles can offer performance benefits in both loss and base pressure—with a change from 8 deg to 14 deg geometry providing up to a 29% reduction in an overall loss for the geometry tested. Finally, as these findings are the result of changes to the unsteady flow phenomena around the trailing edge, they would not be picked up by current low-fidelity, steady design tools. A method is proposed to use data, such as that presented in this paper, to improve low-fidelity modeling.

Flow instability and asymmetry over spiked cylinder

In this paper, supersonic unsteady flows around the spiked cylinder are investigated through two-dimensional (2D) and three-dimensional (3D) unsteady Reynolds-averaged Navier–Stokes (URANS) approaches. Three types of spikes are used to provoke pulsating/oscillating flow in front of the cylinder. Numerical simulations of the flow past the cylinder with a pointed spike are validated against the experimental data of Feszty et al. (2004). Results indicate that the pressure imbalance in the dead-air region can be significantly reduced by increasing the aerodome size, which represents an effective method for controlling periodic flow pulsations. The neglect of real 3D effects in the 2D axisymmetric hypothesis tends to alter the amplitude and frequency of the pressure history, and even overestimate the critical aerodome size at which the flow pattern transitions from violent pulsations to mild oscillations. The discrepancy between the 2D axisymmetric and 3D solutions becomes smaller after activation of pulsation control. Through a comparative study, it is observed that in contrast to the case of zero angle of attack, the instantaneous flow field of the non-zero angle of attack case becomes increasingly symmetrical.

Hypersonic flow control of shock wave/turbulent boundary layer interactions using magnetohydrodynamic plasma actuators

The effect of magnetohydrodynamic (MHD) plasma actuators on the control of hypersonic shock wave/turbulent boundary layer interactions is investigated here using Reynolds-averaged Navier-Stokes calculations with low magnetic Reynolds number approximation. A Mach 5 oblique shock/turbulent boundary layer interaction was adopted as the basic configuration in this numerical study in order to assess the effects of flow control using different combinations of magnetic field and plasma. Results show that just the thermal effect of plasma under experimental actuator parameters has no significant impact on the flow field and can therefore be neglected. On the basis of the relative position of control area and separation point, MHD control can be divided into four types and so effects and mechanisms might be different. Amongst these, D-type control leads to the largest reduction in separation length using magnetically-accelerated plasma inside an isobaric dead-air region. A novel parameter for predicting the shock wave/turbulent boundary layer interaction control based on Lorentz force acceleration is then proposed and the controllability of MHD plasma actuators under different MHD interaction parameters is studied. The results of this study will be insightful for the further design of MHD control in hypersonic vehicle inlets.

Heat transfer and air flow near a pair of circular cylinders

Convective heat transfer is associated with the nature of the flow near the streamlined solid. Since flow properties vary quickly, it is important to fix their momentary values. We propose a technique for studying heat transfer and air flow based on combined use of gradient heat flux measurement and PIV. The paper presents velocity fields near a pair of circular cylinders and distribution of heat flux per unit area on the surface of the second cylinder. Both cylinders were heated with saturated water steam at atmospheric pressure, thereby keeping temperature of the cylinders constant. The experiments were carried out in the range of Reynolds numbers from 480 to 29800. Differences in vortex structure, dead-air region’s length etc., and heat transfer are revealed depending on the velocity of free stream and the distance between the cylinders. Use of gradient heat flux sensors allows us to estimate pulsations of heat flux at various points of the second cylinder and compare them with the pictures of instantaneous velocity fields. The results are consistent with data from other authors and show the prospects of the proposed methodology.

Drag and heat flux reduction mechanism of blunted cone with aerodisks

The major challenge among a number of design requirements for hypersonic vehicles is the reduction of drag and aerodynamic heating. Of all these techniques of drag and heat flux reduction, application of forward facing aerospike conceived in 1950s is an effective and simpler technique to reduce the drag as well as the heat transfer rate for blunt nosed bodies at hypersonic Mach numbers. In this paper, the flow fields around a blunt cone with and without aerodisk flying at hypersonic Mach numbers are computed numerically, and the numerical simulations are conducted by specifying the freestream velocity, static pressure and static temperatures at the inlet of the computational domain with a three-dimensional, steady, Reynolds-averaged Navier-Stokes equation. An aerodisk is attached to the tip of the rod to reduce the drag and heat flux further. The influences of the length of rod and the diameter of aerodisk on the drag and heat flux reduction mechanism are analyzed comprehensively, and eight configurations are taken into consideration in the current study. The obtained results show that for all aerodisks, the reduction in drag of the blunt body is proportional to the extent of the recirculation dead air region. For long rods, the aerodisk is found not that beneficial in reducing the drag, and an aerodisk is more effective than an aerospike. The spike produces a region of recirculation separated flow that shields the blunt-nosed body from the incoming flow, and the recirculation region is formed around the root of the spike up to the reattachment point of the flow at the shoulder of the blunt body. The dynamic pressure in the recirculation area is highly reduced and thus leads to the decrease in drag and heat load on the surface of the blunt body. Because of the reattachment of the shear layer on the shoulder of the blunt body, the pressure near that point becomes large.

