High-speed flow over an open cavity is associated with dynamically complex unsteady events, which warrant the need to better understand the flow in a reduced-order manner. The current work performs a modified version of Spectral Analysis Modal Method (Zhang et al. 2020) on supersonic cavity flows which enables significant computational speed-up and a more optimal way of encoding the large-scale structures. Synchronized measurements of non-time-resolved velocity field using particle image velocimetry and time-resolved single-point pressure measurements were conducted on an open cavity at a nominal free-stream Mach number of 1.4. The test section comprises a rectangular cavity of length-to-depth ratio of 6 and width-to-depth ratio of 6, which spans the full width of the wind tunnel. The datasets are analyzed using a combination of Proper Orthogonal Decomposition and stochastic estimation techniques to obtain the reduced-order modes. Reduced-order reconstructions at high-energy Rossiter frequencies show large-scale unsteady vortical structures indicative of Kelvin–Helmholtz shear layer instabilities convecting down the cavity. The coherence-based reconstruction enhances the extraction of Kelvin–Helmholtz instability dynamics by reducing contamination from incoherent turbulent motions. Furthermore, these interactions highlight significant shear layer–aft wall interactions along with flow ejection and in-sweep events, emphasizing the aft wall interactions of supersonic flow over an open cavity.

Tonal acoustic predictions of a fixed pitch rotor in edgewise flight are developed based on rotor inflow measurements. The induced velocity of a fixed-pitch rotor in hover and edgewise flight was measured with stereoscopic particle image velocimetry (sPIV) in an anechoic wind tunnel. The edgewise flight measurements covered advance ratios ranging from 0.065 to 0.268. The ensemble averaged sPIV measurements showed that an increase in advance ratio led to a skewing of the maximum induced velocity towards the advancing side of the rotor and an increased area of upwash velocity at the front of the rotor disk. The measured mean velocity field was combined with blade element theory and Sears Unsteady Airfoil Theory to estimate the unsteady blade forces, which were used in analytical tonal noise predictions. The predictions were on average within 5 dB of measured sound pressure levels at the rotor blade passage frequency.

We present a phase-based framework for reducing the pressure fluctuations within a spanwise-periodic supersonic turbulent cavity flow with an incoming free-stream Mach number of 1.4 and a depth-based Reynolds number of 10,000. Open cavity flows exhibit large fluctuations due to the feedback between the shear layer instabilities and the acoustic field. The dominant flow physics includes the formation, convection, and impingement of large-scale spanwise-oriented vortical structures. We formulate a flow control strategy to effectively modify the vortex convection frequency, thereby disrupting the feedback loop and suppressing pressure fluctuations within the cavity. We implement a phase-reduction approach to identify the flow response about the time-varying convective process by defining a phase variable using dynamic mode decomposition. Three-dimensional impulse perturbations are introduced from the cavity leading edge to characterize phase response in terms of advancement or delay of convection. We perform open-loop flow control through unsteady blowing using actuation frequencies slightly different from the vortex convection frequency to disrupt the feedback loop. After designing a phase-sensitivity-based actuation waveform optimized for quick flow modification, we investigate the speed of fluctuation reduction and compare it to a sinusoidal waveform. At a spanwise actuation wavenumber of β=2π, both perform similarly, achieving a 46 within five convective times. At β=π, the phase-sensitivity-based actuation performs better by achieving a 40 a sinusoidal waveform within six convective times. This study shows the potential of phase-based analysis for timing-based flow control of unsteady turbulent flows.

Wing-body junction flows are prevalent in many low and high speed aerospace applications. However, our understanding of such flows is grossly limited. In the current work, the flow ahead of a wing-body junction model with NACA 16-006 at supersonic speed of Mach 1.4 is studied. The flow is inspected using a combination of techniques such as Schlieren/shadowgraph imaging and Computational Fluid Dynamics. Particle Image Velocimetry (PIV) is then applied on streamwise and spanwise aligned planes. PIV measurements reveal the existence of large-scale flow structures which are further inspected by applying Proper Orthogonal Decomposition. Overall, the study provides a detailed understanding of large-scale structures historically observed in such flows.

