Evaluation of a Thermal-Tuft Probe for Turbulent Separating and Reattaching Flows

Schwaab, Quentin; Weiss, Julien

doi:10.1115/1.4027642

Cited by 8 publications

(8 citation statements)

References 10 publications

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“…The forward-flow fraction γ is defined as the percentage of the time that the flow moves in the main (downstream) direction. The design and testing of the thermal tuft was reported in Schwaab and Weiss [32], and the first experimental results in the TFT Boundary-Layer Wind Tunnel are provided in Mohammed-Taifour et al [30]. The probe's uncertainty was estimated at 1.5% at the 95% confidence level [32].…”

Section: B Instrumentationmentioning

confidence: 99%

Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

2015

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Wall static-pressure and longitudinal-velocity fluctuations are measured in a pressure-induced turbulent separation bubble generated on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness upstream of separation, is Re θ ≃ 5000 at a free-stream velocity of U ref 25 m∕s. The results indicate that the flow is characterized by two separate time-dependent phenomena: a lowfrequency mode, with a Strouhal number St 1 ≃ 0.01, which is related to a global "breathing" motion (i.e., contraction/ expansion) of the separation bubble, and a higher-frequency mode, with a Strouhal number St 2 ≃ 0.35, which is linked to the roll-up of vortical structures in the shear layer above the recirculating region and their shedding downstream of the bubble. These two phenomena are reminiscent of the "flapping" and "shedding" modes observed in fixed-separation experiments, though their normalized frequencies are different. The breathing mode is also shown to be strikingly similar to the low-frequency unsteadiness observed in shock-induced separated flows at supersonic speeds. Nomenclaturepower spectrum of movable sensor G xx = measured power spectrum of reference (fixed) sensor G yy = measured power spectrum of movable sensor h = maximum height of dividing streamline L = length scale in shock-induced separated flows L b = length of separation bubble, which is equal to 0.42 m p = static pressure p 0 = fluctuating static pressure Re θ = Reynolds number based on momentum thickness St, St 1;2 = Strouhal number, which is equal to fL b ∕U ref St f , St s = Strouhal number of flapping and shedding modes, which is equal to fx R ∕U ∞ St L = Strouhal number in shock-induced separated flows, which is equal to fL∕U ∞ St δ ω = Strouhal number of free shear layers, which is equal to fδ ω ∕ U T C = time constant of constant-voltage anemometer circuit Tu = turbulence level U = longitudinal velocity U = average velocity in the shear layer, which is equal to U max ∕2 U c = convective velocity U s = mean longitudinal velocity of inviscid flow at separation V w = voltage across hot-wire probe x = longitudinal position in test section x R = length scale in fixed-separation flows x det = longitudinal position of transitory detachment on test-section centerline, which is equal to 1.75 m x = nondimensional longitudinal position in test section, which is equal to x − x det ∕L b x D = short-time average of nondimensional instantaneous detachment position x R = short-time average of nondimensional instantaneous reattachment position y = vertical position under test surface, positive going down z = spanwise position in test section δ = 99% boundary-layer thickness δ ω = shear-layer vorticity thickness, which is equal to U max − U min ∕∂U∕∂y max γ = forward-flow fraction γ 0 = short-time average of forward-flow fraction ν = kinematic viscosity θ = momentum thickness ρ = air density Subscripts ref = measurement at wind-tunnel reference location (center of contraction exit area) rms = root mean...

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Section: B Instrumentationmentioning

confidence: 99%

Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

2015

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“…The forward-flow fraction, γ, is defined as the percentage of the time that the flow moves in the main (downstream) direction. It was measured using a thermal-tuft probe, a full description of which is provided in Schwaab and Weiss (35) . The thermal tuft is composed of three parallel wires (one central wire and two sensing wires placed on each side) mounted 1mm under the test surface, perpendicular to the main flow direction.…”

Section: Separated Flow Region: Thermal-tuft Measurementsmentioning

confidence: 99%

“…The thermal tuft was sequentially placed at 50 positions on the top wall of the test section, in the region of recirculating flow. At each position, measurements were obtained for two orientations of the probe (θ and 0° and θ = 180°, where θ is defined in Schwaab and Weiss (35) ). At these orientations, the thermal tuft measures the sign of the longitudinal velocity component.…”

Section: Separated Flow Region: Thermal-tuft Measurementsmentioning

confidence: 99%

A new wind tunnel for the study of pressure-induced separating and reattaching flows

et al. 2015

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The design, construction, and validation of a new academic wind tunnel is described in detail. The wind tunnel is of a classical, blow-down type and generates a pressure-induced, turbulent separation bubble on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness just upstream of separation, is Re θ  5,000 at a free-stream velocity of U ref = 25ms -1 . The length of the separation bubble is estimated at 0·42 ± 0·02m by three different methods. Results of a numerical simulation demonstrate the absence of flow separation in the wind-tunnel contraction. This results in a turbulence level of about 0·05% in the test section. Oil-film visualisation experiments show that the flow near the wall is strongly three-dimensional in the recirculating region and that the topology of the limiting streamlines is consistent with experiments performed on configurations with fixed separation. Finally, spatial variations of the forward-flow fraction have been documented using a thermal-tuft probe and are shown to compare well with the results of the oil-film visualisation.

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“…There is also some uncertainty (about 2 cm) in the position of the iso − γ lines, which depends on the interpolation scheme used between the measurement points. 30 Nevertheless, measurements obtained with the thermal-tuft probe are a useful complement to the oil-film experiment and confirm the generation of a separation-bubble flow by the pressure gradient imposed in the test section. …”

Section: Separated Flow Region: Thermal-tuft Measurementsmentioning

confidence: 96%

“…It was measured using a thermal-tuft probe, a full description of which is provided in a companion paper. 30 The thermal tuft is composed of three parallel wires (one central wire and two sensing wires placed on each side) mounted 1 mm under the test surface, perpendicular to the main flow direction. The middle wire is heated by an electric current which generates a hot wake.…”

Section: Separated Flow Region: Thermal-tuft Measurementsmentioning

confidence: 99%

Design, Construction, and Validation of a New Wind Tunnel for the Study of Pressure-Driven Separating and Reattaching Flows

Mohammed-Taifour

Schwaab

Pioton

et al. 2014

52nd Aerospace Sciences Meeting

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The design, construction, and validation of a new academic wind tunnel is described in detail. The wind tunnel is of a classical, blow-down type and generates a pressure-driven, turbulent separation bubble on a flat test surface. The Reynolds number, based on momentum thickness just upstream of separation, is Re θ 5000 at a free-stream velocity of U ref = 25 m/s. The length of the separation bubble has been estimated at 0.42 ± 0.02 m by three different methods. Experimental and numerical results demonstrate the absence of flow separation in the wind-tunnel contraction. This results in a turbulence level of about 0.05% in the test section. Oil-film visualization experiments show that the flow near the wall is strongly three-dimensional in the recirculating region and that the topology of the limiting streamlines is consistent with experiments performed on configurations with fixed separation. Finally, spatial variations of the forward-flow fraction have been documented using a thermal-tuft probe and are shown to compare well with the results of the oil-film visualization.

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Evaluation of a Thermal-Tuft Probe for Turbulent Separating and Reattaching Flows

Cited by 8 publications

References 10 publications

Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

A new wind tunnel for the study of pressure-induced separating and reattaching flows

Design, Construction, and Validation of a New Wind Tunnel for the Study of Pressure-Driven Separating and Reattaching Flows

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