Relevance of local parallel theory to the linear stability of laminar separation bubbles

AbstractLaminar separation bubbles are thought to be highly non-parallel, and hence global stability studies start from this premise. However, experimentalists have always realized that the flow is more parallel than is commonly believed, for pressure-gradient-induced bubbles, and this is why linear parallel stability theory has been successful in describing their early stages of transition. The present experimental/numerical study re-examines this important issue and finds that the base flow in such a separation bubble becomes nearly parallel due to a strong-interaction process between the separated boundary layer and the outer potential flow. The so-called dead-air region or the region of constant pressure is a simple consequence of this strong interaction. We use triple-deck theory to qualitatively explain these features. Next, the implications of global analysis for the linear stability of separation bubbles are considered. In particular we show that in the initial portion of the bubble, where the flow is nearly parallel, local stability analysis is sufficient to capture the essential physics. It appears that the real utility of the global analysis is perhaps in the rear portion of the bubble, where the flow is highly non-parallel, and where the secondary/nonlinear instability stages are likely to dominate the dynamics.

Investigations into incompressible steady base flows

The turbulent incompressible two-dimensional base flow was investigated in order to get a closer understanding of the laws which determine the base pressure and pressure drag of the body. To this end, the ratio of the turbulent shear stress to the kinetic energy of the turbulent velocity fluctuations, i.e. the parameter α, was predicted for various wedges, and it was detected that in an actual flow α depends solely on the wedge angle and not on the base pressure and pressure drag of the body. The parameter α increases as the wedge angle increases and is for the wedge angle φ = 180 ° (flat plate perpendicular to the freestream) about 1.89 times the value which it has for the wedge angle φ = 0 ° (flow past a backward facing step). The shear stress on the dividing streamline, i.e. ρ u ′ v ′ ¯ , increases even faster with the wedge angle than α, and is for φ = 180 ° about 2.60 times the value which is valid for φ = 0 ° . It was also observed that the length of the dead-air region doesn't depend only on the wedge angle, but also on the ratio of the turbulent shear stress to the kinetic energy of the turbulent velocity fluctuations. Finally, a new method was developed for base pressure prediction of blunt bodies. When the method was applied on wedges, the theoretical values agreed well with the experimental data.

On the origin of the inflectional instability of a laminar separation bubble

This is an experimental and theoretical study of a laminar separation bubble and the associated linear stability mechanisms. Experiments were performed over a flat plate kept in a wind tunnel, with an imposed pressure gradient typical of an aerofoil that would involve a laminar separation bubble. The separation bubble was characterized by measurement of surface-pressure distribution and streamwise velocity using hot-wire anemometry. Single component hot-wire anemometry was also used for a detailed study of the transition dynamics. It was found that the so-called dead-air region in the front portion of the bubble corresponded to a region of small disturbance amplitudes, with the amplitude reaching a maximum value close to the reattachment point. An exponential growth rate of the disturbance was seen in the region upstream of the mean maximum height of the bubble, and this was indicative of a linear instability mechanism at work. An infinitesimal disturbance was impulsively introduced into the boundary layer upstream of separation location, and the wave packet was tracked (in an ensemble-averaged sense) while it was getting advected downstream. The disturbance was found to be convective in nature. Linear stability analyses (both the Orr–Sommerfeld and Rayleigh calculations) were performed for mean velocity profiles, starting from an attached adverse-pressure-gradient boundary layer all the way up to the front portion of the separation-bubble region (i.e. up to the end of the dead-air region in which linear evolution of the disturbance could be expected). The conclusion from the present work is that the primary instability mechanism in a separation bubble is inflectional in nature, and its origin can be traced back to upstream of the separation location. In other words, the inviscid inflectional instability of the separated shear layer should be logically seen as an extension of the instability of the upstream attached adverse-pressure-gradient boundary layer. This modifies the traditional view that pegs the origin of the instability in a separation bubble to the detached shear layer outside the bubble, with its associated Kelvin–Helmholtz mechanism. We contend that only when the separated shear layer has moved considerably away from the wall (and this happens near the maximum-height location of the mean bubble), a description by the Kelvin–Helmholtz instability paradigm, with its associated scaling principles, could become relevant. We also propose a new scaling for the most amplified frequency for a wall-bounded shear layer in terms of the inflection-point height and the vorticity thickness and show it to be universal.