The far-field noise and forces of a subscale two-blade rotor were measured in an anechoic wind tunnel for both hover and edgewise flight conditions. The mean thrust and torque coefficients increased with the edgewise advance ratio in relation to the hover coefficients. The tonal and broadband noise contributions were separated and analyzed in both the time and frequency domains. The sound pressure level (SPL) at the blade pass frequency (BPF) had the greatest contribution to the overall SPL, and the dominant broadband noise source changed from high-frequency trailing edge noise to midfrequency blade wake interaction noise with an increasing edgewise advance ratio. The tip Mach number scaling of the noise sources was examined and found to be highly dependent on the oncoming freestream velocity. The BPF SPL directivity showed a dependency on the advance ratio, with a peak magnitude below and upstream of the rotor’s retreating side. The broadband noise had a dipole directivity with a minimum in the rotor plane. The midfrequency broadband noise had signatures indicative of blade wake interaction noise, and the high-frequency noise had signatures indicative of amplitude modulated trailing edge noise. The results demonstrate the importance of unsteady loading noise and broadband noise for subscale urban air mobility rotors.

A common design of many of the proposed generation of electric Vertical Takeoff and Landing (eVTOL) vehicles involves fixed pitch rotors in edgewise flight. These applications typically have significantly lower tip Mach numbers than conventional rotor-craft and involve distributed smaller rotors making them operate at lower Reynolds numbers and hence likely having a greater sensitivity to gusts and environmental turbulence. This work will present results of studies with fixed pitch rotors conducted in the UF Anechoic Wind Tunnel Facility. The studies involve radiated noise, thrust and stereoscopic Particle Image Velocimetry to measure the inflow velocity to the rotor plane. The measurements were taken for un-altered free stream conditions as well as ones subjected to three different turbulence generation grids. The results provided show that there can be effects at moderate levels of free stream to turbulence to the rotor efficiency and radiated noise.

An experimental investigation of the noise and forces generated by tandem eVTOL rotors in simulated forward flight was performed in the University of Florida Anechoic Wind Tunnel. The campaign consisted of 48 test conditions sweeping through the tip Mach numbers and freestream velocity to cover a range of realistic eVTOL flight conditions for two rotors. The mean rotor forces and noise at 90 microphone locations below the rotor plane were measured for each test condition. The results showed that the noise and forces of the rear rotor were effected by the wake of the front rotor. Additionally, the inclusion of a wing and boom reduced overall noise levels and increased the rear rotor thrust coefficient. The results emphasize the importance of rotor-rotor and rotor-airframe interactions on eVTOL noise.

The induced velocity field of three different sub-scale eVTOL rotors were measured using a newly implemented stereoscopic particle image velocimetry (sPIV) system in the UF Anechoic Wind Tunnel Facility. The measurements were performed in the test section of the open jet wind tunnel for hover and edgewise flight conditions with advance ratios ranging from 0.065 to 0.268. The ensemble averaged sPIV measurements showed that an increase in advance ratio leads to a skewing of the maximum induced velocity towards the advancing side of the rotor disk and an increased area of upwash velocity at the front of the rotor disk. The results were further compared to the induced velocity field modeled with Peters-He Generalized Dynamic wake model. The modeled induced velocity was shown to be in poor agreement to the measured velocity field pointing to the limitations of the cylindrical wake assumption for a fixed pitch rotor in edgewise flight. The measured velocity field was utilized in both blade element theory and Sears unsteady airfoil theory to estimate the unsteady rotor blade forces. These forces were then utilized in an analytical noise model to predict unsteady loading noise and the results showed good agreement to measured sound pressure levels.

Electric vertical take-off and landing (eVTOL) aircraft are characterized by their unconventional wing and electric rotor configurations, which involve both side-by-side and tandem rotor configurations. These configurations create unique aerodynamic and acoustic flow-fields. We numerically investigate the interaction effects between rotor pairs as well as their individual and combined acoustic radiation. We examine horizontal and vertical spacing, rotor tilt angles, and forward flight effects. Performance is characterized by thrust coefficient, blade passage frequency (BPF) sound pressure level (SPL), and overall sound pressure level (OASPL). This study is performed with a mid-fidelity aerodynamic solver, Dust, which is used to predict the aerodynamic flow-field. The tonal acoustic pressure at observer positions is predicted via the Farassat F-1A solution of the Ffowcs Williams and Hawkings equation utilizing the aerodynamic flow-field. The configurations studied show strong aerodynamic interaction effects in thrust, as well as out-of-plane acoustic radiation from the aft rotor. Base predictions of thrust and noise are validated via experimental measurement. As rotor separation decreases, we observe that aft rotor thrust decreases and BPF SPL increases. The most forward rotor, however, is marginally impacted by the interactions. [This research is supported by Archer Aviation Inc.]

