High-speed, time-resolved particle image velocimetry with a pulse-burst laser was used to measure the gas-phase velocity upstream and downstream of a shock wave-particle curtain interaction at three shock Mach numbers (1.22, 1.40, and 1.45) at a repetition rate of 37.5 kHz. The particle curtain was formed from free-falling soda-lime particles resulting in volume fractions of 9% or 23% at mid-height, depending on particle diameter (106-125 and 300-355 μm, respectively). Following impingement by a shock wave, a pressure difference was created between the upstream and downstream sides of the curtain, which accelerated flow through the curtain. Jetting of flow through the curtain was observed downstream once deformation of the curtain began, demonstrating a long-term unsteady effect. Using a control volume approach, the unsteady drag on the curtain was estimated from velocity and pressure data. The drag imposed on the curtain has a strong volume fraction dependence with a prolonged unsteadiness following initial shock impingement. In addition, the data suggest that the resulting pressure difference following the propagation of the reflected and transmitted shock waves is the primary component to curtain drag.
The resonance modes in Mach 0.94 turbulent flow over a cavity having a length-to-depth ratio of five were explored using time-resolved particle image velocimetry and time-resolved pressure sensitive paint. Mode-switching occurred in the velocity field simultaneous with the pressure field. The first cavity mode corresponded to large-scale motions in shear layer and in the vicinity of the recirculation region, whereas the second and third modes contained organized structures associated with shear layer vortices. Modal surface pressures exhibited streamwise periodicity generated by the interference of downstream-traveling disturbances in shear layer with upstream-traveling acoustical waves. Because of this interference, the modal velocity fields also exhibited local maxima at locations containing pressure minima and vice-versa. Modal convective (phase) velocities, based on cross-correlations of bandpass-filtered velocity fields, decreased with decreasing mode number as the modal activity resided in lower portions of the cavity. These phase velocities also exhibited streamwise periodicity caused by wave interference. The measurements demonstrate that despite the complexities inherent in compressible cavity flows, many of the most prevalent resonance dynamics can be described with simple acoustical analogies.
Time-resolved PIV has been accomplished in three high-speed flows using a pulse-burst laser: a supersonic jet exhausting into a transonic crossflow, a transonic flow over a rectangular cavity, and a shock-induced transient onset to cylinder vortex shedding. Temporal supersampling converts spatial information into temporal information by employing Taylor’s frozen turbulence hypothesis along local streamlines, providing frequency content until about 150 kHz where the noise floor is reached. The spectra consistently reveal two regions exhibiting power-law dependence describing the turbulent decay. One is the well-known inertial subrange with a slope of-5/3 at high frequencies. The other displays a-1 power-law dependence for as much as a decade of mid-range frequencies lying between the inertial subrange and the integral length scale. The evidence for the-1 power law is most convincing in the jet-in-crossflow experiment, which is dominated by in-plane convection and the vector spatial resolution does not impose an additional frequency constraint. Data from the transonic cavity flow that are least likely to be subject to attenuation due to limited spatial resolution or out-of-plane motion exhibit the strongest agreement with the-1 and-5/3 power laws. The cylinder wake data also appear to show the-1 regime and the inertial subrange in the near-wake, but farther downstream the frozen-turbulence assumption may deteriorate as large-scale vortices interact with one another in the von Kármán vortex street.
Time-resolved PIV has been accomplished in three high-speed flows using a pulse-burst laser: a supersonic jet exhausting into a transonic crossflow, a transonic flow over a rectangular cavity, and a shock-induced transient onset to cylinder vortex shedding. Temporal supersampling converts spatial information into temporal information by employing Taylor’s frozen turbulence hypothesis along local streamlines, providing frequency content until about 150 kHz where the noise floor is reached. The spectra consistently reveal two regions exhibiting power-law dependence describing the turbulent decay. One is the well-known inertial subrange with a slope of-5/3 at high frequencies. The other displays a-1 power-law dependence for as much as a decade of mid-range frequencies lying between the inertial subrange and the integral length scale. The evidence for the-1 power law is most convincing in the jet-in-crossflow experiment, which is dominated by in-plane convection and the vector spatial resolution does not impose an additional frequency constraint. Data from the transonic cavity flow that are least likely to be subject to attenuation due to limited spatial resolution or out-of-plane motion exhibit the strongest agreement with the-1 and-5/3 power laws. The cylinder wake data also appear to show the-1 regime and the inertial subrange in the near-wake, but farther downstream the frozen-turbulence assumption may deteriorate as large-scale vortices interact with one another in the von Kármán vortex street.