Heat transfer in a laminar separation bubble affected by oscillating external flow

A three-dimensional Direct Numerical Simulation (DNS) of passive heat transfer in a Laminar Separation Bubble (LSB) over a flat plate affected by oscillating external flow is presented. The oscillation imposes a periodicity which is employed for phase-averaging. The flat plate is kept at a uniform, low temperature. The local Nusselt number, Nu, is determined as a function of phase. In the dead-air region of the bubble Nu is found to be relatively small, while it peaks in the recirculation region where hot outer fluid gets entrained and is transported towards the flat plate. Each period a new separation bubble is formed, that merges with the old separation bubble. The reverse flow inside the separation bubble reaches values of up to 60% of the local free-stream velocity, which is sufficient to make the separation bubble absolutely unstable such that self-sustained turbulence can exist. For the phase-averaged flow, neither the turbulent viscosity hypothesis nor the temperature gradient-diffusion hypothesis is found to hold.

Driving Mechanisms of High-Speed Unsteady Spiked Body Flows, Part I: Pulsation Mode

The driving mechanism of the unsteady e ow mode pulsation arising over axisymmetric spiked bodies has been analyzed by using computational e uid dynamics as a tool. Laminar, axisymmetric e ow at Mach 2.21 and Reynolds number (based on the blunt-body diameter) of 0.12 £106 was simulated by a spatially and temporally second-order-accurate e nite volume method. The model geometry was a forward facing cylinder of diameter D equipped with a spike of length L/D=1.00. After reviewing previous pulsation hypotheses, the numerical results were analyzed in detail. A new driving mechanism was proposed, its main features being the creation of a vortical region in the vicinity of the foreshock-aftershock intersection causing mass ine ux into the dead-air region, the existence of supersonic e ow within the dead-air region, the liftoff of the shear layer from the spike tip, and the collision of the recirculated and penetrating e ows within the expanded separated region.

Turbine Blade Trailing Edge Flow Characteristics at High Subsonic Outlet Mach Number

The paper presents an experimental investigationof the effect of the trailing edge vortex shedding on the steady and unsteady trailing blade pressure distribution of a turbine blade at high subsonic Mach number M2,is=0.79 and high Reynolds number RE=2.8×106. The vortex formation and shedding process is visualized using a high-speed schlieren camera and a holographic interferometric density measuring technique. The blade is equipped with a rotatable trailing edge cylinder instrumented side-by-side with a pneumatic pressure tap and a fast response pressure sensor for detailed measurements of the trailing edge pressure distribution. The experiments demonstrate that contrary to the isobaric dead air region demonstrated at low subsonic Mach numbers the data reveal the existence of a highly nonuniform trailing edge pressure distribution with a strong pressure minimum at the center of the trailing edge. This finding is significant for the determination of the base pressure coefficient that is in general measured with a single pressure-sensing hole at the trailing edge center. The paper investigates further the effect of the vortex shedding on the blade rear suction side and discusses the superposition of unsteady effects emanating from the trailing edge and from the neighboring blade. The experimental data are a unique source for the validation of unsteady Navier-Stokes codes.

An Analysis on Flow Field Structures of Two Opposed Supersonic Free Jets. Numerical Simulations by PLM.

The structures of two opposed supersonic free jets with the same and different source pressures are studied by numerical simulation using the piecewise linear method (PLM) based on Godunov's method. For different source pressures, the ratio between the source pressures is set at 1.0 or 2.0. The ratio of the higher source pressure to the back pressure is varied between 50 and l00. The simulation results are analyzed and show agreement with the theoretical and experimetal results with respect to the flow field structures, internal structure of the interacting region and the positions of the shock waves. A dead air region is formed in the vicinity of the slip line. The flow field structure depends significantly on the pressure in the interacting region, where the pressures of the two jets balance.

Experimental Investigation into the Mechanism of Discrete Frequency Noise(DFN) Generation from a NACA 0012 Blade.

An experimental investigation was made into the mechanism of discrete frequency noise ( DFN )generation from a NACA 0012 blade placed in a uniform oncoming flow. It was clarified that DFN was generated when the boundary layer on the blade surface was laminar and separated at the location upstream of the trailing edge. A dead air region was formed just after the trailing edge of the blade. Therefore a Karman vortex street occurred. Meanwhile we could not observe a close relationship between the existence of a separation bubble and the generation of DFN. That is, DFN was generated even when the attack angle of the flow was nearly zero and no separation bubble was observed. When a separation bubble existed, the boundary layer became turbulent and the flow did not separate up to the trailing edge, so a dead air region was not formed behind the blade, which resulted in the disappearance of DFN. The prediction of the sound pressure level of DFN generated from this stream lined blade by Fukano's theory for a flat plate model agreed well with the measured value.

