Time-dependent Surface Pressure Research Articles

Dynamics of a single cavitation bubble near a cylindrical blind hole

Cavitation bubble dynamics, such as microjets and moving shock waves, are strongly influenced by their surroundings. We investigated the complex bubble behavior during the growth and collapse stages near a blind hole. The simultaneous study, which is a combination of experiment and numerical methods, was performed. The single bubble was induced by a laser focus beam near a blind hole in the cuvette and measured by a high-speed camera with an image processing method. Numerical predictions were conducted for a detailed further investigation of the pressure and velocity fields near and inside the bubble. A fully compressible two-phase Navier-Stokes solver based on the homogeneous flow assumption was used. Both experimental and numerical results showed good agreement with each other. Strong deformation of the bottom bubble surface evolution was observed at low standoffs above the hole entrance. An extremely high-velocity microjet with a mushroom cap-shaped bubble was predicted under high standoffs below the hole entrance. Secondary cavitation in the blind hole was observed experimentally, and we discussed the low-pressure region caused by the interaction among the shock wave, bubble, and blind hole. Consequently, the tendencies of maximum microjet velocity and time-dependent surface pressure in the collapse phase are suggested.

Numerical prediction of propeller-induced noise in open water and ship behind conditions

In this article, the underwater noise of non-cavitating propellers is investigated using a boundary element method (BEM) coupled with the Ffowcs Williams–Hawkings (FW–H) formulation in the presence of the hull. The propeller is mounted on the vessel with an infinite plane assumed at the design draft to consider the free surface or tunnel wall effect. The hydrodynamic BEM solves the flow field for the analysis of propeller performance, then the time-dependent surface pressures are taken as the known noise sources for the FW–H equation in the form proposed by Farassat. The hull geometry and its disturbance to the medium are considered by solving the boundary integral equations, and the half-space medium is modeled by adding an additional term to the full space Green’s function for an image body beyond the infinite plane. The propeller performance in open water condition is first investigated to validate BEM predictions against experimental measurements. Along with the overall noise patterns in the flow field, decay rates of noise with the distance from the propeller and limited influence of the linear noise terms of the FW–H equation are discussed. Afterward, the propeller-induced hull pressure fluctuations are investigated, and the time history of acoustic pressures shows reasonable agreement with experimental measurements. Direct noise from the propeller, its scattered noise by the hull, and their sums as the total noise are discussed. This study identifies the dominating noise source around the hull, which could provide a basis for the noise control strategy for a ship propulsion system.

Instrumentation development and testing of a wind turbine blade for sub-scale wake studies

Wind turbine blade aerodynamics and the resulting wake flow are complex, and wind tunnel testing of these flows can provide critical insight. The data from such tests are also valuable for validation of numerical models. For experiments using sub-scale turbines to be useful, the blade aerodynamics and wakes must exhibit the important features observed in utility-scale turbine flows. In this work, ∼1 m blades designed to produce physics relevant to larger scale turbines were manufactured and affixed to an existing turbine. One of the blades was instrumented for blade surface pressure measurements. Time-dependent surface pressure measurements coupled with instantaneous measurements of the inflow with Cobra probes and the wake with particle image velocimetry allowed for characterization of the inflow, blade flow, and near wake. The results demonstrate that the instrumentation was effective in characterizing the blade loading and the flow field. For the one test case discussed in this paper, the measurements of inflow, blade loading, and wake properties facilitate understanding of the wake's behavior.

Control of cellular separation using adaptive surfaces

We report results from an experimental investigation of the three-dimensional separation produced by a high-lift aerofoil at moderate incidence, with constant section, where the separation is controlled by the implementation of an adaptive surface. Mean and time-resolved measurements are made using a NASA GA(W)-1 aerofoil with AR=6 at Rec=3.5×105. Surface oil visualisation and stereo Particle Image Velocimetry (PIV) are used to explore the flow field. The mean topology of the flow identifies characteristic spanwise periodic behaviour, “stall cells”, along the surface of the model. Analysis of the time-dependent surface pressure shows two distinct frequencies within the flow field. The higher frequency appears at a Strouhal number, St≈0.2, representative of vortex shedding, and the typical von Kármán vortex street. The lower frequency appears at St≈0.02, observed as a global fluctuation in stall-cell extent. This lower frequency is apparent in many separated flows, but in the present context, appears to have received only little attention. It correlates with widely observed low-frequency unsteadiness in the wing loading around stall. While this mode is analogous to that observed in other types of separation, here the streamwise extent of the separation varies because the flow is separating from a curved surface rather than from a sharp edge; the width of the separated region also varies. We show that fully-reversible point actuations of an actuated surface with auxetic structure, introduced immediately upstream of the saddle point at the leading edge of the stall cell, reduce the extent of the separated region.

