5 results
Cross-flow instability on a swept-fin cone at Mach 6: characteristics and control
- John B. Middlebrooks, Thomas C. Corke, Eric Matlis, Michael Semper
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- Journal:
- Journal of Fluid Mechanics / Volume 981 / 25 February 2024
- Published online by Cambridge University Press:
- 21 February 2024, A18
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Experiments were performed to document the complex flow field around and over a $70^{\circ }$ swept fin mounted on a $7^{\circ }$ half-angle right-circular cone in a Mach 6 free-stream. Of particular interest is the turbulent transition of the boundary layer over the swept fin, which is expected to be dominated by a cross-flow instability. Stationary features in the surface temperature distribution over the fin are documented using infrared thermal imaging. These were processed further to determine average spatial Stanton number distributions over the fin. Wavelet analysis of the Stanton number distributions revealed stationary patterns with wavelengths near the fin leading edge that were consistent with linear theory predictions of stationary cross-flow modes. Further from the leading edge, the wavelength of the stationary patterns was observed to increase prior to turbulence onset. Based on these observations, specially designed arrays of discrete roughness elements (DREs) were investigated as a means of delaying turbulence transition with the objective of reducing surface heat flux on the swept fin. The DRE designs followed our previous approach used for cross-flow transition control (Corke et al., J. Fluid Mech., vol. 856, issue 10, 2018, pp. 822–849; Arndt et al., J. Fluid Mech., vol. 887, 2020, A30). These focused on either the shorter wavelengths near the leading edge, or the longer wavelengths that developed near turbulence onset. With regard to delaying transition and reducing the surface heat flux, the DREs that focused on the larger wavelengths of stationary modes were most effective. The fin included an array of pressure sensors that were used to document travelling disturbances that could include those associated with travelling cross-flow modes. Phase analysis of the pressure fluctuation time series was used to determine the wavelength, wave angle and phase speed that were consistent with the travelling cross-flow modes. Cross-bicoherence analysis between the stationary and travelling disturbances indicates a nonlinear phase locking that can account for the development of the longer-wavelength stationary features in the surface heat flux, presumed to be due to stationary cross-flow modes, prior to turbulence onset.
Controlled stationary/travelling cross-flow mode interaction in a Mach 6.0 boundary layer
- Alexander Arndt, Thomas Corke, Eric Matlis, Michael Semper
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- Journal:
- Journal of Fluid Mechanics / Volume 887 / 25 March 2020
- Published online by Cambridge University Press:
- 30 January 2020, A30
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Experiments were performed to further investigate a quadratic interaction between stationary and travelling cross-flow modes that was revealed in the previous experiment by Corke et al. (J. Fluid Mech., vol. 856, 2018, pp. 822–849) in the boundary layer on a sharp right-circular cone at an angle of attack at Mach 6.0. As with the previous experiment, passive discrete roughness was applied near the cone tip, just upstream of Branch I of the linear stability neutral curve for stationary cross-flow modes. The passive roughness consisted of indentations (dimples) that were evenly spaced azimuthally to excite a specific azimuthal wavenumber. A plasma actuator was located just downstream of the discrete roughness array. This was designed to produce an azimuthally uniform unsteady disturbance with a frequency that was at the centre of the band of most amplified travelling cross-flow modes. Measurements consisted of off-wall azimuthal profiles of mean and fluctuating total pressure at different axial locations. Spectra of total-pressure fluctuations verified the receptivity of the boundary layer to the unsteady excitation. This affected the azimuthal and streamwise development of the stationary cross-flow modes, with a general effect to move the transition location upstream by approximately 16 %. The quadratic interaction between the stationary and travelling cross-flow modes was further enhanced by the excitation of the travelling cross-flow mode. This was particularly evident by an enlarged band of azimuthal wavenumbers over which a significant triple phase locking existed. The significantly enlarged range of wavenumber sum and difference interactions offered a mechanism for rapid spectral broadening that could account for the hastened transition by the addition of unsteady disturbances.
Control of stationary cross-flow modes in a Mach 6 boundary layer using patterned roughness
- Thomas Corke, Alexander Arndt, Eric Matlis, Michael Semper
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- Journal:
- Journal of Fluid Mechanics / Volume 856 / 10 December 2018
- Published online by Cambridge University Press:
- 11 October 2018, pp. 822-849
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Experiments were performed to investigate passive discrete roughness for transition control on a sharp right-circular cone at an angle of attack at Mach 6.0. A cone angle of attack of $6^{\circ }$ was set to produce a mean cross-flow velocity component in the boundary layer over the cone by which the cross-flow instability was the dominant mechanism of turbulent transition. The approach to transition control is based on exciting less-amplified (subcritical) stationary cross-flow modes through the addition of discrete roughness that suppresses the growth of the more-amplified (critical) cross-flow modes, and thereby delays transition. The passive roughness consisted of indentations (dimples) that were evenly spaced around the cone at an axial location that was just upstream of the first linear stability neutral growth branch for cross-flow modes. The experiments were performed in the air force academy (AFA) Mach 6.0 Ludwieg Tube Facility. The cone model was equipped with a motorized three-dimensional traversing mechanism that mounted on the support sting. The traversing mechanism held a closely spaced pair of fast-response total pressure Pitot probes. The measurements consisted of surface oil flow visualization and off-wall azimuthal profiles of mean and fluctuating total pressure at different axial locations. These documented an 25 % increase in the transition Reynolds number with the subcritical roughness. In addition, the experiments revealed evidence of a nonlinear, sum and difference interaction between stationary and travelling cross-flow modes that might indicate a mechanism of early transition in conventional (noisy) hypersonic wind tunnels.