Experimental study on the characteristics of the near wake of a rotating flat plate. (1st report. Influences of the tip clearance flow with a large attack angle).

The characteristics of the wake of a rotating flat plate with a large attack angle were investigated experimentally with special attention of the flow near the tip of the plate. When the tip clearance is small, the dead air region becomes large towards the tip of the plate and the shear layer of the suction side of the plate curves outward as a result of the accumulation of low-energy fluid. On the other hand, when the tip clearance is large, the leakage flow is so large that the leakage vortex is generated from the trailing edge of the suction side and the shear layer forms by contacting the leakage flow and the dead air region. The variations of the flow rate and the momentum of the wake flow are closely related to those of the leakage flow.

Optimum configuration of high-lift aeromaneuvering orbital transfer vehicles in viscous flow

The results of an analysis to determine the geometrical configuration of an aeroassisted transfer vehicle with a high lift-to-drag ratio (L/D) are described and the constraints imposed on this type of entry vehicle are considered. The aerodynamic characteristics of three configurations, a flat-plate delta wing, a truncated straight cone, and a truncated bent biconic are compared. The effect of viscosity is included in the analysis which examines the rounding of the sharp leading edges. It is shown that, under the constraints of carrying a given volume in the dead air region, the values of L/D are similar for each configuration and that a small blunt leading edge only slightly affects each vehicle's aerodynamic performance, causing less than a 5 percent drop in L/D. The truncated bent biconic is found to be the only configuration that provides the necessary stabilizing moments.

Experimental study on the characteristics of the near wake of a blade-like body with an axisymmetric cross-section rotating about one end. (2nd report. Influences of the tip clearance flow).

The characteristics of the wake of a blunt body near its tip were investigated experimentally. When the tip clearance is large, the downwash flow is so large that the radial component of the velocity in the wake promotes mixing and diffusion of the wake. Therefore the velocity recovery is rapid and the width of the wake increases quickly in the wake. The velocity fluctuation in the wake becomes irregular, and vortex shedding disappears near the tip of the body. On the other hand when the tip clearance is small, the downwash flow is weak and near the tip of the body the dead air region becomes large as a result of gathering the low energy fluid. So the mixing and diffusion in the wake are inactive, and the regularity of the velocity fluctuation in the wake is kept up to the tip of the body.

Self-excited Oscillations of Supersonic Flow in a Sudden Enlargement of a Duct

The behavior of a shock wave in a supersonic outflow at a sudden enlargement of a duct has been clarified experimentally by a schlieren optical system and static pressure measurements. A self-excited oscillation of flow with a shock wave has been observed at the downstream duct at a particular range of supply pressures in which range the outflowing jet is underexpanded at the sudden enlargement and furthermore the jet does not reattach in the downstream duct. In the oscillating flow, compression and expansion waves propagate back and forth between the sudden enlargement section and thickset of the duct, and the base-pressure in the dead-air region downstream of the sudden enlargement and the width of the outflowing jet oscillate periodically.

Theoretical prediction of base pressure for steady base flow

The present paper gives a review of various theories developed for the prediction of base pressure in two-dimensional steady base flow. Theories based on the Chapman-Korst flow model are compared with one another and with experimental results. The new theory of Tanner, based on the concept of mass outflow from the dead-air region, is explained. The various achievements in this field will be pointed out and so also those problems which still should be investigated.

Observations of turbulent reattachment behind an axisymmetric downstream-facing step in supersonic flow.

Supersonic flow over a downstream-facing step on the circumference of a large, ducted, axisymmetric body was used to study flow reattachment. Step heights h were 0.25, 1.00, and 1.68 in., compared to a body radius of 6 in. Freestream Mach numbers were in the range 2 to 4.5. Theturbulent boundary-layer thickness just ahead of the step varied from 0.14 to 0.19 in. (momentum thicknesses of about 0.01 in.). Surface pressure distributions throughout the region of separation and reattachment were measured, and points of reattachment were determined. Comparison of the shapes of the pressure distributions for various step heights shows that the initial (steepest) parts of the reattachment pressure rise, up to the point of reattachment, tend to become superimposed when plotted against x/h. Downstream reattachment the curves branch out, exhibiting a dependence on geometry and probably on initial shear layer profile. In the region of the initial pressure rise (near the end of the dead air region) dynamic pressures are low; the pressure rise there apparently is balanced by turbulent shear stress.