Showing papers on "Freestream published in 2020"

On the unsteady throttling dynamics and scaling analysis in a typical hypersonic inlet–isolator flow

Abstract: The flow field in a two-dimensional three-ramp hypersonic mixed-compression inlet in a freestream Mach number of M∞ = 5 is numerically solved to understand the unsteady throttling dynamics. Throttling conditions are simulated by varying the exit area of the isolator in the form of plug insets. Different throttling ratios between 0 ≤ ζ ≤ 0.7 in steps of 0.1 are considered. No unsteadiness is observed for ζ ≤ 0.2, and severe unsteadiness is found for 0.3 ≤ ζ ≤ 0.7. The frequency of unsteadiness (f) increases rapidly with ζ. As ζ increases, the amount of reversed mass inside the isolator scales with the frequency and the exit mass flow rate. A general framework is attempted to scale the unsteady events based on the gathered knowledge from the numerical study. The inlet–isolator flow is modeled as an oscillating flow through a duct with known upstream design conditions such as the freestream Mach number (M∞) and the isolator inlet Mach number (Mi). Factors such as the mass occupied by the duct volume, the characteristic unsteady frequency, the throttling ratio, and the exit mass flow rate through the duct are used to form a non-dimensional parameter β, which scales with the upstream design parameter ξ = Mi/M∞. The scaling parameters are further exploited to formulate a semi-empirical relation using the existing experimental results at different throttling ratios from the open literature. The unsteady frequencies from the present two-dimensional numerical exercise are also shown to agree with the proposed scaling and the resulting semi-empirical relation.

100 kHz PLEET velocimetry in a Mach-6 Ludwieg tube

Abstract: Picosecond laser electronic-excitation tagging (PLEET) was demonstrated in a Mach-6 Ludwieg tube at a repetition rate of 100 kHz using a 1064 nm, 100 ps burst-mode laser The system performance of high-speed velocimetry in unseeded air and nitrogen Mach-6 flows at a static pressure in the range of 5-20 torr were evaluated Based on time-resolved freestream flow measurements and computational fluid dynamics (CFD) calculations, we concluded that the measurement uncertainty of 100 kHz PLEET measurement for Mach 6 freestream flow condition is ∼1% The measured velocity profiles with a cone-model agreed well with the CFD computations upstream and downstream of the shockwave; downstream of the shockwave the discrepancy between the CFD and experimental measurement could be attributed to a slight nonzero angle of attack (AoA) or flow unsteadiness Our results show the potential of utilizing 100 kHz PLEET velocimetry for understanding real-time dynamics of turbulent hypersonic flows and provide the capability of collecting sufficient data across fewer tests in large hypersonic ground test facilities

Measurements of freestream density fluctuations in a hypersonic wind tunnel

Abstract: Density disturbances in the freestream of the University of Southern Queensland’s Mach 6 wind tunnel ( $$\rho _{\infty } \approx {34\, \hbox {gm}^{-3}}$$ ) have been measured using a focused laser differential interferometer (FLDI) The direct contribution of the turbulent shear layer from the Mach 6 nozzle to the FLDI signal was largely eliminated by mechanically shielding the FLDI beams from these effects This improvement significantly enhanced the low wavenumber FLDI spectra which allowed a von Karman spectrum fit and demonstrated a $$-5/3$$ roll-off in the inertial subrange and enabled the identification of the integral length scale (28–29 mm) The normalised root-mean-square density fluctuations were found to change over the flow duration (typically between 04 and $$06\%$$ ) for the 1–250 kHz frequency range which corresponds to the wavenumber range of 6– 1600 $${\mathrm{m}}^{-1}$$ in this Mach 6 flow Previous disturbance measurements using intrusive methods have identified a narrowband 3–4 kHz disturbance that is first measured in the core flow about 65 ms after the flow begins and remains until the flow terminates The onset of this narrowband disturbance was previously correlated with transition-to-turbulence in the subsonic test gas supply to the nozzle This correlation was investigated further herein, and the 3–4 kHz feature was inferred to be entropy mode disturbances by showing the departure of the FLDI measurements from Pitot pressure measurements Through the comparison of FLDI and Pitot pressure data, Pitot pressure probes were demonstrated to produce a poor estimate of the static pressure fluctuations when non-isentropic disturbances are non-negligible

On the unsteady throttling dynamics and scaling analysis in a typical hypersonic inlet-isolator flow.