Experimental investigation of flowfield over an iced aerofoil

ABSTRACTWind-tunnel measurements were used to study the characteristics of the unsteady separation bubbles on a NACA 0015 aerofoil with simulated two-dimensional leading-edge glaze ice accretions. The unsteadiness present in the iced-aerofoil flowfield was determined using measurements of the time-dependent aerofoil surface pressure distribution at Reynolds number of 1.0 × 106. Additionally, the unsteady flow features were investigated through the power spectrum of the stream-wise velocity fluctuations using a hot-wire anemometry. The results showed that the highest value of root-mean-square fluctuation of the surface pressure consistently occurred upstream of the mean shear-layer reattachment location. Spectral analysis of stream-wise velocity fluctuations near reattachment location revealed evidence of the regular frequency at Strouhal numbers of 0.5-0.63. Moreover, the low-frequency oscillations associated with shear-layer flapping was also identified in the wake velocity spectra on the order of 10 Hz that resulted in Strouhal numbers of 0.0186-021.

Modeling of ultra-high-speed impact at the surface of an elastic half-space

A Semi-Analytical (SA) method is used to model impact loading at the free surface of an elastic half-space. The inertial effects are taken into account in the form of Green’s functions, also called influence coefficients. The influence coefficients are first computed and the simulation of an elastic shock with time-dependent surface pressure is performed. The effect of material properties, characteristic wave speed, the spatial distribution of the load, the impact size and the temporal form of the load is analyzed. It is observed that the dynamic response of a material which is submitted to dynamic loading depends strongly on the characteristic wave speed. The influence of the pressure pulse duration and peak magnitude is found to be also very significant, whereas the effect of the spot size and spatial distribution of the pressure is less significant.

Behaviour of cyanidin-3-glucoside, β-lactoglobulin and polysaccharides nanoparticles in bulk and oil-in-water interfaces

Particle size distributions as well the interfacial and rheological properties of the films at the oil/water interface were used to study the effect of the interacting system between β-lactoglobulin, cyanidin-3-glucoside and pectin or chitosan in buffer solutions. The particles obtained were smaller with cy-3-gluc-β-lg-pectin and had reduced polydispersity with cy-3-gluc-β-lg-chitosan. Based on time dependent surface pressure results, β-lg-pectin mixtures showed a slower increase at the beginning of the measurement, while β-lg-chitosan mixtures showed no differences with β-lg alone. Contrarily, dilatational properties increased for ternary chitosan mixtures, but they remained similar to the pure protein in ternary pectin mixtures. Cy-3-gluc interfacial properties were reduced by the presence of pectin and chitosan.The interactions between cy-3-gluc and the biopolymers that have been selected in the present work resulted in a lower content of free polyphenol, reduced antioxidant properties as well as free β-lg. The impact of this effect was more relevant when pectin was used.

Biosensing at Drop Scale Assisted by Electrowetting at Moderate Frequency

This paper addresses surface ageing of a functionalized substrate in digital microfluidics. A model system based on bovine serum albumin (BSA)-laden sessile drop is selected. AC ElectroWetting-On-Dielectrics (EWOD), increasingly used in digital micro-systems for medical assays, is implemented in coplanar configuration. The small frequency re- quired by EWOD actuation (f =100 - 1000Hz) enables shape oscillations of the latter sessile drop and internal streaming. As demonstrated from dynamic contact angle (DCA) imaging, adsorption of BSA at the solid substrate is promoted un- der EWOD, as also shown from the time-dependent solid-liquid surface pressure. For an oscillating mode, n = 6, our numerical simulations show that corner eddies located near the oscillating contact line yield an efficient molecular re- newal. In this way, oscillating EWOD could be used purposely as a tool to speed up label-free detection from DCA measurements.