Effect of wall suction on rotating disk absolute instability
- Joanna Ho, Thomas C. Corke, Eric Matlis
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- Journal:
- Journal of Fluid Mechanics / Volume 791 / 25 March 2016
- Published online by Cambridge University Press:
- 24 February 2016, pp. 704-737
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This research investigates the effect of uniform suction on the absolute instability of Type I cross-flow modes in the boundary layer on a rotating disk. Specifically, it is designed to investigate whether wall suction would transform the absolute instability into a global mode, as first postulated in the numerical simulations of Davies & Carpenter (J. Fluid Mech., vol. 486, 2003, pp. 287–329). The disk is designed so that with a suction parameter of $0.2$, the radial location of the absolute instability critical Reynolds number, $Re_{c_{A}}=650$, occurs on the disk. Wall suction is applied from $Re=317$ to 696.5. The design for wall suction follows that of Gregory & Walker (J. Fluid Mech., 1960, pp. 225–234) where an array of holes through the disk communicate between the measurement side of the disk and the underside of the disk which is inside of an enclosure that is maintained at a slight vacuum. The enclosure pressure is adjustable so that a range of suction or blowing parameters can be investigated. The holes in the measurement surface are covered by a compressed wire porous mesh to aid in uniformizing the suction on the measurement surface of the disk. The mesh is covered by a thin porous high-density polyethylene sheet featuring a $20~{\rm\mu}\text{m}$ pore size which provides a smooth finely porous surface. A companion numerical simulation is performed to investigate the effect that the size and vacuum pressure of the underside enclosure have on the uniformity of the measurement surface suction. Temporal disturbances are introduced using the method of Othman & Corke (J. Fluid Mech., 2006, pp. 63–94). The results document the evolution of disturbance wavepackets in space and time. These show a temporal growth of the wavepackets as the location of the absolute instability is approached which is in strong contrast to the temporal evolution without suction observed by Othman and Corke. The results appear to support the effect of wall suction on the absolute instability postulated by Thomas (PhD thesis, 2007, Cardiff University, UK) and Thomas & Davies (J. Fluid Mech., vol. 663, 2010, pp. 401–433).
Control of stationary cross-flow modes in a Mach 3.5 boundary layer using patterned passive and active roughness
- Chan Yong Schuele, Thomas C. Corke, Eric Matlis
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- Journal:
- Journal of Fluid Mechanics / Volume 718 / 10 March 2013
- Published online by Cambridge University Press:
- 08 February 2013, pp. 5-38
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Spanwise-periodic roughness designed to excite selected wavelengths of stationary cross-flow modes was investigated in a three-dimensional boundary layer at Mach 3.5. The test model was a sharp-tipped $1{4}^{\circ } $ right-circular cone. The model and integrated sensor traversing system were placed in the Mach 3.5 supersonic low disturbance tunnel (SLDT) equipped with an axisymmetric ‘quiet design’ nozzle at NASA Langley Research Center. The model was oriented at a $4. {2}^{\circ } $ angle of attack to produce a mean cross-flow velocity component in the boundary layer over the cone. The research examined both passive and active surface roughness. The passive roughness consisted of indentations (dimples) that were evenly spaced around the cone at an axial location that was just upstream of the first linear stability neutral growth branch for cross-flow modes. The active roughness consisted of an azimuthal array of micrometre-sized plasma actuators that were designed to produce the effect of passive surface bumps. Two azimuthal mode numbers of the passive and active patterned roughness were examined. One had an azimuthal mode number that was in the band of initially amplified stationary cross-flow modes. This was intended to represent a controlled baseline condition. The other azimuthal mode number was designed to suppress the growth of the initially amplified stationary cross-flow modes and thereby increase the transition Reynolds number. The results showed that the stationary cross-flow modes were receptive to both the passive and active patterned roughness. Only the passive roughness was investigated at a unit Reynolds number where transition would occur on the cone. Transition front measurements using the Preston tube approach indicated that the transition Reynolds number had increased by 35 % with the subcritical wavenumber roughness compared with the baseline smooth tip cone, and by 40 % compared with the critical wavenumber roughness. Based on the similarities in the response of the stationary cross-flow modes with the active roughness, we expect it would produce a similar transition delay.