Abstract: The flow field in a two-dimensional three-ramp hypersonic mixed-compression inlet in a freestream Mach number of $M_\infty=5$ is numerically solved to understand the unsteady throttling dynamics. Throttling conditions are simulated by varying the exit area of the isolator in the form of plug insets. Different throttling ratios between $0\leq \zeta \leq 0.7$ in steps of 0.1 are considered. No unsteadiness is observed for $\zeta\leq 0.2$ and severe unsteadiness is found for $0.3 \leq \zeta \leq 0.7$. The frequency of unsteadiness ($f$) increases rapidly with $\zeta$. As $\zeta$ increases, the amount of reversed mass inside the isolator scales with the frequency and the exit mass flow rate. A general framework is attempted to scale the unsteady events based on the gathered knowledge from the numerical study. The inlet-isolator flow is modeled as an oscillating flow through a duct with known upstream design conditions like the freestream Mach number ($M_\infty$) and the isolator inlet Mach number ($M_i$). Factors like the mass occupied by the duct volume, the characteristic unsteady frequency, throttling ratio, and the exit mass flow rate through the duct are used to form a non-dimensional parameter $\beta$, which scales with the upstream design parameter $\xi=M_i/M_\infty$. The scaling parameters are further exploited to formulate a semi-empirical relation using the existing experimental results at different throttling ratios from the open literature. The unsteady frequencies from the present two-dimensional numerical exercise are also shown to agree with the proposed scaling and the resulting semi-empirical relation.

A bioinspired Separated Flow wing provides turbulence resilience and aerodynamic efficiency for miniature drones

Abstract: Small-scale drones have enough sensing and computing power to find use across a growing number of applications. However, flying in the low–Reynolds number regime remains challenging. High sensitivity to atmospheric turbulence compromises vehicle stability and control, and low aerodynamic efficiency limits flight duration. Conventional wing designs have thus far failed to address these two deficiencies simultaneously. Here, we draw inspiration from nature’s small flyers to design a wing with lift generation robust to gusts and freestream turbulence without sacrificing aerodynamic efficiency. This performance is achieved by forcing flow separation at the airfoil leading edge. Water and wind tunnel measurements are used to demonstrate the working principle and aerodynamic performance of the wing, showing a substantial reduction in the sensitivity of lift force production to freestream turbulence, as compared with the performance of an Eppler E423 low–Reynolds number wing. The minimum cruise power of a custom-built 104-gram fixed-wing drone equipped with the Separated Flow wing was measured in the wind tunnel indicating an upper limit for the flight time of 170 minutes, which is about four times higher than comparable existing fixed-wing drones. In addition, we present scaling guidelines and outline future design and manufacturing challenges.

Time-Accurate Experimental Investigation of Hypersonic Inlet Buzz at Mach 5

Abstract: Hypersonic inlet buzz is investigated experimentally in a ramjet intake at Mach 5. A two-dimensional planar inlet featuring a double-compression ramp with 10 and 22° inclination to the freestream d...

Transition scenario in hypersonic axisymmetrical compression ramp flow

Abstract: A high-fidelity simulation of the shock/transitional boundary layer interaction caused by a 15-degrees axisymmetrical compression ramp is performed at a freestream Mach number of 5 and a transitional Reynolds number. The inlet of the computational domain is perturbed with a white noise in order to excite convective instabilities. Coherent structures are extracted using Spectral Proper Orthogonal Decomposition (SPOD), which gives a mathematically optimal decomposition of spatio-temporally correlated structures within the flow. The mean flow is used to perform a resolvent analysis in order to study non-normal linear amplification mechanisms. The comparison between the resolvent analysis and the SPOD results provides insight on both the linear and non-linear mechanisms at play in the flow. To carry out the analysis, the flow is separated into three main regions of interest: the attached boundary layer, the mixing layer and the reattachment region. The observed transition process is dependent on the linear amplification of oblique modes in the boundary layer over a broad range of frequencies. These modes interact nonlinearly to create elongated streamwise structures which are then amplified by a linear mechanism in the rest of the domain until they break down in the reattachment region. The early nonlinear interaction is found to be essential for the transition process.

Parametric Experimental Studies on Supersonic Flow Unsteadiness over a Hemispherical Spiked Body

Abstract: Experimental studies are carried out to investigate the effects of the geometrical parameters with a drag reducing spike on a hemispherical forebody in a supersonic freestream of M∞=2.0 at 0 deg an...

Expansion Tube Freestream Disturbance Measurements using a Focused Laser Differential Interferometer

Abstract: A focused laser differential interferometer (FLDI) is applied to Caltech's Hypervelocity Expansion Tube (HET). FLDI is shown to be sensitive enough to make high-bandwidth measurements of flow features at low-density conditions. The freestream disturbance level is quantified as a function of the sound speed ratio across the primary contact surface. This is done to experimentally investigate whether the freestream disturbances obey the acoustic wave theory proposed by Paull and Stalker. It is found that disturbance magnitudes generally decrease with reductions in this sound speed ratio, as predicted by the theory, but some anomalous results are observed that suggest additional controlling parameters beyond the acoustic model.

Direct Numerical Simulations of Bypass Transition over Distributed Roughness

Abstract: Bypass transition in a boundary layer subjected to freestream turbulence and distributed surface roughness is studied numerically. The distributed surface roughness is reproduced with an immersed b...