Study and Comparison of Aerodynamic Forces on an Aircraft and Suspension Bridge

Aerodynamic forces are time varying forces experienced by objects undergoing air flow. The effect of these forces depends upon the shape and size of the objects. The aerodynamic force acting on a bluff body like bridge deck is different from that acting on a stream line body like an aircraft.This paper focus on the study and comparison of aerodynamic forces acting on an air craft and a bridge deck. I. INTRODUCTION Researches are booming in the area of aero-dynamics of civil structures, which are not usually designed to influence or accommodate the airflow over them, but rather with other objectives in view. The aero-dynamics of such structures is characterized by separated flow and turbulent wakes exhibiting widely varying degrees of flow organizations. A body immersed in a fluid flow is subjected to surface pressures induced by the flow. If the oncoming flow is turbulent, this will be one of the sources of time dependent surface pressure. If the body moves or deforms appreciably under the induced surface pressure, these deflections, changing as they do the boundary conditions of the flow, will affect the fluid forces, which in turn will influence the deflections. Aero- elasticity is the discipline concerned with the study of the phenomenon wherein aero-dynamic forces and structural motions interact significantly. If the body in the fluid flow deflects under some forces and the initial deflection gives rise to successive deflections of oscillatory and/or divergent character, aero-elastic instability is said to be produced. All aero-elastic instabilities involve aero-dynamic forces that act on the body as a consequence of its motion. Such forces are termed self-excited. A body is said to be aerodynamically bluff when it causes the wind flow around it to separate from its surface leaving a significant trailing wake. In contrast, wind flow around a streamlined body remains tangential and attached to its entire surface, leaving a narrow trailing wake. Most civil engineering structures, including the bridge sections of the long span bridges qualify as bluff bodies, while the shapes of an airfoil belong to the category of a streamlined body. The fundamental aspects of aero-elastic phenomena that need to be taken into account in the design of certain structural members, towers, stacks, tall buildings, suspension bridges, cable roofs piping system and power lines are not completely understood. Without aerodynamics airplanes cannot fly. Thus while considering performance of an aircraft it is no surprise that aerodynamics is a vital aspect. The philosophy of good aerodynamics is primarily derived from low drag. Aerodynamics is dynamics related with flow of air. It is classified into two external aerodynamics and internal aerodynamics. External aerodynamics deals with flow of air over a body. Internal aerodynamics deals with flow of air with in ducts.

Hypersonic Experimental Analysis of Impulse Generation in Airbreathing Laser Thermal Propulsion

This experimental campaign focused on the analysis of the feasibility for impulse generation of a repetitively pulsed airbreathing laser lightcraft at hypersonic speeds. The future application of interest for this basic research endeavor is the laser launch of nano- and microsatellites (i.e., 1–100 kg payloads) into low Earth orbit, at low cost and on demand. These laser propulsion experiments employed a hypersonic shock tunnel, integrated with twin gigawatt pulsed Lumonics 620-TEA lasers to produce the required test conditions. Time-dependent surface pressure distributions were recorded together with schlieren movies of the flowfield structure resulting from the laser energy deposition, at nominal Mach numbers ranging from 6 to 10. The laser induced breakdown occurring off-surface and across the inlet’s midchannel was analyzed and discussed against previous theoretical and experimental results. The data indicated laser induced pressure increases of 0.7–0.9 bar with laser pulse energies of , on an off-shroud induced breakdown condition and a freestream Mach number of six. The results of this research corroborate the feasibility of laser powered airbreathing flight with infinite specific impulse ().

Tip Speed Ratio Influences on Rotationally Augmented Boundary Layer Topology and Aerodynamic Force Generation

Under zero yaw conditions, rotational effects substantially and routinely augment HAWT blade aerodynamic response. To better comprehend the fluid dynamic mechanisms underlying this phenomenon, time dependent blade surface pressure data were acquired from the National Renewable Energy Laboratory (NREL) Unsteady Aerodynamics Experiment (UAE), a full-scale HAWT tested in the NASA Ames 80 ft×120 ft wind tunnel. These surface pressure data were processed to obtain normal force and flow field topology data. Further analyses were carried out in a manner that allowed tip speed ratio effects to be isolated from other confounding influences. Results showed clear correlations between normal forces, flow field topologies, and tip speed ratios. These relationships changed significantly at different blade radial locations, pointing to the complex three-dimensional flow physics present on rotating HAWT blades.