This work aims to estimate time-resolved velocity field that is directly associated with pressure fluctuations in a subsonic round jet. To achieve this goal, synchronous measurements of the velocity field and in-flow pressure fluctuations were performed at Mach number 0.3. Two different experiment campaigns were conducted, the first experimental campaign aims to explore the time-resolved dynamics of the axisymmetric velocity components, and second experiment focuses on the time-resolved, 2D velocity estimates on a streamwise plane. Two different methods were utilized to estimate the input-output relation between velocity and in-flow pressure measurements. A hybrid approach based on the spectral linear stochastic estimation and the proper orthogonal decomposition was applied to setup the model in a linear manner, and a wavelet-based filter was implemented to attenuate the noise level in the cross-correlation functions. In addition, the pressure-velocity relationship was also described by neural network architectures based on the multi-layer perceptron (MLP) and bidirectional long-short-term-memory (LSTM). In both experimental sets, pressure fluctuations inside the flow are found to be connected to the streamwise convection of large-scale coherent structures in the flow. A unique advantage of the bidirectional LSTM method was found among all estimation schemes is also reported in this work. The estimation result represents the space-time dynamics of the acoustic sources in the jet flow field, and it is of great importance to understand the noise generation mechanism.

The acoustics and forces of a sub-scale 2-blade rotor were measured in an anechoic wind tunnel for both hover and edgewise flight conditions. The edgewise flight conditions included four tip Mach numbers, three freestream velocities, and two disk incidence angles representing realistic flight conditions for an Urban Air Mobility (UAM) vehicle. Forces were measured with a 6-axis load cell and far-field noise was measured with 1/4 in free-field microphones at 175 locations around the rotor. The tonal and broadband noise contributions were separated and analyzed in both the frequency and time domains. The mean thrust coefficient and torque coefficient were found to be proportional to the advance ratio squared. The sound pressure level (SPL) at the blade pass frequency (BPF) and the overall SPL (OASPL) increased for both increasing tip Mach number and increasing freestream velocity. The directivity of the noise displayed a dependency on advance ratio with a peak SPL below the retreating side. This mimics the unsteady loading noise directivity for a low tip speed rotor in edgewise flight. The broadband noise had a dipole directivity with a minimum in the rotor plane. The low/mid frequency broadband noise below30 times the BPF had time signatures indicative of blade wake interaction noise, while the high frequency noise above 30 times the BPF had time signatures and directivities indicative of trailing edge noise. These results demonstrate the importance of unsteady noise sources for an isolated UAM rotor in edgewise flight.

Motivated by the successful application of active control using segmented spanwise blowing, this work investigates passive flow control for a rectangular cavity in supersonic flow using a spanwise array of leading-edge tabs to reduce unsteady pressure loads on the cavity surface. The cavity studied here has a length-to-depth ratio of 6 and width-to-depth ratio of 6 with a turbulent boundary layer at the leading edge and a freestream Mach number of 1.4. Fluctuating surface pressure measurements within the cavity show that the leading-edge tabs have similar effectiveness as leading-edge blowing of the same spanwise wavelength in reducing pressure fluctuations on the cavity surfaces. Streamwise-aligned particle image velocimetry (PIV) measurements show evidence of deflection of the shear layer at the cavity opening and modification of recirculation characteristics associated with the controlled flow. Furthermore, cross-stream stereoscopic PIV measurements reveal counter-rotating streamwise vortices, which persist throughout the cavity length. Proper orthogonal decomposition modes of cross-stream-aligned PIV measurements show that leading-edge tabs are successful in breaking up the flow into smaller-scale modes. Surface flow visualization of the cavity floor and aft wall show differences in the recirculation patterns for the baseline and controlled flows. The present analysis provides insights on how the passive tabs effectively control high-speed cavity flow and demonstrates that the control mechanisms are analogous to steady leading-edge blowing strategies.