Universal scaling parameter for a counter jet drag reduction technique in supersonic flows

Abstract: The reduction in aerodynamic drag by injecting a gaseous jet from the nose of a blunt body into a supersonic stream is investigated numerically. The penetration of the jet into the supersonic flow modifies the shock structure around the body and creates a low pressure recirculation zone, thereby decreasing the wave drag significantly. Combining various theoretical estimates of different flow features and numerical simulations, we identify a universal parameter, called the jet to freestream momentum ratio (RmA), which uniquely governs the drag on the blunt body. The momentum ratio fundamentally decides the penetration of the jet as well as the extent of low pressure envelope around the body. In addition, various influencing parameters reported in the literature are reviewed for different steady jet flow conditions. Furthermore, their limitations in regulating the flowfield are explained by correlating the facts with the jet to freestream momentum ratio. We perform the simulations for various combinations of physical and flow parameters of the jet and the freestream to show a universal dependence of drag on the momentum ratio.

Turbulent flow and heat flux analysis from validated large eddy simulations of flow past a heated cylinder in the near wake region

Abstract: We conduct non-isothermal large eddy simulations (LESs) of flow past a heated cylinder (Re = 3900) to investigate flow physics throughout the wake region and develop a foundation upon which future heat flux wall models can be built (both for wall-modeled LES and other lower fidelity models) for mathematical closure of the energy equation. A rigorous validation of the mesh is made under isothermal conditions with results showing a closer match to experimental data than any other LES studies to date. The insights gained into the mesh design and approach are discussed. Simulation of non-isothermal flow is performed on the validated mesh for temperature differences between the cylinder surface and the freestream of 25 K and 300 K. The mesh design and realistic (temperature-dependent) thermodynamic property variations play key roles in predicting delayed separation, larger re-circulation zones, and enhanced turbulence intensity for the higher temperature difference case. The effect of both temperature differences on the flow is analyzed, and a new scaling of the flow domain is proposed to gain further insight into non-isothermal flow physics. Key scaling variables, friction temperature and friction velocity, are able to reduce nearly all of the temperature dependence of first and second order flow statistics, including turbulent heat fluxes. This leads to the finding that the turbulent heat flux in the wake region scales with the wall heat flux irrespective of the temperature difference in the flow.

Dynamic Stall Control around Practical Airfoil Using Nanosecond-Pulse-Driven Dielectric Barrier Discharge Plasma Actuators

Abstract: The flow control effects of a nanosecond-pulse-driven dielectric barrier discharge plasma actuator (ns-DBDPA) in dynamic stall flow were experimentally investigated. The ns-DBDPA was installed on the leading edge of an airfoil model designed in the form of a helicopter blade. The model was oscillated periodically around 25% of the chord length. Aerodynamic coefficients were calculated using the pressure distribution, which was obtained by the measurement of the unsteady pressure by sensors inside the model. The flow control effect and its sensitivity to pitching oscillation and ns-DBDPA control parameters are discussed using the aerodynamic coefficients. The freestream velocity, the mean of the angle of attack, and the reduced frequency were employed as the oscillation parameters. Moreover, the nondimensional frequency of the pulse voltage, the peak pulse voltage, and the type and position of the ns-DBDPA were adopted as the control parameters. The result shows that the ns-DBDPA can decrease the hysteresis of the aerodynamic coefficients and a flow control effect is obtained in all cases. The flow control effect can be maximized by adopting the low nondimensional frequency of the pulse voltage.

Time-Dependent Aerodynamic Loads on Single and Tandem Stores in a Supersonic Cavity

Abstract: To understand time-dependent aerodynamic loads on single and tandem (fore/aft) slender cylinders translated out into the supersonic freestream from a cavity at Mach 1.5, the normal force and pitchi...

Evaluation of a full-scale helium-filled soap bubble generator

Abstract: Various aspects of the design and operation of a full-scale helium-filled soap bubble generator are studied. Shadowgraphy, particle image/tracking velocimetry, hotwire anemometry, and Monte Carlo simulations are employed to investigate bubble production regimes, diameters, production rates, time responses, and the flow quality downstream from the full-scale system. Modifications to internal nozzle geometry are found to greatly impact the production regimes that the nozzles operate within. Specifically, improving the axisymmetry of the air flow within a nozzle leads to desirable bubble formation over a larger range of input combinations and the ability to operate at larger input rates in general. The input of bubble film solution (BFS) is also found to be important for ensuring proper operation, as both small and large inputs lead to undesirable production. A previously defined theoretical relationship (Faleiros et al., Exp Fluids 60:40, 2019) between input parameters and the mean bubble time response is confirmed but found to vary depending on nozzle operation, as spilled BFS and leaked helium during bubble formation cause deviation from theoretical operation. Monte Carlo simulations reveal that the spatial filtering of particle image velocimetry (PIV) reduces the standard deviation of the effective distribution of the bubble time responses by a factor of $${\text{PPIR}}^{ - 1/2}$$, where PPIR is the number of particles per interrogation region. This power law is used to derive an equation for estimating the minimum time scale of the flow that can be resolved using the bubbles from a given generator during applications of PIV. Finally, the wind tunnel flow downstream from a full-scale generator is found to be affected by the blockage of the structure, with the freestream deficit increasing by at most 1.2% of the mean and the freestream turbulence intensity increasing by at most 0.3% for freestream velocities of 6 m/s or greater.