Interfacial and foaming properties of prolylenglycol alginates: Effect of degree of esterification and molecular weight

In the present work we have studied the characteristics of propylene glycol alginates (PGA) adsorption at the air–water interface and the viscoelastic properties of the films in relation to its foaming properties. To evaluate the effect of the degree of PGA esterification and viscosity, different commercial samples were studied—Kelcoloid O (KO), Kelcoloid LVF (KLVF) and Manucol ester (MAN). The temperature (20 °C) and pH (7.0) were maintained constant. For time-dependent surface pressure measurements and surface dilatational properties of adsorbed PGA at the air–water interface an automatic drop tensiometer was used. The foam was generated by whipping and then the foam capacity and stability was determined. The results reveal a significant interfacial activity for PGA due to the hydrophobic character of the propylene glycol groups. The kinetics of adsorption at the air–water interface can be monitored by the diffusion and penetration of PGA at the interface. The adsorbed PGA film showed a high viscoelasticity. The surface dilatational modulus depends on the PGA and its concentration in the aqueous phase. Foam capacity of PGA solutions increased in the order KO > MAN > KLVF, which followed the increase in surface pressure and the decrease in the viscosities of PGA solutions. The stability of PGA foams monitored by the drainage rate and collapse time follows the order MAN > KLVF > KO. The foam stability depends on the combined effect of molecular weight/degree of esterification of PGA, solution viscosity and viscoelasticity of the adsorbed PGA film.

Boundary Layer State and Flow Field Structure Underlying Rotational Augmentation of Blade Aerodynamic Response

Blade rotation routinely and significantly augments aerodynamic forces during zero yaw horizontal axis wind turbine (HAWT) operation. To better understand the flow physics underlying this phenomenon, time dependent blade surface pressure data were acquired from the National Renewable Energy Laboratory (NREL). Unsteady Aerodynamics Experiment (UAE), a full-scale HAWT tested in the NASA Ames 80-by-120-foot wind tunnel. Time records of surface pressures and normal force were processed to obtain means and standard deviations. Surface pressure means and standard deviations were analyzed to identify boundary layer separation and shear layer impingement locations. Separation and impingement kinematics were then correlated with normal force behavior. Results showed that rotational augmentation was linked to specific separation and impingement behaviors, and to associated three-dimensionality in surface pressure distributions.

Large eddy simulation of flow over a wooded building complex

An efficient large eddy simulation algorithm is used to compute surface pressure distributions on an eleven story (target) building on the NIST campus. Local meteorology, neighboring buildings, topography and large vegetation (trees) all play an important part in determining the flows and therefore the pressures experienced by the target. The wind profile imposed at the upstream surface of the computational domain follows a power law with an exponent representing a suburban terrain. This profile accounts for the flow retardation due to friction from the surface of the earth, but does not include fluctuations that would naturally occur in this flow. The effect of neighboring buildings on the time dependent surface pressures experienced by the target is examined. Comparison of the pressure fluctuations on the single target building alone with those on the target building in situ show that, owing to vortices shed by the upstream buildings, fluctuations are larger when such buildings are present. Even when buildings are lateral to or behind the target, the pressure disturbances generate significantly different flows around this building. A simple grid-free mathematical model of a tree is presented in which the trunk and the branches are each represented by a collection of spherical particles strung together like beads on a string. The drag from the tree, determined as the sum of the drags of the component particles, produces an oscillatory, spreading wake of slower fluid, suggesting that the behavior of trees as wind breakers can be modeled usefully.

A model of roof-top surface pressures produced by conical vortices : Model development

The objective of this study is to understand the flow above the front edge of low-rise building roofs. The greatest suction on the building is known to occur at this location as a result of the formation of conical vortices in the separated flow zone. It is expected that the relationship between this suction and upstream flow conditions can be better understood through the analysis of the vortex flow mechanism. Experimental measurements were used, along with predictions from numerical simulations of delta wing vortex flows, to develop a model of the pressure field within and beneath the conical vortex. The model accounts for the change in vortex suction with wind angle, and includes a parameter indicating the strength of the vortex. The model can be applied to both mean and time dependent surface pressures, and is validated in a companion paper.

Oscillating Cascade Aerodynamics at Large Mean Incidence

The aerodynamics of a cascade of airfoils oscillating in torsion about the midchord is investigated experimentally at a large mean incidence angle and, for reference, at a low mean incidence angle. The airfoil section is representative of a modern, low-aspect-ratio, fan blade tip section. Time-dependent airfoil surface pressure measurements were made for reduced frequencies of up to 1.2 for out-of-phase oscillations at a Mach number of 0.5 and chordal incidence angles of 0 and 10 deg; the Reynolds number was 0.9 × 106. For the 10 deg chordal incidence angle, a separation bubble formed at the leading edge of the suction surface. The separated flow field was found to have a dramatic effect on the chordwise distribution of the unsteady pressure. In this region, substantial deviations from the attached flow data were found, with the deviations becoming less apparent in the aft region of the airfoil for all reduced frequencies. In particular, near the leading edge the separated flow had a strong destabilizing influence while the attached flow had a strong stabilizing influence.