We use resolvent analysis to develop a physics-based, open-loop, unsteady control strategy to attenuate pressure fluctuations in turbulent flow over a rectangular cavity with a length-to-depth ratio of $6$ at a Mach number of $1.4$ and a Reynolds number based on cavity depth of $10\,000$. Large-eddy simulations (LES) of the baseline uncontrolled flow reveal the dominance of Rossiter modes II and IV that generate high-amplitude unsteadiness via trailing-edge impingement and oblique shock waves that obstruct the free stream. To suppress the oscillations, we introduce three-dimensional unsteady blowing along the cavity leading edge. We leverage resolvent analysis as a linear model with respect to the baseline flow to guide the selections of the optimal spanwise wavenumber and frequency of the unsteady actuation input for a fixed momentum coefficient of 0.02. Instead of choosing the most amplified resolvent forcing modes, we seek a disturbance that yields sustained amplification of the primary response mode-based kinetic energy distribution over the entire cavity length. This necessary but not sufficient guideline for effective mean flow modification is evaluated using LES of the controlled cavity flows. The most effective control case reduces the pressure root mean square level up to $52\,\%$ along cavity walls relative to the baseline and is approximately twice that achievable by comparable steady blowing. Dynamic mode decomposition on the controlled flows confirms that the optimal actuation input indeed suppresses the formation of the large-scale Rossiter modes. It is expected that the present flow control guideline derived from resolvent analysis will also be applicable at higher Reynolds numbers with the aid of physical insights and further validation.

The goal of this study is to estimate aspects of the time-resolved (TR) velocity field that is associated with pressure fluctuations measured in a subsonic jet using machine learning (ML) approaches. The experiments were conducted in the Anechoic Jet Test Facility at the University of Florida using a round converging nozzle operated at at a Mach number of 0.3 and ReD = 3.8 × 105. Planar PIV was utilized to record nonTR, 2D velocity snapshots on the streamwise plane. A B&K 4138 1/8” microphone and a GRAS 46DD 1/8” microphone were employed to measure inflow pressure fluctuations synchronously with the PIV. Both microphones were equipped with aerodynamically-shaped nosecones and were placed on the upper and lower jet liplines. The nosecone tips were streamwisely aligned and were placed just downstream of the PIV window (see Figure 1(a)). Pressure signals were recorded synchronously with PIV, but at different sampling rates, 80 kHz and 12 Hz, respectively. A total of 8000 PIV snapshots were acquired in the experiment.

The noise from large-scale coherent turbulent structures within jets remains the dominant source. For the purpose of developing future control systems for the large-scale noise source, we investigate the statistics between upstream and downstream radiating waves. We investigate two off-design supersonic jet flows with instability theory and associated noise radiation, large-eddy simulation (LES), and experiments. We compare the auto-correlation, cross-correlation, coherence, and other statistics predicted by aeroacoustic instability theory. As instability waves are closely connected with the formation of large-scale turbulent structures, they yield insight into large-scale noise statistics. We investigate two nozzles at two supersonic off-design conditions. The first is a biconic nozzle operating at an unheated condition, and the second is a NASA nozzle operating at a heated condition. We find that for these jets, the noise from instability waves is coherent between 0.40 to 0.70 at large-scale radiation frequencies between the downstream and upstream radiation directions.

Measuring the fluctuating static pressure within a jet has the potential to depict in-flow sources of the jet noise. In this work, the fluctuating static pressure of a subsonic axisymmetric jet was experimentally investigated using a 1/8” microphone with an aerodynamically shaped nose cone. The power spectra of the fluctuating pressure are found to follow the -7/3 scaling law at the jet centerline with the decay rate varying as the probe approaches the acoustic near field. Profiles of skewness and kurtosis reveal strong intermittency inside the jet shear layer. By applying a continuous wavelet transform (CWT), time-localized footprints of the acoustic sources were detected from the pressure fluctuations. To decompose the fluctuating pressure into the hydrodynamic component and its acoustic counterpart, two techniques based on the CWT are adopted. In the first method the hydrodynamic pressure is isolated by maximizing the correlation with the synchronously measured turbulent velocity, while the second method originates from the Gaussian nature of the acoustic pressure where the separation threshold is determined empirically. Similar results are obtained from both separation techniques, and each pressure component dominates a certain frequency band compared to the global spectrum. Furthermore, cross-spectra between the fluctuating pressure and the turbulent velocity were calculated, and spectral peaks appearing around Strouhal number of 0.4 are indicative of the footprint of the convecting coherent structures inside the jet mixing layer.