Volumetric measurements of the flow within four open cavities were made using stereoscopic particle image velocimetry at a freestream Mach number of 0.8. The cavities nominally had a length-to-diameter ratio, L/D = 7, along with an aspect ratio, b/L = 0.5. The three complex cavity geometries were selected to model features representative of real aircraft bays and compare them to a finite-span rectangular cavity: these included features such as leading edge and side ramps, a scooped leading edge ramp, and a jagged leading edge. Flow is drawn into the cavity at the edges due to a lack of pressure recovery within the cavity. Due to the influence of the leading edge shape and side edges, three-dimensionalities are formed within the cavities that influence the development of the Rossiter tones. In the rectangular cavity, these three-dimensionalities lead to the formation of a set of counter-rotating streamwise-oriented vortices, which create a nearly-sinusoidal, spanwise waviness within its mixing layer. The addition of leading edge and side ramps disrupt the formation of these vortical structures and displace the mixing layer vertically, reducing Rossiter modal amplitudes. The leading edge ramp accelerates the oncoming flow, resulting in a shift in the Rossiter frequencies. A scooped leading edge reintroduced streamwise vorticity, increasing cavity turbulence, whereas an overhanging jagged leading edge reduced cavity velocity fluctuations while increasing the strength of the second Rossiter mode.
High-speed, time-resolved particle image velocimetry with a pulse-burst laser was used to measure the gas-phase velocity upstream and downstream of a shock wave-particle curtain interaction at three shock Mach numbers (1.19, 1.40, and 1.45), at a sampling rate of 37.5 kHz. The particle curtain, formed from free-falling soda-lime particles with diameters ranging from 300 - 355 μm, had a streamwise thickness of 3.5 mm and volume fraction of 9% at mid-height. Following impingement by a shock wave, a pressure difference was created between the upstream/downstream sides of the curtain, which accelerated flow through the curtain. Jetting of flow through the curtain was observed downstream once deformation of the curtain began, demonstrating a long-term unsteady effect. Using a control volume approach, the unsteady drag on the curtain was determined from velocity and pressure data. Initially, the pressure difference between the upstream and downstream sides of the curtain was the largest contributor to the total drag. The data suggests, however, that as time increases, the change in momentum flux could become the dominant component as the pressure difference decreases.
Pulse-burst particle image velocimetry (PIV) has been used to acquire time-resolved data at 37.5 kHz of the flow over a finite-width rectangular cavity at Mach 0.6, 0.8, and 0.94. Power spectra of the PIV data reveal four resonance modes that match the frequencies detected simultaneously using high-frequency wall pressure sensors. Velocity resonances exhibit spatial dependence in which the lowest-frequency acoustic mode is active within the recirculation region whereas the three higher modes are concentrated within the shear layer. Spatio-temporal cross-correlations were calculated from velocity data first bandpass filtered for specific resonance frequencies. The low-frequency acoustic mode shows properties of a standing wave without spatial correlation. Higher resonance modes are associated with alternating coherent structures whose size and spacing decrease for higher resonance modes and increase as structures convect downstream. The convection velocity appears identical for the high-frequency resonance modes, but it too increases with downstream distance. This is in contrast to the well-known Rossiter equation, which assumes a convection velocity constant in space.
The breakup of liquids due to aerodynamic forces has been widely studied. However, the literature contains limited quantified data on secondary droplet sizes, particularly as a function of time. Here, a column of liquid water is subjected to a step change in relative gas velocity using a shock tube. A unique digital in-line holography (DIH) configuration is proposed which quantifies the secondary droplets sizes, three-dimensional position, and three-component velocities at 100 kHz. Results quantify the detailed evolution of the characteristic mean diameters and droplet size-velocity correlations as a function of distance downstream from the initial location of the water column. Accuracy of the measurements is confirmed through mass balance. These data give unprecedented detail on the breakup process which will be useful for improved model development and validation.
Stereoscopic particle image velocimetry was used to experimentally measure the recirculating flow within finite-span cavities of varying complex geometry at a freestream Mach number of 0.8. Volumetric measurements were made to investigate the side wall influences by scanning a laser sheet across the cavity. Each of the geometries could be classied as an open-cavity, based on L/D. The addition of ramps altered the recirculation zone within the cavity, causing it to move along the streamwise direction. Within the simple rectangular cavity, a system of counter-rotating streamwise vortices formed due to spillage from along the side wall, which caused the mixing layer to develop a steady spanwise waviness. The ramped complex geometry, due to the presence of leading edge and side ramps, appeared to suppress the formation of streamwise vorticity associated with side wall spillage, resulting in a much more two-dimensional mixing layer.