Effects of Metal Cations on the Dynamic Characteristics of a Phospholipid Thin Film at an Air/Water Interface

Abstract We measured the time-dependent surface pressure of a dioleoylphosphatidylcholine film at an air/water interface. Our results indicate that: (i) the surface pressure of the thin film traces a closed hysteresis loop accompanied by repetitive cycles of compression and expansion, and (ii) the area of the hysteresis loop markedly increases in the presence of sodium ions. We propose a kinetic model for this phenomenon which includes cooperative aggregation/dissociation at the air/water interface. The experimental findings have been successfully reproduced with the aid of a computer simulation based on this theoretical model.

Time-dependent supersonic separation of tangent bodies

An experimental, time-dependent separation of tangent bodies was performed in a supersonic wind tunnel (Mach 1.5 and 1.9) to investigate the significance of transient effects and the suitability of using steady-state assumptions to predict a dynamic separation event. Model configurations consisted of two bodies placed in a near tangent position. A stationary body, a plate or ogive, was instrumented to obtain dynamic surface pressures, while a second body, a wedge attached to an air cylinder, was plunged in a constrained motion away from and toward the stationary model. Three-dimensional flow expansion around the edge of the wedge reduced the strength of incident shock waves and created a region of low pressure, near freestream static, on body surfaces between the incident and reflection shock waves. Dynamic motion of the wedge did not significantly affect shock-wave development between the bodies, and steady-state corrections that accounted for the motion-induced wedge angle were appropriate for predicting time-dependent surface pressures induced by the incident shock wave. However, unsteady pressures caused from the motion of the wedge were evident when separation distances were less than 20% of the wedge width.

Acoustic time-dependent loading on elastic shells of revolution using the internal source density and singular value decomposition method

A new representation for the time-dependent surface pressure on a fluid-loaded shell of revolution with a prescribed space and time-dependent axisymmetric velocity distribution is presented. This representation, which is based on expanding the surface traction and velocity vectors into in-vacuo modal vector expansions with time-dependent coefficients, results in each time-dependent modal pressure coefficient being expressed as a sum of temporal convolution integrals which involve time-dependent modal radiation impulse responses and modal velocities. After presenting the formulation of the appropriate initial-boundary value problem, Fourier transform methods are introduced to obtain the corresponding Neumann boundary value problem. The solution of this latter problem is formulated as a minimum mean square error problem in which linear monopole harmonic distributions along the axis of the shell are introduced to determine the surface pressures resulting from the normal components of the modal vectors of interest. Singular value decomposition methods are then used to determine the harmonic monopole distributions. Modal decomposition and inverse Fourier transform methods are then used to evaluate the time-dependent modal radiation impulse responses. Numerical results are presented for the self and mutual modal radiation impulse responses for both spherical and spheroidal shells.

Acoustic transient radiation from fluid-loaded shells of revolution using time-dependent in vacuo eigenvector expansions

A new method is presented to evaluate the acoustic transient radiation from fluid-loaded shells of revolution which are excited by axisymmetric mechanical or acoustical excitations. This method is based on the use of an in-vacuo modal vector (eigenvector) expansion with time-dependent coefficients to describe the velocity field of the fluid-loaded shell. Modal acoustic impulse responses which account for the coupling between the in-vacuo modal vectors for the fluid-loaded shell are introduced in the formulation of the general solution. The time-dependent modal velocity coefficients are expressed as the solution of a set of coupled convolution integral equations which are readily solved by marching forward in time. The associated time-dependent surface pressure is subsequently expressed as a modal vector expansion in which the time-dependent coefficients are readily related to the modal velocity coefficients. Since the surface velocity and pressure are then known, the time-dependent pressures in the field are simply obtained via quadrature methods from the space-time Kirchhoff integral solution. The special case of a fluid-loaded spherical shell is addressed to illustrate the application of the method to determine the general characteristics of the transient velocity response of the shell and the associated pressure field resulting from axisymmetric mechanical force excitations of spherical shells. Numerical results are presented to illustrate the effects on the velocity response and pressure field of time-dependent fluid coupling between the modal vectors corresponding to the upper and lower branches of the in-vacuo frequency spectrum of the spherical shell.